Virginia Beach Combats Flooding

Heavy rain, hurricanes and Nor’easters threaten the low-lying coastal city of Virginia Beach year after year. The area has struggled with flooding from record rainfalls, including Hurricane Matthew that impacted over 1,000 homes in 2016. Addi­tion­ally, it has seen recorded sea level rise of nearly one foot over the past 50 years. In response, the city has expedited plans to invest in resilience for its vulnerable neigh­bor­hoods.

Working with the city, CDM Smith used detailed stormwater modeling to develop compre­hen­sive flood mitigation plans and design alternative solutions to address limited stormwater management system (SMS) capacity and impacts from downstream tidal conditions. The completed plans are the basis for ongoing engineering design the city is undertaking to implement infra­struc­ture improve­ments in excess of $275 million.

The project focused on three Virginia Beach neigh­bor­hoods expe­ri­enc­ing a mix of challenging conditions, including low elevations, intense storms and coastal flooding, coupled with inadequate stormwater pipe capacity and storage, and 1950s-1960s-era drainage infra­struc­ture. 

CDM Smith developed hydrologic and hydraulic (H&H) models for each neigh­bor­hood to explore alter­na­tives to alleviate flooding and promote resiliency. The H&H models were developed at an unprece­dented level of detail for the large study area to accurately evaluate SMS performance and formulate infra­struc­ture improve­ments. Stormwater models were developed using Compu­ta­tional Hydraulics Inter­na­tional’s Personal Computer Stormwater Management Model (PCSWMM) software. Due to the extent of recurring flooding and the flat topography in the study area, both one-dimensional (1D) and two-dimensional (2D) models were developed. The modeling approach was specif­i­cally tailored to capture the dynamic interaction between precip­i­ta­tion stormwater runoff and tidal conditions – an interaction that is often simplified in practice, but in this case, substan­tially influences existing flood extent and the ability for stormwater improve­ments to function effectively. 

Models were calibrated to multiple storms over a range of rainfall intensities and boundary conditions and validated with available storm data. Data included the city’s storm complaints database, photographs and video of flooding during rain events and field-surveyed peak flood elevations. The models simulated the SMS response to rainfall events—a range of 2-, 10-, 25-, 50- and 100-year design storms—SLR conditions and correlated downstream tidal boundary conditions.

Neighborhood-specific mitigation plans included analysis of the existing SMS and evaluation of alternatives to reduce flooding. Alternative solutions were evaluated on their ability to meet a practical, achievable minimum level of service: 

• Limit peak stages to three inches of water or less above the road crown for the 10-year design storm
• Prevent flooding of structures for the 100-year design storm

This approach offers substantial benefits with significant cost savings compared to the funding that would be required to meet all current design standards.

The completed engineering evaluations and recommended mitigation plans assisted Virginia Beach to quantify infrastructure improvements necessary to protect highly vulnerable neighborhoods, understand the financial investment in critical stormwater management infrastructure and promote the public stability necessary for public/private renewal, redevelopment and reinvestment.