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