Model Simulation

GeoSWMM can be used to model storage facilities that capture runoff from different design storms and release it to a receiving channel at a controlled rate. This Tutorial demonstrates how the design of a storage pond is an iterative process in which the dimensions of the pond and its outlets are changed to satisfy the design criteria and constraints for the design storms considered. The three main steps to design the storage pond are:

1. Estimate the water quality capture volume (WQCV).
2. Size the storage volume and the outlet to control the release rate of the WQCV.
3. Size the storage volume and the outlet to control the peak runoff rates from the 2-year, 10-year and 100-year design storms.

The final design will be a storage unit with a shape specific to its location, rainfall and climate conditions; a defined relationship between its surface area and storage depth; and a multi-outlet structure designed to control different runoff events. Figure 2.1 shows the schematic layout of a detention pond, its outlets designed to control a WQCV, and the peak discharges for three design storms. The stacked trapezoidal prism shape shown in this figure will be used in this Tutorial; the upper prism will control the major storms (10-yr and 100-yr) while the lower prism will control the minor storms (WQCV and 2-yr).

Note that the discharge for different storms is controlled by a combination of orifices and weirs rather than a single unique outlet. Orifice 1 in Figure 2.1 controls the release of the WQCV; orifices 1 and 2 control the release of the 2-yr storm; orifices 1, 2, and 3 control the release of the 10-yr storm and all the orifices combined together with the weir (4) control the release of the 100-yr storm.

Estimation of the Water Quality Capture Volume

The WQCV is the critical runoff volume to be used in the design of stormwater quality enhancement facilities. Detailed investigation based on calibrated long-term runoff simulations is the preferred method to determine this volume for a given site (Guo and Urbonas, 1996). However, several methodologies or “rules of thumb” have been proposed to estimate the WQCV that are simpler to use but are still reliable when long-term records are not available (see for instance Guo and Urbonas, 1995, 1996 and 2002; Water Environment Federation, 1998). This Tutorial will use the methodology proposed by the UDFCD (2001). Figure 2.8 shows the curves defined in this methodology to estimate the WQCV as a function of the tributary catchment’s total imperviousness and the drain time of the capture volume. The following steps have been used to estimate the WQCV for the detention basin being designed in this tutorial.

1. First, determine the developed site’s average Directly Connected Impervious Area (DCIA). DCIA is the impervious area that is directly connected to the stormwater drainage system; it does not include rooftops, patios, etc. that drain to lawns or other pervious areas, and is smaller than the gross or total impervious area that is typically estimated through aerial photography. These areas were previously estimated for each of the seven subcatchments defined for the post-development site condition in Tutorial 01 and are presented in Table 2.9.

Table 2.9 : Post-development Subcatchment Data

2. Next calculate the site’s average percent imperviousness by weighting the imperviousness of each subcatchment by its area and dividing by the total area (48.79 acres) of the study area. The average percent imperviousness of the site determined by this method is 37%.
3. The next step is to determine the WQCV in watershed inches. Assume that the tutorial site is located in Colorado’s high plains near the foothills and that the storage unit is to have a 40 hour drain time. From Figure 2.8 the corresponding WQCV in watershed inches is 0.1718 in. Thus, the total water quality control volume is 48.79 acres * (43560) * 0.1718 in/12 = 30,427.10 ft3.
4. As the design location is not in Colorado’s high plains near the foothills, the WQCV determined from Figure 2.8 needs to be adjusted. The curves shown in Figure 2.8 are defined to control the 80th percentile runoff event and are appropriate for use in Colorado’s high plains near the foothills. For other locations, the WQCV from Figure 2.8 can be adjusted to obtain an appropriate volume, WQCVo, using Equation 2.1. In this equation, d6 is the average precipitation depth of the runoff-producing storms. Storm events for Equation 2.1 were defined for a 6-hr inter-event period and have a minimum depth of 0.1 in. Figure 2.9 shows estimate of d6 for the contiguous United States (UDFCD, 2001).

The value e of d6 of the location used in this Tutorial is 0.48. The site adjusted value WQCV0 = 0.48 * (0.1718/0.43) = 0.1918 inches in watershed inches. Thus, the site adjusted total water quality control volume is 48.79 ac * (43560) * 0.1918 in/12 = 33,965.14 ft3.

Pond Geometry and Dimensions

The shape of the storage unit will depend on the regulations in the location where the structure will be constructed. Generally, it is recommended that the distance between the inlet and outlet of the facility be maximized; a length to width ratio of 2:1 to 3:1 is adequate. This tutorial will use a length to width ratio of 2:1, a WQCV depth (h1) of 2 ft, and a side slope of 4:1 (H: V). Figure 2.10 shows the geometry of the WQCV, and equations developed based on the length to width ratio (2:1) and the storage unit side slope (4:1) that describe the unit’s geometry. The steps used to determine the dimensions of the WQCV are described below.

In this Tutorial, a trial-and-error approach to find the required size of the pond was selected. After some iterations, it was found that W1 = 86 ft meets the required dimension.

Calculation steps are as followed:

L1        = 2W1,

L2        = 2W2

W2       = 2 * 4h1 + W1

= 2 * 4 * 2 + W1 (h1 = 2ft)

The volume of pond = 0.5 * depth * (Pond Bed Area + Pond Top Area)

or, 33,965.14 = 0.5 * h1 * (W1 * L1 + W2 * L2)

Here, W1 = 86, W2 = 102, L1 = 172, L2 = 188

The volume of the pond is calculated as follows:

0.5 * depth * (Pond Bed Area + Pond Top Area)

= 0.5 * 2 * (14792+19176) cft

= 33968 cft, which meets the criteria.

Then, the storage curve for the WQCV portion of the storage unit will be defined. At 0 depth, the area is L1 * W1 = 172*86 = 14,792 ft²; at the full depth of 2.00 ft, the area is L2*W2 = 188*102 = 19,176 ft². These pairs will be entered into the model in the following section together with new points in the surface area-depth curve representing the shape shown in Figure 2.1 to control larger volumes.

Adding a Storage Unit to the Model

The Tutorial_02.inp file will be used as a starting point to add a storage unit into the model that represents the detention pond. The following steps are taken to define the storage unit.

1. The two previously determined depth-area points are entered into the Curve Editor dialog for curve SC1. These two points are depth = 0, Area = 14,792 ft² and depth = 2 ft, Area =19,176 ft².
2. A new storage unit node, also named SU1, is placed onto the study area map as shown in Figure 2.4 and is left disconnected from the drainage system. The following properties are assigned to SU1; Storage Curve = Tabular; Curve Name = SC1; Invert Elevation = 363 ft (seven feet lower than the outfall node elevation defined in the previous Tutorials); Initial Depth = 0 and Maximum Depth = 2 ft (the maximum allowable depth defined to control the WQCV).
3. Junction J25, instead of outfall O1, is proposed as the outlet node for the control structures because GeoSWMM doesn’t allow more than one link to be connected to the outfall from upstream. Then, a roughness of 0.01 and 100 ft long conduit C24 is provided to connect junction J25 to outfall O1.
4. Figure 2.11 shows the finalized storage unit system SC1 upon completion of the design of the three orifices and the weir. The tabular storage curve SC1 for the WQCV and the storage unit’s Properties table.

Initially, the storage unit and its WQCV orifice Or1 are modeled independent of the watershed, to size the WQCV orifice to drain in 40 hours. Although the storage unit and the watershed are shown in the same input file in Figure 2.4, they will run as independent systems in the model because they are not hydraulically connected. To serve this purpose, a temporary outfall for the outlet node of conduit C23 is assigned to keep SU1 and Or1 hydraulically isolated. The location of the pond in Figure 2.4 will be its final location in the model. The pond could have been placed in the park area since there is significant open space for it, but for clarity, it was placed at the downstream end of the park.

Sizing the WQCV Orifice

The next step is to design the pond outlet so that the entire WQCV is released within 40 hours. The outlet will be an orifice connecting the storage unit to the downstream outfall O1. This orifice could be located at the bottom or side of the storage unit and be either circular or rectangular in shape. The following steps are used to size the orifice, so the WQCV drains in 40 hours.

1. A side orifice (Or1) is added between the storage unit (SU1) and the node (J25), leading to the outfall node. It is given a rectangular shape and assigned an inlet offset of zero so that its invert is the same as that of the storage unit’s. Its discharge coefficient is assumed to be the default value of 0.65.
2. The simulation time step options are set as follows: reporting, wet-weather and routing time steps to 15 seconds and the dry-weather time steps to 1 hr. The simulation duration must be longer than 40 hours so that the performance of the orifice can be properly evaluated; hence this Tutorial uses 84 hours.
3. A temporary outfall for the outlet node of conduit C23 is assigned to keep SU1 and Or1 hydraulically isolated.
4. The final dimensions of orifice Or1 are determined by running SWMM several times using Dynamic Wave flow routing while iteratively changing the orifice dimensions until a size is found that drains the WQCV in approximately 40 hours (39hr 53 min). For each run, the dimensions of the orifice are varied while keeping the initial water depth in the storage unit at the depth of the WQCV, i.e., 2 ft. One can assume that the basin is essentially empty once the water depth is 0.05 ft. Note that the runoff discharge generated by the rainfall falling on the subcatchments does not affect the storage unit during this part of the Tutorial because it is not connected to the drainage system.
5. Figure 2.12 shows the drainage time for three iterations as well as for the final design. Table 2.10 shows the dimensions assigned to the orifice by iteration. The final orifice design has a height of 0.29 ft and a width of 0.29 ft. This small size is typical of a WQCV orifice. That is why the orifice must be protected by a screen to prevent plugging during the storm and maintenance must be done regularly to ensure the screen remains free of debris.

Table 2.10: Design of the WQCV outlet (Or1)

Sizing the 2-yr Design Storm Orifice

The runoff volume generated by the 2-yr storm will be larger than the WQCV volume designed for in the previous section. The volume of the storage unit must now be enlarged and a new outlet must be defined. This new outlet, orifice Or2 which will be placed at a height of 2 ft above the basin floor as an inlet offset, will begin to discharge when the runoff volume from any storm just exceeds the WQCV. This outlet will control not only the peak runoff rate of the 2-yr storm but also the runoff rate from storms greater than the 2-yr storm partially. The required increase in storage volume will be achieved by extending the sides of the storage unit above the WQCV depth, while keeping a lateral slope of 4:1 (H:V) as shown in the basin schematic in Figure 2.1. The following steps outline how the storage unit is sized for the 2-yr design storm orifice.

1. The storage unit is first connected to the rest of the drainage system. This can be done by changing conduit C23’s outlet from the temporary outfall to SU1 and deleting the temporary outfall node. Conduit C23 is given an outlet offset of 1 ft so that for minor storms it has no backwater but still has its crown below the top of the storage pond.
2. Next the size of the pond is enlarged for flood control by expanding its height while keeping a constant slope (refer to Figure 2.1 for an illustration). This is done by entering a new set of surface area-depth pairs to the storage curve SC1. The values for this new set are depth = 0, Area = 14,792 ft², depth = 2 ft, Area =19,176 ft² and depth = 6 ft, Area = 29480 ft². The initial depth of the storage unit is set to zero and its maximum depth to 7 ft to account for the new volume.
3. The model is then run for the 2-yr storm using only the WQCV orifice, Or1 to determine the maximum depth in the storage unit, the peak discharge of the WQCV orifice (Or1), and the time it takes for the storage unit to empty. The results show a maximum storage unit depth of 3.92 ft, a maximum orifice discharge of 0.85 cfs, and an emptying time (for SU1 depth to reach 0.05 ft) of 58:31 (hr:min).
4. Based on the results in step 3, the peak outflow for the 2-yr storm can be increased because the pre-development 2-yr peak runoff (6.95 cfs from Table 2.2) is larger than the discharge through the WQCV orifice (0.85 cfs). Increasing this discharge is advantageous because it will reduce the final volume of the storage required and save in costs. To increase the 2-yr storm pond outflow, a second orifice (Or2) is added directly above the WQCV depth as illustrated in Figure 2.1. This orifice is assigned to a rectangular shape with an inlet offset of 2 ft and a discharge coefficient of 0.65. It should be drawn on the map with at least one intermediate vertex so that it can be distinguished from the existing orifice Or1.
5. An initial estimate of Or2’s area A is made using the orifice equation Q = CA(2gh)^1/2

with C = 0.65, Q = (6.95 – 0.85) cfs = 6.1 cfs and h = (3.92 – 2) ft = 1.92 ft. This produces an orifice area of 0.84 ft². Assume Or2 has an initial height of 0.84 ft and a width of 1 ft.

6. Running the model with these dimensions for Or2 produces a discharge of 4.14 cfs. This value is less than the target discharge (6.95 cfs). Therefore, iterations must again be used to size orifice Or2 (4.14 cfs) as was done for Or1 (0.85 cfs) until the combined peak discharge of the two orifices is equal to or a little less than the 2-yr pre-development peak discharge (6.95 cfs).
7. To simplify the iterations for sizing Or2, its height is fixed at 0.5 ft, and its width is varied in 0.05 ft increments until the combined discharge of both orifices is close to 6.95 cfs. A size of 0.5 height by 2.25 width ft produces a peak discharge of 6.49 cfs and a maximum storage unit depth of 3.20 ft. These dimensions will thus be used for the 2-yr orifice.

Sizing the 10-yr Design Storm Orifice

Up to this point, the storage unit has been represented as a single trapezoidal prism. This shape was determined for the WQCV back in the “Pond Geometry and Dimensions” section and increased in size (keeping the 4:1 side slope) to contain the 2-yr storm runoff. In this section, the storage unit shape is redefined by adding an additional trapezoidal prism over the minor storm prism to contain the 10-yr and 100-yr storm volumes (See Figure 2.1). The steps below show how this is done to size the 10-yr storm orifice.

1. The storage curve SC1 is modified by replacing the surface area-depth with the following three surface area-depth pairs: depth = 0, Area = 14,792 ft², depth = 2 ft, Area =19,176 ft², depth = 3.20 ft, Area= 22176 ft² and depth = 6 ft, Area = 29480 ft². Note that the new depth = 3.20 ft, Area = 22176 ft² point is the highest point of the original shape (the maximum height of the 2-yr storm). The initial depth of the storage unit is set to zero and its maximum depth to 7 ft to account for the new volume.
2. The model is run with the 10-yr storm and the existing orifices, Or1 and Or2 to determine if a 10-yr storm orifice is needed. The resulting maximum storage unit depth is 6.09 ft and the combined peak discharge from both existing outlets Or1 and Or2 is (1.07 + 11.5 = 12.57) cfs. The pre-development peak discharge for the 10-yr storm is 16.62 cfs which means that the storage unit volume can again be decreased, by adding another orifice.
3. A new 10-yr storm orifice (Or3) is added directly above the depth of the volume designed to control the 10-yr storm runoff (inlet offset = 3.20 ft). As with Or2, Or3 is drawn with intermediate vertices so that it can be seen easily on the system map. The orifice equation (2.2) is used to estimate its required area. For C = 0.65, Q = (16.62 – 12.57) cfs = 4.05 cfs and h = (6.09 – 3.20) ft = 2.89 ≈ 2.9 ft the resulting orifice area is 0.46 ft2. A height of 1 ft and a width of 0.46 ft are used as an initial estimate of the orifice’s size.
4. When the model is run with the 10-yr storm for this size of Or3, the combined discharge is (1.05+11.2+3.55) = 15.8 cfs. As this discharge is less than the pre-development discharge (16.62 cfs), the orifice’s width is increased to 0.58 ft and the model is re-run. The new combined discharge is 16.61 cfs and the maximum depth in the storage unit is 5.85 ft. This is nearly the target discharge to accept this orifice size (height = 0.58 ft, width = 1.0 ft).

Designing the 100-yr Weir

The model can now be run with the 100-yr storm using the combined WQCV, 2- yr and 10-yr orifices to determine if the 100-yr weir is needed. The peak discharge of these combined orifices for the 100-yr storm is (1.15+12.79+5.5) = 19.44 cfs, which is not enough to pass the 100-yr storm’s runoff (55.85 cfs), and the storage unit floods. A weir will be designed to control this extreme event so that the pre-development discharge is matched and the total water depth in the pond does not exceed the 7 ft depth that was given as the maximum depth for safety reasons. This is calculated as follows.

1. A new weir link Wr1, drawn with intermediate vertices, is added between the storage unit and the node (J25), leading to the final outfall. It is specified as a transverse weir whose inlet offset is 5.85 ft above the storage unit bottom (the maximum depth reached by the volume controlling the 10-yr storm runoff), and whose discharge coefficient is 3.3. The height of the weir opening is set at 1.15 ft which is the distance between the volume controlling the 10-yr storm runoff (5.85 ft) and the maximum depth of the storage unit (7 ft).

Using Q = (55.85 – 19.44) cfs = 36.95 cfs, C = 3.3 and h = (7.0 – 5.85) = 1.15 ft, a width of 8.95 ≈ 9 ft is calculated.

2. With weir dimensions of height = 1.15 ft, width = 9 ft, and invert offset = 5.85 ft the model is run for the 100-yr storm. The resulting peak total discharge from the storage unit is (1.13+12.42+5.44+24.4) = 43.39, which is below 54.85 cfs. The maximum water depth at SU1 is 6.73 ft.
3. The final step is to ensure that adequate freeboard is maintained in the storage unit. The current design provides 7.0 – 6.73 = 0.27 ft. The required amount will depend on the local design guidelines. For Tutorial, the UDFCD (2001) requires a freeboard of 1 ft above the maximum water surface elevation when the weir is conveying the maximum discharge.