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