2012-02-28_REVISION - M1980244 (80)

Home Browse Search

2. SEDIMENT REMOVAL EFFICIENCY AND PERFORMANCE Performance -based pond sizing criteria was developed for the MLE1 storm water assessment that would ensure that consistent and reasonable removal of sediment will occur in the detention ponds. Two functional types of ponds are generally constructed at the mine. Simple detention ponds (Best Management Practice or BMP Ponds) are built based on runoff within sub - watersheds where functionality is important but that conservative design criteria is not regulated or justified. Such ponds are typically designed for lower magnitude, more frequent storms and are sized for the discharge volume from a single storm. Terminal Enhanced Management Practice detention ponds ( "EMP Ponds ") are constructed for twice the volume of the 10- year /24 -hour storm. Implicit within those criteria are several factors to ensure that sediment and water release will be automatically controlled without administrative intervention. For example, the second storm "volume" provides assurance that inevitable accumulations of sediment from previous storm events will not hinder the ability to store the first 10 -year storm. The beginning condition for these MLE2 SEDCAD computations includes the assumption that two feet of sediment accumulation would be present before the storm. In addition, the provision of two storm volumes will ensure that effective sediment removal can occur, even if a second storm occurs on the tail of the first. Sediment samples from undisturbed areas of Grassy Valley were analyzed by conducting hydrometer settling tests as part of the MLE1 storm water assessment to determine the expected performance of detention ponds for settling and removing sediment from storm water. (The characteristics of those samples were consistent with the descriptions of the soils report.) It was concluded, and adopted as a procedure for MLE1, that by conforming to certain geometric relationships (shapes) in the detention pond design and by adopting the double 10- yr /24 -hr volume criteria a predictable performance for sediment removal can be anticipated. The two geometric properties are simply their construction with 2:1 interior slopes and a 15 -foot depth. From those two properties and by knowing the required storage volume the surface area and base area can be calculated and the following performance can be expected: o If two 10- yr /24 -hr storms occur in quick succession (a matter of a day or two) the EMP Ponds should remove essentially all but colloidal clays (98% total removal) within a day after cessation of runoff. Client: CC &V Gold Mining Co. 2/2/12 Title: Storm Water Management, MLE2 Page 5