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Laboratory experiments have been carried out to investigate the erosion behaviour of a number of marine sediments which were reconstituted and sieved in various ways to ensure a large variation in soil properties such as (i) particle size distribution (including median grain size and fines content), (ii) bulk density, and (iii) hydraulic permeability. Based on the experimental results, it was found that for finer marine sediments the erosion rate at a given shear stress was sensitive to changes in these soil properties. No unique relationship was found between changes in the erosion rate and changes in the bulk density or fines content. More specifically, we find that the erosion rate at a given shear stress reduces with increases in density and fines content, but by an amount that is different for different sediments. In contrast, we show that there appears to be a unique relationship between permeability and erosion rate, such that permeability may be used to predict the erosion rate for the marine sediments at any density. By reinterpreting existing experimental results in the literature, we find that this same relationship between permeability and erosion rate is apparent for quartz sediments. We propose an empirical relationship, which fits well the erosion rate behaviour of finer sediments close to the threshold shear stress.

Under cyclonic conditions, sediment on the North West Shelf (NWS) of Australia may become mobile in shallow water due to classical sediment transport or local liquefaction, and this can affect, for example, the on-bottom stability of subsea pipelines. In this paper, three calcareous sediments sampled from the NWS are analysed, together with realistic metocean data, to illustrate this potential for sediment mobility on the NWS. Specifically, experiments are performed in a recirculating flume (known as an O-Tube) to measure the erosional behaviour and an additional series of experiments are performed using a shaking table, on which each of the sediments have been liquefied and excess pore pressure measurements recorded to back calculate the consolidation coefficient. Soil characterisation data, threshold velocity measurements and shaking table results have then combined to illustrate the potential for sediment mobility for each of the NWS sediments. Best practice models are used to calculate wave and current combined shear stress at the seabed and excess pore pressure accumulation. We find that for these sediments, freshly deposited in laboratory samples, mobility due to sediment transport or liquefaction is very likely in cyclonic conditions on the NWS. Liquefaction is most likely for loosely packed silt, whilst sediment transport is most likely for sand. However, we also show that in more extreme cyclonic conditions there are a subset of sediments that can become mobile due to both sediment transport and liquefaction.

An O-tube flume is a horizontal closed-circuit flume that can be driven by an inline impellertype pump to produce steady and/or oscillatory flow over a mobile seabed. An O-tube has the ability to reproduce large combined wave and current conditions near the seabed, typical of (cyclonic) storm conditions. In this paper, we investigate the hydrodynamics of an O-tube and show applications of this technology. First, we derive a dynamic equation to explain the coupling between pressure and flow rate within the O-tube. This result can be thought of as an extension of the classical hydrodynamic equations developed for U-tubes and allows for improved control of the flow within the O-tube by providing a prediction of the non-linear interaction between steady flow (i.e. currents) and unsteady flow (i.e. waves). We demonstrate this improved control by comparing measurements of flow rate taken from an O-tube with the dynamic equation. Secondly, we present velocity measurements to give a detailed description of the flow field within the O-tube, including mean flow profiles and seabed shear stress. Finally, we conclude the paper by providing two example applications of the facility to study sediment transport and scour.