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Design of a Detention Pond
Peak outflow ison recessionlimb of inflow.
Inflow
Outflow
Time
Q(t)
Volume
DischargeOutflow
Inflow
WL
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
Theory of Reservoir Routing
t
QI1
QI2
QO1
QO2 = ?
Inflow Outflow
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
Theory of Reservoir Routing (2)
12112
222
21
12
2
21
2
21
QOQOt
SQO
t
SQIQI
t
SSQOQOQIQI
1221)1()2( QOQIQIQOfQOf
Inflow = Outflow + Rate of change of storage
Outflow QO
f(QO)
QI1 + QI2 - 2QO1
Outflow Orifice Controls
dHfordHgdCQ c 322
42
5.057.1
25
04.0496.0
d
H
d
H
d
Hfwhere
dHfordgCd
HfQ c
Submerged orifice
Non-submerged orifice
Hd
Ccd
Hd
Outflow Weir Controls
Rectangular weir
Hy
ygBCQ
cr
crdcr
3
2
23
Hy
ymgCQ
cr
crdcr
5
422
25
Triangular weir
HYcr
HYcr
Storage Models
MIDUSS 98 provides 4 tools to assist in defining the depth-storage relation.
“Rectangular” reservoir or pondOversized storm sewersWedge shaped storage (parking lots)Rooftop storage
Rectangular Pond storage
H
Aj+1 = Lj+1 x Bj+1
Aj = Lj x Bj
Lj
Lj+1
mAm
mHBmHLA
mHBmHLA
BLA
AAAH
V
jjj
jjm
jjj
jmj
22
46
1
1
Aspect ratio R = L/B
For irregularly shaped ponds the aspect ratio R is defined by:
Area
PerimeterRRRf
2
844)(
Oversized Storm Sewers
Weir & orificeoutflow control
Datum
WL
IL
D
S0
Wedge shaped Storage
Parking lot storage created by restricting capacity of catch basins
32221
2118
HggggV
Typical depth of exit pipe below rim elevation
Ponding depth H
3 ft/ 0.92 m
R2
R1
g1
g2
Roof top Storage
Roof slope S0
HL/2 L/2
H
Q = K.H
e.g. Q = 24 litres/min/25mm head
Vol = f(H, L S0)
Linear Discharge weir
Recommended