Tidal scour

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Tidal scour is “sea-floor erosion caused by strong tidal currents resulting in the removal of inshore sediments and formation of deep holes and channels”. [1] Examples of this hydrological process can be found globally. [2] [3] [4] Two locations in the United States where tidal scour is the predominant shaping force is the San Francisco Bay and the Elkhorn Slough. [2] [5] [6] Tidal force can also contribute to bridge scour. [2]

Erosion Processes which remove soil and rock from one place on the Earths crust, then transport it to another location where it is deposited

In earth science, erosion is the action of surface processes that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location. This natural process is caused by the dynamic activity of erosive agents, that is, water, ice (glaciers), snow, air (wind), plants, animals, and humans. In accordance with these agents, erosion is sometimes divided into water erosion, glacial erosion, snow erosion, wind (aeolic) erosion, zoogenic erosion, and anthropogenic erosion. The particulate breakdown of rock or soil into clastic sediment is referred to as physical or mechanical erosion; this contrasts with chemical erosion, where soil or rock material is removed from an area by its dissolving into a solvent, followed by the flow away of that solution. Eroded sediment or solutes may be transported just a few millimetres, or for thousands of kilometres.

United States Federal republic in North America

The United States of America (USA), commonly known as the United States or America, is a country comprising 50 states, a federal district, five major self-governing territories, and various possessions. At 3.8 million square miles, the United States is the world's third or fourth largest country by total area and is slightly smaller than the entire continent of Europe's 3.9 million square miles. With a population of more than 327 million people, the U.S. is the third most populous country. The capital is Washington, D.C., and the most populous city is New York City. Forty-eight states and the capital's federal district are contiguous in North America between Canada and Mexico. The State of Alaska is in the northwest corner of North America, bordered by Canada to the east and across the Bering Strait from Russia to the west. The State of Hawaii is an archipelago in the mid-Pacific Ocean. The U.S. territories are scattered about the Pacific Ocean and the Caribbean Sea, stretching across nine official time zones. The extremely diverse geography, climate, and wildlife of the United States make it one of the world's 17 megadiverse countries.

San Francisco Bay bay on the California coast of the United States

San Francisco Bay is a shallow estuary in the US state of California. It is surrounded by a contiguous region known as the San Francisco Bay Area, and is dominated by the large cities of San Jose, San Francisco and Oakland.


Historical Perspective and Relevance

Research on tidal scour is largely centered at Elkhorn Slough in California. [2] The slough was directly exposed to tidal flux beginning in 1947 with the creation of the Moss Landing Harbor. [2] [5] Multiple studies have been done on the slough since tidal exposure to catalog the morphological change and determine how long it will take for the system to reach equilibrium. [2] [5]

Moss Landing, California Census designated place in California, United States

Moss Landing is a census-designated place (CDP) in Monterey County, California, United States. Moss Landing is located 15 miles (24 km) north-northeast of Monterey, at an elevation of 10 feet. It is located on the shore of Monterey Bay, at the mouth of Elkhorn Slough, and at the head of the submarine Monterey Canyon.

Elkhorn Slough is the focus of many tidal scour studies. Tidal scour became prevalent with the creation of the Moss Landing Harbor seen toward the bottom of the image. Moss Landing California aerial view.jpg
Elkhorn Slough is the focus of many tidal scour studies. Tidal scour became prevalent with the creation of the Moss Landing Harbor seen toward the bottom of the image.


Tidal Scours are formed in tide-dominated deltas and estuaries with the changing of the tide. As the tide changes from low to high or high to low, water is transported through the channel taking sediment with it. With increasing erosion, there is increasing tidal volume creating a self-perpetuating system. [2] Tidal scour is most apparent when a barrier is breached due to natural or anthropogenic forcing. [2] [3]

River delta Silt deposition landform at the mouth of a river

A river delta is a landform created by deposition of sediment that is carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or another river that cannot carry away the supplied sediment. The size and shape of a delta is controlled by the balance between watershed processes that supply sediment, and receiving basin processes that redistribute, sequester, and export that sediment. The size, geometry, and location of the receiving basin also plays an important role in delta evolution. River deltas are important in human civilization, as they are major agricultural production centers and population centers. They can provide coastline defense and can impact drinking water supply. They are also ecologically important, with different species' assemblages depending on their landscape position.

Estuary A partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea.

Barrier island A coastal dune landform that forms by wave and tidal action parallel to the mainland coast

Barrier islands are coastal landforms and a type of dune system that are exceptionally flat or lumpy areas of sand that form by wave and tidal action parallel to the mainland coast. They usually occur in chains, consisting of anything from a few islands to more than a dozen. They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where wetlands may flourish. A barrier chain may extend uninterrupted for over a hundred kilometers, excepting the tidal inlets that separate the islands, the longest and widest being Padre Island of Texas. The length and width of barriers and overall morphology of barrier coasts are related to parameters including tidal range, wave energy, sediment supply, sea-level trends, and basement controls. The amount of vegetation on the barrier has a large impact on the height and evolution of the island.



Tidal scour can be determined by looking at the change in bathymetry over time. Bathymetry of tidal channels is determined using multi-beam sonar or LiDAR. By comparing cross-sections of channel bathymetry over several years and at various distances in the tidal channel, the amount of tidal scour can be quantified. [2] [4] [7]

Bathymetry The study of underwater depth of lake or ocean floors

Bathymetry is the study of underwater depth of lake or ocean floors. In other words, bathymetry is the underwater equivalent to hypsometry or topography. The name comes from Greek βαθύς (bathus), "deep", and μέτρον (metron), "measure". Bathymetric charts are typically produced to support safety of surface or sub-surface navigation, and usually show seafloor relief or terrain as contour lines and selected depths (soundings), and typically also provide surface navigational information. Bathymetric maps may also use a Digital Terrain Model and artificial illumination techniques to illustrate the depths being portrayed. The global bathymetry is sometimes combined with topography data to yield a Global Relief Model. Paleobathymetry is the study of past underwater depths.

Sonar technique that uses sound propagation

Sonar is a technique that uses sound propagation to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels. Two types of technology share the name "sonar": passive sonar is essentially listening for the sound made by vessels; active sonar is emitting pulses of sounds and listening for echoes. Sonar may be used as a means of acoustic location and of measurement of the echo characteristics of "targets" in the water. Acoustic location in air was used before the introduction of radar. Sonar may also be used in air for robot navigation, and SODAR is used for atmospheric investigations. The term sonar is also used for the equipment used to generate and receive the sound. The acoustic frequencies used in sonar systems vary from very low (infrasonic) to extremely high (ultrasonic). The study of underwater sound is known as underwater acoustics or hydroacoustics.

Lidar A method of spatial measurement using laser scanning

Lidar is a surveying method that measures distance to a target by illuminating the target with pulsed laser light and measuring the reflected pulses with a sensor. Differences in laser return times and wavelengths can then be used to make digital 3-D representations of the target. The name lidar, now used as an acronym of light detection and ranging, was originally a portmanteau of light and radar. Lidar sometimes is called 3D laser scanning, a special combination of a 3D scanning and laser scanning. It has terrestrial, airborne, and mobile applications.

Grain Size Distribution

Sediment grab samples show that in areas demonstrating tidal scour, there is an increase in grain size from surrounding areas. [5]

Grain size diameter of individual grains of sediment, or of lithified particles in clastic rocks

Grain size is the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be applied to other granular materials. This is different from the crystallite size, which refers to the size of a single crystal inside a particle or grain. A single grain can be composed of several crystals. Granular material can range from very small colloidal particles, through clay, silt, sand, gravel, and cobbles, to boulders.

Ecological Significance

With direct influence from the Ocean, slough morphology can change significantly making it difficult for native species to persist. [2] This can be seen in:

Geomorphology The scientific study of landforms and the processes that shape them

Geomorphology is the scientific study of the origin and evolution of topographic and bathymetric features created by physical, chemical or biological processes operating at or near the Earth's surface. Geomorphologists seek to understand why landscapes look the way they do, to understand landform history and dynamics and to predict changes through a combination of field observations, physical experiments and numerical modeling. Geomorphologists work within disciplines such as physical geography, geology, geodesy, engineering geology, archaeology, climatology and geotechnical engineering. This broad base of interests contributes to many research styles and interests within the field.

· Increased transport of agricultural runoff, such as DDT-laden sediment, introduced by increased tidal scour. [8]

· The erosion of marshes and eelgrass beds. [2]

· The loss of benthic organisms as the channels deepen more than organisms can persist. [6]

See also

Bridge Scour

River Delta


Sediment Transport

Related Research Articles

Beach Area of loose particles at the edge of the sea or other body of water

A beach is a landform alongside a body of water which consists of loose particles. The particles composing a beach are typically made from rock, such as sand, gravel, shingle, pebbles. The particles can also be biological in origin, such as mollusc shells or coralline algae.

Sediment Particulate solid matter that is deposited on the surface of land

Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand and silt can be carried in suspension in river water and on reaching the sea bed deposited by sedimentation. If buried, they may eventually become sandstone and siltstone through lithification.

Lagoon A shallow body of water separated from a larger body of water by barrier islands or reefs

A lagoon is a shallow body of water separated from a larger body of water by barrier islands or reefs. Lagoons are commonly divided into coastal lagoons and atoll lagoons. They have also been identified as occurring on mixed-sand and gravel coastlines. There is an overlap between bodies of water classified as coastal lagoons and bodies of water classified as estuaries. Lagoons are common coastal features around many parts of the world.

Salt marsh A coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides

A salt marsh or saltmarsh, also known as a coastal salt marsh or a tidal marsh, is a coastal ecosystem in the upper coastal intertidal zone between land and open saltwater or brackish water that is regularly flooded by the tides. It is dominated by dense stands of salt-tolerant plants such as herbs, grasses, or low shrubs. These plants are terrestrial in origin and are essential to the stability of the salt marsh in trapping and binding sediments. Salt marshes play a large role in the aquatic food web and the delivery of nutrients to coastal waters. They also support terrestrial animals and provide coastal protection.

Tidal power Technology to convert the energy from tides into useful forms of power

Tidal power or tidal energy is a form of hydropower that converts the energy obtained from tides into useful forms of power, mainly electricity.

Zmudowski State Beach is located on Monterey Bay, in Moss Landing, Monterey County, northern California.

Tidal marsh Marsh subject to tidal change in water

A tidal marsh is a marsh found along rivers, coasts and estuaries which floods and drains by the tidal movement of the adjacent estuary, sea or ocean. Tidal marshes experience many overlapping persistent cycles, including diurnal and semi-diurnal tides, day-night temperature fluctuations, spring-neap tides, seasonal vegetation growth and decay, upland runoff, decadal climate variations, and centennial to millennial trends in sea level and climate. They are also impacted by transient disturbances such as hurricanes, floods, storms, and upland fires.

Suisun Marsh

Located in northern California the Suisun Marsh is the largest brackish water marsh on west coast of the United States of America. The marsh land is part of the San Francisco Bay tidal estuary, and subject to tidal ebb and flood. The marsh is home to many species of birds and other wildlife, and is formed by the confluence of the Sacramento and San Joaquin rivers between Martinez and Suisun City, California and several other smaller, local watersheds. Adjacent to Suisun Bay, the marsh is immediately west of the legally defined Sacramento-San Joaquin Delta as well as part of the San Francisco Bay estuary.

Goleta Beach

Goleta Beach is a region of coastline located near Goleta, California, just east of the University of California, Santa Barbara (UCSB) campus. A portion of the shore of Goleta Bay is managed by the County of Santa Barbara as Goleta Beach County ParkGBCP. The beach itself is partly man-made as sand was spread onto an existing sandspit in 1945. The beach is a seasonal habitat for migrating shorebirds including the snowy plover, an endangered species. The beach is occasionally closed due to nourishment efforts.

Yaquina Bay small bay partially within Newport, Oregon, United States

Yaquina Bay is a coastal estuarine community found in Newport, Oregon, United States. Yaquina Bay is a semi-enclosed body of water, approximately 8 km² (3.2 mi²) in area, with free connection to the Pacific Ocean, but also diluted with freshwater from the Yaquina River land drainage. The Bay is traversed by the Yaquina Bay Bridge. There are three small communities that border the Yaquina River and Bay; Newport, Toledo and Elk City. The Yaquina Bay in Newport is a popular tourist destination along the Pacific Coast Highway. It is also an important estuary for the ecology and economy of the area.

Great Bay (New Hampshire)

Great Bay is a tidal estuary located in Strafford and Rockingham counties in eastern New Hampshire, United States. The bay occupies over 6,000 acres (24 km2), not including its several tidal river tributaries. Its outlet is at Hilton Point in Dover, New Hampshire, where waters from the bay flow into the Piscataqua River, thence proceeding southeast to the Atlantic Ocean near Portsmouth. The northern end of the bay, near its outlet, is referred to as Little Bay.

Sedimentary budget

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.

Elkhorn Slough Body of water in Monterey County, California

Elkhorn Slough is a 7-mile-long (11 km) tidal slough and estuary on Monterey Bay in Monterey County, California. The community of Moss Landing and the Moss Landing Power Plant are located at the mouth of the slough on the bay.

Moro Cojo Slough State Marine Reserve

Moro Cojo Estuary State Marine Reserve (SMR) is a marine protected area established to protect the wildlife and habitats in Moro Cojo Slough. Moro Cojo Slough is located inland from Monterey Bay on the central coast of California, directly south of the more widely known Elkhorn Slough. The area covers 0.46 square miles (1.2 km2). The SMR protects all marine life within its boundaries. Fishing and take of all living marine resources is prohibited.

A tidal prism is the volume of water in an estuary or inlet between mean high tide and mean low tide, or the volume of water leaving an estuary at ebb tide.

Marine habitats A habitat that supports marine life

Marine habitats are habitats that support marine life. Marine life depends in some way on the saltwater that is in the sea. A habitat is an ecological or environmental area inhabited by one or more living species. The marine environment supports many kinds of these habitats.

Moss Landing Wildlife Area

Moss Landing Wildlife Area is a California State wildlife preserve on the shore of Elkhorn Slough.

Slough (hydrology) type of wetland

A slough is a wetland, usually a swamp or shallow lake, often a backwater to a larger body of water. Water tends to be stagnant or may flow slowly on a seasonal basis.

Carneros Creek (Monterey County, California) river in the United States of America

Carneros Creek is a westward flowing stream and is the primary source of freshwater flowing into Elkhorn Slough. The Carneros Creek official mainstem is 9.8 miles (15.8 km) long. Its source is in the northern Gabilan Range along Highway 101/156. After it waters transit Elkhorn Slough, the historic mouth of the Salinas River, Carneros Creek empties into Monterey Bay at Moss Landing, California.


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  3. 1 2 Lewis, Keith; Carter, Lionel; Davey, Fred (1994). "The Opening of Cook Strait: Interglacial tidal scour and aligning basins at a subduction to transform plate edge". Marine Geology. 116 (3–4): 293–312. doi:10.1016/0025-3227(94)90047-7 . Retrieved 2015.Check date values in: |access-date= (help)
  4. 1 2 Shaw, John; Todd, Brian; Li, Michael; Wu, Yongsheng (2012). "Anatomy of the tidal scour system at Minas Passage, Bay of Fundy, Canada". Marine Geology. 323-325: 123–134. doi:10.1016/j.margeo.2012.07.007.
  5. 1 2 3 4 Malzonen, Christopher (1999). "Tidal scour and its relation to erosion and sediment transport in Elkhorn Slough". Master's Theses. Retrieved 2015.Check date values in: |accessdate= (help)
  6. 1 2 Silberstein 1989 M, Campbell E. 1989. Elkhorn Slough. Monterey, CA: Monterey Bay Aquarium. 64 p.
  7. McMullen, Katherine; Poppe, Lawrence; Parker, Castle (2015). "Character, distribution, and ecological significance of storm wave-induced scour in Rhode Island Sound, USA". Geo-Marine Letters. 35 (2): 135–144. doi:10.1007/s00367-014-0392-0. hdl:1912/7246.
  8. "Caspian terns: the world's biggest tern nests at Elkhorn Slough!". Elkhorn Slough National Estuarine Research Reserve (ESNERR). 2001. Retrieved 2015.Check date values in: |accessdate= (help)