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i� <br /> " ' . -- i• '_' .. . . .. •;?::_'. " ' _ . ' . . . ' _ . ' '_'_ _ _ . . .- __'__._ .-__ .- _'_ - . ' <br /> . � <br /> � H N Lehtinen. . <br /> StructuraU Civil Engineers , <br /> 3216 Wetmore Ave. <br /> Everett, WA 98201 -� � - - • - <br /> (206J252-2�73 <br /> Feb. 5, 1998 . <br /> Basis: Use SBUH/SCS iviethod for sizing and design of storm v��ater detention sysfem <br /> with allowable peak discharge flow rate being squal to 50% of the "2 YR. Developed" <br /> 24 Hour peak rate,whil� maintainin8 the peak rates of both the "10 Yr. And la0 Yr. <br /> Developed" 24 Hour storm events, by use of on-site storage.and controlled release.' <br /> a ; <br /> Determine the total precipation from the isographs for a . <br /> " 2 Yµ;24 Hr. Storm"= 15 Hundreds = 1.5 Inches. - - - � ' <br /> "lOYr.-24 Ar. Storm" =28 Hundreds =2.5 Iaches. <br /> "100 Yr.-24 Hr. Storm"= 39 Hundreds=3.5 Inches. ' <br /> Calculate the "Time of Concentration"for both the "Undeveloped" and the <br /> "Developed" site for each of the three storm events using Manning's Equation. <br /> 1'c�= 0.42 ns L a8. <br /> �sa,��So)°'t . � <br /> Where To= Time of Concentration in Minutes. <br /> ns=The "sheet flow"Manning "Roughaess Coefficient" . . <br /> L= Length of flow of the site <br /> �r=�as�a�iA&H€-a=`x"Yrar Crn,,,, -}�u. -}Sme"o-F rc.,cen'�ra-lion <br /> So= Slope of ttie site in ft./ft. 5 hw�d n + v�r y b e-�w�en <br /> 1 . � <br /> s�rms oF d �fFerent �e�rn , <br /> � � -�r2q,uenc �es , ftll o�- rn� - <br /> - . ' re ffrences u5e P2y, -For <br /> . . �¢ T� d�2'I"f rrn�n'a'F-i or1 o-'na� <br /> ' � ��.e� -Nu samR T -Fo� s.�I . . <br /> �„ ... ,. � , , , __ <br /> �..- - .�s,.�:;,,�;. <br /> � StvrmS . ' <br /> � �::�. . . . . � . .. . . . _ :r. :��.=:t.r.�:.�.;+,ytt„ <br /> i�`Fi_.�_-:.�. . r_..._ .. . . ' - _ . <br />