Porous check dams and the MERGE gully erosion model

: Gullies are hot spots of erosion. Gullies are the majority source of sediment that ultimately reaches the Great Barrier Reef despite occupying less than 1% of the catchment. Consequently, considerable investment and effort has focussed on preventing gully erosion through on-site remediation activities. Porous check dams (PCDs) are a common tool in erosion mitigation activities. PCDs are designed to slow the velocity of water through a channel, promoting the deposition of sediment, nutrients and seeds above the dam. Field observations suggest that, in some cases, PCDs can lead to increased scouring below the dam, risking a net increase in erosion relative to pre-intervention conditions. This paper uses the MERGE gully erosion model to explore whether the installation of a PCD can trigger increased scouring below the dam, and consequently a net increase in the amount of sediment delivered to receiving waters. Eight scenarios, covering four flow regimes and two boundary conditions, are explored. We simulate constant depth flows of 0.1 m and 0.5 m depth in a reference gully channel with inflow concentrations from the head of 50 kg/m 3 and 100 kg/m 3 . Varying depth flows are simulated with a sinusoidal function with amplitudes of 0.1 m and 0.5 m depth with the two different inflow concentrations. The reference gully is a small linear gully of 2 m width, 60 m long channel and 2% slope. The sediment is easily eroded, with a density of 1330 kg/m 3 , and 10 µ m particle size and with low cohesion. The PCD is installed 40 m from the start of the channel. The effect of the PCD is explored considering the growth of a depositional layer, and changes in the sediment delivery rate, that is the net sediment flux exiting the gully. This modelling investigation demonstrates that the installation of a PCD can lead to an internal step (or head/waterfall) forming below the PCD. In all simulations the PCD reduced the sediment delivery rate at early times, however in five of the eight scenarios the PCD resulted in a net increase in the sediment delivery rate by the end of the simulation. The increased sediment delivery rate is a direct consequence of accumulation behind the sediment creating a step, or internal head, at the PCD. This introduces an increase in the power available to erode, and therefore a greater rate of entrainment below the PCD. These results highlight the importance of ongoing monitoring and maintenance of PCDs to ensure they continue to operate as intended.