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. 

Download: SWMM_Extran_Examples.zip

This example model serves as a comparison of results between EPA SWMM and HEC-RAS. EPA-SWMs Extran test suite is a collection of SWMM models that are used in the EPA's quality assurance testing for their dynamic wave routing model. The SWMM Extran tests are described in the Quality Assurance Report for Dynamic Wave Routing on the EPA SWMM website and the datasets are also available for download there. In addition brief descriptions of the datasets and and a SWMM *.inp file for them can be found on OpenSWMM.

The HEC-RAS datasets were created by importing the SWMM Extran test models into HEC-RAS via the SWMM Import Tool (described here). Since the SWMM models are not georeferenced, and HEC-RAS relies on the the georeferenced lengths of conduits in the computations, the XY point data for the nodes were manipulated to ensure the conduit lengths were correct in the coordinate reference system. After importing the pipe network geometry, a terrain was created using terrain modifications to match the SWMM Max Depth attributes for each node, and a downstream channel was burned in to model the open channel where the pipe network discharges with a 2D area.
An additional small length of conduit and another node was added to the HEC-RAS geometry at the outfall of the pipe network. This was done because the node where the two pipe branches come together is a Junction type node and cannot also serve as an outfall to an open channel. The extra conduit and additional node (Culvert Opening type) allows the pipe network to outfall into the receiving 2D channel. This was preferred over having the conduits outfall individually to the 2D channel for comparison purposes.

The boundary conditions for the HEC-RAS simulation were extracted from the SWWM model and added to the appropriate nodes as Flow Hydrographs. In Addition, the SWMM model results were brought into HEC-RAS as observed data for easy results comparison using the Import EPA SWMM Output as Observed Data... tool (described here) in the Unsteady Flow Data Editor.

SWMM Extran1 Test Example

In the SWMM Extran1 test, surcharge flooding occurs at node 80608 when the Max Depth is exceeded, allowing water to leave system. To allow water to surcharge out of a node in HEC-RAS a connection to a surface element is required, so drop inlets were added to nodes and Storage Areas were placed to capture the surcharging.

The HEC-RAS stage flow results at each node location compare well to the results in the EPA SWMM model as shown in the examples below. 

SWMM Extran7 Test Example - Inline Pump

The Extran7 test is a modification on the Extran1 dataset to include a inline pump at that pumps water from one branch to the other. The pump draws water from surcharging branch (top) to the branch with additional capacity, preventing surface flooding on the top branch. This dataset was built using the Extran1 dataset that was converted to HEC-RAS as a starting point, then adding a pumping station connected from node 82309 to node 15009. The pump station is controlled by a set of Rule Operations defined in the Unsteady Flow Data Editor that aim to mimic the step function pump curve from the EPA SWMM model. 

On the whole, flow results between the two models agreed well. The figure below shows flow results at the outlet node with HEC-RAS flows in green and EPA SWMM flows in pink. Note there is a slight difference in stage between the two models with the falling limb of the hydrograph which is due to differences in the open channel routing of the downstream channel between the two software packages.

A few differences were noted between EPA SWMM results and HEC-RAS results for this dataset. First, the timing of the pump was slightly off between the two datasets which is likely a result in differences between how each software package handles pump operations (on/off, transitions etc.) and slightly different hydraulic conditions in the system.

Another difference was that at peak flows the HEC-RAS results show a steady state in the system where flows and stages stop changing until the inflow hydrographs recede. Whereas the EPA SWMM model shows flow and stage increasing slightly on the upper branch until the inflow hydrographs recede. This difference can be seen in the hydrograph plot for node 82309 where EPA SWMM flows (pink) and stages (black) are increasing while HEC-RAS flows are steady.