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HomeMy WebLinkAbout2800 and 2804 Calloway Stormwater Runoff 5509 Young Street Bakersfield, CA 93311 Tel: 661.325.9474 - Fax 661.322.0129 www.cornerstoneeng.com Engineering: Civil Electrical Mechanical Process Structural Surveying Project Management Staff Augmentation Drone Services November 6, 2020 File: [727-01-03] Manpreet Behl 1501 Truxtun Ave Bakersfield, CA 93301 RE: 2800 and 2816 Calloway Stormwater Runoff The purpose of this letter is to evaluate the runoff from the development of Parcel A of Lot Line Adjustment 02-0204 (LLA). The project area is located on the southeast corner of Calloway Drive and Flushing Quail Road. The project runoff area consists of three parcels; parcels A, B and C of the LLA. Two of these, parcels B and C are fully developed and are referenced in the attached Storm Drain Master Plan done by Porter Robertson Engineering (2005). These parcels are already developed and the storm water runoff flowing to the existing catch basin at Node181 in the PR study, located on the north side of the project site, on the south side of Flushing Quail. As Parcel A will be improved, we have reviewed and updated the drainage calculations completed by Porter Robertson Engineering. We have determined that the existing catch basin and downstream pipe have sufficient capacity to handle the incremental runoff from the development of Parcel A. Existing Condition: The existing catch basin on Flushing Quail (node 181) is a standard catch basin with a 3.5 ft opening. According the PR study this catch basin has a capacity of 6.5 cfs and the existing Q5 at basin 181 is 3.64 cfs. Downstream of the catch basin is an 18’ SD pipe. The existing pipe flow (Q10) is 4.20 cfs. Proposed condition: The addition of Parcel A to the existing condition will result in a total flow to the catch basin of (Q5) of 5.27 cfs and a total flow in the SD pipe of (Q10) of 6.1 cfs. Catch Basin: The catch basin 181 has an inlet capacity of 5.6 cfs per the PR study. The proposed Q5 to the catch basin is 5.27 cfs. The catch basin is properly sized for the additional runoff. Downstream Pipe: Factoring in the additional runoff from the Parcel A results in the addition of 1.9 cfs to the downstream pipe network. This raises the HGL at basin 181 by a total of 1.05’ which reduces the available freeboard to 1.66’. The additional flow to the network does not adversely affect the collection pipe system. The impact on the rest of the system is negligible because all but 0.09’ of HGL rise is in the last two 18” pipe runs. The overall rise in the rest of the collection system is only 0.09’. Attached you will find the following exhibits to support these conclusions: Runoff drainage calcs for the Parcel A, B and C study area. HGL analysis with revisions to the PR study. 5509 Young Street Bakersfield, CA 93311 Tel: 661.325.9474 - Fax 661.322.0129 www.cornerstoneeng.com Attachments Exhibit of Parcels Drainage Calculations for Parcels B & C of LLA 02-0204 5509 Young Street Bakersfield, CA 93311 Tel: 661.325.9474 - Fax 661.322.0129 www.cornerstoneeng.com Parcel Exhibit 1.54 Acres Calculations Existing Conditions: A= 3.46 Acres C=0.9 Per attached Porter Robertson Engineering Drainage Calculation for Parcel B & C of LLA 02-0204: C*A = 3.11 Tc = 17 min I5 = 1.17 I10 = 1.35 Proposed Conditions A = 3.46 + 1.54 = 5.00 Acres Q5 = (0.9)(1.17)(5.00) = 5.27 cfs Q10 = (0.9)(1.35)(5.0) = 6.1 cfs (proposed) Per attached Porter Robertson Engineering Drainage Calculation for Parcel B & C of LLA 02-0204: Existing CB full capacity is 1.6 cfs/ft * 3.5 ft wide = 5.6 cfs Proposed Runoff = 5.27 cfs Existing Pipe per the Drainage Calculations for Parcel A, B & C of LLA 02-0204: See attached HGL calculations with additional 1.9 cfs to the pipe system