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blocks and will encompass approximately 36 years. During this 36 yeaz <br />period it is likely that flood events for the main stem and south branch of <br />the St. Vrain River of various return intervals (frequencies) will occur. <br />On page G-6, the applicant states that overflow structures will be <br />provided for each of the ponds to be constructed within the reclaimed <br />area. Please provide engineering designs for each of the structures to be <br />constructed and provide the location of the structures on a revised map <br />(Reclamation Plan Map or Water Information Map). Please indicate <br />whether the overflow structures will be temporary or permanent <br />structures. If temporary, the structures should be designed to safely pass <br />the peak flow expected from a 25-yeaz storm event from their respective <br />watershed. If permanent, the structures should be designed to safely <br />pass the peak flow expected from a 100-yeaz storm event from their <br />respective watershed. The operator should use the greater of the peak <br />flows expected from a 6-hour vs 24-hour storm event and should <br />provide supporting hydrologic calculations. The operator should also <br />describe the watershed used to size the structures and specify whether <br />the watershed includes the entire basin for the main stem and south <br />branch of the St. Vrain or just the immediate area within the site itself. <br />Response: The Division is referred to Attachment #11, "Floodplain Impact Analysis <br />for the Lyons Pit-Rockin' WP Sand and Gravel Mining and Reclamation <br />Plan," prepazed by CP Engineering. <br />26. Comment: As required by Rule 6.4.7(b), please identify all known aquifers that may <br />be directly or indirectly affected by the proposed mining and reclamation <br />activities at the Lyons Pit-Rockin' WP operation The applicant should <br />also chazacterize the geology and depths of the aquifers as well as <br />pertinent aquifer characteristics, if known. <br />Response: The Division is referred to Attachment #8, "An Analysis of Possible <br />Groundwater and Surface Water Impacts Resulting from the Lyons- <br />Rockin' WP Gravel Pit," prepazed by Bishop Brogden Associates, Inc. <br />27. Comment: On page G-5, the applicant states that in order to ensure that the <br />proposed mining does not adversely impact the prevailing hydrologic <br />balance in the vicinity of the Lyons site Western Mobile will employ a <br />slurry wall and compaction of backfilled slopes as mitigation techniques. <br />DMG Adequacy Review Response Page 10 <br />