2D Rain-on-Grid Modeling for an Urban Resilience Plan

Chesapeake’s Greenbrier Drainage Area experienced significant street and parcel flooding during Hurricane Matthew, when more than 11 inches of rain fell on October 8th and 9th, 2016. Some streets became impassible, and key roadway intersections experienced prolonged flooding. The floodwaters were slow to recede. In response to this and other severe storm events, engineers from the City of Chesapeake and GKY & Associates, Inc. prepared hydraulic models and produced a resiliency plan that identified three cost-effective projects to reduce the magnitude and duration of future flooding.

The engineering approach is different from routine master drainage plans and stormwater management designs because the scale of the problem is so much larger. This study evaluated rainfall events having average recurrence intervals on the order of 100 to well over 1,000 years.

The project team used the U.S. Army Corps of Engineers HEC-RAS software to create 2D Rain-on-Grid (ROG) models of existing and proposed scenarios, including the Hurricane Matthew storm. The models were built using 3D LiDAR-derived terrain data, so flood depths and overland flows are much more reliably computed than by simpler methods. Construction of the models required careful terrain processing and data entry of 50 significant culvert crossings. The substantial assumptions required in 1D SWMM modeling—when hydraulic grade lines rise above the ground—are eliminated in ROG modeling. Likewise, flows are computed over the entire terrain, not merely along assumed 1D flow paths.

Aerial photographs were taken during Hurricane Matthew, showing flooding on the ground during the event. Rainfall was reliably recorded near the center of the drainage area. The rainfall record and aerial photographs were used to calibrate the ROG model—which was then used to configure and evaluate the benefits of potential flood improvement projects.

The engineering team identified three potential improvement projects that can alleviate future flooding by reducing the elevation and duration of street and parcel flooding in the Greenbrier Drainage Area. The results are useful not only for showing what would work but also for avoiding potential improvement projects that would not provide appreciable benefits.