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I I I I I I I I 1 I I I ,I I I I I I I Alternative NO.3. This alternative is to completely remove and replace the existing structure and gates. 1. Completely remove the existing structure, including the concrete base slab. 2. Construct a new concrete base slab with a cutoff into bedrock. 3 . Install a control gate and bay on the east end of the structure similar to the existing sand gate. Operator would be motor driven. 4. Install a concrete wall overflow section in place of the existing gates and stop logs. 5. Install a downstream concrete, erosion-control apron and stilling basin with energy dissipation blocks. 6. Rebuild and raise the abutments at each end and provide riprap erosion control protection to reduce the potential for removal of the abutment areas during a flood. 7. Construct a measuring flume and concrete training walls downstream of the new control gate to allow measurement of flows passed through the gate. The top of the spillway would be set at elevation 98.0, which would provide the maximum head required to deliver water into the main canal at any time during the operating season. Peak flows (3,170 cfs) would pass a 20-foot salld gate and flow over the uncontrolled spillway crest. Also, the diversion structure will be designed to pass a flow of 3,170 cfs. This flow would be distributed through the enlarged sand gate and also over the top of the new spillway section. 2. Foundation PreDaration. If a new structure is constructed at the site, the base slab shall be required to bear on the unweathered blue-gray Pierre shale. This material is expected at approximately an eleven-foot (11 ') to thirteen-foot (13 ') depth based on the test borings at the abutments. The exact depth to rock should be determined prior to fmal design by drilling on a pad constructed in the river. The upper portions of the shale will be excavated and removed until hard competent shale is encountered, estimated to be approximately fifu:en feet (15') deep. It is expected this material can be excavated with a backhoe and will not require blasting. The shale must be protected during and after excavation as it looses strength upon drying. 3. Concrete Reauirements. Concrete for the structure will be designed to be highly impermeable and to have a high resistance to freeze-thaw action. This will require a mix with a high air content, low water-to-cement ratio, high paste content, and high sand content. The PAGE 12. CbapcerV