Istokpoga Marsh Watershed Improvement District (IMWID) Minor Impoundment Project. May 23, 2012

Transcription

1 STORMWATER INFORMATION AND CALCULATIONS FOR ENVIRONMENTAL RESOURCE PERMIT Istokpoga Marsh Watershed Improvement District (IMWID) Minor Impoundment Project Prepared by Chastain-Skillman, Inc. Prepared for May 23, 2012 Certificate of Authorization No. CA US Highway 27 South (33870) Post Office Box 7036 Sebring, Florida [863] Highlands County Board of County Commissioners 4344 George Boulevard Sebring, Florida Submitted to South Florida Water Management District 205 North Parrott Avenue, Suite 201 Okeechobee, Florida ENGINEER: Wendy G. Contner, P.E. Fla. Reg. No CSI File No Copyright 2012

2 Minor Impoundment Project Stormwater Calculations Table of Contents Page No. STORMWATER CALCULATIONS REPORT List of Attachments Attachment No. ICPR MODEL INPUT/OUTPUT DATA...1 CSI File No i

3 Minor Impoundment Project Stormwater Calculations 1.0 PROJECT SUMMARY The attached ICPR model is provided to determine the maximum water depth of the proposed minor impoundment and to size an emergency overflow structure. The design is based upon the guidelines provided in Appendix 6, Section 2.2 of the Basis of Review for Environmental Resource Permits. 2.0 MODEL INPUT BASIN: The basin area of acres represents the top of bank area of the reservoir. It is conservatively assumed that this entire acreage is open water retention with a Curve Number equal to 100. NODE: The node represents the stage/storage areas of the impoundment. The initial stage elevation is set at the overflow weir elevation of A base flow of 78 cfs is presented here to account for the maximum amount of water being pumped into the reservoir when all pumps are operating. This pumpage amount of 78 cfs or 35,000 gpm was established by the modeling efforts of Del Bottcher. Del s report titled Operation Protocols is also provided under separate cover. WEIR: The weir represents a 300 ft wide structure with invert elevation equal to The height of opening is the difference between the structure invert and top of bank elevation of 36. TAILWATER: Per Appendix 6, Section , the tailwater elevation must be set assuming a 100-year downstream condition. Per Del s modelings (see attached supporting ), it is estimated that 100-year elevation of the receiving canal is 31.4, utilized elevation 32.0 in the model. ROUTING/HYDROLOGY: Per Appendix 6, Section , a three day 25-year storm event was routed to determine the maximum water depth also known as the design water depth for minor impoundments. A three day 25- year storm event produces 9 inches of rainfall per Figure C.10 of the Basis of Review Design Aids. CSI File No

4 Minor Impoundment Project MODEL RESULTS Stormwater Calculations The maximum water depth a.k.a. design water depth equals elevation This elevation is based upon all pumps running with a three day 25-year rainfall event on the impoundment area. The impoundment s top of bank elevation is 36.0, therefore a two foot freeboard is provided above the maximum water depth. FLOODPLAIN The site lies within a ZONE A floodplain. Per the guidelines of the Environmental Resource Permit Information Manual Volume IV 2011, the site is determined to be an Exporter. 1. Site pre-development 100-year, 3-day storm runoff volume: computed by using the SCS method of volume estimation given by: Runoff Depth Over Basin (Q) = (P- 0.2*S) 2 / (P+ 0.8*S) where P = Rainfall Depth (11.0") S = Potential Storage Abstraction = 1000/CN 10; CN = 84 Q = ac-ft 2. Site pre-development storage available to the basin during the 100-year flood. V = 9.48 ac-ft 3. Since the difference between (1) and (2) is positive ( = ), thus the runoff volume is larger than the available storage; the site is considered an exporter of basin runoff. In the post-development conditions, the site s contributing runoff downstream during the 100-year, 3-day storm event will be eliminated since the impoundment s overflow is set at 2.1 foot above the estimated 100-year elevation of the surrounding canal. CSI File No

5 Nodes A Stage/Area V Stage/Volume T Time/Stage M Manhole Basins O Overland Flow U SCS Unit CN S SBUH CN Y SCS Unit GA Z SBUH GA Links P Pipe W Weir C Channel D Drop Structure B Bridge R Rating Curve H Breach E Percolation F Filter X Exfil Trench T:TW-100 year W: OVERFLOW A: impoundment U:impound. basin Interconnected Channel and Pond Routing Model (ICPR) 2002 Streamline Technologies, Inc.

6 ==== Basins ============================================================================== Name: impound. basin Node: impoundment Status: Onsite Group: BASE Type: SCS Unit Hydrograph CN Unit Hydrograph: Uh256 Peaking Factor: Rainfall File: Sfwmd72 Storm Duration(hrs): Rainfall Amount(in): Time of Conc(min): Area(ac): Time Shift(hrs): 0.00 Curve Number: Max Allowable Q(cfs): DCIA(%): 0.00 ==== Nodes =============================================================================== Name: impoundment Base Flow(cfs): Init Stage(ft): Group: BASE Warn Stage(ft): Type: Stage/Area BASE FLOW REPRESENTS THE INFLOW FROM ALL PUMPS RUNNING Stage(ft) Area(ac) Name: TW-100 year Base Flow(cfs): Init Stage(ft): Group: BASE Warn Stage(ft): Type: Time/Stage 100-year elevation in canal per Del Bottcher's study Time(hrs) Stage(ft) ==== Weirs =============================================================================== Name: OVERFLOW From Node: impoundment Group: BASE To Node: TW-100 year Flow: Both Count: 1 Type: Vertical: Mavis Geometry: Rectangular Span(in): Rise(in): Invert(ft): Control Elevation(ft): Bottom Clip(in): Top Clip(in): Weir Discharge Coef: Orifice Discharge Coef: TABLE ==== Hydrology Simulations =============================================================== Name: 25-yr, 3-day Filename: I:\LKDCIV\PUBLIC\Sebring\Istokpoga\sfwmd.R32 Override Defaults: Yes Storm Duration(hrs): Rainfall File: Sfwmd72 Rainfall Amount(in): 9.00 Time(hrs) Print Inc(min) ==== Routing Simulations ================================================================= Interconnected Channel and Pond Routing Model (ICPR) 2002 Streamline Technologies, Inc. Page 1 of 2

7 Name: 25-yr, 3-day Hydrology Sim: 25-yr, 3-day Filename: I:\LKDCIV\PUBLIC\Sebring\Istokpoga\sfwmd.I32 Execute: Yes Restart: No Patch: No Alternative: No Max Delta Z(ft): 1.00 Delta Z Factor: Time Step Optimizer: Start Time(hrs): End Time(hrs): Min Calc Time(sec): Max Calc Time(sec): Boundary Stages: Boundary Flows: Time(hrs) Print Inc(min) Group Run BASE Yes Interconnected Channel and Pond Routing Model (ICPR) 2002 Streamline Technologies, Inc. Page 2 of 2

8 Max Time Max Warning Max Delta Max Surf Max Time Max Max Time Max Name Group Simulation Stage Stage Stage Stage Area Inflow Inflow Outflow Outflow hrs ft ft ft ft2 hrs cfs hrs cfs impoundment BASE 25-yr, 3-day TW-100 year BASE 25-yr, 3-day Interconnected Channel and Pond Routing Model (ICPR) 2002 Streamline Technologies, Inc. Page 1 of 1

9 about:blank Page 1 of 1 5/23/2012 Wendy Contner - Fwd: RE: Istokpoga Above Ground Impoundment From: Wendy Contner Subject: Fwd: RE: Istokpoga Above Ground Impoundment >>> "Del Bottcher" <dbottcher@swet.com> 4/17/2012 7:25 PM >>> Wendy and Ron, I have completed a number of simulations and it s clear we can go even lower with the pump sizes. My current recommend is for a two 15,000 gpm pumps and one 5,000 gpm duty pump. These pumps will work with or without the neighboring farmer pumping in, so I suggest you design using a total of 35,000 gpm for your outlet. As far as the 100-yr tailwater elevation, my results show a maximum stage of 31.4 feet over the 43-year simulation for the reach receiving the impoundment s outflow. Therefore, it would be safe to assume that the 100-yr stage would be a little less than 32 feet. Del Del Bottcher, Ph.D. P.E. Soil and Water Engineering Technology, Inc NW 12 th Ave Gainesville, FL Office PH: (352) Cell PH: (352)