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APPROACH / METHODOLOGY Suitability Of the Methodologies and Armroaches Used In Achieving the Tasks E; Drainage Analysis: The project is located on NE 174th Street between Collins Avenue and approximately 700 ft west of Collins Avenue. The existing drainage system consists of nine catch basins maintained by the City (the two catch basins exist near the radius returns at Collins Avenue are maintained by FDOT and are part of their system). The two catch basins at the west end of the project are connected to a 30" outfall pipe that drains due south along the western property line of the Winston Towers condominium and outlets to a finger of Dumbfoundling Bay. The remaining catch basins utilize French drain to exfiltrate stormwater runoff locally. This project proposes to improve the drainage system along the NE 174th Street to relieve the flooding generated after high intensity, short duration rainfalls. As presented in the RFQ, there are three potential options to resolve the drainage issues: I. Exfiltration System (French Drain) 2. Deep Wells 3. Outfall Line The roadway elevation for this segment of NE 174th Street ranges from approximately 3.5' to 5.0'. Considering this low elevation and the high water table due to proximity to Dumbfoundling Bay and the Atlantic Ocean (high/ low tides), it is unlikely that an exfiltration system, or French drains, will provide adequate drainage to alleviate stormwater ponding on the street. For these reasons we eliminated Option 1 from consideration. Option 2 will require two deep ells and p approximately 600 LF of storm drain pipe. The cost of this deep well drainage system alone, excluding roadway and other improvements, would be approximately $150,000. Option 3, outfall line, will require one 10ft x 4ft control structure, four manholes and approximately 800 LF of i storm drain pipe. This option has the advantage of utilizing an existing 30" outfall pipe to Dumbfoundling Bay. The cost of this outfall line storm drain system alone, excluding roadway and other improvements, would be r t approximately $100,000. Based on our initial analysis, BHA favors the outfall line option, which will cost approximately $50,000 less to y construct than the deep well option. BHA proposes to tie the existing catch basins into a single system that utilizes the existing 30" outfall to f Dumbfoundling Bay. To accomplish this, a 24" storm drain line will be installed beneath the northernmost westbound travel lane of NE 174th Street, immediately south of the existing communications duct bank that runs J adjacent to the shoulder. Existing catch basins on the south side of NE 174th Street will be connected to the system via laterals that tie into the new manholes. A control structure will be constructed just east of the existing 30" outfall line, which will accept stormwater from the new system. This project must satisfy Miami Dade County guidelines for water quality. The County has specific requirements 1 pertaining to the water quality volume as explained in Section D -4 of the Miami Dade County Public Works Water Control Design Manual. A detention volume equal to the runoff generated by a 5 -year storm (Miami Dade County IDF Curve), and total duration T, of t1"+ tc. tc = 10 minutes 2940x F-"-'' 308.5 x C - 60.5 x (0.5895 + F-'-") Tt =tc +t" The storm intensity will be computed by the formula: t „= 2940x F -o.i i 1 308.5 x C - 60.5 x (0.5895 + F -0 `1 ) BHA . Request for 9ualifications City of Sunny Ines Beach ® RFQ 11 -11-02 beiswenger Nosh b Associates ■ Civil Engineering Services To design Street and Drainage improvements