REV04005 - Laserfiche WebLink

Home Browse Search

Mathews Page 2 November 13, 2000 ld. The pond embankment designs for Pond #1 and Pond #2 do not show sufficient detail. The pond designs should provide enough detail so that an As-Built Certification can be made by the operators engineer and verified by the Division. Please provide a plan view drawing showing detail on pond geometry and capacity. le. Please clarify Note 3 on Map 89 for Ponds #1 and #2. This note indicates that the embankment will be breached for both ponds. The ponds should be designed so that the embankments are not breached. 1 f. A Colorado Discharge Permit and associated monitoring will be required for each of these proposed ponds. lg. As indicated in the SEDCAD 4 Design Manual and User's Guide, it is assumed that all sediment deposited across the elevation designed for sediment storage is retained and no resuspension of sediment is allowed. One implication of this is to locate the spillways sufficiently above the top of the sediment storage elevation such that velocities into the spillway are relatively small. The User's Guide suggests that a minimum two foot elevation difference exits between the invert of the principal spillway and the top of the sediment storage area. If a slow passive dewatering system is employed, the elevation difference can be reduced to 0.5 to 1.0 ft, depending on the size and flow rate of the dewatering inlet. Pond#1 and Pond #2 at the East Portal Area show that the top of the sediment storage is 0.82 ft and 0.73 ft below the primary spillway, respectively. The Division is concerned that sediment laden water out of compliance with the 0.5 ml/I settleable solid limitation may be discharged during a significant storm event as a result of resuspension of previously deposited sediment. The Division recommends that a minimum two foot elevation be maintained between the primary spillway and the top of the sediment storage area. Otherwise, please provide verification and sufficient detail to show that a slow passive dewatering system will be employed. A horizontal 4 inch discharge pipe does not represent a slow passive dewatering system. Please also provide a detailed plan view drawing of the proposed ponds showing pond geometry and location of inlet and outlet structures, and topographic information for SEDCAD Areas 1 and lA. ~ap~ ~, ~ T.4,,,k ~~„Lj As further justification for the Division's concern, a SEDCAD error message was obtained when f/,y,~.r attempting to duplicate the SEDCAD run for Pond #1. Utilizing the same input data that BME presented and a sediment capacity based on Average Annual R of 16.6 for 3 years, an error message was given indicating that there is not enough volume in the pond to contain the predicted sediment. Railroad Loop Area lh. Please provide the assumptions used in selecting the runoff curve numbers utilized in each of the SEDCAD runs provided for the Railroad Loop Area. This information should include the