Environmental Engineering Reference
In-Depth Information
River Energy Audit Scheme (REAS)
applied only to alluvial (self-formed) channels
with discernible floodplains. Secondly, it is best
to apply streampower analysis at the reach scale so
that the length of the channel unit used to calcu-
late stream power is sufficient to avoid 'noise'
associated with local features, but short enough
that the spatial distributionof reaches captures the
degree of complexity necessary to characterize the
fluvial sediment transfer system (Worthy 2005).
Thirdly, itmust be borne inmind that there are,
in fact, no universal, threshold values of stream
power that may be used to predict channel re-
sponse to perturbation a priori. For example,
Brookes (1988) reported a 'grey area' between
about 25 and 35W/m
2
within which rivers might
respond to channelization through either erosion
or deposition-led adjustments, and experience has
shown that threshold bands vary significantly
between rivers of different types located in differ-
ent morphogenetic regions.
A fourth limitation is that the empirical data-
sets and indicative rules that currently underpin
interpretation of observed stream power values
reflect responses to channelization, adjustments
occurring within channelized reaches (especially
in straightened channels) and the impacts of river
restoration in perennial streams. These data and
the rules based on them cannot be used to predict
adjustments in other environments or to other
types of channel disturbance.
Finally, the results of a stream power analysis
cannot be used in isolation to make reliable pre-
dictions of morphological responses to distur-
bance. At least a basic knowledge of the bed
sediment characteristics and bank material prop-
erties is also required as these variables condition
the morphological outcomes of process-response
mechanisms and strongly influence the pattern
and sequence of channel adjustments in perturbed
streams (Simon and Thorne 1996).
In dealing with these limitations, Brookes and
Wishart (2006) recommend that end users develop
their own personal approaches to the accurate
interpretation of the outcomes of stream power
analysis and compile regional datasets appropriate
to the river environments, boundary conditions
and sediment issues in question.
Background and development
A component of the research undertaken during
Phase 1 of the FRMRC and led by Wallerstein and
Soar (2006) centred on developing a catchment-
scale approach for characterizing river reaches in
terms of their potential to erode, deposit and trans-
fer sediment, achieved by validating and building
on the insights gained fromqualitative assessment
of sediment dynamics using a Fluvial Audit. The
aimwas to produce a practical and robust tool that
could bridge the gap between simple approaches,
such as the stream power screening tool, and com-
plex sediment transport modelling in providing a
scheme for identifying locations of potential insta-
bility within the fluvial system where more de-
tailedanalysisandresources couldthenbedirected.
The theoretical basis for the REAS stems from
the idea first proposed by Bagnold (1966) that
excess specific streampower can be used to predict
a stream's capacity to performwork on its channel
boundaries through eroding and transporting sed-
iment. However, recognizing the practical con-
straints and uncertainties associated with
predicting sediment transport rates, REAS does
not employ sediment transport calculations or
attempt to route sediment through the river chan-
nel network. Instead, it calculates a measure of
'Annual Geomorphic Energy' (AGE) for each de-
lineated reach by integrating excess stream power
available for performing geomorphological work
with flow duration (represented by a flow frequen-
cy histogram). Balances and imbalances in AGE
between consecutive reaches are considered to be
indicative of channel stability or potential for
morphological change through processes domi-
nated by scour (an excess in AGE) or those dom-
inated by deposition of sediment (a deficit inAGE).
For each reach, the excess specific streampower
for the median discharge in each discrete class in
the flow frequency histogram is calculated and
thenmultiplied by the water surface width to give
excess total stream power per unit length of chan-
nel. These values are then multiplied by their
respective discharge's decimal
frequency of
