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I I I I II I I I I I I I I Ii I I I I I 10. Drawings GHP-I (Sheet 14 of 30) and GHP-2 (Sheet 15 of 30), Gronge HolI Creek These two drawings are included to i lIustrote the extent of the shallow flooding along the lower port of Grange Hall Creek. The flood plain area shown on these drawings is very broad ond flat with subsequent shallow flooding for the 100-year flood. The potential flood damages consist of erosion, loss of crops and livestock, damage to farm equipment, and limited residential inundation. A portion of this area is also subject to flooding from the South Platte River ond lies within the currently defined South Platte River flood plain. There are no proposed improvements illustroted on these two drawings as the channelization is illustrated on Drawings GH-9, GH-IO, and GH-II. II. Drawing TS-I (Sheet 16 of 30), Tributary S, Stotion 140+00 to 114+00 The channel in this reach depicted by this drawing has 0 low flow channel and a braad overflow area in the upper and lower portions of the reach. The channel in the middle portion of the reach consists of a 60" RCP and a 31'2' x 8' reinforced concrete box culvert under the Tuck Loteral and Melody Drive. Tributary S crosses Huron Street in an existing 54" CMP which is inadequate to poss the 100-year storm and results in overtopping of the street. The proposed improvements in this reach consist of the following: (I) Construction of a 60" RCP culvert under Huron Street with a rip-rapped outlet stilling bosin. (2) A grass-lined improved channel with a concrete low flow (trickle) channel from Huron Street to the Tuck Lateral. (3) A drop inlet structure at the entrance to the 60" RCP and proposed addition of a 48" CMP culvert. The 48" CMP is proposed to tie into the existing 3.5' x 8' RCBC at the existing junction structure. The 3.5' x 8' RCBC will remoin intact with no additional culverts proposed across Melody Drive. The purpose of the additional 48" CMP across the Tuck Lateral is to fully uti Iize the capacity of the box culvert under Melody Drive. The capocity of the RCBC is approximately equal to runoff peak flow from a 3-year storm. 12. Drawing TS-2 (Sheet 17 of 30), Tributary S, Station 114+00 to 87+00 The channel in the upper portions of the reach has a low flow channel with a broad overbank area. The flooded areas near 1-25 are created by the inadequacy of the existing culverts (48" RCP and 24" RCP) to pass the 100-year storm. The reach contains one pond without a formal spillway. The improvements consist of the addition of one 63" x 98" HE-RCP under 1-25 to olleviate the flooding problem west of 1-25. An option which can be considered is to substitute two 60" RCP's in place of the 63" x 98" HE-RCP under 1-25. Rip-rap is to be ploced at the culvert outlets to reduce outlet velocities and control erosion. There are no improvements below 1-25 for the remainder of the reach except flood plain zoning with flood insurance. With the improvements at 1-2S significant flood storage is eliminated which will increase the peak flow below 1-25 for the 100-year flood for future development conditions from 280 to 730 cfs. This results in 0 rise in the 100-year flood profile below 1-25 and is indicated as such on the profile drawing. However, the rise is one foot or less downstream of 1-25 ond the resulting flood plain is indistinguishable from the flood plain without improvements. However, all future flood plain zoning and regulation is to be based on the flood flows and flood elevations with improvements at 1-25 since the improvements ore imminent. This holds for the entire orea from 1-25 to the confluence with Grange Hall Creek. 13. Drawing TS-3 (Sheet 18 of 30), Tributary S, Station 87+00 to 58+00 The channel in this reach is characterized by a series of ponds above and below Washington Street. The pond above Washington Street and the ponds below Woshington 17