# Design of a Detention Pond

## Presentation on theme: "Design of a Detention Pond"— Presentation transcript:

Design of a Detention Pond
Volume Inflow WL 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 ‘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

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

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

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

Outflow Weir Controls Rectangular weir H Ycr Triangular weir H Ycr

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

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:

Oversized Storm Sewers
Weir & orifice outflow control D S0 WL IL Datum

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

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)