Department of Geography & Environmental Engineering
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(Sediment) Accounting 101: An Example part of Vignettes:Vignette Collection
I. Introduction Rivers transport both water and sediment [rock fragments such as sand and gravel] from hillsides to the oceans. As sediment moves downstream it creates river bars, accumulates into floodplains, and often forms the bed of a river itself. One way to track the movement of sediment in a particular area is through a sediment budget. This is an expression of the conservation of mass for a reach [portion] of river, in which the difference between the sediment fluxes [rate of flow through a surface] into and out of the reach are balanced by a change in the mass of sediment stored within the reach. A sediment budget is not unlike a piggy bank–the change in value (storage) is determined by the difference between deposits (flux in) and withdrawals (flux out). An example of the application of a sediment budget comes from the 2007 removal of the Marmot Dam from the Sandy River, Oregon (described in the vignette titled An Example of One River's Response to a Large Dam Removal). The budget is presented here to provide a brief overview of the data collection and synthesis used in the construction of a sediment budget. II. Changes in Storage a. Remote Sensing To calculate changes in sediment storage, measurements of topography were made at multiple points in time using several instruments. Airborne LiDAR [Light Detection And Ranging] is a method of collecting large amounts of topographic data from a landscape using a laser system. An airborne platform flies multiple 'swaths' across the area of interest from an altitude of several hundred meters. By using records of the aircraft's GPS [Global Positioning System] location, the angle of the laser, and the time for the laser pulses to return to the aircraft, a set of spatial coordinates for the surface of interest is generated. For the Sandy River, there were multiple pre- and post-dam-removal LiDAR flights flown from 10 km upstream to 48 km downstream of the dam, measuring at least 2 km on either side of the river (Figure 1 shows a portion of the data collected). b. Ground Surveys In order to measure bathymetry [underwater topography], two types of ground surveys with different methods of selecting points were made (Figure 2). The first type was a cross-section-based survey using fixed reference points, allowing comparisons from one survey to the next. Points were selected along each cross section so as to define the channel geometry. This type of survey resulted in an average point density of one point every 2.4 m for cross sections spaced an average of 13 m apart (on a river with a width of 20-40 m). The second type of survey involved tracing features of geomorphic significance such as tops of banks, channel thalweg [path of the deepest flow], and the water's edge. Cross section surveys were repeated annually from 2005 through 2009, while the topographic surveys were opportunistic and based on the occurrence of large storms and the low-water season. c. Volume calculation Spatial coordinates from LiDAR and ground surveys were interpolated to create digital elevation models [DEM]. Topography changes (height and volume differences) between surveys were identified by subtracting respective DEMs. A 5-km-long reach approximately centered on the dam site was mapped using a combination of ground surveys and LiDAR, then values of sediment erosion and deposition, reflecting the spatial and temporal changes in storage, were derived (Figure 1). d. Grain Size In addition to measuring channel shape, the channel-bed texture (or grain size distribution [GSD] of the bed material) was measured. Because different grain sizes (for example sand [0.0625-2 mm] and gravel [2-64 mm]) interact with the river bed differently, the budget accounted for various size classes separately. Two types of grain size measurements were made. First, facies [areas perceived to have the same GSD] on the bed surface were mapped, and a pebble count [size measurements of a random sample of grains from the surface] was conducted. Second, a bulk sample (Figure 3) involved excavating a pit, then sieving and weighing the excavated sediment (1500-2500 kg). The results (Figure 3c) show armoring [a surface layer coarser than the subsurface layer] and a pebble count (GSD by area) that differs slightly from the surface bulk sample (GSD by weight), both of which are typical findings when comparing GSD computed from bulk samples and pebble counts. III. Sediment Fluxes The other part of the budget is the measurement of fluxes in and out of the reach. The bed of the Sandy River is composed primarily of sand and gravel. Bed material moves down the river both as suspended load [carried in suspension throughout the water column] and bedload [moving in near-continuous contact with the bed surface]. On the Sandy River, the high-discharge flows that move most of the bedload occur throughout the winter. Samples were taken during most of the high-flow events for the first two winters following the dam removal at multiple locations along the river (Figure 4) which allowed measurements of sediment moving into and out of the dam area. At each site, suspended sediment and bedload samples were collected concurrently with measurements of water discharge to produce paired values of sediment flux and water discharge. A regression curve was fit to the discharge / sediment transport relationship (Figure 4D) at each site. This regression relationship [rating curve] was used with a continuous record of water discharge at each site to calculate the annual sediment load at each site. IV. Putting it all Together–A Balancing Act A sediment budget for one reach encompassing Marmot Dam shows the increase in sediment flux due to the dam removal and the deposition of the sediment downstream of the dam (Figure 5). The sediment budget approach together with the monitoring data proved to be an effective way to measure the fate of the sediment released by the Marmot Dam removal. This study serves as just one example of how various field data can be combined with the simple concept of mass continuity to construct a sediment budget of a particular river reach.
An Example of One River's Response to a Large Dam Removal part of Vignettes:Vignette Collection
Damming a river can provide hydropower, water storage, and/or flood control, but at the same time may have adverse effects on fish and other aquatic species, flood land behind a dam, or degrade a river downstream of a dam. Many dams are undergoing a required relicensing process, during which time a dam's particular advantages and disadvantages are weighed. One way to mitigate the negative effects of dams is to remove them. However, dam removal itself can be problematic, especially when dealing with sediment stored behind the dam, which erodes and moves downstream. Marmot Dam The monitoring of the 2007 removal of Oregon's 15-m-tall Marmot Dam from the Sandy River (Figure 1), is a case study that examines the effects of removing a dam and releasing stored sediment. During the relicensing process, it was decided that the costs associated with maintaining and upgrading the Marmot Dam were higher than the benefits it generated and it was selected for removal. At the time of its removal, the 94-year-old dam was one of the tallest, and largest (in terms of stored sediment) dams to be removed in the United States. Documenting the response of the Sandy River provides valuable information for future large dam removals. This vignette provides a short description of the monitoring efforts on the Sandy River near the Marmot Dam. The data collection techniques are detailed in another vignette: (Sediment) Accounting 101: An Example. Sediment Rivers carry water and sediment [rock fragments such as sand and gravel], and dams affect the transport and storage of both. The Sandy River transports sediment ranging in sizes from fractions of a millimeter to half of a meter or larger. These sediment grains are carried suspended in the water column [suspended load] and along the bed of the river [bedload]. As the suspended load and bedload of a river encounter the slower, deeper water behind a dam, some or all of the particles stop moving and deposit on the river bed. Over the life of Marmot Dam, approximately 750,000 m3 of sediment had accumulated behind the dam, filling it nearly to the brim. At the time of removal, it was believed that this amount of sediment was roughly equivalent to 5-20 times the amount of sediment the river moved annually. The impounded sediment could be visualized as a 2.5 km long upstream-tapering wedge 15 m thick at the dam. Once the dam was removed, this reservoir sediment was once again subject to transport by the river, and had the potential to dramatically change the character of the Sandy River. Upstream Erosion The dam removal process involved constructing a small cofferdam [temporary dam] 70 meters upstream of the dam, removing the concrete dam with heavy equipment, leaving just the cofferdam atop the impounded sediment, and then breaching the cofferdam as the winter high flows arrived. Erosion started immediately upon its breaching on October 19, 2007. Within hours, the river had cut down several meters through the cofferdam and into the reservoir deposit. While the initial response was incision [the river cutting down through the sediment], the river rapidly migrated laterally [moved back and forth eroding banks] and cleared the sediment at the former dam site from bank to bank (Figure 2). A longitudinal profile of the river [elevation plotted versus downstream direction] shows the upstream progression of erosion (Figure 3). This erosion occurred by a combination of three processes: incision, lateral migration, and knickpoint migration [the moving upstream of a steep break, or step, in the bed slope]. Within one month, 25% of the reservoir sediment had been eroded, and by the end of the first year 50% had eroded. Two years after the dam was removed, the river had a similar width and similar banklines to its pre-dam state, but the bed elevation was still slightly higher than it was before the river had been dammed. Downstream Deposition Much of the eroded sediment did not travel far. As a result of the sediment released from the reservoir, the bed of the river immediately downstream of the former dam aggraded [increased in elevation] by as much as 4 m. The river initially changed from a single-channel river to a multiple-channel one due to the sediment introduced from upstream (Figure 4). By the end of the first wet season, the sediment immediately below the dam stabilized into one new large bar and the river went back into a single-channel configuration. The small sediment sizes (<2 mm) moved down the river as suspended load. Much of the larger sediment (2 mm-20 cm) was deposited in the 2 km immediately downstream of the dam. Very near the dam, the sediment deposited uniformly across the river; cross sections show this section-wide aggradation (Figure 5). However, farther downstream the sediment deposited less uniformly–primarily on top of existing gravel bars. Most of the monitoring effort focused on the reach [portion of the river] near the dam, where the large changes occurred; annual surveys 14 and 20 km below the dam failed to show change in the two years after the dam was removed. Overall Observations The Marmot Dam removal provided an opportunity to monitor the response of a portion of the Sandy River to a sudden increase in the sediment supplied to it. The reservoir bed showed three erosion processes–incision, widening, and knickpoint migration. The downstream deposition differed based on grain size and proximity to the dam. Much of the sediment quickly stabilized into the bed near the dam. Over the remainder of the river there was no measurable change in the river bed. This response is similar to pre-removal predictions using a numerical sediment transport model (Cui and Wilcox, 2008). The multi-year monitoring project has documented how the Sandy River has processed the large amount of sediment that was made available to it by the removal of the Marmot Dam and should help inform future dam removal decisions.