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<br />~i <br />Mr, Brent Anderson - 3 - April 24, 1991 <br />6. Ensure that all ditches are designed in a configuration which is <br />operationally feasible. The Division seriously questions the projected <br />installation of ditches with slopes of less than 2a and the construction <br />of trapezoidal ditches with bottom widths less than 6'. <br />7. Pease document the source of the riprap sizing equation found on page 13 <br />and re-examine the equation to ensure that it is correct. Values that <br />the Division derives from the OSM Surface Water Diversion Design Manual <br />and SEDCAD+ are not comparable. <br />8. Document the use and sizing of either a granular filter layer or filter <br />fabric in the armored channels and road crossing installations. <br />9. The Division would like to reiterate that we have not approved any <br />permanent impoundments per the ACZ Engineering demonstrations at the <br />Grassy Gap site at this time. This is in sharp contrast to the statement <br />on page 14 of Attachment 2. The Division has seen no adequacy responses <br />to our October 23, 1990 letter. <br />10. The emergency spillway calculations for Pond 2 indicate that the existing <br />spillway is three feet deep. The Division would like to reiterate from <br />the October 23, 1990 letter that we need to see plan designs for this <br />• site. In addition the MLRD field inspection dated October 10, 1990 noted <br />that the Pond 2 emergency spillway was only 1,33' deep. This is too <br />shallow to pass the 100-year 24-hour event as calculated and does not <br />allow adequate freeboard. <br />11. The calculations for the Pond 5/6 emergency spillway need to be revised <br />to accommodate the additional drainage area contributing to the watershed <br />fallowing the removal of the upper collection ditch. <br />12. Please provide design certification for all analyses associated with the <br />ponds per Rule 4.05.6(7). <br />13. Please provide 100-year 24-hour culvert designs for the culverts beneath <br />the access roads going to Pits 1 and 2/3 per item 6 of NOY C-89-034. <br />14. Rock castle/ACZ is proposing to allow the upper diversion ditches on <br />Pits 4, 5 and 6 to silt in. As the Division indicated in the October 23, s <br />1990, engineering study review, this proposal is unacceptable per <br />Rule 4.05.3(5). The Division does not agree to the concept that <br />disturbance associated with such reclamation activities is 'significant, <br />unwarranted disturbance'. Rather, we feel that directed reclamation <br />activities will accelerate the revegetation at the site. <br />15. There is no discussion about the fate of the lower diversion ditch <br />immediately above Pit 6 which was noted as part of NOY C-89-034, item lA, <br />Please provide complete documentation. <br />U <br />