Board of Adjustment Agenda and Packet 2003 04 16

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As presented in the application information, the Water Resources Engineer report states the following: "The analysis shows a flow depth (existing 100 year flood elevation) of 5312.47 feet at the proposed bridge site.... The proposed condition (construction) analysis shows the proposed roadway fills and bridge construction of the project will increase the base flood elevation .6 feet (.58 actual) ... with a new flow depth of 5313.05 feet." Further, "Flood flow velocities in the (project area) increase 5 to 9 feet per second. These velocities will not have significant negative erosion or sedimentation effects considering the existing conditions...." As a result of this proposal, the flow characteristics of the proposed channel (flow velocity) appears to be in an acceptable range of a 5 to 9 cubic foot increase, (then at 3905 to 3909 cubic feet per second). Secondly, the base flood elevation of 5312.47 feet is only increased to a 5313.05-foot depth, just under the permitted increase of one foot at any point within the project as listen in Section 17.56.160 A. The requirement of certification by a registered professional engineer is included in the application information, but due to it's size, (over 150 pages) the full report was not included as a document to review. Staff has confirmed that an Engineer has certified the report, which can be found in the March 2003 study on record in the Public Works Department. • 17.56.160 B. Structures: 1. "Structures will have low flood damage" 2. "The structure will be constructed and placed on the building site so as to offer the minimum obstruction to the flow of floodwaters...." 3. "Structures will be firmly anchored as required to prevent the structure from floating away...." As previously described, the structures will have "low flood damage", the velocity of water and the flow increase is within the permitted limits of Section 17.56.160 A. and B. During a 100-year flood, the bridge is designed to maintain it's structural integrity and should be secured into the sandstone and claystone bedrock found at the base of the floodway. • 17.56.160 C. Fills or Deposits: 1. "Any fill or deposit of materials shall comply with subsection A of this section." 2. "The fill or deposit materials shall have some beneficial purpose (and the amount shall not exceed what is needed during construction) " 4