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value of 0.035 was sclcclcd; for the overbanks, a
<br />roughness value of 0.1 was sclcclcd.
<br />Steady-state water-surface elcvalions were
<br />modeled for low flow (225 ft;/sec), the 1.5-year flood
<br />(4,000 ft t/sec), [he 10-year flood (10,500 fe /sec), the
<br />50-year flood (19,100 fl;/sec), and the 100-year flood
<br />(24,600 ft~/sec). Water-surface elevations along the
<br />profile of the Sandco reach arc shown in figure 8.
<br />Output tables from [[EC-RAS for the 100-year flood
<br />calculations are shown in Appendix 1 at the end of this
<br />report. The drops inwater-surface elevations at higher
<br />flows are locations where the channel narrows and the
<br />energy increases.
<br />Water-surface elevations for the 100-year flood
<br />at the surveyed cross sections are shown in figures 9
<br />through 11. At all of the cross sections, the water-
<br />surface elevations do not exceed the height of the east
<br />bank. This indicates that even during a 100-year
<br />flood, the river may not top over its banks and flood
<br />the future use lake area. At some of the cross sections,
<br />the water-surface elevation appears to be higher than
<br />the west bank. Because the cross section data were
<br />obtained from available files and not collected by the
<br />author of Phis report, it is unknown whether the bank
<br />continues above these survey stations, or whether the
<br />water-surface elcvalions would exceed the bank
<br />heights on the west side during the 100-year flood.
<br />ENVIRONMENTAL EFFECTS OF THE
<br />FUTURE USE AREA
<br />Concerns have been expressed about the effects
<br />of the future use area on water quantity, aquatic envi-
<br />ronment, water quality, and erosion. Available data
<br />have been compiled and new interpretations provided
<br />to address these concerns.
<br />Water quantity
<br />In 1989, the Colorado Legislature passed Senate
<br />Bill 120 that affects gravel pits in operation after
<br />September 3l, 1980. §37-90-137(1 l)(a)(ll), C.R.S.,
<br />requires any gravel pit that exposed ground water to
<br />the atmosphere after December 31, 1980, to replace all
<br />out-of-priority depletions of ground water The opera-
<br />tions at the future use area is not expected to have any
<br />direct effect on the ground-water system of the area.
<br />No tributary watercourse, wells, stock water ponds, or
<br />reservoirs exist on dtc afl~cled land. "fhc water table
<br />ranges from 8-12 ft below land surface depending on
<br />seasonal weather and subsurface water conditions;
<br />however, dcwatering of the future use area is not antic-
<br />ipated due to the method of gravel extraction that will
<br />be utilized. A minimum riparian buffer of 300 to 500
<br />ft will be maintained between the future use area and
<br />the Animas River to act as a vegetative buffer strip.
<br />Evaporative losses from the lake are the only
<br />expected out-of-priority depletions from the future use
<br />area. Penman published an important paper (Penman,
<br />1948), which allows calculation of the rate of evapora-
<br />tion from a water surface like that of a lake. It is diffi-
<br />cult to acquire all of the data for variables such as
<br />surface roughness, wind speed, and the saturation
<br />water vapor pressure. A common alternative is to
<br />measure the evaporation from a pan, such as a U.S.
<br />Class-A pan, and then multiply the pan evaporation
<br />measurement Ep by a factor. The factor (the ratio Eo /
<br />Ep, called the pan coefficient) is often taken as about
<br />0.7.
<br />Evaporative losses from have been calculated as
<br />part of this report using the pan evaporation data from
<br />Vallecito Dam (the nearest and most representative
<br />pan evaporation site in the region). Pan evaporation
<br />data at Vallecito Dam ranges from 30 to 40 inches per
<br />year, and averages about 37 inches per year. The
<br />exposed lake surface area will increase annually at 2
<br />acres per year until the project has been completed at
<br />about 44 acres. Using the pan coefficient of 0.7 and
<br />distributing over the area of the lake, evaporation
<br />depletions by year are shown in figure 12. Evapora-
<br />tion rates are shown in acre-feet per year to indicate
<br />the total depletions, and are shown in cubic feet per
<br />second to indicate the effects of the lake evaporation
<br />on flows in the Animas River. The total evaporation
<br />depletion after completion of the project at 44 acres
<br />would be about 96 acre-ft per yeas This is verified by
<br />calculations done by the Colorado Division of Water
<br />Resources concluding that the total evaporation deple-
<br />tion for 45 acres would be about 97.5 acre-ft per year
<br />(John Bilisoly, Colorado Division of WaterResources,
<br />Office of the State Engineer, written commun.,
<br />February 26, 2003). Assuming the ground water in the
<br />lake would eventually discharge to the river, evapora-
<br />tion from the lake may decrease flows in the Animas
<br />River by about 0.13 ft3lsec. Flow depletions of this
<br />small magnitude would probably not impact aquatic
<br />life or endangered fish species.
<br />Hydrologic Study of the Sandco Reach, Animas River Valley, La Plata County, Colorado, 2004 12
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