PROJ00435 - Laserfiche WebLink

Home Browse Search

Project Formulation floodwater, erosion and sediment damage is such that land treatment measures alone will not achieve the desired level of protection. Because of the numerous sources of possible runoff located above the irrigated lands, consideration was first given to enlarg.ing the Highline Canal to intercept and then convey flood peaks and sediment, to a point at which a floodway channel could take floodwater to the South Platte River. No reservoirs were to be used with this program. Investigations using weighted peak runoff of 712 c.f.s. per square miles 'from the 10-year frequency storm with land treatment appl ied, determined that the Highline Canal capacity would have to be tripled for each square mile of runoff intercepted. The normal operating capacity is about 180 c.f.$. in the western portion of the water- shed down to about 25 c.f.$. in the eastern portion at the Nebraska state line. Without reservoirs to reduce peak runoff into the Highline Canal, a floodway would have to be provided to convey runoff to the South Platte River at least every 1-1/2 to 2 miles along the canal. Est imated costs for ,these fJoodways proved to be excess ive when consideration was given'to: (I) stabilization of steep floodway gradient of 150 feet of fall in 2-1/2 miles; (2) land rights, including relocation assistance for several of the floodway routes, and--{-3-)-cana+-eA-la'Fgemen+--ees,t-s. Thef"efore, it was-de l", "" i lied lI'd l , proJect obJectives would best be achieved by providing floodwater retardation structures and to select routes for floodways 'that would not involve unnecessary costs for construction and relocation assistance. Structural measures believed to meet obJectives of the sponsors in Colorado were: a combination of floodwater retarding structures with principal spillway release rates restricted to the safe disposal capa- city of the Highline Canal; grade stabilization structures at several channel headcuts above the Highline Canal; drop inlet structures into the canal below each reservoir and floodways to carry the principal spillway releases to the river. Measures to control damages in the Nebraska portion of the watershed were investigated. Two floodwater retarding structures and an attendant floodway to reduce floodwater, erosion and sediment damages were not economically feasible. The flood plain of the Nebraska portion of the proJect Is separate from that of the Colorado structures. The desires and needs of the Board of Directors of the Julesburg Irriga- tion District to repair and enlarge the embankments of the Julesburg Reservoir were discussed with representatives of the U.S. Bureau of Reclamation, state agencies, Julesburg Irrigation District sponsors, and 32