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Craig Ranch Pit — Response to Adequacy Stormwater Comments <br />Page 2 <br />October 27, 2011 <br />3. Page E -5, first paragraph/precipitation — The Applicant states the 100 -year, 24 -hour <br />precipitation event is 2.2 inches. The Ellis Letter (p. 2, Item 5) states the value is 2.6 <br />inches as requested by the Division. Please revise the text in the Application accordingly. <br />4. Page E -5, first paragraph/Curve Number — The Applicant states a conservative curve <br />number (CN) of 85 is used for calculations. The Ellis Letter (p. 2, Item 6) states the curve <br />number used is 88 as requested by the Division. Please revise the text in the Application <br />accordingly. <br />5. Page G -1, second paragraph: This paragraph states "all surface water will be contained <br />on site ". This statement should be revised to indicate the stormwater management system <br />is designed to control and contain runoff from the 100 -year, 24 -hour precipitation event. <br />6. Page G -1, Hydrology and Sediment Control — The Division previously requested separate <br />calculations for each sediment pond (Comment 7a of the Division's June 23, 2011 <br />Memorandum). There should be a separate runoff calculation for the Western and <br />Eastern Sediment Ponds shown on Map G. Furthermore, the 119.56 acres is less than the <br />sum total of all seven of the stormwater basins labeled on Map G (159.1 acres). This <br />calculation should be revised to include all 159.1 acres, but with separate calculations for <br />each sediment pond, based on the appropriate contributing areas. <br />a. Note — The Ellis Letter states the 119.56 acres is the contributing area for the <br />Western Sediment Pond. However, assuming SWB3 through SWB7 are the <br />contributing areas, the total is 118.84 acres. Please be specific about which basins <br />contribute to which sediment pond and that the total contributing area is <br />consistent. <br />b. The Ellis letter states the Eastern Sediment Pond has 40.26 contributing acres. <br />This is consistent with the assumption that SWB1 and 2 are the only contributing <br />basins. <br />7. Page G -2, first paragraph: The descriptions for depth and surface area of the two <br />sediment ponds have reversed the names "west" and "east" when compared to the labels <br />on Map G. Please correct this and use consistent nomenclature as in "Western" and <br />"Eastern". <br />a. Also note the 24 -hour duration of the 100 -year design storm is not stated in this <br />paragraph. Please be specific that the 100 -year design storm for the project is 24 <br />hours when revising the text. <br />8. Page G -2, second paragraph: The descriptions for the collection ditches states they will <br />be constructed by building gravel berms. The Division is concerned about the <br />effectiveness and sustainability with gravel as the construction material. A gravel berm is <br />typically porous and would be expected to leak. Also, the design calculation figure <br />(second page after page G -11) shows a "zero" top width of the gravel berm. This narrow <br />peak will accelerate the time to erode the height of the berm and make compaction during <br />construction difficult. Please provide a discussion on how gravel berms will be <br />constructed so as to prevent leakage and be sustainable (i.e., minimize piping and erosion <br />failures). <br />c:\ users \thm \appdata \local\microsoft \windows \temporary internet <br />files \ content. outlook\ 3nyv6den\stormwatercomments mem27oct11.docx <br />