Dredging measurement starts with the quantity you need to know. The depth of the bed, the flow through a pipe and the dry mass of sediment transported are different measurements. Choosing the right combination helps an operator interpret a reading and an engineer define the reporting boundary.
Choose the quantity before the instrument
A survey describes the bed and changes in its level or shape. A flow measurement describes the slurry moving through a line. A density meter describes the mass of a given volume of that slurry. Solids concentration describes how much of the mixture is solid, while production combines measurements over a defined time.
For hydraulic dredging, a useful first question is whether you need a live operating trend, a transferred slurry quantity, a dry-solids estimate or evidence of material removed from the bed. An inline density reading cannot answer all four questions on its own.
Slurry density and solids concentration
A dredging slurry contains water and sediment. Density is normally expressed in kg/m³ or tonnes/m³. It changes with the liquid, solids and their proportions, and can also be affected by entrained air and dissolved material.
Percentage solids by volume describes the share of mixture volume occupied by solids. Percentage solids by mass describes the share of mixture mass contributed by solids. They differ because solids and liquid have different densities.
A simple two-phase calculation can estimate concentration from measured slurry density when the liquid and effective solids densities are known and representative. Those assumptions need checking for saline water, mixed sediment, temperature changes and gas. The existing slurry density and percent-solids calculator explains the relationships and accepts an optional volumetric flow input.
Flow and production
A volumetric flowmeter describes how much slurry passes a measuring point per unit time, for example in m³/h. Velocity in m/s is a different quantity; converting velocity to volume flow also requires the effective pipe area.
Multiplying aligned volumetric flow by representative slurry density gives a wet slurry mass rate. A dry-solids rate needs an additional validated concentration relationship. The dredging production and mass-flow guide shows a worked example and defines its assumptions.
The measurement locations and timestamps matter. Water addition, branches, transport delay and gaps in either signal can change the interpretation. PMA’s density and flow reconciliation guide explains how to keep a transfer record traceable.
What an ultrasonic slurry density meter measures
The Rhosonics SDM ECO uses ultrasonic acoustic impedance at the sensor–slurry interface to determine density against its configured calibration. The selected fitting places the sensing face at the process boundary. This is a process-contact density measurement without a radioactive source.
It is distinct from an external clamp-on Doppler flowmeter, which uses reflected ultrasound to measure velocity. The word ultrasonic describes several measurement principles; it does not mean that instruments measure the same quantity or use the same installation.
A suitable density trend can support a review of solids loading, dilution or transport conditions. It does not replace a flowmeter, soil characterization, survey evidence or a complete pump-protection system. Review the SDM ECO product, installation guide and non-nuclear versus nuclear comparison for the next selection step.
A manufacturer example from Beens Dredging
Rhosonics reports that Beens Dredging has used SDM instruments since 2017 and subsequently added SDM ECO. Its February 2024 account describes density measurement of material being pumped into a hopper alongside flowmeters.
That is a useful example of density and flow working together in a dredging process. It describes the manufacturer’s reported installation in the Netherlands; it is not a PMA customer claim or a performance guarantee for another vessel. Read the Rhosonics account.
Keep process monitoring and environmental monitoring distinct
A measurement inside a dredging transport pipeline describes the material at that point. It does not establish the condition of an environmental plume outside the line. Turbidity, water-quality monitoring and bed surveys need their own measurement methods and evidence.
Likewise, transported slurry volume, wet slurry mass and dry-solids mass must not be labelled as the in-situ volume removed from the bed. Define the result, units and acceptance evidence before using an estimate in a project report.
Prepare a useful application review
Tell PMA the dredger type, sediment and carrier liquid, expected operating range and the quantity you need to report. Include pipe dimensions, fill conditions, pressure, temperature, entrained-air behaviour, measuring locations, existing flow data and the reference-sampling method.
For a density application, start with the dredging slurry density library. It links to existing guides for pipelines, cutter suction and hopper dredgers, booster pumps, land reclamation, maintenance work and reservoirs. Discuss your measurement with PMA.
Sources
Rhosonics SDM ECO product information, Rhosonics dredging applications, Rhosonics Beens Dredging account, Pulsar UK dredging flowmeter case, and PMA’s linked calculator and reconciliation guide.
