Sedimentary budgets are a coastal management tool used to analyze and describe the different sediment inputs (sources) and outputs (sinks) on the coasts, which is used to predict morphological change in any particular coastline over time. Within a coastal environment the rate of change of sediment is dependent on the amount of sediment brought into the system versus the amount of sediment that leaves the system. These inputs and outputs of sediment then equate to the total balance of the system and more than often reflect the amounts of erosion or accretion affecting the morphology of the coast. To assess the sedimentary budget the coast has to be divided into two separate morphologies, commonly known as littoral cells and compartments. Sediment compartments can usually be defined as two rocky barriers which mark the ends of a beach and have a fixed sediment budget, although usually leaky to some extent. Littoral cells can either be free or fixed and can occupy a hierarchy of scales, from individual rip cells to entire beaches. There are various types of natural sources and sinks within a coastal system. Sediment sources can include river transport, sea cliff erosion and longshore drift into an area. Sediment sinks can include longshore drift of sediment away from an area and sediment deposition into an estuary. Anthropogenic activities can also influence sedimentary budgets; in particular damming of a river and in stream gravel mining of a river bed can reduce the sediment source to the coast. In contrast beach nourishment can increase sediment source. In 1966, Bowen and Inman defined a littoral cell and separated sediment inputs, accretion by longshore drift and outputs. Sedimentary budgets are used to assist in the management of beach erosion by trying to show the present sediment movement and forecast future sediment movement.
Feedback mechanisms In order to understand the sedimentary budget of a coastal environment it is important to know the different types of feedback that can determine whether there is stability. When a beach environment is effected by wind, wave and tidal energy it responds with either positive or negative feedback which determines whether the system is balanced and in equilibrium. Negative feedback is a stabilising mechanism acting to oppose changes to coastal morphology and establish equilibrium. A coastal environment in equilibrium is able to dissipate or reflect incoming energy without the occurrence of sediment input or output and change to morphology. For example; when a beach in equilibrium erodes during a storm it forms an offshore bar that in turn forces waves to break over it. By doing this the waves lose a lot of energy and dissipate before reaching the shoreline, significantly reducing further erosion. When the storm calms, the bar is then re-worked back on to the beach. In contrast positive feedback pushes a coastal system away from equilibrium by modifying its morphology until a threshold is reached, whereby a different type of response occurs. For example; if a storm event was to breach the foredune of a beach that is not in equilibrium, a vulnerable area would be created, which in turn would become susceptible to the formation of a blowout due to wind exploiting the absence of vegetation.
Sediment compartments and littoral cells Compartmentalisation of the shore occurs where there are major obstacles or objects, especially headlands on deeply embayed coasts. The beaches that are the most enclosed are commonly known as pocket beaches. On these types of beaches the volume of sand remains constant and are closed compartments. Littoral cells can be defined as sediment within a coast that is circulated e.g. rip currents. Littoral cells usually develop on coast which are not impeded by headlands and where longshore currents are allowed to develop. Identifying littoral cells is crucial to determine the sediment budget of sandy coasts. In south-west Western Australia, large cuspate forelands and rocky headlands are thought to be boundaries for littoral cells. Boundaries of littoral cells have been defined using tracer studies of sediment movement, geomorphological observation and sedimentological description, heavy mineral sourcing, and analysis of the spatial distribution of wave flux along the shore. Littoral cells are usually an area where changes in the volume of sediment directly affects changes in the coastline, and ideally they are defined to minimise longshore sediment exchange with other littoral cells, for example, a pocket beach surrounded by rocky headlands (which are presumed to exclude sediments). Sub-cells are usually defined to better measure the sediment budget of a coast with varying rates of accretion and erosion. The landward boundary of a littoral cell is usually the foot of a dune or cliff, however, the seaward boundary is difficult to define as mechanisms of sediment transport here are poorly understood. There are three kinds of boundaries between littoral cells: longshore, landward, and seaward; across which sediment may enter the littoral cell or leave it by various processes. It is important to identify which processes operate on a particular littoral cell and also important to identify sediment sources and sinks, as by measuring the sediment gained or lost by these sources and sinks, a sediment budget can be determined.
Sources
Rivers
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