Fort Walton Beach Council agenda — 09-13-11 regular — reading text

pulvial line exhibits for the various design storm events as provided in Appendix B of NRCS Technical Release 55 ( TR55). Based upon these scenarios, HMM will provide critical storm data, directly from the model ( if compatible) or in tabular format to the City' s GIS department. The objective is for the City GIS department to create a map of the various flood areas for the critical storm event( s). HMM services in this task shall include the following efforts: Model Calibration Adjustment Historic storm events shall be reviewed relative to storm rainfall volumes, measured stream flow rates and documented flooding extents and timing. Hydrologic and hydraulic parameters, including unit hydmgraph/ peak rate factors, runoff CN numbers, channel roughness coefficients ( Manning' s N), initial water surface elevations and base flow rates, shall be modified based upon collected gauge data, historic or measured data and questionnaire responses as furnished by area residents. Parameter adjustments shall be made to calibrate the model results to match documented timing of water flows, surface elevations and extents of flooding. A meeting shall be held with City staff to discuss calibration efforts and results and identify final model revisions that are required prior to proceeding to the Design Storm Simulation phase. Design Storm Simulations Model simulations shall be generated for the 10, 25, 50 and 100 year critical storm frequencies. For each of these critical return periods, several storm duration simulations shall be modeled to determine the controlling critical slam, event for each node. Maximum stages for each return frequency and duration shall be compared to warning stages identified for each node. All nodes which experience flooding conditions shall be identified showing the maximum height reached above the warning stage and the duration and extents of flooding for each of the critical design storms. Model results shall be checked for continuity and stability concerns. Nodes or links within the model that exhibit anomalous behavior in stage or flow ( i. e. oscillations, divergence, etc.) shall be identified and addressed as necessary. Model Results Meeting and Design Criteria Finalization A meeting shall be held with City staff to review results, model simulations and identify required revisions to the final model prior to analysis for design alternative development. One component of this meeting will also be to compare model results against the current City Land Development Code so that the City may determine if code revisions may be desired. This may affect the future scenarios used to identify and prioritize capital improvements. page 9 City of Fort Walton Beach SW. amr Master Plan Scope.docx Packet Pg. 165  F 4.2.. HMM will present design criteria for City consideration. The City will approve the design criteria used for design alternative development and prioritization. Examples of criteria established at the meeting are: 1. Level of service desired 2. Concurrence on Critical Storm events 3. Prioritization of non/ transportation related flooding 4. Prioritization Categories and Weightings ( as described below) Task 4 Design Alternative Develonmenh. This task shall involve the development of design alternatives to address capacity and flooding issues, preparation of opinions of probable construction costs for the various alternatives and the evaluation and prioritization of the recommended drainage improvements. The following efforts will be completed by HMM under this task: Development of Design Alternatives: The final design storm simulation results shall be used to identify areas flooding in the existing conditions analysis. A conceptual design alternative for each area shall be developed to determine necessary drainage infrastructure improvements required to provide for the following: Eliminate roadway over -topping at culvert(bridge crossings during the 100 year storm event for all primary arteries and evacuation routes. Eliminate roadway over -toppings at culvertfbridge crossing during the 25 year stone event for all secondary arteries and collectors. Contain the 25 -year storm event within the banks of all open channels. Limit flooding during the 100 -year storm event to streets and yards. Minimize street and yard flooding during the 25 year storm event. Provide for future growth/ development in the basin where feasible. Opinions of Probable Costs: Preliminary Opinions of Probable Costs shall be prepared for all the design alternatives which were considered. Construction costs with quantities and unit prices, costs for land acquisition ( i.e., pond sites and easements or right -of -way), engineering costs including design, surveying, permitting and inspection, and contingencies such as mobilization, traffic control and utility relocations shall be included in the Opinions of Probable Costs. Unit price data shall be obtained either from the City of Fort Walton Beach or the FDOT Historic cost data base. Land acquisition costs shall be estimated jointly by City and HMM staff and are acknowledged to be rough order of magnitude costs. Pa €c 7 City of Fort Walton Beach Stormwatcr Master Plan Scope.docx Packet Pg. 166  Alternatives Evaluation and Prioritization: All proposed design alternatives shall be " scored" using a decision matrix consisting of seven major categories similar to that identified as follows: 1. Flooding Improvement 2. Provides for Future Growth 3. Constmctability 4. Permit- ability 5. hmpacts to Environmentally Sensitive Areas 6. Dependency /Independency ( on other design altematives) 7. Cost Versus Benefit. Categories are typically given a particular weight based upon importance and then scored on a scale from 1 to S. For example, categories 1 and 7 may be determined by the City to be twice the relative importance as categories 2 through 6. Therefore, their weighting would be 2, while categories 2 through 6 would have a weighting of 1. Total points are summed for each design alternative. The matrix scores shall then be utilized to prioritize the proposed improvements within a basin and compare them to proposed improvements from other basins. The resulting prioritizations shall be used to add projects to the City Capital Improvements Plan ( CIP) list in a logical manner consistent with the City' s needs, goals, and funding. Design Alternative Results Meeting: HMM shall meet with City staff to discuss the results of each design alternative proposed, the decision matrix rankings and identify any required modifications to the evaluations prior to preparation of the stormwater master plan document. Task 5 City of Fort Walton Beach Smrmwater Master plan Document: This task will involve the preparation of a comprehensive stormwater master plan document to be provided to the City of Fort Walton Beach. This document shall at a minimum detail the data collected, data inputs and input development information, existing conditions analysis, design alternatives analysis and rankings resulting From the modeling effort. N, City of Fort Walton Beach Stomawater Master Plan Scope. docx Packet Pg. 167  14. 2. 1 Attachment B City of Fort Walton Beach Stormwater Master Plan PmjBel NamB: City WFWBSWMP CG.. City W F. I W3ton 6— m Date September1, 3011 em Man -hour Estimate iaL_ a Rlndpala PrEn9es.p<L. 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