Can a Doppler radar flowmeter still measure accurately under backwater from a downstream gate?

Gate-induced backwater is the classic open-channel measurement problem: once the downstream gate throttles or the tailwater rises, the measuring section passes from free flow into the backwater zone, the velocity profile flattens, local recirculation appears, and the surface-to-mean velocity coefficient K drifts. Every method built on a fixed K loses accuracy, and Doppler radar flowmeters are no exception — but three measures bring the error back into an acceptable band.

Pick the section location first. Place the measuring section outside the backwater influence: compute the backwater profile at maximum tailwater and site the section above the afflux line with enough straight reach upstream of the gate. Where an existing station inside the backwater zone cannot be moved, go to the second measure.

Calibrate K in segments. The COSP model of the HZ-KSLF accepts K as a function of stage (or of the head difference across the gate). During calibration, add dedicated gaugings under throttled high-tailwater conditions on top of the routine mid-flow ones: set the gate to several openings, gauge with a current meter once each stage stabilises, and regress a separate K curve for the backwater regime. Free flow then uses the free-flow curve, backwater the backwater curve, with switching decided automatically from real-time level.

Monitor head on both sides. Install a visual millimetre-wave radar level meter downstream (80 GHz FMCW, ±1 mm resolution) to read tailwater, and wire both levels into an EC32/EC60 series RTU. The head difference classifies the flow regime in real time: above the critical head the section is in free flow; below it, the backwater K curve takes over. Discharge records from gate-controlled reaches also carry a regime flag for water-allocation audits.

Irrigation networks, urban drainage gate stations and tidal barriers all see periodic backwater. This combination — section siting, segmented calibration and dual-level regime detection — has been proven at multiple gate-controlled sites, holding flow deviation within ±8% under backwater.

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