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This plan would require the cooperation of the irrigation ditch company that owns Terry Lake. Negotiation of the types and costs of i~provements would have to take place with the ditch company. In addition, storage rights would have to b~ obtained, and some agreement would be needed to define the ownership and, therefore, the liability for the dam embankment. Therefore, the recommended plan of improvement is Alternative #5. This plan has net benefits of $723,700 before considering the benefits of increased property values for lands removed from the regulated lOO-year flood plain. In addition, drainage flows up to the lOa-year event are separated from the Larimer-Weld Canal, very much limiting the probability of large spills from the canal and possible embankment failure. Preliminary Design The location and type of improvements covered by Alternative #5 are shown on the drawings, Sheets 8 through 12. Those improvements that are required in Reach A are described on Sheet 12. The existing culvert at Meadow Lane is not adequate to handle the IO-ye~r design flow of 720-cfs and a larger concrete box culvert, 12' x 6', was designed as a replacenent. An embankment used as a farm road would be removed as a restriction to flood water upstre~rn of the Larimer and Weld Canal. Improvements for the bypass to the poudre River are detailed on Sh",..t 11. To sc.paratc. flood viaters in Dry Creek frol'1 the LJ.rimcr 3.nc Weld Canal, the canal would be routed through a siphon with discharye capacity of 1,000-cfs. Dry Creek would be designed to flow above it. A div8rsion from tilt! natural channel is nade just upstream of the crossing. ^ weir would be designed at this point of diversion to allow flood waters in excess of the laO-year e~ent to be directed back into Dry Creek. - 44 - " In addition, a low-flow culvert would be provided to pass flows up to the 2-year frequency event directly down the Dry Creek channel. Two new triple 12' x 8' concrete box culverts would be required where the diversion channel crosses the Union Pacific Railroc~ and willox Road. A trapezoidal-shaped, grass-lined channel was designed for the remainder of the route. The channel would be designed for the 100-year discharge of 2,700-cfs with one-foot of freeboard. The bypass terminus is a gravel pit just north of the poudre River. Minor excavation would be required in these gravel pits to allow flood water to reach the river. Diversion of flows to the poudre River appears to have minimal impact on the poudre River from the point of diversion to the normal point of inflow at the Dry Creek confluence. The 2,700-cfs diverted to Dry Creek is only 13% of the 100-year discharge of 19,620-cfs on the Poudre River. It is not likely that a 100-year event will peak on the poudre River simultaneously with a lOa-year event on Dry Creek, but should this occur, ch~nges in flood depths would be minimal because of the wide poudre River flcod plain. If a 100-year event occurred on Dry Creek during a normal summer flow on the poudre River, the poudre River discharqe would increase from l,500-cfs to 4,300-cfs. This is well below bankful capacity of approximately 8,000-cfs in this reach. DeLailed on Sheet 10 arc i~provemonts r~q~ired in reach R. ~n improved concrete box culvert, Ii' Yo.';', \;as designed for the d~veloped area just upstream and downstrea~ of College Avenue. 'rhe culvert was uesiyned to handle a flow of ~40-cfs in this lIre~, the residual lOO-year flood event belav.' the l,arimer-Weld Canal. - 45 -