final Sedimentation Report - City and Borough of Sitka

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In lakes and reservoirs with fluctuating water levels, some material that was deposited at higher lake levels is entrained and moved out along the delta within the re-established stream channel, creating a complex and dynamic pattern of sediment deposits within the active channel portion of the fluctuation zone. The lake level at the time of sediment inflow controls the elevation at which the deposits occur. This pattern can be seen in the existing Lake Creek delta deposits on a profile of the stream channel and adjacent delta surface measured during low lake levels in 2008 (Figure 8). Note that while the active stream channel is graded to the lake level at the time of the survey, there are several remnant mini-delta features evident on the adjacent delta surface, small delta-shaped lobes of cobble, gravel, and sand that were deposited when the lake level was at specific elevations when large amounts of cobble/gravel bedload entered the lake or was reentrained. The top of each of these deposits represents the approximate lake surface elevation when the deposition occurred. Elevation (ft) 300 295 290 285 280 275 270 265 260 255 Remnant mini-deltas 250 11,000 11,500 12,000 12,500 13,000 13,500 14,000 Distance from Upper Barrier Falls (ft) Remnant mini-deltas Blue Lake Streambed Delta Surface adjacent to Streambed Figure 8. Surveyed Longitudinal Profile of Blue Lake Creek and Adjacent Delta Deposits within the Current Reservoir. The area and volume of each of the existing Blue Lake Creek delta map units was estimated to provide an idea of sediment input rates. Table 1 shows the area, average depth, and volume of the current Blue Lake Creek delta. Areas/volumes of the Becky Creek delta are not included in the table. Blue Lake Reservoir Sedimentation Report 12 Blue Lake Project Expansion Watershed GeoDynamics FERC No. 2230 December 2009
Table 1. Estimated Area, Depth, and Volume of Existing Lake Creek Delta Deposits Map unit Area (acres) Average depth (ft) Estimated volume (cu yd) Active channel (cobble/gravel/sand) 40 5 323,000 Cobble/gravel/sand 3 15 73,000 Gravel/sand 4 5-8 40,000 Sand/silt 57 2.5 231,000 Total 104 -- 667,000 The grain size distribution across and within the delta varies greatly, but grab samples of representative surficial material were taken in the cobble/gravel/sand, gravel/sand, and sand/silt map units to provide some information on the particle size distribution (Figure 9, Appendix A). The cobble/gravel/sand and gravel/sand samples were dominated by gravel size material (70-80 percent) with the remainder of the samples sand and fines. The sand/silt sample was dominantly sand and fines. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Delta cobble/gravel/sand Fines (64mm) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Fines (64mm) Figure 9. Grain Size Distribution in Existing Delta Deposits. 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Fines (64mm) Expansion-related Blue Lake Creek Delta (Potentially Inundated Area) The proposed change in dam height and reservoir operations will inundate areas up to elevation 425 feet. As a result, Blue Lake Creek will enter Blue Lake approximately 1.9 miles upstream of the current location. It is anticipated that Blue Lake will fluctuate between El 365 and 425 under the proposed Expansion (Figure 5, above). The distribution and depth of the Blue Lake Creek Expansion delta deposits 50 years after the change in operations were estimated based on the following assumptions: 1. The rate of sediment input will remain constant, so the volume and grain size of sediment in the Expansion delta after 50 years will be approximately equal to sediments in the existing Blue Lake Creek delta (52 years of accumulation). Blue Lake Reservoir Sedimentation Report 13 Blue Lake Project Expansion Watershed GeoDynamics FERC No. 2230 December 2009
Page 1: FINAL RESERVOIR SEDIMENTATION STUDY
Page 4 and 5: Figure 11. Cross Sectional Profiles
Page 6 and 7: Brad Creek Figure 1. Proposed Expan
Page 8 and 9: 2. Evaluate probable future fish ha
Page 10 and 11: The mapping units were transferred
Page 12 and 13: in June flows were greater than 541
Page 14 and 15: Peak flows in Blue Lake Creek were
Page 18 and 19: 2. The operations model provides a
Page 20 and 21: Elevation (ft) Elevation (ft) 370 3
Page 22 and 23: Under expansion operation condition
Page 24 and 25: Relative Elevation (ft) Relative El
Page 26 and 27: 100% 90% 80% 70% 60% 50% 40% 30% 20
Page 28 and 29: Table 5. Estimated Spawning Use, Me
Page 30 and 31: 3500 3000 2500 2000 1500 Distance a
Page 32 and 33: 3500 3000 2500 2000 1500 Distance a
Page 34 and 35: volume of sediment deposition and e
Page 36 and 37: Comment Number Comment Summary Resp
Page 38 and 39: Blue Lake Creek Delta Sediment Samp
Page 40 and 41: pan 3,876 14.36 mm 0.0471 ft Blue L
Page 42 and 43: December 2009 Blue Lake Creek Reser
Page 44: Blue Lake Creek Delta Sediment Samp

final Sedimentation Report - City and Borough of Sitka

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