Open-water disposal

ERDC TN-DOER-E8

June 2000

4. Plot TSS as the y-axis versus turbidity as the x-axis.

5. If the plot reveals a relationship that looks linear, calculate the equation for the line of best

fit, of the form y = ax + b and plot it on the graph with the data points, along with the 95

percent confidence limits. If it is not linear, use a nonlinear curve-fitting program.

Open-water disposal. The TSS-turbidity correlation curve for open-water disposal should be

produced in the same manner as that for the effluent from a diked containment area. In this case,

the larger, heavier solids will begin to settle to the bottom immediately upon exiting from the dredge

discharge pipe, hopper, or barge usually in a more or less well-defined plume. The permit limitation

will frequently be on the TSS at the limit of a mixing zone at or near the surface a specified distance

from the discharge. Only those solids that have not settled in the time it takes for water to flow

from the discharge point to the measurement point at the prevailing surface velocity will be present.

The best way to obtain a representative sample of solids that are likely to be in suspension at that

point is with a laboratory column settling test as described previously.

Use of correlation curve for field monitoring. When the dredging or disposal operation is

in progress, samples of water can be taken at the points of compliance specified in the permit and

quickly analyzed for turbidity, even on the sampling boat. TSS can then be estimated directly from

the correlation curve.

The curve produced by this procedure can be used in the beginning phases of the operation to monitor

compliance with TSS standards on a routine basis, and the operation can be stopped or modified if

the turbidity values indicate a TSS standard violation. Some samples will have to be analyzed for

TSS in addition to turbidity to check for actual compliance with the standards, in addition to

estimated compliance. Any samples suggesting noncompliance must be checked. These samples

should also be analyzed for turbidity and the data plotted on the laboratory-produced TSS-turbidity

correlation curve. When sufficient field data are available, the correlation curve should, if necessary,

be recalibrated including the new field data. The recalibrated curve can then be used for the

remainder of the project with even greater confidence.

EXAMPLE: A laboratory column settling test was performed to obtain data for the design of a

dredged material containment area for sediments from Mobile Harbor. Both TSS and turbidity were

measured on the samples taken from the supernatant. The circles plotted in Figure 1 represent these

data, and the solid line is the linear regression line of best fit and represents the laboratory-developed

correlation curve.

When the dredging project was underway, effluent samples were also analyzed for TSS and

turbidity. These data are plotted as triangles in Figure 1, and the dotted line represents the linear

line of best fit.

Using the laboratory-derived solid line, a turbidity of 40 NTU is estimated to represent a TSS of

about 70 mg/L with a range of about 55 to 90 mg/L. The dotted line representing the field data

indicates that a turbidity of 40 NTU represents a TSS of about 52 mg/L with a range of about 25 to

60 mg/L. Although there are obvious differences in the two sets of data, the laboratory and field

correlations are in closer agreement than similar data sets produced by Earhart by analyzing whole