SITE ANALYSIS

Waipahu

Figure 1. Oahu map highlighting Waipahu.

Site and Demographics

Waipahu is located on the south shore of Oahu on Pearl Harbor, between Middle Loch and West Loch (see Figure 1). The half-mile radius study area is centered at Pouhala Rail Station and encompasses varying types of properties such as parks, commercial business, apartments, and single-family homes (see Figure 2). The study area also includes the new rail station, guideway, Farrington Highway, and proposed transit-oriented development (TOD). Of the 1139 buildings within the study area, 324 buildings lie within the Waipahu Neighborhood TOD plan as depicted in the 2016 Waipahu TOD brochure.

In 2012, the City and County of Honolulu’s Department of Planning and Permitting randomly surveyed residents living within the study area. Of the 2,700 mailed surveys, they received 21 percent completed and delivered. Responding residents provided the following data:

• Nearly 40% live in walk-up apartments. • 24% stated that flooding affects their properties at least once a year. An additional 10% are affected once every two to three years. • 24% report that their main mode of transportation is public transportation.

These statistics are valuable when considering future sea level rise. Unlike single family homes, many apartment and commercial buildings are unable to be elevated. In the case of the 34% of survey respondents who experience flooding at least once every two to three years, it is important to consider alternatives to mitigate flood damage as sea levels elevate. As nearly a quarter of respondents report that public transportation is their main mode of transportation, protecting low lying bus stops, bus routes, and rail guideway infrastructure can ensure continued use of public transportation.

Sites of Interest

The following water bodies and open spaces within the Waipahu TOD study area offer opportunities for flood mitigation: Pouhala Marsh; Waipahu District Park; Kapakahi Stream; and Waipahu Drainage Canal. In addition, this study is written for the State of Hawaii Office of Planning, therefore, the six state-owned parcels near Waipahu District Park are of interest and described here. Other sites addressed in this report include the rail guideway along Farrington Highway and residences located along the waterways. Photos of the area and a site map key to the photos are provided (see Figure 3).

Pouhala Marsh

Located across Waipahu Depot Street from the Waipahu Refuse Convenience Center lies a 70acre tidal marsh (see Figure 2). Managed by the Department of Forestry and Wildlife, Pouhala Marsh is a wildlife sanctuary that has been undergoing improvements and restoration efforts due to significant ecological disturbance. Because of its proximity to the Waipahu Refuse Convenience Center, considerations for the site once included turning the area into a landfill, and eight acres of artificial fill was added to the marsh. This affected natural freshwater flows from Kapakahi Stream. As a result, the area is typically dry. Invasive plants, particularly mangrove and pickleweed dominate the landscape. Kapakahi Stream begins at the Hawaiian Plantation Village and flows along Waipahu Depot Street into Pouhala Marsh. In a 2006 watershed study, Kapakahi Stream was found to have unacceptable levels of nutrients such as nitrites, nitrates, and phosphates and trash. In a 2010 article published in the John Hopkins University’s journal,

Figure 2. Waipahu Transit Oriented Development map showing current and future development.

E

A F B

C D

EXISTING CONDITIONS PHOTOS SPRING 2018

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.

Credit: K. Yamamotoya

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.

Credit: K. Yamamotoya

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.

Credit: L. Gonzales

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.

Photo credit: N. Nishimura

Photo D

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 F

Soil in portions of Waipahu contain high amounts of clay, which results in low water absorbtion rates.

Photo credit: H. Au

“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

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.

Figure 4. Waipahu TOD FEMA Flood Insurance Rate Map

OCEAN

COASTLINE

coastal erosion annual high wave flooding

passive flooding

SLR exposure area

ISLAND

Figure 5. Schematic diagram of the Sea Level Rise Exposure Area (SLR-XA) as the combined exposure to sea level rise from passive flooding, annual high wave flooding, and coastal erosion. Source: Hawaii Climate 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.

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

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

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) 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.<?>

X “unshaded” (Orange solid)

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.<?>

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

Figure 8. Storm drains act in reverse during king tides in Waikiki in 2018. Photo: W. Meguro. 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).

Hans L’Orange Park

Clinic

Waipahu District Park

FARRINGTON HWY

Wastewater Pump Station Wailani Stream Drainage Canal

Waikele Stream

Pouhala Marsh Wildlife Sanctuary

Kapakahi Stream LEGEND

3.2 Ft. SLR-XA exposure area 4 Ft. Inundation above mean higher high water 6 Ft. Inundation above mean higher high water State parcels State parcels

Critical facilities Waipahu Transit Center Station Proposed Land Use (June (2016) Pouhala Station (NTS)

0.5 mi radius

±0 0.05 0.1 0.2 0.3 0.4 0.5