Geoscience Reference
In-Depth Information
(a)
180000
150000
120000
90000
60000
30000
0
0
5
10
15
20
25
30
(b)
20000
15000
10000
5000
0
0
2
4
6
8
10
Water discharge, m
3
s
-1
Figure 13.22
Suspended sediment concentration versus water discharge for a sequence of samples taken during two flash floods
in the Nahal Eshtemoa, northern Negev: (a) 18 October 1997 event arising from high-intensity, convectively enhanced, cellular
storm, showing clockwise hysteresis as a result of sediment flushing; (b) 5 November 1994 event arising from low-intensity, frontal
storm, showing counterclockwise hysteresis in both waves of the flood hydrograph (after Alexandrov, Laronne and Reid, 2003).
important in controlling downstream displacement dur-
ing moderate floods in which values of excess shear stress
are comparatively small. In this context, of interest is a
comparative study of perennial and ephemeral channels
(Wittenberg
et al
., 2007), which finds the spatial density
of pebble/cobble clusters (implicated in Church and Has-
san's 'locked' category of surface clasts) to be lower in
ephemerals, where abundant sediment supply and lack of
low-flow winnowing ensures a smaller grain sorting index
and, therefore, less tendency to form grain clusters. The
implication is that bed structure is less of a restraint on
entrainment, a factor contributing to high bedload flux in
ephemerals.
These studies provide valuable information about bed
nature in that they report displacement of clasts after the
event. Indeed, in some cases (e.g. Schick and Lekach,
1987), even the flood hydrograph has been reconstructed
from slackwater deposit evidence of maximum water-
stage rather than recorded information. Live-bed infor-
mation in ephemeral streams is inevitably rare given the
dangers of wading in flash floods with portable bedload
samplers, let alone the fact that the chance of being on-site
during an event is extremely small (Lekach and Schick,
1983).
This absence of information has been remedied in
recent years by the installation of Birkbeck-type bedload
samplers in the Nahal Yatir and Nahal Eshtemoa, two
gravel-bed ephemeral streams in the northern Negev
