Bay mud

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Richardson Bay mudflats show exposed layers of bay mud Pickleweedecreekerazorback.jpg
Richardson Bay mudflats show exposed layers of bay mud

Bay mud consists of thick deposits of soft, unconsolidated silty clay, which is saturated with water; these soil layers are situated at the bottom of certain estuaries, which are normally in temperate regions that have experienced cyclical glacial cycles. Example locations are Cape Cod Bay, Chongming Dongtan Reserve in Shanghai, China, Banc d'Arguinpreserve in Mauritania, The Bristol Channel in the United Kingdom, Mandø Island in the Wadden Sea in Denmark, Florida Bay, San Francisco Bay, Bay of Fundy, Casco Bay, Penobscot Bay, and Morro Bay. Bay mud manifests low shear strength, high compressibility and low permeability, making it hazardous to build upon in seismically active regions like the San Francisco Bay Area.


Typical bulk density of bay mud is approximately 1.3 grams per cubic centimetre. Bay muds often have a high organic content, consisting of decayed organisms at lower depths, but may also contain living creatures when they occur at the upper soil layer and become exposed by low tides; then, they are called mudflats, an important ecological zone for shorebirds and many types of marine organisms. Great attention was not given to the incidence of deeper bay muds until the 1960s and 1970s when development encroachment on certain North American bays intensified, requiring geotechnical design of foundations. [1]

Bay mud has its own official geological abbreviation. [2] The designation for Quaternary older bay mud is Qobm and the acronym for Quaternary younger bay mud is Qybm. An alluvial layer is often found overlying the older bay mud.

In relation to shipping channels, it is often necessary to dredge bay bottoms and barge the excavated material to an alternate location. In this case chemical analyses are usually performed on the bay mud to determine whether there are elevated levels of heavy metals, PCBs or other toxic substances known to accumulate in a benthic environment. It is not uncommon to dredge the same channel repeatedly (over a span of ten to thirty years) since further settling sediments are prone to redeposit on an open estuarine valley floor.

Depositional scenarios

Decomposed diatoms are one component of bay mud Diatoms through the microscope.jpg
Decomposed diatoms are one component of bay mud

Bay muds originate from two generalized sources. First alluvial deposits of clays, silts and sand occur from streams tributary to a given bay. The extent of these unconsolidated interglacial deposits typically ranges throughout a given bay to the extent of the historical perimeter marshlands. Second, in periods of high glaciation, deposits of silts, sands and organic plus inorganic detritus (e.g. decomposition of estuarine diatoms) may form a separate distinct layer. Thus bay muds are important time records of glacial activity and streamflow throughout the Quaternary period.

Some depositional formation is quite recent, such as in the case of Florida Bay, where much of the bay mud has accumulated since 2000 BCE, and consists of primarily decayed organic material. [3] In the case of Florida Bay these bay muds can accrete as much as 0.5 to 2.0 centimeters per annum, although the dynamic equilibrium of erosion, wave action redistribution and deposition complicate the net rate of layer growth. In the case of the Bristol Channel in the United Kingdom bay, mud formation has been occurring at least since the Eemian Stage (known as the Sangamonian Stage in North America), or about 130,000 years ago. [4] In other cases such as with San Francisco Bay, deposition has been interrupted by sea-level changes, and strata of vastly different vintages are found. In the San Francisco Bay Area, these are called Young bay mud and Older bay mud by geologists. Human activities can also affect deposition; close to half of the Young Bay Mud in San Francisco Bay was placed in the period 1855–1865, as a result of placer mining in the Sierra Nevada foothills.

Geotechnical factors

Dakin Building, a national headquarters use, built on driven piles over bay mud Dakinbldg.jpg
Dakin Building, a national headquarters use, built on driven piles over bay mud

Construction on bay mud sites is difficult because of the soil's low strength and high compressibility. Very lightweight buildings can be constructed on bay mud sites if there is a thick enough layer of non-bay-mud soil above the bay mud, but buildings which impose significant loads must be supported on deep foundations bearing on stiffer layers below the bay mud, or obtaining support from friction in the bay mud. Even with deep foundations, difficulties arise because the surrounding ground will likely settle over time, potentially damaging utility connections to the building and causing the entryway to sink below street level.

A number of notable buildings have been constructed over bay muds, typically employing special mitigation designs to withstand seismic risks and settlement issues. Complicating design issues, fill (beginning about 1850 AD) is sometimes found deposited on the surface level. For example, the Dakin Building in Brisbane, California was designed in 1985 [5] to sit on piles 150 feet deep, anchoring to the Franciscan formation, below the bay muds and through an upper fill layer. Furthermore, the structure's entrance ramp has been set on a giant hinge to allow the surrounding land to settle, while the building absolute height remains constant. The Crowne Plaza high-rise hotel in Burlingame, California [6] was also designed to sit over bay muds, as was the Westin Hotel in Millbrae, California and Trinity Church in Boston's Copley Square. Indeed, Boston's entire Back Bay district is named for the tidal bay that it now covers. Logan International Airport and the San Francisco International Airport are also constructed over bay mud.

Black-necked stilts foraging on Pickleweed Inlet mudflats Bnstiltpair.jpg
Black-necked stilts foraging on Pickleweed Inlet mudflats


When the mud layer is exposed at the tidal fringe, mudflats result affording a unique ecotone that affords numerous shorebird species a safe feeding and resting habitat. Because the muds function much like quicksand, heavier mammalian predators not only cannot gain traction for pursuit, but would actually become trapped in the sinking muds. The muds are also an important substrate for primary marsh productivity including eelgrass, cordgrass and pickleweed. Furthermore, they are home to a large variety of molluscs and estuarine arthropods. Richardson Bay, for example, exposes one third of its areal extent as mudflat at low tide, which hosts a productive eelgrass expanse and also a large shorebird community.

Mammals such as the Harbor seal may use mudflats to haul out of estuary waters; however, larger mammals such as humpback whales may become accidentally stranded at low tides. Note that normally humpback whales do not frequent estuaries containing mudflats, but at least one errant whale, publicized by the media as Humphrey the humpback whale, became stuck on a mudflat in San Francisco Bay at Sierra Point in Brisbane, California.

Worldwide occurrences

Bay muds occur in bays and estuaries throughout the temperate regions of the world. In North America, prominent instances are: (a) the Stellwagen Bank formed 16,000 to 9000 BCE by glaciation of Cape Cod Bay in Massachusetts, (b) Florida Bay, (c) in California Morro Bay and San Francisco Bay and (d) Knik & Turnagain Arms in Anchorage, Alaska. In the United Kingdom large bay mud occurrences are found at Morecambe Bay, Bridgwater Bay and Bristol Bay. Straddling Denmark, the Netherlands and Germany is the Wadden Sea, a major formation underlain by bay muds.

In Asia the Chongming Dongtan Nature Reserve in Shanghai, China is an example of a large scale bay mud formation. The Atlantic coast of Africa holds the Banc d'Arguin, a World Heritage nature preserve in the country of Mauritania. Banc d'Arguin is a vast area underlain by bay mud.

Regulatory issues and actions

When building on top of bay mud layers or when dredging estuary bottoms, a variety of regulatory frameworks may arise. Normally in the United States, an Environmental Impact Report as well as a geotechnical investigation are conducted precedent to any major construction over bay mud. Combined, these reports have developed much of the data base extant on bay mud characteristics, frequently yielding original field data from soil borings. These data have demonstrated that in many locations the shallower bay muds contain concentrations of mercury, lead, chromium, petroleum hydrocarbons, PCBs, pesticides and other chemicals which exceed toxic limits: a geological record of human activities of the last century. These data are particularly important to consider when dredging of bay muds is contemplated as part of a development project. Such dredging can have impacts to receiving lands as soil contamination, but also water column impacts from sediment disturbance.

In the case of dredging within the United States, a permit is almost always required from the United States Army Corps of Engineers, after submission of extensive data on the project limits, chemical properties of the bay muds to be disturbed, a dredge disposal plan and often a complete Environmental Impact Statement pursuant to the National Environmental Policy Act. Further review by the United States Coast Guard would normally be required. Within individual state jurisdictions, such as California, an Environmental Impact Report must be filed for dredging of any significance; furthermore, agency reviews by the California Coastal Commission and the Regional Water Quality Control Board would normally be mandated. All of these regulatory bodies serve an important role in deciding whether an area may be dredged or not. However, the most important body is CEQA, or the California Environmental Quality Act. This guiding legislation is the reason for Environmental Impact Reports, costly mitigation measures and arduous review processes. One of CEQA's main goals is to promote interagency cooperation in the review process of a project. This is one of the main reasons why it is the overseer of all projects in California. [7]

For buildings proposed over bay mud layers, typically the municipality involved will, in addition to the usual engineering and design review issues common to all building projects (which are more complicated because of the site conditions), require an Environmental Impact Report . This process would include reviews by that city's building department, as well as applicable regional and state agencies such as those cited above for dredging projects, except that Coast Guard agencies would not typically be concerned. In developing in California, proposed development over bay mud layers would also have to go through a planning commission and a city council in order to be allowed. This process would respect the EIR, CEQA, and all the other bodies discussed above. [8] In the case of San Francisco the project would have to get approved by the San Francisco Board of Supervisors. [9]

In regards to San Francisco, some interesting negative externalities are being experienced due to the dredging of land by the Bay. The Millennium Towers, which were completed in 2008, are currently sinking. This has had a negative impact on the residents of this building. In response to this subsidence, San Francisco's city attorney has decided to file a lawsuit against the developer, because the developer failed to inform the residents of the accelerated speed that the building was sinking at. [10] If this lawsuit results in success for San Francisco this could lead to policy changes being made in the future surrounding transparency between developers and potential buyers of the developed land.

Sea level rise

Sea level rise will have a huge impact on the ecosystems surrounding and within bays all across the globe. Sea level rise in California will completely engulf bay mud that makes up San Francisco Bay. In order to deal with sea level rise the California Coastal Commission has adopted policy guidelines to help California. [11]

See also

Related Research Articles

San Francisco Bay Shallow estuary on the coast of California, 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 Francisco, San Jose, and Oakland.

Yellow Sea Sea in Northeast Asia between China and Korea

The Yellow Sea is a marginal sea of the Western Pacific Ocean located between mainland China and the Korean Peninsula, and can be considered the northwestern part of the East China Sea. It is one of four seas named after common colour terms, and its name is descriptive of the phenomenon whereby fine sand grains from the Gobi Desert sand storms, that descend annually from the northwest, turn the surface of its waters a golden yellow.

Estuary Partially enclosed coastal body of brackish water with river stream flow, and with a free connection to the 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.

Salt marsh Coastal ecosystem between land and open saltwater that is regularly flooded

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.

Mudflat Coastal wetlands where sediments have been deposited by tides or rivers

Mudflats or mud flats, also known as tidal flats, are coastal wetlands that form in intertidal areas where sediments have been deposited by tides or rivers. A recent global analysis suggested they are as extensive globally as mangroves. They are found in sheltered areas such as bays, bayous, lagoons, and estuaries; they are also seen in freshwater lakes and salty lakes alike, wherein many rivers and creeks end. Mudflats may be viewed geologically as exposed layers of bay mud, resulting from deposition of estuarine silts, clays and aquatic animal detritus. Most of the sediment within a mudflat is within the intertidal zone, and thus the flat is submerged and exposed approximately twice daily.

Sacramento–San Joaquin River Delta Inland river delta and estuary in Northern California

The Sacramento–San Joaquin River Delta, or California Delta, is an expansive inland river delta and estuary in Northern California. The Delta is formed at the western edge of the Central Valley by the confluence of the Sacramento and San Joaquin rivers and lies just east of where the rivers enter Suisun Bay. The Delta is recognized for protection by the California Bays and Estuaries Policy. Sacramento–San Joaquin Delta was designated a National Heritage Area on March 12, 2019. The city of Stockton is located on the San Joaquin River on the eastern edge of the delta. The total area of the Delta, including both land and water, is about 1,100 square miles (2,800 km2). Its population is around 500,000 residents.

The California Environmental Quality Act (CEQA) is a California statute passed in 1970 and signed in to law by then-Governor Ronald Reagan, shortly after the United States federal government passed the National Environmental Policy Act (NEPA), to institute a statewide policy of environmental protection. CEQA does not directly regulate land uses, but instead requires state and local agencies within California to follow a protocol of analysis and public disclosure of environmental impacts of proposed projects and, in a departure from NEPA, adopt all feasible measures to mitigate those impacts. CEQA makes environmental protection a mandatory part of every California state and local (public) agency's decision making process. It has also become the basis for numerous lawsuits concerning public and private projects.

Richardson Bay arm of San Francisco Bay

Richardson Bay is a shallow, ecologically rich arm of San Francisco Bay, managed under a Joint Powers Agency of four northern California cities. The 911-acre (369 ha) Richardson Bay Sanctuary was acquired in the early 1960s by the National Audubon Society. The bay was named for William A. Richardson, early 19th century sea captain and builder in San Francisco. It contains both Strawberry Spit and Aramburu Island.

Yaquina Bay

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.

Humphrey the Whale

Humphrey the Whale is a humpback whale which twice deviated from his Mexico to Alaska migration by entering San Francisco Bay. This behavior is unusual for a humpback whale, and Humphrey attracted wide media attention when entering the bay in both 1985 and 1990. Both of his bay incursions resulted in rescue by the Marine Mammal Center, based in Marin County, California, assisted by the United States Coast Guard and hundreds of other volunteers.

San Rafael Creek

San Rafael Creek is a watercourse in Marin County, California, United States that discharges to San Rafael Bay, a small embayment of the San Francisco Bay. The mouth of San Rafael Creek is a channelized estuary through an industrial area. San Rafael Creek has a designation under Federal Law Section 303(d) as impaired by diazinon, the principal pollutant causing impairment designations for streams discharging to San Pablo Bay, which is the northern arm of San Francisco Bay. In September 2007, the organization Save The Bay designated San Rafael Creek as one of the top ten "worst trash hot spot" waterways flowing into the San Francisco Bay. The channel portion of San Rafael Creek below the Grand Street Bridge is dredged on a regular maintenance schedule to keep the shallow draft channel navigable. Dredge spoils are disposed of at a site near Alcatraz Island.

Seal Slough

Seal Slough is a narrow winding tidal channel through a tidal marsh in San Mateo and Foster City, California. This slough has been the object of a wetland restoration project in recent years to enhance habitat value. Dredging has been carried out in Seal Slough since at least 1954. When the original sewage treatment plant for the city of San Mateo was constructed in 1935, its discharge was directed to Seal Slough.

Castro Cove

Castro Cove is an embayment of the San Pablo Bay in Richmond, California between Point San Pablo and the confluence of Wildcat Creek into Castro Creek.

Ecology of the San Francisco Estuary

The San Francisco Estuary together with the Sacramento–San Joaquin River Delta represents a highly altered ecosystem. The region has been heavily re-engineered to accommodate the needs of water delivery, shipping, agriculture, and most recently, suburban development. These needs have wrought direct changes in the movement of water and the nature of the landscape, and indirect changes from the introduction of non-native species. New species have altered the architecture of the food web as surely as levees have altered the landscape of islands and channels that form the complex system known as the Delta.

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.

San Francisco Estuary Partnership

The San Francisco Estuary Partnership (Partnership) is one of the 28 National Estuary Programs created in the 1987 Amendments to the Clean Water Act. The Partnership is a non-regulatory federal-state-local collaboration working to restore water quality and manage the natural resources of the San Francisco Bay-Sacramento–San Joaquin River Delta estuary. The Partnership works with over 100 municipalities, non-profits, governmental agencies, and businesses and helps develop, find funding for, and implement over 40 projects and programs aimed at improving the health of the estuary. The partnership either directly implements these projects, or administers and manages grants, holds educational workshops and highlights project results. The Partnership is also the official representative for the San Francisco Bay region to the Most Beautiful Bays in the World.

Greco Island

Greco Island is a wetland island in Redwood City, California. Greco Island is part of the larger Don Edwards San Francisco Bay National Wildlife Refuge. Westpoint Slough follows the South side of the island while Redwood Creek is along the West. The San Francisco Bay bounds the North and East sides of the island.

South Puget Sound Region in Washington, United States

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Restore the Delta is a campaign, based in Stockton, California that advocates for restoring the Sacramento-San Joaquin Delta also known as the San Francisco Bay-Delta Estuary. It began in 2006 working towards education and outreach to help Californians recognize the Delta as part of California's heritage. Currently, they are up to 40,000 members throughout California of both residents and various organizations working towards the same goal.

Sedimentation enhancing strategy

Sedimentation enhancing strategies are environmental management projects aiming to restore and facilitate land-building processes in deltas. Sediment availability and deposition are important because deltas naturally subside and therefore need sediment accumulation to maintain their elevation, particularly considering increasing rates of sea-level rise. Sedimentation enhancing strategies aim to increase sedimentation on the delta plain primarily by restoring the exchange of water and sediments between rivers and low-lying delta plains. Sedimentation enhancing strategies can be applied to encourage land elevation gain to offset sea-level rise. Interest in sedimentation enhancing strategies has recently increased due to their ability to raise land elevation, which is important for the long-term sustainability of deltas.


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