Cliffs are dangerous due to the hazard of falling off the edge, especially in areas with thick, unstable soil, and where debris slides downhill to the base as waves continually undermine the cliff (Skinner, 1992). The base of the cliff also experiences a process known as quarrying, in which compressed air and the impact of water due to wave action dislodges fractured rock and other loose particles (Summerfield, 1991). However, storm events periodically do the most work in eroding the cliff. At the base of the cliff, storm waves undermine the cliff face, while at the top of the cliff, high pore pressures build up from groundwater accumulation can cause severe landslides, dependent upon the mixture of air, water, and sediment in the soil. Along the coastline of California, much of the coastal rock is the Franciscan Complex, a melange of marine sediments and ocean crust scraped off the sea floor during subduction. Water percolates easily through rocks such as sandstone and graywacke, and these rocks tend to be easily eroded and are prone to mass wasting. For example, the cliffs at Black's Beach in San Diego tend to collapse periodically in sections, dropping tons of dirt and rocks in sometimes highly populated areas, creating danger at the top, middle and bottom sections of the cliff.
Cliffs can be dangerous due to other circumstances, such as lack of lighted signs at night, lack of guard rails, trails composed of poorly consolidated sand, and fog, caused by the uplift of air over the land surface. But human judgment and awareness play a key role in cliff safety. For example, many visitors to the Santa Barbara coast fall off cliffs each year due to intoxication and lack of familiarity with their surroundings (Tkachuk, personal experience). Along the Mendocino Headlands, sneaker waves often surprise unsuspecting tourists, as happened last year when two adults spreading their mother's ashes into the sea were swept off the cliffs and into the ocean (Siem, personal experience). Another factor that makes these areas dangerous is that, should there be accident, a rescue will likely take longer than normal because of the lack of accessibility to many of these areas. In addition to this, there are usually fewer rescue personnel available in these remote areas.
Geologic hazards are also present in the surf zone. In the temperate zones of the world, coral reefs are absent due to the low water temperature and lack of sunlight, and instead rock reefs become more prevalent. A rock reef becomes dangerous when a swimmer driven under water by a wave comes into proximity with mobile material ranging from pebbles to boulders. Another geologic hazard found in Hawaii is basaltic reefs formed from the cooling of lava that has flowed into the ocean. This type of reef is dangerous due to the sharp exposures formed as the supercooled lava fractures in glassy shards. Rock and lava reefs are usually only dangerous to those who choose to pursue activities in their vicinity, in contrast to the risk associated with less predictable landslides and seismic activity.
|When entering the water at the beach we are entering an entirely
different ecosystem. Living within these ecosystems is an broad
array of marine life. Some of these organisms can cause injury
or even death to humans. Some of these dangerous organisms include
sharks, rays, fish, Portuguese-man-of-war, jellyfish, and even
marine mammals in certain situations.
Shark attack is viewed by most beach users as the most threatening risk to their lives. It is estimated that there are up to 75 shark attacks each year with up to 10 resulting in death. Of the 250 species of sharks, only a few are considered a threat to humans. The three most frequent sharks to attack humans are the tiger shark, bull shark, and white shark.
Most recorded shark attacks occur in nearshore areas due to the following reasons. First, this is the area most frequented by humans. The most common site in the United States for shark attack is Florida, which is probably due to the high annual water attendance. Sharks are also drawn to these areas because their natural prey can be found there.
One area of the world with a high number of shark attacks is South Africa. At Umhlan Rocks, a shark shield has been set up to try to cut down on fatal shark attacks. The nets were implemented because in 1957 five people were killed in the area by sharks in a span of 100 days.
For further information on shark attacks in California, please visit Great White Shark Attacks in California, a part of the Conservation Connection.
Other dangerous aquatic organism is jellyfish. They are normally free swimming, colorless and can range in size from a couple of inches to three feet in diameter. Their occurrence is seasonal, most likely being found in the spring and summer. Jellyfish use nematocysts in their tentacles to catch their prey. Humans can get stung when they come into contact with the tentacles. A severe example of dangerous jellyfish is the sea wasp, which can be found off the coast of Australia. Each year it kills one or two people and seriously injures many more. It contains up to 500 feet of tentacles with an extremely potent and fast acting venom, able to stop an adult human's heart within three minutes.
Hazardous surf conditions are often the most threatening aspect of a dangerous beach. The majority of injuries and fatalities at the beach are drowning related. The erratic nature of coastal processes makes recreational use of the ocean hazardous for local beach bums as well as for unfamiliar tourists. The strength of unidirectional ocean currents and the power of breaking waves is often underestimated and unpredictable, due to variability, temporally and spatially.
Ocean processes are far from being fully understood. The explanations and models currently used to make predictions are based on universally accepted theories. Wave generation is one of the fundamental ocean processes. Other than ship waves and tidal waves, all waves are generated by wind. This mechanism is not fully understood and many of the predictions for wave heights and speeds are based on empirical relationships. The wind transfers energy to the water through boundary shear stress. As wind blows over water, the wind disturbs the water surface causing waves.
Waves pose a threat to swimmers because of their intensity and the energy they harness. Some of the most dangerous waves are the surging and collapsing waves. They appear to not be intense, but these waves exist on very steep slopes which reflect most of the waves energy, resulting in a doubling of the incoming wave height. This creates very strong surf. The other wave types are associated with milder slopes, but the energy they release as they break is often too intense for swimmers. Large waves are also threatening because they are so attractive to surfers, and their strength is often underestimated.
More threatening than the waves are the currents, which cannot be predicted or detected by the casual observer. There are several types of currents. Most common are tidal currents, which are associated with the sea level fluctuations and occur diurnally. These are referred to as ebb and flood tides. Usually these tides typically produce very weak currents, though in some waters they produce strong currents with tidal bores close to five feet in height. The currents under the Golden Gate Bridge in San Francisco can reach ten miles per hour during the flood and ebb tides.
Another familiar current is the longshore current which transports sand along the beaches. This current only exists in the surf zone and it is caused mainly by the oblique incidence of waves. Longshore current is sometimes a result of nearshore circulation cells which also produce rip currents.
Rip currents are the most dangerous and most prevalent current. Rip currents are a result of uneven wave setup. As a wave breaks it raises the level of the water surface allowing larger waves to travel shoreward. This temporary elevation in water level is wave setup. A phenomenon known as edge waves (waves which travel longshore, causing unevenness in water level) is suspected to cause variation in wave setup. Water from regions of high setup flows into the regions of low setup causing a seaward current (this is not undertow). The rip currents are visible as evenly spaced and uniformly sized brown turbulent streaks oriented perpendicular to the shore.
The shape of the ocean floor near the beach causes currents to form in the nearshore zone. Undertow, one of the most hazardous elements of the surf, is an effect of the wave hydrodynamics. As the mass of a wave moves up onto the beach face, there must be conservation of mass, therefore water from the previous wave rushes down the beach face, forming a seaward current along the ocean floor . If a swimmer gets caught under a wave, they may be dragged seaward by this current. The strength of the undertow is dependent on the incoming wave height, the geomorphology of the beach, and the presence of rip currents.
Pollution is one of the greatest hidden hazards at a beach. People often choose beaches for aesthetic reasons. To most, a trip to the beach is a way to reach one of the last unspoiled places on the earth and walk along the shores of a mysterious world , the ocean. People believe that the ocean starts where their land world leaves off and that the two are unrelated. Little do most know how deeply the coastal zone is interrelated to it's adjacent land mass. In the nearshore waters people come into contact with a very misunderstood water hazard, Pollution.
Beach going people don't select a beach to visit that is often strewn with trash. Visible trash, oil spills, and sewage are what most people consider to be dangerous to themselves. Humans will avoid these areas. The problem is that most of the harmful pollution to humans is invisible and often goes undetected. It is this undetected pollution that, at various levels, cause are harm to humans that come into contract with it.
There are three separate categories of pollution that are considered to be of direct concern to humans in the near coastal waters by the USEPA, 1990. They are:
Human Pathogens: These are bacteria and viruses that are often transferred through human and animal fecal matter.
Heavy Metals: Metals occur in the coastal waters naturally and in low levels are harmless to humans. However it is the non-natural anthropogenic introduction of heavy metals that is of concern.
Toxins: Due to the complexity of toxins both organic and synthetic they often go undetected and unmonitored. The organic toxins occur naturally from red tides. Red tides are periods of time where copious amounts of dinoflagellate algae bloom in the shallow nearshore waters. To humans direct water contact at these times should be avoided.
The reality of pollutants and beach users: Currently, little is done to keep humans in coastal zone from being poisoned by pollution. As defined above with the complexity of all the various types of pollution and their sources it is near impossible to monitor all types of pollution in the nearshore marine environment. Fecal coliform is the least expensive and most widely monitored pollutant. While current monitoring techniques are inadequate a few precautionary steps that should be taken before entering the coastal waters.
Think about it. Pollution runs down hill. Check and see when and how much rain has fallen in the past few days. If no measurable precipitation has occurred then most likely the surf zone won't be contaminated and will be safe for swimming. In the case of either a deluge or persistent rains of more than a quarter inch per day, coastal waters should be avoided.
Look around. If there is a storm drain dumping onto the beach, dog feces strewn about, a livestock yard, chemical plant discharge pipe, or a sewage treatment plant within a quarter mile of you don't go in. You'll likely become sick.
Think Again. Before consuming any seafood caught or found near a highly populated area, or industrial area check and see where your meal may have come from and what it may have consumed. You may be poisoning yourself.