7052 - Laserfiche WebLink

Home Browse Search

r 468 Topographic Response of a Oar 70 percent of the days with mean daily discharge greater than 510 m3/s during the period from 1978 to 1986 occurred in 1983 and 1984. The mean annual load of sand transported past the Jensen gage during the period 1983 to 1986 was 2.7xl06 ton/yr, or more than 3 times the mean annual quantity transported during the period immediately after the reservoir was constructed, 1963 to 1982. The relatively large discharges and sediment transport rates during the period from 1982 to 1986 ire comparable to the condition which existed prior to the reservoir. Changes in the Green River channel since the construction of Flaming Gorge Reservoir must be deduced primarily from a comparison of large scale aerial photographs (Andrews, 1986]. All of the Green River gages downstream from the reservoir are ocated in stable reaches, where coarse gravel and cobble bed-material as well as bedrock severely limit the adjustment of channel width and depth. PuchereUi Jwritten communication, 1988] analyzed recent changes in bankfull channel wi th as well as the number and area of islands in several reaches of the Green River, using aerial photographs taken in 1963, 1974, 1978, and 1986. One of the reaches he investigated extends approximately 18 km upstream from the mouth of the Duchesne River, and includes our study reach. Average bankfull channel width in this part of the Green River decreased from ...215 m to 189 m during the period from 1964-78. During the subsequent period, 1978-86, however, bankfull channel width increased to 198 m. Although gradual accretion of bank material was common in the Green River between 1964 and 1978, the most Sipificant process narrOwing the channel was aggradation of channel bars, result1'llg in their attachment 1.0 the bank and incorporation into the floodplain. In nearly all instances, thick vegetation became established before the bar became attached. This narrowing of the channel since the beginning of flow regulation appears to involve the same processes responsible for forming the pre-reservoir floodplain. Inspection of the floodplain adjacent to the study reach shows that it has been constructed by the aggregation of islands as well as the gradual accretion of scroll bars. . The remainder of this chapter will consider in detail the nature of flow and sediment transport in the vicinity of a mid-channel bar. In addition, the evolution of bar topography over time at various discharges will be described. Sand bars are quite common in alluvial reaches of the Green River between the Jensen and Ouray gages. The particular bar we have selected appears to be a typical example in every way. The fluvial nature of this bar and associated reach will be investigated by means of a fluid dynamic model, which will be described in the following section. Model of Flow and Sediment Transport The morphology of alluvial channels is ultimately the product of a strong nonlinear coupling between channel geometry, the flow field in the channel, and the transport of bed material by the flow. The first step in understanding the development of channel topography IUld its response to viUiations in discharge is a careful treatment of the various aspects of this coupling. The approach we have taken treats the interactions between the flow; bed topography, and sediment transport by combining three separate components of the full coupling, as follows: (1) a flow algorithm capable of making accurate predictions of the velocity and boundary shear stress fields in channels of known topography and planform, ill) a method for determining sediment fluxes over the bed given the flow field, and 3) a method for determining temporal changes in channel topography due to th sediment transport and changes in stage. In this section, each of these three components is briefly described, with emphasis on the physical effects which are incorporated and, importantly, those which are neglected in our formulation. J