Transcript Designing an exfiltration trench

Exfiltration Trench
Perforated distribution pipe
Outflow Control Device
Outflow Q
Exfiltration X
Water table
Idealized diagram of exfiltration trench
Purpose of Exfiltration
Encourage return of storm runoff to the
groundwater
Reduce the hydraulic load on the minor
(e.g. piped) system
Improve quality of runoff by removal of some
particulate matter
Reduce thermal impact on the runoff
Split inflow hydrograph into two components
of Outflow and Groundwater recharge
Basic Theory
Inflow = Outflow + Exfiltration + Change of storage
I1  I 2 Q1  Q2 X 1  X 2 V2  V1



2
2
2
t
I
V
X
 2V2
  2V1

I1  I 2  
 Q2  X 2   
 Q1  X 1   2Q1  2 X 1
 t
  t

I1  I 2  f V2 , Q2 , X 2   f V1 , Q1 , X1   2Q1  2 X1
f (Q2 )  f Q1   2Q1  2 X1  I1  I 2
Q
Trench Cross-section
Topwidth T
For laminar flow:
Filter
Q
 q  KS f
A
Clear stone
Height H
P + y/2
yeff
Invert
elevation IL
B
P=IL-G
Water table elevation G
where
Sf 
IL  G  y
2
IL  G
y
 1
2IL  G 
K = hydraulic
conductivity
Define Trench parameters
Data window is opened by using Geometry/Trench
menu command from Trench Design form.
Define Trench parameters
Define outflow control
[Compute]
Plot V,Q=f(H)
Defining the Trench Pipes
Main storm sewer is
solid 450 mm pipe
with invert =100.85
Two 200 mm diam.
perforated pipes are
plugged at down stream
end to distribute inflow
along trench
Press [Compute]
to update volume
table
Pipes positioned
graphically with
clearance shown.
Position refined
by editing table.
Results of Routing
The Etobicoke Trench