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require replacement cribbing (1,000 total ft assuming new cribbing for both banks). For cost estimating purposes, all new cribbing is assumed to be stone masonry. In practice, cribbing type (stone, timbers, or concrete) will be coordinated with the individual property owners on a case- by-case basis. Modi!'ication or replacement of existing cribbing will require approval from the State and National Historic Preservation Offices; if such approval can not be 'obtained, this alternative is infeasible. of Lower Clear Creek. Therefore, the flood hazard mitIgation plan includes dredging and removal of accumulated sediment from the Lagoon at the mouth of Clear Creek. 6.2.4 Implementation Issues Alternative B (IO-Year Dredging + 100-Year Bvpass Conduit). Alternative B for South Clear Creek consists of dredging the channel invert to a depth sufficient to provide lO-year capacity in the channel and constructing a bypass conduit to carry flows in excess of the lO-year event. Dredging to provide lO-year capacity in South Clear Creek would extend from the confluence point with Clear Creek upstream for a distance of 725 ft to the Taos Street Bridge. The maximum required depth of dredging would be 1.0 ft between the Clear Creek confluence and the Rose Street Bridge. From the Rose Street Bridge upstream to the limit of dredging at Taos Street, typical dredging depths would be 0.5 ft. The proposed dredging profile for Alternative B is presented in Figure 6-3. A cost estimate for providing lO-year capacity in South Clear Creek through dredging is presented in Table 6-1. Note that because dredging depths at all locations are 1.0 ft or less, the cost estimate assumes that no cribbing will be undermined or require replacement as a result of the dredging work. Clear Creek and South Clear Creek are naturally aggrading channel systems. Therefore, maintenance-level channel dredging activities will be necessary from time to time after implementation of the flood mitigation projects. Maintenance frequency will vary depending on hydrologic conditions. Staff gages will be installed at the following locations to monitor sediment accumulation: South Clear Creek at 8th Street South Clear Creek at Rose Street Clear Creek at 15th Street When sediment has accumulated to a depth of 0.5 feet above the design channel invert, maintenance dredging activities will be scheduled. The capacity of the bypass conduit would be approximately 220 cfs, which is the difference between the 10- and 100-year discharge rates for South Clear Creek. In order to keep costs to a minimum, it has been assumed that the bypass conduit would be corrugated metal pipe (CMP) with a smooth interior. One example of such pipe is called Ultraflow and is produced by Contech in Denver. The required pipe diameter is 60 inches. Excess flows in South Clear Creek would be diverted into the conduit through a gated inlet structure at Griffith Street. The conduit alignment would follow Griffith Street north to Park Street, cross the ball field to 11th Street, and then extend west on 11th Street to Rose Street. From here, it would diagonal across the train station parking lot to Clear Creek. A cost estimate for the Alternative B b~ypass conduit is provided in Table 6-1. The proposed alignment for the conduit is illustrated in Figures 6-5 and 6-6. Removal of the top I to 2 feet of channel bed material may remove an armor layer and expose smaller bed material. The armor layer is formed naturally by the stream based on its sediment carrying capacity, and establishes a stable channel bottom. If smaller material is exposed, it could be mobilized by annual discharges, leading to channel instability problems. An analysis of bed material will be necessary to project potential impacts, It may be necessary to sort dredged bed material and salvage the larger size particles, which would then be replaced to restore the armor layer and prevent excessive erosion. There may be an opportunity to construct a debris basin on South Clear Creek near the Town water tank. The debris basin would reduce the need for dredging maintenance in the South Clear Creek channel. This option will be considered further by the Town in the future. Final design of a bypass conduit should include consideration of the corrosion potential of soils along the proposed alignment. In certain aggressive soil environments, the lifespan of concrete pipe c()uld be substantially larger than CMP. Consideration should also be given to the placement of weep holes in the pipe along Griffith Street south of II th Street. Groundwater levels are often high here and the pipe could serve as a drain to help mitigate that problem. The weep holes would also eliminate the tendency of the pipe to "float" during high groundwater periods. A concern relative to weep holes is the presence of an old sewer line in Griffith Street. Weep holes in the proposed CMP bypass conduit could intercept any potential leakage from the sewer line and thus deliver raw sewage to Clear Creek. The initial channel dredging work will generate up to 12,500 cubic yards of river rock (under Alternative A for South Clear Creek). The Town will develop a plan for disposal of this dredged material. The river rock may be a desirable resource for private construction and landscaping companies or County departments, and thus may represent a source of revenue for the Town if it can be stockpiled and sold. 6.2.3 Georgetown Lake At present, the Town Charter does not give the Town ownership or control of the banks (cribbing) on Clear Creek or South Clear Creek. The preferred alternative includes new or restored cribbing or floodwalls on both channels. In order to assure that the Town has the authority to construct adequate bank stabilization measures and maintain them to necessary standards, the Town must acquire responsibility for the channel banks. This may require modifying the Town Charter, which in turn may require an act of the State Legislature. The Town will investigate its options in this regard. No structural flood control improvements are required for Georgetown Lake. The primary concern at the Lake is deposition of sediment in the Lagoon at the mouth of Clear Creek. Sediment deposition is adversely impacting the Lagoon and could eventually reduce the capacity 6-4 6-5