Model Setup

A GeoSWMM model for Tutorial 04 can be developed using the GIS shape files supplied with this manual (Table 2.1) by applying Import Layer technique. Detailed model development techniques are demonstrated in the User's Manual of GeoSWMM. It is assumed that readers have sufficient knowledge on model developing procedure, hence contexts of this section start with the GeoSWMM model geodatabase provided for Tutorial 04 e.g. Tutorial_04.gdb. Users should keep a backup of Tutorial_04.gdb before working with it.

When opened in ArcGIS Pro, the model geo-database should appear like following Figure 2.2.

Both the geodatabase and model object panel contain network element information (e.g. raingage, subcatchments, junctions, conduits and outfall). In addition, model object panel contains non-visual object information (e.g. transects, time series, time patterns, land use, pollutants, calibration data etc.). Users can read or edit object attributes either from the GIS feature attribute table or from the element table of the model object panel.

Before running the model, users need to provide (or check) following input data.

Rain Gage Properties

A rain gage provides precipitation or rainfall data to a GeoSWMM model. Rainfall in the study area in this tutorial is measured at 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 the above figure, the Data Source is specified as TIMESERIES and the Series Name is specified as 2-yr. This series is created under Time Series block in the Model Object Panel (see Conduit Properties for details). To learn more on rainfall data types that can be assigned GeoSWMM, review the User’s Manual.

In SWMM, every subcatchment must be linked to a rain gage for the model to run. However, no rainfall should be applied directly to subcatchments representing LID features like filter strips and infiltration trenches, since they are considered part of their parent subcatchments. To address this, a special Time Series named "Null" is created with zero rainfall values. A corresponding rain gage, also named "Null", is linked to this series and assigned to all LID subcatchments. Meanwhile, the main runoff-generating subcatchments remain connected to the actual rain gage (e.g., "Rain Gage") used in the model.

Subcatchment Properties

The subcatchment properties in this tutorial will be the same as they were used in Tutorial 02 except that the widths will be different due to the addition of LIDs. For better estimate by the infiltration trenches, W16 has been divided into two subcatchments: W16_1 and W16_2. Figure 2.4 shows the discretization of subcatchment W16 into two subcatchments and the associated infiltration trench. Table 2.3 lists the subcatchments to which LIDs have been added where filter strips and infiltration trenches are symbolized as “FS” and “IT” respectively.  The properties of the two discretized subcatchments of W16 are listed in Table 2.4. Note that the widths for all the subcatchments, including the discretized subcatchments, have been calculated by the Width Calculation Tool; a preprocessing tool of GeoSWMM.

Filter Strips Properties

As listed in Table 2.2, seven filter strips are added to the model. Seven separate pervious subcatchments have been created to represent these filter strips. Table 2.3 lists the properties of these strips.

Table 2.3 : Properties of the Filter Strips

After adding the filter strip properties (as listed in Table 2.3) to the model, each filter strip subcatchment is set to 0% imperviousness. The impervious roughness is assigned a value of 0.015, and impervious depression storage is set to 0.1 inches, though these values are unused due to the absence of impervious area. The pervious roughness is set at 0.24, and pervious depression storage at 0.3 inches, matching the values used for other pervious areas in the watershed. For infiltration, the Horton method is applied with both the maximum and minimum infiltration rates set to 0.28 inches/hour, which corresponds to the minimum infiltration rate of the local soil. This conservative setup accounts for potential reductions in infiltration capacity and assumes the soil may already be saturated when a storm begins. The rain gage assigned to each filter strip subcatchment is the newly created “Null” rain gage so that no rainfall occurs directly over the strip’s area. 

Infiltration Trenches

Previously listed in Table 2.2, seven infiltration trenches are added to the model. Seven rectangular pervious subcatchments have been created to represent these trenches. Table 2.4 lists the properties of these trenches.

Table 2.4 : Properties of the Infiltration Trenches

When the variant properties of the infiltration trenches provided in Table 2.4 are added to the model, all trenches are assigned a common slope of 0.1%, an imperviousness of 0%, an impervious roughness of 0.05, an impervious depression storage of 0.1 in., and a pervious roughness of 0.24. A constant capacity of 2.5 and 1 in/hr will be used, respectively, as the maximum and minimum infiltration rate for all the infiltration trenches. This ignores any horizontal infiltration that might occur through the sides of the trench. The rain gage assigned to each trench subcatchment is the newly created “Null” rain gage so that no rainfall occurs directly over the trench’s area.

Junction Properties

Conduit ends and their confluences are represented by simple junctions. Locations of these nodes in this tutorial are shown in Figure 2.1. List of these 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 385.12 ft.

Conduit Properties

Figure 2.1 shows the layout of the runoff conveyance network in the study area. The conduit properties also remain the same as they were in Tutorial 02 except that two extra conduits have been added in this tutorial. Physical properties of all conduits are listed in the following table.

Table 2.6 : Conduit Properties

NB:

• Maximum Depth represents vertical distance from the invert to the top width level, in the cross section for an irregular channel. For a circular pipe, it is the internal diameter.
• Inlet and outlet offsets of the conduits are set to zero e.g. conduit bottoms coincide with the invert of inlet and outlet nodes.
• Length of the conduits used in this model is 2D e.g. elevation difference 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

Transects section in the Hydraulics block contains cross sectional data for the irregular channels. In this tutorial, two irregular channels are used with transect data named TRSECT4 and TRSECT12. In transect editor, these data can be inserted manually, or it can be directly imported from an external file. Note that a .xlsx file, as listed in Table 2.1, contain the transect properties for the current model. However, they have been provided with this manual, and the Transect Editor and Viewer of conduit C4 should look like Figure 2.5.

Time Series Data

In the project file- Tutorial_04.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 e.g. 2-year, 10-year and 100-year. The rain gage is set to an “Intensity” format with a interval of 5 minutes. The Time Series editor and the chart viewer for Tutorial 04 should appear like Figure 2.6.

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

Simulation Option Setting

The Options block in the Model Object Panel enables simulation settings. There are five tabs in Options editor. In this tutorial a 12-hour simulation has been carried out at 5 minutes Time Step to examine the effects of LIDs. Table 2.7 lists the primary simulation settings which are set for Tutorial 04.

Table 2.7 : Simulation Options for Tutorial 04

Loss Parameters

In a catchment hydrologic process, major water losses accounted are infiltration and evapotranspiration. To account for infiltration loss from the subcatchments, Horton model has been applied in this tutorial. Parameter values, which have been assigned to theall subcatchments except for filter strips and infiltration trenches, used in this model are listed below in Table 2.8. For Horton’s parameters of filter strips and infiltration trenches, see Filter Strips Properties and Infiltration Trenches respectively.

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.