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~. 3. Aesthetic Environment: The Big Thompson Canyon is a rugged, rocky and narrow valley in a National Forest. Certainly one of the most scenic places of interest in Colorado, its' natural beauty attracts thousands of tourists each year. The Big Thompson flood plain generally is narrow (100 to 150 feet) and confined between steep rock-faced slopes. The normal river channel is only 10 to 30 feet wide, and only a few feet deep. Locally, the flood plain expands into broader park-like areas. The generating station and reservoir sites are located in two of those park-like areas. The surrounding area, a National Forest can best be described as natural, with relatively undisturbed rugged mountains and forests. There are no known properties listed on, or eligible for listing on, the National Register of Historic Places. There are also no known archaeological sites within the project area. ,- 17 G. How will the environment described in F affect the project? 1. Physical Environment: The dam, aqueduct and power plant had all been in existence in the canyon for over 50 years at the time of the flood. Flood flows during the 1976 flood were estimated to be in excess of 31,000 cubic feet per second. This extremely high flow accompanied by velocities never before experienced in the recorded history of the Big Thompson River totally destroyed the dam and power plant, including the washing out of approximately 1,700 lineal feet of pipeline. In addition to actually breaking up the concrete dam, the torrent of water scoured out the river bottom at the dam site an additional ten feet in depth. As a result, the new dam will, by necessity, measure additional height to maintain the same crest, elevation of 6,024. ~ The statistical chances of a flood of the magnitude experienced in 1976 in any given year is estimated to be less than two tenths of one per-cent. To design the new dam to withstand this flood magnitude is not economically feasible. A more reasonable approach to protection against future floods in the Big Thompson Canyon is to design for pro- tection against the statistical 100 year flood which has been done in this case. The dam will withstand the flows of the 100 year flood without measurable damage. (Flows in the area south Drake for a 100 year flood are calculated to be 10,000 cubic feet per second). .-- This will require keying the structure in to bed-rock which will further increase the total vertical dimension of the new dam. As a result, instead of a 24 foot high dam (height of the old dam) it will be necessary to construct a dam 35-45 feet high to maintain .a ,-