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I I I I I I I I I I I I I I I I I I I CHAPTER V - FLOOD HAZARD AREA DELINEATION case, the capacity of the culverts (see Table IV-I) is inadequate and street overtoppi n9 occurs. The channel way between the crossi ngs generally has adequate capacity to carry the lOO-year flood. The 76" x 48" concrete cu1 vert beneath Hampden Avenue (Station 36+50 to Station 40+00) has an existing capacity of approximately 260 cfs before overtopping of the frontage road. The 100-year flood flow at thi s poi nt is about 870 cfs. Pondi ng wi 11 occur upstream of the cu1 vert, inundating the parking lots and flooding the Farmers Insurance Building. Ponding will continue up to elevation 5461.2 at which point the runoff will spill onto the south service road causing a flow spl it to the east. The flood hazards caused by this flow split are discussed in detail in the next section of this chapter. A. General The floodway is the main channel of a stream, plus those adjacent floodplain areas that must be kept free of obstructions in order that the 100-year flood be carried without significantly increasing the energy grade line. Based upon Urban Drainage and Flood Control District (UD&FCD) criteria, the floodway was defined as being the limit of encroachment into the floodplain where the existing energy grade line profile is not increased more than 0.5-feet. The HEC-2 computer program (Reference-7) was utilized to determine the floodway limits. B. Henry's Lake Basin Drainageway The drainageway within Academy Park (Station 40+00 to Station 52+00) is characterized by surface flow along the south service road and the roadside ditches. L i tt 1 e flood damage occurs in thi s reach except for some eros i on and road inundation. The present capacity of the 72" x 44" culvert beneath Wadsworth Boulevard is 75 cfs. The peak runoff at this point is 635 cfs. Ponding upstream of the culvert occurs up to elevation 5474 plus or minus at which point the flow splits to the north along Wadsworth Boul evard. Most of thi s flow overtops Wadsworth Boulevard and flows into the Academy Park area. The remainder of the flows go north under Hampden Avenue and meander northeasterly back into Bear Creek. The open channel upstream of Wadsworth Boulevard (Station 53+25 to Station 60+00) is of adequate capacity to contain the 100-year flood. The culvert beneath the service road (Station 61+00) does not have adequate capacity to pass the 100-year runoff. Consequently, ponding occurs upstream of the service road compl etely fi 11 ing the existing RTD detention site and the low lying area north of the pond. Some overtopping of the service road also occurs resulting in some shallow flooding at the Park-n-Ride facility. Floodway widths, which represent the maximum 1 imits of encroachment into the floodplain, are tabulated and presented in Table V-I. The 100-year floodplain limits and water surface profiles are shown on the flood hazard area delineation maps, drawings 2 through 8. The computed 100-year flood elevations at each reference section are tabulated on Table V-I. Descriptions of the extent of flooding for both the Henry's Lake Basin drainage and the flow split drainageway are discussed in the following sections. The drainageway below West Floyd Avenue (Station 3+00 to Station 9+60) has an average slope of approximately 6.5%, whi ch resu1 ts in flood depths of generally I-foot to 2-feet with velocities of 6.0 fps. Although no residences are inundated, the floodplain does encroach on the properties along both sides of South Pierce Street. Above West Floyd Avenue, (Station 9+60 to Station 28+20), the drainageway passes through four small ponds with earthen embankments. For all ponds, the 100-year flood will overtop these embankments in excess of 1.0-foot. The potential hazard in this reach is breaching and subsequent failure of the embankments which will add to the downstream flood hazards. Upstream of the ponds to Hampden Avenue (Stati on 28+20 to Station 36+15), the drai nageway is characteri zed by the open channe 1 through the Hampden Vi11 a resident i a 1 comp1 ex. There are three road/driveway crossings in this reach at Station 28+10, 31+30, and 35+05. In each In the reach extending from the service road up to Carr Street the floodplain wi dt his partially caused by the back water effects of the cu1 vert beneath the service road, but is primarily due to the broad and flat character to the drainageway. Typical 100-year flood depths are approximately 1.0-foot with corresponding velocities in the range of 1 to 2 fps. A single residence in the middle of the drainageway is completely inundated and several residences along the north experience minor flooding. -23-