Application of Storm Water Model

One of the major elements affecting Jefferson Parish or any urban area is flooding caused by runoff from excessive rainfall. One of the factors that affects the hydraulic design is the amount of rainfall used in the calculation for sizing of the drainage system. Currently, Jefferson Parish uses 24 hour, 10 year rainfall frequency in design of drainage system. The design rainfall is 9.2 inches. This policy has been in place since 1980.

To evaluate the effectiveness of the current rainfall frequency curve that the Parish uses, a typical urban development is used which consists of 23 acres with several catch basins as shown in Figure 5.26. XP-SWMM Storm Water Management Model (Storm Water Management Model, 2003) with rainfall frequencies that were calculated from the Audubon rain gauge station was used to determine the hydraulic effects of runoff. Additionally, XP-SWMM Model was run with the existing curve for comparison purposes.

XP-SWMM is a software package for modeling storm water and wastewater flows and pollutants. It is used to simulate a natural rainfall-runoff process. It can perform continuous or event simulations with or without the inclusion of water quality computations. To simulate the hydrologic cycle, the user may select the design storm or actual storm event. Design storms for any duration and return periods may be created from:

• SCS Method

• Chicago Storm

• Huff Distributions

• ARDR Temporal Patterns

• User Defined Distributions.

There are numerous methods available for computing storm runoff hydrographies for events such as:

• Non-linear runoff routing

• SCS unit hydrograph using a curve number with curve linear or triangular unit hydrographs

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• Nash unity hydrograph

• Rational method

• Modified rational method

• EPA RTK unit hydrograph for RDI.

To set up the storm water model, the model basin is divided into several catchments and physical attributes are entered into tables. These data include size, length or shape of pipes, elevation invert slopes, outfalls and any other existing conditions that would affect the runoff. Then the data are processed with one of the rainfall events that was presented earlier.

Since Jefferson Parish uses a 10 year, 24 hour rainfall for the design of their drainage system, only results from a 24 hour rainfall analysis are discussed in this section. Output data include numerous graphs for the flow, velocity and hydraulic grade line for each node. Results can be viewed in three dimensions. Users may navigate by zooming or changing the viewing location.

The limits of the study area (approximately 23 acres) is shown on Figure 5.26. Based on the field information, the entire watershed was divided into sub-areas to identify the theoretical overland flow patterns. Each sub-area contains a reference number which will be used to identify hydraulic data in the model analysis.

The Chicago storm was selected for the rainfall block of XP-SWMM model. The synthetic hyetograph computed by the Chicago method (Modern Sewer Design, 1980) is based on the parameter of an assumed intensity-duration-frequency relationship in the form: a i = (5-2) (t + c)b where

i = average rainfall intensity (inch/hour) t = storm duration (minutes)

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a, b, c = constants depending on the units employed and return frequency of the storm The rising and receding legs of hyetograph are described by the following equations:

After the peak (5-3a)

Before the peak (5-3b)

Where

ta = time after peak

tb = time before peak

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Figure 5.27 Synthetic Rainfall Hyetograph (XP-SWMM)

The current Jefferson Parish 10 year, 24 hour design storm of 9.2 inches was used with XP-SWMM Model. A copy of input and output results have been included in Appendix “D.” Additional figures have been provided to summarize the maximum flow, stage outfall and Chicago Storm Distribution. The maximum stage at the outfall is 574.96 feet and the maximum flow is 61.87 cubic feet per second. The maximum intensity of rainfall is 3.8 inches per hour.

Since the 24 hour duration rainfalls are being evaluated, rainfall values for 2, 5, 10, 25, 50 and 100 year return periods were used with XP-SWMM Model. These rainfall values were calculated using the Log Pearson Method from rainfall data for the Audubon rain gauge station. The rainfall hyetograph and maximum flow volumes are included in Appendix “C.” Table 5.4 summarizes the output results for the six return periods.

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Table 5.4 Summary Results of Storm and Wastewater Management Model for 24 hour 10 Year Rainfall Frequency

Return Period Rainfal Rainfall Intensity Outfall Max Flow Flooded Volume Year (inches) (inch per hour) (ft) (cu. ft. per second) (cu.ft.)

2 6.64 2.8 574.88 58.83 5,150 5 8.40 3.5 574.94 61.58 49,669 10 9.73 4.1 575.00 62.58 187,229 25 11.58 4.8 575.00 62.98 224,690 50 13.10 5.5 574.98 62.71 285,920 100 14.75 6.2 575.00 62.71 335,814

Rainfall from Log Pearson Type III is applied to each sub-area. The XP-SWMM Model produces a hydrograph at each node or junction (entrance to the pipe network). The hydraulic block of the program was used to route the rainfall through the network of pipes. All culverts sizes, types and length were entered into the model. The volume of flooding in Table 5.4 refers to the total volume of water above the ground elevation where the flooded pond storage area starts. Ground elevations vary from a high of 582 feet to a low of 580.5 feet.

The results indicate an increase in rainfall contributed to an increase in the volume of flooding. For the 5 year return period, maximum flooding occurs at node 211 with 1.2 inches of flooding. For the 100 year return period, the volume of flooding results in 6 to 9 inches of flooding over the entire study area.

Figure 5.27 shows actual rainfall records in which there are many shorter duration rainfalls than the current Jefferson Parish 24 hour design storm. However, these storms have caused major flooding throughout Jefferson Parish.

57 - 1.0 2.0 3.0 4.0 5.0 6.0

Hour 1 Hour 3 Hour 5 Hour 7 Hour 9

Hour 11 Hour 13 Hour 15 Hour 17 Hour 19 Hour 21 Hour 23

Ra infa ll (inc he s ) 11/7/1989 5/9/1995 5/8/1995 11/8/1989 5/31/1959 6/1/1959 9/26/2002

Figure 5.28 Historical Short Duration Rainfall Events

To evaluate the affects of short duration rainfall, a rainfall event that occurred on November 8, 1989 resulting in 5.8 inches of rainfall in 2 hours was selected for analysis. The XP-SWMM Model was used with the 5.8 inch rainfall. The output file with hyetograph, stage elevation and maximum flows are provided in Appendix “E.” Table 5.5 compares the results that were generated by the model for a 10 year return period, a 24 hour Jefferson Parish design storm and the two hour duration rainfall that was selected from historical records.

Table 5.5 Summary Results of 10 Year, 24 Hour Rainfall Duration

Rainfall (Inches)

Rainfall Intensity

(inch/hour) Outfall (ft)

Max Flow (cubic ft/second)

Flooded Volume (cu-ft)

Jefferson Parish 9.20 3.9 574.96 61.87 79,042

Log Pearson Method 9.73 4.1 575.00 62.58 187,229

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The intensity for the short duration rain is very largely due to its being less than an hour. This illustrates that there are shorter duration events that can result in a significant increased flooding.

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