The formation of river longitudinal profile under the significant influence of slope processes (on the example of the Waipaoa river, New Zealand)

The article studies sediment accumulation processes in the upper reaches of a river with abundant sediment supply from slopes and subsequent river incision into the accumulative strata when the supply of slope sediments ceases. The object of study is the upper reaches of the Waipaoa River (New Zealand), where such processes are well studied and morphometric, hydrological and sedimentological information is available. The deformation equation using the Grishanin formula for calculating the transport rate of bedload was solved using numerical methods, regional morphometric relationships and an empirical formula for the rate of sediment supply from slopes to the head of the river. The result is a quantitative description of both sediment accumulation processes and the processes of river incision into them. Under conditions of sediment supply from slopes, a sediment accumulation area was formed, which expanded over time both upwards (regressively) and downwards (transgressively) along the river. The longitudinal profile of the channel straightened along the chord. Subsequent incision upon cessation of sediment supply occurred transgressively, starting from the upper section of the accumulation area. As a result, a cyclic chord terrace with time-transgressive downstream incision was formed. During the incision, sediments accumulated on the surface of the terrace in the middle and lower parts, which, despite subsequent erosion, could be preserved in the section of the alluvial layer. The use of numerical methods allows us to trace the details of the processes of formation of accumulative deposits of cyclic terraces and the incision of river flows into these deposits.

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