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Design of a Detention Pond Peak outflow is on recession limb of inflow. Inflow Outflow Time Q(t) Volume Discharge Outflow Inflow WL

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Types of Detention Pond On rooftops of proposed new commercial buildings On-site storage on parking lots or below ground in oversized storm sewers or trench ‘Off-line storage reservoir with connection above normal hydraulic grade line ‘In-line’ storage reservoir with outflow control device to reduce peak flow

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Theory of Reservoir Routing tt QI1 QI2 QO1 QO2 = ? InflowOutflow Inflow = Outflow + Rate of change of storage Assume:- (1) Storage depends only on outflow (2) Reservoir surface is horizontal (3) Water surface elev. is function of outflow Law of Continuity

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Theory of Reservoir Routing (2) Inflow = Outflow + Rate of change of storage Outflow QO f(QO) QI1 + QI2 - 2QO1

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Outflow Orifice Controls Submerged orifice Non-submerged orifice H d CcdCcd H d

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Outflow Weir Controls Rectangular weir Triangular weir H Ycr H

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Storage Models MIDUSS 98 provides 4 tools to assist in defining the depth-storage relation. z“Rectangular” reservoir or pond zOversized storm sewers zWedge shaped storage (parking lots) zRooftop storage

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Rectangular Pond storage H A j+1 = L j+1 x B j+1 A j = L j x B j L j L j+1 m AmAm Aspect ratio R = L/B For irregularly shaped ponds the aspect ratio R is defined by:

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Oversized Storm Sewers Weir & orifice outflow control Datum WL IL D S0S0

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Wedge shaped Storage Parking lot storage created by restricting capacity of catch basins Typical depth of exit pipe below rim elevation Ponding depth H 3 ft/ 0.92 m R2 R1 g1 g2

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Roof top Storage Roof slope S 0 H L/2 H Q = K.H e.g. Q = 24 litres/min/25mm head Vol = f(H, L S 0 ) Linear Discharge weir

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