Brown and Caldwell; 1997 (03 June); Preliminary Feasibility Study of Large-Scale Detention Structures; 40p + attachments.
Results of a study evaluating the use of in-reservoir detention structures in Cascade Reservoir at Gold Fork, Lake Fork and North Fork (of the Payette River) tributaries. The report specifically covers the potential for reduction in total suspended solids (TSS), and total and soluble phosphorus if such detention systems are utilized. Considerations in the performance of such structures include the length of time water is detained, settling characteristics of particulate involved, and the proportion of dissolved and particulate phosphorus in the detained water. (Very small particles and dissolved phosphorus are not typically reduced by simple detention.) Data supplied and utilized for evaluation was from water years 1994 and 1995, and does not include particle size, settling rate, or particle-size based phosphorus concentrations. Spring flow-required detention volumes were calculated for Gold Fork, 400 acre-feet (design flow 800 cfs), Lake Fork, 450 acre-feet (design flow 900 cfs), North Fork, 1000 acre-feet (design flow 2000 cfs) (based on a 6 hour detention time). Runoff volumes for average storms were calculated for Gold Fork, 364 acre-feet, Lake Fork, 190 acre-feet,' North Fork, 410 acre-feet (resulting in a detention time greater than 6 hours). Annual average treatment rates (combining both spring and average flow rates) estimated for phosphorus and suspended sediments were 35 to 50% and 60 to 90% respectively, resulting in a total load reduction of up to 7% for Gold Fork, 3% for Lake Fork, and 15% for North Fork Payette (as total phosphorus). It was suggested that these values would be enhanced by concurrent chemical and/or wetland treatment to 9% for Gold Fork, 4% for Lake Fork, and 19% for North Fork Payette (as total phosphorus). Cost evaluations are listed as 2.2 million dollars for the three tributaries discussed. Additional costs associated with concurrent chemical or wetland treatments were assessed at a per- application cost of $156,000, Environmental impacts listed include creation and destruction of wetlands with resulting changes in water level, alterations in tributary access, barriers to fish movement, and poor water quality within the detention volume. Conclusions should be drawn with careful consideration of the assumptions made and limitations of the data available. This report does not account for any instream sediment deposition currently occurring, and does not address sedimentation or temperature issues for the detention structures if constructed.
Brown and Caldwell; Pending; Cascade Reservoir Performance Criteria, Technical Memorandum. (Not available at this time.)
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