A sedimentary investigation into the Camel Estuary, North Cornwall Navigational Dredging in Camel Estuary.

The Camel estuary, stretching seaward from the mouth of the River Camel, may be found situated on the North Cornwall coast, a relatively unspoilt area. Characteristic land marks distinguishing the area include: the headland Pentire Point as well as the headland, Streppire Point, the Doom Bar and the beach at Polzeath. The Camel River catchment is one of two large catchment areas that drain a large area of the east and north east Cornwall region. Into the River Camel and therefore the Camel estuary, run many rivers and streams, including the River Amble as well as the Allen, offering a large catchment area. Due to the size and area of the Camel catchment, it may often respond slowly to large rain events and from its sheer headwaters down to the sea going estuary, it runs through and over a varied morphology of rock. At the town of Padstow the estuary is more than half a mile wide and then reaches toward the market town of Wadebridge where it stretches inland for five miles.
Estuaries are unique natural settings where freshwater environments meet ocean or salt water environments, in a usually semi-enclosed body of water along the coast. These two entities mingle and exchange elements including sediment loads and water types. The variable and sometimes erratic environment requires the animals that inhabit it to be able to cope with irregular environmental circumstances and many animals including salmon and shell fish have developed bodily systems to be able to cope efficiently with varying levels of salinity in their surroundings. Estuaries are commonly seen as areas with a high level of biological productivity.
Recent research on the Camel estuary, its surroundings and habitats, has covered a range of topics including issues such as the conditions and threats to Biological Action Plan (BAP) habitats as seen in Bronwen Gibbard and David Brew’s work, ‘The impact of geological change on Biodiversity Action Plan habitats in the Camel estuary, Cornwall’, (2006) which, discusses the distribution of sediment throughout the Camel Estuary and the importance of the sediment input and distribution to estuary habitats and the wildlife concerned.
Additionally, the work by R, Uncles and J, Stephens, ‘Turbidity and Salinity of some estuaries of South West England’, describes the tidal and sediment tendencies of waters in the Camel estuary. In addition, there are many more works on the general conditions of estuaries in the North Cornwall area of Britain and around the world. The Environment Agency also publishes ongoing information covering changes and issues regarding the camel estuary and the associated region especially those affecting human settlements and habitats of interest. English Heritage also publishes information of interest regarding the Camel and its catchment area.
There are many types of estuaries and each river’s estuary may vary quite differently. Estuary types can be determined by either their water features or their geological features. Types of identifiable water features include, The salt wedge estuary where the mouth of the river flows directly into the sea, forcing the salt water back and generating a less salty upper layer and a more salty under layer. There are also partially mixed estuaries that occur in deeper estuaries where the salt water mixes upwards and the freshwater mixes downwards. A well mixed estuary, mostly occurs in shallower waters where the salinity mix is of a uniform distribution ranging from top to bottom but decreases from the direction of the ocean towards the river, due to a slow flowing river and strong tidal mixing. Fjord type estuaries are often found along glaciated coasts, particularly in Scandinavian countries. Fjords contain a relatively high river input but this is conjoined with little tidal mixing in the deeply voluminous and extended basin of fjord estuaries, that will normally, also contain a ledge or barrier separating it from the sea.
Types of geological features that determine an estuary type include: Tectonic, Coastal Plain, Bar built and Fjord estuaries. Coastal plain estuaries developed when global waters warmed after the last ice age, allowing sea levels to rise, and inundating shallow valleys. The depth of this type of coastal plain estuary normally increases further towards the river mouth. There are also estuaries known as tectonic estuaries which, gain formation after areas of land sink or subside due to movements of the earth’s crust, after which, the oceans waters fill in the depression or gap, left in place. Bar built estuaries develop as sand bars form along a given coastline, leaving the water on the river side of the sand bars, partly enclosed, away from the sea. Bar built estuaries have a tendency to retain shallow waters with a low tidal velocity, as a consequence, the wind is often an important mixing factor.
The Camel Estuary is a ria type estuary and an extremely dynamic system, with similar geological formations to that of a fjord. Its characteristics include a deep valley, flooded by post glacial sea rise, typical of the aforementioned coastal plain estuary. Additionally, the estuary is shallow, and containing sandy sediment, although it deepens at the river mouth and displays a narrow meandering channel that constantly changes position, shifting across the channel. The estuary is flanked by many small woods and creeks as well as the irregular pattern of medieval farmland areas surrounding the estuary landscape alongside more modern looking field parcels.
In addition to these land features, the Camel estuary is the site of a drowned forest, at the mouth of the estuary, similar to others found along the Cornish coast. The drowned valley was once home to a forest of hazel, oak, alder and birch trees, swamped as the post-glacial waters rose up, between a period ranging between 4000 to 7000 years ago. As the water level began to rise, it inundated the forest which, was then covered in layers of sand, after the forest was submerged underneath the then residing sand dunes(Bristow, C, 1996). Previously, to this event it appears as though the coast line looked some what different. The coast of Cornwall, including the area we now know as the Camel estuary, according to the best morphological evidence, was surrounded by a coastal plain further offshore than the line of cliffs now acting a s a coastal barrier. The then shore line probably took the form of a beach backing onto sand dunes. Over the last 7000 to 8000 years, the sands forming the beach at the time have gradually been forced further ashore, inland. The geology of the Camel estuary, its catchment and the surrounding area tend to revolve around the devonian types, quite common in Devon and North Cornwall, including the lower devonian, upper, middle and the named mixtures (Starrier, P, 1990). There are also middle devonian slates present (Bristow, C, 1996), featuring slates of a folded or faulted nature from the devonian period approximately 400 million years ago.
The Camel estuary has a wide and varied range of habitats and is considered to be of prime biological importance, shown by its designation as an Area of Outstanding Natural Beauty (AONB). It possesses, salt marshes, mudflats, tidal rivers, beaches, sand flats, sand dunes, grazing marshes and sub tidal channels as well as harbouring species such as golden plover, lapwing and shelduck at various times of the year. Due to the Camel estuary’s fluctuating dynamics, these habitats are often prone to changes in area shape and the extent of the range of individual habitats and the long-term increase in sea levels, more recently, due to global warming, is likely to increase pressure on habitats of a sensitive nature, especially around the river estuary, and those dependent on a limited fluvial (fluvial is used to describe characteristics relating to flowing water, including water flowing overland, percolating, as well as through flow and generally relating to streams and rivers) sediment supply or that are restricted by artificial stabilisation. When the Camel’s tides are low, the mudflats and sand banks become exposed. In the more sheltered areas, notably, the creeks, reeds and other vegetation colonise these flats and banks, often providing important centres for wildlife. The Camel estuary area has seen some significant interference from human development particularly in the inner estuary. Negative Environmental Impacts have been caused by armoured embankments, tidal barriers and bridges, the construction of some of which have altered the estuary quite extensively. One habitat that has suffered in particular is the salt marshes. Large areas that were isolated from the tidal flow due to human developments have become freshwater marshes or grazed farmland, although projects have been put in place to reclaim some of this land to revert back to salt marshes. Of the salt marshes that have existed to date, they mostly exist behind sheltered mudflats that are still open to the tidal flow.
Throughout the last hundred years or so, the estuary has seen some fairly radical changes of a natural persuasion. Particularly, during the 1940s , the middle of the estuary observed an important change when the main sub tidal channel experienced a shift from the west bank (adjacent to the town of Padstow) and moved across to the east bank (close to Rock) similar to its current location. Early studies have shown this type of movement of the channel to be a frequent occurrence and that the channel naturally changes again as it reaches what may be considered a critical point in its stability (Gibbard, 2006). Further examples include changes in the intertidal sandbars based in the channel, also in the mid estuary, across the past 30 years, suggesting a progressive movement of the main subtidal channel towards the former formation held before the channels migration in the 1940s (Gibbard, 2006).
Historical trends have shown that the type of channel migration already described has led to the formation of sediment features such as sand dunes at Harbour Cove and north of the town of Rock which has additionally, caused the erosion of sand dunes to the South of Rock. Since this occurrence, the outer extremes of the estuary have remained relatively stable and at present there is no morphological evidence to suggest that further major changes will occur. Since then, the outer sector of the estuary has also remained quite stable with a positive sediment budget, with the main input of sand sized sediment most likely to be from marine sources.
In the past, mining and dredging activities have both made contributions to the sediment budget and also removed large quantities of sediment from the estuary system. In total, there has been a net artificial removal of sediment from the estuary, removed for the purposes of sand mining and also navigational dredging. As this has had no apparent major effects on the estuary in terms of a significant reduction of sediment as to cause degradation to estuary features and habitats, there appears to be a considerable amount of sediment being transported into the estuary from the ocean, on the tidal flow. It follows then, that there must be a positive marine sediment budget.
The Camel estuary’s littoral drift patterns appear to constantly keep the estuary replenished with a large supply of sediment. Littoral drift involves the distribution of sediment provided from a variety of sources including the erosion of cliffs and the oceans currents transporting sediment from areas such as the sea bed for direct input into the estuary. The arrival of sediment into the estuary means that it has to be stored and that normally takes place in the form of a depositional or near shore feature such as bank or offshore bar. Alternatively it is transported straight through the estuary as a “through put” and becomes “output” (exiting) from the system, being deposited either out in deeper water or away from the coastal area in question, such as the Camel Estuary. There are two main types of sediment. The first type is clastic sediment taking the form of fragments of rock in various sizes formed from erosion and the weathering process. These may take the shape of tiny clay or mud particles to sand, pebbles or may be as big as boulders. A boulder is generally regarded to be 256 mm or more, a cobble is approximately 64-256 mm, a pebble is 4-64 mm a granule 2-4mm, a grain of sand is ¹/₁₆ -2mm, silt is ¹/₂₅₆ –¹/_16mm and clay is > ¹/_{256 mm} (anon, 2007). The second type of sediment is biogenic sediment which includes shells and skeletons of marine organisms.
The transport of sediment and its deposition is affected by weather and natural occurrences such as the wind, waves, tides and currents. It is these occurrences that provide the energy for the erosion process and the transportation for the sediment from its source to an area of low energy where it is deposited to create coastal landforms that often exist in a state of dynamic equilibrium with the local conditions. The Cornwall and Devon coastlines, including the Camel estuary region can be subject to strong prevailing winds from the south west which can affect the deposition of sediments, especially air-borne sand particles that the wind can transport and deposit to create sand dunes. Each estuary has its own individual local environmental conditions that ultimately affect and are affected by tidal and riverine flows. For the Camel estuary, the following are amongst the approximate condition; spring tidal range: 6.5 metres, tidal length: 6m, maximum depth average SPM (usually owing to suspended sediment) a low figure of 28 compared with the Tamar’s 700, residence time (flushing): 2.5 (Uncles, R & Stephens, J, 2006).
Depositional landforms such as beaches, mudflat, sand flats, sand dunes are created through the transportation and deposition of sediment. To this end, they act as constantly shifting (dynamic) sediment storage areas whilst also allowing sediment to be transported onshore, offshore and alongshore through them, to transport sediment to alternative or additional sediment stores along the coast. Sand, which is a non-cohesive sediment, is transported in individual grains. The energy needed to transport such particles and indeed to erode them in the first place depends on the size of the sediments (also known as the calibre). Although the size of the particle is not always the deciding factor. The tiniest particles, the clays and muds are not necessarily the easiest to erode and transport. They tend to stick together by force of electromagnetic bonding making them sticky or cohesive shown by the separate sediment and sink formations present in the Camel estuary. As such, these particles of sediment require far more energy and therefore more velocity than their individual sizes might otherwise suggest. However, once they have been finally dislodged from their sediment sink and entrained (set in motion) they are moved very easily with just a small force needed. Once in the process of transportation, irritation of the particles (attrition) causes them to take on a more rounded shape.
Because of the difference in energy requirements needed to transport different types of sediment, they often end up in different areas. The larger sediments such as sand and pebbles are deposited in areas with high wave energy, forming beaches and smaller particles such as silt and clays form deposition areas such as salt marshes and mud flats after being carried in suspension and settling in an area of low wave energy but with a high tidal energy. The uniquely local conditions which create these landforms are in a state of dynamic equilibrium. In areas of mobile clastic sediment these adjustments take place continuously in response to short term wave and tidal conditions as well as eventually, reaching a long-term equilibrium. The erosion, transport and deposition processes sustain the equilibrium both offshore and onshore. At low tide and on sandy beaches, the wind can act as a transport agent by entraining and transporting small sand and silt particles. These particles are quickly deposited again and may form sand dunes. Sediment sinks are locations in the estuary where the sediment paths of transport meet causing sediment8 to build up in zones disposed for sediment build up. Sediment sinks occur in deeply indented bays and estuaries. The estuaries of Cornwall are generally, and this is true of the Camel estuary, partly mixed for salinity. This is characterised by their tendency to show a gradual decrease in salinity from sea or river bed to the surface although they can be very stratified (with the salinity near the surface tapering off toward the edge of the tidal range).
The movement of sand and shingle by long-shore drift (also known as littoral drift) has been found to occur in distinctly separate “sediment cells”. These “littoral drift dividers” occur where the coastline abruptly changes direction such as at a major headland (Pentire point or Streppire Point could be such headlands) and also where wave conditions cause a change in the long-shore drift direction. Since material is moved outward from a drift divide there is a net output of sediment from the area. This results in a dominance of erosional processes and landforms, for example, eroding beaches and cliffs. The littoral drift at Camel estuary must either be in small quantities or far outweighed by by the input from the ocean. This could be quantified by a sediment budget which shows the additions and subtractions of sediment at a given location.
In conclusion, the distribution of sediment throughout the estuary is dependent on the tidal and wave forces and patterns as well as the calibre of the sediment particles discussed. In addition to this, depending on the cohesive properties of each type of sediment, different amounts of wave and tidal energy are needed at each stage of the transportation process. The tiniest particles of sediment, clay and silts, require the most energy input at the beginning of the transportation process to entrain them, due to their strong cohesive qualities but once they have been put in motion, these particles are easily moved, needing little energy to sustain their transportation until they are deposited in areas of low tidal energy such as the Camel estuary’s mudflats. Sediments of a larger size such as grains of sand and pebbles, however, can require a relatively small amount of energy to entrain them but require high energy to hold them in suspension and are washed up in areas with high wave energy such as the beach at Polzeath or with wind assistance, the sand dunes at Rock.
The camel estuary receives and offloads an entire range of sediment sizes, this is displayed by the wide variety of habitats and land and water formations present throughout the Camel estuary’s distribution area. The ample abundance of formations and habitats (where human development has not intervened and disturbed natural distribution patterns) is indicative of a high level of sediment input from terrestrial and more probably, marine sources. As sediment is still being extracted from the estuary, including in the form of sand mining and observations have shown that this is not having a negative affect on the estuary’s sediment sinks, it is clear that the estuary must, possess a net sediment intake .