HomeMy WebLinkAboutCD 2020-0022; THE BEACON PHASE 3; DRAINAGE MEMO CONSISTENCY DETERMINATION; 2021-12-16Blue Peak Engineering, Inc. • 18543 Yorba Linda Blvd., #235 • Yorba Linda, CA 92886 • 714.749.3077
www.bluepeakeng.com
December 16, 2021
City of Carlsbad 1200 Carlsbad Village Drive Carlsbad, CA 92008
Re: Drainage Memo Consistency Determination Beacon Phase III – El Camino Real GR2021-0024, DWG 532-1A
To Whom It May Concern:
In reference to the request for a Drainage Report for the above-referenced project, the following is a summary of the proposed drainage impacts from the project:
•The proposed project will maintain the existing drainage pattern which surface flows from the existingbuildings, drive aisles and parking areas out onto El Camino Real and into the public storm drain systemlocated in the public right-of-way.
•The proposed project will provide a reduction of the impervious area of the site by replacing existingimpervious surfaces with landscaping and pervious paving, thereby decreasing the runoff coefficient, C,included in the rational method equation. The Grading Plan included in this submittal provides additionaldetails as to the quantities of impervious and pervious surfaces as well as the project drainage patterns.
•The rational method is commonly used for determining peak discharge from relatively small drainage areas.The rational method is based on the following equation:
Q = C I A
Where:Q = peak discharge, in cubic feet per second (cfs)
C = runoff coefficient, proportion (impervious surface vs. pervious surface) of the rainfall that runs off thesurface (no units)
I = average rainfall intensity (in/hr)
A = Drainage Area (acres)
•Since the rainfall intensity, I, and the drainage area, A, in the rational method equation remain the same for the
pre- and post-development and the runoff coefficient, C, is decreased by reducing the project impervioussurface area, the peak discharge, Q, from the proposed project will be less than that of the existing condition.
Drainage Memo – Beacon Phase III Page 2 of 2
Blue Peak Engineering, Inc. • 18543 Yorba Linda Blvd., #235 • Yorba Linda, CA 92886 • 714.749.3077
www.bluepeakeng.com
•The proposed flow from DMA-1 (as shown on the Single Sheet BMP Site Plan) is calculated as follows:
Q100 = C I A = 0.85 X 4.21 X 0.18 = 0.64 CFS
The proposed outlet piping for the water quality basin is a 6" pipe from the overflow riser down to the proposed sidewalk underdrain. The 6" pipe at a minimum slope of 2.0% has a capacity of 0.84 CFS, which is greater than the 0.64 CFS calculated above. The sidewalk underdrain, San Diego Regional Standard D-25A, a 3’ wide by 3” high rectangular channel at a minimum slope of 2.0%, has a capacity of 4.3 CFS, which is greater than the 0.64 CFS calculated above. The water quality basin’s outlet piping is sufficient.
Respectfully submitted,
Robert J. DePrat, PE President/CEO
100
2.5
10
4.21