Environmental Engineering Reference
In-Depth Information
Short Exercise 7: Estimation of Diffuse Overland Flow
The amount of diffuse overland flow can be estimated using a wetland as a natural
overland flow trap. If the wetland does not have inflow or outflow streams, and the
contribution of groundwater flow is negligible during a short-duration storm, then
the water balance equation for the wetland pond is given by Eq.
3.32
. Total
overland flow during the storm (
O
ftot
) is estimated from measuring the volume of
pond water before (
V
ini
) and after (
V
fin
) the storm. The figure embedded in Table
3.8
shows the pond stage and cumulative precipitation in Wetland 109 in the St. Denis
National Wildlife Area in Saskatchewan, Canada, on July 4-5, 1996 (see Hayashi
et al.
1998
for a site description). The cumulative precipitation (
p
cum
) during the
entire storm was 51 mm. The pond stages recorded at 21:00 and 02:00 are listed in
Table
3.8
. Water depth (
H
) at the deepest point in the pond is given by subtracting
551.68 m from the pond stage. The area of pond surface (
A
) and the volume of pond
water (
V
) can be estimated using Eqs.
3.4
and
3.5
with
s
3,180 m
2
and
p
1.61
(Hayashi and van der Kamp
2000
). The effective drainage area (
A
eff
) of Wetland
109 is 20,100 m
2
.
¼
¼
1. Calculate the initial (21:00) and final (02:00) pond area and volume from the
stage data.
2. Calculate the total amount of precipitation (
P
tot
) falling within the pond by
multiplying
p
cum
by the pond area (
A
fin
) at 02:00.
3. Using Eq.
3.32
, determine
O
ftot
.
4. Runoff-contributing area to the pond is given by
A
eff
A
fin
. From
O
ftot
, estimate
the areal average runoff (mm) in the contributing area.
5. Estimate the runoff coefficient (
R
c
¼
runoff/precipitation) for this storm.
Table 3.8 Pond stage in Wetland 109 in the St. Denis National Wildlife Area, Saskatchewan,
Canada on July 4, 1996
Stage
(m)
H
(m)
A
(m
2
)
V
(m
3
)
Time
21:00 552.41
60
552.70
02:00 552.62
50
552.65
552.60
40
552.55
30
552.50
20
pond stage
cum. pcp.
552.45
10
552.40
0
15:00
18:00
21:00
00:00
03:00
06:00
July 4
July 5
