Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 64

draining stream. Waves and currents are generated in the pool during high dis-
charge events and these transport sediment, usually sands, from the floor of the
pool to form high water stage beaches surrounding the pool.
Most plunge pools form at the heads of gorges where the waterfall, through
knick point retreat, is in the process of extending a gorge into a highland. The
streams draining the pool in these circumstances are confined within the gorge.
In these geomorphic settings, high discharge events are responsible for deposit-
ing the beach surrounding, or to one side of, the pool; however, subsequent
higher discharge events have the capacity to erode and remove that beach. The
plunge pools examined by Nott and Price (1994, 1999)aredifferentastheyare
not located at the heads of gorges. At Gunlom in Kakadu National Park and
Wangi Falls in Litchfield National Park, Northern Territory, Australia (Fig. 3.4),
theplunge pools are located at the base of long escarpments where the waterfall
and associated stream have not incised a gorge into the highland. The streams,
after plunging over the escarpments, turn sharply and follow the base of the
escarpment. These streams drain to the side of the plunge pools. High discharge
events cause a rapid rise in plunge pool water levels but the waters then spread
out across the lowland plain rather than being confined within a gorge. The
wavesand currents generated in these plunge pools during floods also spread
out some distance from the normal plunge pool beach and generate another
beach, or beach ridge, that marks the stage level of these higher discharge events.
Gunlom and Wangi Falls have two beach ridges at progressively higher elevations
away from the normal pool beach. Luminescence (TL) dating of the sediments
in the ridges at Gunlom showed that Ridge 1, closest to the plunge pool beach,
wasdeposited between approximately 2000 years BP and present (Fig 3.7). This
ridge still receives sediment during very high magnitude discharge events. Ridge
2isapproximately3mhigherthanRidge1,and150mawayfromthisridge
and 300 m away from the plunge pool beach. Ridge 2 was deposited between
22 000 and 5000 years BP. Sedimentation on Ridge 2 was not continuous during
this period but occurred during two distinct phases: the first for approximately
4000 years around the Last Glacial Maximum (22 000--18 000 years BP) and then
again from approximately 9000 to 5000 years BP corresponding to the Holocene
Climatic Optimum. Ridge 2 has not been accumulating substantial quantities
of sediment, indeed any at all, since about 5000 years BP suggesting that
floods have been significantly smaller in magnitude (5--7 times smaller) since
this time.
The two ridges at Wangi Falls revealed a similar story to Gunlom (Fig. 3.7).
Wangi Falls is approximately 300 km east of Gunlom so it would seem unlikely
that the similarity in flood histories between the two sites is a coincidence or
a function of a localised catchment phenomenon such as a temporary natural