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<br />A spreadsheet was used to calculate inflow, outflow, volume, and submergence on the <br />overflow weir. An initial detention pond size, as well as an overflow location and elevation, <br />was assumed. A volume-elevation table was created for use in the spreadsheet. An HEC-2 <br />backwater model was used to develop the discharge-elevation rating curve at the weir. The <br />HEC-2 model began downstream from the U.S. Highway 138 bridges, with a starting water <br />surface elevation of 3914.8 feet mean sea level (ms1) corresponding to the South Platte River <br />lo-year event and a starting discharge of 1212 cfs corresponding to the full discharge expected <br />at node 5. The HEC-2 split flow method was used to divert flow by energy grade line into the <br />detention area for free flow over the weir.' For submerged flow, the overflow discharges were <br />decreased according to the head differential as shown in Brater & King, H"nclhook of <br />Hyclr:mHcs, Figure 5-5. Initial HEC-2 iterations involved changing the weir size and elevation <br />to get the desired magnitude of flow over the weir. Initial spreadsheet iterations involved <br />changing the detention pond size along with the HEC-2 data until the desired results were <br />obtained. These included keeping the weir from submerging before the peak of the channel <br />hydrograph could be reduced by the detention pond. Once these areas were narrowed down, <br />an outlet downstream from the pond was added to the spreadsheet to simulate an outlet conduit <br />from the pond. Putting flows back into the channel downstream from the detention pond caused <br />additional backwater effects, which were accounted for in the channel HEC-2 model and the <br />spreadsheet. <br /> <br />3.2.6 The above analysis resulted in a detention pond taking almost the entire 40 <br />acres of available land northeast of North Division Avenue and Pioneer Road. The plot of land <br />was assumed to be square and measure 1320 feet per side. The detention pond was sized to be <br />square as well and to measure 1200 feet per side. A 6O-foot-wide area around the pond was left <br />for berm construction along the ditch, if needed, and for access, if desired. The interior <br />sides10pe is 1V:3H from a maximum elevation of 3922.5 feet ms1 to the pond floor. The pond <br />floor slopes from north to south and from both sides to the middle to allow for proper drainage. <br />The maximum floor elevation is at 3919.5 feet ms1 and the minimum is at 3916.0 feet ms!. <br />The total volume of the pond is 208,716 cubic yards (129.4 acre-feet). Plates 3-6 show a plan <br />view, cross sections, and profiles of the proposed detention pond. Two-foot contour maps from <br />the City of Sterling flood plain study were used to calculate cut and fill. For the detention pond <br />alone, 435 cubic yards of fill and 208,282 cubic yards of cut are required. <br /> <br />3.2.7 The detention pond would take 740 cfs from the 1040 cfs peak at node 4 by <br />diverting flows over a 7S-foot-10ng rock weir at elevation 3920.0 feet ms!. The resulting peak <br />water surface elevation in the ditch at the weir location is 3922.4 feet ms!. A flap gate should <br />be used on the outlet of the pipe to prevent channel flows from backing into the detention pond. <br />The maximum elevation of the detention pond wall is 3922.5 feet ms!. It is recommended that <br />a berm be constructed between Pioneer Ditch and the detention pond to elevation 3923.S feet <br />ms1 to keep flows from overtopping into the detention pond at places other than the weir. A <br />!-foot-high berm with 1 V:3H sides10pes, a IO-foot top width, and a length of approximately <br />1400 feet would require 674 cubic yards of fill material. <br /> <br />6 <br />