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Chapter 4. Sediment Transport and Deposition

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Chapter 4. Sediment Transport and Deposition CHAPTER 4. SEDIMENT TRANSPORT AND DEPOSITION INTRODUCTION Purpose An analysis was performed to characterize sediment conditions of the Lower Puyallup River, quantify sediment inflow to the study reach, assess recent changes in the river cross section due to sediment deposition and aggradation, and develop a numerical sedimentation model for forecasting future deposition along the Lower Puyallup. A HEC-RAS sediment transport model (Hydrologic Engineering Center, Version 4.0, 2006) was developed to predict the location, volume and depth of sediment deposition or erosion through the study reach. The primary objective of the sediment investigation was to provide a 50-year forecast of bed adjustments related to sediment deposits that may affect the river channel’s flood carrying capacity. The sediment model was developed and calibrated using information from surveyed channel cross sections, site specific measurements of sediment transport rates, measurements of existing bed material characteristics, and analysis of hydrologic conditions. It was then used to estimate future channel aggradation in the study area. The results provide estimates of future channel geometry, which when input into the HEC-RAS hydraulic model reveal how flood profiles are likely to be affected by sediment deposition 50 years in the future. The analysis assumes that no significant sediment maintenance activity (i.e. channel dredging) occurs during the 50-year forecast period. Prior Studies Previous sediment and hydraulic studies of the Puyallup River include the following: • Flood-Carrying Capacities and Changes in Channels of the Lower Puyallup, White, and Carbon Rivers in Western Washington (USGS, 1988) • Sediment Transport in the Lower Puyallup, White and Carbon Rivers of Western Washington (USGS, 1989) • Flood Insurance Mapping Study for Puyallup River (FEMA, 2007). Information presented in these studies includes surveyed channel cross sections, measurements of sediment transport rates, measurements of bed material characteristics and analysis of hydrologic and hydraulic conditions on the Lower Puyallup River. Definitions Sediment is present in the Puyallup River in a wide range of particle sizes. Coarse gravels and cobbles make up the majority of the channel bed upstream of the confluence with the White River. Sands, silts, and clays tend to deposit in the lowermost, tidally influenced reach of the river. The total sediment load of a stream consists of sediment from the following sources: • Wash load sediment is the finest portion of sediment, generally silt and clay, that is carried through the system with inappreciable quantities found in the channel bed. Typically wash- load sediments are derived from watershed soil erosion process (rill and gully erosion) and 4-1 Lower Puyallup River Flood Protection Investigation; Without-Project Analysis… channel bank erosion. The discharge of wash load depends primarily on the rate of supply from the watershed and is not generally correlated with flow characteristics. • Bed material load sediment is sediment found in the bed of the channel. The process of transport can be through either bed load or suspended load transport. Typically, the total magnitude of bed material load increases with increasing stream flow. Finer bed material load is transported as suspended sediment load and coarser sediments are transported as bedload. Larger bed materials (i.e. gravels and cobbles) can be mobilized as flow strength increases. The following are the two primary modes of sediment transport: • Bedload transport is sediment that is moving on or near the bed by rolling, bouncing or sliding. Movement can be either continuous or intermittent but is generally much slower than the mean velocity of the stream. In the upper Puyallup River watershed, bedload consists primarily of coarse sands, gravels and cobbles. • Suspended sediment is supported by the turbulent motion in the stream flow and is transported at a rate approaching the mean velocity of flow. In the Puyallup River watershed, suspended sediment consists primarily of fine sands, silts and clays. The boundary between bedload and suspended load can change between low flows and high flows as material that was being transported as bedload at low flows becomes suspended when velocities and turbulence increase sufficiently during high flows. EXISTING CONDITIONS River Morphology The sediment model was developed for the lower 37,800 feet of the Puyallup River, from Commencement Bay to a few miles upstream of USGS Gage 12101500 (Puyallup River at Puyallup). Lower Puyallup streambed elevations range from -10 feet (NAVD88) at the river mouth to about 25 feet (NAVD88) near the confluence with the White River (RM 10.1). Slopes range from 0.00035 feet/foot near the mouth to 0.0006 feet/foot near the White River confluence, with a break in gradient near RM 3.75. Samples indicate that bed materials in the study reach are primarily medium and fine sand with gravel. Field observations show that the median particle size decreases in the downstream direction. Surveyed Cross Sections Five sets of surveyed cross sections were used to estimate the amount of deposition and/or erosion that has occurred in the study reach of the Lower Puyallup River over the past 27 years (USGS, 1980; W&H Pacific, 2001; COE, 2001; NHC, 2002; Minister and Glaeser, 2007). Due to the dates in which data was collected, two slightly different time intervals were examined to estimate the average rates of deposition and erosion: 1980 to 2002 and 2002 to 2007 for the lower 30,000 feet of the study reach and 1980 to 2001 and 2001 to 2007 for the remainder. Survey points in the USGS 1980, W&H Pacific 2001 and NHC 2002 surveys were originally referenced to NGVD29 and were converted to NAVD88 by adding 3.49 feet. The COE 2001 survey was referenced to the mean lower low water (MLLW) elevation and was converted to NAVD88 by subtracting 2.68 feet. All surveyed data was spatially referenced to Washington State Plane South NAD 83/91. Surveyed cross section comparison plots are attached in Appendix B. Appendix C provides a channel stationing baseline indicating the channel section identifier. 4-2 …4. SEDIMENT TRANSPORT AND DEPOSITION Bed Material Bed material samples were collected throughout the study reach during a field reconnaissance in May 2007. Samples were collected by boat from the channel thalweg (the lowest-elevation point of each cross section) at approximately 1 mile increments starting in Commencement Bay and ending at the confluence with the White River. Each sample was collected with a drag sampler consisting of a 6-inch-diameter, 2-foot-long steel pipe with one end open as a cutting edge. The other end had a removable filter to allow water to drain out while lifting the sampler from the water. Each sample was taken by lowering the sampler to the channel bottom then dragging it along the channel bed. When retrieving the sampler, the boat was held stationary while the sampler was brought up slowly to ensure that no fines were washed out. Once the sampler was brought on deck, time was allowed for excess water to drain through the filter, then a representative subsample of the bed material was placed into a plastic bag which was sealed and labeled for shipment for laboratory sieve analysis. Laboratory test results are provided in Appendix D. Each of the 10 sampling sites was entered in a sample log with a corresponding handheld GPS waypoint; sampling locations are provided in Appendix C. Subsurface samples were collected to characterize the materials in transport during bed mobilization flood events (Parker 1991). Subsurface samples were analyzed for grain size distribution by Shannon & Wilson, Inc. Laboratory test results are provided in Appendix D. Figures 4-1 and 4-2 show the resultant bed material size distributions used for the HEC-RAS simulations. Figure 4-1 shows the bed material size distribution for the lower 6.87 miles. This size distribution was adopted for the sediment inflow size distribution as well. The sediment is characterized as a poorly graded sand, with a median particle diameter of 0.35 mm (medium sand). Figure 4-2 shows the bed material size distribution for the upper 3.32 miles. The sediment has a bimodal distribution containing a poorly graded fine sand and a poorly graded gravel. 60 50 40 30 20 Percent Occurrence (%) Occurrence Percent 10 0 1-2 2-4 4-8 .5-1 8-16 16-32 32-64 .25-.5 <0.125 .125-.25 Sediment Diameter (mm) Figure 4-1. Bed Material Characteristics for Lower 6.87 Miles of Study Reach 4-3 Lower Puyallup River Flood Protection Investigation; Without-Project Analysis… 60 50 40 30 20 Percent Occurrence (%) Occurrence Percent 10 0 1-2 2-4 4-8 .5-1 8-16 .25-.5 16-32 32-64 <0.125 .125-.25 Sediment Diameter (mm) Figure 4-2. Bed Material Characteristics for Upper 3.32 Miles of Study Reach Observed Sediment Load Figure 4-3 shows a sediment rating plot created from suspended sediment discharges (tons/day) and stream flow (cfs) both recorded at USGS Gage 12101500 from January 1978 through July 1994. Only the fraction of sediments coarser than 0.063 mm (very fine sand) is plotted, because this closely matches the minimum size of the sampled bed material. It is assumed that the finer silts and clays do not deposit but are carried downstream to Commencement Bay. This information was used, together with the flow duration curve presented in Chapter 2, to estimate the initial sediment load in the HEC-RAS model. 1000000 100000 10000 1000 USGS Suspended Data >0.063mm 100 Sediment Load (Tons/Day) Sediment Load 10 1 0 100 1000 10000 100000 Flow (cfs) Figure 4-3. Sediment Rating Curve for Puyallup River at Puyallup Gage (12101500), 1978—1994 4-4 …4. SEDIMENT TRANSPORT AND DEPOSITION Stage-Discharge Rating Curve Shifts Shifts in a river’s stage-discharge relationship over time are good indicators of changes in channel geometry.
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