GROUNDWATER RECHARGE. Clear Creek Solutions, Inc., 2010

Transcription

1 GROUNDWATER RECHARGE Clear Creek Solutions, Inc., 2010 Groundwater provides the base flow to streams and wetlands during dry periods. Groundwater also recharges aquifers and provides water for residential, commercial, and industrial uses. When land use development occurs and impervious surfaces are added the amount of groundwater recharge decreases. The amount of the decrease depends on many factors: soil, vegetation, climate, and percent of impervious area. Some jurisdictions require that land use development maintain the predevelopment groundwater recharge. WWHM4 is designed to help the user quantify the amount of predevelopment groundwater recharge occurring on the project site and how much recharge will change with development. WWHM4 provides the user with recharge duration information along with monthly and annual recharge total volume. This information helps the user determine the appropriate mitigation facilities to maintain the predevelopment groundwater recharge while changing land use conditions. In this example we will model a land use basin draining to a stormwater pond. We will compute the groundwater recharge for both the predevelopment and developed land use conditions. We will model a 100-acre commercial development in Thurston County, Washington, near the Olympia Airport. The first thing that we will do is to locate our project on the project map. 1

2 The Olympia Airport is located in Thurston County, Washington, just south of the city of Olympia. We click on the map to select the project location. Based on our project location WWHM4 selects the appropriate precipitation record and precipitation multiplication factor. We then have the option to fill in the Site Information boxes. 2

3 For the Predevelopment scenario we select two standard land use basins. We have to decide the appropriate predevelopment land use for the project site. The project site is on a mixture of soils. Of the total 100-acre project site 60% of the soils are high infiltrating outwash (A/B) soil and 40% are low infiltrating till (C) soil. The predevelopment vegetation for the entire project site is forest and the land slope is flat (0-5%). Basin A represents the outwash soil area; Basin C represents the till soil area. NOTE: We could place both the outwash and till soil areas in a single land use basin, but for this example we are dividing them into two separate basins solely for the purpose of whether or not we connect the basin s groundwater to the adjacent stream (discussion below). Note that the Predeveloped land use is defined by the local permitting agency. If unsure, contact the local permitting agency and find out what they want you to use. The project site is 100 acres. 3

4 Here is the Basin C information. 4

5 The project site has a small stream channel exiting the property. This stream channel is a loosing channel (in other words, water infiltrates into the ground through the bottom of the channel). We input the channel dimensions and infiltration data. 5

6 We connect Basin A to the channel element. All land use basins produce surface runoff, interflow (shallow, subsurface runoff), and groundwater. We select which of these three types of runoff go to the stream channel. Surface runoff and interflow always goes to the stream channel. However, with groundwater things can get a bit more complicated. Groundwater can either go to the stream channel or it can go directly to the aquifer. If the groundwater box is checked then the groundwater goes to the conveyance element (in this case, the stream channel); otherwise it goes directly to the aquifer. 6

7 We leave the groundwater box unchecked and the groundwater from Basin A goes directly to recharge the aquifer. We can look at the Flows To boxes on the screen to confirm that the Basin A surface runoff and interflow are connected to Channel 1 and the groundwater is not connected. 7

8 For Basin C we will connect all three runoff components to Channel 1. To do this we make sure that all three boxes are checked, including the groundwater box. 8

9 We see that for Basin C that all three flow components (surface runoff, interflow, and groundwater) are connected to Channel 1. This means that the Basin C groundwater will be routed to the stream channel rather than directly to the aquifer. 9

10 We connect the outlet from Channel 1 to Point of Compliance 1 (POC 1). Note that Channel 1 has two outlets. The standard convention in WWHM4 is that outlet 1 is always the surface outlet (in other words, discharges on the surface even if it is via a pipe or other enclosed conveyance outlet configuration). Outlet 2 is subsurface infiltration and is usually NOT included in the POC calculations. For this project we will check only Outlet 1 and then click on the Connect button. 10

11 At this point we can run the predevelopment scenario. However, no recharge calculations will be made unless we first designate for which elements we want to calculate recharge. 11

12 To turn on the recharge calculations we go to View, Options. 12

13 We select the Recharge tab. We now see all of the elements contributing to POC 1. We can select individual elements for which we want recharge calculations made, or we can click on the Select All button to select all of the elements. 13

14 To turn on the recharge calculations we check the Compute Recharge box. We then select all of the contributing elements for the recharge calculations by clicking on the Select All button. 14

15 The contributing elements for which recharge will be computed are designated with a small R. We can now run the predevelopment scenario. 15

16 We set up the developed mitigated scenario by first adding Basin A s developed land use. 16

17 We do the same for Basin C. 17

18 Channel 1 (the stream channel) does not change from the predeveloped to the mitigated scenario. 18

19 We make the identical connections from Basin A and Basin C to Channel 1 that we made for the predeveloped scenario. To save us from having to flip back in the documentation to review those connections, they are: Basin A: Surface runoff connected to Channel 1. Interflow connected to Channel 1. Groundwater NOT connected to Channel 1. Basin C: Surface runoff connected to Channel 1. Interflow connected to Channel 1. Groundwater connected to Channel 1. 19

20 To mitigate the stormwater impact of this land use change we need to add a stormwater control facility. For this example we will add a stormwater trapezoidal pond downstream of Channel 1. We only connect Outlet 1 from the channel element to the pond element. Outlet 1 is the stream channel s surface discharge; Outlet 2 is the channel s subsurface infiltration. The subsurface infiltration does not go to pond; it goes to the aquifer as groundwater recharge. 20

21 We connect the pond to POC 1. Now we normally would size the pond using AutoPond (see Stormwater Pond Sizing documentation for details). For the purposes of this example I am going to skip the pond sizing exercise and march straight forward to computing recharge. 21

22 We have our pond dimensions and outlet structure configuration. For the moment we are going to assume that there is no infiltration from the pond. All of the flow out of the pond goes out through the riser (the riser discharge includes the bottom orifice and the riser notch discharge). 22

23 As with the predeveloped contributing elements, we need to go to View, Options, Recharge to select which mitigated scenario elements we want recharge calculated for. The mitigated elements are identified by the -> notation in front of each mitigated scenario element in the Contributing Elements and Point of Compliance columns. 23

24 We click the Select All button to select recharge calculations for all of the mitigated scenario elements (regardless of whether or not they include infiltration). As with the predevelopment scenario elements, all of the groundwater recharge that goes directly to the aquifer either as groundwater from a land use basin or as infiltration from a conveyance element is included in the recharge total. Now we can run the mitigated scenario and compare predevelopment and mitigated recharge. 24

25 We have three recharge analysis options: 1. Recharge Duration 2. Recharge Predeveloped 3. Recharge Mitigated 25

26 Recharge Duration compares the groundwater recharge rates for both the predevelopment recharge and the mitigated development recharge for the elements contributing to POC 1. For the recharge duration comparison the goal is to maintain the predevelopment recharge rates. In other words, we want the mitigated recharge rates to be at least equal to the predevelopment recharge rates. The mitigated recharge rates can be larger than the predevelopment recharge rates, but they can t be smaller. This means the Percentage values in column 4 of the duration table all should be 100 percent or greater. For this example almost all of the Percentage values are less than 100 percent. For this reason the Pass/Fail column (column 5) is full of Fail. 26

27 Recharge Predeveloped produces monthly and annual total predevelopment recharge volumes (in units of acre-feet) and compares the groundwater recharge volumes for both the predevelopment recharge and the mitigated development recharge. For this recharge volume comparison the goal is to maintain the predevelopment recharge volumes. In other words, we want the mitigated recharge volumes to be at least equal to the predevelopment recharge volumes. The mitigated recharge volumes can be larger than the predevelopment recharge volumes, but they can t be smaller. For this reason the Pass/Fail column (column 4) in the table on the right is full of Pass. The Copy to clipboard button is on the left. 27

28 The monthly predevelopment recharge values can be copied to the Windows clipboard and then pasted into an Excel spreadsheet for additional analysis outside of WWHM4. The same can be done for the mitigated recharge values. 28

29 If we decide to allow infiltrate through the bottom of the stormwater pond then we can change the pond s infiltration from NO to YES and input a measured infiltration rate (10 inches per hour) and a reduction factor (1/safety factor, where safety factor equals 4) of After we run the mitigated scenario we see that the percent of flow entering the pond that is then infiltrated through the pond bottom is percent. 29

30 We return to the Recharge Duration and see that the mitigated recharge has greatly increased with percentage values greater than 100 percent. The recharge duration values pass at all lower recharge flow levels. They only fail at the highest flow levels. Whether or not this failure at the highest recharge flow levels is allowed is dependent on the permitting agency. This completes the recharge example. 30

31 SUMMARY: 1. Locate project site on map. 2. Place Predeveloped basin element on schematic grid. 3. Input Predeveloped land use information. 4. Select the Predeveloped point of compliance (POC 1). 5. Turn on recharge calculations and recharge for individual predevelopment elements by going to View, Options, Recharge. 6. Run the predeveloped scenario. 7. Click on Mitigated scenario. 8. Place Mitigated (developed) basin element on schematic grid. 9. Input Mitigated land use information. 10. Add a pond to Mitigated schematic grid. 11. Select the Mitigated point of compliance (POC 1). 12. AutoPond or manually size the pond. 13. Turn on recharge calculations and recharge for individual mitigated elements by going to View, Options, Recharge. 14. Look at the Analysis screen Recharge Duration table results. 15. Look at the Predeveloped Recharge monthly and annual volumes. 16. Look at the Mitigated Recharge monthly and annual volumes. 17. Finished. 31