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The most common method for estimating gravel volume is the ‘End Area Method’, which is widely used by many councils. This method calculates gravel volume changes between upstream and downstream cross sections using a volumetric balance equation. It relies on surveyed cross sections to assess changes in channel geometry, based on a predetermined active channel width that is assumed to remain constant over time. This assumption can create challenges in braided rivers, where the main channel may shift laterally across the river corridor.

The Rivers team at Marlborough has developed a new method to quantify total gravel volume in braided rivers. This method is based on the principle that conveyance water volume is conserved along the channel length. As a result, channel characteristics such as active width, wetted cross-sectional area and mean bed level are functions of discharge, velocity and channel geometry.

This newly developed method has been used to estimate mean bed levels and gravel volumes in the Wairau River, Marlborough, since 2020. By representing the channel as trapezoidal with varying active width, rather than as a fixed rectangular section, the method provides a more reliable estimate of gravel volumetric changes.

Presented by Duc Nguyen, Marlborough District Council.