|
<br />:5
<br />
<br />area above the reservoir. The second study was prepared to show the
<br />floodplain through Bayfield, which is about 12 miles below Vallecito
<br />Dam and 8 miles below the subdivision. The Bayfield study assumed
<br />that Vallectio Reservoir would coy,taiy, part of the flood and
<br />resulted in a 100 year flood of 6000 cfs for a drainage area of 349
<br />square mi les.
<br />The sections below describe the 100 year flood
<br />River Cattle Ranch with and without the effects
<br />RI:?set"Vo i r.
<br />
<br />at
<br />of
<br />
<br />the PiY"le
<br />Vallecito
<br />
<br />100 year Flood Without Vallecito Reservoir
<br />
<br />The potential 100 year flood at the subdivision can be
<br />estimated from the Vallecito Creek flood study through area
<br />cor,'elations. The description of the process and result is below.
<br />The 100 year flood for the Pine River, which drains the other
<br />1:50 square miles of the basin above Vallecitio Reservoir, has not
<br />been determined. A reasonable estimate of the flood peak for the
<br />Pine River can be made using the l"at"o of the drainag.e areas. The
<br />estimated food peak from the Pi,",e FHver would be :l5'~/100 times
<br />11,000 cfs which is 15,000 cfs. l~is methodology, though crude, is
<br />r.easonable because both draiy,age,. have neal"ly idey,tical slopes,
<br />vegetation, elevations, exposure, and other characteristics. The
<br />rainfall and the resultant runoff should be the same.
<br />There is also an intermediate area between the dam and the
<br />subdivision tha~ will experience flood runoff, this area is about 18
<br />square miles. The Corp of Engineers study esti,mated a flood peak of
<br />1100 cfs for a small tributary nam.~d Grimes Cre,ek which is about the
<br />same size, 8.4 square miles, and adjacent to the intermediate area.
<br />A~lain the drainages''-'are nearly the safl,e ay,d it is reasonable to
<br />estimate the flood peak in the intermediate area by correlation with
<br />the ratio of the areas. The resulting flood peak is 18/8.4 times
<br />11. 00 cfs, wh i ch is 2400 cfs.
<br />The above flood peak est imates ar-e 11,000, 15,000 ar',d 2400 cfs
<br />for Vallectio Creek, the Pine River, and the intermediate area
<br />re,spectively. These three subbpsins m,ake up the total dr-ainage area
<br />above the subdivision. If the three peaks were added together the
<br />flood would be 28,400 cfs. Tne peaks are not directly additive
<br />be,cause the time of concentrations for the thl"ee subbasins a,'e
<br />different, in other words the p",aks f:('om the three areas would not
<br />reach the subdivision at the spme time. Generally the flood peak
<br />occurs the soonest on basins ",ith s,horter chay,y,el ler,gths. The
<br />channel lengths are 26, 34 and 7 miles for Vallecito C:,'eek, Pine
<br />River, and the intermediate are respectively. Using the SCS
<br />methodology for the time of concentration, the times are 8, 9, and 6
<br />hours. The peaks for the Vallecito Creek an;d intermedeiate area
<br />would have passed when the pepk for the Pine River reaches the
<br />property. The resultant flood peak with the times of concentration
<br />factored in is 26,000 cfs which is a small difference from the
<br />28,4iZiiZi cfs.
<br />A second flood estimation methodology was used to estimate the
<br />flood at the subdivision. Thi. methodology was developed, tested,
<br />and is being used in flood studies performed by the U. S. Forest
<br />Service in the San Juan Nation.al Forest and is entitled "Region 2
<br />Water Resource Analysis Syst.m, Channel Geometry and Culvert
<br />Sizing". The procedure was d.veloped by Mark Story et,nd Michael
<br />Burke for the San Juan National Forest. The procedure is generally
<br />
|