HomeMy WebLinkAbout2014-10-28_cityofsanrafael_83017f0875d0c5f731775b9d5d188448Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)
Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)GENERAL The existing and proposed site drainage is split between the two existing catch basins located at the corner of both Fourth Street and G Street. The drainage from the catch basins then drain into a storm drain manhole located near the northeasterly corner of Fourth and G Street, from here the drainage is contained in the City's existing storm drain facilities. The drainage for the proposed project is based upon the County of Marin Department of Public Works Hydrology Manual (Simplified Instruction, Revised 8-2-00) and the Design Review Plans prepared by SGP A Architecture and Planning and prepared for Epstein Properties, LLC. Q=CIA C value Site Area 1 curve per the manual it is suggested to assume that the ground is saturated, it is our opinion that all saturated ground will react to storm water as an impervious surface, therefore, we have assumed that the entire site, both existing and proposed, will yield a C value equal to 0.9 we are only including the on site area, 7,498 SF or 0.17 acres. The off site area will contribute to be concrete sidewalk and will continue to drain toward the street; therefore, there is no change to this area. refer to attached Maps V, I and Chart K to determine Zone C, design rainfall intensity 1.3 and i curve, respectively. Existing & proposed C = Existing & proposed A = Existing & proposed i = 0.9 0.17 acres from Chart "K" Zone C , EXISTING CONDITIONS tc = 1.8 (1.1-C) ~Ly/2 + 5 min (S (100)i/ L = 114' +/-; S = (42.16 -40.96)1114 = 0.0105 Y :\14064\Documents\Drainage\141 001 DRBHydrol.doc
Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)tc= 3.84/1.0164 + 5 min = 8.8 min; from Chart K, i = 3.1 QEX = 0.9 (3.1) (.17) = 0.47 cfs PROPOSED CONDITIONS A flat roof is proposed, roof plans have not been prepared for the design review submittal (Conceptual Plans), therefore, we have assumed that the longest roof run is 110' and will slope at an average of 1.5%. tc = 3.84/1.14 + 5 min = 8.3 min; from Chart K, i = 3.2 QProposed = 0.9 (3.2) (.17) = 0.49 cfs CONCLUSION The existing flow from the site is 0.47 cfs and the proposed flow from the site is 0.49 cfs. This represents a less than significant increase in flow. It is our opinion that there is no measurable change in the hydraulic grade line in the storm drain catch basin or piping and that no downstream improvements to the City's storm drain system will be required as a result of the construction of this project. FINAL CONSTRUCTION DESIGN This project is not fully designed and preliminary assumptions have been made. During the final construction document preparation, we will review these preliminary calculations and assumptions and adjust our findings and conclusions, accordingly. The owner is aware and amenable to including on site detention, as necessary on the final design plans, to detain the storm water flow so that there is no increase in flow from the site. Some forms of possible detention/retention could include piping or a storage tank installed in the parking area. Y:\14064\Documenls\Drainage\141001DRBHydrol.doc
Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)
Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)
Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)
Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)
Exhibit 5.dAugust 28, 2014 PC Meeting Exhibit 5.d (Hydrology)