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Offices in Illinois, Iowa, Minnesota, and Wisconsin 2901 International Lane, Suite 300, Madison, WI 53704-3133 (608) 242-7779 (800) 446-0679 FAX: (608) 242-5664 WEB ADDRESS: www.msa-ps.com H:\TEMP\Middleton\Confluence pond memo.doc MEMO To: City of Middleton Water Resources Commission From: Uriah Monday, PE, CFM and Eric Thompson, PE, CFM Subject: Middleton Confluence Pond – Outlet Capacity Analysis Date: July 7, 2009 Purpose This memo documents the performance of the weir at the outfall of the Confluence Pond in the City of Middleton. In this analysis, the possible effects of tailwater in the reach of the weir are considered. The water quality analysis of the confluence pond performed for the 2007 Storm Water Quality Plan used the DNR-approved WinDETPOND model. The WinDETPOND model, however, can only operate under the assumption that the V-notch weir had a free outfall uninfluenced by tailwater in the channel. The presence of tailwater would decrease flows out of the pond for a given pond surface elevation, i.e. the outlet is essentially more restricted. This restriction increases the residence time of flows within the pond; in turn, this extended residence time allows more solids to settle out in the pond. A V-notch concrete weir controls flows out of the Confluence Pond. The notch of the weir is set at elevation 907 and has a 120 degree opening angle. The notch is set two feet higher than the channel bottom. The notch is set in a one-foot-thick concrete wall which has a top elevation of 912. A sketch of the weir is attached to this memo. Methods To assess the influence of tailwater on the pond’s outlet, a HEC-RAS model of the channel was constructed. This HEC-RAS model uses the same cross-section geometry data found in the model created by RS Grant Consulting for their 1999 design of the Confluence Pond, which is also the current regulatory FEMA model for floodplain along Pheasant Branch Creek. The RAS model output indicates that for flows ---PAGE BREAK--- Offices in Illinois, Iowa, Minnesota, and Wisconsin 2901 International Lane, Suite 300, Madison, WI 53704-3133 (608) 242-7779 (800) 446-0679 FAX: (608) 242-5664 WEB ADDRESS: www.msa-ps.com H:\TEMP\Middleton\Confluence pond memo.doc greater than 25 CFS, the tailwater in the stream exceeds elevation 907 and therefore begins to affect the weir. To verify that the HEC-RAS model generates elevations similar to actual observations, a comparison was made of the RAS results at the location of the USGS streamflow gage at USH 12 with actual results from the gage. The elevations compare favorably (within 0.1’) for flow rates greater than about 20 CFS – see chart below. RAS vs. Gage Rating at USH 12 Gage 903 903.5 904 904.5 905 905.5 906 906.5 907 0 20 40 60 80 100 120 Flow (CFS) Water Surface Elevation (NAVD 88) RAS Results Gage Rating Additionally, at flows less than 25 CFS, the weir takes on characteristics of a broad-crested weir (as opposed to sharp crested) due to the low ratio of the flow depth to weir thickness. When the pond exceeds elevation 912, the weir is overtopped. The higher flows resulting from the overtopping condition creates a much higher tailwater condition and restricts the outflow to an even greater extent. A graph is presented below which compares the elevation-outflow relationship of the RAS model vs. a weir with a free outfall condition (similar to that produced by the WinDETPOND model). ---PAGE BREAK--- Offices in Illinois, Iowa, Minnesota, and Wisconsin 2901 International Lane, Suite 300, Madison, WI 53704-3133 (608) 242-7779 (800) 446-0679 FAX: (608) 242-5664 WEB ADDRESS: www.msa-ps.com H:\TEMP\Middleton\Confluence pond memo.doc RAS vs. WinDETPOND Weir Rating 906 907 908 909 910 911 912 913 914 915 916 0 500 1000 1500 Pond Outflow (CFS) Pond Elevation HEC-RAS DETPOND Findings The effects of the tailwater present in the channel generally reduce the outlet capacity by about 25% to 30% compared to a free-outfall condition. The HEC-RAS hydraulic rating curve described above was entered into a P8 model of the confluence pond. The P8 model estimated a TSS reduction rate of 77.5%vs. the 58.2% previously indicated by the WinSLAMM model. The 2007 Storm Water Quality Plan was amended by replacing the WinSLAMM-reported reduction with the P8 model result. Note, however, that the loads originally reported in the 2007 document were left unchanged from the WinSLAMM results so as to maintain a constant point of comparison. Pond Elevation WinDETPOND HEC-RAS 907 0.0 0.0 907.6 1.3 0.9 908 4.5 3.2 908.5 12.5 8.8 909 25.7 18.0 910 70.8 49.6 911 145.3 102.0 912 253.8 178.0 912.5 346.4 253.3 913 472.0 356.5 913.5 626.4 425.6 914 808.7 614.7 914.5 1019.2 765.6 915 1258.3 920.0 Flow Out of Pond (CFS) ---PAGE BREAK---