Geoscience Reference
In-Depth Information
be misleading regarding the characteristics of dynamic bedforms, as could subaque-
ous measurements of the bed surface by mechanical probing (e.g. using point
gauges), a sonic depth sounder was developed to allow the non-intrusive measure-
ment of bed levels in shallow water. Measurements of dynamic bed-surface varia-
tions in space and time were readily obtainable using the new depth sounder and its
relative the dual-channel stream monitor (Karaki et al.
1961
;Richardsonetal.
1961
),
from which dynamic data on bedform types, shapes, dimensions, and migration and
transport rates could also be easily determined. This development served to revolu-
tionise the measurement and analysis of dynamic subaqueous bed topography in the
following decades, becoming the principal experimental tool of use and enabling
landmark advances in understanding and description of bedforms (e.g. ASCE
1966
;
Guy et al.
1966
; Nordin and Algert
1966
; Ashida and Tanaka
1967
; Crickmore
1967
;
Jain and Kennedy
1971
,
1974
; Wang and Shen
1980
; Willis and Kennedy
1980
;
Nakagawa and Tsujimoto
1984
; Bridge and Best
1988
). Later studies used similar
approaches for non-intrusive bed measurements that utilised different instruments,
e.g. an infra-red probe (Richards and Robert
1986
; Robert and Richards
1988
)anda
laser displacement meter (e.g. Nairn
1998
; Coleman et al.
2003
; Tuijnder et al.
2009
).
1.3 Outstanding Gaps in Understanding and Recent Research
Despite the highlighted studies and advances up until the 1960s, and the significant
efforts over the subsequent years to the start of the twenty-first century, ASCE
(
2002
) observed that even then it remained difficult to give assured answers to basic
questions such as how to characterise a dune-covered bed, including bedform
shapes, sizes, three-dimensionality, and their statistical natures.
In the following, recent research is presented into conjecture that has been put
forward regarding gaps in the understanding of fluvial bedforms. In each case, the
postulated understanding is outlined, and then experimental investigations involv-
ing the writer that test the hypotheses are described. With this approach, it is
intended that the use of developments in instrumentation and experimental methods
to gain insight into longstanding research questions will be highlighted, along with
resulting progressive advances in understanding.
2 Bedform Generation
The generation of bedforms from plane-bed conditions is typically attributed to one
of three phenomena: (a) turbulent fluid motions, (b) instability of the fluid-sediment
flow system when perturbed, and (c) granular transport mechanics. Through signif-
icant research efforts, there has been a wide spectrum of theories developed, with
all of these theories still presenting unresolved inconsistencies. Postulated theories
are discussed below along with investigations designed to test the hypotheses and
resulting findings.
