Laserfiche WebLink
<br />the mai n stem ranges from steep slopes of 2% in the upper reach to <br />about 0.5% near the South Platte River confluence. <br /> <br />approximately five feet above that level shown on the <br />date of survey. This increase, however, does not <br />inundate any dwellings. Refer to Sheets 5, 6, and 23. <br /> <br />Development along the Todd Creek main stem is essentially all <br />agricultural. Scattered rural residential development also exists. <br />With the exception of street crossings and irrigation reservoirs, no <br />significant encroachment into the floodplain has occurred along the <br />5.9 mile study reach. <br /> <br />1. There are several roads that are overtopped from one to <br />six feet during the 100-year event. These include <br />Holly Street (2.5 feet), Quebec Street (1.2 feet), <br />Yosemite Street (2.5 feet), Elmira Drive (4 feet), <br />Havana Street (2 feet), and the private road just <br />upstream of the Brantner Ditch Flume (6 feet). All <br />street overtoppings result from inadequate culvert <br />capacities. Only the Elmira Drive overtopping causes <br />resi denti a 1 fl oodi ng to occur. Because the 100-year <br />flows are much 'larger than the culvert capacities, <br />debris blockage is probably of greater concern for the <br />smaller runoff events. All the stream crossings listed <br />above are gravel or di rt roads, and hence erosion and <br />road washout potential from overtopping is high. Refer <br />to Sheets 2, 4, 8, 10, 11, and 12. <br /> <br />3. The embankments of both the unnamed reservoir <br />immediately downstream of Smith Reservoir and the <br />reservoir at the confluence of Tributaries 1 and 4 will <br />be compl ete 1y overtopped duri ng the 100-year event. <br />Embankment erosion potential is very high. The water <br />level for both reservoirs increases about six feet <br />above that level shown on the date of survey, although <br />no flood damage to housing is sustained. Refer to <br />Sheets 7, 9, and 24. <br /> <br />Specific flooding problems along the Todd Creek main stem are <br />described following: <br /> <br />4. The Brantner Ditch Fl ume creates a flow constriction <br />and backwater effect in the main stem. The structure <br />totally conveys the 100-year flood with about one foot <br />of freeboard, and the concrete wingwalls should offer <br />erosion protection from the high water velocity. No <br />residential flooding results from this flow condition. <br />Refer to Sheet 12. <br /> <br />5. The breached reservoir just upstream of Riverdale Road <br />creates a backwater due to flow constriction, but the <br />remaining embankment does not overtop. No residential <br />flood damage is sustained. Refer to Sheet 13. <br /> <br />2. Smith Reservoi r does not have adequate spillway <br />capacity for the 100-year event. Although the entire <br />embankment does not overtop, erosion potential is high <br />in the spillway and at the left abutment where shallow, <br />high velocity overflows will occur. The backwater <br />effects in the reservoir will raise the water surface <br /> <br />6. Ri verdale Road at the Todd Creek confluence is a <br />particularly troublesome area for several reasons. The <br />Riverdale Road bridge does not have adequate capacity <br />to convey the 100-year event. Todd Creek also <br />dead-ends into the Brighton Ditch, immediately <br />downstream of this crossing, creating a barrier between <br />the Todd Creek floodwaters and the South Platte River. <br /> <br />15 <br /> <br />