Improving Our Understanding of Surface Flooding with 1D-2D SWMM Modeling: Elbow Road Drainage Study

As part of the City of Chesapeake’s Elbow Road improvement efforts, GKY and Associates, Inc.
was tasked with creating an existing condition Stormwater Management Model (SWMM). The primary
goal of this modeling effort was to help the City and its roadway designers better understand and
accommodate drainage in the highly undeveloped, low-lying, and flat watershed. A secondary goal was
to evaluate alternative design standards, including a 20% increase in rainfall depths and 1.5- and -3ft sea
level rise (SLR) conditions.
Traditional 1D modeling was deemed inadequate due to watershed conditions; thus integrated
1D-2D SWMM modeling was introduced. Engineers worked to incorporate ditches, driveway culverts,
and inlet structures as the 1D roadway drainage system. These elements were then connected to a “2D
mesh” made of finite 1D elements which mimic the ground surface. The models analyzed 10- and 100-yr
design storms, with and without a 20% increase in rainfall depth, using 3-, 4.5-, and 6-ft tailwater
elevations. GKY engineers provided the City with 21 SWMM 1D-2D models, showing both 1D ditch
hydraulics and surface flooding. Conclusions were drawn from plan view flood maps, hydraulic grade
line plots along the roadside, and cross-sectional figures capturing road overtopping at key stations.
Ultimately, roadway designers were provided a table summarizing the resultant water surface
elevations at key drainage outlet points along the road. These elevations may be used as boundary
elevations for hydraulic grade line computations in other projects.
1D-2D SWMM modeling has not always been a feasible solution, but with continued
technological developments, these models are increasingly practical and highly effective. The
topography of this watershed is so flat and low that assumptions required for traditional 1D modeling
can subjectively skew the results. Unfortunately, this situation is common across much of coastal
Virginia. Additionally, increased design rainfall depths only intensify the need to evaluate surcharged
flow paths. This roadway improvement study was a useful example to explore the versatility and power
of 1D-2D SWMM modeling and serves as a lens through which we can highlight the importance of this
modeling method moving forward.