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- II I I I I I I I I I I I I- I I I I II II Lower Big Dry Creek Hydrologic Study Weld County line (168th Avenue). The study area encompassed mne major tributaries, including: Wadley South Creek, Wadley North Creek, Short Run, Elms Run, Morris Creek, Shay Ditch, Mustang Run, Sack Creek, and Preble Creek. In 1989, the study area was essentially undeveloped cultivated land with only three low-density residential developments. The study showed that the existing culverts did not have adequate capacity to convey even the minor storm runoff without road overtopping. The 100-year floodplain was identified as very wide, and increased velocity due to higher future peak flows was anticipated to cause extensive channel erosion. The three major problems identified were lack of channel conveyance, potential for erosion and sedimentation, and road overtopping during minor floods (WWE 1989b). To help solve these problems, the selected outfall systems included the use and/or expansion of existing inadvertent detention areas and channelization from the upstream to the downstream limit. Road crossings were recommended to be designed to convey the 100-year flow without road overtopping. The open channels selected for the outfall system included engineered wetland bottom, engineered wetland bottom with bench, and engineered grass-lined channels (WWE 1989b ). This plan will be updated during 2005 and presents a significant opportunity for recognition of downstream flooding and erosion problems. 5.3.4 Flood Hazard Area Delineations Five floodplain studies have been conducted in the Big Dry Creek watershed, as summarized in Table 6. Figure 7 shows the mapped floodplains in the watershed. The Greiner (1988) study included the Big Dry Creek reach from the headwaters to 1-25, followed by the WWE (1989a) study from 1-25 to the Weld County line. Greiner (1988) and WWE (1989a) indicated that because of the historically undeveloped nature of the floodplain areas, relatively little flooding history was available :at the time of the study. Two floods that were documented included the May 1973 flood in the vicinity of Huron Street and the June 1984 flood along the middle branch of Hy1ands Creek at Sheridan Boulevard. Damage was confined to the culvert crossings during these historic events (Greiner 1988). Further downstream, flooding was noted at 160th Avenue and 162nd Avenue where runoff overflows from the stream channel, purportedly due to 971-179.092 June 2005 Wright Water Engineers. Inc. Page 37