EXISTING CONDITIONS PHOTOS SPRING 2018
Lack of effective grading and stormwater conveyance infrastructure results in persistant ponding during heavy rain events, which is problematic for pedestrians, drivers, and residents.
Photo B
The smooth, vertical-walled drainage canals causes floodwater to flow rapidly downstream, transporting pollutants and sediment with little chance to settle or be filtered.
Photo C
A different section of the same canal pictured above has a natural, rather than hard edge. Vegetation along the canal edge provides increased surface area to slow down water movement.
Photo E (Off map)
Aging infrastructure may not be able to withstand the increased projections of flooding in Waipahu.
The Pearl Harbor bike path bridge over Wailani Canal may be deteriorating due to accumulation of stagnant water.
Soil in portions of Waipahu contain high amounts of clay, which results in low water absorbtion rates.
Photo A
Vegetation along stream edges provides habitat to support bio-diversity. However, allowing vegetation to grow rampant and pollution to accumulate can cause upstream backup of flood water as it travels downstream.
Photo F
Photo D
Credit: K. Yamamotoya
Credit: S. Mendes
Photo credit: H. Au
Photo credit: N. Nishimura
Credit: L. Gonzales
Credit: K. Yamamotoya
FLOOD MITIGATION 09
“Water”, several watersheds were assessed using landscape development indices (LDI) on a scale of 1 (natural landscapes) to 10 (central business districts). Pouhala Marsh was rated the highest value of all study sites: 6.9. In the 2006 watershed study, it is stated that “before Kapakahi Stream was isolated from Waikele Stream by the flood control berm, occasional high flows in the stream would help transport trapped sediment out of the stream and into Pearl Harbor. Because the flushing does not occur, the sediment accumulates in the stream”. Improvements to the watershed such as dechannelizing Kapakahi stream, removal of invasive species, and clearing of artificial fill may allow natural ecological functions to return to Pouhala Marsh and improve the site’s overall health. Both Pouhala Marsh and lower Kapakahi Stream mark the end point of the Pearl Harbor Historic Bike Path. Designating and improving the area to become a destination point may increase desirability and use of the recreational path.
Waipahu Drainage Canal
This water body has many names; commonly referred to as the Waipahu drainage canal as it runs near the Waipahu District Park, Wailani Stream or Wailani Stream drainage channel as it passes residences near the Ted Makalana Golf Course, and less commonly, E`o Stream (see Figure 2). It is a channelized concrete canal beginning at Waikele County Club and flowing to its outlet in Pearl Harbor. The waterway runs between Waipahu District Park and the parcel housing Waipahu Civic Center and Waipahu Public Library. The community has expressed interest in bridging over the canal as it currently separates the recreational park from the Waipahu town core. Several articles posted by Hawaii News Now,, in recent years sited the chronic illegal dumping and wastewater spillage problems along the canal. Quantifiable data of pollution levels for the Waipahu drainage canal is limited, but verbal opinions from residents suggest that the waterway may be a health concern.
Waipahu District Park
Waipahu District Park is a fourteen acre recreational facility that supports baseball, football, swimming, tennis, and more. It is bordered by Waipahu Drainage Canal to the west, Paiwa St.to the east, and Kahuamoku St. to the south. The eastern and southern edges of the park are lined be large monkeypod trees and while the park is mostly covered in a permeable grass lawn, little other vegetation exists.
State-owned Parcels
Six State-owned parcels on the northeast side of the study area (see Figure 2) are described below with their function and vegetated area, and named by the tax parcel number. The current programs for the six parcels include vehicle parking, multi-family housing, library, civic center, and adult day care. For more information on the property owners and current conditions, see Appendix C: State-Owned Parcels.
Flooding Background Information
This section provides background information on flooding that relates to the Waipahu TOD site. The TOD plans and interviews with residents illustrate the need to address flooding. The Sea Level Rise Viewers and Hawaii Sea Level Rise report estimate projected potential damage, and potential financial incentives or savings are described.
The September 2017 Waipahu Town Action Plan identified three priority actions, including “Address Areawide Flooding”. In the June 2016 Waipahu Transit Center Station Brochure, one of seven plan highlights is, “Address the flood hazard limitations of the transit station area.”
The newly released Hawaii Sea Level Rise Vulnerability and Adaptation Report recommendation highlights for Oahu include, “Develop design standards to increase flood resiliency for existing
10 WAIPAHU TOD COLLABORATION
and new development within the SLR-XA that cannot be relocated.”
During a March 2017 TOD focused community workshop, Hawaii News Now interviewed residents for recommendations on improvements. Upgrading the Waipahu Transit Center and addressing area-wide flooding issues were two of the most supported recommendations during the workshop. Waipahu resident Ryan Ng said, “What I focused on mainly was the infrastructure. I think without infrastructure you’re not going to be able to do a lot of the things that we want to do.”
In addition to property damage above grade, utilities that lie below grade are also in critical danger. “Utilities, such as water, wastewater and electrical systems often run parallel and underneath roadways, making lost road mileage a good indication of the extent of lost utilities. This chronically flooded infrastructure would have significant impacts on local communities as well as reverberating effects around each island through loss of commerce, loss of access to emergency services, and increased traffic on other roads and highways.” Please see a later section in this report, Appendix B: Micro Maps, which illustrate where future SLR affects storm sewer lines and inlets as well as sanitary sewers.
In parts of Waipahu, low lying areas close to Pouhala Marsh, streams, and canals, will be the first areas to experience SLR. However, it is important to note that damage due to SLR will affect more than just the localized area. “Hot spots for potential economic loss across the State are centered in urban areas with the greatest potential loss in Honolulu on the Island of O‘ahu, with 66% of the total statewide economic loss, due to the density and economic assets potentially exposed to SLR. As a result, the impacts of SLR on O‘ahu could generate substantial social, infrastructure, and economic impacts with ripple effects throughout the State.” The Waipahu TOD study area provides an excellent opportunity to use resilient design planning and building methods presented in this report and beyond.
The Waipahu TOD study area may also gain financial benefit from flood mitigation planning through flood insurance discounts and reduced disaster recovery costs. As mentioned in the Hawaii Sea Level Rise Vulnerability and Adaptation Report, areas in Hawaii may pursue the Federal Emergency Management Authority (FEMA) Community Rating System (CRS), which encourages floodplain management activities beyond National Flood Insurance Program (NFIP) standards, and, “depending upon the level of participation, flood insurance premium rates for policyholders can be reduced up to 45%.” This report is not geared toward the purpose of pursuing the FEMA Community Rating System.
In addition, FEMA reports that “for every $1 spent on mitigation, it saves society an average of $4.…Creditable activities include: land acquisitions and restoration to open space uses, relocation, flood-proofing, open space preservation, and other measures that reduce flood damages.”
The Hawaii Sea Level Rise Vulnerability and Adaptation Report includes a list of nine recommendations for addressing future impacts of SLR in the State. This report is related to three selected recommendations from the study:
1. Recommendation 1: Support sustainable and resilient land use and community development;
2. Recommendation 2: Prioritize smart urban redevelopment outside the Sea Level Rise Exposure Area (SLR-XA) and limit exposure within the area; 3. Recommendation 3: Incentivize improved flood risk management.
FLOOD MITIGATION 11
Extents of Projected Flooding
When researching flood mitigation strategies for this or other sites, one must first understand the causes of flooding and the extent of flooding. Occurring in isolation or simultaneously, temporary flood shocks include storm surge, king tides, intense rainfall, and stormwater runoff; as well was the long-term flood stressor of sea level rise and groundwater inundation. The following maps illustrate anticipated flooded areas depicting FEMA Flood Insurance Rate Maps; maps of various levels of sea level rise; and maps of riverine flooding. The maps show that flooding threatens the existing and proposed roadways, buildings, and some of the rail transit area. Impervious buildings, roads, and slow-draining clay soils exacerbate flooding in the area.
FEMA FLOOD INSURANCE RATE MAPS
The FEMA FIRM flood zones are used to identify flood risk for flood insurance rate purposes. The FEMA FIRMs are of interest in order to understand the extent of anticipated flooding and BFEs for buildings where available.
It is important to recognize the limitations of the FEMA FIRMs. As of 2018, the FEMA FIRMs for the Waipahu TOD study area do not include flooding from future sea level rise. El Niño events are associated with more intense rainfall in Hawaii and these events will be more common in the future. According to a 2017 study on El Niño frequency predictions, “extreme El Niño events continue to increase after the GMT (global mean temperature) peaks (at 1.5°C) and stabilizes beyond 2050, from about 10 events per 100 years … at 1.5°C warming … to about 14 events per 100 years beyond 2050.” The study was conducted after the Paris Agreement established the goal of limiting global warming to 1.5°C.
Within the study site, there are five FEMA flood zones that are described below. Definitions of other flood zones are omitted for brevity.
SEA LEVEL RISE
Figure
the combined exposure
level
flooding, annual high wave flooding, and coastal
Change Mitigation and Adaptation Commission.
* Hawaii Sea Level Rise Vulnerability and Adaptation Commission. 2017. Hawaii Sea Level Rise Vulnerability and Adaptation Report. Prepared by Tetra Tech, Inc. and the State of Hawaii Department of Land and Natural Resources, Office of Conservation and Coastal Lands, under the State of Hawaii Department of Land and Natural Resources Contract no. 64064.
This study provides the following replicable process to map estimated sea level rise flooding from various sources of information, including the Pacific Islands Ocean Observing System (PacIOOS) and the National Oceanic and Atmospheric Administration (NOAA). Next, this study references recent planning directives from the Honolulu Mayor, the Hawaii Climate Change Commission, and the Hawaii Sea Level Rise Reports, 2017 and future updates. It is important to note that this report does not address flooding due to natural disasters such as hurricane or tsunami, but hurricane inundation maps
Figure
4.
Waipahu TOD FEMA Flood Insurance Rate Map
COASTLINE OCEAN ISLAND coastal erosion annualhighwaveflooding passiveflooding SLR exposure area
5. Schematic diagram of the Sea Level Rise Exposure Area (SLR-XA) as
to sea
rise from passive
erosion. Source: Hawaii Climate
*
12 WAIPAHU TOD COLLABORATION
may be found in the Appendix.
In July 2018, Honolulu Mayor Kirk Caldwell issued a “formal directive to all city departments and agencies to take action in order to address, minimize the risks from, and adapt to the impacts of climate change and sea level rise.”
“In its Sea Level Rise Guidance, the commission emphasized that the city should be planning for high tide flooding associated with 3.2 feet of sea level rise by mid-century, and, because of continued high global carbon emissions, take into consideration 6 feet of sea level rise in later decades of the century, especially for critical infrastructure with long expected life spans and low-risk tolerance. The sea level rise guidelines recommended by the commission are consistent with findings by the Intergovernmental Panel on Climate Change and the National Oceanic and Atmospheric Administration (NOAA).” The new rail under construction, and the planned new buildings in the TOD have life spans into the latter decades of this century, so should be planned for 6 feet of sea level rise.
The impact of sea level rise at Waipahu is mainly due to passive flooding. The study area’s location inland of Pearl Harbor and with protection from Pouhala marsh shelters it from annual high wave flooding and erosion (see Figure 6). The following series of maps illustrate the extents of anticipated flooding with 3.2, 4, and 6 feet of sea level rise.
With 3.2 feet of sea level rise, the elevated water levels in streams and canals result in flooding of smaller roadways and buildings adjacent to the streams and in unattached low-lying areas. The flooded areas include smaller roads, detached homes, and low-rise commercial buildings. (See Figure 7). The map showing the 3.2 foot Sea Level Rise Exposure Area (SLRXA), include passive flooding, annual high wave events, and coastal erosion (see Figure 5) from the PacIOOS viewer.
With 4 feet of sea level rise, a significant number of residential roads and detached homes will be flooded near the intersection of the Wailani Stream Drainage Canal and the unnamed stream. Waipahu District Park further inland near Wailani Drainage Canal and Farrington Highway will also flood.
With 6 feet of sea level rise, there is widespread flooding of roads, detached homes, and commercial areas, future Transit Oriented Development areas, and parks and golf courses (see Figure 7). As previously stated, the 6 foot sea level rise maps from NOAA show modified bathtub flooding including “local tidal variability and hydrological connectivity.”
Please note the different mapping methods. The map showing the 3.2 foot and 4 foot Sea Level Rise Exposure Area (SLR XA), include passive flooding, annual high wave events, and
Zone Definition
AE Areas subject to inundation by the 1-percentannual-chance flood event determined by detailed methods. Base Flood Elevations (BFEs) are shown. Mandatory flood insurance purchase requirements and floodplain management standards apply.<?>
“Flood fringe area” means a special flood hazard area consisting of the area of the flood fringe designated on the flood insurance rate map as zone AE, AO and AH.<?>
AEF Floodway area” or “AEF” means a special flood hazard area consisting of the portion of zone AE designated on the flood insurance rate map as a floodway.<?>
X “shaded” (Orange dot)
X
“unshaded” (Orange solid)
Moderate flood hazard areas, labeled Zone B or Zone X (shaded) are also shown on the FIRM, and are the areas between the limits of the base flood and the 0.2-percentannual-chance (or 500-year) flood.<?>
The areas of minimal flood hazard, which are the areas outside the SFHA and higher than the elevation of the 0.2-percent-annual-chance flood, are labeled Zone C or Zone X (unshaded).<?>
D The Zone D designation is used for areas where there are possible but undetermined flood hazards, as no analysis of flood hazards has been conducted.<?>
FLOOD MITIGATION 13
coastal erosion from PacIOOS Hawaii Sea Level Rise Viewer. The 6 foot sea level rise maps from NOAA’ Sea Level Rise Viewer show modified bathtub flooding including “local tidal variability and hydrological connectivity.” Please note that the PacIOOS Hawaii Sea Level Rise Viewer data and maps “illustrate the scale, not the exact location, of potential flooding and erosion with sea level rise. The Hawaii Sea Level Rise Viewer should be used only as a screening-level resource to support management decisions to address exposure and vulnerability to coastal hazards with sea level rise.”
A new study recently published by researchers at the University of Hawaii estimate that twice the amount of land will be impacted by SLR than was previously modeled. The new data set includes annual high wave events and coastal erosion which provides additional information about future sea level rise impacts. The results from the modified simulations showed that, “depending on the island and SLR scenario, our results show that relying solely on the passive model results in missing 35–54 percent of the total land area that is exposed to one or more hazards.” Sea Level Rise simulations provided by NOAA’s Sea Level Rise Viewer uses passive, “bathtub” flooding models and does not take annual high wave events and coastal erosion into account. However, due to Waipahu’s location protected by Pearl Harbor and Pouhala Marsh, we anticipated less damage from wave events than the island as a whole.
RIVERINE FLOODING
In collaboration with the University of Hawaii’s Department of Urban Planning’s Fall 2017 practicum students, flood simulations for the study site were run through FEMA’s software, HAZUS, “a GIS-based software model which produces loss estimates for earthquakes, floods, hurricanes, and tsunamis based on state-of-the-art scientific and engineering knowledge and software architecture.” The riverine flood maps show flood events of varying severity: 1% annual chance rainfall event (100-year flood), 3% annual chance rainfall event (30-year flood), and 100% annual chance rainfall event (1-year flood). The 1 year river flood area primarily affects properties adjacent to the streams. However, the 30 year and 100 year riverine flood events cause widespread flooding in the future TOD area south of the rail station.
Rainfall that typically accompanies low pressure systems can cause flash flooding that begins far above in Oahu’s mountain ranges and flow heavily through waterways. The study site is particularly vulnerable due to its location between Waikele stream, Kapakahi stream, Waipahu drainage canal, and an unnamed waterway that runs parallel to the Pearl Harbor Historic Bike Path. Anecdotes of soil with high clay content limits water infiltration and exacerbates ponding and flooding.
FEMA produces flood zone maps based on riverine flooding data. Though these maps are typically used for establishing flood insurance rates through the NFIP, they show areas of flood risk in the event of heavy rainfall and provide information about the BFEs that will become a critical element for flood mitigation strategies in upcoming sections.
STORM SURGE AND KING TIDES
Additional flood hazards include storm surge and king tides.
“Storm surge is the abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide. The surge is caused primarily by a storm’s winds pushing water onshore.” Although publicly available information on storm surge predictions is limited, it is mentioned here because it could occur in the near future, e.g., in the event of a hurricane. However, data from Hurricane Iniki in 1992 can offer worst-case scenario input. “The stronger the storm, the higher the storm surge. Iniki created storm tides between 4–6
14 WAIPAHU TOD COLLABORATION
Figure
6. Waipahu existing
aerial
map. FLOOD MITIGATION 15
feet above normal. If a hurricane that strong struck Oahu, many areas of the south shore would be underwater.” “Storm tide is the total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide.” Severe storm surge would likely affect properties typically unaffected by coastal threats.
Regarding storm surge, the LEED resilience credit released in early 2019 suggests using the “NOAA Sea, Lake, and Overland Surges from Hurricanes (SLOSH) Model to interpolate storm surge” but it appears that model is only available when there is an impending hurricane near Hawaii, therefore additional depth for anticipated storm surge is unknown at this time.
More frequent king tides, which are exceptionally high tides, are anticipated in the coming century. The number of days per year with king tide flooding is anticipated to increase throughout the century. Sweet et al. estimated the arrival of king tide flooding at different frequencies: six, twelve, and twenty-four days per year. Flooding six days per year is estimated to arrive as early as 2024 and as late as 2038. Flooding twenty-four days per year is estimated to arrive as early as 2028 and late as 2045. One observed impact of the king tides is that they cause storm drains to work in reverse and act a conduit for ocean water to the streets of Waikiki in 2018 (see Figure 8).
Figure 8. Storm drains act in reverse during king tides in Waikiki in 2018. Photo: W. Meguro.
16 WAIPAHU TOD COLLABORATION
Waipahu District Park Pouhala Marsh Wildlife Sanctuary KapakahiStream Waikele Stream FARRINGTON HWY Wailani Stream Drainage Canal Wastewater Pump Station Clinic 3.2 Ft. SLR-XA exposure area LEGEND 6 Ft. Inundation above mean higher high water Waipahu Transit Center Station Proposed Land Use (June (2016) 0.5 mi radius 4 Ft. Inundation above mean higher high water Pouhala Station (NTS) State parcels Critical facilities State parcels ± 0 0.1 0.2 0.3 0.4 0.50.05 Miles SCALE 1:10,000 Figure 7. Waipahu 3.2 ft and 6 ft SLR and TOD plan FLOOD MITIGATION 17
FLOOD RESILIENCE DESIGN CRITERIA
This section of the report focuses on potential flood mitigation design criteria for the buildings, roads, and rail in the Waipahu TOD area at three scales: planning, site, and building scale. Addressing flood mitigation at the planning scale promotes the most comprehensive flood mitigation strategies. The discussion and illustrations are site-specific, but the process and references provided are applicable to other sites.
Planning-Scale Strategies
Comprehensive flood hazard mitigation strategies begin with an analysis of the site, as demonstrated in the replicable process of mapping earlier in this report. Design teams should then proceed as follows:
1. Consider phased retreat of buildings and infrastructure from areas that will be chronically flooded. Prioritize areas that will flooded soonest, such as those affected by 3.2 feet of sea level rise. Consider how the future flooded areas may be used for multiple benefits, such as human recreation and habitat creation.
2. Prioritize smart urban redevelopment outside of the SLR-XA and limit development within the area, as recommended by the Hawaii Sea Level Rise Vulnerability and Adaptation Report 3. Determine if FEMA flood zones should be used as criteria for the location of new development.
4. Identify large open spaces for temporary above grade water detention and potential for below grade water detention.
The diagrammatic map (see Figure 9) illustrates some of the planning-scale recommendations above, tailored for the Waipahu site.
Retreat should be considered for existing buildings within the areas affected by 3 feet of sea level rise, which primarily consists of buildings located immediately adjacent to waterways. These properties will experience flooding from sea level rise first, and roads and infrastructure serving these areas are expected to become permanently flooded as sea levels continue to rise. In the mid and long-term, retreat should be considered from areas affected by 4 feet and 6 feet of sea level rise. Avoiding new development within these areas is consistent with the RELi standard to avoid building on sites affected by 2’6” of sea level rise. When retreat occurs, consider using the areas for multiple benefits, as described in the upcoming section on widening stream floodways and creating a multi-use berm. If retreat is not a viable strategy, the building-scale adaptation with wet or dry-flood proofing strategies discussed later in this report may be considered to prolong the use of and access to those properties.
Second, “Prioritize smart urban redevelopment outside of the SLR-XA and limit development within the area,” as recommended by the Hawaii Sea Level Rise Vulnerability and Adaptation Report. The proposed Waipahu Neighborhood Plan created in 2014 should be re-evaluated to incorporate new knowledge about sea level rise. A portion of the new development is predicted to experience flooding with four to six feet of sea level rise. For example, it is evident that the proposed new residential neighborhoods near the Waipahu Refuse Convenience Center will experience flooding. A different site should be considered for development, outside of the areas affected by sea level rise.
Third, “Determine if FEMA flood zones should be used as criteria for location of new
18 WAIPAHU TOD COLLABORATION
Figure
9
Waipahu TOD Area Potential Future Flood Mitigation Strategies Diagram
(E) Waipahu Refuse Convenience Center
Pouhala Station
FLOOD MITIGATION 19
development.”
The diagrammatic map of potential strategies (see Figure 9) encourages dense development in the areas outside of the 100-year flood zone (1-percent-annual-chance flood event in FEMA FIRM Zone AE). This is a challenge because much of the study area is within the 100year flood zone. For reference, the RELi rating system takes an even more stringent approach, avoiding sites within a 500-year flood plain (Zone X).
Fourth, “Identify large open spaces for temporary above grade water detention, and potential for below grade water detention.” During a large rain event, riverine flooding event, or storm surge, these temporary detention areas could hold large volumes of water to reduce the flood depths at buildings, roads, and rail. Principles and precedent projects are presented in an upcoming section of this report.
Figure 9 (continued) Waipahu TOD Area Potential Future Flood Mitigation Strategies Diagram - Breakdown 20 WAIPAHU TOD COLLABORATION
RETREAT, EXPAND FLOODWAY, CREATE MULTI-USE BERM
At the Waipahu TOD site, there is an opportunity to retreat buildings on properties adjacent to streams and rivers, many of which are affected by 3.2 feet of sea level rise, create a wider floodway, and a multi-use berm. Modification to the Wailani and Kapakahi streams widths and banks presents an opportunity for flood protection, ecological restoration, human transportation, and recreation. The potential retreat, floodway, and berm are described and illustrated in this section of the report.
Retreat
Widening the floodways and creating a multi-use berm will encroach onto private property. This presents the opportunity for managed retreat from properties that will experience flooding from sea level rise soonest.
Buildings within areas of permanent inundation by the mid to end of the century may find that floodproofing methods may no longer be an option for extending the use of the structure. Property owners may incur costly maintenance or difficulty in accessing the structure. In this case, a government-led retreat would be a strategy aligned with the Hawaii Sea Level Rise Vulnerability and Adaptation Report recommendation to “incentivize improved flood risk management.”
Some houses that can be lifted may have the ability to be relocated to a new suitable location. Other houses may be “bought out.”
Although the financial mechanisms for retreat or buy out are not the focus of the report, they are briefly mentioned to show their potential and precedent. For houses that cannot be moved, funding to aid flood victims exist such as: “reverse mortgages offered by the state and funded by general obligation bond issues wherein owners turn over homes at the end of receiving a fixed period of payments; a homeowner donation program; tax relief for businesses or families in exchange for later ownership transfer…; transferrable development rights for businesses; county, state, and federal land conservation funds; land swaps…; conservation easements that pay owners to manage their land for the environment; and others”.
One precedent for government buy-out in the aftermath of several flood events occurred in Wisconsin, when the entire business district of Soldier’s Grove experienced multiple flooding events and decided to relocate. “The relocation cost of $7.1 million was raised primarily through innovative financing. Federal funds of just under $1 million were contributed only after the town refused to accept disaster relief funds slated for standard reconstruction”.
Figure 10. The section diagram above from Building Green describes the engineering terms for segments of a river basin, including the floodway. *
* Malin, Nadav. Letting Floodplains Do Their Job. Building Green. September 1, 1993. https://www.buildinggreen.com/feature/letting-floodplainsdo-their-job.
FLOOD MITIGATION 21
* Green, Jared. “New Park in Singapore Shows What Rivers Can Do.” The Dirt. https://dirt.asla.org/2012/07/25/new-park-in-singapore-shows-whatrivers-can-do/
One example of guidelines that discourage building within high flood risk areas is the resilient design rating system, RELi, which suggests that building within the 500-year flood zone or coastal zones affected by 2’-6” of SLR inundation be prohibited. Within the study area, properties higher than the 500-year flood zone are identified as Zone X, minimal flood hazard (solid orange areas in Figure 4). Much of the proposed new TOD buildings are in the 500-year flood zone, therefore, building scale flood mitigation strategies will be needed and are discussed later in this report.
Allow the Floodway to Flood
A potential strategy to prevent overtopping of stream and canal banks is to increase the width of the floodway adjacent to the waterways. Challenges include modification of the existing stream channel or bed.
Figure 12 Before and after aerial photos of the Kallang River de-channelization.
* Wikimedia Commons. Author Pagodashophouse. https://commons. wikimedia.org/wiki/File:Before_and_After_Aerial_View_of_Kallang_River.jpg
Widening the flood way could increase the volume of the stream, as compared to the current channelized streams. See Figure 10 for a section diagram of the floodway, the “expanded channel in which water flows during floods.” In addition, the stream banks could be elevated in an effort to protect surrounding areas from stream overtopping. The potential expansion of the floodway and elevated bank height would require calculation of anticipated water volumes, and the conceptual idea is presented here. The expanded floodway would be a strategy aligned with the Hawaii Sea Level Rise Vulnerability and Adaptation Report recommendation to “support sustainable and resilient land use and community development.”
Potentially changing portions of Kapakahi and Wailani streams from concrete channels to natural stream banks may have potential for ecological restoration. Concrete channelized streams are problematic because faster moving water swiftly carries sediment and pollutants downstream, to the detriment of estuaries, coastal fisheries, reefs, and offshore species. The channelized stream “offers no habitat to native species” and encourages development within adjacent flood plains. Future modified stream banks could be modified to restore the natural ecology. “Ideally, streams in Hawaii should have natural beds and banks. Cool, clear water should meander through features such as pools and riffles that also provide habitat to native species. Naturally overhanging vegetation should provide shade from the hot subtropical sun for the indigenous ecology.”
In natural floodplain areas, streams and rivers are allowed to overflow. This process has numerous benefits for the surrounding habitat including benefits to plants, invertebrates, birds, and animals that live on the banks. This also improves habitats, natural erosion and deposition, and potential removal of fine silt, which increases water clarity, in riparian and
Figure 11 A stream de-channelization and park project in Singapore is a useful precedent for Wailani Stream and Kapakahi Stream in Waipahu.*
*
22 WAIPAHU TOD COLLABORATION
Figure 13. Proposed changes to the existing Kapakahi Stream along Waipahu Depot Road include features that serve the natural ecology from Pouhala Marsh as well as pedestrians.
FLOOD MITIGATION 23
Figure 14 Penn/Olin’s design for a multi-use levee for the Rebuild by Design competition.
* PennDesign/Olin. “Hunts Point Lifelines”. Rebuild by Design, http:// www.rebuildbydesign.org/data/files/677.pdf
floodplain zones.
One challenge to modifying the existing Kapakahi and Wailani streams include relocation of existing buildings adjacent to the streams, which primarily consist of detached homes. The other challenge is the engineering and logistical planning required, which is addressed effectively in the following precedent project.
A built precedent project shows past success dechannelizing a stream and designating a flood plain adjacent to the stream. In Singapore, 1.7 miles of the previously channelized Kallang River was revived into a 1.8 mile, natural meandering waterway, in order to reduce peak flooding while increasing water security by slowing and collecting storm water runoff. “In a feat of sequenced engineering, [landscape architect Herbert] Dreiseitl managed to re-engineer soils, add bio-engineered plant systems along with trees, break up the existing concrete channel and reuse the rubble to stabilize the entire system — all while the river was still running” (see Figure 11). In addition to reducing dangerous flooding, biodiversity increased, adjacent property values rose, and people in the city have much-desired access to nature. This is an excellent example of public infrastructure that also serves ecological and human recreation purposes.
Figure 15 The competition winning entry by Third Nature for Enghaveparken shows a sunken space used for recreational public gathering when dry, and to hold stormwater during heavy rains. Image credit: Third Nature. *
* Tredje Natur. “Enghaveparken Now”. http://tredjenatur.dk/en/portfolio/ enghaveparken-now/
The canal resembles the channelized stream in Waipahu between to Waipahu District Park and the State owned land parcels. One may envision the stream de-channelized, with planned temporary storm water flood detention in Waipahu District Park to prevent or reduce downstream flooding near the new rail stop.
Figure 16 Analysis image from MIT’s “Strategies for Urban Stormwater Wetlands” shows a portion of a redesigned stream corridor utilizing optimized topographical elements that support human and ecological functions.*
* MIT Center for Advanced Urbanism, Nepf, Heidi, et al. “Design Guidelines for Urban Stormwater Wetlands.” MIT, http://lcau.mit.edu/project/ strategies-urban-stormwater-wetlands. Fall 2016.
Even at the federal level, the Army Corps of Engineers has begun to shift their focus away from grey-infrastructure elements. In Massachusetts, a plan for damming the Charles River basin was altered from a $100 million plan to install greyinfrastructure elements such as dams and channelizing the river, to a $10 million plan to protect upstream wetlands. “The money was used to buy acreage and easements preventing landowners from making alterations to the wetlands. According to Corps studies, this approach has performed quite well at preventing floods, and a large area of wetlands in New England’s most densely populated river basin is protected from development.”
Section drawings later in this report illustrate widening the floodway and adding a vegetated multi-use berm concept at the Waipahu TOD study area (see Figure 43).
Vegetated Multi-Use Berm
After increasing the width of the floodway adjacent to the Kapakahi and Wailani streams, another potential strategy to minimize overtopping of the stream and canal banks is to
24 WAIPAHU TOD COLLABORATION
increase the height/elevation of the stream banks and create a vegetated multi-use berm.
The vegetated multi-use berm is primarily used to reduce stream overtopping onto surrounding properties.
The term “multi-use” refers to its capacity for varying uses including storm water management, ecological habitat and corridor, and recreational use properties. Terracing along the berm could allow for recreational paths to be installed for biking and walking, nature walk with signage, and for public access to the waterway (see Figure 13). The streams could serve as a mauka to makai, mountains to ocean, contiguous green way, habitat corridor, and alternative transportation way.
The multi-use berm diagram references the RELi set of resilient design guidelines to identify and visualize storm water management goals for Waipahu. The section Hazard Preparedness (PH) addresses floodplain functions in PolyRequirement 2: Minimum Protection for Prime Habitat & Floodplain Functions. The sub-heading titled Preserve Floodplain Functions references the Envision NW1.3 document that suggests “four main ways to improve ecosystem functions: 1. Maintain or enhance hydrologic connection, 2. Maintain or enhance water quality, 3. Maintain or enhance habitats, and 4. Maintain or enhance sediment transport.” It also suggests to “avoid or mitigate impacts, maintain infiltration and water quality, enhance riparian and aquatic habitat, and enhance connectivity and sediment transport.”
Another precedent project, is a winning Rebuild By Design competition entry called, “Hunts Point Lifelines” by PennDesign/Olin et. al. (see figure 14). The process is a useful precedent because of the analysis of flood depths and integration of waterfront functions to create a floodable green space along the water’s edge. A ¬part of the multi-faceted design features a levee built adjacent to a river. It features walkable space and access to the water on dry days and a protected walkway during flooding events (see Figure 12).
Large, Above-Grade Water Detention
When re-developing the half-mile radius surrounding the new rail station, there is an opportunity to utilize large open spaces for multiple benefits, including stormwater detention, flood mitigation, recreation, and habitat creation. Parks or plazas are ideal amphibious spaces – serving as public space during dry times and floodable space during rainy times to store, infiltrate, and/or slowly release significant volumes stormwater to municipal systems. One may consider redesigning Waipahu’s major green spaces to better receive and detain stormwater runoff from surrounding areas. Areas of interest include Hans L’Orange Park, Waipahu District Park, Hawaii’s Plantation Village, and Ted Makalena Golf Course (see Figure 9 for map). The following local, national, and international precedent projects provide excellent examples for Waipahu of
Figure 17 Green space with swale in Honolulu, HI. Photo credit: W. Meguro.
Figure 18 Floodable landscape for stormwater management in Brooklyn Bridge Park, NY.
Photo credit: W. Meguro
FLOOD MITIGATION 25
Figure 19.
*
Square
Figure 20.
* Landscape Architecture Foundation.
Performance
Boneyard Creek Restoration: Scott Park and the Second Street Detention Basin”.
creek-restoration.
infrastructure with multiple protective, ecological, and social benefits.
Copenhagen’s Enghaveparken public park is an example of a multi-functional park that is designed to accommodate community gathering during dry seasons, and water during rainy seasons. The park can hold 24,000 square meters of storm-water runoff from nearby neighborhoods and an additional 11,500 cubic meters underground, and pumps are required for restoring recreational spaces to non-flood conditions. Along three of its lowest edges, are levees that serve as the park’s main storm water management element and also serves as seating and provides activities “such as a bulkhead and small openings for play.” Currently under construction, and estimated to be completed in 2019, the design for the park was envisioned through a design competition and won by the team Third Nature (see Figure 15). The design aims to maintain the original structure of the park and visibly manage stormwater by using the park as a retention basin and directing remaining water to a closed underground reservoir.
The following study method is a useful precedent for iterative design, analysis, and resulting guidelines for urban stormwater wetlands in Waipahu. The MIT Center for Advanced Urbanism study considers regional-scale digital simulation to design engineered green spaces, to the “capture and purify stormwater while delivering ecosystem and recreational benefits.” Researchers at MIT released the “Design Guidelines for Urban Stormwater Wetlands” report in 2018 a study on maximizing efficiency of wetlands through digital modeling and simulation for Los Angeles, California and Houston, Texas sites. A large portion of this study looks at the engineered approach to “optimal topographic forms” (see Figure 16) to minimize water pollution. However, their guidelines also encourage the combination of three elements: new habitats, public programming, and recreation to create a “unique palette for design” which results in “programming opportunities for people and nature”.
The following photos demonstrate public green spaces that support human recreation while directing stormwater runoff to slow and infiltrate in low-lying areas. There is a bioswale adjacent to playing fields at Manoa District Park in Honolulu, HI (see Figure 17). There is a constructed wetland with pedestrian bridges (not shown) for stormwater management at Brooklyn Bridge Park in New York City, NY (see Figure 18).
Floodable public spaces comprised of hardscape may be more appropriate than a green space in some urban contexts. Water Square Benthemplein in Rotterdam, NL was designed and constructed by De Urbanisten for the Rotterdam Climate
Floodable urban space, Water Square, in Rotterdam, NL Square by De Urbanisten.*
“Water
Benthemplein.” De Urbanisten, http://www.urbanisten. nl/wp/?portfolio=waterplein-benthemplein
The basin project in Illinois “provides 100-year flood protection by containing the 15 million gallons of stormwater generated during a 100-year storm event.”*
“Landscape
Series:
https://www.landscapeperformance.org/case-study-briefs/boneyard-
26 WAIPAHU TOD COLLABORATION
Initiative and demonstrates how a municipality can use infrastructure to serve multiple benefits (see Figure 19). The square “combines water storage with the improvement of the quality of urban public space.” The square is used for recreation, in dry weather. In wet weather, gutters direct water to three basins in a “dramatically gush the rain water visibly onto the square.” Stormwater either “flows into an underground infiltration devices” or is detained and slowly released to the municipal combined sewer system. The description of the project time line is helpful in understanding the steps and schedule relevant for similar studies and potential application at the Waipahu Transit Oriented Development (TOD) area. The Water Square project moved from research in 2005, to policy in 2007, to a pilot study in 2008, to a publication in 2010, to design in 2011-2012, to opening in 2013.
The Second Street Basin project is a small portion of the larger Boneyard Creek Restoration effort in Champaign, Illinois. The once channelized stream resembled the Waipahu Drainage Canal that runs through the study site. Efforts to de-channelize the stream pushed designers to increase the allowable floodplain and make the stream accessible to the public. Completed in 2010, the final design is capable of containing 15 million gallons of storm-water caused by a 100-year storm (see Figure 20). Like the soil of Waipahu, the soil within the Second Street Basin parcel has a high clay content. To facilitate better drainage into the ground, pea gravel and installation of drains were added to the design to also aid vegetation survival. The entirety of the Boneyard Creek restoration effort connects large green spaces to open up floodplains that total 47 acres of run-off management and 5 acres of recreational use during non-flood conditions.
Large, Below-Grade Water Detention
Utilizing large underground cisterns to temporarily store large quantities of stormwater runoff, riverine flooding, or storm surge water may mitigate flooding in the Waipahu TOD Water would be pumped out and the space cleaned after a flood event. Section drawings illustrating potential new underground cisterns are illustrated in Section A-D.
One of the world’s most well-known examples of underground cisterns is Japan’s G-Cans project. The system consists of five silos, each 65 meters deep and 32 meters in diameter, and 6.5 kilometers of connecting tunnels that lead to a 25.4 meter high by 177 meter long “Underground Temple.” Tokyo’s G-Cans have a capacity to store a 200-year flood event below city streets (see Figure 21 and Figure 22). After a storm event, large pumps actively pump the floodwater into the Edogawa River.
Another example of below-grade large volume water storage is a museum carpark in Rotterdam (see Figure 23). The car park
Figure
21.
Water storage tank in
Japan’s G-Cans project.* * Water Technology, “G-Cans Project, Kasukabe, Saitama, Greater Tokyo Area” https://www.water-technology.net/projects/g-cans-project-tokyojapan/.
Figure
22.
Diagram of Japan’s
G-Cans
Project.
* * Hill, Ed. “G-Cans Project, Tokyo – Japan’s $2.6 Billion Flood Tunnel.” Flood List, http://floodlist.com/protection/g-cans-project-tokyo-flood-tunnel
Figure
23 MuseumPark Car Park water storage in Rotterdam, Netherlands* * Rotterdam Climate Initiative. “Underground Water Storage Facility with a Capacity of 10 million litres” http://www.rotterdamclimateinitiative.nl/ uk/news/underground-water-storage-facility-with-a-capacity-of-10-millionlitres?news_id=743&p=5 FLOOD MITIGATION 27
Section Drawing A Existing Conditions and Proposed Future Design at the rail guideway/Farrington Hwy.
28 WAIPAHU TOD COLLABORATION
accommodates 1,150 cars and also houses “one of the largest underground water reservoirs in the Netherlands, a reservoir with a capacity of 10,000 m3” (over 2.64 million gallons).
Site-Scale Strategies
At the scale of a building site in the Waipahu TOD area, on-site storm water infiltration and detention should be incorporated to mitigate flooding of the built environment.
Future design teams may refer to existing, well-developed stormwater and green infrastructure best management practices in the following resources.
• Honolulu Stormwater Best Management Practices (BMP) Guide for New and Redevelopment
• US Green Building Council Leadership in Energy and Environmental Design (USGBC LEED) for New Construction Sustainable Sites credit on Rainwater Management
• The separate section of this Waipahu TOD report on green infrastructure by author Dr. Suwan Shen of the University of Hawaii Department of Urban and Regional Planning
The above resources include design requirements or criteria that limit surface runoff into neighboring properties, maintain or decrease peak runoff volume into municipal stormwater systems, and maintain or improve runoff water quality. For example, future building and site design teams should set targets for stormwater management, such as the LEED criteria to, “In a manner best replicating natural site hydrology processes, manage on site the runoff from the developed site for the 95th [or 98th] percentile of regional or local rainfall events using low-impact development (LID) and green infrastructure.” Stormwater management requirements specific to Waipahu TOD area could be incorporated into the area’s new and re-development requirements.
This section of the report illustrates, at a proof of concept level, how several green infrastructure stormwater management strategies could be integrated into the Waipahu TOD study area, including bioswales and permeable pavement. When designing on-site stormwater infiltration, clay-based soils with low permeability indicate some areas may require installation of a more permeable gravel bed below grade to facilitate drainage. Future design teams would need to determine if the top of the below-grade water table is below the potential the gravel storage bed.
Bioswales and detention areas
A bioswale is a “stormwater control feature that uses a combination of an engineered basin, soils, and vegetation to slow and detain stormwater, increase groundwater recharge,
* DLANDSTUDIO, et al. Hold System Flushing Bay Regional Plan Association.
Figure 24. Bioswale between the roadway, sidewalk, and parking lot in Milwaukee, WI. Photo credit: W. Meguro
Figure 26. Vegetated stormwater detention basin in Seattle, WA. Photo credit: W. Meguro
Figure 28. The sidewalk, bioswale, bicycle, filter strip, and road in Long Island, NY is a useful precedent for Waipahu TOD multi-modal transportation. Photo credit: W. Meguro
Figure 25. Bioswale with curb cuts adjacent to urban buildings, sidewalk, and road in Milwaukee, WI. Photo credit: W. Meguro
Figure 27. Bioswale, curb, cut, and pervious asphalt at the Center for Sustainable Landscapes in Pittsburgh, PA. Photo credit: W. Meguro
Figure 29. HOLD, by DLANDSTUDIO directs stormwater runoff from downspouts to gravel sedimentation basins and remediative plants.*
FLOOD MITIGATION 29
and reduce peak stormwater runoff.” Bioswales filter sediment and pollution that flow off streets before they enter storm drains, streams, and ultimately the ocean. The following “Section Drawing A” illustrates conceptual incorporation of bioswales immediately adjacent to a roadway and in a parking lot and illustrates potential vegetated bioswale below the elevated rail guideway below structural columns. Section A also illustrates how new bioswales (with gravel beds) adjacent to the roads can filter and absorb runoff from the road. A relocated sidewalk or elevated walkway enhances the pedestrian experience by providing shade trees and a buffer from Farrington Highway. On the right side of Section A the existing impermeable surface parking lots is redesigned to include permeable paving and cisterns for water detention below grade for slow release to the storm sewer.
Consideration of creating interspersed containment spaces over large surfaces such as parking lots and plazas may capture urban run-off and lower the total volume of storm-water that may affect downstream areas. In areas of extensive flooding such as the residential neighborhood north of the Ted Makalena Golf Course, bio-retention using permeable paving atop a gravel storage bed may store storm water runoff and flooding from riverine overtopping. There are many areas with light vehicular traffic, such as residential driveways and residential road shoulders used for walking and parallel parking.
Several images of precedent projects (right) demonstrate how bioswales are integrated into roadway and elevated highway designs, as part of a complete street that accommodates pedestrians, bicyclists, and parking. Figure 24 shows a heavily vegetated bioswale at a lower elevation than the surrounding the large roadway, sidewalk, and parking lot in Milwaukee, WI. Figure 25 shows a bioswale with curb cuts integrated in a dense urban area between buildings, sidewalk, and road in Milwaukee, WI. Figure 26 shows a bioretention basin approximately four feet deep with curb cuts and overflow outlet to storm sewer in a detached house residential community in Seattle, WA. Figure 27 shows a bioswale, curb, cut, and pervious asphalt at the Center for Sustainable Landscapes in Pittsburgh, PA. Figure 28 shows the sidewalk, bioswale, bicycle lanes, filter strip, and road in Long Island, NY, which is a useful precedent for Waipahu TOD to plan for pedestrians, bicyclists, cars, and stormwater. Figure 29 shows a project named “HOLD” by DLANDSTUDIO direct stormwater downspouts runoff to gravel sedimentation basins and remediative plants, and is a useful precedent for the structural columns for the elevated rail guideway in Waipahu.
Another relevant precedent for the Waipahu TOD area is the method used to identify the best performing stormwater management designs to guide future development produced
Figure 30. Illustration section drawing of permeable paving stormwater infiltration by the Pennsylvania Department of Conservation and Natural Resources. Photo credit: W. Meguro
Figure 31. Permeable paving in parking lot in Pennsylvania. Photo credit: W. Meguro
Figure 32. Permeable grass-crete in parking lot in Honolulu, HI. Photo credit: W. Meguro
30 WAIPAHU TOD COLLABORATION
through a municipal study in Wisconsin. The city of La Crosse, Wisconsin published a study, “Using Green Infrastructure to Mitigate Flooding in La Crosse, WI: An Assessment of Climate Change Impacts and System Wide Benefits” in which they utilized the EPA’s Storm Water Management Model to test the effectiveness of permeable pavement and bio-retention areas under different storm scenarios. Their study concluded that a deeper storage gravel bed in bio-retention areas and wider use of permeable streets mitigate flooding from increasingly intense storm events. Future Waipahu TOD design team members may provide design guidance in a format similar to the La Crosse study’s resulting clear, tangible design guidance below.
• Permeable pavement with a 4 foot gravel storage bed was determined to be the highest performing system in the basin that can be used to meet the City’s objectives for flood control.
• Implementing permeable pavement on 25 percent of the potential street area represents the knee-of-the-curve solution to mitigate flooding from the 3-month, 2-hour storm event.
• Implementing permeable pavement on 75 percent of the potential street represents the knee-of-the-curve solution to mitigate flooding from the 10-year, 2-hour storm event.
Permeable Pavement
Permeable pavement is “a paved surface with a higher than normal percentage of air voids to allow water to pass through it and infiltrate into the subsoil”. A section drawing on signage by the Pennsylvania Department of Transportation illustrates the layers below permeable paving including a base course, uncompacted soil, compacted soil, and finally the water table (see Figure 30). Materials such as grass pavers or permeable pavers may be used in lieu of typical asphalt (see Figure 31 and Figure 32).
When re-developing the Waipahu TOD area, large impermeable areas such as less-trafficked roads, parking lots, residential street shoulders, sidewalks, driveways, alleys, bike paths, patios, and public plazas should be evaluated for permeable paving. In Waipahu’s residential neighborhoods, there are many areas with light vehicular traffic, such as residential driveways and residential road shoulders used for walking and parallel parking.
Because permeable paving may be more susceptible to compression and its air voids are necessary to allow water to pass through it, only light vehicular traffic areas are recommended.
As mentioned earlier, anecdotes of soils with high clay content and low water permeability, as well as the location of the water
Figure
33. NOAA Sea Level Rise Viewer year 2080 High Scenario estimates 5.77 feet of sea level rise, and the map illustrates anticipated flooded areas in light blue with 6 feet of sea level rise.* * NOAA, https://coast.noaa.gov/digitalcoast/tools/slr.html FLOOD MITIGATION 31
Sea level rise (for year 2080 high scenario from NOAA)
5.7 feet, water depth on land varies and is qualitatively expressed with a blue gradient
+ Storm surge (from NOAA SLOSH model)
+ Base Flood Elevation (from FEMA
FIRM, 1 in 100 year)
__ feet ___ feet above local tidal datum
= Total water level per LEED credit calculation
table would need to be confirmed. Dredging and installing a gravel storage bed below new permeable paving in order to create space for water infiltration may address low permeability. Sub-surface perforated pipes may also slowly transport water to stormwater sewers or retention basins.
Building-Scale Flood Strategies
___ feet above local tidal datum
Figure 34 LEED Credit Total Water Level Calculation
Note: Values would need to be determined for each specific site.
Total water level per LEED credit calculation
- Elevation at the building site = Elevation above grade for habitable floors
__ feet above local tidal datum __ feet above local tidal datum __ feet above grade at building site
Figure 35 Design Flood Elevation Example
Note: Values would need to be determined for each specific site.
This section summarizes resources for a building owner to: estimate the flood related hazards on the building’s land parcel; consider the BFE and potential design flood elevation; consider wet floodproofing strategies; and consider dry floodproofing strategies. This section includes design criteria that go beyond current requirements of the Revised Ordinances of Honolulu (ROH) for the voluntary use by building design teams. In addition, the City of Honolulu or State of Hawaii may consider developing incentives or requirements using the strategies and references in this section.
SITE-SPECIFIC ASSESSMENT OF NATURAL HAZARDS
At the beginning of a building and site design, the site analysis includes an assessment of site-specific flood-related hazards. This section summarizes flood-related portions of emerging voluntary resilient building design rating systems, which may be used by future design teams to conduct a natural hazard assessment. Working with a hydrological engineer is necessary for design calculations, but the following guidance is useful when starting the design process.
The USGBC LEED pilot credits on Resilient Design were updated and re-released in early 2019, while this report was being finished. Design teams should consider using the LEED pilot credit, “IPpc98 Assessment and Planning for Resilience”, which requires teams to consider and plan for their site’s vulnerability to natural hazards and changing climate conditions. The required Hazard Assessment includes the following specific requirement, “Sea Level Rise and Storm Surge for 2022 and beyond should use the NOAA 2012 Sea Level Rise year 2080 High Scenario combined with a 1 in 100year floodplain (Extreme Flood) to determine water levels for planning purposes.”
Regarding SLR, for Honolulu, the NOAA Sea Level Rise viewer estimates 5.77 feet of SLR in the 2080 High Scenario (see Figure 33). As discussed earlier, portions of the proposed new construction in the Waipahu Action Plan would be flooded with 6 feet of sea level rise. The depths of the anticipated flooding are shown in the blue gradient, although the flooding depth is not quantitatively expressed in the NOAA tool.
Regarding storm surge, the LEED credit suggests using the “NOAA Sea, Lake, and Overland Surges from Hurricanes
32 WAIPAHU TOD COLLABORATION
(SLOSH) Model to interpolate storm surge” but it appears that model is only available when there is an impending hurricane near Hawaii, so the additional depth for anticipated storm surge is unknown at this time.
The 1 in 100-year BFE varies from approximately seven to eleven feet above the local tidal datum in Waipahu TOD study area (see the FEMA FIRMette in Figure 36). The FIRMs are based on hydraulic modeling of present day flood risk and do not include future increases in flood hazards with SLR. In the Waipahu TOD study areas that experience flooding with three to six feet of SLR, the land elevation on the topography map is between zero to five feet (see Figure 7).
If the depth of the sea level rise at a particular site and the storm surge were known, they could be added to the BFE to determine the total water level (see Figure 34). If the elevation at the building site is subtracted from the total water level, then the elevation above grade for habitable floors could be calculated.
After documenting the flood hazards at the building site, the LEED credit IPpc98 requires the design team to “assess potential exposure, sensitivity and vulnerability to each hazard based on the project goals, program and intended service for the life of the building” and implement “Climate Resilient Planning” or “Emergency Preparedness Planning”.
Design teams should also consider that future rainfall may not follow historical patterns of intensity, duration, frequency, due to climate change. “It is projected that Hawaii will see more drought and heavy rains causing more flash flooding, harm to infrastructure, runoff, and sedimentation.” Design team engineers may consider using sources of information for projected future rainfall. One information source is WeatherShift Rain, which “uses data from 21 global climate models (GCMs) to generate projected climate change-shifted [in years 2035 and 2090] rainfall Intensity-Duration-Frequency (IDF) curves and 27 GCMs to generate daily time series rainfall data for use in drainage infrastructure design, such as sizing storm drain networks, pump stations, treatment plants, and rainwater harvesting tanks.”
BASE FLOOD ELEVATION AND DESIGN FLOOD ELEVATION
FEMA’s FIRMs include flood zones and a property’s BFE, which is “the computed elevation to which floodwater is anticipated to rise during the base flood”. A base flood is defined as “having a one percent chance of being equaled or exceeded in any given year (also called the “100-year flood”)”.
“Freeboard” is defined by FEMA as “a factor of safety usually expressed in feet above a flood level for purposes of floodplain
FLOOD MITIGATION 33
Figure 36 FEMA FIRMette shows the Base Flood Elevation (BFE) measured from the local tidal datum.*
*
FEMA Flood Map Service Center, https://msc.fema.gov/portal/home
34 WAIPAHU TOD COLLABORATION
Figure 37 An excerpt from the ASCE 24-14 reference sheet shows habited floor elevations based on Flood Design Classes
Standard FEMA LEED IPpc98
Freeboard for Design Flood Elevation (DFE) Suggestion (feet above FEMA BFE)
+1 to +2 depending on building “class”, or at 500-year flood (if known), according to ASCE 24-14
Varies depending on 1% chance annual flood, sea level rise, and storm surge
LEED IPpc99
+5
RELi Fortified Enterprise MultiFamily Housing
Plan for 500-year floods
+3 or 500year flood elevation (if known)
NIBS
+1 to +2 Depending on region (up to +6 in areas around Hurricane Sandy devastation)
Figure 38 Suggested elevation above base flood elevation (BFE)
FLOOD MITIGATION 35
Figure 40 The rock Environmental Center in Virginia is elevated to exceed the standard for a 500-year storm event and future projected sea level rise flooding.*
* Chesapeake Bay Foundation. “Making of a Green Building”. N.d. Accessed November 1, 2018. http://www.cbf.org/about-cbf/locations/virginia/ facilities/brock-environmental-center/about/making-of-a-green-building/index. html
management. Freeboard tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood and floodway conditions, such as wave action, bridge openings, and the hydrological effect of urbanization of the watershed”. In some cities across the United States, building codes reference design flood elevations (DFE). DFEs are typically higher than BFEs and are “the minimum height at which residential units may be constructed and utilities like the boiler, the water heater and electrical equipment may be located. It also sets the minimum height for dry or wet flood-proofing measures for buildings generally”.
The Hawaii State Office of Planning may consider establishing freeboard and DFEs that address flooding from simultaneous sea level rise, storm surge, and heavy rainfall events.
ELEVATION
A logical approach to flood-proofing is to elevate existing or new habited spaces and critical infrastructure above the BFE. This chapter addresses the resources design teams or policy makers may refer to when determining the height to elevate a structure above the BFE. This section on elevation will discuss existing codes and regulations, explore resiliency guidelines, provide suggestions, as well as a case study.
FEMA determines minimum elevations based on building function and location/hazard exposure (see Figure 37). The table below is an excerpt from the ASCE 24-14 reference sheet from the FEMA website. Higher flood class numbers indicate more critical building functions. For example, sheds and storage fall under Flood Design Class 1, while hospitals fall under Flood Design Class 4. FEMA references ASCE 24-14 and deems the document to “meet or exceed the minimum National Flood Insurance Program (NFIP) requirements for buildings and structures”. IBC 2006-2012 “requires dwellings in floodways to be designed in accordance with ASCE 24” and the 2015 edition of the IBC allows use of ASCE 24 for dwellings in any flood hazard area.
Current building code and regulation in the Revised Ordinances of Honolulu (ROH) Chapter 21 Article 9 require buildings within flood fringe zones to elevate habited floors up to BFE. The table below (see figure 38) compares the design flood elevation (DFE) from various reference documents and resiliency guidelines for consideration by planning officials and design teams. Resiliency guideline sources exceed the ASCE 24-14, but as the NIBS elevation requirement suggests, each region of the country experiences different levels of storm surge.
The City of Boston’s Climate Resiliency Guide provides a useful precedent for Honolulu. Boston requires large projects to take
36 WAIPAHU TOD COLLABORATION
sea level rise, local land subsidence, and rainfall into account in determining BFE and DFE. If the project site is located in a Sea Level Rise Flood Hazard Area (SLR-FHA), teams use an online mapping tool, which takes into account a 1% annual chance flood event with 40 inches of sea level rise. Next, to calculate the Sea Level Rise-Design Flood Elevation (SLR-DFE), a minimum 24” of freeboard is added for critical facilities and infrastructure and buildings with ground floor residential units or a minimum 12” of freeboard for all other buildings and uses. Flooding and other climate change related issues for large projects are reported to city officials through a Climate Resiliency Checklist, which is due with the initial project filing, design/building permit filing, and construction/certificate of occupancy filing.
Owners of detached houses in the Waipahu TOD study area anticipated to experience flooding with three feet of sea level rise may consider elevating the houses to or above the BFE. FEMA 347 Above the Flood: Elevating Your Flood Prone House provides several techniques, each with corresponding case studies:
1. Extend the walls of the house upward and raise the lowest floor;
2. Convert the existing lower area of the house to non-habitable space and live in the existing second or third story space (see Figure 41);
3. Lift the entire house, with the floor slab attached and build a new foundation to elevate the house.
The Chesapeake Bay Foundation’s Brock Environmental Center was the world’s first LEED Platinum building and designed to be self-sustaining and resilient. The shoreline setback distance, storm water management, and building elevation are useful examples for the Waipahu TOD study area. In addition to passive daylighting, heating and cooling, and on site energy production, the center’s site is located 200 feet from the shoreline, which is “100 feet … beyond Virginia’s standard 100-foot Chesapeake Bay Preservation Act Resource Protection Area”. The site also treats run off on site and “prevents increases in peak runoff volume and net increases in runoff volume and pollutants. Permeable roads and paths allow water to filter into the soil, while rain garden treatment, rain water harvesting, and water re-use reduce the volume of runoff and meet pollution-reduction objectives. The site design ensures that all runoff on CBF’s property is filtered through the vegetation and the soil, not flowing untreated into local waters”. The building itself is elevated 13.8 feet above sea level “to exceed the standard for a 500-year storm event and future sea level rise projected flooding” (see Figure 40.)
Existing and new buildings should elevate utilities (e.g., water
Figure 41 One wet-flood proofing option is to abandon the lowest floor and replace it with non-occupied space and install flood openings and/or break away walls.*
* FEMA. “P-1037 Reducing Flood Risk to Residential Buildings that Cannot Be Elevated”. September 2015.
FLOOD MITIGATION 37
Figure
heaters, air conditioning equipment, electric meters, and septic tanks) above the BFE to protect them from damage or loss of function from flooding. Homeowners may elevate outdoor equipment on platforms, or move indoor equipment to higher floors or attics, or build an elevated utility room.
WET FLOODPROOFING
After elevating critical building functions, design teams may consider wet floodproofing methods to allow water to pass through non-critical areas of the building with minimal harm. As described by FEMA, the benefits of wet floodproofing are that “if floodwaters are allowed to enter the enclosed areas of the home and to quickly reach the same level as the floodwaters outside, the effects of hydrostatic pressure, including buoyancy, are greatly reduced”. Flood proof materials should be used to minimize structural damage and critical machinery and equipment should be moved to higher floors. Electrical systems including outlets should be elevated to avoid contact with flood waters.
Other benefits of wet floodproofing include becoming NFIP compliant only if: “(1) the area is limited to parking, access, or storage, (2) designed to allow for automatic entry and exit of flood waters through the use of flood openings, and (3) uses only flood damage-resistant materials below the DFE”.
General wet floodproofing guidelines include the following.
1. Spaces below the DFE should not contain critical equipment, mechanical or electrical systems, or be a habitable space.
Functions such as garage, storage, or building access are appropriate for floors below the DFE. “Discounts are currently available under the NFIP for elevating building utilities in the V Zone. There are also discounts available for this activity in the A Zone when utilizing the NFIP’s Submit for Rate Guide. Unlike other alternative mitigation measures, elevating building utilities is effective against coastal floods or high velocity riverine flood forces, provided that indoor utilities are elevated to a higher floor or inside an elevated utility room supported on piles.” (FEMA P-1037)
2. If the area must be enclosed, walls should be constructed to break away during a flood event or should include flood vents to allow water to enter.
“The benefit of wet floodproofing is that, if floodwaters are allowed to enter the enclosed areas of the home and to quickly reach the same level as the floodwaters outside, the effects of hydrostatic pressure, including buoyancy, are greatly reduced.” (FEMA P-312) A break away wall is a “wall that is not part of the structural support of the building and is intended to collapse under specific lateral loading forces, without causing damage
Grate style
Louver style
42.
Four examples of wet-floodproofing fixtures.
Hinge style
Permeable/Break away walls
38 WAIPAHU TOD COLLABORATION
to the elevated portion of the building”. Alternatively, one may install flood vents in existing walls that allow for the automatic entry and exit of floodwaters. Several different flood vent options are shown in Figure 42.
3. Materials below the DFE should also be flood damage resistant.
Building materials installed in floodable spaces—including framing, wallboard, flooring and ceiling paneling—should be able to survive water exposure without major damage, promoting mold or mildew, or absorbing contaminants. Building materials under the DFE should also be able to withstand contact with floodwaters for up to 72 hours without requiring more than cosmetic repairs. Examples include flood damage-resistant building finish materials such as non-paperfaced gypsum board and terrazzo tile flooring versus traditional drywall and carpeted flooring, vinyl flooring over a concrete slab, metal doors, glass block windows, and concrete walls. Wet floodproofing is also considered a relatively low cost measure with an expected useful life ranging from 15 to 20 years with some limited annual maintenance costs. In coastal areas with salt water, corrosion of metals may be a problem. Materials that might dry out and be usable after exposure to fresh water may be damaged beyond repair by salt water.
For buildings that cannot be elevated, several interior modifications may be possible, along with wet floodproofing techniques.
1. Abandon the lowest floor. Convert the existing lower area of the house to non-habitable space and live in the existing second or third story space (see Figure 41).
2. Elevate the lowest interior floor to make floor(s) with lower floor to ceiling heights (e.g., modifying from ten feet to eight feet).
DRY FLOODPROOFING
Dry floodproofing involves “sealing your home to prevent floodwaters from entering” and may be necessary for buildings that cannot be elevated. Building owners can dry flood proof their homes by:
• using waterproof coatings or coverings to prevent floodwater from passing through the walls;
• installing waterproof shields or flood gates over doors and openings;
• installing devices that prevent sewer and drain backup
• sealing cracks and openings in walls and foundation;
• protect against seepage by installing a sump pump
Dry floodproofing can withstand hydrostatic loads up to a height of three feet. Dry floodproofing is not recommended for homes with frame walls at low levels or with basements or crawlspaces.
Below the DFE, equipment should be waterproofed. Existing or new building utilities (e.g., water heaters, air conditioning equipment, electric meters, septic tanks) may also be modified with dry floodproofing. For example, outdoor equipment may be placed behind a wall or watertight, passive utility enclosure up to or above the BFE to protect them from damage or loss of function from flooding.
Below the DFE, materials should be flood proof for easy clean up. Because floodwaters often carry debris and hazardous materials, cleaning up a dry floodproofed home after a flood may involve removing mud and disinfecting surfaces.
Dry floodproofing may not be as cost effective as other methods because “construction of a passive dry floodproofing system is generally considered a relatively high-cost measure with an expected useful life ranging from 15 to 30 years and extensive annual maintenance costs needed to maintain the various elements of a floodproofing system”. The cost of the system is highly dependent on the area size, wall type, number of existing openings, plumbing and electrical lines, etc. However, there are currently no flood insurance premium rate discounts available under the current program.
Before finalizing a plan for dry floodproofing, an engineer should take into consideration the expected flood depth, flow velocity, erosion and scour, debris impact, wave action, flood duration, post-flood clean up, and active human interaction required for setting up barriers such as shields.
FLOOD MITIGATION 39
APPLICATIONS FOR WAIPAHU TOD
The following portion of this report includes section drawings to illustrate potential applications of the green infrastructure strategies discussed in selected areas of the Waipahu TOD site. The Waipahu site map shows the location of the section drawings with a line (see Figure 43). Section A is a section drawing of Kapakahi Stream near the Waipahu Convenience Center. Section B addresses the residential section with high levels of predicted flooding. These areas are selected for illustration because they will experience flooding from sea level rise or storm surge soonest.
At the top of each page, a section drawing depicts the existing site conditions. Each section drawing below it displays potential green infrastructure strategies for flood mitigation at roadways.
Each section drawing is divided into portions denoted with vertical lines. Each portion should be considered as a singular strategy. The entirety of a section drawing is not a cohesive prescription, but rather a series of options. For example, permeable pavement is a singular design strategy and can be applied in combination with a terraced floodway, or with a simple sloped floodway. The following is a summary of the design strategies.
Impervious surface lots may benefit from:
• Replacing impervious hardscape with pervious paving;
• Providing un-lined gravel beds for infiltration or underground cisterns beneath paving;
• Installation of bioswales;
• Modification of topography to guide floodwaters toward waterways; and
• Minimizing parking area footprint to maximize area for flood mitigation systems.
Vegetated areas near waterways may benefit from:
• Dechannelizing waterways;
• Extending the floodway adjacent to the stream;
• Providing vegetation along the edges of waterways for water absorbance, filtration, creation of habitat, and shading; and
• Vegetated, multi-use berm.
Large, permeable surfaces may benefit from:
• Modification of topography to accept large amounts of flood overflow; and
• Vegetation to provide water absorbance, filtration, and creation of habitat and shading.
40 WAIPAHU TOD COLLABORATION
Figure
43 Waipahu TOD
Location
of Sections FLOOD MITIGATION 41
Section Drawings to Illustrate Potential Strategies
SECTION A-A EXISTING CONDITION
Current conditions along Kapakahi stream vary greatly as portions are underground, severely overgrown, or heavily polluted. Sea level rise, river, and storm surge flooding occurs around streams and their neighboring low-lying areas. Flooding will negatively impact residences, commercial businesses, and public infrastructure if current conditions are not adapted accordingly. High clay content soil in combination with expansive asphalt coverage prevents proper drainage of water. Streams are clogged with vegetation and trash, and roads are susceptible to disuse due to elevated flooding.
The following proposal strategies should be considered individually, not grouped per section. Strategies should be considered on a siteby-site basis.
Please refer to Appendix A for small scale maps of flood impacts on infrastructure, TOD plan, and existing FEMA zones.
SECTION A-B MULTI-USE BERM/PERMEABLE PAVEMENT
By extending the floodplain around Kapakahi stream, the stream will be able to handle a higher volume of flood water, thereby decreasing velocity. A berm is proposed in the remaining schemes as elevated walkways provide a means of transportation when vehicular roads become flooded. Permeable pavement should be incorporated in low lying areas to reduce ponding on large lots. Native vegetation well suited for saline environments should be given highest priority when selecting species for planting. Trees such as hala, and shrubs such as ‘ohelo kai and ‘ahu‘awa are hardy species.
SECTION A-C TERRACED MULTI-USE BERM/BIO-SWALE
Terraced floodplains allow for use of elevated platforms when dry, and permit the flow of water during flood events. Benefits are similar to the extended floodplain mentioned above. Bioswales are forms of green infrastructure that have been widely implemented on urban streets across the globe. Swales are miniature ecologies that capture and filter water through microbial processes and the natural uptake of water by plants. Many designs of bioswales exist and each is created to achieve different outcomes for site specific needs. Bioswale implementation is versatile and can be implemented on paved lots, along streets, for residential use, and more.
SECTION A-D SHIFTED AND EXPANDED FLOOD-PLAIN/MULTI-USE BERM/CISTERN
The entire floodplain can be extended into Pouhala Marsh and allow for greater volume capacity. Although sections of Kapakahi stream are underground, expanding the stream can be advantageous along all sections to reduce bottlenecking.
Gray infrastructure should be considered in conjunction with green infrastructure. Cisterns and pumps may be useful around soils with high clay content to absorb excessive levels of flooding.
42 WAIPAHU TOD COLLABORATION
FLOOD MITIGATION 43
SECTION B-A EXISTING CONDITION
Section drawings “B” are spread over two pages and should be viewed on pages facing each other. Flooding along the canal between Wailani stream and nearby residences is a big contributor to widespread flooding in the nearby low-lying areas. This channel is often filled with large debris and trash, has low flow rates, and is both narrow and shallow. Many residences (refer to Appendix A-Waipahu RES maps) will experience flood-related damage. Redesigning the site may open up opportunities for improved public experiences on the bike path and improved health and safety.
SECTION B-B BIO-SWALE/ELEVATION ABOVE BASE FLOOD ELEVATION
Elevated buildings and roads should allow water to flow beneath, especially for buildings within FEMA Zone AEF where development within the floodway (F) must ensure “no increases in upstream flood elevations” (FEMA). Bioswales may be installed along the bike path and within the golf course to aid in storage of floodwater.
SECTION B-C SUB-SURFACE/RETENTION AND FLOODPROOFING
Opportunity lies within the large expanse of the Ted Makalena Golf Course. Allowing the golf course to flood can ease pressures on the small channel. By allowing flood design on golf course property, the channel may be given space to be properly designed. Buildings may also be designed to be dry or wet floodproofed. Buildings within floodways must be wet floodproofed to allow movement of water. However, dry floodproofing requires sealing lower levels in buildings to prevent water infiltration.
SECTION B-D EXPANDED FLOODPLAIN/RETREAT
Berms may be able to protect residences from stream overtopping. However, simply increasing the height of stream banks may cause an increase in stream velocity. Therefore, a combination of widening the channel and increasing the height of a berm may help contain those floodwaters. The elevated berm may be outfitted with recreational elements such as a bike or jogging lane. In cases of chronic flooding, property owners may choose to retreat and relocate activities.
44 WAIPAHU TOD COLLABORATION
FLOOD MITIGATION 45
SECTION C-A
Section C-A illustrates the existing condition with an impervious parking lot on a State-owned land parcel, channelized drainage canal, and Waiphau District Park.
SECTION C-B
Section C-B illustrates a potential future pervious parking lot with gravel bed below, expanded floodway and vegetated berms adjacent to the canal.
SECTION C-C
Section C-C illustrates a potential future pervious parking lot with gravel bed below, bioswales and shade trees, and an expanded floodway with a vegetated multi-use berm.
SECTION C-D
Section C-D illustrates a potential future parking garage with temporary water detention cisterns below with pumps. This strategy also features an expanded floodway, vegetated multiuse berm on only one side of the canal, and intentional use of Waipahu District Park as a floodable open space.
46 WAIPAHU TOD COLLABORATION
FLOOD MITIGATION 47
SECTION D-A EXISTING CONDITION
Section drawing D-A illustrates the existing rail guideway at Farrington Highway.
SECTION D-B SWALES, UNDERGROUND CISTERN
Section D-B illustrates a potential vegetated bioswale below the elevated rail guideway and structural columns (see Figure 45). New bioswales (with gravel beds) adjacent to the roads can filter and absorb runoff from the road. A relocated sidewalk or elevated walkway enhances the pedestrian experience by providing shade trees and a buffer from Farrington Highway.
On the right side of Figure 45, the existing impermeable surface parking lot is redesigned to include permeable paving and cisterns for water detention below grade for slow release to the storm sewer.
48 WAIPAHU TOD COLLABORATION
SECTION D-A EXISTING CONDITION
SECTION D-B SWALES, UNDERGROUND CISTERN
FLOOD MITIGATION 49
POTENTIAL FLOOD MITIGATION STRATEGIES FOR TWO TYPICAL BUILDING TYPOLOGIES IN WAIPAHU
The following illustration identifies strategies from flood mitigation standards and resiliency benchmarks that may be relevant for detached homes (see Figure 44) and multi-story buildings (see Figure 45) in the Waipahu TOD study area.
• Elevate the building (if possible)
• Elevate or dry flood-proof critical machinery and equipment, and utilities
• Elevate low lying electrical outlets
• Allow floodwaters to pass through floors below the DFE. Install flood vents, breakaway or permeable walls, or remove walls to prevent damage from hydrostatic and -dynamic loads
• Floors below the DFE should be designated as nonhabitable spaces such as parking and storage.
• Foundations should be engineered and approved by a professional
• Dry flood-proof entries and openings below DFE and provide alternative access
• Dry flood-proof below grade elevator shafts and provide sump pumps
For more information on elevation, wet flood-proofing, and dry flood-proofing, please see the previous section of this report.
50 WAIPAHU TOD COLLABORATION
Elevate low-lying electrical outlets
Relocate critical equipment above the Base Flood Elevation (BFE)
Select flood-resistant building materials below the BFE
Permeable or break-away walls
Elevate occupied floors
Provide access above BFE
Structural foundation should resist hydro-dynamic and hydro-static loads
Figure 44 Illustration of flood mitigation strategies relevant for a detached home.
Wet flood-proofing allows flood waters to move through building below the base flood elevation
Elevate critical equipment and programs
Design elevators with dry-flood proofing below BFE
Dry-flood proofing includes armored open ings below the BFE
Provide alternative, elevated access
Figure 45 Illustration of flood mitigation strategies relevant for a multi-story building.
FLOOD MITIGATION 51
STANDARDS AND BENCHMARKS FOR RESILIENCY
The following sections summarize standards and benchmarks that pertain to flood mitigation and resilience at the time of writing this report in 2018 in order to help future design teams quickly locate relevant guidance from various sources. The following resources were available as of November 2018, and design teams should check for new or updated resources in the future.
FEMA FLOOD-RELATED RESOURCES
The following section is intended to help design teams quickly locate relevant flood-related guidance from FEMA. The various FEMA documents provide information on floods and floodproofing strategies aimed at different audiences. For example, FEMA P-312 Homeowner’s Guide to Retrofitting introduces flood damage mitigation concepts and overviews strategies for the homeowner. In contrast, FEMA P-259 Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures is a reference book for engineers, architects, and contractors that provides detailed specifications for tools, computations, and building techniques.
Several FEMA documents cover topics related to retrofitting existing residential and nonresidential buildings for to mitigate damage from both riverine and coastal floods. Topics include sea level rise specific flood-proofing design, floodwalls, pumps and backflow valves, and structural calculations related to static and dynamic flood loads.
• FEMA P-1037 Reducing Flood Risk to Residential Buildings that Cannot Be Elevated (September 2015)
• FEMA Highlights of ASCE 24-14 (July 2015)
• FEMA P-312 Homeowner’s Guide to Retrofitting, 3rd Edition (June 2014)
• FEMA P-936 Floodproofing Non-Residential Buildings (July 2013)
• FEMA P-259 Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures (January 2012)
• FEMA P-55 Coastal Construction Manual, 4th Edition (August 2011)
• FEMA P-347 Above the Flood: Elevating your Floodprone House (May 2000)
FEMA documents have abundant and descriptive diagrams to visualize all strategies and related equipment. Though some documents, such as P-55 and P-259, discuss strategies in the context of residential buildings, many strategies and techniques are still applicable to non-residential buildings particularly wet and dry floodproofing methods. As described in the beginning of this report, some buildings in Waipahu are anticipated to experience inundation from SLR and FEMA P-55 Coastal Construction Manual may be referenced.
FORTIFIED GUIDELINES
FORTIFIED Home™ is a program of the Insurance Institute for Business and Home Safety and provides a voluntary set of standards to help improve a home's resilience by adding systemspecific upgrades to minimum code requirements. FORTIFIED provides a series of documents including standards, fact sheets, and technical guides. For the purpose of this report, reference documents include:
• FORTIFIED for Safer Business: Standards Volume I (2014) and
• FORTIFIED for Safer Living: Standards (2008).
Within these standards, natural disaster hazards are addressed, including flood, earthquake, wildfire, extreme weather, etc. The section on flood criteria is concise and uses FEMA P-55 as a benchmark.
52 WAIPAHU TOD COLLABORATION
FORTIFIED Safer Business builds on this benchmark as follows:
• “Foundations in the Coastal A Zone shall be the same as required in the Coastal V Zone. (See figure 46)
• For flood prone areas (not X unshaded or C) the finished floor elevations must be equal the FDFE (FORTIFIED design flood elevation), which shall be greater than or equal to the highest of the following:
3ft. above the BFE
3 ft. above the Advisory Base Flood Elevation (ABFE)
The 500-year flood elevation (if known)
• Buildings located in flood prone areas will have a check valve or similar back flow device installed at the point of entry into the building on the main discharge sewer line to prevent sewage from potentially flowing back into the building during a flood event. An alternative is to provide a drain plug device for all floor drains located in basements and first floor.
• All mechanical equipment and utility connections shall be above the FDFE. Vertical runs shall be protected by columns, or other structural elements that are not part of any break away wall system, and shall not be connected to any break away elements.”
ENTERPRISE GREEN COMMUNITIES: MULTIFAMILY BUILDING RESILIENCE MANUAL
Enterprise Community Partners is an affordable housing nonprofit with nationwide markets. In 2015, Enterprise created a retrofit manual called, “Ready to Respond: Strategies for Multifamily Building Resilience” to guide building owners to make their buildings more resilient against the effects of extreme weather events in the Northeastern United States.
The guidelines cover a wide range of resilience topics. The flood-related topics are listed below (some sub-topics were omitted as they do not reflect flood-related strategies).
Protection
Wet Floodproofing
Dry Floodproofing
Perimeter Floodproofing
Elevators
Valves
Pumps
Adaptation
Efficiency
Equipment
Living
Stormwater
◦
◦
◦
•
◦
◦
◦ Site
◦ Resilient
◦ Backwater
◦ Sump
•
◦ Envelope
◦ Elevated
◦ Elevated
Space ◦ Surface
Management Figure 46 Typical shoreline elevation showing flood zones V, Coastal A and X.* * Coastal Construction Manual, 3rd edition, FEMA 55 FLOOD MITIGATION 53
Three additional categories beyond the scope of this report are “Back-up”, “Community” and “Putting It All Together”, which address ensuring power back-ups for critical needs, developing community organization and responsiveness, disaster funding, and pre-development strategies.
Written for communities, the nineteen strategies are described and displayed graphically in short, easily understood sections. Each strategy includes discussion on cost, maintenance, and complimentary strategies. However, cost analysis and flood risk are written for the Northeastern United States and may not reflect Hawaii’s property and construction costs.
EMERGING RESILIENCY GUIDELINES: LEED AND RELI
The US Green Building Council’s LEED voluntary green building rating system is now widely used. In the 2009 LEED version 3, three pilot credits, IPpc98 Assessment and Planning for Resilience, IPpc99 Design for Enhanced Resilience, and IPpc100 Passive Survivability and Functionality
During Emergencies paved the way toward incorporating resilient design during the pre-design phase for new construction.
The credits address a variety of natural hazards such as flooding, earthquakes, tsunamis, and wildfire. The pilot credits address resiliency broadly and are briefly summarized in the following sections.
LEED IPpc98 Assessment and Planning for Resilience
An assessment to identify the top three priority hazards that affect the site (such as flood, hurricane, or earthquake) must be conducted. The assessment should summarize the hazard and provide references for addressing the selected hazards. After the assessment, one of two options may then be completed.
Option One: Climate Resilient Planning
A vulnerability assessment for the selected hazards must be conducted with the use of state, regional, or local climate change studies, or through consultation with a climate scientist to assess the additional impacts to existing hazards caused by climate change.
Option Two: Emergency Preparedness Planning
This measure is designed to ensure that emergency preparedness has been considered. Compliance is provided by completing two forms, which are available through the American Red Cross Ready Rating program and available online at no cost. The two forms are the Red Cross 123 Assessment Form and the Red Cross Facility Description Form.
A full description for the fulfillment of LEED v4 IPpc98 can be found on the USGBC website at https://www.usgbc.org/credits/assessmentresilience.
LEED IPpc99 Design for Enhanced Resilience
Based on the hazards identified in IPpc98, this credit implements hazard-specific mitigation strategies. For example, if the hazard identified is flooding, the following list is an excerpt to illustrate sample strategies to implement:
• All flood resistant provisions of ASCE 24-14 must be followed.
• The lowest structural member of an occupied floor must be a minimum of five feet above the FEMA-defined base flood elevation, or dry floodproofing measures must be implemented for applicable commercial buildings.
• All sewer connections must include sewer backflow preventers.
54 WAIPAHU TOD COLLABORATION
Full description for the fulfillment of LEED v4 IPpc99 can be found at the USGBC website: https:// www.usgbc.org/credits/enhancedresilience
LEED IPpc100 Passive Survivability and Functionality During Emergencies
Like the Enterprise Green Communities document, LEED IPpc100 ensures “that buildings will maintain reasonable functionality, including access to potable water, in the event of an extended power outage or loss of heating fuel.” Two of three options must be completed to earn the LEED point:
• Option One: Thermal Resilience
• Option Two: Back-Up Power
• Option Three: Access to Potable Water
Full description for the fulfillment of LEED v4 IPpc100 can be found at the USGBC website: https://www.usgbc.org/credits/passivesurvivability
RELI ACTION LIST AND CREDIT CATALOG REFERENCE BRIEF ONLINE FLIPBOOK, PILOT VERSION 1.2.1
The RELi guidelines provide resiliency standards and integrated design processes. The RELi Action List and Credit Catalog have been in development for several years and were adopted by the USGBC and Green Building Certification Institute (GBCI) in 2017. The RELi credits are intentionally modeled after LEED to ease adoption by the design community. As of the RELi Pilot Version 1.2.1, the draft of resiliency strategies consists of four main topics and a total of eight subtopics.
• Panoramic Approach
Panoramic Approach to Planning, Design, Maintenance, & Operations (PA)
• Risk Adaptation & Mitigation for Acute Events
Hazard Preparedness (HP)
Hazard Adaptation and Mitigation (HA)
• Comprehensive Adaptation & Mitigation for a Resilient Present & Future
Community Cohesion, Social & Economic Vitality (CV)
Productivity, Health & Diversity (PH)
Energy, Water & Food (EW)
Materials & Artifacts (MA)
• Applied Creativity & Contextual Factors for Resiliency
Applied Creativity, Innovation & Exploration
Each subtopic consists of numerous credits, and requisites however, points have not yet been allocated as a part of the RELi pilot phase. Relevant material found in the RELi action list are extracted and listed here for reference:
Hazard Adaptation & Mitigation (HA)
1: Sites of Avoidance & Repair: 500-Year Flood Plain, Storm Surge & Sea Rise
2: Adaptive Design for Extreme Rain, Sea Rise, Storm Surge & Extreme Weather, Events & Hazards
Health & Diversity (PH)
Poly-Req 2: Minimum Protection for Prime Habitat & Floodplain Functions
Poly-Credit 5: Ecological PHD: Protect Wetlands & Avoid Slopes and Adverse Geology
• Energy, Water & Food (EW)
Poly-Req 1: Minimum Water Efficiency & Resilient Water and Landscapes
◦
◦
◦
◦
◦
◦
◦
◦
•
◦ Req
◦ Poly-Credit
• Productivity,
◦
◦
◦
FLOOD MITIGATION 55
The GBCI adopted RELi in 2017. In collaboration with the USGBC, GBCI is working toward further refining RELi to synthesize the LEED Resilient Design pilot credits with RELi’s Hazard Mitigation and Adaptation credits. The GBCI and the RELi resilience standard will work together to develop buildings and communities that offer greater adaptability and resilience to weather and natural disasters.
56 WAIPAHU TOD COLLABORATION
CONCLUSION
This study examines flood mitigation design criteria for a new rail transit hub community that is undergoing rapid urban planning and development—the Waipahu Transit Oriented Development (TOD) area on Oahu.
The report includes maps showing the existing and new planned development that are anticipated to experience temporary flood shocks from storm surge, king tides, intense rainfall, and storm water runoff. Additional challenges for the area also include the long-term flood stressor of SLR and groundwater inundation in low-lying areas. The September 2017 Waipahu Town Action Plan identified three priority actions, which included addressing area-wide flooding.
This study provides a replicable methodology to analyze flooding from various sources: comparing flood maps to existing and planned development; identifying relevant flood mitigation strategies and built examples; and surveying flood mitigation design criteria. It offers potential planning scale flood mitigation measures for future development at the town planning scale, city block scale, and building scale. The report presents concepts, shares built examples, and shows potential future designs specific to the Waipahu TOD site.
Town planning scale strategies include retreat from flooded areas, development on higher ground, widened floodways, intentionally floodable open spaces, and large below grade water detention areas. City block planning scale strategies include bioswales and pervious pavement. At the building scale, various guides on DFEs are compared. At the Waipahu TOD site, flooding from rainfall, SLR, and storm surge should be considered simultaneously when determining the appropriate building occupied floor elevation. Also at the building scale, techniques for elevation, wet floodproofing, and dry floodproofing are presented.
Relevant flood related portions of existing standards and benchmarks are summarized for ease of reference by future design teams including FEMA, LEED, RELi, Fortified, Enterprise Housing, and NIBS.
This document should be a useful reference for future planners, policy makers, and design teams addressing the Waipahu TOD area.
FLOOD MITIGATION 57
58 WAIPAHU TOD COLLABORATION
FLOOD RELATED REGULATIONS
MAPS
STATE-OWNED PARCELS
IN EFFECT
APPENDIX A
CURRENTLY
B MICRO
C
FLOOD MITIGATION 59
APPENDIX A: FLOOD RELATED REGULATIONS CURRENTLY IN EFFECT
Current state regulations that define flood damage protection requirements and recommendations are stated in material sources referenced in this document. The City and County of Honolulu’s Revised Ordinances of Honolulu (ROH) focuses on building code requirements and flood districts in chapters 16 and 21, respectively.
Building Code Amendments to Reduce Existing and Future Building Stock Vulnerability to Coastal Hazards and Climate Impacts In the City and County of Honolulu
Gary Chock’s report to the State of Hawaii Office of Planning was prepared in 2015 to amend existing regulations regarding climate change impacts including flooding. New information regarding climate change and associated impacts is continuously updating; therefore regulations regarding the mitigation of climate change impacts should likewise reflect that knowledge. Listed below is an excerpt from Gary Chock’s report listing relevant regulation and those of which may consider further amendments.
i. Building Codes and Regulatory Standards Currently in Effect
1. Land Use Ordinance, ROH Chapter 21 (amended 2014)
2. Honolulu Building Code, ROH Chapter 16 (October 18, 2012) 3. Honolulu International Residential Code, ROH Chapter 16 (October 18, 2012) 4. Shoreline Setbacks, ROH Chapter 23
5. Special Management Area, ROH Chapter 25
6. Flood Hazard Area, ROH Chapter 21A
7. National Flood Insurance Program (NFIP)
8. Honolulu Plumbing Code, ROH Chapter 19
9. Honolulu Electrical Code, ROH Chapter 17
10. State of Hawaii Coastal Management Program, HRS Chapter 205A
11. State of Hawaii Coastal Lands Program of DLNR Land Division
12. State of Hawaii Land Use Commission, HRS Chapter 205 13. State of Hawaii Certified Shoreline, HRS Chapter 205A
14. Mandatory Seller Disclosures in Real Estate Transactions, HRS Chapter 508D
15. Uniform Land Sales Practices Act, HRS Chapter 484
16. Hawaii State Building Code, HRS Chapter 107, Part II
17. PUC 2007 Adoption of the 2002 National Electrical Safety Code
18. Standards for Civil Works Under the Jurisdiction of the USACE
19. The U.S. Navy Climate Change Roadmap (2010
20. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers Standard 169, Weather Data for Building Design Standards (ASHRAE 2006)
21. The National Fire Protection Association NFPA 1144: Standard for Reducing Structure Ignition Hazards from Wildland Fire (NFPA 2013)
ii. Adapting Building Construction and Civil Infrastructure to Climate Change iii. Draft list of Proposed code amendments
1. Additional Codes and Regulatory Standards not previously mentioned: a. For Risk Category III and IV structures, any proposed Hawaii codes and standards incorporating climate change effects should reference the USCOE technical guidelines for determining relative sea level rise. b. ROH Chapter 32 Building Energy Conservation Code: Adopt American Society of Heating, Refrigerating, and Air-Conditioning Engineers Standard 169, Weather Data for Building Design Standards for use in the application of ANSI/ASHRAE/IESNA Standard 90.1-2004, Energy Standard for Buildings Except Low-Rise Residential Buildings
60 WAIPAHU TOD COLLABORATION
c. ROH Chapter 20 Fire Code: Create regulatory maps of historic burn areas for use in these regulations and for mandatory seller disclosures
In addition to code updates should reflect the current knowledge on climate change and flooding. Identifying and approaching flooding as an area-wide problem instead of a building-scale problem would promote comprehensive flood mitigation strategies.
FLOOD MITIGATION 61
APPENDIX B: MICRO MAPS
Location near Waipahu Refuse and Convenience Center
62 WAIPAHU TOD COLLABORATION
Location near the Pearl Harbor Historic Bike Path
FLOOD MITIGATION 63
Location near Waipahu District Park
64 WAIPAHU TOD COLLABORATION
Location near Pouhala Station
FLOOD MITIGATION 65
APPENDIX C: STATE-OWNED PARCELS
Six State-owned parcels on the northeast side of the study area (see Figure 2) are described below with their function and vegetated area, and named by the tax parcel number.
1. 9-4-017:065, Undefined address
A three-acre lot with even level topography. Mostly paved with interspersed grass strips, trees, and shrubs. East face is lined by the Waipahu drainage canal. Currently serves as vehicle parking for Plantation Town Apartments and is owned by Aoao of Plantation Town Apartments and Hawaii Housing Finance and Development Corporation.
2. 9-4-017:001, 94-941 Kauolu Pl.
A four-acre lot that houses two buildings that comprise Kamalu Hoolulu Elderly Housing. Contains a parking lot but maintains large vegetated surfaces with grass, shrubs, and large canopy trees. Currently owned by Hawaii Public Housing Authority.
3. 9-4-017:063, Undefined address, currently parking for 94-941 Kauolu Pl.
A one-acre lot, with even level topography. Includes an unpaved strip that parallels Waipahu drainage canal. Currently owned by Hawaii Housing Finance and Development Corporation, includes a roundabout at the end of Kau Olu Place serves as a parking lot. The north end of the lot is paved and has interspersed grass strips and trees.
4. 9-4-017:062, 94-333 Mokuola St
A one-acre lot with even topography. Currently contains the Mokuola Vista apartments. Majority of the ground surface is paved and used as parking. Edges of the property are not paved and contain grass strips with trees.
5. 9-4-017:052, 94-275 Mokuola Street
A four-and-a-half-acre lot which contains Waipahu Public Library, Waipahu Civic Center, and a parking lot. Much of the property is developed and serves as roads or parking. In areas of vegetation, there is grass with trees and shrubs of varying sizes.
6. 9-4-017:051, 94-830 Hikimoe Street
A 0.8 long lot that houses the Waipahu Community Adult Day Care Center. The lot is largely developed and paved. Currently owned by Waipahu Community Adult Day Care Center and Hawaii Housing Finance and Development Corporation.
66 WAIPAHU TOD COLLABORATION
Figure
