Channel Geometry for Radar Open-Channel Flow Measurement

A non-contact radar flow system can measure water level and, depending on the design, surface velocity. It still needs a defensible channel model to calculate discharge. Incorrect width, side slope, invert elevation or water-level datum can create a systematic flow error even when both sensors are working correctly. Geometry must therefore be surveyed, documented and maintained as part of the measuring system.

Define the hydraulic section

Identify whether the site is a rectangular channel, trapezoidal canal, circular conduit, natural section, flume or weir. Record the exact cross-section used by the calculation and its relationship to the sensor location. Do not substitute nominal construction dimensions when lining, erosion or deposits have changed the as-built profile.

Survey enough points to represent corners, benches, side slopes and irregular surfaces. For natural channels, define the stable control section and the range over which the survey remains valid. Photograph the section and reference permanent marks so it can be resurveyed consistently.

Use one vertical datum

Tie the radar reference plane, channel invert, staff gauge and calculation zero to the same elevation datum. Record antenna offset and any bracket or adapter dimensions. A small datum shift affects cross-sectional area at every reading and is especially important in shallow flow.

After the sensor or bracket is replaced, verify the reference rather than copying the old empty distance. Keep raw measured distance and calculated water level as separate values so a survey or configuration error can be traced.

Account for changing geometry

Sediment raises the effective invert; vegetation, debris and scale reduce area or disturb velocity. Erosion can enlarge a natural channel. Establish inspection triggers after floods, construction, dredging and seasonal growth, and schedule routine checks based on how quickly the section changes.

If the calculation cannot be updated immediately, flag the discharge as estimated or invalid rather than silently using obsolete geometry. Retain geometry revisions with effective dates so historical data remain interpretable.

Check hydraulic controls

Backwater from a gate, downstream obstruction or high receiving-water level can change the level-flow relationship. A flume or weir must operate within its specified submergence and approach-flow conditions. A rating curve developed in free flow is not automatically valid during submerged flow.

Map upstream bends, drops, gates, pumps and confluences. Use a second level point or another hydraulic method where backwater is credible and consequential.

Verify the calculation

Test surveyed points through the area algorithm and compare results with independent calculations. Confirm units, interpolation and out-of-range behaviour. If velocity is measured, ensure the velocity and area refer to a compatible section and timestamp.

Validate discharge across low, normal and high flows using a suitable reference such as current-meter traverse, acoustic method, calibrated structure or controlled volume. One test point cannot prove the entire range.

Worked maintenance example

A lined channel reports increasing flow although pump operation is unchanged. The radar level remains stable, but a survey finds sediment has raised the invert by 80 mm beneath the sensor. The calculation still uses the original construction datum and therefore overstates area.

The section is cleaned, geometry is resurveyed and the datum is verified against the staff gauge. The team adds a post-storm inspection trigger and keeps both old and new geometry versions with the affected data period.

Engineering checklist

Survey the as-built hydraulic section.
Use one datum for sensor, invert and gauge.
Record sediment, vegetation and erosion limits.
Identify backwater and submergence conditions.
Verify units, interpolation and validity flags.
Validate low, normal and high discharge.

Frequently asked questions

Can nominal channel width be used?

Only when it represents the maintained as-built section over the full depth range.

Why does a small invert error matter?

It changes calculated depth and area at every reading, with a large relative effect at low flow.

When should geometry be resurveyed?

After floods, dredging, construction or visible change, and at an interval supported by site stability.

Need a project-specific review? Send drawings, operating data, photographs and acceptance criteria through our contact page.