Laserfiche WebLink
Response to Division M- 2005 -050 TR02 Review Letter of September 26, 2011 <br />The Division's comments from the Division's M- 2005 -050 Technical Revision 02 review letter dated <br />September 26, 2011 are listed below. Responses by Rimrock (James Pierce 970 - 864 -7455) to <br />each specific Division comment have been inserted following the relevant Division comment. <br />Division Comment 1) There are no conceptual drawings of the spillways. However, the <br />"Revised Drainage" drawing suggests they are to be riprap -lined structures. The weir <br />coefficient used in the calculations implies a smooth concrete spillway section. A riprap- <br />lined spillway will be irregular and have more head loss, therefore be less efficient. As <br />such they would be undersized. The Division is requesting drawings to clarify the intended <br />spillway design concept and revised calculations if the spillway is not a concrete section. <br />Response - Per the Division's request spillway drawings are being submitted. As <br />shown in the attached drawings the throat of the weirs will be a Geo- textile lined <br />section. If the Division has a more relevant coefficient for this, please forward it. <br />This is also detailed more fully in the response to 6) below where the actual flow <br />quantities will be two- thirds to half of the design quantities based on the TR -20 <br />analysis. <br />Division Comment 2) If the spillways are in fact to be riprap - lined, the effective spillway crest <br />(for storage, not discharge) is at the elevation of the bedding layer below the riprap, not <br />the top of riprap (Le., flows will seep /leak through the riprap). This effectively reduces the <br />available storage capacity of the pond to the bedding layer elevation unless a cutoff wall is <br />installed in the spillway. The Division requests a clarification (again of the spillway design <br />concept) and if no cutoff wall is to be included the design, the available storage should be <br />adjusted accordingly. <br />Response - As depicted in the submitted sections the riprap lined portion of the <br />spillway occurs after the spillway has passed through the dam and the discharge <br />drops into it. <br />Division Comment 3) There are no riprap size calculations. Given the low expected flows, a <br />six -inch D50 is almost certainly adequate, but we should request some calculations and <br />design specs (to include gradation, shape, and density, layer thickness, etc.). CDOT's <br />Standard Specifications for Road and Bridge Construction, Division 506 (or equivalent <br />would suffice). <br />Response - As per the riprap design for the Prince Albert Mine 110d application, the <br />BURec PAP 790 'Simplified Design of Riprap Subject to Overtopping Flows' <br />procedure was followed. <br />Division Comment 4) The calculations do not address weir depth, only flow depth. We need <br />enough information to assess freeboard. I think 6 inches (above flow depth) is adequate in <br />this case if there is no cutoff wall, a foot should be required if there is a cutoff wall. <br />Response - No cutoff wall was intended nor has one been added. Additionally, based <br />on the Division recommended recalculation of the storm data using the TR -55 <br />methodology, pond volume requirements are significantly reduced. Therefore, the <br />spillway elevations were lowered and additional freeboard is available. <br />Page 1 of 2 <br />