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F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 1 2532 Durant Avenue, Suite 201, Berkeley, CA 94704 Phone ( 510) 848 8098 Fax ( 510) 848 8398 Draft Technical Memorandum Date: March 12, 2002 To: Portland General Electric, Portland, Oregon Subject: Sediment transport modeling following the removal of Marmot Dam with 125,000 and 300,000 cubic yards of dredging prior to dam removal Introduction On February 19, 2002, Portland General Electric requested that Stillwater Sciences run sediment transport simulations for two additional alternatives for removing Marmot Dam on the Sandy River, Oregon. For this analysis, Stillwater Sciences performed model runs for dredging 125,000 and 300,000 cubic yards of sediment from the reservoir upstream of the dam prior to dam removal. These volumes represent the worst and best possible dredging volumes for one in water working season, respectively ( Kevin Marshall, personal communication, Feb. 22, 2002). We previously estimated that 970,000 cubic yards of sediment are stored in the reservoir ( Stillwater Science 2000), so the two new alternatives represent 31 % and 13 % removal of the sediment from behind the reservoir, respectively. This technical memorandum presents the results of the new simulations and compares them with the results of earlier simulation results for reference conditions ( no sediment release), Alternative B ( minimal dredging), and Alternative D ( dredging sediment to a point 2,700 ft upstream of the Marmot Dam, or about 720,000– 510,000 cubic yards). Portland General Electric engineers have determined that removal of the sediment as called for in Alternative D is not feasible in one season, but the results are presented here so that the new alternatives can be easily compared to alternatives presented in Stillwater Sciences ( 2000a, b). To facilitate interpretation of the different modeling results, both the current and the earlier simulations used the same average hydrologic series and average grain size distribution as input data. These input data are described in detail in Stillwater Sciences ( 2000a, b) and Cui and Wilcox ( in press). Detailed background information about the geological and hydrological conditions of the Sandy River and the removal of Marmot Dam can also be found in Stillwater Sciences ( 2000a, b). Detailed information about the sediment transport model can be found in Cui and Wilcox ( in press) and in Stillwater Sciences ( 2000b). Draft Technical Memorandum F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 2 Table 1. Dam removal options presented in this report. Dam Removal option* Volume of sediment available for downstream transport ( cubic yards) Stillwater Sciences ( 2000a) PGE Current Single season dam removal with minimal sediment removal 970,000 Alternative B Alternative 3 remove sediment to the bottom of the sand layer with sand layer excavation to a point 2,700 ft upstream of Marmot Dam 250,000– 460,000 Alternative D N/ A Remove 125,000 cubic yards of sediment 845,000 N/ A Alternative 1A Remove 300,000 cubic yards of sediment 670,000 N/ A Alternative 1B Leave dam in place 0 reference condition Alternative 2 * The names of the alternatives have changed since our report ( Stillwater Sciences 2000a). This memorandum uses the alternative names in Stillwater Sciences ( 2000a) so that readers can easily cross reference this memorandum and Stillwater Sciences ( 2000a). In order to assess the upstream extent of excavation of the reservoir deposit, we assumed an average width of 50.5 m and depth as shown in Figure 1. Using these assumptions, sediment excavation was estimated to extend to 200 m and 600 m upstream of the dam for removal volumes of 125,000 and 300,000 cubic yards, respectively. This analysis presents results from coarse and fine sediment transport models. Results of the coarse sediment simulation are presented as the annual change in bed elevation in Figures 2 to 9, and as deposition thickness in Figures 10 to 17. These model runs show that the bed dynamics downstream will be very similar under the dam removal scenario with minimal sediment removal ( Alternative B) and scenarios with removal of 300,000 and 125,000 cubic yards of sediment. Results of the simulations for fine sediment are presented as area and magnitude of deposition in Figures 18 to 22 and as total suspended sediment concentration ( TSS) in Figures 23 to 27. In this document, explanations of the results and the comparisons among the alternatives are kept very brief because the diagrams are rather self explanatory. Coarse Sediment Modeling Results The downstream impact of sediment released from behind the dam will include both deposition ( as the sediment comes into a site) and erosion ( as the sediment wave disperses downstream). The most important factor when considering the potential impact of a coarse sediment pulse to aquatic habitat is the annual change in bed elevation ( shown in Figures 2 9). Figures 2 to 6 are plots of annual change in bed elevation under the assumed reference conditions, and for Alternative B ( minimal dredging), dredging 125,000 cubic yards, dredging 300,000 cubic yards, and Alternative D. These figures are plotted at the same scale so that they can be easily compared. Additionally, the reference condition and Alternative B ( minimal dredging) are plotted on the same graph in Figure 7, and Alternative B, removal of 125,000 cubic yards, and removal of 300,000 cubic yards are shown on the same graph in Figure 8. Figure 9 shows the annual change in bed elevation for the reference condition and each of the removal alternatives for the first ten years following dam removal. This figure shows that for Alternative B and the removal of 125,000 and 300,000 cubic yards, the annual change in bed elevation ( essentially an envelope of erosion and deposition) is similar in all reaches. The coarse sediment dynamics following each of the alternatives is described separately for each reach below. Draft Technical Memorandum F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 3 Figures 10 through 14 are plots of deposition thickness under the assumed reference conditions, Alternative B ( minimal dredging), 125,000 cubic yard dredging, 300,000 cubic yard dredging and Alternative D. These plots clearly show the evolution of the reservoir deposit, following dam removal, but the actual impacts to aquatic habitat are better represented by Figures 2 8. Comparisons between Alternative B ( minimal dredging) and dredging 125,000 cubic yards, and between Alternative B ( minimal dredging) and dredging 300,000 cubic yards are presented in Figures 15 and 16, respectively. These three removal options are shown on the same graph in Figure 17, but the large amount of data makes the graph difficult to interpret. Figure 15 shows that the erosional and depositional patterns for Alternative B ( minimal dredging) and dredging 125,000 cubic yards prior to dam removal are very similar, and the difference in the magnitude of channel deposition downstream of the dam is insignificant. Similarly, Figures 16 and 17 show that for dredging 300,000 cubic yards, the erosional and depositional patterns are very similar to those shown for Alternative B ( minimal dredging), and that the difference between the two alternatives, in terms of the magnitude of sediment deposition downstream of Reach 1, is insignificant. The amount of deposition in Reach 1 is greater for Alternative B than for dredging of 300,000 cubic yards for the first two years following dam removal. After, Year 2, the difference between the two alternatives is insignificant in Reach 1 ( Figures 8, 16, and 17). Reach by Reach Descriptions The reservoir reach will experience rapid erosion following dam removal for each of the removal alternatives. The amount of sediment in the reservoir reach is essentially the same after year 4 for the 125,000 cubic yard removal alternative and Alternative B ( Figure 15). Similarly the deposit morphology is the same after year 6 for the 300,000 cubic yard removal alternative and Alternative B ( Figure 16). After four years the difference between these three removal alternatives is less than 1 m in the reservoir reach ( Figure 17). Reach 1 will be the most seriously impacted of the reaches for each removal option. This reach will be very dynamic, with extensive deposition in the first year following dam removal, and erosion in subsequent years ( figures 3 6). Sediment dynamics in Reach 1 are similar to reference conditions starting in year 8. The remainder of the sediment wedge in year 8 may be a permanent deposition that fills in the area where either the spillway caused erosion or sediment was excavated during the construction of the dam. Generally, the steep slope and narrow width of Reach 2 ( the gorge) prevent sediment deposition ( and subsequent erosion), rather sediment is delivered to downstream reaches relatively quickly. Some deposition does occur at the upper and lower ends of Reach 2 ( Figures 3 6, 10 14). Both locations have lower slopes and slightly wider channel widths, and therefore, sediment deposition is more likely than in the remainder of the gorge. The annual change in bed elevation at the entrance to the gorge under reference conditions is within 1.3 ft ( 0.4 m) of the current bed elevation. Under Alternative B ( minimal dredging), the maximum change in bed elevation at this location increases to about 2.6 ft ( 0.8 m) in year 2 after dam removal ( Figure 3). The sediment would be eroded in subsequent years. The magnitude of deposition at the top of Reach 2 for the alternatives where 125,000 and 300,000 cubic yards of sediment were removed was similar to the amount of deposition under Alternative B. The frequency of deposition and erosion will also increase from the reference conditions. The annual change in bed elevation at the exit of the gorge under reference conditions is within 1.3 ft ( 0.4 m). The maximum change in bed elevation at this location under Alternative B is about 4 ft ( 1.2 m), but is generally less than 2.6 ft/ yr ( 0.8 m/ yr) ( Figure 3). The annual change at this site for removal of 125,000 and 300,000 cubic yards of sediment is similar to Alternative B ( Figure 8). Draft Technical Memorandum F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 4 With the exception of over 3.3 ft ( 1 m) of deposition at the upstream end of Reach 3, sediment dynamics in Reach 3 are similar to reference conditions for the three excavation alternatives ( Figures 3 6, 10 14). Additionally, there is little difference between the amount of deposition in Reach 3 under Alternative B versus removing 125,000 or 300,000 cubic yards of sediment ( Figures 8 and 17). The magnitude of annual erosion and deposition under Alternative B, however, is similar to that of the reference condition, both at about 3.3 ft ( 1 m). The upper 3 miles of Reach 3 would be more dynamic following dam removal, with an increased frequency of erosion and deposition for the dam removal alternatives than under reference conditions ( Figures 7 and 8). The magnitude and frequency of coarse sediment dynamics in Reaches 4 and 5 are similar to reference conditions for Alternative B and removal of 125,000 and 300,000 cubic yards of sediment. ( Figures 8 and 17). Fine Sediment The extent and magnitude of sand deposition are given in Figures 18 22 for the assumed reference conditions, Alternative B ( minimal dredging), 125,000 cubic yards of dredging, 300,000 cubic yards of dredging and Alternative D ( dredging approximately 730,000 cubic yards). The results indicate that none of the alternatives would result in significant sand deposition from the reference conditions. Modeling results of TSS concentrations are given in Figures 23  27 for the assumed reference conditions, Alternative B ( minimal dredging), 125,000 cubic yard dredging, 300,000 cubic yard dredging and Alternative D. The results indicate that the dam removal alternatives resulted in similar TSS for the first two years after the dam removal. The increases of TSS over the assumed reference conditions due to the removal of the dam are relatively low. For example, the maximum TSS downstream of Marmot Dam during the first year increased from under 200 ppm to about 550 ppm, i. e., an increase of less than 300 ppm. Errata An error was discovered in Stillwater Sciences ( 2000b) during the current modeling exercise. Figure 28 in Stillwater Sciences ( 2000b) is incorrect. A correct replacement for this figure can be found in Stillwater Sciences ( 2000a, Figure 20), or in this technical memorandum ( Figure 24). There are no implications to the explanations and conclusions in Stillwater Sciences ( 2000b) from this correction. References Cui, Y., and A. Wilcox. 2002 ( in press). Development and application of numerical modeling of sediment transport associated with dam removal. Sedimentation engineering. ASCE Manual 54, Volume 2, M. Garcia, editor. American Society of Civil Engineers. Kevin Marshall, personal communication, Feb. 22, 2002. Portland General Electric Engineer Stillwater Sciences. 2000a. Evaluation of geomorphic effects of removal of Marmot and Little Sandy dams and potential impacts on anadromous salmonids. Preliminary final technical report. Prepared by Stillwater Sciences, Berkeley, California for Portland General Electric Company, Portland, Oregon. Stillwater Sciences. 2000b. Numerical modeling of sediment transport in the Sandy River, Oregon following removal of Marmot Dam. Technical report. Prepared by Stillwater Sciences, Berkeley, California for Portland General Electric Company, Portland, Oregon. Draft Technical Memorandum Stillwater Sciences Figure 1. Initial profile in the reservoir for the alternatives presented in this memorandum 210 215 220 225 230 235 240 4 3.5 3 2.5 2 1.5 1 0.5 0 Distance upstream from the dam ( km) Elevation ( m) Alternative B ( minimal dredging) Dredging 125,000 cubic yards Dredging 300,000 cubic yards Alternative D Estimated pre dam channel bed Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 2. Annual change in bed elevation under reference conditions Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 3. Annual change in bed elevation for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 4. Annual change in bed elevation for 125,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 5. Annual change in bed elevation for 300,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 6. Annual change in bed elevation for Alternative D Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Reference Condition Alternative B ( minimal dredging) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Figure 7. Annual change in bed elevation: comparison between reference conditions and Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Alternative B ( minimal dredging) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Dredging 125,000 cubic yards Dredging 300,000 cubic yards Figure 8. Annual change in bed elevation: Comparison between Alternative B ( minimal dredging), dredging 125,000 cubic yards, and dredging 300,000 cubic yards. Draft Technical Memorandum Stillwater Sciences Horizontal Scale: Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) 10 km 10 mile Reference Condition Alternative B ( minimal dredging) Dredging 125,000 cubic yards Dredging 300,000 cubic yards Alternative D Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Figure 9. Comparison of 10 years of annual change in bed elevation following dam removal for the different alternatives Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 10. Deposition thickness under reference conditions Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 11. Deposition thickness for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 12. Deposition thickness for 125,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 13. Deposition thickness for 300,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 14. Deposition thickness for Alternative D Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Alternative B Dredging 125,000 yd3 Figure 15. Deposition thickness: Comparison between Alternative B ( minimal dredging) and dredging 125,000 cubic yards Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Alternative B Dredging 300,000 yd3 Figure 16. Deposition thickness: Comparison between Alternative B ( minimal dredging) and dredging 300,000 cubic yards Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial year 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 Alternative B Dredging 125,000 yd3 Dredging 300,000 yd3 Figure 17. Deposit thickness: Comparison between Alternative B ( minimal dredging), dredging 125,000 cubic yards, and dredging 300,000 cubic yards. Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 18. Area and magnitude of sand deposition under reference conditions Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 19. Area and magnitude of sand deposition for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 20. Area and magnitude of sand deposition for dredging 125,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 21. Area and magnitude of sand deposition for dredging 300,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 22. Area and magnitude of sand deposition for Alternative D Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 23. Suspended sediment concentration under assumed reference conditions Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 24. Suspended sediment concentration for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 25. Suspended sediment concentration for dredging 125,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 26. Suspended sediment concentration for dredging 300,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 27. Suspended sediment concentration for Alternative D
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Title  Sediment transport modeling following the removal of Marmot Dam with 125,000 and 300,000 cubic yards of dredging prior to dam removal draft technical memorandum 
Subject  Sediment transportOregonSandy RiverComputer simulation.; Dam retirementOregonSandy RiverMethodology.; DredgingOregonSandy RiverComputer simulation.; Sandy River (Or.)Environmental conditions.; G291 P22 Web Resource 
Description  Draft.; "March 12, 2002."; Includes bibliographical references (p. 4). 
Publisher  Stillwater Sciences 
Contributors  Portland General Electric Company.; Stillwater Sciences. 
Type  Text 
Language  eng 
Relation  http://www.stillwatersci.com/resources/2002sandydredging.pdf; http://worldcat.org/oclc/639025501/viewonline 
DateIssued  2002 
FormatExtent  31 p. : digital PDF file, ill., charts, tables ; 4.26 MB. 
RelationRequires  Mode of access: World Wide Web. 
Transcript  F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 1 2532 Durant Avenue, Suite 201, Berkeley, CA 94704 Phone ( 510) 848 8098 Fax ( 510) 848 8398 Draft Technical Memorandum Date: March 12, 2002 To: Portland General Electric, Portland, Oregon Subject: Sediment transport modeling following the removal of Marmot Dam with 125,000 and 300,000 cubic yards of dredging prior to dam removal Introduction On February 19, 2002, Portland General Electric requested that Stillwater Sciences run sediment transport simulations for two additional alternatives for removing Marmot Dam on the Sandy River, Oregon. For this analysis, Stillwater Sciences performed model runs for dredging 125,000 and 300,000 cubic yards of sediment from the reservoir upstream of the dam prior to dam removal. These volumes represent the worst and best possible dredging volumes for one in water working season, respectively ( Kevin Marshall, personal communication, Feb. 22, 2002). We previously estimated that 970,000 cubic yards of sediment are stored in the reservoir ( Stillwater Science 2000), so the two new alternatives represent 31 % and 13 % removal of the sediment from behind the reservoir, respectively. This technical memorandum presents the results of the new simulations and compares them with the results of earlier simulation results for reference conditions ( no sediment release), Alternative B ( minimal dredging), and Alternative D ( dredging sediment to a point 2,700 ft upstream of the Marmot Dam, or about 720,000– 510,000 cubic yards). Portland General Electric engineers have determined that removal of the sediment as called for in Alternative D is not feasible in one season, but the results are presented here so that the new alternatives can be easily compared to alternatives presented in Stillwater Sciences ( 2000a, b). To facilitate interpretation of the different modeling results, both the current and the earlier simulations used the same average hydrologic series and average grain size distribution as input data. These input data are described in detail in Stillwater Sciences ( 2000a, b) and Cui and Wilcox ( in press). Detailed background information about the geological and hydrological conditions of the Sandy River and the removal of Marmot Dam can also be found in Stillwater Sciences ( 2000a, b). Detailed information about the sediment transport model can be found in Cui and Wilcox ( in press) and in Stillwater Sciences ( 2000b). Draft Technical Memorandum F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 2 Table 1. Dam removal options presented in this report. Dam Removal option* Volume of sediment available for downstream transport ( cubic yards) Stillwater Sciences ( 2000a) PGE Current Single season dam removal with minimal sediment removal 970,000 Alternative B Alternative 3 remove sediment to the bottom of the sand layer with sand layer excavation to a point 2,700 ft upstream of Marmot Dam 250,000– 460,000 Alternative D N/ A Remove 125,000 cubic yards of sediment 845,000 N/ A Alternative 1A Remove 300,000 cubic yards of sediment 670,000 N/ A Alternative 1B Leave dam in place 0 reference condition Alternative 2 * The names of the alternatives have changed since our report ( Stillwater Sciences 2000a). This memorandum uses the alternative names in Stillwater Sciences ( 2000a) so that readers can easily cross reference this memorandum and Stillwater Sciences ( 2000a). In order to assess the upstream extent of excavation of the reservoir deposit, we assumed an average width of 50.5 m and depth as shown in Figure 1. Using these assumptions, sediment excavation was estimated to extend to 200 m and 600 m upstream of the dam for removal volumes of 125,000 and 300,000 cubic yards, respectively. This analysis presents results from coarse and fine sediment transport models. Results of the coarse sediment simulation are presented as the annual change in bed elevation in Figures 2 to 9, and as deposition thickness in Figures 10 to 17. These model runs show that the bed dynamics downstream will be very similar under the dam removal scenario with minimal sediment removal ( Alternative B) and scenarios with removal of 300,000 and 125,000 cubic yards of sediment. Results of the simulations for fine sediment are presented as area and magnitude of deposition in Figures 18 to 22 and as total suspended sediment concentration ( TSS) in Figures 23 to 27. In this document, explanations of the results and the comparisons among the alternatives are kept very brief because the diagrams are rather self explanatory. Coarse Sediment Modeling Results The downstream impact of sediment released from behind the dam will include both deposition ( as the sediment comes into a site) and erosion ( as the sediment wave disperses downstream). The most important factor when considering the potential impact of a coarse sediment pulse to aquatic habitat is the annual change in bed elevation ( shown in Figures 2 9). Figures 2 to 6 are plots of annual change in bed elevation under the assumed reference conditions, and for Alternative B ( minimal dredging), dredging 125,000 cubic yards, dredging 300,000 cubic yards, and Alternative D. These figures are plotted at the same scale so that they can be easily compared. Additionally, the reference condition and Alternative B ( minimal dredging) are plotted on the same graph in Figure 7, and Alternative B, removal of 125,000 cubic yards, and removal of 300,000 cubic yards are shown on the same graph in Figure 8. Figure 9 shows the annual change in bed elevation for the reference condition and each of the removal alternatives for the first ten years following dam removal. This figure shows that for Alternative B and the removal of 125,000 and 300,000 cubic yards, the annual change in bed elevation ( essentially an envelope of erosion and deposition) is similar in all reaches. The coarse sediment dynamics following each of the alternatives is described separately for each reach below. Draft Technical Memorandum F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 3 Figures 10 through 14 are plots of deposition thickness under the assumed reference conditions, Alternative B ( minimal dredging), 125,000 cubic yard dredging, 300,000 cubic yard dredging and Alternative D. These plots clearly show the evolution of the reservoir deposit, following dam removal, but the actual impacts to aquatic habitat are better represented by Figures 2 8. Comparisons between Alternative B ( minimal dredging) and dredging 125,000 cubic yards, and between Alternative B ( minimal dredging) and dredging 300,000 cubic yards are presented in Figures 15 and 16, respectively. These three removal options are shown on the same graph in Figure 17, but the large amount of data makes the graph difficult to interpret. Figure 15 shows that the erosional and depositional patterns for Alternative B ( minimal dredging) and dredging 125,000 cubic yards prior to dam removal are very similar, and the difference in the magnitude of channel deposition downstream of the dam is insignificant. Similarly, Figures 16 and 17 show that for dredging 300,000 cubic yards, the erosional and depositional patterns are very similar to those shown for Alternative B ( minimal dredging), and that the difference between the two alternatives, in terms of the magnitude of sediment deposition downstream of Reach 1, is insignificant. The amount of deposition in Reach 1 is greater for Alternative B than for dredging of 300,000 cubic yards for the first two years following dam removal. After, Year 2, the difference between the two alternatives is insignificant in Reach 1 ( Figures 8, 16, and 17). Reach by Reach Descriptions The reservoir reach will experience rapid erosion following dam removal for each of the removal alternatives. The amount of sediment in the reservoir reach is essentially the same after year 4 for the 125,000 cubic yard removal alternative and Alternative B ( Figure 15). Similarly the deposit morphology is the same after year 6 for the 300,000 cubic yard removal alternative and Alternative B ( Figure 16). After four years the difference between these three removal alternatives is less than 1 m in the reservoir reach ( Figure 17). Reach 1 will be the most seriously impacted of the reaches for each removal option. This reach will be very dynamic, with extensive deposition in the first year following dam removal, and erosion in subsequent years ( figures 3 6). Sediment dynamics in Reach 1 are similar to reference conditions starting in year 8. The remainder of the sediment wedge in year 8 may be a permanent deposition that fills in the area where either the spillway caused erosion or sediment was excavated during the construction of the dam. Generally, the steep slope and narrow width of Reach 2 ( the gorge) prevent sediment deposition ( and subsequent erosion), rather sediment is delivered to downstream reaches relatively quickly. Some deposition does occur at the upper and lower ends of Reach 2 ( Figures 3 6, 10 14). Both locations have lower slopes and slightly wider channel widths, and therefore, sediment deposition is more likely than in the remainder of the gorge. The annual change in bed elevation at the entrance to the gorge under reference conditions is within 1.3 ft ( 0.4 m) of the current bed elevation. Under Alternative B ( minimal dredging), the maximum change in bed elevation at this location increases to about 2.6 ft ( 0.8 m) in year 2 after dam removal ( Figure 3). The sediment would be eroded in subsequent years. The magnitude of deposition at the top of Reach 2 for the alternatives where 125,000 and 300,000 cubic yards of sediment were removed was similar to the amount of deposition under Alternative B. The frequency of deposition and erosion will also increase from the reference conditions. The annual change in bed elevation at the exit of the gorge under reference conditions is within 1.3 ft ( 0.4 m). The maximum change in bed elevation at this location under Alternative B is about 4 ft ( 1.2 m), but is generally less than 2.6 ft/ yr ( 0.8 m/ yr) ( Figure 3). The annual change at this site for removal of 125,000 and 300,000 cubic yards of sediment is similar to Alternative B ( Figure 8). Draft Technical Memorandum F:\ PGE\ 2100_ add'l_ sandy\ Sandy River 2002\ dredging 125 and 300k\ Sandy Technical memo Final. doc Stillwater Sciences March 2002 4 With the exception of over 3.3 ft ( 1 m) of deposition at the upstream end of Reach 3, sediment dynamics in Reach 3 are similar to reference conditions for the three excavation alternatives ( Figures 3 6, 10 14). Additionally, there is little difference between the amount of deposition in Reach 3 under Alternative B versus removing 125,000 or 300,000 cubic yards of sediment ( Figures 8 and 17). The magnitude of annual erosion and deposition under Alternative B, however, is similar to that of the reference condition, both at about 3.3 ft ( 1 m). The upper 3 miles of Reach 3 would be more dynamic following dam removal, with an increased frequency of erosion and deposition for the dam removal alternatives than under reference conditions ( Figures 7 and 8). The magnitude and frequency of coarse sediment dynamics in Reaches 4 and 5 are similar to reference conditions for Alternative B and removal of 125,000 and 300,000 cubic yards of sediment. ( Figures 8 and 17). Fine Sediment The extent and magnitude of sand deposition are given in Figures 18 22 for the assumed reference conditions, Alternative B ( minimal dredging), 125,000 cubic yards of dredging, 300,000 cubic yards of dredging and Alternative D ( dredging approximately 730,000 cubic yards). The results indicate that none of the alternatives would result in significant sand deposition from the reference conditions. Modeling results of TSS concentrations are given in Figures 23  27 for the assumed reference conditions, Alternative B ( minimal dredging), 125,000 cubic yard dredging, 300,000 cubic yard dredging and Alternative D. The results indicate that the dam removal alternatives resulted in similar TSS for the first two years after the dam removal. The increases of TSS over the assumed reference conditions due to the removal of the dam are relatively low. For example, the maximum TSS downstream of Marmot Dam during the first year increased from under 200 ppm to about 550 ppm, i. e., an increase of less than 300 ppm. Errata An error was discovered in Stillwater Sciences ( 2000b) during the current modeling exercise. Figure 28 in Stillwater Sciences ( 2000b) is incorrect. A correct replacement for this figure can be found in Stillwater Sciences ( 2000a, Figure 20), or in this technical memorandum ( Figure 24). There are no implications to the explanations and conclusions in Stillwater Sciences ( 2000b) from this correction. References Cui, Y., and A. Wilcox. 2002 ( in press). Development and application of numerical modeling of sediment transport associated with dam removal. Sedimentation engineering. ASCE Manual 54, Volume 2, M. Garcia, editor. American Society of Civil Engineers. Kevin Marshall, personal communication, Feb. 22, 2002. Portland General Electric Engineer Stillwater Sciences. 2000a. Evaluation of geomorphic effects of removal of Marmot and Little Sandy dams and potential impacts on anadromous salmonids. Preliminary final technical report. Prepared by Stillwater Sciences, Berkeley, California for Portland General Electric Company, Portland, Oregon. Stillwater Sciences. 2000b. Numerical modeling of sediment transport in the Sandy River, Oregon following removal of Marmot Dam. Technical report. Prepared by Stillwater Sciences, Berkeley, California for Portland General Electric Company, Portland, Oregon. Draft Technical Memorandum Stillwater Sciences Figure 1. Initial profile in the reservoir for the alternatives presented in this memorandum 210 215 220 225 230 235 240 4 3.5 3 2.5 2 1.5 1 0.5 0 Distance upstream from the dam ( km) Elevation ( m) Alternative B ( minimal dredging) Dredging 125,000 cubic yards Dredging 300,000 cubic yards Alternative D Estimated pre dam channel bed Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 2. Annual change in bed elevation under reference conditions Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 3. Annual change in bed elevation for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 4. Annual change in bed elevation for 125,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 5. Annual change in bed elevation for 300,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Figure 6. Annual change in bed elevation for Alternative D Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Reference Condition Alternative B ( minimal dredging) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Figure 7. Annual change in bed elevation: comparison between reference conditions and Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) Year 1 Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9 Year 10 Alternative B ( minimal dredging) Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Dredging 125,000 cubic yards Dredging 300,000 cubic yards Figure 8. Annual change in bed elevation: Comparison between Alternative B ( minimal dredging), dredging 125,000 cubic yards, and dredging 300,000 cubic yards. Draft Technical Memorandum Stillwater Sciences Horizontal Scale: Vertical Scale: 1 grid = 0.4 m ( 1.31 ft) 10 km 10 mile Reference Condition Alternative B ( minimal dredging) Dredging 125,000 cubic yards Dredging 300,000 cubic yards Alternative D Reach 2 Reach 3 Reach 4 Reach 5 Reach 1 Reservoir Figure 9. Comparison of 10 years of annual change in bed elevation following dam removal for the different alternatives Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 10. Deposition thickness under reference conditions Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 11. Deposition thickness for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 12. Deposition thickness for 125,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 13. Deposition thickness for 300,000 cubic yards of dredging prior to dam removal Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Figure 14. Deposition thickness for Alternative D Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Alternative B Dredging 125,000 yd3 Figure 15. Deposition thickness: Comparison between Alternative B ( minimal dredging) and dredging 125,000 cubic yards Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 year Alternative B Dredging 300,000 yd3 Figure 16. Deposition thickness: Comparison between Alternative B ( minimal dredging) and dredging 300,000 cubic yards Draft Technical Memorandum Stillwater Sciences Horizontal Scale: 10 km 10 mile Vertical Scale: 1 grid = 1 m ( 3.28 ft) Reach 3 Reach 4 Reach 5 Reach 1 Reach 2 Reservoir Initial year 1 year 2 year 3 year 4 year 5 year 6 year 7 year 8 year 9 year 10 Alternative B Dredging 125,000 yd3 Dredging 300,000 yd3 Figure 17. Deposit thickness: Comparison between Alternative B ( minimal dredging), dredging 125,000 cubic yards, and dredging 300,000 cubic yards. Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 18. Area and magnitude of sand deposition under reference conditions Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 19. Area and magnitude of sand deposition for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 20. Area and magnitude of sand deposition for dredging 125,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 21. Area and magnitude of sand deposition for dredging 300,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 0 0.1 0.2 0.3 0.4 0.5 30 35 40 45 50 Distance from the dam ( km) Thickness of Sand Deposition ( m) Figure 22. Area and magnitude of sand deposition for Alternative D Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 23. Suspended sediment concentration under assumed reference conditions Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 24. Suspended sediment concentration for Alternative B ( minimal dredging) Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 25. Suspended sediment concentration for dredging 125,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 26. Suspended sediment concentration for dredging 300,000 cubic yards of sediment prior to dam removal Draft Technical Memorandum Stillwater Sciences 1 10 100 1000 0 365 730 Time from the removal ( days) Total Suspended Sediment Concentration ( ppm) Downstream of Marmot Dam Downstream of Bull Run Confluence Downstream of Dabney Park Figure 27. Suspended sediment concentration for Alternative D 
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