In the stormwater management system, on the one hand, the stormwater collection is often done via catch basin. Catch basins are used to catch overland flow and transport the water to underground sewer system. On the other hand, the overland inflow can also entre sanitary sewer via sewer manholes, reducing sanitary sewer capacity. In the VO-SWMM, we provide users a set of default capture curves (i.e. depth-flow table) for catch basins based on their types, multipliers, and locations and we also provide a set of default manhole rating curves based on the manhole cover types and holes. However, to use those capture curves or rating curves and therefore to model properly the flow from overland system e.g. street or road to underground sewer, the model needs to know the surface water depth at that location e.g. on the street or road. Therefore, only knowing capture curve or manhole rating curve is not enough. The modeling of overland system is needed to tell the water depth and flow of the overland system e.g. on street or road.

Therefore, in the modeling of dual-drainage system, modeling the overland system and the underground sewer system simultaneously has a significant meaning in the stormwater management. Based on that, we recommend the dual-drainage modeling procedure as below. There are in general five major steps to create a dual-drainage model by using VOSWMM. In each step, there are tools to facilitate the model setup and tuning process. In general, the overland flow is modelled by creating nodes and conduits on the ground. The inflow from the overland system to the sewer systems are modelled by outlet links with rating curves. The hydraulics of those nodes and conduits are simulated, together with the sewer hydraulics, by SWMM5 engine.

The first step is to build the sewer system, which in many situations is straightforward importing the shp file. In VO-SWMM, the Import/Export GIS tool can help users to import layers and map parameters. Users can also save the parameter mapping for the next-time use.

As a preparation to build and set up the overland (i.e. major) system, VO-SWMM offers users a set of DEM Processing (note: DEM means digital elevation model) tools to process and obtain the key information for the overland system, e.g. flow path, sag area location, etc.

Based on the results of the previous two steps, users can create and set up overland system by adding overland nodes and links. User can drag-and-drop Junction from the Toolbox and hold CTRL key to add Junction and Conduit on the map. In the micro-drainage modeling procedure, the Sag area is modelled as a Storage Unit. Use the Create Sag Nodes tool to add and obtain rating curves for the Sag node. Please find the tool in Nodes and Links.

There are several rules recommended that users can follow when adding overland nodes and conduits.

  • It is recommended to place overland nodes on the flow path
  • It is recommended to make overland conduits match with flow path direction.

Regarding the locations of overland nodes, it is recommended to add overland nodes:

  • at the starting point of the flow path
  • when there is a catch basin / inlet along the flow path
  • when there is a sag / pond area
  • when there is a sewer manhole and at the curb of the road transect across the manhole
  • on the crest of sag/pond area
  • at a junction point when two flow paths meet
  • when there is a change in cross sections
  • when there is a change in slope
  • when there is a change in roughness of the road / ground
  • at the outlet point

In order to model how much water flow goes from the overland system to the sewer system, we need to build a connection between the overland and the sewer system. In the micro-drainage modeling procedure, this connection is modelled by outlet links that connect overland (i.e. major) nodes to sewer (i.e. minor) nodes and the flowrate from the overland to the sewer system is modelled by the rating curve. The rating curve of those outlet links can be obtained from the capture curves of the catch basins.

In the micro-drainage modeling procedure, an overland node is suggested to be placed next to a catch basin and then connected to a sewer node by an outlet link. Using the Assign Catch Basin tool, the system builds an internal connection between the overland node and the catch basin, so the system will treat the overland node as a pseudo of the catch basin. Using the Generate Capture Curves tool, based on connection built between the overland node and the catch basin, the outlet link rating curve will be obtained from the catch basin capture curve. In some situations, users may want to simplify the system by assigning several catch basin to one overland node, instead of placing major nodes next to each of those catch basins. By doing that, the rating curve will take a sum of a group of capture curves based on the catch basins’ type, multipliers, and locations.

The capture curve defines the overland flow drainage via catch basin, while for inflow and infiltration via sewer manhole, the similar method can be used to define the rating curves at the manhole. In the micro-drainage modeling procedure, an overland node is suggested to be placed at the curb of the road transect across the sewer manhole and then connected to the manhole by an outlet link. The outlet link’s rating curve is a result of manhole cover’s rating curve and the elevation distance from the manhole cover to the overland node (i.e. Channel Depth before Inflow). For more information, please refer to Manhole Inflow and Infiltration Modeling.

After the overland nodes and links are built, use the Generate Subcatchment tool to delineate the entire area into several subcatchments with slope and dimension calculated automatically based on the DEM file. If there are extra shp files or information available, use the Imperviousness Calculation tool to calculate the imperviousness for each subcatchment.

Finally, set up rain data, run simulation and analyze the results. The rain data are added via Resource Library. Users can also use the DRMT or the Obtain Rainfall From Raster tools to obtain rainfall for each subcatchment. Set up the simulation Engine Option similarly as in EPA SWMM5. Users can use the Surcharge Analysis to analyze the sewer surcharge level and the Flood Mapping tool to produce a flood mapping raster based on the overland system modeling results.