Model Setup

GeoSWMM model for Tutorial 03 can be developed using the GIS shape files supplied with this manual (Table 2.1), by Import Layer model setup. Details of these methods are demonstrated in the User's Manual of GeoSWMM. It is assumed that readers have sufficient knowledge of the model developing procedure, hence contexts of this section start with the GeoSWMM model geodatabase provided for Tutorial 03 e.g. Tutorial_03.gdb. A backup of Tutorial_03.gdb should be stored separately before working with it.

When opened in ArcGIS Pro, the model geodatabase should appear like following Figure 2.3.

Both the Geodatabase and Model Object Panel contain network element information (e.g. Rain gages, subcatchments, junctions, storage units, conduits, orifices, weirs, and outfalls). In addition, the Model Object Panel contains non-visual object information (e.g. time series, transect and storage curve data). Users can read or edit object attributes either from the GIS feature attribute table or from the Properties Editor of the Model Object Panel. In a SWMM model, generally, natural channels are represented as conduits (links); manholes as junctions (nodes); while weirs can be represented as both links and nodes.

Representation of Detention Pond

After creating a GeoSWMM project database using the supplied shape files, or after opening Tutorial_03.gdb, notice that a significant change has been made at the site outfall location in comparison to previous Tutorial 02. This is illustrated in the following Figure 2.4.

The outfall O1 is moved to a new position (downstream of the detention pond) in Tutorial 03. It connects the pond to the receiving water channel. The previous outfall node of Tutorial 02 is replaced with a simple junction J24. The storage unit SU1 represents the pond. The Invert of the pond is set 7 ft below the ground elevation of 370.0 ft at 363 ft. Thus, initially, a design pond depth of 7 ft is considered including 6 ft of stored water (at maximum level) and 1 ft of freeboard. For safety purposes, it is better to keep the water level as low as possible in the pond. However, lowering depth would require a larger surface area, which might not be always feasible. The shape of the pond is described in the model using a storage curve, SC1. Note that the final shape of the pond will depend on how it has been designed to capture and discharge runoff flow rates.

A 140.3ft long circular conduit C23 is provided to connect storage unit SU1 to junction J24. Other properties of conduit C23 are identical to those of conduit C1. However, an outlet offset of 1 ft was provided for C23 to keep its outlet above the invert of the pond.

Multiple outlets have been proposed and designed for SU1 in the model. Each of these outlets is subjected to a specific runoff event. After model building, the user will find three orifices and one weir connected from SU1 to the downstream junction J25. Among them:

• Orifice Or1 is proposed to release WQCV,
• Orifice Or2 is proposed to release 2-year storm runoff,
• Orifice Or3 is proposed to release 10-year storm runoff,
• Weir Wr1 is proposed to release 100-year storm runoff

In case of larger storms, the subject outlet and all other outlets located below it operate together. For Tutorial, in case of a 10-year storm, orifices Or1, Or2 and Or3 will operate together to discharge flow from the pond.

Junction J25, instead of outfall O1, is proposed as the outlet node for the control structures because GeoSWMM doesn’t allow more than one link to be connected to the outfall from upstream. Then, a roughness of 0.01 and 100 ft long conduit C24 is provided to connect junction J25 to outfall O1. The physical properties of this conduit are similar to those of conduit C23. New inverts of the nodes J25 and O1 are 325.0 ft and 320.0 ft respectively.

After the GeoSWMM model network is developed and the detention pond elements are created in the project, please take note of the following:

• Shape files of the orifices and weir do not contain designed information. They will populate only some arbitrary values during model building. While going through the design procedure, orifice and weir properties will be designed and updated.
• All orifices and the weir share common end nodes (e.g. SU1 and J25) in this model. So, the elements are represented in parallel to distinguish their positions. Otherwise, all orifices and weirs will align vertically on top of one another. Therefore, it would be useful to display them separately (by creating additional vertices) on the map for ease of identification and selection (Figure 2.4).

Finally, before the model simulation, provide (or check) the following input data.

Rain Gage Properties

A rain gage provides precipitation or rainfall data to a GeoSWMM model. In this Tutorial, rainfall in the study area is measured at a Rain gage. The Property Editor of this gage is shown below.

A 2-hour synthetic storm event with three different return periods i.e. 2-year, 10-year and 100-year has been assigned as the rainfall Data Source. To set rainfall data in the gage, the type of Data Source (e.g. time series or external file) and the Series Name need to be assigned. For Tutorial, in above figure, the Data Source is specified as TIMESERIES and the Series Name is specified as 2yr. This series is created under Time Series block in the Model Object Panel (see Time Series Data for details). To learn more on rainfall data types that can be assigned GeoSWMM, review the User’s Manual.

Subcatchment Properties

There are 16 subcatchments in the GeoSWMM model of Tutorial 03. Table 2.4 summarizes the physical properties of their drainage areas.

Table 2.4 : Subcatchment Properties

Junction Properties

The endpoints of the conduits and their confluences are represented by regular junctions. Locations of these nodes in this Tutorial are shown in Figure 2.1. A list of the junctions and their invert elevations are listed in Table 2.5.

Table 2.5 : Invert Elevation of Junctions

NB: Maximum Depth of all junctions is set to zero. This will allow GeoSWMM to set the depth of each junction as the distance from the junction’s invert to the top of the highest conduit connected to it. Thus, junction flooding will occur as soon as flow exceeds the channel capacity.

Outfall Properties

The entire study catchment drains to the FREE type outfall O1. It is connected to the dendritic conduit network, and acts as the outlet node for subcatchment W1. Invert of this outfall is 320.00 ft.
To facilitate the design of orifice Or1, the drainage system cannot be directly connected to storage unit SU1. Therefore, a temporary outfall will be used as C23's outlet. Upon Or1 design completion, the temporary outfall will be removed, and the outlet node of conduit C23 will be assigned to storage unit SU1, establishing hydraulic connectivity.

Conduit Properties

Figure 2.1 shows the layout of the runoff conveyance network in the study area. In this Tutorial, the conduit sizes will be designed to avoid any flooding. A total of 24 conduits have been modeled in this Tutorial, among which two are natural channels with irregular cross section and the remaining 22 conduits have circular cross section. Physical properties of all the conduits are listed in the following table.

Table 2.6 : Conduit Properties

NB:

• Maximum Depth represents the vertical distance of the top width level from the invert in the cross section for an irregular channel. For a circular pipe, it is the internal diameter.
• The diameters listed in the above table are initial values. These values will be hydraulically designed in this Tutorial (see Estimation of the Water Quality Capture Volume).
• Except for conduit C23, inlet and outlet offsets of the conduits are set to zero i.e., conduit bottoms coincide with the invert of inlet and outlet nodes.
• The length of the conduits used in this model is 2D i.e., elevation differences in inlet and outlet nodes are not considered in length computation.
• Note that no minor loss, storage, and transport are considered in the conveyance network to keep the analysis simple.

Transect Properties

The Transects section in the Hydraulics block in Model Object Panel contains cross sectional data for the irregular channels. In this Tutorial, two irregular channels are used with transect data named TRSECT4 and TRSECT12. In the Transect Editor, these data can be inserted either manually or directly imported from an external file. Note that the two CSV files as listed in Table 2.1 contain these transect data. After the data has been inserted, the Transect Editor and Viewer of conduit C4 should look like Figure 2.6.

Time Series Data

In the project file Tutorial_03.gdb, three time series datasets are provided under Time Series block in the Model Object Panel. These datasets represent a 2-hour synthetic storm event with three return periods i.e. 2-year, 10-year, and 100-year. They are assigned as a Rain Gage property (e.g. rainfall data) in three analysis scenarios. The rain format is Intensity with a Time Interval of 5 minutes. The Time Series editor and the chart viewer should appear as the following Figure 2.7.

Under Time Series block, users can create, import, or edit time series data for any object (e.g. node inflow) as well. For details on working with time series data in GeoSWMM, see the User’s Manual.

Simulation Options Setting

The Options block in the Model Object Panel enables simulation settings in GeoSWMM. There are five tabs in Options editor. In this Tutorial, an 84-hour simulation was carried out at 15 second Time Step to compare the peak-runoffs for different hydraulic routing methods. Table 2.7 lists the primary simulation settings.

Table 2.7: Simulation Options for Tutorial 03

Loss Parameters

In a catchment hydrologic process, major water losses accounted for are Infiltration and Evapotranspiration. To account for infiltration loss from the subcatchments, Horton model has been applied in this Tutorial. Parameter values used in this model, which have been assigned to all subcatchments, are listed below.

Table 2.8: Horton Infiltration Model Parameters

Note that evapotranspiration and other loss properties are not assigned to the current model. For details on these loss parameters, review the User’s Manual.