Model Results
The final GeoSWMM model for the post-development site with the detention pond is shown in Figure 2.13. Table 2.11 summarizes the characteristics of the different discharge elements included in the pond’s outlet, which are illustrated in Figure 2.13.
Table 2.11 : Characteristics of the outlet structure
ID | Type of Element | Event Controlled | Shape | Height h (ft) | Width b (ft) | Invert Offset Z (ft) | Discharge Coefficient |
|---|---|---|---|---|---|---|---|
Or1 | Orifice | WQCV | Closed Rectangular | 0.29 | 0.29 | 0 | 0.65 |
Or2 | Orifice | 2 yr | Closed Rectangular | 0.50 | 2.25 | 2.00 | 0.65 |
Or3 | Orifice | 10 yr | Closed Rectangular | 0.58 | 1.0 | 3.20 | 0.65 |
Wr1 | Weir | 100 yr | Rectangular | 1.15 | 9.00 | 5.85 | 3.3 |
The outflow hydrographs produced by this final design for the detention pond were compared against those resulting with no control as well as against the pre-development discharge targets. Those target discharges were 6.95, 16.62 and 55.85 cfs for the 2-yr, 10-yr and 100-yr storms, respectively. The post-development-controlled hydrographs were generated using the input file for the final design of this Tutorial, while the post-development uncontrolled discharges were generated using the final design developed in Tutorial 2. The pre-development hydrographs were generated using the final design developed in Tutorial 01 for the pre-development watershed.
The resulting sets of hydrographs are shown in Figures 2.14 through 2.16. Once again, a spreadsheet program was used to combine results from different GeoSWMM runs onto one graph. They verify that the detention pond was able to control post-development peak discharges from the site to their pre-development levels. Note, however, that the storage unit had no effect on reducing the total volume of post-development runoff that resulted from the large increase in impervious area.
Major Outcomes
The major outcomes from the analyses carried out in this Tutorial are presented below:
- A detention pond here helps to detain the water for a specific period (40 Hours) and then releases the water after some degree of settlement in the pond.
- The use of a storage unit with 3 orifices and 1 weir helps in reducing the peak flow and hence reduces the probability of flash flooding.