Charleston Peninsula Storm Surge Wall: A Critical Assessment by Robinson_Design_Engineers

Charleston Peninsula Storm Surge Wall: A Critical Assessment Prepared for

The Community of Charleston, South Carolina June 2020

Sea oxeye daisy along Gadsden Creek Photo credit: Ben Jack

Charleston Peninsula Storm Surge Wall: A Critical Assessment Prepared for

The Community of Charleston, South Carolina June 2020

Prepared by Emma Collins, MS Philip Ellis, MS, PE Rebecca Fanning, MPA Christopher Grose, MS, EI Joshua Robinson, MS, PE Lucy Rummler, MLA Nolan Williams, EIT

Contents

Executive Summary

Natural Context

Critique

Conclusion

Black skimmer at Waterfront Park Photo credit: Nolan Williams

Executive Summary

Robinson Design Engineers is a Charleston-based team of hydrologists, ecological engineers, and hydraulics engineers engaged in thoughtful and creative planning, research, analysis, and design for aquatic ecosystems. In our view, the US Army Corps of Engineers 3x3 proposal offers the City of Charleston a failure-prone, robust approach to a challenge that requires resilience. We served as active participants in the Dutch Dialogues and we acknowledge the need for a storm surge barrier. This particular manifestation of that proposed structure, however, contradicts the core principles of building resilient cities. We have therefore crafted this report to elucidate our concerns, which can be summarized by the following three critiques: 1. 2. 3.

The proposed wall creates a false sense of security, It doesn’t solve flooding, and It diminishes our cultural and ecological assets.

This report serves as our official public comment to the USACE and is intended to provide our fellow Charlestonians and colleagues with our perspective as engineers and experts in methods of flood potential reduction and management of aquatic resources. In our twelve-year history, Robinson Design Engineers has contributed to a variety of projects including urban stream daylighting, dam removal, flood hazard mitigation, watershed instrumentation, hydrology and hydraulic modeling, urban wetland creation, biotechnical bank stabilization, river engineering, salt marsh restoration, and lake management. Our work is predicated upon a shared conviction that each person, community, and ecosystem is unique and intrinsically valuable. It is important to us that our projects maintain a high quality over time, and our ideals ask that we seek self-renewing solutions where possible to reduce future burdens on land managers. As a result, we have reclaimed the service-oriented professional practice model, in contrast to the recent industrialized firm model. This means that we work to make impact through long-term relationships, not through transactions. We are an intentionally-small firm with deep roots in the region. To us, this isn’t a “market”; this is home, and our clients and colleagues are our neighbors. The Tentatively Selected Plan consists of a robust system of concrete, pilings, riprap, and diesel-powered pumps that conjure unfathomable cost and concern of failure. The proposed 7.8-mile long dam is incredibly simple in its approach, and yet the troubles we face are enormously complex. These structures will do very little towards protecting us from the damage caused by flooding, addressing only one rare causal mechanism of floodwater—that of moderate storm surge—in a system dominated by frequent precipitation and daily tide cycles. Furthermore, this permanent disfigurement of Charleston will disconnect residents and visitors alike from our waterways, held in the public trust.

Natural Context

“We cannot know what we are doing until we know what nature would be doing if we were doing nothing.”

Our work is guided by the above refrain, written in 1979 by the farmer-poet, Wendell Berry. In our work to manage and restore water resources, we strive to emulate and catalyze the natural processes of selfrenewing ecosystems. Historic Charleston is a unique coastal city because it is situated atop an ancient depositional bar at the confluence of two major rivers. For centuries, as the waters of the Ashley and Cooper Rivers mixed and met the tides of the Atlantic Ocean, the sediments they carried deposited and formed land. Layer by layer, the Charleston Peninsula was formed by water, waves, and wind—the sediment and debris they carried, and the plants and animals that they invited. No wonder the tip of the peninsula was naturally composed of large oyster shoals, i.e. the “White Point”, as this confluence zone is where meroplanktonic oyster spat was sheltered from strong currents. The process of land formation was punctuated by coastal storms, where violent wave and wind activity delivered large quantities of sediment, shell, and organic matter, and where the the physics of erosion and deposition helped to form the ridges of high ground where King and Meeting now lie. What would nature be doing If we did nothing to the Charleston Peninsula, and what can we learn from this understanding? These natural forces are clearly still at work, as each tropical storm surge deposits sediment, debris, and unmoored boats along the margins of the Peninsula and low areas in the landscape. Left alone, these natural layers of sediment and wrack would naturally elevate the Peninsula. Along its margins, the Peninsula would build a natural depositional bar, anchored by self-colonizing native vegetation, subject to shift as it absorbs the energy of wind and wave. A natural breakwater would emerge in the form of dense oyster clusters growing from woody vegetative anchors. This unbroken, living shoreline would meet waves with a soft ramp. The dissipating energy would in turn, cause more material to accumulate. The fascinating mechanisms of this natural defense are the storms themselves. With every surge, the Peninsula would become more resilient. Instead of natural berm-building, however, each surge event understandably catalyzes a clean up response to scrape away all deposits, returning the ground to a base level of zero. We have manipulated our natural estuarine system perhaps beyond repair. We cannot not deepen the harbor, for instance, as shipping traffic demands an unnaturally deep channel in an embayment that would otherwise form a shoal. Historic hydromodification dammed and diverted rivers to accommodate shipping routes and electricity demand, and these modifications have permanently altered the interaction of sediment and water. Furthermore, the artificial jetties along our coast have altered the natural cycles of sedimentation—depleting our natural barrier islands.

This “plan of the town, bar, harbour, and environs of Charlestown” issued in 1780 by William Faden documents the shifting sediment deposits once typical of Charleston’s harbor, reaching a maximum depth of just 13 feet.

We do not know whether an honest approach to emulating these natural processes exists, and whether the Peninsula could once again build naturally resilient depositional features. But we are sure that the proposed storm surge wall directly opposes these natural processes, and no matter what we do, the forces of nature will continue their attempt—with each tide and with each storm—to gradually elevate the ground of the Peninsula. The USACE proposed storm surge wall has its own unintended consequences, some of which we will never be able to imagine, let alone model and understand. The TSP would inflict a permanent scar and would simply be the latest episode in Charleston’s long history of oppositional acts. If we build a large wall, we sever ourselves from the natural, inherent resilience of this place. We blockade ourselves against any hope of the restorative forces of what would happen, if we were doing nothing. But the natural geomorphic context begs several questions, including: Could the City of Charleston be gradually elevated, slowly and carefully? Could naturally-scaled, nature-based, self-sustaining breakwaters—composed of stone, soil, plant, and shell—be constructed along the margins of the Peninsula? Could the remaining tidal creeks of the Peninsula be restored and expanded over time, both to absorb storm surge and to attenuate rainwater and large tides? Could Charleston embrace its natural context, and use this opportunity to enhance ecology, improve access for all, beautify the landscape, and reduce flood risk?

“It is critical that people understand that the storm surge wall would not completely The USACE Tentatively Selected Plan proposes to use storm surge walls, a eliminate storm inundation wave surge attenuation structure, and buyouts of vulnerable properties to reduce risk to public health and safety. In their own analysis of risk or flood risk. The intention is to reduction (see their report’s Appendix C) they demonstrate that the storm surge wall would result in a 0% change to loss of life and that the reduce damages from more incorporation of a wave attenuating structure would increase that level to a 18.7% reductionevents, in lives lost. Theynot do not adequately explain the frequent storm surge mechanism of this reduction in lives lost, but it is our understanding that the wave is deemed necessary to defend the old extreme events. Inattenuation otherstructure words, battery wall. flood risk remains after The percent reduction in anticipated lives lost over the fifty year period construction…projection corresponds to ~80 people saved. For comparison, Charleston County (population 411K) versus the City’s population of 138K records about 60 fatalities from car accidents each year. So if this number is extrapolated over the same period as the life-loss analysis (50 years), the accumulated lives lost to car crashes would be 3,000 lives. Culturally, we have accepted this loss of life, widely understood to be avoidable, but also mundane and not worth large scale disruption. 3,000 fatalities is accepted, but this wall is spending $2B dollars to save 80 of us.

Additionally, there is likely to be rainfall associated with any it is event unclear whether this lossif of life analysis incorporated the coastal storm Moreover, surge that, concern that people may be lured into a false sense of security, choosing in excess of pump capacity, to evacuate to the peninsula rather than far inland in the event of a catastrophic hurricane. The USACE report identifies this concern in its would flood streets low-lying discussion of and ringwalls (see pg. 57) and reduced compliance with evacuation orders (see pg. 147). They define ringwalls as essentially the areas, makingsame transit within the tactic of their perimeter wall, only on a smaller scale used to protect a complex of buildings, such as a hospital district, for example. They screen city a life safety hazard (much like ringwalls from consideration because it could induce risk by “entic[ing] people to seek shelter in in a structure the city has experienced theinstead of evacuating…. present[ing] a life safety concern in the event a ringwall is overtopped or fails” (pg. 57). past).” USACE Draft Feasibility Report, pg. 147

Critique

The Tentatively Selected Plan (TSP) seeks to effectively improve Public Health and Safety, Economic Sustainability, and Access to Critical Infrastructure. In our view, however, the proposed storm surge wall, wave attenuation structure, and buyouts not only lack demonstrable improvements to these three concerns, but may worsen the problems they seek to solve. In the Executive Summary of the Draft Feasibility Report (pg. 12), the USACE identifies three overarching problems the TSP seeks to redress: 1.

Storm surge inundation on the Charleston Peninsula places people at risk, including the potential for loss of life and declines in public health;

Access to critical facilities, emergency services, and evacuation routes is limited or cut off entirely during coastal storm surge events on the Charleston Peninsula;

The Charleston Peninsula experiences storm surge inundation that adversely affects the economic sustainability of Charleston, including: impacts to businesses, organizations, and industry; critical facilities and infrastructure; and residents.

In the following three sections, we outline our specific concerns and describe some of the unintended consequences of the TSP. In Section 1: A False Sense of Security, we discuss the problem statement on Public Health and Safety. In Section 2: Doesn’t Solve Flooding, we outline our concerns regarding Access to Critical Infrastructure. In Section 3: Diminishes Cultural & Ecological Assets, we elucidate our critique of the plan’s attempt to increase Economic Sustainability of the city.

A False Sense of Security

We foresee that the TSP would increase risk to public safety and health, forcing floodwaters at increasingly high velocities and depths, impairing natural waterways, and luring residents into a false sense of security raising the stakes of the life-loss analysis should the TSP infrastructure be compromised during a catastrophic storm.

Key Concerns §

The perimeter wall could induce risk by luring area residents into a false sense of security, who might be tempted to ignore evacuation orders, or worse— it could entice Charleston residents to evacuate to the peninsula instead of away from it in the event of a catastrophic coastal storm event.

The storm surge wall will create high velocity flow conditions as stormwater is siphoned out of pump stations instead of dispersing evenly and ponding as it currently does.

Ponding of water behind the storm surge wall could increase the duration and depth of stormwater flooding for perimeter locations.

Summary The USACE Tentatively Selected Plan proposes to use storm surge walls, a wave attenuation structure, and buyouts of vulnerable properties to reduce risk to public health and safety. The analysis of risk reduction (Appendix C) demonstrates that the storm surge wall would result in a 0% change to loss of life and that the incorporation of a wave attenuating structure would increase that level to a 18.7% reduction in lives lost. The report does not adequately explain the mechanism of this reduction in lives lost. The percent reduction in anticipated lives lost over the fifty-year period projection corresponds to approximately 80 people saved. For comparison, Charleston County (population 411K) versus the City’s population of 138K records about 60 fatalities from car accidents each year. So if this number is extrapolated over the same period as the life-loss analysis (50 years), the accumulated lives lost to car crashes would be 3,000 lives. Culturally, we have accepted this loss of life, widely understood to be avoidable, but also mundane and not worth large-scale disruption. 3,000 fatalities is accepted, but the proposed wall requires $2B dollars to save 80. If the primary objective is to increase public health and safety, then, this enormous investment makes very little headway in achieving that aim. Moreover, it is unclear whether this loss of life analysis incorporated the concern that people may be lured into a false sense of security, and choose to evacuate to the Peninsula rather than far inland in the event of predicted storm surge. The USACE report identifies this concern in its discussion of ringwalls (see pg. 57) and reduced compliance with evacuation orders (see pg. 147). Ringwalls are simply a small-scale storm surge wall, and are used to protect a complex of buildings, such as a hospital district, for example. The report screens ringwalls from consideration because it could induce risk by “entic[ing] people to seek shelter in a structure instead of evacuating…. present[ing] a life safety concern in the event a ringwall is overtopped or fails” (pg. 57). We submit the same logic as evidence against the storm surge wall.

Doesn’t Solve Flooding

People rely on infrastructure. Failing or low-performing infrastructure has an intangible negative effect on society. The proposed project would be one of the largest investments in infrastructure in South Carolina. The purpose of this infrastructure would be to reduce flooding. And yet, an investment in the USACE TSP still leaves significant risk during a catastrophic hurricane and ignores nuisance flooding entirely, a major source of floodwater in Charleston. Many of the very transportation routes and property the plan aims to protect during coastal storm surge will still be susceptible to stormwater flooding on a day-to-day basis.

Key Concerns §

The probability that a storm surge will overtop the proposed wall at least once during its 50-year lifespan is more than 40%.

The TSP proposes to funnel stormwater runoff to remnant natural creek systems and floodgate access points—which will inevitably increase the duration and depth of floodwater from normal rainfall events.

Summary The USACE Draft Feasibility Report specifies that the tentatively selected plan (TSP) serves to address storm surge flooding only. Storm surge occurs when high winds of hurricanes and tropical storms push water inland, creating significantly higher tidal flooding. The proposed storm surge wall would block this water from the shoreline up to an elevation of twelve feet. The wall would follow the perimeter of the Peninsula, tracing Morrison Drive, Murray Boulevard, and Lockwood Drive, and then would follow the marsh up to the I-26 Rutledge Avenue southbound exit. The TSP makes no mention of how much water storage capacity of marshland is lost by displacing it with the combo walls. The TSP addresses storm surge, one aspect of a much larger problem, but even closing floodgates and damming riverine and harbor waters behind a concrete barrier will not eradicate storm surge flooding. Remnant natural creek systems on the Peninsula will be used as a temporary holding place for stormwater runoff while flood gates are closed. This will overburden an already degraded natural resource and fail to provide adequate storage capacity given the extent of the wall and its floodplain and the limited capacity for stormwater pumping stations. USACE is transparent about shortcomings of the TSP, admitting that the storm surge wall will not solve even this one component of the larger flooding problem. The 1% Annual Exceedance Probability (AEP) for Base Flood Elevation (BFE) at the proposed location for the wall is 15’ elevation according to a 2016 FEMA analysis. Even without accounting for sea level rise, this translates to a 40% probability the wall will be overtopped at least once in its 50-year design life. Note that by using 15’ overtopping threshold we receive a grossly conservative estimate, as the proposed height of the wall is 12’. In the event it does overtop, the damage caused by floodwaters will be increased as water will have nowhere to drain from the peninsula for an extended period as the stormwater holding capacity in the remnant natural creek systems would likely already be filled. Although the proposed wall’s deep foundation makes it unlikely to crumble if overtopped, the USACE report highlights the floodgates as most likely to fail when subjected to an intense storm event (see report pg. 146).

Sunny day tidal flooding along Morrison Drive. Photo credit: Joshua Robinson

Failure of floodgates during a storm event would cause a wave of intense, localized flooding to hit areas adjacent to the gated rivers. Consequences range from flood related damages to loss of life and property of those who did not take appropriate storm precautions due to a false sense of security. It is important to note that while the wall would be effective in blocking storm surge less than 12’ elevation provided no failures occur, it does not aim to solve everyday stormwater flooding, and in fact, will worsen such nuisance flooding in some areas. The last several years have seen a handful of devastating coastal storm events, but the ever more frequent “small” rain events have been equally if not more disruptive. Pavement and rooftops coat the peninsula in impervious surfaces that force stormwater to flood low lying areas including roads, courtyards, and base floors of buildings. When rainfall coincides with King Tide events, the compound flooding heightens the damage. The riverine floodgates proposed in the TSP would remain open most of the time, closing only when the National Weather Service issues a “major flooding” advisory, which is far less often than coastal flooding occurs on the Peninsula. Coastal flooding will be heightened in some areas as high tide builds against the storm surge wall, its only release being through the floodgates. From there water will be forced up our remnant natural creek systems, with increased force of flow, damaging this vital ecological asset. The TSP does not attempt to address these types of nuisance stormwater flooding, which cause property damage and road closures on a regular basis.

Diminishes Cultural & Ecological Assets

The historical haunted feeling of Charleston has long been intertwined with its natural beauty and complex culture. Charleston owes much of its charm to its long history of resilience in the face of impending disaster. Charlestonians have endured terrible natural disasters and rebuilt on scarred ground. Some of our growth has been contorted by this scarring, and we have made decisions that perhaps we would take back in hindsight. The design philosophy of these federally administered funds prioritizes economic conservatism over indirect costs to ecosystem services and cultural expectations of Charleston aesthetics. Any amenities beyond the brutalist hard structures will necessarily incur cost to the City of Charleston both directly in producing the upfront funding, and indirectly as the City grapples with unintended consequences lasting long past the end of the wall’s design life.

Key Concerns §

The remaining tidal creeks and open waters currently enjoy semi-diurnal tidal exchange. The wall threatens to choke tidal flow to these creeks and destroy their natural form and function.

Aesthetic incongruity of a waterfront wall will likely not be acceptable to residents, requiring the City of Charleston to undertake significant beautification improvements along the 7.8-mile perimeter.

The ongoing long-term costs associated with wall maintenance and repair will marginalize flood hazard mitigation funding for the entire City of Charleston.

Charleston currently lacks meaningful opportunities to engage with its waters. The proposed wall threatens to preclude the possibility of Charleston developing a healthy relationship to its harbor and rivers.

Summary The 3x3 study uses Cost Benefit Analysis (CBA) to determine the final degree of conservatism, ensuring that the proposed burden on federal funds is achieving the highest benefit-to-cost ratio. Direct benefits comprise the predicted savings in damages from deflecting storm surge inundation. And direct costs include those entailed by construction, procurement of materials, and buyouts of impacted properties. An example of indirect costs, not accounted for in their analysis, would be reduced property tax revenues to the City, reduced livability for residents, impacts to businesses adjacent to construction, and reduced ecosystem services of marsh habitat. This economic feasibility analysis, then, comes at the expense of maximizing social benefit and minimizing social cost. Ideally, CBA incorporates indirect, or non-price factors to maximize social benefit and minimize social cost—thereby creating the most public value for the costs incurred. The plan fails in this regard. We must consider the substantial ongoing financial burden this infrastructure will inevitably entail. While the initial City investment of $600M does include expected cost of acquiring properties within the buffer zone of the proposed storm surge wall, as well as the City’s portion of the construction costs, it does not include the many mitigating factors the City will likely need to consider, such as the cost of replacing underground infrastructure to accommodate the storm surge wall, the increase in stormwater runoff generated by the footprint of the structure itself, the hazard posed to water resources by constricting flows through floodgates, and considerable aesthetic blight it will impose on our shorelines. Furthermore, the ongoing maintenance of this structure by the City will constrain funds to the Peninsula instead of providing avenues of funding that can be applied to the greater area comprising the City of Charleston.

Great egret over Gadsden Creek Photo credit: Blake Suarez

Conclusion

The Tentatively Selected Plan consists of a robust system of concrete, pilings, riprap, and dieselpowered pumps that conjure unfathomable cost and concern of failure. The proposed 7.8-mile long dam is incredibly simple in its approach, and yet the troubles we face are enormously complex. These structures will do very little towards protecting us from the damage caused by flooding, addressing only one rare causal mechanism of floodwater—that of moderate storm surge—in a system dominated by frequent precipitation and daily tide cycles. Furthermore, this permanent disfigurement of Charleston will disconnect residents and visitors alike from our waterways, held in the public trust.

The storm surge wall, if implemented, will fail within its design life: it will undoubtedly fail to decrease flooding the vast majority of the time; it will fail to protect the public, our health and safety; and it will in no way secure the economic sustainability of the City of Charleston, as it purports to do. Moreover, the risk of structural failure through overtopping or an actual breach would constitute a tremendous waste of resources and render meaningless the sacrifices we must make to clear a path for it: the homes demolished, the financial resources reallocated, and the salt marsh squandered, all in vain. We fear that more harm than good would come of this wall, and that it would prove a lasting burden and wreak unforeseeable havoc on this, our home.

The United States Army Corps of Engineers has little capacity to adapt this plan towards a more resilient, less destructive approach. This $3M study will not provide the City of Charleston with a viable solution, but only distract us from the array of alternatives we’ve already considered and made little headway towards implementing. Sea level rise is an inevitable existential threat that we must confront; however, we should be wary of modifications that have the capacity to make matters worse. This plan demonstrates a remarkable capacity to do just that. There is such a long history in Charleston of consequential manipulations to the natural environment, we must move forward with integrity. The land where Charleston now lies was resilient in the face of natural disaster, and it can become so again, if natural processes are embraced instead of ignored.

We encourage City leaders and the community at large to reject this proposal and to consistently reject others like it that induce risk by raising the stakes of failure. Instead, we must consistently embrace those alternatives that promote the ecological integrity of our natural defenses against floodwater and build self-renewing systems of resilience that recognize our city’s unity with the waters that give us definition.

Gadsden Creek Photo credit: Gately Williams