SIR 2004-5028

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Patino, Eduardo, and Byrne, M.J., 2004, Application of Acoustic and Optic Methods for Estimating Suspended-Solids Concentrations in the St. Lucie River Estuary, Florida: U.S. Geological Survey Scientific Investigations Report 2004-5028, 23 p.


Acoustic and optic methods were applied to estimate suspended-solids concentrations in the St. Lucie River Estuary, southeastern Florida. Acoustic Doppler velocity meters were installed at the North Fork, Speedy Point, and Steele Point sites within the estuary. These sites provide varying flow, salinity, water-quality, and channel cross-sectional characteristics. The monitoring site at Steele Point was not used in the analyses because repeated instrument relocations (due to bridge construction) prevented a sufficient number of samples from being collected at the various locations. Acoustic and optic instruments were installed to collect water velocity, acoustic backscatter strength (ABS), and turbidity data that were used to assess the feasibility of estimating suspended-solids concentrations in the estuary. Other data collected at the monitoring sites include tidal stage, salinity, temperature, and periodic discharge measurements.


Regression analyses were used to determine the relations of suspended-solids concentration to ABS and suspended-solids concentration to turbidity at the North Fork and Speedy Point sites. For samples used in regression analyses, measured suspended-solids concentrations at the North Fork and Speedy Point sites ranged from 3 to 37 milligrams per liter, and organic content ranged from 50 to 83 percent. Corresponding salinity for these samples ranged from 0.12 to 22.7 parts per thousand, and corresponding temperature ranged from 19.4 to 31.8 C. Relations determined using this technique are site specific and only describe suspended-solids concentrations at locations where data were collected. The suspended-solids concentration to ABS relation resulted in correlation coefficients of 0.78 and 0.63 at the North Fork and Speedy Point sites, respectively. The suspended-solids concentration to turbidity relation resulted in correlation coefficients of 0.73 and 0.89 at the North Fork and Speedy Point sites, respectively. The adequacy of the empirical equations seems to be limited by the number and distribution of suspended-solids samples collected throughout the expected concentration range at the North Fork and Speedy Point sites. Additionally, the ABS relations for both sites seem to overestimate at the low end and underestimate at the high end of the concentration range.


Based on the sensitivity analysis, temperature had a greater effect than salinity on estimated suspended-solids concentrations. Temperature also appeared to affect ABS data, perhaps by changing the absorptive and reflective characteristics of the suspended material. Salinity and temperature had no observed effects on the turbidity relation at the North Fork and Speedy Point sites.


Estimates of suspended-solids concentrations using ABS data were less "erratic" than estimates using turbidity data. Combining ABS and turbidity data into one equation did not improve the accuracy of results, and therefore, was not considered.




Purpose and Scope

Description of Study Area


Methods of Investigation and Data Analysis

Measurement of Acoustic Backscatter Strength

Measurement of Turbidity

Sampling and Determination of Suspended-Solids Concentrations

Collection of Tidal Stage, Discharge, Salinity, and Temperature Data

Analysis of Laboratory and Field Data

Estimation of Suspended-Solids Concentrations Using Acoustic and Optic Methods

Suspended-Solids Concentration to Acoustic Backscatter Strength Relation

North Fork Site

Speedy Point Site

Suspended-Solids Concentration to Turbidity Relation

North Fork Site

Speedy Point Site

Qualification of Empirical Equations

Sensitivity Analysis

Time-Series Estimates of Mean Suspended-Solids Concentrations

Summary and Conclusions

References Cited

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