Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 66

Falls suggesting that the highest section of the ridge has not been experiencing
sedimentation since approximately 4000 years BP.
All the plunge pool sites here in the Northern Territory highlight a syn-
chronous pattern of flooding over the past 25 000 years. Nott and Price (1999)
were able to demonstrate that these phases of enhanced flooding, especially
that related to the HCO, were due to relatively minor shifts in average climatic
conditions. The plunge pool records emphasise that the changes in regime and
magnitude of flood flows may be disproportionate, with the latter (Holocene)
experiencing much greater relative change than the former. Similar conclusions
have also been observed in the USA by Ely et al .(1993) and Knox (1993). These
flood records, though, were not in the form of plunge pool sediments.
Geobotanic indicators
Geobotanic indicators of palaeofloods can be grouped into four main
classes: corrasion scars, adventitious sprouts, tree age and tree-ring anomalies
(Wohl, 1995). Corrasion scars occur when part of the cambrium wood is destroyed
by theimpact of flood-carried debris. The wood tissue is scarred and this mark
can provide a date for the flood event. The height of the scar can also be used to
estimate the height of the flood waters although this may be a minimum stage
height depending upon the nature of the debris impacting the tree. For example,
if the debris were large rocks then it is unlikely that such rocks would have been
transported near the water surface but at some depth below. Alternatively, if the
impacting debris was something that normally floats such as tree branches then
the corrasion scar may be an indicator of the flood stage height. It is important
to assess the likely nature of the impacting debris before using this type of data
to estimate the flood height.
Tree rings, like scar tissue, can also provide data on the age of a flood. Trees
can be tilted by flood waters and this can cause eccentric ring patterns. Also,
ring suppression and release patterns and variations in ring width can be caused
by changes in river flow patterns. The age of trees growing on flood sedimentary
sequences or in zones of erosion caused by high flood waters can also help date a
flood. Likewise, the age of adventitious sprouts growing from broken tree stems
due to floods can be determined by examining the ring patterns toward the base
of the sprout.
Flow competence measures
The size of sedimentary clasts transported by flood flows can be used
to gain a measure of the velocity, discharge and stream power of past floods.