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4. Riprap Specification - The specification on Drawing 2, Upper Pond Details proposes using "well cemented sandstone or <br />conglomerate with a minimum specific gravity of 2.24. <br />a. Sandstone is typically not a satisfactory material for riprap applications due to its low durability, as it tends to <br />weather when exposed to water and freeze -thaw cycles. Please demonstrate the proposed material meets the <br />standard of care with respect to durability. <br />b. Specific gravity - Typical values for specific gravity are 2.4 to 2.65. However, given the low design peak flow <br />and the ability to account for the specific gravity in the selected methodology (see equation in Item 3 above }, <br />DRMS will allow a 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 limestone riprap being used <br />in the area. This may be a viable alternative.) <br />Response: See attached emails for discussion of appropriateness of sandstone as a riprap. As noted in <br />previous response, the DRMS supplied equation has been used to size the riprap. <br />5. Spillway Riprap and Filter Design - The longitudinal sections on Drawings 2 and 3 (Upper Pond Details and Lower <br />Pond Details) do not show riprap on the crest of the spillway and a geotextile filter fabric is proposed as underlayment <br />for the Lower Pond spillway. No filter is addressed for the Upper Pond spillway. DRMS experience suggests that the <br />use of geotextile as a filler for riprap on sleep slopes frequently results in failure. This is substantiated (for slopes <br />steeper than 25 %) in the Frizell Paper outlining the method used to size the riprap. DRMS will require a granular filler <br />for the riprap spillway. Furthermore, the riprap section needs to be extended across the entire crest of the spillway weir <br />section upstream lo downstream). Please revise the spillway designs for both the Upper and Lower Ponds to include <br />an appropriately designed granular filler and fully lined riprap spillway. <br />Response: A 3 -inch layer of 1 -1/2-inch D50 has been added at the crest of the spillway extending to the <br />upstream face of the berm. The missing label for the geotextile filter on the Upper Pond spillway has been <br />added. See the attached emails for discussion of the Frizell paper and geotextile filters. Due to price <br />increases for geotextile a granular filter for the primary riprap has been added as an option. <br />6. Spillway Cross Section Design — The lateral section on Drawing 3 (Lower Pond Details) shows a one -foot high vertical <br />section at the outside edge of the spillway section. This vertical slope is not sustainable using earthen materials for <br />construction. Please clarify what material is to be used to maintain this profile or revise the lateral section lo show a <br />side slope of 2H:IV or flatter. <br />Response: Additional detail has been added to the lateral spillway cross - section. <br />