HomeMy WebLinkAbout26_ Detention Analvsis
The drainage system for the project site was designed in accordance with the 1990 King County
Surface Water Design Nlanual (KCSWD l) The peak flows for the associated hydraulic design
were determined using the Santa Barbara Urban HYdrogtaph (SBLW) Method with Type IA
rainfall distribution- The hydrologic computer program Stormshed -as used to develop desi
hydrographs (stormwater calculations are included in Appendix B). gn
-- Impervious and pervious surface areas were determined for both the existing and proposed site
conditions as shown in the following table-
'.. Drainage Basin Areas Table
Area ExistingConditions Pro Conditions
ous Area ac) 13-02 acres 12.77 acres
3 Pervious Area (ac) 0.0 acres ' ��
° Total Area ac ! 13.02 acres 0�_ acres
13.02 acres
Peak flow rates for the 100-yri24-hr design storm were determined for both the existing and
Proposed site conditions as shown in the following table -
Peak Flow Rates Table
Peak Flow Rate Existing Conditions Proposed Conditions
I00-vr/24-hr storm (cfs) 11.33 cfs 1 128 cfs
The decrease in peak runoff rate for the 100-yr,24.hr desigr. storm for
conditions was 0-05 cfs. Proposed developed
In accordance with the -'Exemptions From On -Site Peak Rate Runoff Control" section of the
KCSWDRL detention will not be required for this project due to a negligible increase in peak
runoff rate for the 100-year, 24-hour storm event. The increase in the peak runoff rate for the I00-
Yr/24-hr storm event is less than 0.5 cfs-
Water Quality 4natvsis
Imperious surface areas subject to vehicular use were determined for both the existing and
proposed site conditions as shown in the following table- -
Impervious Areas Subject to Vehicular Use Table
Conditions ' Pro Conditions
j Vehicular Area (sfl 567,151 sf 405,108 sf
DLR Group