Model Results

Runoff from the Undeveloped Site

Figure 2.8 shows the outfall hydrographs (Total Inflow at node O1) obtained for all storm events. This plot has been prepared in Microsoft Excel using the Time Series Output of both models. The hydrographs show sensitivity of peak discharge with increase in return period and with the rainfall intensity of a particular storm event. The rate at which the discharge volume increases is much greater than the rate at which the rainfall volume changes for different return periods. This is because the soil becomes more saturated during larger storms resulting in more of the rainfall becoming runoff.

The vertical scale for runoff in the above plot is kept relatively high (e.g. identical to the plot for developed site as shown in Figure 2.9) to understand how the magnitude of peak discharge increases from the undeveloped to developed condition at the same site.

Table 2.6 lists the summary of runoff results generated at the undeveloped site. It also shows the amount of infiltration loss and surface storage (depressions) generated at each storm event.

Table 2.6 : Runoff Result Summary of Undeveloped Site

Runoff from the Developed Site

Figure 2.9 shows the outlet hydrographs for developed condition in following. Unlike Figure 2.8, this plot shows a sharp rise in peak discharge, and a rapid decline after the rainfall is ceased. These results demonstrate the consequence of significant increase in overland surface imperviousness.

Table 2.7 lists the runoff result summary obtained for developed scenario. It demonstrates a sharp rise in peak discharge, compared to the undeveloped scenario, when the imperviousness is increased on the site overland surface.

Table 2.7 : Runoff Result Summary of Developed Site

Comparison of Runoff between Undeveloped and Developed Scenarios

Table 2.8 compares runoff volume and peak discharge between undeveloped and developed conditions as per peak total inflow at O1. For large storm events, where infiltration plays minor role in runoff generation, the site responses become similar between two cases.

Table 2.8: Comparison of Runoff Results

Major Outcomes

In this tutorial, GeoSWMM has been used to estimate the runoff response to different rain events for a 48.8 acres development that is built in a natural area. Comparisons are then made between the runoff peaks and total volume for three storm events for both undeveloped and post-developed conditions. The major outcomes of the analyses made are listed below:

• Overland flow of a drainage area can be measured by modeling a single or multiple subcatchments connected to an outfall. For calculation of only the amount of runoff generated, hydraulic routing is not essential.
• A large catchment can be divided into a number of subcatchments to better represent the drainage area properties. The Watershed Delineation Tool of GeoSWMM can be used for the task.
• Imperviousness is the key factor behind increase in runoff in the tutorial site.
• Characteristics of soil infiltration and the storm event affect the runoff hydrograph pattern. When soil becomes saturated, infiltration is reduced and overland flow increases. This effect is evident especially in case of larger storm events (e.g. 100-year) which usually have higher rainfall intensity.
• Runoff declines rapidly from a developed surface after the rainfall has ceased. This is because overland flow on an impervious surface is mostly rainfall driven.
• As the influence of soil infiltration on outflow hydrograph gradually decreases with larger storm events, the gap between the peak discharges of the undeveloped and developed scenarios also decrease.