Change in sediment behaviour of steep channel in modified bed conditions, University of Auckland, New Zealand

Steep channels (>10% slope) in mountain environment frequently face sediment overloading conditions. Mass wasting processes in the form of landslides supply materials to the channels. This results in the channel bed rising and widening. An interesting aspect is to understand the response of the modified wider channel in comparison to the original low-elevated narrow channel bed. We are working on this taking a 5.5 km long channel section of the Hapuku River, South Island, New Zealand. The Hapuku River was badly affected during the 2016 7.8 Mw Kaikōura Earthquake. Biggest mapped landslide for the earthquake event formed a landslide dam lake at the headwaters. The sediment transfer events initiated from the dam breach overloaded the channel downstream. This changed the channel morphology from a single thread to braided in the upper section of the study area.

We are working on understanding the sediment behaviour for the original and modified channel conditions using the morphodynamic model of Delft3D FM. We are using two Delft3D FM morphodynamic models with original and modified channel conditions. We are using a 45-hour design hydrograph generated from the long term (30 years) discharge data. Field collected sediment data was used in the initial bed composition and sediment files. We are running each model with and without sediment feed. We are planning to generate outcomes for 4 scenarios: a) original channel without sediment feed b) original channel with sediment feed c) modified channel without sediment feed d) modified channel with sediment feed. Our goal is to analyse the results and see how the sediment behaviour changes for each scenario. It would be a good opportunity to see how the Delft3D FM performs in such steep channel conditions as well.