Laserfiche WebLink
,-, <br />Memo - Grassy Gap File - 3 - April 16, 1991 <br />10. Provide calculations to document that the reconstructed natural <br />drainageway (No. 1) can handle the 100-year 24-hour event. <br />11. Commit to removal of the diversion ditch on the northeast end of Pit 5 <br />and provide quantitative documentation tied to a 10-year 24-hour event <br />that the surrounding area qualifies as a small area exemption per <br />IV (4)(a) and Rule 4.05.2(3). <br />12. All permanent drainageways at the site must be designed and constructed <br />to handle the 100-year 24-hour event, per Rule 4.05.4(2)(1). Revise the <br />ditch, riprap and culvert calculations to reflect this sizing, <br />13. 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 2% and the construction <br />of trapezoidal ditches with bottom widths less than 6'. <br />14. Please 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 />15. Document the use and sizing of either a granular filter layer or filter <br />fabric in the armored channels and road crossing installations. <br />16. 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 />17. 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 artd does not <br />allow adequate freeboard. <br />18. 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 />following the removal of the upper collection ditch. <br />19. Please provide design certification for all analyses associated with the <br />ponds per Rule 4.05.6(7). <br />20. 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 NOV C-89-034. <br />