San Antonio Water System Mitchell Lake Constructed Wetlands Below the Dam Preliminary Hydrologic Analysis

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2 San Antonio Water System enhancement. This recommendation was based on limited water quality data provided by SAWS and using a free-water surface constructed wetland with approximately 112 acres of wetted area. The analysis presented in this technical memorandum includes consideration of some of the hydrologic aspects of this approach and an evaluation of the feasibility of utilizing the storage capacity of the lake coupled with controlled flows to the wetland to manage stormwater discharges from the lake. An initial assessment of the lake identified the storage capacity available between ft msl (minimum operating elevation) and a spillway set at ft msl (lake full without spilling) and compared that volume to the runoff expected from various storm events, based on a statistical return frequency interval. This assessment is referred to as the single-event model. The number of days it would take to reduce the lake volume down to the minimum operating elevation was then calculated in days, based on various pumping rates. A second assessment utilized a water balance approach to estimate inflows to, outflows from, and end-of-month storage in the lake based on normalized monthly precipitation and lake evaporation data. A minimum pumping rate from the lake to the wetland was estimated based on the objective of preventing spillway discharges, utilizing the water balance approach. 3. SINGLE-EVENT MODEL A single-event model for the Mitchell Lake watershed was developed using the United States Army Corps of Engineers Hydrologic Engineering Center Hydrologic Modeling System (HEC-HMS). The purpose of this model was to evaluate whether stormwater runoff could be contained within Mitchell Lake without discharging over the spillway given an initial water surface elevation of ft msl and a spillway elevation of ft msl. Various return storm frequencies were considered. A second purpose of the single-event model was to calculate the time (in days) it would take to restore the original storage capacity given pumping rates of 7, 14, and 21 MGD. The pumped stormwater would be routed through the wetland downstream of the dam for quality improvement. Since no reliable stream gauge data were available for the Mitchell Lake watershed to calibrate the single-event model, initial efforts included developing HEC-HMS models for the nearby Polecat Creek watershed. Various hydrograph transform methods were used and the modeled peak discharges for existing conditions were compared to the Federal Emergency Management Agency (FEMA) effective discharges as published in the Bexar County Flood Insurance Study (2011). Of the Polecat Creek models evaluated, the Snyder Unit Hydrograph method provided the best comparison. This model included four subbasins with flow routing incorporated through the two downstream subbasins. This same approach was selected for the Mitchell Lake single-event model. Previous HEC-HMS models developed by Arcadis and Pape-Dawson Engineers (PDE) for Mitchell Lake were provided by SAWS and were also reviewed during setup of the single-event model. The watershed area for the Mitchell Lake model was delineated using 2011 Light Detection and Ranging (LiDAR) data obtained from SAWS. As with the Polecat Creek model, four subbasins were delineated and routing of flow was incorporated through the downstream basins. The polders and decant basins (located in the north portion of Mitchell Lake) were excluded from the contributing watershed, as it was assumed that the berms surrounding the polders/decant basins would be improved such that they would be hydrologically isolated from the lake and surrounding area. Alan Plummer Associates, Inc. Page 2 of 6 Firm Registration #13

3 San Antonio Water System Weighted runoff curve numbers (CNs) for Antecedent Condition II (ARC II) representing existing land use conditions were developed for each subbasin from the 2011 National Land Cover Database (NLCD 2011) using the Natural Resources Conservation Service (NRCS) TR-55 methodology. ARC II is generally defined as the average runoff condition for annual floods. Land use representing full-build conditions and corresponding weighted CNs were derived from SAWS GIS zoning data. Stage-storage-area relationships for Mitchell Lake were developed from Merrick 2 and from the 2011 LiDAR contour data. Using these data, the total capacity available for storing runoff between elevations ft msl and ft msl (spillway) was estimated to be 4,644 acre-feet. This is shown schematically in Figure 1. Figure 1. Schematic of Mitchell Lake Stage- Relationship Used in Models An artesian well 3 exists within the watershed and discharges approximately MGD into Ballestral Lake, which drains into Canvasback Lake. This well was modeled as an inflow and was routed through Canvasback Lake. Inflows from LCWRC into Mitchell Lake were set to zero. Daily evaporation loss from the lake surface was modeled using the long-term annual average 4 for the San Antonio area. Flows from the lake into the wetland were also set to zero, except during the pumpdown portion of the evaluation. Attachment 1 shows the Mitchell Lake watershed, subbasins, inflows/outflows, and flow routing geometry used in the single-event model. The HEC-HMS model was run using precipitation inputs representing 25-year/24-hour, 50-year/24- hour, and 100-year/24-hour return events. The peak flow rates and peak storage volumes for each event were calculated using CNs for existing and full build-out land use conditions. The model utilized a 5-minute time-step for calculations and was allowed to run for 72-hours so that all runoff in the trailing leg of the hydrograph would be included in the peak storage total. The single-event model indicated that the peak storage required for a 100-year/24-hour event under both existing and full build-out conditions could be contained within the lake, given the criteria described above. Tables 1 2 Conceptual Design Report for Mitchell Lake Dam, Merrick & Company. Table 4.6. December Communication from SAWS indicated the well was completed in the Trinity Aquifer and produces an estimated 300 gallons per minute (432,000 gallons per day) under artesian flow conditions. 4 Mean Crop Consumptive Use and Free Water Evaporation for Texas, Borrelli, J. et al Table 26. Alan Plummer Associates, Inc. Page 3 of 6 Firm Registration #13

4 San Antonio Water System and 2 provide the peak storage volume requirements and the percent of available storage capacity of the lake that is used under existing and full build-out land use conditions, respectively. APAI used the HEC-HMS model developed by PDE for comparison purposes 5 and documented the runoff volumes for the same storm events. Differences 6 in the PDE and APAI models resulted in slight differences in the runoff volumes calculated. The results of the PDE model are also shown in Tables 1 and 2. Table 1. Mitchell Lake Single-Event Model Existing Land Use Conditions Model 25-year 50-year 100-year (ac-ft) (ac-ft) (ac-ft) APAI (2017) 2,630 57% 3,330 72% 3,790 82% PDE (2008) 2,830 61% 3,310 71% 3,890 84% Table 2. Mitchell Lake Single-Event Model Full Build-Out Land Use Conditions Model 25-year 50-year 100-year (acre-feet) (acre-feett) (acre-feet) APAI (2017) 2,930 63% 3,640 78% 4,110 89% PDE (2008) 3,490 75% 3,980 86% 4,590 99% With the peak storage volumes identified for the scenarios described above, the number of days it would take to restore the lake to the original minimum operating level was estimated using pump rates of 7, 14, and 21 MGD. Average evaporation losses and inflow from the artesian well were considered during the pump-down period, but no additional runoff contribution was included. The results for the APAI single-event drawdown model are shown in Table 3, with units provided in million gallons (MG). 5 The PDE model was more comparable to the APAI model than the Arcadis model, which included 33 subbasins (PDE and APAI both included 4 subbasins). 6 The PDE model was initially developed in 2008 and utilized data available at that time. Some updates were made to the PDE model in The APAI model applies the current (NLCD 2011) existing condition land use values (vs. 1992). APAI also excluded the polders/decant basins and added inflows from the artesian well. Alan Plummer Associates, Inc. Page 4 of 6 Firm Registration #13

5 San Antonio Water System Table 3. Estimated Days to Restore Mitchell Lake to Minimum Operating Level Return Event (24-hour) Volume above (MG) Remaining Volume in Lake (MG) Estimated Days to Restore to Normal Operating Level (517.5 ft msl) Existing Land Use Conditions 7 MGD pump rate 14 MGD pump rate 21 MGD pump rate 25-year year 1, year 1, Full Build-Out Land Use Conditions 25-year year 1, year 1, MONTHLY WATER BALANCE AVERAGE YEAR CONDITIONS To assess general trends using long-term average conditions, a spreadsheet-based monthly water balance calculation was developed to evaluate the relationship between monthly inflows, outflows, and reservoir storage. Inflows included runoff from the watershed, direct rainfall on the lake surfaces, artesian well inflow, and inflows from LCWRC. Outflows included lake evaporation, pumping to the wetland, and spillage from the lake. Seepage losses were excluded from the calculation. Monthly precipitation and lake evaporation data were obtained for Quad 809 (San Antonio) from the Texas Water Development Board website for the period of 1954 to 2015 and the monthly averages were used in the spreadsheet model. from the watershed was calculated using the SCS Curve Number method as described in the NRCS National Engineering Handbook, Part 630 Hydrology, Chapter 10. Curve numbers were taken from the single-event HEC-HMS model and adjusted to account for a monthly time-step. Rainfall contribution to and evaporation losses from Canvasback and Mitchell Lakes were calculated using the combined areas of the lakes. The area of Canvasback Lake (66 acres) was considered to be constant due to continuous inflow from the artesian well (through Ballestral Lake). The surface area for Mitchell Lake was also held as a constant and was based on using the lake surface area corresponding to the average of the end-of-month storage volumes for all twelve months. Inflows from watershed runoff, direct precipitation, and the artesian well were summed and lake evaporation was subtracted to arrive at a potential change-in-storage for a given month. If the potential change-in-storage was positive (inflows exceeded outflows), then water would be pumped from Mitchell Lake to the wetland. An input cell defined the dewater rate for the pump. The volume of water pumped to the wetland was constrained by not allowing the volume to exceed the monthly pumping capacity entered into the spreadsheet. If the potential change-in-storage exceeded the lake dewatering pumping capacity, the excess volume would be added to the previous month s storage to arrive at the current month s ending storage. If the ending storage exceeded the available lake storage capacity of 4,644 acre-feet, spillage would occur. If the potential change-in-storage for a Alan Plummer Associates, Inc. Page 5 of 6 Firm Registration #13

6 San Antonio Water System month was negative, then water would be added from LCWRC to Mitchell Lake to refill the lake to the minimum operating level (517.5 ft msl). Calculated end-of-month storage values equaling zero indicate that the lake could be maintained at the minimum operating level of ft msl for the month. End-of-month storage values greater than zero indicate that water would be stored above the minimum operating elevation. Spreadsheet calculations began with an end-of-month storage manually set to zero for the month of August. August was selected as a starting point for the annual water balance because it and the preceding month (July) both had negative values for the calculated monthly potential change-instorage. Calculations proceeded from August through December, then from January to July, with January using December s end-of-month values. A check cell was used to ensure that the July to August calculation would still result in a zero end-of-month storage value for August. If the check cell resulted in a value greater than zero, then the spreadsheet would not result in a closed calculation; or stated differently, the spreadsheet would indicate that inflows could not be managed through normal evaporation and pumping to the wetland and that water stored within the lake would continuously increase to a point at which spillage would occur. In this case, a higher pumping rate to the wetland would be needed to offset calculated inflows. The spreadsheet indicated seasonal trends with negative potential changes in storage in July and August and positive the remainder of the year. The highest potential change in storage occurred in May which corresponds to the month with the highest rainfall. Pumping rates of 4 and 7 MGD were tested in the spreadsheet and both resulted in a sustainable solution with no spillage. The minimum pumping rate with no spillage was 1.3 MGD. Spreadsheet results are included as Attachments 2, 3 and CONCLUSIONS AND RECOMMENDATIONS Based on the stage-storage-area relationships from Merrick and 2011 LiDAR data, Mitchell Lake appears to have sufficient storage capacity to contain runoff expected from a 100-year, 24-hour storm for existing and full build-out land use conditions when the runoff volume is captured in the lake and the lake begins at the minimum operating elevation of ft msl. A monthly water balance calculation using the SCS Curve Number Method suggests that stormwater runoff can be appropriately managed within Mitchell Lake between a proposed minimum operating level of ft msl and spillway elevation of ft msl, given average precipitation and lake evaporation conditions and pumped flows to the wetland of at least 1.3 MGD. It is recommended that a more extensive water balance simulation be developed utilizing available daily precipitation and evaporation data over a 50-year period to identify the probability of discharge during periods of chronic rainfall and extreme events. Alan Plummer Associates, Inc. Page 6 of 6 Firm Registration #13

7 ATTACHMENTS San Antonio Water System March 30, Mitchell Lake HEC-HMS Basin Model 2 Monthly Water Balance with Wetland Pump Rate = 1.3 MGD 3 - Monthly Water Balance with Wetland Pump Rate = 4.0 MGD 4 - Monthly Water Balance with Wetland Pump Rate = 7.0 MGD

8 ATTACHMENT 1 Mitchell Lake HEC-HMS Basin Model

9 MONTHLY WATER BALANCE - SCS CURVE NUMBER METHOD Date: 3/6/2017 Location: Mitchell Lake (Bexar County) Daily inflow (a) : MGD 39.8 ac-ft/30-day mo 41.1 ac-ft/31-day mo Pump to Wetland (b) : 1.30 MGD ac-ft/30-day mo ac-ft/31-day mo area (c) : 5,498 acres Lake area (d) : 511 acres Lake storage capacity (e) 4,644 acre-feet CN (f,g) 1-day day 63 Ia Coefficient (h) : 0.2 Month Precip Lake Evap Direct Rainfall on Lake Artesian Well Inflow Lake Evaporation Potential Change in Water to Wetland Water from LCWRC Spillage End of Month above in/mo in/mo in/mo ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total , , , , , Input Notes: (a) Daily inflow is from artesian well that enters Ballestral Lake and flows into Canvasback Lake. (b) This is the minimum pump rate required to establish system equilibrium and prevent spillway discharge given normalized monthly data. (c ) Contributing watershed area excluding Mitchell and Canvasback Lakes. (d) Combined water surface areas for Mitchell and Canvasback Lakes. Canvasback Lake fixed at 66 acres (primary spillway elev). Mitchell Lake area based on the surface area corresponding with the average end of month storage volume for all 12 months. (e) capacity available in Mitchell Lake between normal pool (517.5) and spillway (525.8). (f) Composite curve number for watershed taken from NLCD land use cover database, ARC II, average watershed runoff conditions. CN represents existing conditions. (g) curve number adjustment ref: USDA NRCS TR-60 Earth Dams and Reservoirs. Table 2-3. (h) Coefficient used to calculate initial abstraction, Ia, from the maximum potential retention, S. Calculation Notes: (1) Month of the year (2) Average monthly precipitation. Ref. TWDB Precipitation Quad 809 for 1954 to (3) Average monthly lake evaporation. Ref. TWDB Lake Evaporation Quad 809 for 1954 to (4) Direct runoff from watershed in inches, where Q= (P-Ia) 2 / (P-Ia)+S. Ref: USDA NRCS National Engineering Handbook, Part 630 Hydrology, Chapter 10. (5) volume from watershed converted to acre-feet prer month. (6) Direct rainfall on water surface, converted to acre-feet per month. (7) Artesian well inflow converted to ac-ft/mo, based on number of days per month. (8) Evaporation losses from Canvasback and Mitchell Lakes. (9) Change in storage prior to any pumping to/from the lake or spillage. Col. 5 + Col. 6 + Col. 7 - Col. 8. Seepage losses neglected. (10) Volume of water to be pumped from Mitchell Lake to wetland to restore available storage capacity above Cannot exceed monthly pumping capacity. (11) Volume of water to be pumped from LCWRC to Mitchell Lake for evaporation loss makeup. Occurs when no water is pumped to wetland. (12) Spillage occurs when inflows - outflows + previous months storage exceed the available storage capacity. (13) Volume of water stored above End of month storage from previous month + current month change in storage + water from LCWRC - water to wetland. Calculation began in August as the baseline start month. Water Balance SCS Monthly V4

10 MONTHLY WATER BALANCE - SCS CURVE NUMBER METHOD Date: 3/6/2017 Location: Mitchell Lake (Bexar County) Daily inflow (a) : MGD 39.8 ac-ft/30-day mo 41.1 ac-ft/31-day mo Pump to Wetland (b) : 4.00 MGD ac-ft/30-day mo ac-ft/31-day mo area (c) : 5,503 acres Lake area (d) : 506 acres Lake storage capacity (e) 4,644 acre-feet CN (f,g) 1-day day 63 Ia Coefficient (h) : 0.2 Month Precip Lake Evap Direct Rainfall on Lake Artesian Well Inflow Lake Evaporation Potential Change in Water to Wetland Water from LCWRC Spillage End of Month above in/mo in/mo in/mo ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total , , , , , Input Notes: (a) Daily inflow is from artesian well that enters Ballestral Lake and flows into Canvasback Lake. (b) This is the minimum pump rate required to establish system equilibrium and prevent spillway discharge given normalized monthly data. (c ) Contributing watershed area excluding Mitchell and Canvasback Lakes. (d) Combined water surface areas for Mitchell and Canvasback Lakes. Canvasback Lake fixed at 66 acres (primary spillway elev). Mitchell Lake area based on the surface area corresponding with the average end of month storage volume for all 12 months. (e) capacity available in Mitchell Lake between minimum operating elevation (517.5) and spillway (525.8). (f) Composite curve number for watershed taken from NLCD land use cover database, ARC II, average watershed runoff conditions. CN represents existing conditions. (g) curve number adjustment ref: USDA NRCS TR-60 Earth Dams and Reservoirs. Table 2-3. (h) Coefficient used to calculate initial abstraction, Ia, from the maximum potential retention, S. Calculation Notes: (1) Month of the year (2) Average monthly precipitation. Ref. TWDB Precipitation Quad 809 for 1954 to (3) Average monthly lake evaporation. Ref. TWDB Lake Evaporation Quad 809 for 1954 to (4) Direct runoff from watershed in inches, where Q= (P-Ia) 2 / (P-Ia)+S. Ref: USDA NRCS National Engineering Handbook, Part 630 Hydrology, Chapter 10. (5) volume from watershed converted to acre-feet prer month. (6) Direct rainfall on water surface, converted to acre-feet per month. (7) Artesian well inflow converted to ac-ft/mo, based on number of days per month. (8) Evaporation losses from Canvasback and Mitchell Lakes. (9) Change in storage prior to any pumping to/from the lake or spillage. Col. 5 + Col. 6 + Col. 7 - Col. 8. Seepage losses neglected. (10) Volume of water to be pumped from Mitchell Lake to wetland to restore available storage capacity above Cannot exceed monthly pumping capacity. (11) Volume of water to be pumped from LCWRC to Mitchell Lake for evaporation loss makeup. Occurs when no water is pumped to wetland. (12) Spillage occurs when inflows - outflows + previous months storage exceed the available storage capacity. (13) Volume of water stored above End of month storage from previous month + current month change in storage + water from LCWRC - water to wetland. Calculation began in August as the baseline start month. Water Balance SCS Monthly V4

11 MONTHLY WATER BALANCE - SCS CURVE NUMBER METHOD Date: 3/6/2017 Location: Mitchell Lake (Bexar County) Daily inflow (a) : MGD 39.8 ac-ft/30-day mo 41.1 ac-ft/31-day mo Pump to Wetland (b) : 7.00 MGD ac-ft/30-day mo ac-ft/31-day mo area (c) : 5,503 acres Lake area (d) : 506 acres Lake storage capacity (e) 4,644 acre-feet CN (f,g) 1-day day 63 Ia Coefficient (h) : 0.2 Month Precip Lake Evap Direct Rainfall on Lake Artesian Well Inflow Lake Evaporation Potential Change in Water to Wetland Water from LCWRC Spillage End of Month above in/mo in/mo in/mo ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft ac-ft (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Total , , , , , Input Notes: (a) Daily inflow is from artesian well that enters Ballestral Lake and flows into Canvasback Lake. (b) This is the minimum pump rate required to establish system equilibrium and prevent spillway discharge given normalized monthly data. (c ) Contributing watershed area excluding Mitchell and Canvasback Lakes. (d) Combined water surface areas for Mitchell and Canvasback Lakes. Canvasback Lake fixed at 66 acres (primary spillway elev). Mitchell Lake area based on the surface area corresponding with the average end of month storage volume for all 12 months. (e) capacity available in Mitchell Lake between minimum operating elevation (517.5) and spillway (525.8). (f) Composite curve number for watershed taken from NLCD land use cover database, ARC II, average watershed runoff conditions. CN represents existing conditions. (g) curve number adjustment ref: USDA NRCS TR-60 Earth Dams and Reservoirs. Table 2-3. (h) Coefficient used to calculate initial abstraction, Ia, from the maximum potential retention, S. Calculation Notes: (1) Month of the year (2) Average monthly precipitation. Ref. TWDB Precipitation Quad 809 for 1954 to (3) Average monthly lake evaporation. Ref. TWDB Lake Evaporation Quad 809 for 1954 to (4) Direct runoff from watershed in inches, where Q= (P-Ia) 2 / (P-Ia)+S. Ref: USDA NRCS National Engineering Handbook, Part 630 Hydrology, Chapter 10. (5) volume from watershed converted to acre-feet prer month. (6) Direct rainfall on water surface, converted to acre-feet per month. (7) Artesian well inflow converted to ac-ft/mo, based on number of days per month. (8) Evaporation losses from Canvasback and Mitchell Lakes. (9) Change in storage prior to any pumping to/from the lake or spillage. Col. 5 + Col. 6 + Col. 7 - Col. 8. Seepage losses neglected. (10) Volume of water to be pumped from Mitchell Lake to wetland to restore available storage capacity above Cannot exceed monthly pumping capacity. (11) Volume of water to be pumped from LCWRC to Mitchell Lake for evaporation loss makeup. Occurs when no water is pumped to wetland. (12) Spillage occurs when inflows - outflows + previous months storage exceed the available storage capacity. (13) Volume of water stored above End of month storage from previous month + current month change in storage + water from LCWRC - water to wetland. Calculation began in August as the baseline start month. Water Balance SCS Monthly V4