Introduction

The purpose of this page is to provide an example demonstrating how to model a detention pond riser structure using HEC-RAS pipe networks. A brief description of how the model was created and a look at simulation results will be provided. The dataset can be downloaded below for reference. 

The example dataset is provided 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: RiserStructureExample.zip

Geometry Setup

The model domain in this example includes a short section of creek with a detention pond in the right overbank. The detention pond receives inflows through a conduit representing the outfall of a large stormwater system. The detention pond is drained by a riser structure with multiple inlets that discharge into the creek via a conduit.  The geometry for this dataset includes a small 2D mesh developed in RAS 2025 beta and imported into HEC-RAS 7.0.

After the mesh was brought into HEC-RAS 7.0, the inflow and the outflow conduits to the pond were drawn and attributed in HEC-RAS Mapper.

Since these two conduits are not connected, they can each be a standalone Pipe Network (analogous to having separate 2D Areas).  Though not required, this can be beneficial in a few ways: it allows for separate computation options (i.e. timestep, equation set, Courant target, etc. ), and if one of the networks is iterating or requiring very small timesteps, the other pipe network won't be affected by it. To designate these as two networks, unique System Names were given to each conduit as shown below:

Looking in the Computation Options, you can see that the 'Inflow' and 'Outflow' networks each have computation options.

Riser Structure Setup

To create the riser structure, the 'Riser' node was given a Base Area representing the area of the standpipe. Since the top of the structure is above the terrain elevation, the 'Riser' node's Terrain Elevation Override was set to the elevation of the rim of the structure. Setting the Base Area and Terrain Elevation Override for a riser structure is not required, however, doing so will account for the volume of the structure itself in the pipe network computations, and it makes for better visualization in the results profile plot.

The profile plot of the Det-Out conduit shows the width and height of the node after setting the Base Area and Terrain Elevation Override for the 'Riser' node.

 One Top Inlet and one Side Inlet were defined in their respective layers and attached to the 'Riser' node. The Top Inlet serves as the overflow weir on the top of the structure during high flow events, and the Side Inlet is the restrictor or bleeder inlet which is smaller for a slower release of flow. The Top Inlet elevation was set to that of the rim of the structure and 

Top Inlet and Side Inlet tables showing newly defined Overflow and Restrictor inlets:

Node Attributes table showing the new inlets and their elevations assigned to the 'Riser' node:

Replotting the profile plot for the conduit now shows the Top Inlet and Side Inlet on the node. 

Simulation Setup

To introduce inflows into the detention pond, an Inflow Hydrograph boundary condition was attached to the upstream 'Inflow-US' node in the Unsteady Flow Data Editor.  This is a hypothetical hydrograph that is intended to represent the outflow of a large stormwater system upstream. The 2D area has two External boundary conditions for the creek - an upstream flow hydrograph and a downstream Normal Depth. The simulation was run with the default computation options for Pipe Networks and 2D Areas.  

Results

The results profile plot for riser structure and conduit is shown below. As the water surface elevation in the detention pond rises the side and top inlets begin to flow, allowing water to enter the conduit and outfall into the creek downstream. The conduit is relatively steep, producing supercritical flow, so the HGL is below the critical depth profile.  Flow and velocity profiles through the conduit are at the bottom of the plot. 

The animation below shows a spatial plot of surface depth and pipe velocity next to the detention pond inflow (green) and outflow (pink) hydrographs. The riser structure and detention pond are performing well to cut the peak off from the hypothetical stormwater inflow hydrograph, reducing the 120 cfs peak flow to just 25 cfs. This reduces the peak flow in the receiving creek , spreading out the volume of the hydrograph over many hours.  

Since the 'Restrictor' Side Inlet on the riser structure is still a few feet above the bottom of the detention pond, some water remains stored there when the simulation is complete. If it is desired to drain the pond to a lower depth, another Side Inlet with a lower invert can be added to an auxiliary node. To set this up a new node and conduit can be drawn upstream of the 'Riser' inlet. The conduit's Computation Method should be set to Instantaneous so that flow through the inlet is routed instantaneously to the 'Riser' node. Then new low flow inlet can be attached to the new node. The images below demonstrate this setup.