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Memo to Kate Pickford 2 March 27, 2006 <br />Detention Pond Desi Ig ], 2ni Review File No. M-2005-045 <br />telephone discussion DMG had with Matt Carnahan, agent for Gravel LLC, during January 2006. (As <br />opposed to the affected runoff routing ditches included in the designs in the Applicant's letter report <br />dated February 27, 2006). If the Applicant were to use overburden derived from stripping in the pit area, <br />the afore-quoted DMG requirement could not be met. <br />c) In order to be effective for run-on diversion, perimeter berms must be continuous, and there is currently <br />insufficient subsurface stratigraphic information in the permit application materials to determine if <br />sufficient overburden is available to construct effective run-on control berms. <br />d) The access road(s) to the pit must at all times during the life of the operation pierce the perimeter berms <br />at locations where runoff from the road(s) and any areas that would contribute runoff to gaps piercing the <br />perimeter berms would not be tributary to the detention pond. <br />If the Applicant does intend to use perimeter berms, the foregoing list ofpotentially problematic issues must be <br />addressed. Item b) could be addressed by a commitment to strip the Phase A area and complete the construction <br />of the berms during the portion of a single year when the potential for flooding is low, i.e. from November 1 s` to <br />March 1~`. In addition, DMG will require a surface water hydrology report with a determination of the peak nin- <br />on flows to be diverted around the proposed pit area from the up gradient basin for the 100-year, 24-hour <br />precipitation event. DMG further requires designs for the berms including berm sizing, construction <br />specifications, and armoring designs for the outslopes to resist erosion during the design run-on event. <br />Alternatively, if the Applicant chooses to use perimeter diversion ditches, as was discussed in the January 2006 <br />telephone conversation with Mr. Carnahan, the same type of hydrology report will be necessary, as well ditch <br />designs sufficient to assure diversion of the design event around affected land. Also, access road ditch crossing <br />details must be provided. <br />3. Detention Pond Construction Specifications <br />Detailed earthwork specifications and a quality assurance plan must be provided for DMG review and approval. <br />Specifications and testing for the placement of dam embankment fill must include, at a minimum: <br />a) Grain size distribution constraints. <br />b) Minimum allowable density specified as a percentage of maximum dry density measured by the <br />Standard Proctor Test. At least 95 percent compaction must be specified unless that Applicant can <br />provide sufficient rationale that a lower percentage will be sufficient. <br />c) Maximum lift thickness; no more than 8-inch loose lift thickness should be specified. <br />d) Riprap sizing specifications sufficient to prevent erosion during design event maximum flow velocities. <br />e) Riprap bedding specification (not necessary if all armoring is to be gabions). <br />f) Riprap soundness specification stating that the riprap to be installed will be sound and durable, and will <br />be non-slaking. <br />g) The Applicant must provide a general plan for testing of fill and riprap to assure that the specifications <br />will be met. This plan will describe the tests to be employed with reference to applicable ASTM or other <br />standards, the frequency of testing to be conducted, and a commitment to provide DMG with a <br />construction report, including test results, within 45 days following completion of the detention pond. <br />cc: Harry Posey, DMG (via email) <br />Bruce Humphries, DMG (via email) <br />u~DOCUmenrs and SettingsWCSU-Iy DocunentsUock dog detention pond 20° review.doc <br />