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<br />HYDROLOGY <br /> <br />Creek through this ~rea is largely a drainage swale with flat, <br />gently sloping overbank areas. <br /> <br />overview <br />This scope of work required an analysis to "detemine runoff rates <br />and volumes for the 2-, 10-, 50-, 100-, and SOO-year flood <br />frequencies for selected basin conditions using basin growth <br />projections from planning and zoning maps of local governments". <br />The hydrology determined in this study was used to develop water <br />surface elevations for these flood events throughout the study reach. <br /> <br />In the College Avenue area extending upstream to the Larimer-Weld <br />Canal, (Eaton Ditch), the Dry Creek channel is very small or <br />non-existent, the result of heavy development. The area is again <br />very flat with no defined flow path. <br /> <br />From the Larimer-Weld Canal upstream to the study limit, the channel <br />again becomes well-defined. Overbank areas slope toward the channel <br />and provide large flow capacity in a relatively narrow area. <br /> <br />Recent Flood History <br /> <br />The Storm Water Management Model (SW~1M) (Reference 2) ....as selected <br />for use in this study and was approved for use by the Technical <br />Committee at an initial meeting. The model accounts for channel <br />and overbank storage, lagging and combining of hydrographs, and <br />reservoir routing. The model can be easily altered to look at <br />varying basin conditions and will be available to the City and County <br />to evaluate the impact of future development projects ....ithin the <br />basin. <br /> <br />Recent history has shown that no serious flooding has occurred in <br />the Dry Creek Basin below the Eaton Ditch in the last 25 to 30 years. <br />This absence of even minor flooding ~s generally attributed to the <br />network of irrigation canals that have some capacity for intercepting <br />Dry Creek flows. <br /> <br />Presently, the impact of new development in the lower basin is already <br />increasing the magnitude and fr<lquency of drainage flows. In the <br />past, much of the excess drainage was intercepted by irrig~tion canals. <br />Today, these canals no longer have the capacity to intercept increns~d <br />drainage flc".s and the canals shuul<.l Le bypassed with [Jropetc conveyance <br />provided in the <.lrainageway. <br /> <br />Irrigation Canals <br />~umerous irrigation canals cross the Dry Creek channel and directly <br />intercept drainage flows as mentioned previously. Several of these <br />canals, including the Larimer and Weld Canal (Eaton Ditch), the <br />Larimer County Canal, the Terry Inlet, the Poudre Valley Canal and <br />the North poudre Ditch, have large enough flow capacity to signifi- <br />cantly impact flood magnitudes on Dry Creek. For the more frequent <br />flood events, the Larimer and Weld Canal (Eaton Ditch). the Larimer <br />County Canal and the Terry Inlet were assumed to intercept all Dry <br />Creek flows reaching their channels. <br /> <br />Flood hiOltury prior to this time period is .:t little lc~s clear. <br />Before substantial development occurred in the area, flows could often <br />be ignored or overlooked where little damage was incurred. Floodir.g <br />did occur in the Dry Creek basin in 1924 with flows severed. felet deep <br />at the present location of Jax Surplus; however, it is unclear as to <br />whether this was actually duE' to overflows from the poudre River. <br /> <br />As development within the basin continues, these canals will <br />int_ercept greater flows on a more frequent basis. This will <br />increase the probability of sporadic overflows from the canals <br />which can cause severe damage particularly if an emba~kment fails. <br />For t~is reason, separation of drainage flows and irriqation flows <br />should be considered for at least the lower frequency events. <br /> <br />-10 - <br /> <br />- 11 - <br />