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r <br /> Y�i torian Plaa Hvdroloev RryiQA dc Calculations M94-4506 <br /> � WETPOND DESIGN <br /> ( 1. Csiculate the wetpond design volume. <br /> The wetpond design volume outside of the Silver Lake drainage basin shall be equal to the <br /> I stortn water runoff from the 6-month/24-hour design storm. The wetpond design volume <br /> is calculated by multiplying the total precipitation associated with the 6-month/24-hour <br /> design storm (0.99 inches)by the amount of impervious surface contributing runoff to the <br /> � wetpond Q.92 acres). <br /> The required wetpond design volume is calculated to be: <br /> (0.99 in)(1.92 acres)(43560 ft2/Ac)(ft/12 in)= 6,900 CF <br /> I 2. Determine the wetpond permanent atorage volume required and provided. <br /> � Permanent Storage Volume Requirod - 50% - 6,900 CF=3,450 CF (minimum) <br /> Wetland Treatment Cell =(0.30)3,450 CF= 1,035 CF(minimum) <br /> � Open Water Cell = 3,450 CF-1,035 CF=2,415 CF(minimum) <br /> � <br /> Permanent Storage Volume Provided = 3,454 CF> 3,450 CF <br /> � Wetland Treatment Cell = 1,036 CF> 1,035 CF <br /> Open Water Cell =2,418 CF> 2,415 CF <br /> I3. Determine the live atorage volume rcquircd and provided. <br /> Live Storege Vol. Req'd= Total Wetpond Design Volume -Permanent Storage Volume <br /> I = (6,900 CF-3,454 CF)=3,446 CF <br /> Live Storage Vol. Prov'd=3,543 CF(at water surface elevation 694.00') <br /> l4. Route the 6-month/21-hour design storm through the wetpond to demonetrate that <br /> the full wetpond design volume ia provided in the pond. <br /> ' � Using the SBUH Method with Type lA Rainfall Distribution and 24-hour duration for the <br /> 1 tia �u�}� 6-month stortn (P=0.99"), the totel volume of runoff generated by the 1.92 acre basin <br /> �fl„�, tributary to the wetpond is determined to be 0.1250 Ac-ft or 5,445 CF(see attached Basin <br /> ' h�' L��I '� Summary). This is significantly less than the volume calculated using the City of Everett <br /> �y U.1 F A��aw� Method 4-10.2(4a). Therefore, the wetpond will be designed to provide the total wetpond , <br /> 1;�� �+ x� design volume computed using the City of Everett Method 4-10.2(4a). The volume of { <br /> ' �J ��^ !'��� runoff calculated using the SBIJH Method is significantly less than the volume calculated � <br /> �r � ,�tei using Method 4-10.2(4a), consequently it will be difficult to show that the }'ull wetpond <br /> �1 e`'' � �a � design volume is being reached during the 6-month/24-hour design storm. The level pool <br /> � ��,,,x Q� �P�'y routing analysis indicates that an oriBce diameter of less than the minimum 0.50" would be <br /> �ve��„fi � necessary to achieve this requirement. As e compromise, the peak stage and live storage � <br /> ( � , volume will be computed using an orifice diameter of 0.50" and the top of the overflow <br /> J���� h� riser will be set at elevation 694.00. See the following level pool routing for the results of <br /> ( '' c�' � �n this analysis. � jh�P 1h � <br /> ( F <br /> h c �y ��� G .y- �c <br /> w, �I r h r,�n ., � �,1 �,�' u �j'' e'�' <br /> �'�� 1t�° I) �- ' �„ � ,y '' ,, �^ . �� `J <br /> Yw 1� f 1 � `1��u u{-�(� I _� _ <br /> I � ,� � <br /> �� rr r'� ,� �� GROUP FOUR,INC. <br /> * <br /> oq `�' ��, ��� � <br /> E ' � <br />