The Geomorphic Impact of Rock Avalanches on Landscape Evolution.

Rock-avalanches (RAs) are large erosional events which, in a matter of minutes, can deliver acatastrophic volume of debris (≥ 1×106m3) directly to a fluvial system, often dammingmountainous rivers. RAs have a boulder carapace protecting readily transportable sedimentsizes, differing from landslides. Evidence suggests that the point-load sediment delivery ofrock-avalanche deposits (RADs) on fluvial systems can force rivers into a chronicallydisturbed state for up to 104 years. Fieldwork and micro-scale modelling are used to trace RAsediment dispersion in Ram Creek, New Zealand. The data forms a new conceptual model ofriver erosion and depositional response, changes to river planform and sedimentyield, and an effective model timescale of perturbation for both RA and landslidedams.Fieldwork investigation shows large scale aggradation from RA outburst flood sedimentdispersion, buffering vertical bedrock incision. The system is unable to transport the quantityof dam-derived sediments, and will not attain equilibrium before the next major event; it is ina state of persistent disturbance where localised reworking dominates. Lateral bevelling in theslot-type bedrock gorge topography appears to be the dominant form of erosion promotinggorge widening of the weak bedrock. It is theorised that aggradation of RA debris and lateralbevelling may result in the formation of a strath terrace if sediment cover persists,independent of external tectonic and climatic forcing. Normalised steepness and concavityindices of the Ram Creek river profile were compared to four neighbouring streams. TheRAD perturbation to profile metrics is not substantial enough to be discerniblefrom neighbouring streams containing large tectonically induced knickpoints. Thissuggests RA frequency estimates may be underrepresented in landscape evolutionmodels.Micro-scale modelling of the responses to a catastrophic sediment input into Ram Creek;either a RA (including carapace) or a landslide, each show different geomorphic responses.The RA model shows an un-breached carapace acts to trap fluvial sediments, resulting inpartial burial. Further downstream, initial sediment starved incision is followed by a slowerrecovery to pre-RA bed elevations, and continued aggradation of fluvial sediment tovertically displace the profile up towards the RA. In the landslide model, immediate breachchannel formation results in a sediment pulse downstream. Dispersed sediment forcesaggradation, controlling fluvial geomorphology.Fieldwork and modelling both show catastrophic sediment inputs control fluvialplanform, sediment yield, calibre, and storage, and geomorphic forms throughout theaffected fluvial profile. This contradicts the view that rivers in the Southern Alps, areunder-loaded and transport all sediment that is supplied to them. The inability of theriver to remove both the RA and landslide deposits suggests that the river will notrecover before the return period for the next large sediment input, and is a scenariowitnessed across numerous other feeder catchments to larger rivers in New Zealand. Itis suggested here that RA and landslide sediment storage from multiple eventscontrols the geomorphic evolution of smaller mountainous catchments, and RAforced dis-equilibrium may actually be the 'normal’ state in many mountainousstreams. [ABSTRACT FROM AUTHOR]