Virginia Beach Public Works Design Standards Manual - Sea Level Rise and Precipitation Adjustments for Stormwater Management Design

In 2019, the City of Virginia Beach updated its Public Works Design Standards Manual (Manual) to account for climate change impacts in drainage system design requirements that help control stormwater runoff. The Manual provides minimum design standards for all City public works projects within City rights-of-way, easements, and City-owned properties (e.g., roadway, drainage, and traffic signal design standards). Section 8 of the Manual addresses stormwater management, and unlike most of the rest of the Manual, its standards apply to all private projects as well as City construction projects relating to stormwater management. Section 8 was updated to include design factors that account for increased rainfall, sea-level rise, and recurrent flooding in the criteria for designing critical and non-critical drainage systems. The updates implement an action item in the City’s Sea Level Rise Policy Response Report and will help mitigate stormwater-driven flooding under future climate conditions.

Section 8 of the Manual identifies minimum design standards for drainage systems within the City (i.e., the pipes, channels, ditches, and other infrastructure designed to hold or carry stormwater in order to reduce risk of stormwater-driven flooding). The Manual notes that drainage systems in coastal areas have additional factors to consider and evaluate beyond those of non-coastal drainage systems - including tidal impacts, sea level rise (SLR) and climate-change-driven precipitation changes, low elevations, and groundwater tables. Therefore, the City included modifications to its drainage design standards to account for SLR and precipitation increases among the many updates to Section 8 of the Manual.

First, the Manual was updated to adjust the design rainfall depths for drainage infrastructure, based on observed and anticipated increases in precipitation due to climate change. The Manual notes that design depths are typically generated from NOAA Atlas 14 precipitation frequency data, but that the City has already observed rainfall frequency depths around 10% greater than NOAA projections. Additionally, the City anticipates further increases in these average values over the next 30 years due to climate change. As a result, the Manual has been updated to increase the NOAA Atlas 14 values by 20%, meaning that drainage infrastructure design will now have to accommodate a larger amount of rainfall regardless of the storm frequency (e.g., 2-year storm, 100-year storm) that the infrastructure is designed for.

The Manual also now requires design considerations for sea level rise. For drainage systems draining into tidal waters, the water surface level of the receiving waters (known as “tailwater level”) affects the ability of the infrastructure to convey water effectively to mitigate localized flooding, and is therefore an important consideration in the design of the infrastructure. Tailwater levels can be affected by tides, winds, storm events, SLR, and other factors. Accordingly, the Manual incorporates sea-level rise by requiring an adjustment in the design tidal elevations that are used to determine the tidal tailwater levels. In order to account for future SLR, the Manual requires that for non-critical drainage infrastructure, 1.5 feet be added to the existing design tidal elevation (typically taken from the City’s FEMA Flood Insurance Study, or from other City studies). For critical drainage infrastructure (i.e., drainage infrastructure serving critical infrastructure such as major roadways, hospitals, emergency centers, etc.), a value of 3.0 feet is added to the design tidal elevation. The SLR adjustment will help lower the risk of backflow into the drainage system when receiving water levels (tailwater levels) are higher in the future, thereby ensuring that drainage infrastructure will still drain stormwater effectively.

The adjusted rainfall event and tide event levels (adjusted for SLR) are paired together to determine the appropriate tailwater elevation that controls the design of the infrastructure. The larger the area draining into the drainage asset that is under design consideration, the stricter (more infrequent) the design storm frequency should be (e.g., a one in a 10-year storm for a smaller area, compared to a one-in-50-year storm for a larger area). Therefore, larger drainage areas will result in the City building drainage assets that can accommodate larger storms. Drainage systems for “critical infrastructure” (e.g., hospitals, shelters, major roadways, evacuation routes, etc.) are required to be designed to a 100-year storm regardless of the drainage area size. Ultimately, the climate-adjusted tailwater criteria is taken with other criteria, such as seasonal groundwater levels and upstream and downstream drainage improvements, to determine the minimum design requirements for any stormwater conveyance system.

The modifications to account for SLR and increased precipitation implemented an action item under Goal 2 of the City’s Sea Level Rise Policy Response Report. Within Goal 2 (“Enhance the Flood Resilience of Critical Infrastructure and Transportation Systems and Invest in Capital Improvements to Reduce Community Flood Risk”), action item 1 of the stormwater plan and management actions recommends: “Formally adopt the most recent findings regarding sea level rise estimates and increased rainfall provisions into the stormwater design requirements and fully integrate these considerations into stormwater management and design practice.” The City’s drainage systems must also be designed in accordance with the criteria and provisions in various state guidance (Virginia Department of Transportation’s Drainage Manual, the Virginia Erosion and Sediment Control Handbook, and the Virginia Stormwater BMP Clearinghouse and Stormwater Management Handbook), as well as other city, state, and federal codes, ordinances, and regulations, including Virginia Beach’s Stormwater Management Ordinance. 

 

Publication Date: May 2019

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  • City of Virginia Beach, Virginia

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