Model Setup

This tutorial focuses on how to develop a 2D flood model using an existing calibrated 1D stormwater model. The base model, provided in the “Tutorial_1D.gdb”, represents an urban watershed that includes both pipe networks and open channel systems. We will build upon this 1D model to generate a 2D flood simulation.

To begin, open the “Tutorial_1D.gdb” project in GeoSWMM. Then, add additional spatial data layers to start 2D modeling.

Required Data Inputs:

• DEM (Digital Elevation Model)
• 2D Model extent Boundary
• Obstructions (Building footprints)

All of the above datasets are included in the ‘Supporting_Data.gdb’ file provided in the tutorial folder. Once all these layers are loaded to your ArcGIS Pro workspace along with the GeoSWMM project, follow the steps below to build the 2D model from the existing 1D GeoSWMM project.

To ensure accurate simulation of overland drainage, it is recommended to add 2D outfall nodes at appropriate locations within the model domain. These newly added outfall nodes serve as surface drainage exit points, allowing water to flow continuously and naturally out of the system. This maintains hydraulic continuity and prevents artificial ponding along the 2D model boundary.

Tip: Assign 2D outfalls at the lowest elevation points where overland flow would naturally exit the modeled area. Refer to Figure 2.2 below for a visual example.

For this tutorial, an outfall node OF2 was added at the lowest point in the 2D surface grid near the study area outfall. This ensures realistic surface drainage and more accurately reflects real-world flood behavior.

Rainfall Data

In GeoSWMM, a rain gage is used to provide precipitation or rainfall input to the model. For this tutorial, the study area is assigned a 24-hour synthetic storm event with three return periods: 2-year, 10-year, and 100-year. The rainfall data is linked to the rain gage through the Rain Gage Property Editor, as shown in Figure 2.3.

In this tutorial, the data source is set to TIMESERIES, and the series name is “100yr24hr”. This time series is sorted and managed within the Time Series block of the Model Object Panel in GeoSWMM.

For detailed instructions on assigning rainfall data and creating time series, refer to the GeoSWMM User Manual.

Time Series Data

The Tutorial_2D.gdb project includes a predefined time series dataset located under the Time Series block in the GeoSWMM model panel. This dataset represents a 100-year return period, 24-hour duration synthetic storm, and is directly linked to the Rain Gage property. The time series is formatted using intensity values with a 6-minute time interval (Figure 2.4), providing high resolution for simulating detailed rainfall-runoff responses throughout the model domain.

Simulation Options Set Up

Simulation settings are configured in the Options block of the GeosWMM Model Object Panel. The Options editor contains five tabs, each allowing control over different aspects of the simulation.

For this tutorial, a 24-hour long simulation has been carried out at 1 minute reporting time step to support overland flow analysis. Key simulation parameters, including process model, routing methods, and time step controls, are summarized in Table 2.3.

Table 2.3 : Simulation Options for Tutorial 10

Loss Parameters

In hydrologic modeling, two major forms of water losses are infiltration and evapotranspiration. To account for infiltration losses from the subcatchments, Green-Ampt model has been applied in this Tutorial. Uniform parameter values have been assigned across all subcatchments, as shown in Table 2.4.

Table 2.4 : Green-Ampt Model ParametersTable 2.4: Green-Ampt Model Parameters

A monthly average evaporation rate of 0.01 in/hr has been applied to the model. For simplicity, depression storage is assumed to be uniform across all the subcatchments. This approach ensures consistent surface runoff behavior, as overland flow is only generated once depression storage is satisfied.