Geoscience Reference
In-Depth Information
Other parameters required for baseflow computation for both HEC-1 and HEC-HMS are
initial flow and threshold flow.
Routing model
The Muskingum method is used for the flow routing. This method is a commonly used
hydrological routing method for handling a variable discharge-storage relationship
(Chow at al., 1988). It uses a simple finite difference approximation of the continuity equation:
(5.6)
where
I
t
!!
t
and
I
t
are the inflow hydrograph ordinates at times
t
!'
t
and
t,
respectively;
and
O
t
!!
t
and 0t are the outflow hydrograph ordinates at times
t
!'
t
and
t,
respectively.
Similarly
S
t
!!
t
and
S
t
are the storage in the reach at times
t
!'
t
and
t,
respectively. The
storage is modelled as the sum of prism and wedge storage. The prism storage is the
volume defined by the water surface profile at steady stage, while the wedge storage is
the additional volume under the profile of the flood wave. The storage at time
t, St,
is
defined as:
S
t
=
K
[
XI
t
+(1!
X
)
O
t
]
(5.7)
where
K
is the travel time of the flood wave through routing reach and
X
is a
dimensionless weight (0(
X
(0.5).
From Equations (5.6) and (5.7), it follows that
(5.8)
Given inflow and outflow hydrographs, the Muskingum parameter
K
can be estimated.
Once
K
is estimated,
X
can be determined by trial-and-error. These parameters can also
be determined by calibration.
5.1.3
Uncertainty due to precipitation
Forecasting a flood using rainfall-runoff type models requires the forecasted precipitation
over the forecast period. Over the last decade significant progress has been made in the
quantitative forecast of precipitation using sophisticated radar technology. The
uncertainty due to precipitation, however, remains a major part of the input uncertainty in
such models. The impact of the uncertainty in precipitation from radar data on
hydrological models is discussed by Borga (2002), Carpenter et al. (2001), Cluckie and
Search WWH ::
Custom Search