An analytical framework to investigate catchment-scale controls dictating sediment storage patterns within three West Auckland catchments, New Zealand.

Abstract

An analytical framework was created allowing generation of continuous, high resolution datasets pertaining to empirically derived sediment storage volumes. A mixed methodology, combining GIS techniques and field work was employed. Sediment storage volumes were calculated through a morphologically approached sediment budgeting technique, thus allowing the framework to be universally applicable and underpinned by process-based understandings. Sediment storage was differentiated by how it was stored (geomorphic unit type), where it was stored (landscape compartment) and how readily it was available to been reworked (inundation levels of a 100 year ARI event). Sediment storage patterns were investigated in context of the catchment scale controls acting on the system; namely slope, discharge, stream power, bed material substrate size and confinement. In isolation the controls analysed proved insufficient in accounting for the observable patterns of sediment storage. Although broad scale controls provided the context in which across catchment scale patterns emerged, it was the influence of local scale controls that drove diversity between the three West Auckland catchments. The interdependant nature and relative importance of each control in context consequently fashioned the sediment storage patterns. Confinement proved a good indicator of floodplain initiation and storage within floodplain surface and valley floor landscape compartments. Noted distinctions were made between the patterns of bedrock-controlled and planform-controlled confinement settings. Slope and bed material substrate exerted influence over within channel storage and bar, riffle and veneer geomorphic units. Local scale disturbances arose due to antecedent controls, whereby the landscapes historical morphology was actively shaping the contemporary pattern of sediment storage. The sensitivity of a reach to past movement consistently aligned with increases in sediment storage volumes. The storage patterns present were in part, a reflection of the scale in which they were observed. Both systematic across catchment and reach scale patterns presented. There was feedback in the form of trans-scale linkages, whereby contextual controls were able to exert top down influence and localised controls were able to exert bottom up influence over patterns observable at courser scales than which they occupied.