by Bob Horner, Julie Eaton Ernst, and Indrani Ghosh
Each hurricane season, residents in the West Ashley community of Charleston, S.C. pay attention when the weather forecast shows a system heading their way. Having often been impacted by flooding, there is much attention focused on improving drainage in any way possible. The traditional approach to stormwater management has included widening drainage channels and designing and constructing new outfalls and stormwater pump stations.
Engineers, scientists, and landscape architects are now, however, working together on more resilient stormwater management solutions to reduce the frequency and intensity of flooding that causes property damage. These approaches embrace topography and enhancing natural systems that have the capacity to store and absorb water.
Through a FEMA mitigation grant, the City of Charleston purchased and demolished about a dozen residential homes and a townhouse development that have flooded repeatedly over the past 20 years. This reactive “managed retreat” approach will field test the concept of reverting developed land back into pervious areas that can infiltrate water while also increasing basin storage capacity. Plans call for the areas to be repurposed into wetland buffers, walking trails, and transitional land that can provide recreation during dryer periods and stormwater storage during significant storm events. The concept is to incorporate this sustainable approach into other efforts to reduce flooding, with the hope of expanding the approach to better accommodate changing weather patterns and sea level rise, which is expected to increase in the area by about two feet over the next 50 years.
Other communities in South Carolina are alternatively taking a more proactive approach. For example, Marion County and the Town of Pamplico are developing a comprehensive stormwater management plan and conducting stormwater flood modeling efforts to be able to predict flooding hot spots and develop phased and incremental flood mitigation alternatives for future severe weather events.
Here in New England, many communities are also taking a similar proactive approach by planning for the impacts of climate change ahead of time. For example, Weston & Sampson, in collaboration with the Charles River Watershed Association, developed a watershed wide model for more than 15 communities in the Charles River watershed covering about 280 square miles. This model predicts the extent, depth, and flood elevations at different locations in the watershed for different storm events, considering both present and 2070 climate conditions. The model simulated what would happen if 50% of the current undeveloped conservation land in the upper and middle parts of the watershed were to be developed, covering what had been pervious undeveloped land with impervious roads, sidewalks, and roofs. Under such a scenario, the model showed that by 2070, the amount of land impacted by flooding would increase by over 1,700 acres, or 19%, with flooding in some parts of the watershed doubling or more. On average, this increase in impervious land cover would be expected to increase runoff volumes by 41%.
As the impacts of climate change become more severe, the more frequent and extreme weather events that we are all now seeing will become more and more the norm. This will make innovative and effective approaches to stormwater management even more valuable.
Whether in South Carolina or New England, land use is a major determinant of the impacts from flooding. Any increase in the amount of impervious land in a watershed will necessarily result in more impacts from flooding, and the occurrence of more frequent storms with higher intensities will only exacerbate those impacts. Enlightened scientists, municipal planners, and elected officials are already implementing resilient and flexible changes in how they approach land use based on real data, and time will tell if these changes will be aggressive enough to mitigate the continued challenges of flooding in our communities.
Bob Horner, PE is regional manager; Julie Eaton Ernst, PE is project manager; and Indrani Ghosh, Ph.D. is technical leader at Weston & Sampson.