North Poudre Sediment Study Prog Report

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'. RECEIVED APR 2 2 1998 North Fork Poudre Sedimentation Study Progress Report Nov. 1997 Ellen Wohl, Dept. Of Earth Resources, Colorado State University Colorado Water Conservation Board Introduction This progress report summarizes the results from the first phase of a study undertaken to assess the effects of a sediment release from Halligan Reservoir into the North Fork Poudre River in Sep, 1996, Based on the volume of sediment accumulation in pools downstream from the reservoir, we estimate that 7,000 m3 of clay to gravel sized sediment were released from the reservoir. The first phase of the study consisted of field measurements of how this sediment was deposited during the initial reservoir release, and then subsequently mobilized during the 1997 high flow period. The second phase of the study involves testing the ability of three sediment transport models to simulate the observed patterns of sediment movement and deposition, Methods We established three reference sites for detailed study within the first 6 Ian of channel affected by the sediment release, Reach I is located 500 m downstream from the dam (Figure 1), This reach includes a 26 m long pool and the riffle immediately upstream. Reach II is located 2,8 to 3,2 Ian below the dam, and includes a set of two pools and two riffles, Reach III is located 4,9 Ian below the dam, and includes a 40-m long pool. Reaches I-III were surveyed shortly after the sediment release (Oct. 1996), and again during the subsequent autumn low flow (Sep. 1997), Reaches I and II were also surveyed during the rising limb of the snowmelt hydrograph (March 31,1997), In addition to the repeat surveys, we established a sampling cross section in a riffle at both Reach I and II. From 22 February to 3 September 1997 we sampled at these cross sections approximately once a week. Sampling frequency was increased to twice a week during the snowmelt peak (10 May- 24 June), During each sampling, we measured flow velocity at 0.4 and 0.6 of the flow depth at I-m intervals across the channel. Measurement sites were fixed in reference to benchmarks established on both banks, We also measured a thalweg velocity profile at 6-cm vertical intervals, (The thalweg is the line of deepest flow along the channel,) Each velocity point measurement represents an average offive readings taken with a Marsh-McBirney Model 2000 on 6-second fixed-point averaging. The velocity data, in combination with stage measured from a staff gage at the site, were used to establish a hydro graph for each sampling cross section during the study period, Using the bank benchmarks, we established four sediment measuring points at each cross section, evenly spaced at 2-m intervals across the cross section, We used a DH-48 to obtain depth-integrated suspended sediment samples, Bedload samples were collected during 2-minute intervals with a 7.6 cm X 7,6 em Helley-Smith sampler. Suspended sediment samples were filtered, dried, and weighed; sediment concentration in mgfL was calculated, Bedload samples were dried, weighed, sieved in 1 ~ intervals (range 4 ~ to ~6 ~, or 0,0625 to 64 mm), and weighed, (rp = -log2S, where S is grain size in millimeters.) ,