Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 70

flow velocities of 7.5 m s − 1 and flow depths of 15--45 m were able to trans-
port these boulders and probably caused cavitation features such as pot holes
to occur in the floor of the bedrock gorge. These flood flows, along with struc-
tural controls in the bedrock, were also responsible for the size and location
of pool and riffle sequences in the Katherine Gorge. Pools and riffles are the
zones of scour and intervening sediment deposition, or shallows and sometimes
rapids, in rivers. Pool and riffle spacing is typically 5--7 times the channel width
and in alluvial rivers, along with the planform of the stream, is usually seen to
develop by the 2--3 year return interval flood. Bedrock channels, whose channel
boundaries are considerably more resistant to erosion than alluvial channels, do
not appear to conform to this relationship. In the case of the Katherine Gorge,
the spacing of pool and riffle sequences appear to be determined, not by the
2--3 year flows, but by much more extreme flood events. Hence, pool and riffle
dimensions within a bedrock stream system can also be used as a guide to the
size of past floods. Wohl (1992a, b)foundasimilarrelationship between extreme
flood flows (palaeofloods) in the Burdekin and Herbert River Gorges in northeast
Queensland and the location of pools and riffles, boulder sizes, erosional scour
features (Fig. 3.9)and also slackwater sediment deposition zones.
Channel geometry
Like pool and riffle spacing, the wavelength of meanders in rivers can be
used as a guide to the channel forming discharge. Meander wavelength, chan-
nel width and discharge all appear to be interrelated, such that as discharge
increases so does channel width and wavelength. The width of an alluvial chan-
nel is approximately proportional to the square root of discharge ( Q )andlikewise
meander wavelength also varies with Q 0.5 . Which particular discharge is the most
effective, i.e. mean annual flood, mean annual discharge, the 1% AEP discharge
or most probable annual flood is a matter of slight controversy (Knighton, 1984).
But there is little doubt that high channel forming discharges tend to produce
longer meander wavelength (distance between crests or bends of the same ori-
entation). The water carrying capacity, i.e. channel width and depth or cross-
sectional area, of a channel also tends to increase as discharge increases. In
alluvial rivers this can vary from flood to flood (i.e. a flood flow can erode the
banks and bed of a channel in order to accommodate an increased flow capacity)
but the average channel geometry is thought to be determined by the 2--3 year
flood event. The same appears generally true of meander wavelengths in alluvial
channels. Whether this relationship holds true for bedrock channels or, as has
been shown for pools and riffles, tends to be related to much higher discharges,