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I I I I I I I I I I I I I I I I I I I DESIGN ALTERNATIVES This replacement option consists of four design alternatives. They are: Alternative I - original trapezoidal shape, Alternative 2 - rectangular channel shape, Alternative 3 .. partially full flowing pipe, and Alternative 4 - partially full flowing box culvert. These alternatives may be constructed with either a precast or a cast in place concrete construction. The concrete construction for Alternative 1 can incorporate a textured foml to resemble the existing hand placed rock. ALTERNATIVE 1 - TRAPEZOIDAL CHANNEL SHAPE This alternative will resemble the existing channel in dimensions and shape. The advantages are historical resemblance, minimal earthwork, and larger debris passag1e. The disadvantages are construction access and the cost of the temporary bypass for Willow Creek flow during construction. The field cost ofthis design alternative is estimated at $3,120,000 excluding appurtenances. ALTERNATIVE 2 - RECTANGULAR CHANNEL SHAPE This alternative will have a channel width of 16 feet and a depth of approximately 7 feet. The channel has a smaller cross section width than Alternative 1. The advantages are the extra room for construction access within the rights of way and construction of a Willow Creek flow bypass system. The disadvantages are less bypass capacity of large debris, and additional earthwork. The field cost of this design alternative is estimated at $3,040,000 excluding appurtenances. ALTERNATIVE 3 - P ARTIALL Y FULL FLOWING PIPE This alternative will use a precast concrete pipe with a 10 feet inside diameter. The advantage is the smallest cross section width providing room for a Willow Creek flow bypass system and construction access. Other advantages will be the elimination of pedestrian bridges and the safety hardware. The disadvantages are least capacity ofthe four alternatives to pass debris, the potential for debris plugging at the entrance, large construction equipment needed for the installation, and difficult maintenance inspection. The field cost of this design alternative is estimated at $4,150,000 excluding appurtenances. ALTERNATIVE 4 -PARTIALLY FULL FLOWING CONCRETE BOX CULVERT This alternative will use a precast concrete box culvert. The advantages are the same as Alternative 2 with the improvement in safety. The box culvert will eliminated the use of safety cables, safety fences, and pedestrian bridges. The disadvantages are the lower capacity for debris passage, the potential for debris plug at the entrance, large construction equipment needed for installation, and difficult maintenance inspection. The field cost of this design alternative is estimated at $6,750,000 excluding appurtenances. NRCS Northern Plains Engineering Team IV