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, . <br /> H N Lehtinen � <br /> StructuraU Civil Engineers <br /> 3216 Wetmore Ave. <br /> Everett, WA 98201 �� - •- - - <br /> (206)252-2�73 <br /> Feb. 5, 1998 <br /> Basis: Use SBUfi/SCS �vlethod for sizing and design of stomi water detention sysfem <br /> with allowable peak discharge flow rate being equal to 50%of the "2 YR. Developed" <br /> 24 Hour peak rate, while maintaining the peak rates of botli rhe "10 Yr. And 100 Yr. <br /> Developed" 24 Hour storm events, by use of on-site storage and contrulled release. <br /> Determine the total precipation from the isographs for a . <br /> " 2 Y�.-24 Hr. Storm" = 15 Hun�dreds = 1.� Tnches. - - � - <br /> "lOYr.24 Hr. Storm"=28 Hundreds = 2.5 Inches. <br /> "100 Yr.-24 Hr. Storm"= 39 Hundreds = 3.5 Inches. <br /> Calculate the "Time of ConcenL*ation"for both the "LTndeveloped" and the <br /> "Developed" site for each of the three storm events acing Manning's Equation. <br /> Tc = 0.42 nsLag. <br /> �, :i�So)°'t <br /> Where Tc =Time of Concentration in Minutes. <br /> ns = The "sheet flow"Manning"Roughness CoefficienY' . <br /> L= Length of flow of the site <br /> ��=-�r�sa�a#e�-c�€a"x" YPar ctnrm -Fb,.� -�,me �u f rc�cen-ha�i�n <br /> � So = Slope of the site in ft./ft. s ha�d n -F v a r y b e-f-w e�e n <br /> s��rms oF d +-fFere,�-r reivrn <br /> -��c��CnG �ES , f�Yl I o� rn� ' <br /> re ference5 use PZy� -F�or <br /> -�-�te T� al e f-f rrn� na 1-i�n �o nc� <br /> �5� '�"�l S c�lYl¢ T� 7�or a � � <br /> 51-orm5 . <br />