
small (250x250 max)
medium (500x500 max)
Large
Extra Large
large ( > 500x500)
Full Resolution


North Umpqua Hydroelectric Project FERC Project No. 1927 Douglas County, Oregon DRAFT NORTH UMPQUA COOPERATIVE WATERSHED ANALYSIS RESPONSE TO RESOURCE TEAM INFORMATION REQUEST Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal Technical Report Prepared by Stillwater Sciences Berkeley, California Prepared for PacifiCorp Portland, Oregon June 1999 DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 1 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD 1. INTRODUCTION The Dam Removal Subgroup of the North Umpqua watershed analysis Resource Team requested that Stillwater Sciences prepare a preliminary assessment of fine sediment transport prior to the 24– 25 June 1999 Resource Team meeting. Stillwater Sciences is also developing a study plan to assess the environmental effects of the removal of Soda Springs Dam in more detail, particularly the effects of releasing the sediment stored in the reservoir. This report describes the results of preliminary modeling of sand/ silt release from Soda Springs Dam removal and of the associated impacts of suspended sediment on adult salmonids. Given the time constraints in developing the model and the lack of additional field data, modeling results should be considered preliminary. Numerical modeling combines empirical data about physical processes ( e. g., sediment transport) and theoretical governing equations, typically expressed as sets of differential equations, to develop quantitative predictions of how a given system will respond under varying scenarios or conditions. For this study, numerical modeling based on sediment transport equations was used to assess the downstream routing of sediment from Soda Springs reservoir to the North Umpqua River. Although there are important uncertainties in numerical modeling, this approach provides a means to conduct predictive exercises and to make quantitative forecasts for different management options. In this preliminary study, numerical modeling is directed at characterizing the large scale response of the North Umpqua River to dam removal rather than the local sediment size adjustments; hence it portrays the channel system in a relatively simple way. Details about the influence of boulder, bank irregularities, or river bends on rates of processes are not considered. Instead, the model focuses on predicting the time rate of change of sediment discharge and storage downstream of the dam. This preliminary effort was conducted without additional field data on either grain size characteristics of the reservoir sediment or on the downstream channel bed. Instead, we used assumptions about the grain size distribution of reservoir sediment, based on surficial sediment samples, in order to assess the downstream release of the sand and finer fraction of the reservoir sediments. This includes assessment of estimated total suspended sediment ( TSS), length of time required to move fine sediments out of the system, and depositional characteristics of fine sediment released from the reservoir. Information on the concentration of suspended sediment and the duration of elevated suspended sediment levels following dam removal were used to assess mortality risks to adult salmonids. This preliminary effort does not include assessment of the release of coarse sediment from the reservoir and associated gravel transport; infiltration of sand into the downstream channel bed; or changes in the mobility of the existing channel bed downstream of Soda Springs dam. The full study plan will describe a methodology for assessing the environmental effects ( including the short term and long term effects on salmonids and other key species identified in the watershed analysis) of different dam removal scenarios using numerical and physical modeling for both coarse and fine sediment. This preliminary modeling was based on a one shot dam removal method ( i. e., Alternative 5 in the Raytheon report [ PacifiCorp 1999]) in which removal of Soda DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 2 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD Springs Dam and sediment release occur instantaneously. The model was designed to assess release of all the fine sediment in the reservoir, although model runs in which varying percentages of sediment were dredged before dam removal were also conducted to assist assessment of methods of limiting impacts on salmonids. Dam removal methods in which sediment is metered out of the dam at a controlled rate could be assessed in future modeling. 2. BRIEF DESCRIPTION OF THE FINE SEDIMENT TRANSPORT MODEL Because gravel transport and sand transport occur over different time scales ( years vs. days), it is assumed in this modeling effort that the existing gravel bed of the North Umpqua River is immobile as sand is transported downstream in a short period of time following removal of Soda Springs dam. The model used here is therefore equivalent to a model of sand transport over a rough bedrock river. The model is one dimensional with width variation in space. The cross sectional form of the river is assumed to be rectangular for modeling purposes, with width variations based on data from Instream Flow Incremental Methodology ( IFIM) transects surveyed by Harza in 1993 ( PacifiCorp 1995). Flow calculations employ the backwater equation in the case of low Froude number flows ( Fr < 0.75, where Fr denotes Froude number) and the quasi normal assumption in the case of high Froude number flows: backwater equation for ( 1) quasi normal assumption for ( 2) where h denotes water depth, S0 denotes bed slope, Sf is friction slope, and x denotes streamwise coordinates. Froude number Fr is defined as: ( 3) where u is flow velocity, g is acceleration of gravity, Qw is water discharge and B is channel width. A Keulegan type of relation is used to calculate flow resistance in the case where the bed is not covered with sand: ( 4) DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 3 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD where u* is shear velocity, 0 is the thickness of the sand deposit, Dg is geometric mean grain size of the sand, and ks is bed roughness ( which can be characterized by gravel geometric mean grain size in gravel bed rivers). The relation between shear velocity and friction slope is ( 5) If the channel bed is covered with sand, Brownlie's resistance relation is employed. Because of the relatively high slope of the North Umpqua River, only the upperregime equation is needed ( the lower regime equation applies to low gradient systems with bed forms [ e. g., dunes] in the channel bed). The upper regime Brownlie's resistance relation is as follows: ( 6) DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 4 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD where R is submerged specific weight of sand, Fg is geometric standard deviation of sand and Fg is grain Froude number, defined as: ( 7) The potential sediment transport rate is the sediment transport rate a given flow can carry when sediment supply is not limited. Brownlie's bed material equation is used: ( 8) where Qs denotes sediment transport rate ( volume per unit time), Qw denotes water discharge, cF is a coefficient ( assumed to equal 1.268 for field cases), and Fgo is critical grain Froude number, given by ( 9) ( 10) ( 11) where Rg is particle Reynolds number, defined as ( 12) and < is kinematic viscosity of water. Brownlie's equation ( Equation 8) was developed for sand bedded rivers, but it is used here because no sediment transport equations exist to calculate sand transport in a bedrock dominated river. Brownlie’s equation should provide reasonable estimates of potential sediment transport rate ( as defined above) where the bed is not covered with sand ( as well as when the bed is covered with sand), provided appropriate roughness adjustments are included. When there is enough sediment in the channel bed or from upstream supply, the actual sediment transport rate equals the potential sediment transport rate from Equation ( 8). When transport capacity exceeds supply from upstream and in the channel bed, the actual sediment transport rate is lower than the potential transport rate, resulting in erosion of the channel bed to the coarse gravel substrate present prior to the sediment pulse from the reservoir; mass conservation is also preserved. DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 5 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD The channel bed is divided into two sections vertically. In the lower section, where the thickness of the sand deposit is less than the height of bed roughness ( ), sand only fills the existing gravel interstices. The upper section, where the sand deposit is thicker than the bed roughness height ( ), is open space where sand can deposit freely. The porosity of the sand deposit is taken as a constant 8s. The porosity of the existing gravel surface is denoted as 8g. The Exner equation of sediment continuity takes the form: where ( 13) where ( 14) Mass conservation must be satisfied in the case where the thickness of the sand deposit ( 0) is changing between being less than or greater than the bed ( gravel) roughness height ( ks). Description of such a mass balance is lengthy and, because it would not provide additional insight to this modeling effort, is not included here. Effects of sand deposition below the roughness level ( i. e., infiltration into coarsesubstrate interstices) are not modeled here. Much of the bed would likely be in a state of partial filling of the gravel interstices below the top of the roughness level when Brownlie’s equation takes effect. Sand accumulations below the roughness level would reduce the thickness of sand deposits above the coarse surface layer and the TSS ( compared to those suggested by the model results presented below), but this effect is not likely to be very large. In this model, silt is treated as throughput load that is carried in suspension and cannot be deposited in the channel bed. The Soda Springs reservoir deposit, however, likely has a considerable amount of silt in it. This silt deposit is included in the model as part of the volume of eroded material from the reservoir, although as noted above, the silt fraction is treated as throughput load that does not redeposit downstream. The total suspended sediment ( TSS) includes both silt and the portion of sand that is in suspension. The criterion for suspension is set as: ( 15) where vs denotes the particle fall velocity for a given grain size, and 6 is the von Karman constant ( assumed to equal 0.4). To test whether variations in suspended sediment concentration through the water DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 6 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD column would affect the severity of impact on salmonids ( see the discussion of biological modeling below), suspended sediment concentrations in the upper half of the water column were evaluated ( in addition to assessment of TSS in the entire water column). This concentration was calculated with an exponential vertical sediment concentration distribution: ( 16) where . is dimensionless distance from the channel bottom ( equal to z/ h, where z is the upward distance from the channel bed), . b is a near bed reference location normally set as 0.05, C is sediment concentration of a specific grain size as a function of ., Cb is the value of C at . b. The evaluation of suspended sediment concentration in the upper half of the water column involves integration of ( 16) from . b to 1 to determine Cb and then integration of ( 16) from 0.5 to 1. Factors such as cohesion of the sediment deposit are not accounted for in the model. Sediment release out of the reservoir is completely dependent on the transport capacity of the water flowing through the reservoir area. This transport capacity is calculated with Brownlie's equation ( Equation 8). The sediment transport out of the reservoir is assumed to be laterally uniform, given the high degree of confinement of the reservoir reach. It is possible that sediment transport would occur by cutting of a channel through the reservoir deposit rather than by laterally uniform transport. 3. PRELIMINARY RESULTS OF MODELING OF FINE SEDIMENT RELEASE The results of the model depend heavily on the grain size distribution of the reservoir sediment deposit. Currently no data are available on the grain size distribution of the entire reservoir deposit. Stillwater Sciences has proposed a plan to characterize the grain size distribution of the entire reservoir sediment deposit based on collection of sediment core samples; these data will be necessary for more accurate modeling of the effects of sediment release from Soda Spring reservoir ( see Soda Springs Dam Removal Study Plan). Grain size distribution data are currently available only for the upper 0.6 m ( 2 ft) of the reservoir sediment, based on samples collected by Harza Northwest, Inc. in 1993. These data were used in the preliminary modeling effort described here. Figure 1 shows the the grain size distribution curves for the surficial samples recovered in the 1993 survey, as well as a curve constructed by Stillwater Sciences based on the average of the Harza samples. This constructed curve was used in our modeling to represent the assumed grain size distribution of sediment in Soda Springs reservoir. The constructed sediment distribution, referred to below as the “ Average” size distribution scenario, has a geometric mean grain size ( Dg) of 0.7 mm and a geometric standard deviation of 1.4. Two additional grain size distribution scenarios were developed in which the grain size is one standard deviation coarser and one standard deviation finer than the constructed grain size. The “ Average,” “ Coarser,” and “ Finer” grain size distributions are shown in Figure 2. It is assumed DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 7 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD for modeling purposes that 30% of the sediment deposit in the reservoir is silt and 70% is sand; this assumption is arbitrary and omits the gravel fraction of the reservoir sediment, which is not considered in the preliminary modeling. Discharge is another important factor in modeling the downstream transport of the reservoir sediment. For operational reasons, removal of Soda Springs dam would need to occur at discharges below 1,200 cfs; average flows are below this level between July and October. Dam removal during October would likely have the least impact on aquatic species ( compared with dam removal earlier in the summer). We therefore used a typical October discharge record ( from 1953) from the USGS North Umpqua above Copeland gaging station for simulation purposes; discharge records from this station show little variation from year to year in October. The discharge record for October 1953 is shown in Figure 3. In the model runs presented below, the modeled reach extends 16 km downstream of Soda Springs dam ( the Steamboat Creek junction is about 27 km downstream of the dam). The longitudinal profile of the modeled reach prior to release of the reservoir deposit, based on a 1915 USGS long profile of the North Umpqua River, is shown in Figure 4. A modeling test run was conducted for a longer reach, extending 50 km downstream of the dam, using an assumed average slope of 0.007 and a channel width of 35 m. In this test run, no deposition was found beyond 15 km downstream of the dam. Simulations were performed for the three grain size distributions ( Fig. 1) for 30 days from the time of reservoir draw down, except for Run 3 ( finer grain size distribution), in which all the sand and silt was flushed out of the modeled reach in 28 days from reservoir draw down. A test run suggests that the amount of sediment flushed out of the reservoir during the draw down period is small, indicating that the time allowed for demolition preparation will not affect the simulation. For sediment modeling purposes, we assumed a 7 day period between reservoir draw down and dam removal ( i. e., dam removal occurs on day 7 of the model run). The results of each model run are summarized below. Run 1: Average Grain Size The results for this run are shown in Figures 5– 8. The sand deposit was cleared from the reservoir in 13 days following reservoir draw down and in 6 days after removal of the dam. The sand released from the reservoir was almost entirely transported out of the modeled reach in 30 days counting from reservoir draw down. TSS during the reservoir drawdown period remained below 500 ppm, increased to more than 20,000 ppm after the dam is removed, and then gradually decreased in time. The suspended sediment concentration in the upper half of the water column is slightly less than TSS. Run 2: Coarser Grain Size The results for this run are shown in Figures 9– 11. A slightly longer period was required for downstream transport of the sand deposit than in Run 1, reflecting the effects of the coarser grainsize distribution in this run. The sand deposit was flushed out of the reservoir in 15 days from the beginning of reservoir draw down ( 8 days after removal of the dam). DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 8 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD Run 3: Finer Grain Size The results for this run are shown in Figures 12– 14. The sand deposit flushed out of the reservoir in 11 days counting from reservoir draw down, or 4 days after removal. The sand released from the reservoir was cleared out of the modeled reach in 28 days counting from reservoir draw down. Suspended sediment concentrations were about the same in all the runs, with high concentrations lasting a shorter period of time for finer grainsize distributions ( Figs. 15, 16). In order to assess the effect of releasing sediment under higher flow conditions, model runs were also conducted based on a flow regime that varied from about 1,300 to 2,000 cfs ( a flow of 1,500 cfs has an exceedence probability of about 40% at the Copeland gauge). This discharge simulation was based on a flow regime starting March 1, 1956 and was conducted for all three grain size distribution scenarios. These model runs suggest that the effects of higher discharges during and following dam removal are as follows: ( 1) the amount of time required for sand to be cleared from the reservoir and the modeled reach is decreased, ( 2) TSS values are higher immediately following dam removal than with lower discharges but dissipate more rapidly, and ( 3) maximum sand thickness is not substantially different compared to lower discharge conditions. Graphics describing the results of the higher discharge model runs are not included here but are available on request from Stillwater Sciences. In addition, model runs ( based on the October discharge, as in Runs 1– 3) were conducted in which varying percentages of the reservoir sediment were dredged before removal of Soda Springs dam. These model runs were carried out to assess the potential for limiting the downstream impacts of sediment release from the reservoir. As would be expected, these model runs indicate that progressive increases in the amount of sediment dredged from the reservoir before dam removal result in progressive decreases in the thickness of sand accumulations and in TSS levels. The effects of pre removal sediment dredging on suspended sediment impacts to adult salmonids are discussed below. As with the higher discharge model runs, graphics describing the results of the pre removal dredging model runs are not included here but are available on request from Stillwater Sciences. 4. PRELIMINARY CONCLUSIONS OF MODELING OF FINE SEDIMENT RELEASE The preliminary conclusions of the sediment modeling effort are summarized as follows: C The fine sediment deposit would be flushed out of the reservoir in 4– 8 days following dam removal, depending on the assumed grain size distribution of the reservoir sediment, with some sediment flushing occurring before removal during drawdown of the reservoir. C Sand accumulations above the existing channel bed would be mostly cleared from the modeled reach ( which extends about 16 km [ 10 mi] downstream of Soda Springs dam) in about 3– 4 weeks, with variations according to the assumed grain size distribution of the reservoir sediment. DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 9 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD C The reach affected by sand deposition is likely to be short ( less than about 20 km [ 12 mi] from the dam). C Total Suspended Sediment ( TSS) levels will likely be high for a short period of time. Modeling indicates that peak TSS of over 20,000 ppm would occur immediately following dam removal, with subsequent decreases with time. The effects of such TSS levels on adult salmonids are discussed below. C Fine sediment accumulations are most likely to occur in wider reaches of the river where sediment transport capacity is lower. Such accumulations, however, will likely be small and will be cleared in a short period of time. C Higher discharge conditions immediately following dam removal would shorten the period in which impacts of high TSS levels occur. 5. MORE SPECULATIONS ON FINE SEDIMENT RELEASE In addition to the preliminary results presented above, a number of speculations were developed about the potential transport and deposition of fine sediment from Soda Springs reservoir following dam removal: C Sand deposits above the existing gravel surface layer will likely be cleared in a short period of time, given the high transport capacity in the North Umpqua River. Sand that deposits further down in the gravel interstices may have a longer residence time and may alter the mobility of the existing coarse substrate layer. This will need to be verified by physical modeling. C It is likely that the sand/ silt deposit in Soda Springs reservoir will be cleared in a short period of time without leaving a sand wall. This will need to be verified by field observations of existing deposits and possibly physical modeling. C Although sand deposition would not be expected downstream of the modeled reach ( except locally in areas where the channel is wider and has a lower slope), suspended sediment impacts ( TSS) will propagate down to the ocean. TSS levels will decrease as tributaries add water to the North Umpqua River, and the TSS will be inversely proportional to discharge. Incorporation of local variabilities in slope, channel width, and roughness into the modeling effort might change the results to some extent. Some sand deposition could occur in wider reaches or areas with momentum defects beyond the 16 km reach modeled here, but the general trend of sediment transport and deposition modeled here would likely not change substantially. Sediment deposition would not be expected to occur downstream of the modeled reach because sediment supply is reduced compared to transport capacity with increasing distance downstream from the dam ( following dam removal and release of sediment). Sediment deposition and/ or transport at any location are dependent on the difference of sediment supply to that location ( influx) and sediment transport capacity ( outflux). In the case of Soda Springs dam removal, as the reservoir sediment washed downstream, sediment supply ( influx) to downstream reaches would be higher than transport capacity ( outflux), resulting in deposition of sediment on the channel bed. Supply would only be expected to exceed transport capacity for a short reach downstream of the dam, however, given the high transport capacity of the North Umpqua River. Further downstream of the dam, the sediment supply decreases to less the maximum amount that the flow can carry ( i. e., actual transport rate is less than potential sediment transport rate and sediment transport becomes supply limited). In such reaches, the DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 10 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD influx and outflux of sediment are the same as what is supplied from upstream ( i. e., all sediment remains in transport), and deposition does not occur. The results and conclusions of this modeling effort are preliminary and contain considerable uncertainty. For example, sediment flushing from the reservoir could occur more slowly than indicated by this modeling if sediment transport occurred by cutting a channel through the reservoir sediment deposit instead of by laterally uniform sediment transport, as assumed in the modeling This and other assumptions and conclusions of numerical modeling could be tested by physical modeling ( i. e., creating a scale model of the reservoir and downstream reach). Additional information on the grain size distribution of the reservoir sediment, numerical modeling that accounts for the gravel fraction of the reservoir sediment and for different dam removal methods, and physical modeling would be necessary to fully predict the effects of releasing sediment from Soda Springs reservoir. 6. MODELING OF SUSPENDED SEDIMENT EFFECTS ON ADULT FISHES Based on the results of the modeling of fine sediment release described above, Stillwater Sciences developed quantitative estimates of the effects of suspended sediment on adult salmonids in the North Umpqua River following dam removal. This modeling is based on a series of papers in which Newcombe and various co authors define a semi quantitative fourteen point scale for " severity of impact" ( SEV) of suspended sediment on fishes, and develop models for this quantity in terms of suspended sediment concentration ( C) and duration of exposure ( D) ( Newcombe 1986, Newcombe and MacDonald 1991, Newcombe and Jensen 1996). By collecting and analyzing the results of a wide range of studies of the effects of suspended sediment on fishes, Newcombe and his co authors found a highly significant relationship between severity of impact ( SEV) and the natural logarithm of the concentration duration product. The severity of impact ranking scale and descriptions of associated effects are shown in Table 1. The concentration ( C) duration ( D) models developed by Newcombe and Jensen ( 1996) and Newcombe and MacDonald ( 1991) have the forms and . It turns out that these model forms are indistinguishable in practice. Fitting a model of the first form to data on adult salmonids from the literature, as reported in Newcombe and Jensen ( 1996), results in the model , where SI is the natural logarithm of the concentration duration product. For concentrations varying in time, the concentration duration product corresponds to the integral of concentration over the exposure period. Stillwater Sciences combined the data on suspended sediment concentration and duration of exposure developed in the numerical modeling of fine sediment release ( described above) with the data and techniques of Newcombe and Jensen ( 1996) to generate estimates of mortality to adult salmonids that could result from dam removal. We also assessed predicted mortality under various scenarios of sediment dredging from the reservoir prior to dam removal. The results of the impact assessments are shown in Table 2, which shows the basic mortality prediction for DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 11 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD various sediment concentration duration integrals, as well as the upper and lower 90 percent confidence intervals for these predictions. Categories 10 through 14 of the severity of impact scale are defined as ranges of direct mortality; values in these categories have been converted to percent mortality values in Table 2. While the modeled relationship between SEV and SI is highly significant ( p< 10 12), it also leaves a lot of variability in the data unexplained ( r2= 0.59). A prediction confidence interval is . The results shown in Table 2 suggest that for one shot dam removal with no prior dredging, 29% mortality of adult salmonids is predicted, although the 90% confidence interval ranges from 0% to 100% mortality ( i. e., as much as an entire year class of adult fish could be lost due to suspended sediment impacts). These results also indicate that dredging before dam removal has only a limited effect on reducing potential mortality. Even if dredging of all the sediment from the reservoir is attempted, some mortality could occur because it is assumed that maximum dredging efficiency would be approximately 90% ( i. e., 10% of the sediment accumulation would remain). The Newcombe and Jensen ( 1996) modeled relationship between severity of impact and sediment concentration and duration leaves substantial variability in the data unexplained, which accounts for the wide range in the 90% confidence intervals shown in Table 2. The scatter in the data is probably due to a great many factors, none of which are well enough understood at this time to incorporate into the analysis. For example, the tolerance of fish to suspended sediment has been shown to depend on the size of the suspended particles and water temperature, and is likely to depend on particle shape and composition as well. Despite the uncertainty in the model, it represents the best available method for quantifying potential mortality risks associated with suspended sediment impacts on adult salmonids and is based on a wide range of studies from peer reviewed scientific literature. It does not account for other types of impacts ( e. g., infiltration of fine sediment into spawning gravels) or possible long term benefits of dam removal on downstream sediment dynamics; these potential impacts and benefits would require further study. REFERENCES Newcombe, C. P. 1986. Suspended sediments in aquatic ecosystems: a guide to impact assessment. British Columbia Ministry of Environment and Parks, Waste Management Branch, Victoria. Newcombe, C. P., and J. O. T. Jensen. 1996. Channel suspended sediment and fisheries: a synthesis for quantitative assessment of risk and impact. North American Journal of Fisheries Management 16: 693 727. Newcombe, C. P., and D. D. MacDonald. 1991. Effects of suspended sediments on DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 12 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD aquatic ecosystems. North American Journal of Fisheries Management 11: 72 82. PacifiCorp. 1995. Final Technical Report for aquatic resources. North Umpqua Hydroelectric Project, FERC Project No. 1927, Douglas County, Oregon. Portland, Oregon. PacifiCorp. 1999. Soda Springs Dam removal technical feasibility study. Prepared by Raytheon Engineers & Constructors. Portland, Oregon. 0 20 40 60 80 100 120 10 1 0.1 Grain Size ( mm) % Finer SR 1 SR 8 SR 9 SR 10 SR 11 SR 12 SR 13 SR 14 SR 15 SR 16 SR 17 Average Figure 1. Grain size distributions of sediment samples collected from the upper 0.6 m of the Soda Springs reservoir deposit ( PacifiCorp 1995) and a constructed average size distribution based on these curves. 0 20 40 60 80 100 120 10 1 0.1 Grain Size ( mm) % Finer Constructed Coarser Constructed Finer Constructed Original Figure 2. Constructed grain size distribution scenarios used in modeling. 0 500 1000 1500 2000 2500 3000 3500 4000 4500 1 Mar 21 Mar 10 Apr 30 Apr 20 May 9 Jun 29 Jun 19 Jul 8 Aug 28 Aug 17 Sep 1956 Discharge ( cfs) Figure 3. Discharge at USGS Copeland Station, North Umpqua River, 3/ 1  9/ 30, 1956 400 420 440 460 480 500 520 540 560 580  3 0 3 6 9 12 15 Distance ( km) Bed Elevation ( m) Dam Sand Deposit Figure 4. Long profile of the modeled reach, based on USGS 1915 long profile Fig. 5 Long profile, Average grain size scenario, low discharge 400 420 440 460 480 500 520 540 560  3 0 3 6 9 12 15 Distance ( km) Bed Elevation ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 6 Thickness of sand deposit, Average grain size scenario, low discharge 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 7 TSS value, Average grain size with low discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 8 Water depth, Average grain size with low discharge scenario 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1  3 0 3 6 9 12 15 Distance ( km) Water Depth ( m) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 9 Thickness of sand deposit, Coarser grain size with low discharge scenario 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 16 day 17 day 18 day 19 day 20 day 21 day 21.4 day Fig. 10 TSS value, Coarser grain size with low discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 16 day 17 day 18 day 19 day 20 day 21 day Fig. 11 Thickness of sand deposit, Finer grain size with low discharge scenario 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 9.8 day Fig. 12 TSS value, Finer grain size with low discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 9.8 day Fig. 13 Maximum TSS value under different grain size scenarios with low discharge 100 1000 10000 100000 0 100 200 300 400 500 600 Time ( hours) Maximum TSS in the River ( ppm) Average Grain Size Finer Coarser Fig. 14 Thickness of sand deposit, Average grain size with moderate discharge scenario 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 0.5 day 1.0 day 1.5 day 2.0 day 2.5 day 3.0 day 3.5 day 3.6 day Fig. 15 TSS value, Average grain size with moderate discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 0.5 day 1.0 day 1.5 day 2.0 day 2.5 day 3.0 day 3.5 day 3.6 day Fig. 16 Water depth, Average grain size with moderate discharge scenario 0.3 0.5 0.7 0.9 1.1 1.3 1.5  3 0 3 6 9 12 15 Distance ( km) Water Depth ( m) 0.5 day 1.0 day 1.5 day 2.0 day 2.5 day 3.0 day 3.5 day 3.6 day Fig. 17 Maximum TSS value, Average grain size scenario for low and moderate flows 100 1000 10000 100000 0 50 100 150 200 250 300 350 400 Time ( hours) Maximum TSS in the River ( ppm) Flow starting March 1, 1956 ( low) Flow starting May 20, 1956 ( moderate) Fig. 18 Maximum thickness of sand deposit and the time it occurred 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 Distance ( km) Maximum Thickness ( m) 0 100 200 300 400 500 600 Time of Maximum Thickness ( hours) Run 1 Thickness Run 2 Thickness Run 3 Thickness Run 4 Thickness Run 1 Time Run 2 Time Run 3 Time Run 4 Time
Click tabs to swap between content that is broken into logical sections.
Rating  
Title  Preliminary modeling of sand/silt release from Soda Springs Reservoir in the event of dam removal draft North Umpqua cooperative watershed analysis response to resource team information request, technical report 
Subject  Sediment transportOregonNorth Umpqua RiverMathematical models.; Suspended sedimentsEnvironmental aspectsOregonNorth Umpqua River.; Dam retirementEnvironmental aspectsOregonNorth Umpqua River.; SalmonidaeEffect of sediments onOregonNorth Umpqua River.; North Umpqua River (Or.)Environmental conditions.; North Umpqua River (Or.)Effect of sediments on.; G288 N99 Web Resource 
Description  "June 1999."; Includes bibliographical references (p. 1112). 
Publisher  Stillwater Sciences 
Type  Text 
Language  eng 
Relation  http://www.stillwatersci.com/resources/1999sodaspringsreport.pdf; http://worldcat.org/oclc/613722865/viewonline 
DateIssued  1999 
FormatExtent  31 p. : digital PDF file, col. ill., charts ; 372.61 KB. 
RelationRequires  Mode of access: World Wide Web. 
Transcript  North Umpqua Hydroelectric Project FERC Project No. 1927 Douglas County, Oregon DRAFT NORTH UMPQUA COOPERATIVE WATERSHED ANALYSIS RESPONSE TO RESOURCE TEAM INFORMATION REQUEST Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal Technical Report Prepared by Stillwater Sciences Berkeley, California Prepared for PacifiCorp Portland, Oregon June 1999 DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 1 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD 1. INTRODUCTION The Dam Removal Subgroup of the North Umpqua watershed analysis Resource Team requested that Stillwater Sciences prepare a preliminary assessment of fine sediment transport prior to the 24– 25 June 1999 Resource Team meeting. Stillwater Sciences is also developing a study plan to assess the environmental effects of the removal of Soda Springs Dam in more detail, particularly the effects of releasing the sediment stored in the reservoir. This report describes the results of preliminary modeling of sand/ silt release from Soda Springs Dam removal and of the associated impacts of suspended sediment on adult salmonids. Given the time constraints in developing the model and the lack of additional field data, modeling results should be considered preliminary. Numerical modeling combines empirical data about physical processes ( e. g., sediment transport) and theoretical governing equations, typically expressed as sets of differential equations, to develop quantitative predictions of how a given system will respond under varying scenarios or conditions. For this study, numerical modeling based on sediment transport equations was used to assess the downstream routing of sediment from Soda Springs reservoir to the North Umpqua River. Although there are important uncertainties in numerical modeling, this approach provides a means to conduct predictive exercises and to make quantitative forecasts for different management options. In this preliminary study, numerical modeling is directed at characterizing the large scale response of the North Umpqua River to dam removal rather than the local sediment size adjustments; hence it portrays the channel system in a relatively simple way. Details about the influence of boulder, bank irregularities, or river bends on rates of processes are not considered. Instead, the model focuses on predicting the time rate of change of sediment discharge and storage downstream of the dam. This preliminary effort was conducted without additional field data on either grain size characteristics of the reservoir sediment or on the downstream channel bed. Instead, we used assumptions about the grain size distribution of reservoir sediment, based on surficial sediment samples, in order to assess the downstream release of the sand and finer fraction of the reservoir sediments. This includes assessment of estimated total suspended sediment ( TSS), length of time required to move fine sediments out of the system, and depositional characteristics of fine sediment released from the reservoir. Information on the concentration of suspended sediment and the duration of elevated suspended sediment levels following dam removal were used to assess mortality risks to adult salmonids. This preliminary effort does not include assessment of the release of coarse sediment from the reservoir and associated gravel transport; infiltration of sand into the downstream channel bed; or changes in the mobility of the existing channel bed downstream of Soda Springs dam. The full study plan will describe a methodology for assessing the environmental effects ( including the short term and long term effects on salmonids and other key species identified in the watershed analysis) of different dam removal scenarios using numerical and physical modeling for both coarse and fine sediment. This preliminary modeling was based on a one shot dam removal method ( i. e., Alternative 5 in the Raytheon report [ PacifiCorp 1999]) in which removal of Soda DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 2 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD Springs Dam and sediment release occur instantaneously. The model was designed to assess release of all the fine sediment in the reservoir, although model runs in which varying percentages of sediment were dredged before dam removal were also conducted to assist assessment of methods of limiting impacts on salmonids. Dam removal methods in which sediment is metered out of the dam at a controlled rate could be assessed in future modeling. 2. BRIEF DESCRIPTION OF THE FINE SEDIMENT TRANSPORT MODEL Because gravel transport and sand transport occur over different time scales ( years vs. days), it is assumed in this modeling effort that the existing gravel bed of the North Umpqua River is immobile as sand is transported downstream in a short period of time following removal of Soda Springs dam. The model used here is therefore equivalent to a model of sand transport over a rough bedrock river. The model is one dimensional with width variation in space. The cross sectional form of the river is assumed to be rectangular for modeling purposes, with width variations based on data from Instream Flow Incremental Methodology ( IFIM) transects surveyed by Harza in 1993 ( PacifiCorp 1995). Flow calculations employ the backwater equation in the case of low Froude number flows ( Fr < 0.75, where Fr denotes Froude number) and the quasi normal assumption in the case of high Froude number flows: backwater equation for ( 1) quasi normal assumption for ( 2) where h denotes water depth, S0 denotes bed slope, Sf is friction slope, and x denotes streamwise coordinates. Froude number Fr is defined as: ( 3) where u is flow velocity, g is acceleration of gravity, Qw is water discharge and B is channel width. A Keulegan type of relation is used to calculate flow resistance in the case where the bed is not covered with sand: ( 4) DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 3 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD where u* is shear velocity, 0 is the thickness of the sand deposit, Dg is geometric mean grain size of the sand, and ks is bed roughness ( which can be characterized by gravel geometric mean grain size in gravel bed rivers). The relation between shear velocity and friction slope is ( 5) If the channel bed is covered with sand, Brownlie's resistance relation is employed. Because of the relatively high slope of the North Umpqua River, only the upperregime equation is needed ( the lower regime equation applies to low gradient systems with bed forms [ e. g., dunes] in the channel bed). The upper regime Brownlie's resistance relation is as follows: ( 6) DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 4 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD where R is submerged specific weight of sand, Fg is geometric standard deviation of sand and Fg is grain Froude number, defined as: ( 7) The potential sediment transport rate is the sediment transport rate a given flow can carry when sediment supply is not limited. Brownlie's bed material equation is used: ( 8) where Qs denotes sediment transport rate ( volume per unit time), Qw denotes water discharge, cF is a coefficient ( assumed to equal 1.268 for field cases), and Fgo is critical grain Froude number, given by ( 9) ( 10) ( 11) where Rg is particle Reynolds number, defined as ( 12) and < is kinematic viscosity of water. Brownlie's equation ( Equation 8) was developed for sand bedded rivers, but it is used here because no sediment transport equations exist to calculate sand transport in a bedrock dominated river. Brownlie’s equation should provide reasonable estimates of potential sediment transport rate ( as defined above) where the bed is not covered with sand ( as well as when the bed is covered with sand), provided appropriate roughness adjustments are included. When there is enough sediment in the channel bed or from upstream supply, the actual sediment transport rate equals the potential sediment transport rate from Equation ( 8). When transport capacity exceeds supply from upstream and in the channel bed, the actual sediment transport rate is lower than the potential transport rate, resulting in erosion of the channel bed to the coarse gravel substrate present prior to the sediment pulse from the reservoir; mass conservation is also preserved. DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 5 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD The channel bed is divided into two sections vertically. In the lower section, where the thickness of the sand deposit is less than the height of bed roughness ( ), sand only fills the existing gravel interstices. The upper section, where the sand deposit is thicker than the bed roughness height ( ), is open space where sand can deposit freely. The porosity of the sand deposit is taken as a constant 8s. The porosity of the existing gravel surface is denoted as 8g. The Exner equation of sediment continuity takes the form: where ( 13) where ( 14) Mass conservation must be satisfied in the case where the thickness of the sand deposit ( 0) is changing between being less than or greater than the bed ( gravel) roughness height ( ks). Description of such a mass balance is lengthy and, because it would not provide additional insight to this modeling effort, is not included here. Effects of sand deposition below the roughness level ( i. e., infiltration into coarsesubstrate interstices) are not modeled here. Much of the bed would likely be in a state of partial filling of the gravel interstices below the top of the roughness level when Brownlie’s equation takes effect. Sand accumulations below the roughness level would reduce the thickness of sand deposits above the coarse surface layer and the TSS ( compared to those suggested by the model results presented below), but this effect is not likely to be very large. In this model, silt is treated as throughput load that is carried in suspension and cannot be deposited in the channel bed. The Soda Springs reservoir deposit, however, likely has a considerable amount of silt in it. This silt deposit is included in the model as part of the volume of eroded material from the reservoir, although as noted above, the silt fraction is treated as throughput load that does not redeposit downstream. The total suspended sediment ( TSS) includes both silt and the portion of sand that is in suspension. The criterion for suspension is set as: ( 15) where vs denotes the particle fall velocity for a given grain size, and 6 is the von Karman constant ( assumed to equal 0.4). To test whether variations in suspended sediment concentration through the water DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 6 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD column would affect the severity of impact on salmonids ( see the discussion of biological modeling below), suspended sediment concentrations in the upper half of the water column were evaluated ( in addition to assessment of TSS in the entire water column). This concentration was calculated with an exponential vertical sediment concentration distribution: ( 16) where . is dimensionless distance from the channel bottom ( equal to z/ h, where z is the upward distance from the channel bed), . b is a near bed reference location normally set as 0.05, C is sediment concentration of a specific grain size as a function of ., Cb is the value of C at . b. The evaluation of suspended sediment concentration in the upper half of the water column involves integration of ( 16) from . b to 1 to determine Cb and then integration of ( 16) from 0.5 to 1. Factors such as cohesion of the sediment deposit are not accounted for in the model. Sediment release out of the reservoir is completely dependent on the transport capacity of the water flowing through the reservoir area. This transport capacity is calculated with Brownlie's equation ( Equation 8). The sediment transport out of the reservoir is assumed to be laterally uniform, given the high degree of confinement of the reservoir reach. It is possible that sediment transport would occur by cutting of a channel through the reservoir deposit rather than by laterally uniform transport. 3. PRELIMINARY RESULTS OF MODELING OF FINE SEDIMENT RELEASE The results of the model depend heavily on the grain size distribution of the reservoir sediment deposit. Currently no data are available on the grain size distribution of the entire reservoir deposit. Stillwater Sciences has proposed a plan to characterize the grain size distribution of the entire reservoir sediment deposit based on collection of sediment core samples; these data will be necessary for more accurate modeling of the effects of sediment release from Soda Spring reservoir ( see Soda Springs Dam Removal Study Plan). Grain size distribution data are currently available only for the upper 0.6 m ( 2 ft) of the reservoir sediment, based on samples collected by Harza Northwest, Inc. in 1993. These data were used in the preliminary modeling effort described here. Figure 1 shows the the grain size distribution curves for the surficial samples recovered in the 1993 survey, as well as a curve constructed by Stillwater Sciences based on the average of the Harza samples. This constructed curve was used in our modeling to represent the assumed grain size distribution of sediment in Soda Springs reservoir. The constructed sediment distribution, referred to below as the “ Average” size distribution scenario, has a geometric mean grain size ( Dg) of 0.7 mm and a geometric standard deviation of 1.4. Two additional grain size distribution scenarios were developed in which the grain size is one standard deviation coarser and one standard deviation finer than the constructed grain size. The “ Average,” “ Coarser,” and “ Finer” grain size distributions are shown in Figure 2. It is assumed DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 7 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD for modeling purposes that 30% of the sediment deposit in the reservoir is silt and 70% is sand; this assumption is arbitrary and omits the gravel fraction of the reservoir sediment, which is not considered in the preliminary modeling. Discharge is another important factor in modeling the downstream transport of the reservoir sediment. For operational reasons, removal of Soda Springs dam would need to occur at discharges below 1,200 cfs; average flows are below this level between July and October. Dam removal during October would likely have the least impact on aquatic species ( compared with dam removal earlier in the summer). We therefore used a typical October discharge record ( from 1953) from the USGS North Umpqua above Copeland gaging station for simulation purposes; discharge records from this station show little variation from year to year in October. The discharge record for October 1953 is shown in Figure 3. In the model runs presented below, the modeled reach extends 16 km downstream of Soda Springs dam ( the Steamboat Creek junction is about 27 km downstream of the dam). The longitudinal profile of the modeled reach prior to release of the reservoir deposit, based on a 1915 USGS long profile of the North Umpqua River, is shown in Figure 4. A modeling test run was conducted for a longer reach, extending 50 km downstream of the dam, using an assumed average slope of 0.007 and a channel width of 35 m. In this test run, no deposition was found beyond 15 km downstream of the dam. Simulations were performed for the three grain size distributions ( Fig. 1) for 30 days from the time of reservoir draw down, except for Run 3 ( finer grain size distribution), in which all the sand and silt was flushed out of the modeled reach in 28 days from reservoir draw down. A test run suggests that the amount of sediment flushed out of the reservoir during the draw down period is small, indicating that the time allowed for demolition preparation will not affect the simulation. For sediment modeling purposes, we assumed a 7 day period between reservoir draw down and dam removal ( i. e., dam removal occurs on day 7 of the model run). The results of each model run are summarized below. Run 1: Average Grain Size The results for this run are shown in Figures 5– 8. The sand deposit was cleared from the reservoir in 13 days following reservoir draw down and in 6 days after removal of the dam. The sand released from the reservoir was almost entirely transported out of the modeled reach in 30 days counting from reservoir draw down. TSS during the reservoir drawdown period remained below 500 ppm, increased to more than 20,000 ppm after the dam is removed, and then gradually decreased in time. The suspended sediment concentration in the upper half of the water column is slightly less than TSS. Run 2: Coarser Grain Size The results for this run are shown in Figures 9– 11. A slightly longer period was required for downstream transport of the sand deposit than in Run 1, reflecting the effects of the coarser grainsize distribution in this run. The sand deposit was flushed out of the reservoir in 15 days from the beginning of reservoir draw down ( 8 days after removal of the dam). DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 8 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD Run 3: Finer Grain Size The results for this run are shown in Figures 12– 14. The sand deposit flushed out of the reservoir in 11 days counting from reservoir draw down, or 4 days after removal. The sand released from the reservoir was cleared out of the modeled reach in 28 days counting from reservoir draw down. Suspended sediment concentrations were about the same in all the runs, with high concentrations lasting a shorter period of time for finer grainsize distributions ( Figs. 15, 16). In order to assess the effect of releasing sediment under higher flow conditions, model runs were also conducted based on a flow regime that varied from about 1,300 to 2,000 cfs ( a flow of 1,500 cfs has an exceedence probability of about 40% at the Copeland gauge). This discharge simulation was based on a flow regime starting March 1, 1956 and was conducted for all three grain size distribution scenarios. These model runs suggest that the effects of higher discharges during and following dam removal are as follows: ( 1) the amount of time required for sand to be cleared from the reservoir and the modeled reach is decreased, ( 2) TSS values are higher immediately following dam removal than with lower discharges but dissipate more rapidly, and ( 3) maximum sand thickness is not substantially different compared to lower discharge conditions. Graphics describing the results of the higher discharge model runs are not included here but are available on request from Stillwater Sciences. In addition, model runs ( based on the October discharge, as in Runs 1– 3) were conducted in which varying percentages of the reservoir sediment were dredged before removal of Soda Springs dam. These model runs were carried out to assess the potential for limiting the downstream impacts of sediment release from the reservoir. As would be expected, these model runs indicate that progressive increases in the amount of sediment dredged from the reservoir before dam removal result in progressive decreases in the thickness of sand accumulations and in TSS levels. The effects of pre removal sediment dredging on suspended sediment impacts to adult salmonids are discussed below. As with the higher discharge model runs, graphics describing the results of the pre removal dredging model runs are not included here but are available on request from Stillwater Sciences. 4. PRELIMINARY CONCLUSIONS OF MODELING OF FINE SEDIMENT RELEASE The preliminary conclusions of the sediment modeling effort are summarized as follows: C The fine sediment deposit would be flushed out of the reservoir in 4– 8 days following dam removal, depending on the assumed grain size distribution of the reservoir sediment, with some sediment flushing occurring before removal during drawdown of the reservoir. C Sand accumulations above the existing channel bed would be mostly cleared from the modeled reach ( which extends about 16 km [ 10 mi] downstream of Soda Springs dam) in about 3– 4 weeks, with variations according to the assumed grain size distribution of the reservoir sediment. DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 9 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD C The reach affected by sand deposition is likely to be short ( less than about 20 km [ 12 mi] from the dam). C Total Suspended Sediment ( TSS) levels will likely be high for a short period of time. Modeling indicates that peak TSS of over 20,000 ppm would occur immediately following dam removal, with subsequent decreases with time. The effects of such TSS levels on adult salmonids are discussed below. C Fine sediment accumulations are most likely to occur in wider reaches of the river where sediment transport capacity is lower. Such accumulations, however, will likely be small and will be cleared in a short period of time. C Higher discharge conditions immediately following dam removal would shorten the period in which impacts of high TSS levels occur. 5. MORE SPECULATIONS ON FINE SEDIMENT RELEASE In addition to the preliminary results presented above, a number of speculations were developed about the potential transport and deposition of fine sediment from Soda Springs reservoir following dam removal: C Sand deposits above the existing gravel surface layer will likely be cleared in a short period of time, given the high transport capacity in the North Umpqua River. Sand that deposits further down in the gravel interstices may have a longer residence time and may alter the mobility of the existing coarse substrate layer. This will need to be verified by physical modeling. C It is likely that the sand/ silt deposit in Soda Springs reservoir will be cleared in a short period of time without leaving a sand wall. This will need to be verified by field observations of existing deposits and possibly physical modeling. C Although sand deposition would not be expected downstream of the modeled reach ( except locally in areas where the channel is wider and has a lower slope), suspended sediment impacts ( TSS) will propagate down to the ocean. TSS levels will decrease as tributaries add water to the North Umpqua River, and the TSS will be inversely proportional to discharge. Incorporation of local variabilities in slope, channel width, and roughness into the modeling effort might change the results to some extent. Some sand deposition could occur in wider reaches or areas with momentum defects beyond the 16 km reach modeled here, but the general trend of sediment transport and deposition modeled here would likely not change substantially. Sediment deposition would not be expected to occur downstream of the modeled reach because sediment supply is reduced compared to transport capacity with increasing distance downstream from the dam ( following dam removal and release of sediment). Sediment deposition and/ or transport at any location are dependent on the difference of sediment supply to that location ( influx) and sediment transport capacity ( outflux). In the case of Soda Springs dam removal, as the reservoir sediment washed downstream, sediment supply ( influx) to downstream reaches would be higher than transport capacity ( outflux), resulting in deposition of sediment on the channel bed. Supply would only be expected to exceed transport capacity for a short reach downstream of the dam, however, given the high transport capacity of the North Umpqua River. Further downstream of the dam, the sediment supply decreases to less the maximum amount that the flow can carry ( i. e., actual transport rate is less than potential sediment transport rate and sediment transport becomes supply limited). In such reaches, the DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 10 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD influx and outflux of sediment are the same as what is supplied from upstream ( i. e., all sediment remains in transport), and deposition does not occur. The results and conclusions of this modeling effort are preliminary and contain considerable uncertainty. For example, sediment flushing from the reservoir could occur more slowly than indicated by this modeling if sediment transport occurred by cutting a channel through the reservoir sediment deposit instead of by laterally uniform sediment transport, as assumed in the modeling This and other assumptions and conclusions of numerical modeling could be tested by physical modeling ( i. e., creating a scale model of the reservoir and downstream reach). Additional information on the grain size distribution of the reservoir sediment, numerical modeling that accounts for the gravel fraction of the reservoir sediment and for different dam removal methods, and physical modeling would be necessary to fully predict the effects of releasing sediment from Soda Springs reservoir. 6. MODELING OF SUSPENDED SEDIMENT EFFECTS ON ADULT FISHES Based on the results of the modeling of fine sediment release described above, Stillwater Sciences developed quantitative estimates of the effects of suspended sediment on adult salmonids in the North Umpqua River following dam removal. This modeling is based on a series of papers in which Newcombe and various co authors define a semi quantitative fourteen point scale for " severity of impact" ( SEV) of suspended sediment on fishes, and develop models for this quantity in terms of suspended sediment concentration ( C) and duration of exposure ( D) ( Newcombe 1986, Newcombe and MacDonald 1991, Newcombe and Jensen 1996). By collecting and analyzing the results of a wide range of studies of the effects of suspended sediment on fishes, Newcombe and his co authors found a highly significant relationship between severity of impact ( SEV) and the natural logarithm of the concentration duration product. The severity of impact ranking scale and descriptions of associated effects are shown in Table 1. The concentration ( C) duration ( D) models developed by Newcombe and Jensen ( 1996) and Newcombe and MacDonald ( 1991) have the forms and . It turns out that these model forms are indistinguishable in practice. Fitting a model of the first form to data on adult salmonids from the literature, as reported in Newcombe and Jensen ( 1996), results in the model , where SI is the natural logarithm of the concentration duration product. For concentrations varying in time, the concentration duration product corresponds to the integral of concentration over the exposure period. Stillwater Sciences combined the data on suspended sediment concentration and duration of exposure developed in the numerical modeling of fine sediment release ( described above) with the data and techniques of Newcombe and Jensen ( 1996) to generate estimates of mortality to adult salmonids that could result from dam removal. We also assessed predicted mortality under various scenarios of sediment dredging from the reservoir prior to dam removal. The results of the impact assessments are shown in Table 2, which shows the basic mortality prediction for DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 11 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD various sediment concentration duration integrals, as well as the upper and lower 90 percent confidence intervals for these predictions. Categories 10 through 14 of the severity of impact scale are defined as ranges of direct mortality; values in these categories have been converted to percent mortality values in Table 2. While the modeled relationship between SEV and SI is highly significant ( p< 10 12), it also leaves a lot of variability in the data unexplained ( r2= 0.59). A prediction confidence interval is . The results shown in Table 2 suggest that for one shot dam removal with no prior dredging, 29% mortality of adult salmonids is predicted, although the 90% confidence interval ranges from 0% to 100% mortality ( i. e., as much as an entire year class of adult fish could be lost due to suspended sediment impacts). These results also indicate that dredging before dam removal has only a limited effect on reducing potential mortality. Even if dredging of all the sediment from the reservoir is attempted, some mortality could occur because it is assumed that maximum dredging efficiency would be approximately 90% ( i. e., 10% of the sediment accumulation would remain). The Newcombe and Jensen ( 1996) modeled relationship between severity of impact and sediment concentration and duration leaves substantial variability in the data unexplained, which accounts for the wide range in the 90% confidence intervals shown in Table 2. The scatter in the data is probably due to a great many factors, none of which are well enough understood at this time to incorporate into the analysis. For example, the tolerance of fish to suspended sediment has been shown to depend on the size of the suspended particles and water temperature, and is likely to depend on particle shape and composition as well. Despite the uncertainty in the model, it represents the best available method for quantifying potential mortality risks associated with suspended sediment impacts on adult salmonids and is based on a wide range of studies from peer reviewed scientific literature. It does not account for other types of impacts ( e. g., infiltration of fine sediment into spawning gravels) or possible long term benefits of dam removal on downstream sediment dynamics; these potential impacts and benefits would require further study. REFERENCES Newcombe, C. P. 1986. Suspended sediments in aquatic ecosystems: a guide to impact assessment. British Columbia Ministry of Environment and Parks, Waste Management Branch, Victoria. Newcombe, C. P., and J. O. T. Jensen. 1996. Channel suspended sediment and fisheries: a synthesis for quantitative assessment of risk and impact. North American Journal of Fisheries Management 16: 693 727. Newcombe, C. P., and D. D. MacDonald. 1991. Effects of suspended sediments on DRAFT Preliminary Modeling of Sand/ Silt Release from Soda Springs Reservoir in the Event of Dam Removal 23 June 1999 Page 12 Stillwater Sciences C:\ Documents and Settings\ emily\ Local Settings\ Temporary Internet Files\ OLK451\ NEWMODL. WPD aquatic ecosystems. North American Journal of Fisheries Management 11: 72 82. PacifiCorp. 1995. Final Technical Report for aquatic resources. North Umpqua Hydroelectric Project, FERC Project No. 1927, Douglas County, Oregon. Portland, Oregon. PacifiCorp. 1999. Soda Springs Dam removal technical feasibility study. Prepared by Raytheon Engineers & Constructors. Portland, Oregon. 0 20 40 60 80 100 120 10 1 0.1 Grain Size ( mm) % Finer SR 1 SR 8 SR 9 SR 10 SR 11 SR 12 SR 13 SR 14 SR 15 SR 16 SR 17 Average Figure 1. Grain size distributions of sediment samples collected from the upper 0.6 m of the Soda Springs reservoir deposit ( PacifiCorp 1995) and a constructed average size distribution based on these curves. 0 20 40 60 80 100 120 10 1 0.1 Grain Size ( mm) % Finer Constructed Coarser Constructed Finer Constructed Original Figure 2. Constructed grain size distribution scenarios used in modeling. 0 500 1000 1500 2000 2500 3000 3500 4000 4500 1 Mar 21 Mar 10 Apr 30 Apr 20 May 9 Jun 29 Jun 19 Jul 8 Aug 28 Aug 17 Sep 1956 Discharge ( cfs) Figure 3. Discharge at USGS Copeland Station, North Umpqua River, 3/ 1  9/ 30, 1956 400 420 440 460 480 500 520 540 560 580  3 0 3 6 9 12 15 Distance ( km) Bed Elevation ( m) Dam Sand Deposit Figure 4. Long profile of the modeled reach, based on USGS 1915 long profile Fig. 5 Long profile, Average grain size scenario, low discharge 400 420 440 460 480 500 520 540 560  3 0 3 6 9 12 15 Distance ( km) Bed Elevation ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 6 Thickness of sand deposit, Average grain size scenario, low discharge 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 7 TSS value, Average grain size with low discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 8 Water depth, Average grain size with low discharge scenario 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1  3 0 3 6 9 12 15 Distance ( km) Water Depth ( m) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 15.2 day Fig. 9 Thickness of sand deposit, Coarser grain size with low discharge scenario 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 16 day 17 day 18 day 19 day 20 day 21 day 21.4 day Fig. 10 TSS value, Coarser grain size with low discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 10 day 11 day 12 day 13 day 14 day 15 day 16 day 17 day 18 day 19 day 20 day 21 day Fig. 11 Thickness of sand deposit, Finer grain size with low discharge scenario 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 9.8 day Fig. 12 TSS value, Finer grain size with low discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 1 day 2 day 3 day 4 day 5 day 6 day 7 day 8 day 9 day 9.8 day Fig. 13 Maximum TSS value under different grain size scenarios with low discharge 100 1000 10000 100000 0 100 200 300 400 500 600 Time ( hours) Maximum TSS in the River ( ppm) Average Grain Size Finer Coarser Fig. 14 Thickness of sand deposit, Average grain size with moderate discharge scenario 0 2 4 6 8 10 12  3 0 3 6 9 12 15 Distance ( km) Sand Deposit Thickness ( m) initial 0.5 day 1.0 day 1.5 day 2.0 day 2.5 day 3.0 day 3.5 day 3.6 day Fig. 15 TSS value, Average grain size with moderate discharge scenario 100 1000 10000 100000  3 0 3 6 9 12 15 Distance ( km) TSS ( ppm) 0.5 day 1.0 day 1.5 day 2.0 day 2.5 day 3.0 day 3.5 day 3.6 day Fig. 16 Water depth, Average grain size with moderate discharge scenario 0.3 0.5 0.7 0.9 1.1 1.3 1.5  3 0 3 6 9 12 15 Distance ( km) Water Depth ( m) 0.5 day 1.0 day 1.5 day 2.0 day 2.5 day 3.0 day 3.5 day 3.6 day Fig. 17 Maximum TSS value, Average grain size scenario for low and moderate flows 100 1000 10000 100000 0 50 100 150 200 250 300 350 400 Time ( hours) Maximum TSS in the River ( ppm) Flow starting March 1, 1956 ( low) Flow starting May 20, 1956 ( moderate) Fig. 18 Maximum thickness of sand deposit and the time it occurred 0 2 4 6 8 10 12 0 2 4 6 8 10 12 14 16 Distance ( km) Maximum Thickness ( m) 0 100 200 300 400 500 600 Time of Maximum Thickness ( hours) Run 1 Thickness Run 2 Thickness Run 3 Thickness Run 4 Thickness Run 1 Time Run 2 Time Run 3 Time Run 4 Time 
OCLC number  613722865 



W 


