Continuous Simulation Example Problem

Transcription

1 Continuous Simulation Example Problem The following examples provided an introduction to the continuous simulation hydrograph in the PONDS 3.2 Refined Method software, with an emphasis on wetland hydroperiod modeling. Site Description: An existing wetland area in Central Florida (Orlando) Wetland area = 38 acres Contributing runoff basin of 74 acres (upland) Total basin area = 112 acres (wetland plus upland) Postdevelopment: This example assumes that postdevelopment changes in the basin uplands will result in the removal of upland areas (through regrading, etc.) as well as the addition of directly connected impervious area. Page 1

2 Part 1 - Modeling Predevelopment Hydroperiod The predevelopment hydroperiod will be used as the basis by which to assess any postdevelopment changes. Runoff Basin Parameters Curve Number, CN = 83 DCIA = 0 acres Aquifer Parameters Base of aquifer = 68 ft Initial water table elevation = 79 ft Horizontal Saturated Hydraulic Conductivity = 5 ft/day Fillable Porosity = 25% No vertical infiltration. Leakage from surficial to Upper Floridan Aquifer = 2 in/yr Geometry Data Discharge Equivalent Pond Length = 2500 ft Equivalent Pond Width = 400 ft Wetland Discharge Elevation = 80 ft (Assume 50 ft wide broad crested weir) Page 2

3 Stage vs Area Data For continuous simulation modeling, it is often desirable to use an alternative stage vs area relationship which describes the equivalent area from the ground surface down to the base of aquifer (or to some depth which will not be subject to drying). In this relationship, both the open pond volume and the available soil void volume are accounted for. Consider the following illustration: Ac Sc Aopen Sb Soil porisity, η where A c = pond area at control stage A open = pond area at arbitrary stage S c = control stage S b = pond bottom stage η = porosity The equivalent area is calculated as: Stage Area > S c A open S c A c between S b and S c ( A c - A open ) x η + A open < S b A c x η Page 3

4 Therefore, given the S b = 79 ft, A b = 500,000 ft S c = 80 ft, A b = 1,000,000 ft S = 81 ft, A open = 1,500,000 ft 2 The stage vs area is calculated as: Stage (ft) Area (sq-ft) 250, , ,000 1,000,000 1,500,000 Rainfall Data In this example we are using 4 consecutive years of average rainfall for Orlando (51.6 in/yr). This rainfall data was prepared in a spreadsheet and pasted into PONDS. The rainfall data is also saved as: Four Consecutive Average Rainfall Years (Orlando).csr Page 4

5 Results Use the PONDS 3.2 Refined Method software with the continuous simulation hydrograph to calculate the predicted hydroperiod. Plot the resulting data in Excel spreadsheet. Exhibit 1 below shows the predicted predevelopment hydroperiod for the entire duration of the simulation. Note that the model appears to be stable during the latter years of the simulation, i.e., it does not show a pronounced year-on-year change in the predicted high and low water levels, which might indicate that the model was not well calibrated. Exhibit 2 shows the predicted hydroperiod for the last year of the simulation. The normal high water level for this wetland is controlled by the discharge elevation (+80 ft). As seen in Exhibit 2, the wetland becomes inundated during the wet season, by about the last half of June, and remains periodically inundated through October. There is a general drying trend between about the end of October through the last half of March, with the predicted low water elevation of about 77.5 ft in March Predicted Water Level (ft) Exhibit 1. Predicted predevelopment hydroperiod Page 5

6 Predicted Water Level (ft) Jan-01 Feb-01 Mar-01 Apr-01 May-01 Jun-01 Jul-01 Aug-01 Sep-01 Oct-01 Nov-01 Dec-01 Jan-02 Exhibit 2. Predicted average year hydroperiod (year 4 of 4 of simulation) Page 6

7 Part 2 - Postdevelopment Area Reduction Assume that the uplands will be re-graded, and that as a result 50% of the existing uplands will be removed from the runoff basin for this wetland. Evaluate the postdevelopment hydroperiod for this wetland assuming 50% of the uplands are removed. Postdevelopment Upland Area and Total Basin Area Original Upland Area (acres) 74 Percentage of Upland Area Remaining (%) 50 Area of Remaining Uplands (acres) 37.0 Wetland Area (acres) 38.0 Total Basin Area (acres) 75.0 Assume no change in DCIA, i.e., DCIA = 0 acres. Page 7

8 Exhibit 3 below shows a plot showing the predevelopment hydrograph and the postdevelopment hydrograph (50% of uplands removed from basin). For this wetland, the post-development hydroperiod is nearly identical between July and February. The differences in the hydroperiod become more apparent towards the end of the dry season and onset of the wet season Predicted Water Level (ft) Jan-01 Feb-01 Mar-01 Apr-01 May-01 Jun-01 Jul-01 Aug-01 Sep-01 Oct-01 Nov-01 Dec-01 Jan-02 Predevelopment 50% of uplands preserved Exhibit 3. Predicted Postdevelopment Hydroperiod Page 8

9 Part 3 - Postdevelopment With Increase In DCIA Starting with the postdevelopment conditions from example Part 2 above, add a small amount of DCIA to try to compensate for the loss in upland area. Exhibit 4 below shows the results of adding 3 acres if DCIA to the postdevelopment hydrograph. The result is a better match in the hydroperiod between April and June, at the expense of slightly wetter conditions during the dry season from December through February Predicted Water Level (ft) Jan-01 Feb-01 Mar-01 Apr-01 May-01 Jun-01 Jul-01 Aug-01 Sep-01 Oct-01 Nov-01 Dec-01 Jan-02 Predevelopment 50% of uplands, 3 acres DCIA Exhibit 4. Predicted postdevelopment hydroperiod, 50% of uplands preserved and 3 acres of DCIA added This particular wetland appears to be more sensitive to the addition of DCIA than it does to the removal of upland basin area. In these examples, 37 acres of upland were removed, which was roughly compensated for by an increase of 3 acres DCIA. Page 9

10 Hints: For most continuous simulation models, the boundary condition at the perimeter of the model should be set to no-flow. The no-flow boundary condition allows the water level to rise and fall at the model perimeter, which better represents seasonal water table fluctuations. It is a good idea to turn on the groundwater mound output and examine the groundwater mound to make sure it is not diverging. It is possible to have a model that looks reasonable with respect to the pond water levels, while the groundwater mound outside the pond is diverging (either too high or too low). This can a sign of a badly calibrated model, and might indicate that the leakages are not well calibrated. For long duration models, the leakage rates can have a significant impact on the solution. (As opposed to short duration models, such as storm event and recovery models where leakage is typically ignored.) Sometimes, regional leakage values are known in a general sense. However, in some hydrogeologic settings, the leakance can vary significantly. For example, lakes and depressions formed by karst phenomena often have significantly higher leakage within the lake or depression. Whereas, wetlands tend to form in areas where leakage is low, either regionally or locally. The leakage rate is often one of the main parameters which must be iterated on in order to calibrate a continuous simulation model. When modeling for average rainfall conditions, it is often better to run a series of consecutive average rainfall years than to run a single average rainfall year. This allows the model to approach an equilibrium in regards to the groundwater profile conditions, etc. (At the beginning of the simulation, the groundwater mound is assumed to be horizontal, but vary as the simulation progresses.) Page 10