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TETRA TECH <br /> June 28, 2016 <br /> Connie Davis, Land Advisor <br /> Aggregate Industries—WCR, Inc. <br /> 1687 Cole Boulevard, Suite 300 <br /> Golden, Colorado 80401 <br /> RE: DRMS Permit M-1999-034 Technical Revision No. 5, Cooley Reservoir, As- <br /> Constructed Certification; Thornton, Colorado; Tetra Tech Job No. 133-23514- <br /> 15002 <br /> Dear Ms. Davis: <br /> We have prepared this letter in response to the DRMS requirement to submit a certification by a <br /> qualified engineer certifying that the interim mitigation action has been constructed to the <br /> required compaction and specifications, approved through TR-05. Due to factors beyond our <br /> control, some field adjustments were made rapidly during construction to meet the intended goal <br /> of the action, which is to provide a stabilized and controlled inlet for the river into the reservoir <br /> until the permanent construction is completed. <br /> Two factors at the time of construction altered implementation of the interim mitigation action as <br /> originally planned: <br /> 1. The water level within the Cooley south reservoir was rising, which inundated the borrow <br /> source for fill material; and <br /> 2. The water level within the South Platte River was also rising and weather forecasts <br /> called for rain. <br /> The first factor was rising water surface elevations within the Cooley south reservoir. The City <br /> of Thornton, owner and operator of the reservoir facility, elected to fill the reservoir for its <br /> operation. The filling began inundating the alluvium that had been deposited in the reservoir as <br /> a result of the breach. This alluvium was intended to be used as borrow for the interim <br /> mitigation action. The volume of borrow material above the water surface elevation was <br /> estimated to be insufficient to complete the original plan. <br /> The second factor was the rising water surface elevation in the South Platte River. In addition <br /> to the expected increase in river flow in the spring, the weather forecast called for rain. At the <br /> time, we believed there was a credible risk of water re-entering the reservoir through the breach. <br /> Tetra Tech <br /> 1900 S.Sunset Street,Suite I-E, Longmont, CO 80501 <br /> Tel 303-772-5282 Fax 303-772-7039 www.tetratech.com <br />