UPDATED 09 SEPTEMBER 2017

As described in related articles, the LISST-ABS has a near-constant sensitivity (Volts/concentration) for sediment particles in the >30 micron size range [see Features section here]. In contrast, the sensitivity of optical turbidity degrades with increasing grain size. This article illustrates the consequence of this difference from actual field data. Not only does the LISST-ABS see detailed profile, it’s estimated concentrations match physical samples. We have earlier predicted similar consequence using actual Cowlitz river particle size-distribution profile [see here.]

In a river, the typical sediment vertical profile is a sum of a wash-load and a suspended load. The wash-load is fine material that does not settle out, and is simply ‘washed’ downstream. It is well-mixed, i.e. it has a constant concentration throughout the river column. In contrast, the suspended load consists of coarse grains, typically fine to coarse sand, that exists only due to re-suspension from river bottom by turbulence. The concentration of this suspended load is maximum at the river bed, and decreases away vertically. River sedimentologists often refer to this as the Rouse profile.

LISST-ABS vs Turbidity

The data displayed below are reproduced by permission from USGS observations in the Missouri river. The inset shows ADCP acoustic backscatter profiles taken at 3 cross-sections. Along mid-river profiles 3,4 and river-bank profile 5, turbidity and ABS data are shown. In the mid-river profiles (3 &4), the ABS sees a rich vertical structure. Total acoustic backscatter increases by a factor of 3 in the deepest profile (#3). Profile 5 does not show this structure (see explanation below).

Data provided by and reproduced by permission of Molly Wood and Mark Landers, USGS.

A few observations are:

• LISST-ABS clearly reveals Rouse-like vertical profiles along lines 3 and 4, that is missed by optical turbidity.
• Whereas profiles from the middle of the river showed Rouse-like structure, the near-bank profile (#5) did not. This was explained by measurements of particle size effect [particles are smaller near banks.] Read the section below on combined acousto-optic sensor.

Concentration: LISST-ABS vs Physical Samples

Similar comparison of estimates of concentration by LISST-ABS  and from physical samples show a satisfactory agreement, see figure below. This can be interpreted to be consistent with near-constant sensitivity of LISST-ABS to grain sizes greater than about 30 microns.

Data provided by and reproduced by permission of Molly Wood and Mark Landers, USGS.

These data are quite extensive. We have only offered a preview. For more details, we refer interested readers to contact us, or write Drs. Mark Landers and Molly Wood of USGS directly.

Combined Acousto-Optic Sensor

In a concept originated at Sequoia (patent pending), a method of combining optical turbidity and acoustic backscatter has been invented. It takes advantage of the opposing trends in sensitivity with increasing grain size – optical sensitivity declines, acoustic sensitivity increases (till reaching 30 microns). A combining algorithm uses both data and produces an estimate of concentration whose grain-size sensitivity is nearly flat over a 1-500 micron size range. An article on how this works will follow.

This method would bring the errant profile near the river bank also close to true sediment concentration, TSS.

UPDATED 09 SEPTEMBER 2017