Sedimentary budget

The Waimakariri River on the coast of Canterbury, New Zealand produces 77% of sediment supplied to the Pegasus Bay coastline.

Rivers are major point sources of sediment contribution to the coastal sedimentary budget, this being particularly true of coasts with a steep gradient, where rivers directly dump their sediments at the coast. Coasts with a low gradient may lose river sediment to estuaries.[5] Sediment delivery to the shoreline can be very intermittent mostly occurring during floods with the increase in flow typically creating an increase in sediment supplied to the coast. Some rivers are referred to as ‘large’ because they produce high amounts of sediment for which to nourish the coastal environment. For example, the Waimakariri River on the coast of Canterbury, New Zealand produces 77% of sediment supplied to the Pegasus Bay coastline.[6] This is not always the case with some rivers referred to as ‘small’ because they struggle to supply enough sediment to keep their coastlines from eroding, for example, the Rakaia River South of Banks Peninsula in Canterbury, New Zealand.[7]

The construction of river dams for flood control and hydropower reduces sediment supply to many coastlines due to the trapping of sediment and the decrease in flood peaks and flood intensity. In places such as Southern California of the United States, in particular the San Luis Rey River, dams are built to control flooding of properties along the river. Ironically, in doing so, this is contributing to the damage of coastal properties due to a lack of sediment produced to protect the beaches.[8] Another example is the Aswan Dam constructed on the Nile River, Egypt in 1964. Prior to the construction of the Aswan Dam, the Nile River delivered 60-180 million tonnes of sediment and water to the Mediterranean Sea every year. Sediment supply is now almost zero which has produced a significant imbalance to the near shore sedimentary budget, creating major erosion and shifting of sediment along the coast.[9][10]

The effects of sediment trapping due to dams can be exacerbated when combined with other activities such as in-stream gravel mining. Excavation of gravel from a river bed forms pits within the channel profile which can trap much of the incoming bed load sediment, preventing or slowing it from reaching the coastline. Mining can also reducing the amount of overall sediment available for transport, especially when it occurs down stream from dams. For example; approximately 300,000 m³ of gravel is extracted from the San Luis Rey River in South California every year, which is almost 50 times more than the bedload sediment yield after the dam was built.[8] Thus, removing more bedload sediment further decreases the sediment yield available to the coast.

The removal of natural vegetation for cultivation and land use can increase soil erosion resulting in an increase in sediment yield transported by rivers to the coast. For example; in Westland New Zealand this has had a cumulative effect with clear felling of trees increasing in river sediment yield up to eight times.[10]

Sea cliff erosion is a large source of sediment to many coastal sedimentary budgets, initiated by many different processes including wave attack, rainfall and groundwater seepage. Cliff erosion can be influenced by rising sea levels and is magnified with storm surge events.[11] An example of cliff erosion is the erosion of large Pleistocene alluvial fans that span the length of the Canterbury Bight, situated north of the Waitaki River in New Zealand. The erosion of these cliffs, due to high energy wave environments contributes 70% of the overall material supplied to these beaches.[12]

Although less frequently observed than river transport and sea cliff erosion, storms may account for a large percentage of coastal sedimentary budgets. Following Hurricane Katrina and Hurricane Rita in 2005, over 131 x 10⁶ metric tons of sediment were deposited along the Louisiana and eastern Texas Gulf Coast. Study of the western Louisiana coastal wetlands found hurricanes appear to be the "overwhelming pathway" by which sediment is deposited in that location, accounting for amounts of sediment far greater than amounts deposited by the local river systems.[13] Based annualized estimates of magnitude of sediment deposition, hurricanes have been found to deposit hundreds of times more sediment in these coastal wetland regions than man-made river diversions intended to redirect river-transported sediment to starving wetland systems. For salt marsh wetlands, particularly those of coastal Louisiana, sediment accumulation from hurricanes is "sufficient" to account for the entire inorganic sedimentary budget.[13]

Estuaries are an example of a coastal sink in that they tend to trap sediment which can be due to tidal circulations and the mixing of fresh and salt water, the injection of river sediment and the presence of mangroves. As the tide rises and falls water and sediment is pumped into the estuary. Because salt water and sediment particles are heavier than fresh water they tend to be carried the bottom until the sediment sinks to the floor and is trapped within the estuary. The movement of sands and offshore material into an estuary generally depends on the direction of longshore drift and the movement of bottom waters across the continental shelf. Estuaries can often trap a lot of coarse bedload sediments that are fed down rivers, intercepting them before they reach the coast.[14] The North Island of New Zealand experiences sediment sinks into estuaries often, enhanced by the presence of mangroves. Mangroves are sediment hungry and trap a lot of suspended sediment with their complex aerial root structure, thus functioning as land builders.[15]

Sand that is blown by the wind inland to form sand dunes usually develop on shorelines where there is suitably strong winds. This can be a major sink for the sediment budget of a shoreline.

When a coastal environment is in sediment deficit, anthropogenic sediment nourishment is one way in which a balanced sedimentary budget can be maintained. This type of coastal erosion management has been adopted all over the world in order to preserve and protect. An example of this is at Mount Maunganui beach in the North Island of New Zealand which had been experiencing erosion, resulting in coastal dune retreat of almost 20 m. When ongoing dredging at the entrance to the Tauranga Harbour began, it was decided that the sediment removed would be used to re-nourish the Mt. Manganui beach. The sediment was deposited in the nearshore zone promoting beach accretion by offshore berm emplacement. Results show that most of the 80,000 m³ of sediment added to the nearshore zone made it to ashore to re-nourish the beach and even out the past sediment deficit.[18] Nourishment of a coast can be regarded as a fast fix option to reversing a sediment deficit; however, it is important that nourishment is on-going in order to ensure the sedimentary budget remains balanced.

When protecting the coastline it is important to understand the how the sedimentary budget can be affected when implementing appropriate coastal protecting techniques. Often management plans for coastal erosion have seen the use of ‘hard’ engineering structures as a means of protecting the coastline from recession. In particular groynes which are used to trap the longshore drift of sediment that often deprives a beach. Groynes have the ability to change the coast's sediment budget, accreting up drift beaches but at the same time starving down drift beaches.[19] This management approach is not used so much these days, with modern knowledge of coastal dynamics promoting the use of ‘soft’, natural approaches such as nourishment and preservation of natural systems such as dunes.

Being able to incorporate the sedimentary budget into a coastal management plan is becoming critical, especially in today's world where the majority of populations are living and owning property very close to the coast. One of the essential components drawn from sediment budget is being able to predict the morphological change that is likely to occur to a coastline over time, especially when creating plans associated with major environmental change such as sea level rise. Incorporating a sediment budget into a coastal plan has been recognised as highly important in the Hawke's Bay of New Zealand for finding information relating to hazard zones, beach property protection, and coastal erosion as well as assess the successfulness of current management strategies.[20] The major setback with using the sediment budget for management and what is probably the underlining issue regarding the sediment budget is its complexity.