How to Measure Dredged Quantities

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While relaxing at home one day in 1974 thinking of the struggles other engineers were having at the river dredging site, an idea came to me for solving their problem.  They needed a way to measure the three-dimensional change in bed profile between one survey and the next, so that the contractor could be paid for the difference, assuming that this represented the amount of material he had removed from the river bed.

The site for dredging a diversion channel had been mapped.  At least three marker points had been established as sextant targets, and sextant circle charts were generated from them.  Using these, the survey boat would proceed on a series of approximately parallel runs across the channel area.  At a sequence of points a few seconds apart, a co-ordinator would shout, "Fix", and two surveyors would take their angular readings from their sextants at that moment, while the sonar instrument button was pressed to mark simultaneously a depth reading on its output (or other depth reading device was used).  This resulted in a plan with a several zig-zag rows of dots, each dot annotated with a depth.

The problem was how to use the information on two such maps to work out the volume change.  The survey runs were never quite the same on different occasions owing to the effect of current, wind, steering, and so forth.  So, the rows of dots on different survey plans never had any correlation.

The solution I suggested was to transfer the information for each survey onto a fixed grid of standardised cross-section lines, as follows.  Use each surveyed dot plan to construct a map of interpolated contours.  These may rarely show up two possible contour layouts at saddle shapes, but the clients team and the contractor's team had to agree to share the advantages and disadvantages of these occurences.  As contours intersected a standardised cross-section line, values could be assigned to points on the cross-section.  The area of each cross-section could be calculated and thence the volume within the grid on that survey.  On the next survey the same process yielded a different volume.  The difference between the two was a reasonable assessment of the volume of material that had been taken away by the contractor's dredging activities: an acceptable basis for fair payment for his work.

Jasper Burford

Date of last edit: 03 April 2015