PROJ02027 - Laserfiche WebLink

Home Browse Search

I I I I I I I I I I I I I I I I I I t SEGIIDIFIVE Flood DelenDon and Dlnnlln structure to take place. Zone 1 consists of a sandy clay to silty sand core. This zone is designed . to inhibit seepage through the structure. Zone 2 consists of clean sand and gravel and makes-up the majority of material within the fuseplug structure. The Zone 2 material is the primary material that is eroded should the structure be overtopped. Zone 3 consists of a light rock riprap material on the upstream face that is utilized to protect the upstream face from minor erosion created by waves and water movement. Zone 4 consists of a 4-inch layer of crushed gravel or bank run material installed at the crest and downstream slopes of the structure. The Zone 4 material is required to prevent surface erosion on the crest and slopes created by rainfall-runoff A geotextile fabric material is specified on the downstream riprap slope of the existing spillway structure to provide a separation between the existing spillway riprap material and the Zone 2 material. The segmented fuseplug was selected as the preferred alternative for flood attenuation at Douglas Dam. The fuseplug will not affect the day-to-day operations of the reservoir. The cost for the fuseplug was determined to be approximately $300,000. The Larimer and Weld Irrigation Company has indicated that they would be willing to consider this alternative if Dry Creek was upgraded to handle 75 cfs. This cost can be assumed to be in lieu of property or flood easement acquisition within the reservoir. The estimated cost to upgrade the channel and cross culverts to pass 75 cfs is $200,000, which brings the total cost of the fuseplug to $500,000. This results in the cheapest alternative for flood detention at approximately $350 per acre-foot 5.1.2 Purchase of Storage Rights in Douglas Dam Another alternative considered for additional flood storage in Douglas Dam was the purchase of water storage rights in Douglas Reservoir to be used for flood control. The proposed alternative would maintain a pool in Douglas Reservoir to approximate gage height 28 feet with the spillway crest at gage height 30 feet. The additional storage for flood control up to the existing spillway crest would be approximately 1,014 acre-feet. The water right would be obtained by trading a similar quantity ofCBT water stored in Horsetooth Reservoir. The current value of this water is approximately $12,000/acre-feet. Therefore, the total cost of purchasing flood control storage in Douglas Reservoir would be approximately $12,168,000. 5.1.3 Irrigation Conveyance in Dry Creek As part of the proposed agreement between the Larimer and Weld Irrigation Company and Larimer County regarding storage of flows in Douglas Reservoir and activities involving the Larimer and Weld Canal, the irrigation company has requested that the existing Dry Creek channel be upgraded to convey approximately 75 cfs of irrigation flows to the Larimer and Weld Canal. This concept has been discussed with the Cache la Poudre River Commissioner (Shawn Hoff) and they are in general acceptance of this option provided that State Statute 37-87-102 is met and that concerns of adjacent property owners are considered. Also, a provision as to the maintenance of the stream will be necessary. State Statute 37-87-102 allows for up to the 2.333-year frequency discharge to be conveyed in natural streams. For the Dry Creek, this flow rate ranges from approximately 70-150 cfs. A similar level offlows is currently released from Cobb Lake to Boxelder Creek (an average flow rate of approximately 60 cfs). Historically (in the 1930's to 1940's), the Larimer and Weld Irrigation Company used Dry Creek for conveying irrigation water. T:\PlOjects\682466ODry CrkDiversionlarimerCounty\Sub_00I12.0 Word ProcIFIood Control P1an-Rpldoc 04124100(2:04 PM) 5-2