Environmental Engineering Reference
In-Depth Information
without the need for routine collection and analysis
of physical samples other than for periodic calibra-
tion purposes. Selected sediment-surrogate technolo-
gies show varying degrees of promise toward
providing the types, quality, and density of fl uvial-
sediment data needed to improve SSL computations.
Potentially useful instruments and methods for infer-
ring the physical characteristics of fl uvial sediments
(Bogen
et al
. 2003; Gartner
et al
. 2003; Gray
et al
.
2003a,b; Gray 2005; Topping
et al.
2007; Gray &
Gartner 2009) are being developed and tested world-
wide. For example, through the informal USGS
Sediment Monitoring Instrument and Analysis
Research Program (Gray 2003; Gray & Simões
2008), the USGS and collaborators in other govern-
ment agencies, academia, and the private sector are
testing several instruments for measuring SSCs and,
in some cases, PSDs. These instruments, operating
on bulk-, laser-, and digital-optic, pressure-
difference, and acoustic principles are being evalu-
ated in North American rivers and laboratories. To
make the transition from research to operational
monitoring applications, these new technologies
must be rigorously tested with respect to accuracy
and reliability in different physiographic and (or)
laboratory settings as appropriate, and their per-
formances must be compared with data obtained by
the aforementioned traditional methods and to avail-
able quality-control data. In most cases, performance
comparisons should include concurrent collection of
data by traditional and new techniques for a suffi -
cient period - probably years - and in a variety of
river types and fl ow conditions to identify potential
bias and minimize differences in precision between
the old and new technologies.
The
in situ
technologies presented herein require
periodic site-specifi c calibrations to infer the sedi-
mentary characteristics representative of the entire
channel cross section or reach segment. This require-
ment is anticipated to be substantial for new river-
monitoring applications, but may diminish as
comparative data accumulate.
None of the technologies represents a panacea for
sediment monitoring in all rivers under all fl ow and
sediment-transport conditions. However, with
careful matching of surrogate-monitoring technolo-
gies to selected river reaches and objectives, it is
becoming possible to remotely, continuously, and
accurately monitor SSCs and SSLs (and in some
collected nationally consistent daily sediment data in
2006 was about a quarter of the number operated in
1981 (David W. Stewart, USGS, personal communi-
cation 2008) (the USA has never had a federally
funded, national sediment monitoring and assess-
ment program analogous to the National Streamfl ow
Information Program (USGS 2008a) for fl ow moni-
toring). This precipitous decrease in sediment moni-
toring over a quarter century by the USGS - the
Federal agency tasked by the US Department of the
Interior to collect, archive, and disseminate US water
data, including fl uvial sediment (Glysson & Gray
1997; USGS 2008b) - is due to several factors, prin-
cipally cost (Gray
et al
. 2003). The decrease in moni-
toring is of particular concern, given that the physical,
chemical, and biological damages attributable to
fl uvial sediment in North America alone are esti-
mated to range from US$20 billion to US$50 billion
annually (Pimental
et al
. 1995; Osterkamp
et al.
1998, 2004; Gray & Osterkamp 2007). The relative
dearth of adequate, consistent, and reliable data
describing fl uvial-sediment fl uxes hinders develop-
ment of technically supportable management and
remedial plans around the world.
Historically, suspended-sediment fl ux data in the
US have been produced by gravimetric analyses per-
formed on physical samples collected by manual or
automatic samplers (see Edwards & Glysson 1999;
Bent
et al
. 2003; Davis 2005; Nolan
et al
. 2005;
Gray
et al
. 2008). These traditional data-collection
methods tend to be expensive, labor intensive, time-
consuming, diffi cult, and under some conditions,
hazardous. Specialized instruments and considerable
training in their proper use are prerequisites for
obtaining reliable samples. The characteristic paucity
of the derived data - particularly at the higher fl ows
that are most infl uential in mass transport of sedi-
ment - can lead to inadequate defi nition of the tem-
poral variability in SSCs and suspended-sediment
discharges, or loads (SSLs). Consequently, temporal
interpolations and spatial corrections are commonly
required to develop the requisite time series that is
used with an associated time series of water-dis-
charge data to produce sub-daily and daily records
of SSL (Porterfi eld 1972; Koltun
et al
. 2006).
Sediment-surrogate technologies are defi ned as
instruments coupled with operational and analytical
methodologies that enable acquisition of temporally
and (or) spatially dense fl uvial-sediment data sets
