Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 69

associated with using such equations. There are several potential limitations
with using equations describing the relationship between stream velocity and
boulder size to determine the discharge of a palaeoflood. Some of these include
thevariability of the velocity profile in streams and knowing which velocity is
the most appropriate one for the movement of the clasts in question (stream
velocity varies with flow depth in rivers). Also, the effects of particle shape and
packing, boundary roughness and variable fluid density are not well understood.
Often, however, boulder piles in streams may be the only evidence available from
which to gauge the size of a palaeoflood. So in terms of hazard risk assessments,
these equations can provide very useful information on the size of past events,
especially when attempting to ascertain the largest relatively recent flood in a
catchment.
Another way of expressing the relationship between stream flood parameters
and boulder transport is by using stream power defined as

w = γ QS / W = τ V (mean) (3.8)

stream powerperunitarea of bed (W m − 2 ),

where w = is density of fluid,

is boundary shear stress (N m − 2 ), V (mean) is mean flow velocity in m s − 1 , Q =

discharge (m 3 s − 1 ), S = energy gradient or water surface slope and W = water

surface width in metres.
Boundary shear stress is determined by the duBoys equation

τ = γ DS (3.9)

where D = flow depth.
Using these equations, mean flow velocity can also be determined using

V (mean) = w / τ = w / γ DS (3.10)

Simplified relationships using these fundamentals have also been developed for
stream powerandclast transport by Williams (1983)as

w = 0 . 079 d 1 . 27 (3.11)

(where d is the length ofthe b axis) and for a threshold mean velocity by Costa
(1983) which is the critical velocity at which boulder motion will commence.
This can be expressed as

V c (mean) = 0 . 18 d 0 . 49 (3.12)

Baker and Pickup (1987)used stream power relations to determine the flood
flow frequencies required to move sandstone boulders as large as 4 1.5 m 3

within the Katherine Gorge, Northern Territory, Australia (Fig. 3.4). They found
that

× 3 ×

m 3 s − 1 , the 1% AEP (1 in 100 year) event with a discharge of 6000