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<br />3.1.2 Sources of flooding in Sterling include Sand Creek, Pawnee Creek overflows, <br />and local drainage. This report focuses on the future JO-year local drainage flood as outlined <br />in the 1988 Leaf report. <br /> <br />3.2 Detention Pond <br /> <br />3.2.1 The City of Sterling requested that a detention pond alternative be analyzed. <br />A 40-acre site northeast of Pioneer Road and North Division A venue was identified as a prime <br />area for a detention pond because of its natural shape and because the City already owns the <br />property. Plate 1 shows the proposed detention pond site. A detention pond at this location <br />would help alleviate several flooding problems downstream from the pond. <br /> <br />3.2.2 The main problem in this area is due to backwater flooding from the two <br />railroad and two highway bridges that the ditch must pass under before its confluence with the <br />South Platte River. The bridges are capable of passing only half of the flow that would <br />normally be in the channel at this point. F100dflows back up upstream from the bridges and <br />overtop the channel banks, resulting in significant flooding in the surrounding residential areas. <br />The second problem is local drainage during a flood event in the same residential areas. If the <br />ditch is full and backing up, the local drainage outfal1s into the ditch are not able to operate <br />properly. The drainage collects and floods low areas until the ditch water recedes. <br />Theoretically, the detention pond would take the peak off of the channel flows so that <br />backwater effects from the four bridges would not cause flooding. Also, the pond would allow <br />a timing difference between in-channel flows collected upstream and local drainage from the <br />residential areas near the downstream end, so that they do not pass through the bridges <br />simultaneously. <br /> <br />3.2.3 Discharges used for the detention pond analysis are shown in Table 1, and the <br />node locations are shown on Plate 1. These represent 10-year future conditions local drainage <br />discharges taken from Leaf (1988). It was assumed that the discharge shown at each location <br />is with some future channel improvement in place. In other words, it was assumed that no <br />overflow occurs at low-capacity locations of the channel or at bridge constrictions, etc. Also, <br />it was also assumed that the discharges include any coincident flows from collectors or outfal1s. <br />Node 4 represents the discharge location at the detention pond site. <br /> <br />3 <br />