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Design of a Detention Pond

Design of a Detention Pond. Volume. WL. Inflow. Discharge. Outflow. Q(t). Inflow. Peak outflow is on recession limb of inflow. Outflow. Time. Types of Detention Pond. ‘In-line’ storage reservoir with outflow control device to reduce peak flow.

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Design of a Detention Pond

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  1. Design of a Detention Pond Volume WL Inflow Discharge Outflow Q(t) Inflow Peak outflow ison recessionlimb of inflow. Outflow Time

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

  3. Theory of Reservoir Routing QO2 = ? QI2 Law of Continuity Dt Inflow Outflow QI1 QO1 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

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

  5. Outflow Orifice Controls Submerged orifice Ccd H d Non-submerged orifice d H

  6. Outflow Weir Controls Rectangular weir H Ycr Triangular weir H Ycr

  7. Storage Models MIDUSS 98 provides 4 tools to assist in defining the depth-storage relation. • “Rectangular” reservoir or pond • Oversized storm sewers • Wedge shaped storage (parking lots) • Rooftop storage

  8. Rectangular Pond storage Aj+1 = Lj+1 x Bj+1 Lj+1 Aspect ratio R = L/B Am H m Lj Aj = Lj x Bj For irregularly shaped ponds the aspect ratio R is defined by:

  9. Oversized Storm Sewers Weir & orificeoutflow control D S0 WL IL Datum

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

  11. Roof top Storage L/2 L/2 H Roof slope S0 Linear Discharge weir H Q = K.H e.g. Q = 24 litres/min/25mm head Vol = f(H, L S0)

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