WSP00547 - Laserfiche WebLink

Home Browse Search

on033 banks. Mass failure of the banks also causes removal of the bank vegetation in what appears to be a self-regulating feedback mechanism. Since the undercutting of the banks (the mechanism that steepens the bank angle) occurs during relatively low flows, it can be argued that the increased duration of mid range flows following dam emplacement on the Green River (Andrews, 1986) has increased the rate at which the tamarisk is being removed. 2) Channel Capacity Determination of the frequency of overbank flooding along the study reach of the Green River Is dependent on being able to define the channel capacity. To achieve this, a number of cross sections were surveyed during the field inspection. At Cottonwood Bottom, the USFWS had established an IFIM site in 1980 with the additional purpose of having a channel monitoring site to Investigate aggradation/degradation trends in this reach of the river. Six cross sections were surveyed, tied together horizontally and vertically and monumented. between RM 55 and RM 56. The cross sections w,ere surveyed at discharges of 10,000. 5.100 and 2,600 cfs. The plotted cross sections (provided by Mr. George Smith. USFWS) clearly indicate that cross section geometry Is dependent on discharge (Harvey and Watson. 1989; Anthony and Harvey. 1992) and that aggradation/degradation trends should be based only on resurveys conducted at similar discharges. Cross sections (transects) 2. 3. 4, and 6 were relocated and resurveyed on 14 May. 1992 at a discharge of about 9.500 cfs. The plotted cross sections are attached to this report (Appendix A). Also shown on the plotted cross sections are the profiles for the 10.000 cfs survey in .1980. Comparisons of the profiles indicates that there has been very little change at any of the cross sections other than what Is to be expected given the minor differences In discharge between the 2 surveys. Cross sections were also surveyed at 81ocatioris at Unknown Bottom between RM 28.5 and RM 31.3. The subaqueous portions of the cross sectlons were recorded on a fathometer trace and later digitized. Observations of the bank heights and Inclinations were made as well and the surveyed bank profiles for the Cottonwood Bottom cross sections were transferred to the appropriate cross sections (on the basis of locaticn within the bend) at the downstream location. Cross section stationing, and hence the. horizontal relationship between the sections was approximated, from the topographiC map. (The channel slope was also approximated from the topographic ,maps at the Unknown Bottom location.) The plotted cross sections are attached to this report (Appendix B). The surveyed cross sections at the two locations were used to create HEG-2 models for each of the reaches. At the Cottonwood Bottom reach. the bankfull discharge for the individual cross sections ranged from 34,000 to 22,000 cfs for a Manning's n value of 0.03 and from 29,000 to 19.300 cfs for an n value of 0,035. Average values for the reach were 32.980 and 28.167 cfs for n values of 0.03 and 0.035. respectively. At the Unknown Bottom reach the bankfull discharge for the individual cross sections ranged from 41,900 to 34,000 cfs for an n value of 0.03. and 41.900 to 30,200 cfs for an n value of 0.035. Average values for the reach were 36,240 and 32.560 cfs for n values of 0.03 and 0.035, respectively. To determine the frequency of inundation of the bottomlands. a flow duration curve was developed from mean dally flows for the Green River at the Green River. Utah gaging station for the period 1963 to 1990 (Figure 1). Also shown on the figure Is the flow duration curve for the period between 1963 and 1982 developed by Andrews (1986). Comparison of the curves demonstrates the effects of the high discharge years of 1983. 1984 and 1986 on the upper end of the flow duration curves. At the Cottonwood Bottom reach. the exceedence frequencies for the reach averaged bankfull discharges are 1.3 percent and 0.8 percent for n values of 0.03 and 0.035, respectively. The exceedence frequencies translate into 4.7 and 2.9 days per year. At the Unknown Bottom reach the exceedence frequencies for the reach average bankfull discharges are 0.4 percent and 0.3 percent for n values of 0.03 and 0.035, respectively. The exceedence frequencies translate into 1.5 and 1.1 days per year. The exceedence frequencies represent average values and they do not indicate that bankfull discharge is exceeded every year. In fact. as the next paragraph shows. bankfull discharge at the 2 sites has a return period of between 4 and 10 years. 3 Resource Consultants & Engineers, Inc.