Environmental Engineering Reference
In-Depth Information
Fig. 3.19 A simple overland flow trap consisting of the contributing area delineated by concrete
walls and a pit to collect water (Photo by Masaki Hayashi)
If diffuse overland flow occurs in an area of relatively uniform vegetation and
slope, and depth of surface water is measured with reasonable accuracy, approxi-
mate flow rates can be estimated from the empirical Eq.
3.30
described in the
section on streamflow measurements. This method has a large degree of uncertainty
due to uncertainties and errors associated with empirical coefficients, uncertainty in
delineation of the area contributing to overland flow, and uncertainty in estimating
the water depth of the overland flow.
The methods described above can be applied to areas with uniform characteristics;
results from each of those areas are then summed to generate
O
f
for the entire wetland.
Methods described below all provide a single, integrated value for the entire wetland.
If a wetland does not have inflow and outflow streams, and the groundwater flow
rate is much slower than the overland flow rate, the amount of diffuse overland flow
during a short-lasting storm event can be estimated from the water-balance equa-
tion. Omitting stream and groundwater flow terms and assuming negligible evapo-
ration during the event, Eq.
3.1
can be written as
O
f
¼ Δ
V
=
Δ
t
P
(3.31)
Integrating (
3.31
) for the entire event, the total overland flow volume (
O
ftot
)isgivenby
O
ftot
¼
V
fin
V
ini
P
tot
(3.32)
where
V
ini
and
V
fin
are the initial and final volumes of water contained in the
wetland, respectively, and
P
tot
is the total volume of precipitation applied at
