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<br />. <br />. <br />. <br />. <br />. <br />. <br />t <br />t <br />t <br />t <br />t <br />. <br />. <br />. <br />. <br />t <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />t <br />t <br />t <br />t <br />t <br />t <br />. <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br />t <br /> <br />Proiect Description and Alternatives <br /> <br />The purpose of this project is to establish an unregulated spillway at Empire Reservoir in accordance with <br />the requirements outlined in the Dam Safety Branch of the Colorado State Engineer's Office (SEO). <br />Currently, storm flow discharges are routed through outlet conduits and directed to the Bijou Ditch. The <br />current policy of the SEO does not permit inclusion of gated outlets in the analysis of flood routing. In <br />addition, the state has outlined concerns related to increased seepage as a result of increased reservoir <br />head caused by storm runoff. <br /> <br />Three alternatives were considered for the project including: <br /> <br />I. Do nothing <br />2. Construct a new spillway only <br />3. Construct a new spillway and place new toe drains in the Outlet Embankment and McIntyre Dike <br /> <br />· Alternative No.1: Continue the operation of the reservoir at the restricted gage height and <br />disregard the SEO direction to the BID to construct a spillway, the "do-nothing" alternative was <br />not considered. <br /> <br />· Alternative No.2: Construction of a new spillway without other improvements would satisfy the <br />flood routing concerns voiced by the SEO, but would not address the SEO concerns of seepage <br />through the dam and the potential for internal instabilities resulting from such seepage. This <br />alternative does not address all the SEO concerns and was not considered. <br /> <br />· Alternative No.3: Construct a spillway capable of passing storm flows in accordance with the <br />requirements outlined by the SEO. Address the embankment stability concerns as required by the <br />SEO by installing a seepage collection system for the dams. The Outlet Embankment and <br />McIntyre Dike have seepage collection ponds at the toe of the embankments. Adequate records <br />or physical evidence of toe drains in the Outlet Embankment and McIntyre Dike which discharge <br />into the collection ponds could not be found. The East Embankment has working toe drains <br />which were installed in the mid-I 980s. As-built records of these drains are available for review. <br />Additional seepage control work on the East Embankment is not currently proposed. Alternative <br />No.3 appeared to address each area of concern voiced by the SEO; therefore Alternative No.3 <br />was selected. <br /> <br />Selected Alternative <br /> <br />Alternative No.3 involves the construction of a spillway with a crest elevation of 4519.6 feet (Gage <br />Height 30) capable of passing approximately 592 cubic feet per second (cfs). Additionally, <br />approximately 4,400 linear feet total of toe drain is to be constructed at the Outlet Embankment and <br />McIntyre Dike. Each of the toe drains will have cleanout and manholes at regular intervals for inspection <br />and future location confirmation. A Parshall Flume will be installed at each toe drain outfall to quantify <br />discharge to determine seepage flow rates. Feasibility level design drawings can be found in Appendix B. <br /> <br />The emergency spillway will have a concrete crest and two concrete drop structures. The main channel <br />will be grass-lined with riprap reinforcing at key areas for hydraulic stability. The spillway will be <br />located at the Outlet Embankment and will outfall into the main outlet channel and will be directed into <br />the Bijou Ditch. <br /> <br />Feasibility Study <br />Empire Reservoir Spillway Construction <br /> <br />-2- <br /> <br />June 2005 <br /> <br />F:\4763 _00 l\Feasibility Sludy\Feasibility _ SIUdy.1pLdoc <br />