2020 Excellence in Environmental Engineering and Science™ Awards Competition Winner

Honor Award - Environmental Sustainability

City of Virginia Beach Stormwater Management and Flood Mitigation Planning Project

Entrant: CDM Smith
Engineer in Charge: Gary St. John, P.E., BCEE, PMP
Location: Virginia Beach, Virginia
Media Contact: Joanna Samson

Entrant Profile

The City of Virginia Beach is the most populous city in Virginia, located on the state's south- eastern coast along the Chesapeake Bay and Atlantic Ocean. Virginia Beach is almost entirely surrounded by water and permeated with lakes, rivers and marshes. The City's economy mainly relies on tourism, the military and agriculture. Being in the coastal region of Hampton Roads, Virginia Beach understands the threat of sea level rise and is preparing for the future. The City is developing plans for current and future flooding risks after seeing recorded sea level rise of nearly one foot over the past 50 years.

CDM Smith is a privately-owned engineering and construction firm providing legendary client service and smart solutions in water, environment, transportation, energy and facilities. CDM Smith has over 50 years of experience in stormwater and watershed management planning and engineering, and during this time, has developed innovative state-of-the-art methods and tools to analyze water quality and quantity issues for urban development. The City of Virginia Beach selected CDM Smith to develop stormwater management system mitigation plans for multiple neighborhoods that are consistently experiencing critical flooding, including: The Lakes, Princess Anne Plaza, and Windsor Woods. In total, the study areas encompass almost 2,000 acres.

Project Description

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. In response, the City 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.

Integrated Approach

The project focused on three Virginia Beach neighborhoods experiencing a mix of challenging conditions:

Low elevations
Intense storm events
Tidal and coastal flooding impacts
Sea level rise (SLR)
Inadequate stormwater pipe capacity
Lack of stormwater storage
1950s- and 1960s-era drainage infrastructure

These complex challenges required an integrated approach. The project area included approximately 2,000 acres of land, surrounding drainage basins and bodies of water, including the Chesapeake Bay, lakes, ponds, canals, creeks and tributaries. In total, over 12,000 acres—18+ square miles—were analyzed to account for flow interaction, SLR and water quality. Solutions were formulated carefully so benefits to one area would not adversely impact an adjacent area. SMS improvements proposed in the mitigation plans accounted for impact to wetlands and addressed permitting requirements.>

Innovative Technology

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. The high level of detail in the H&H models was purposely selected to address the sensitivity of flood damages. For example, in the Windsor Woods neighborhood, each tenth of a foot in elevation change represented approximately 50 homes.

Stormwater models were developed using Computational Hydraulics International's Personal Computer Stormwater Management Model (PCSWMM) software. PCSWMM uses the U.S. Environmental Protection Agency's SWMM engine, which uses physical H&H parameters based on the best available data, engineering guidelines and judgement. 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 2D model used 20-foot grid spacing to effectively simulate surface storage, flow paths and flood depths at a high level of detail in the most vulnerable, extremely flat areas. Because of the computational requirements of 2D modeling, the remaining study area was modeled in 1D. 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.

Proven Performance

Models were calibrated to multiple storms over a range of rainfall intensities and boundary conditions and validated with available storm data. These data included the City's storm complaints database, photographs and videos of flooding during rain events and field-surveyed peak flood elevations.

CDM Smith successfully calibrated the H&H models to a range of observed flooding conditions caused by notable weather events, including a storm on September 8, 2014; remnants of Tropical Storm Julia on September 19-21, 2016; and Hurricane Matthew on October 8-9, 2016. Simulation of the existing SMS capacity, overland flooding and downstream tidal conditions were critical to producing accurate water surface elevations throughout the study areas.

Once the model matched observed conditions, it was used to simulate the SMS response to other rainfall events—a range of 2-, 10-, 25-, 50- and 100-year design storms—SLR conditions and correlated downstream tidal boundary conditions. The models allowed the City to make informed decisions prioritizing infrastructure resiliency and sustainability projects.

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.

Solutions

Mitigating flooding in a low-lying area 10 miles from the coast without impacting downstream areas posed a complex technical challenge. CDM Smith provided both short- and long-term solutions for Capital Improvement Program planning. Immediate, readily correctible issues would restore the SMS to its original capacity and included:

Dredging multiple canals to reduce head loss, maximize downstream storage and repair canals to conveyance capacity close to the original design
Repairing/replacing damaged/blocked pipes found during the field surveys

Longer-term flood mitigation options, requiring more extensive design, permitting and investment included:

Constructing stormwater pump stations and gated control structures
Directly connecting bodies of water to maximize available storage volume
Constructing approximately 85 acres of new detention storage
Maximizing connectivity and storage in the entire system behind gated structures
Increasing the size of the SMS significantly to improve conveyance toward existing canals
Increasing ditch conveyance capacity
Constructing pipe improvements for better drainage

CDM Smith provided planning-level opinion of probable construction cost (OPCC) for each study based on a conceptual level of detail.

Social and Economic Advancement

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. The mitigation plans were a key component in the City's long-term plan to address SLR, which was necessary to maintain bond ratings, promote confidence with private investors and preserve property values in established neighborhoods within the community. Planned safety measures provided substantial protection from flood damage to homes and structures and maintained emergency vehicle access to roadways.

Click images to enlarge in separate window.


Photo 1: This chart shows the distributions of surface soil types found in the Princess Anne Plaza and The Lakes study area based on spatial data provided by the City. The primary components of the H&H model representation of existing hydrology were soils and land use data. Soils data were used to define the infiltration and stormwater runoff from pervious areas included in the model. Land use data was used to estimate impervious cover, and for parameters that define the routing of stormwater runoff to the SM.	Photo 2: The Windsor Woods Model Schematic includes the modeled SMS network for the Windsor Woods study area. The CDM Smith team analyzed the existing systems in each neighborhood to fully understand conditions and capacity constraints. The Windsor Woods SMS includes open channels, pipes of 15-inch diameter and larger, constructed stormwater facilities, and overland flows.

Photo 3: The City developed a website for their customers and citizens to upload date-stamped photographs of water surface elevations that were used to calibrate models.	Photo 4: CDM Smith reviewed photos documenting the event and estimated peak flood depths at multiple locations. For calibration, model results were compared with estimated flood depths derived from photos of observed water levels. To achieve calibration, model parameters were adjusted to achieve a strong match between observed flood depths and model simulated flood depths.

Photo 5: H&H models were calibrated to historic rain events. Here is an example of calibration for a major storm that occurred March 31, 2017. As shown, the modeled peak stages are within 0.1 feet of observed levels; hence, the model was considered reasonably calibrated.	Photo 6: The topographic digital elevation model (DEM) extracted from LiDAR shows the low-lying, flat nature of the study area. Topographic elevations range from approximately 0 feet NAVD88 along London Bridge Creek to 15 feet NAVD88 within portions of the study area. The highest elevations within the natural topography are approximately 15 feet NAVD, with most stormwater inlets ranging between 5 feet and 11 feet NAVD.

Photo 7: The flood map for the study area of Princess Anne Plaza and The Lakes and a complete list of flood depths and indicator elevations were compiled for the 10-year storm. The results of the existing condition simulations indicated that 116 intersections do not meet the minimum level of service goals for the 10-year storm.	Photo 8: The results of the existing condition simulations for the 100-year design storm are shown here for the same study area. This figure represents a flood extent map, superimposed on an aerial photograph of the study area. The simulated flood extent in- tersects the building footprint for a total of 600 structures for the 100-year design storm. The project team also created animations of the flood extent, providing a dynamic view of flooding during any event simulated. This helped gain support and establish credibility with stakeholders and City Council members, especially for the Hurricane Matthew model calibration.

Photo 9: This map outlines one of four alternatives proposed to mitigate flooding in the Princess Anne Plaza and The Lakes study area. Alternative 3 and Alternative 4 provide suitable options to be considered since both alternatives meet the target 10-year LOS and nearly resolve 100 percent of the simulated flooding locations for the 100-year LOS. The identification and recommenda- tion to transform the Bow Creek Golf Course into a stormwater storage facility was an innovative way to create storage, which is a necessity to overcome the technical challenges associated with downstream tidal conditions.	Photo 10: Near-term and long-term flood mitigation options were tested as part of the various alternatives for all neighborhoods. The options were combined and built upon one another until the target LOS was met and adverse offsite impacts were removed. Depicted here is Alternative 5 presented for the Windsor Woods neighborhood.

Photo 11: Adding to the project's complexity, the recommended Windsor Woods SMS improvements are located in the effective Federal Emergency Management Agency (FEMA) floodway and would impact wetlands. Construction of improvements in the floodway require approval from FEMA and wetland permitting and consultation with governing regulatory agencies, including the US Army Corps of Engineers, the Virginia Department of Environmental Quality, and the Virginia Marine Resources Commission.	Photo 12: The Windsor Woods stormwater pump station located on Thalia Creek will lower upstream water levels prior to storm events and provide up to 750 cubic feet per second (cfs) of capacity to maintain water levels to meet the established level of service.