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Mitigation and Modification Alternatives ENGINEERING CONSIDERATIONS IN STRUCTURE MODIFICATION General Design and Construction Issues Based on cost considerations, structural modifications to diversion structures should be undertaken only where economically and ecologically justified. In light of potential secondary impacts to the river associated with structure modifications, consideration should be given to the net benefit gained from these activities. Potential secondary impacts include: • Increasing diversion efficiency and the potential for localized reach dewatering. • Inflexibility in adapting to natural river morphologic changes (a flood can isolate a constructed diversion as happened at the Walker Ditch); • Short-term construction impacts including water quality degradation and substrate disturbance; • Reduced (but not eliminated) maintenance effort; • Impacts on river morphology by establishment of rigid control points. With the exception of rock structures (e.g., the Maybell Canal), instream structures are generally temporary enough to consider their locations somewhat flexible and limited primarily by the location of the irrigation water delivery systems. To locate the best position for a diversion structure requires detailed site topography along approximately 1,000' of river as well as subsurface soils/geologic information to locate appropriate foundation conditions (drill holes or test pits to bedrock every 200' on both sides of the river). Vegetation and habitat mapping is also needed to optimize ecological benefits associated with structure placement. Similar information is needed for rock structures, though they are more fixed in location and may need to be rehabilitated in place. Bed armoring and/or stable slope analyses are necessary to design the structure for local stability as well as to reduce secondary impacts on the natural stream. Side channel diversions are generally sources of maintenance problems due to sediment and debris accumulation. At locations with that characteristic, the conveyance works must be adjusted to connect directly to the instream diversion feature. Flow duration relationships in the vicinity of each structure are needed to configure approach and downstream passage hydraulic characteristics and the structures lowflow passage feature characteristics and to determine the lowflow bypass needs (and possibly to define supplemental water needs). The ability to effectively pass lowflows and yet to structurally withstand highflows without damage or causing additional local flooding should be evaluated utilizing water surface profile calculations based on the aforementioned topographic information. Construction access and materials availability require significant attention. Since structures of this type are largely constructed with native materials for aesthetic and functional purposes, locating adequate quantities of large rock is a factor which significantly controls cost. Delivery of this material to the site, as well as delivery of such items as plant-mixed concrete 4-9