Introduction 

The purpose of this page is to provide an example demonstrating how to perform urban stormwater flood modeling for a small city using Pipe Networks in HEC-RAS. This example model is a small 2D rain-on-mesh dataset for central Davis that includes a pipe network. A short duration high intensity hypothetical rainfall is applied to the 2D mesh and Top Inlets in the pipe network drain water from the surface throughout the city and direct flow to the subsurface storage on the downstream end. Then a pump station attached to the pipe network draws water from the downstream most pipe node and pumps it to the the outfall channel, a separate 2D area to the north.

Download: DavisStormSystemExample.zip

The example dataset provided is for demonstration purposes only and should not be used for other purposes. Some of the data within the dataset may have been altered specifically for the purpose of testing and demonstration and do not reflect actual conditions at the location. The example model has been simplified where possible for storage size and computation runtime purposes, and is not reflective of detailed study models. 

Geometry Setup

The two 2D meshes in this model were drawn in HEC-RAS Mapper, and refined using Breaklines and refinement regions. The terrain in this model is the USGS Original Product Resolution which is ~3.3ft cell size. This resolution is fine enough that the it captures the crown of the roadways which can be a significant hydraulic control and important for small scale detailed urban modeling. Breaklines were placed down the center of major streets to capture the crown elevations, and ensure appropriate flow paths down the major roadways. The images below show the 2D mesh with faces capturing the crown of the road, and a profile of the road showing the significant cross slope.

The pipe network in the model was developed starting with GIS data for the trunk lines in the city. The point and polyline GIS data was imported in HEC-RAS Mapper  using the import tools described in User's Manual. Then, more detail was added to the pipe network where needed such as lateral lines and top inlets at intersection locations. However, the stormwater network is still skeletonized (trunk lines only) in places where additional detail was not warranted. The laterals in this model were modeled using the Instantaneous Modeling Approach for conduits. This option allows flow into the inlets on the laterals to be instantaneously transferred to the receiving trunk node without performing the hydraulic or including the conduits in the pipe computational mesh. The benefits of doing this are faster computation times and a more stable model. 

To allows flow into (or out of) the pipe network to the 2D surface, Top Inlets were defined in the Top Inlet layer and attached to pipe nodes where inlets were located.


To model the wet well and pumping station on the downstream end of this system, a pump station was created connecting the pipe network to the outfall channel. The downstream end of the pipe network is a a large wet well / sump to store volume while the pumping station relieves the system. In order to model that, a Base Area was set on the downstream node representing the area of the wet well, and a Terrain Elevation Override  was set to represent the height of the structure. Together these dimensions will represent the volume of the the storage in the pipe network mesh.

Results

An animation of the profile plot below show the stormwater system surcharging, then slowly drawing down as the pump relieve the system.

Further Discussion

Though this model has a high resolution of detail in some areas, in order to use it for a detailed study level model a few improvements should be made. 

  • Further refinement of the 2D mesh to capture more streets and other terrain features that control surface water flow in the city
  • Addition of a fine resolution Manning's n layer that includes surface streets, buildings, etc.
  • Addition of a fine resolution infiltration and percent impervious layer to account for precipitation losses
  • More resolved Pipe Network geometry to include more lateral lines and side inlets
    • Note Side Inlets were not available in HEC-RAS when this model was developed.