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Flooded Are~ Description Throughout this reach, topography does not drain overflows back toward the Dry Creek channel. The topography, instead, carries runoff parallel to the channel through overbank areas. The Dry Creek flood plain is very unusual in that although the 100-year discharge is very small for the amount of contributing drainage area, the flood plain is extremely wide, averaging over 2,000-feet in reaches belo~ Willox Lane. In these reaches, the overbank flo~ is very shallow and slow moving. Seldom do depths in overbank areas exceed t,,'o-feet. Vir.e RO<ld to College Avenue (Sheets 2' 4 and S) In some areas, flood depths are less than one foot and are not hydraulically connected to flood elevations along the main channel. These areas are defined as shallOW flooding areas and are identified as such in the flood plain mapping. The flood plain through this reach remains very wide with shallow overbank flooding. The average width is 2,200-feet with overbank depths again less than two-feet. Culvert crossings of Lel1ay Avenue and Vine Road have very limited capacity and these roads are overtopped for considerable distances. The 100-year and SOO-year are presented on Sheets 2 areas in specific reaches flood plains throughout the study reach through 7. ^ description of flood plain follows. The Alta Vista Comnunity the east side of College within the flood plain. areas. and the co~mercial development along Avenue are the only developed areas Flooding is extensive in both these C<lche la t>oudre River to Vine Hoad (Sheets 2 and j) __~u The IOO-year flood plain in this reach is very wide and shallOW. The average flood plain width is 2,SOO-feet and depths are generally less than two-feet. The flood plain encompasses the Valley Airpark Industrial Area almost in its entirety. The existing Dry Creek channel through this area has a very limited c~pacity to contain only minor flood events. The limited capacity of the box culvert under Colorado High,,'ay 14 causes a large backwater <lrea behind this highw<lY ;!,pd sheet flclI' over the highwi'lY. Depths behind the highway embankMent range from onc-foot to six-feet. In the reach immediately downstreaM of College Avenue for approximately 1,200 feet, flows that cross the Josh Ames ditch 1\'111 spill dircc'~ly intc the Caelle la t'oudre River. ;,n estinate of the magnitude of flow spilling into the poudre River ir. this area.is JOO-cfs, 600-cfs, 8S0-cfs and 2,SOO-cfs for the IO-year, 50-year, 100-year and SOO-year events respectively ("''''''' Sh..",t 4). These discharges were deducted from th,-, total Dry Creek flows when analyzing reaches downstream for flood plain analysis only. -13- -19-