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J Birds Mine — Stormwater Comments <br />Page 2 <br />December 6, 2011 <br />Spillway Comments: <br />3. The spillway and riprap sizing methodology used are appropriate. However, there are <br />some significant errors in obtaining values for "D50 *Cu " 5 " from Figure 4 in the Frizell <br />paper. Most notably, the fact that a riprap with a specific gravity of 2.24 is proposed, <br />whereas the selected methodology assumes a specific gravity of 2.65. DRMS notes that <br />an arbitrary size increase is used to account for the significantly lighter rock. Fortunately, <br />DRMS has obtained the equation used to develop Figure 4 from the Bureau of <br />Reclamation, which they obtained from the Ph. D. dissertation used in the Frizell paper <br />(ref. Mishra, Subhendru K., 1998. RIPRAP DESIGN FOR OVERTOPPED <br />EMBANKMENTS. Ph. D. dissertation, Civil Engineering Department, Colorado State <br />University, Fort Collins, CO, July 7, 1998). The equation is as follows: <br />va 4 t <br />0.52 sin a i.i <br />D SO C „ = 0.55 5314 \ (G s cos a- 1)(cosa tan 0 — sin g <br />a. Where: D50 = median stone size (meters); C„ = coefficient of uniformity; of = unit <br />discharge (m /s /m); S = embankment slope (dimensionless); a = embankment <br />slope (in degrees); Gs = specific gravity of the riprap; an cp = angle of repose of <br />the riprap. Note that this equation should be used with metric units as it is not <br />certain what physical parameters are represented by the 0.55 coefficient. <br />b. Using this equation, please provide revised riprap sizes to account for differences <br />in i) specific gravity; and ii) angle of repose for the proposed riprap. Please <br />contact me if you have any questions. <br />4. Riprap Specification — The specification on Drawing 2, Upper Pond Details proposes <br />using "well cemented sandstone or conglomerate with a minimum specific gravity of <br />2.24. <br />a. Sandstone is typically not a satisfactory material for riprap applications due to its <br />low durability, as it tends to weather when exposed to water and freeze -thaw <br />cycles. Please demonstrate the proposed material meets the standard of care with <br />respect to durability. <br />b. Specific gravity — Typical values for specific gravity are 2.4 to 2.65. However, <br />given the low design peak flow and the ability to account for the specific gravity <br />in the selected methodology (see equation in Item 3 above), DRMS will allow a <br />low density (but durable) rock (Gs > 2.24) if incorporated into the riprap sizing <br />methodology. Please revise the D50 accordingly. (Note: DRMS is aware of some <br />limestone riprap being used in the area. This may be a viable alternative.) <br />5. Spillway Riprap and Filter Design — The longitudinal sections on Drawings 2 and 3 <br />(Upper Pond Details and Lower Pond Details) do not show riprap on the crest of the <br />spillway and a geotextile filter fabric is proposed as underlayment for the Lower Pond <br />spillway. No filter is addressed for the Upper Pond spillway. DRMS experience <br />suggests that the use of geotextile as a filter for riprap on steep slopes frequently results <br />in failure. This is substantiated (for slopes steeper than 25 %) in the Frizell Paper <br />outlining the method used to size the riprap. DRMS will require a granular filter for the <br />riprap spillway. Furthermore, the riprap protection needs to be extended across the entire <br />crest of the spillway weir section (upstream to downstream). Please revise the spillway <br />m :\min1tc14n- 2005-050 jbirds \stormwatercomments_memo6decl 1.docx <br />