Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 59

those that measure flow competence, can be regarded as proxy measures of
flood height.
The development of the discipline of palaeoflood hydrology is a classic story
of the reluctance by the scientific community to accept new ideas. J. Harlen
Bretz was the first to argue that the deposits and erosional features present in
theMissoula scabland in Washington State were produced by truly massive flood
flows. These features formed when water was released after ice dammed lakes
burst during the amelioration of the climate following the Last Glacial Maxi-
mum (Bretz, 1928). Bretz was attacked for his views that these features, which
are phenomenally large compared to most fluvially generated forms, could have
been caused by possibly the largest ever floods on Earth in recent geological
time. He was later shown to be correct in his interpretation and as a result
of his early research, and then the subsequent research of many others (prin-
cipally by V. Baker of the University of Arizona and colleagues), the science of
palaeohydrology emerged. Today, palaeohydrology is seen as most credible and a
very valuable tool in the understanding of flood climatology for regions globally.
However, while the science is now well established, its incorporation into flood
risk assessments has not been so readily adopted everywhere.
Slackwater sediments
Slackwater sediments are generally fine-grained sediments deposited in
zones of low velocity flow during a flood. Such zones of low flow velocity include
the mouths of tributaries (Fig. 3.2), alcoves and caves in bedrock gorge walls,
areas where channels expand in width, upstream of channel constrictions and
the sides of river valleys where sedimentary terraces and levees can develop
(Fig. 3.3). Each of these zones represents an area separated from the main flood
flow where circulation eddies or physical obstructions cause the flood flow veloc-
ity to slow and as such deposit the suspended sediment. The fine-grained size
of these sediments shows that the flow at the point of deposition must have
been of relatively low velocity. As a general rule, there is a direct relationship
between the grain size of the sediments deposited and the local flow velocity
such that progressively finer-grained sediments are deposited with decreasing
flow velocity. The height of the sediments above the floor or bed of the river
provides an indication of the stage height of the flood flow. This stage height is
usually only a minimum flood flow height as the real flood water height is at
least 20% higher than the elevation of the slackwater sediments.
The height of a number of slackwater deposits along a river reach, often a
length of gorge, can be used to model the discharge of the palaeoflood event. The
modelling is usually undertaken using a step backwater approach that calculates