Environmental Engineering Reference
In-Depth Information
(b)
(a)
(c)
(d)
Fig. 1.5
Photographs showing
nephelometry sensors. (a) YSI model
6136; (b) Hydrolab turbidity sensor with
wiper; (c) Forrest Technology Systems
model DTS-12; (d) Campbell Scientifi c
Inc. model OBS 3+; (e) Hach Solitax with
wiper. All photographs reproduced with
permission.
(e)
Bulk-optical instruments lack moving parts (unless
outfi tted with optical wipers), can be deployed
in situ
to collect time-series data, and provide rapid-
sampling capability. The technology is relatively
mature, and has been shown to provide reliable data
at several USGS streamgages (Uhrich 2002; Melis
et
al.
2003; Schoellhamer & Wright 2003; Uhrich &
Bragg 2003; Rasmussen
et al
. 2005) and other sites
(Lewis 2002; Pratt & Parchure 2003).
The validity of data produced by bulk-optic instru-
ments can be compromised by at least two in-stream
conditions. Biological fouling (“biofouling”) of the
optical windows of sensors, which results in the ten-
dency for the output to shift from the calibration
curve to spuriously larger values over timescales of
days or more, remains a problem, particularly in
warmer, microbiologically active waters. Commer-
cially available mechanical wiper systems for some
sensors may alleviate this problem.
Additionally, turbidity levels exceeding the instru-
ment's maximum measurement limit results in sensor
saturation. When saturation occurs, constant values
equal to the turbidimeter's upper measurement limit
are output, creating a turbidity trace with a “plateau”
comprising erroneously low turbidity data. This phe-
nomenon tends to occur at the higher fl ows and
higher SSCs that are most infl uential in sediment
transport. Figure 1.6 shows a hydrograph and tur-
bidity trace for the USGS streamgage on the Kansas
River near DeSoto, Kansas, USA, for the period April
12 to May 24, 2002. The turbidity trace for periods
encompassing April 22 and May 14 (Fig. 1.6) show
the characteristic “saturation plateau” when the in-
stream turbidity level exceeded the turbidimeter's
maximum recording level.
Maximum SSC limits for turbidimeters depend in
part on instrument specifi cations and the ambient
PSD. The OBS instrument has a generally linear
