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Knight Piesold <br />co Mau ~rrro <br />2 <br />Mr. Steve Onorofskie <br />Centex Eagle Gypsum Company LLC <br />January 19, 2001 <br />these same purposes. Articulated blocks are generally less expensive than riprap, easy to <br />install and can easily be shown to be more effective than riprap. In either event, a <br />Technical Revision request would be submitted to the DMG. <br />3. SedCad Results: The complete set of SedCad results that Knight Piesold originally <br />provided may have been inadvertently omitted from the package submitted to the DMG <br />or otherwise misplaced. A complete set of SedCad results for all the storm water <br />management structures is attached. Due to revisions of the storm water ponds to <br />conform with the requirements of the State Engineer, the SedCad model was re-run. <br />Thus, the SedCad results provided supercede any that were included with the original <br />permit submittal. <br />4. Sediment Pond Failure Impacts on I-70: We did not investigate the impacts of the <br />unlikely event of a storm water pond embaril~nent failure on structures downstreant. The <br />pond designs include perforated riser pipes that will drain the relatively small volume of <br />water stored in the pond within five days of their filling and the dams will be constructed <br />to State Engineer specifications. The likelihood that [he embankment would fail under <br />these circumstances is remote. <br />5. Earthworks Specifications: Knight Piesold did not develop technical specifications and <br />quality assurance plans for the earthworks associated with the storm water management <br />structures. However, these documents will be developed prior to construction as a <br />Technical Revision to the permit and will tier off the specification requirements of the <br />State Engineer (see response item 1). <br />6. Outlet Works: Exhibit C-7 has been modified to include a perforated riser pipe to drain <br />down impounded storm water in the five proposed erosion control ponds. The modified <br />drawing is attached. There are no water rights issues associated with these storm water <br />management structures. <br />7. Design Storm: Storms with a 6-hr duration were also considered in the analysis; <br />however, since these storms provided lower peak runoff rates as well as lower sediment <br />concentrations, the 24-hr storm distribution was used to develop the design flow for the <br />hydraulic structures. The NRCS Type II storm distribution is used to represent storms in <br />Colorado; [his distribution is characterized by a brief period of high intensity rainfall near <br />the temporal center of the storm. Since this high intensity rainfall dictates the peak runoff <br />from the watershed, the storm with the greatest depth typically generates the larger peak <br />flow rates. From the NOAA Atlas, the 100-yr/6-Its and 100-yr/24-hr design storm depths <br />were determined to be 1.9 inches and 2.8 inches, respectfully, thus causing runoff to be <br />greater for the 24-hr storm event. <br />1\FST\GILLIGAN\t600S\1695A\VJP\State response tloc <br />