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sharply to zero. The hydraulic method has the
advantage over the historic method that it takes into
account the actual streambed morphology which
may differ markedly between rivers.
The habitat method extends the hydraulic method
by taking the hydraulic information and combining
it with knowledge of how different aquatic species
survive in those flow regimes. In this way the
appropriate flow regime can be designed with
particular aquatic species in mind. In the case of fish,
some prefer shallow turbulent streams compared
to deep, slow moving rivers. The habitat method
allows differentiation between these so that a flow
regime can be set with protection, or enhancement,
of a particular species in mind.
The most common use of the habitat method
is the Instream Flow Incremental Methodology
(IFIM; Irvine et al ., 1987; Navarro et al ., 1994)
which has been developed into computer models
such as PHABSIM (Physical HABitat SIMulation;
Milhous et al ., 1989; Gallagher and Gard, 1999)
and RHYHABSIM (River HYdraulic HABitat
SIMulation; Jowett, 1997).
The habitat method focuses on a particular
species and life stage at a time, and investigates its
response at a particular flow. For each cell in a two-
dimensional grid, velocity, depth, substrate and
possibly other parameters (e.g. cover) at the given
flow are converted into suitability values, one for
each parameter. These suitability values are com-
bined (usually multiplied) and multiplied by
the cell area to give an area of usable habitat (also
called weighted usable area, WUA). Finally, all the
usable habitat cell areas are summed to give a total
habitat area (total WUA) for the reach at the given
flow. The whole procedure is repeated for other flows
until a graph of usable habitat area versus flow
for the given species has been produced. This
graph has a typical shape, as shown in Figure 6.19,
with a rising part, a maximum and a decline. The
decline occurs when the velocity and/or depth
exceed those preferred by the given species and
life stage. In large rivers, the curve may predict that
physical habitat will be at a maximum at less than
naturally occurring flows (Jowett, 1997).
Historic
Hydraulic
Habitat
Flow
Figure 6.19 Hypothetical relationships showing
biological response to increasing streamflow as
modelled by historic, hydraulic and habitat methods.
Source : Adapted from Jowett (1997)
environment. With relatively easily derived flow
information (i.e. average flow) a new flow regime
can be set for a river that takes into account the
instream values. However this approach precludes
the possibility that a stream could be enhanced by
a non-natural flow regime. This is especially true
where there is an upstream reservoir, in which case
flows can be manipulated to improve the aquatic
environment, not just maintain what is presently
there.
The hydraulic method requires measurements of
hydraulic data such as wetted perimeter, width,
velocity and depth at a series of cross sections. Then,
using either rating curves (i.e. the stage-discharge
relationship described in Chapter 5) or an equation
such as Manning's (see Chapter 5), the variations in
a hydraulic parameter with flow can be derived. The
most commonly used hydraulic parameter is the
wetted perimeter because it takes into account the
area of streambed where periphyton and inverte-
brates live. A healthy periphyton and invertebrate
community generally leads to a healthy river eco-
system. The variation in wetted perimeter with flow
is drawn in the same way as represented by the
broken line in Figure 6.19. The minimum flow for
river is normally defined by where the hydraulic
parameter (e.g. wetted perimeter) starts to decline
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