Stumpy Lake Dam Spillway Capacity Improvement Alternatives

The City of Virginia Beach identified the need to refine the existing HEC-HMS/RAS model for Stumpy Lake Dam to evaluate future dam and spillway improvements per the DCR conditional operations and maintenance certificate. Lying within a 10 square mile, highly urbanized drainage area, Stumpy Lake Dam is classified as a Significant Hazard Potential Dam.

Significant refinements to the existing probable maximum flood event (PMF) model included two main parameters: developing an alternative unit hydrograph (UH) approach to the SCS default peak rate factor of 484, and revising the time of concentration runoff parameters by leveraging the City’s highly detailed and validated PCSWMM stormwater drainage model (containing 1200 subcatchments, 2800 conduits, and 2200 nodes). Revised UHs were based on USBR’s lag time and shape factor nomographs, which are appropriate for a wider range of watersheds, specifically flat, coastal regions such as Virginia Beach. The model revisions resulted in a higher (and earlier) peak discharge that more accurately simulated regulatory large storm events. The calibrated model also better matched observed Stumpy Lake water levels during the historic 2016 Hurricane Matthew event (500-year recurrence), by not only matching peak levels, but also the rising and falling limbs.

In addition, review of LiDAR data revealed the potential of flow to bypass or circumvent the spillway by overtopping in an uninhabited, wooded area west of the spillway, which was previously not accounted for. The updated model provided a full and accurate picture of how the dam functions during regulatory storm events by quantifying the bypass depth-velocity impacts to better support dam safety decisions.

To meet the Half PMF event spillway design flood capacity, four alternatives were evaluated: widen existing spillway, construct an auxiliary spillway, armor entire dam embankment with HydroTurf™ protection, and raise the dam embankment. The low-relief terrain, presence of nearby residences, and downstream wetland swamps were the primary evaluation factors.

The detailed modeling revealed that due to the flat topography, marginal increases to spillway discharge capacity triggered significant downstream tailwater impacts during the Half PMF event, rendering several options as ineffective. Ultimately, the selected rehabilitation alternative, which also was the most cost-effective, was to raise the embankment while also providing increased impoundment capacity.