Explain how the factors identified by the cartoon can cause positive feedback.
Positive feedback is when one factor (X) causes an increase in another factor (Y) which inturn causes an increase in the first factor (X). It is also known as a (Negative in most cases) multiplier. There are many factors that can cause positive feedback in a beach system
One of these is the possible effects of global warming. A temperature rise globally of around 0.6’c since 1900, a figure which is increasing has led to a rise in sea levels. This rise in sea level could plausibly spell disasters for Californian coasts and beaches such as the Santa Barbara beach and harbour due to the positive feedback created by sea level rise. If sea levels rise then there will be more intense waves and they will happen more frequently, this will erode more sand due to the waves having a strong backwash. This in turn will create deeper waters which will cause even more strong waves which will further erode more beach.
This links to El nino events also which cause the changes in the jet stream and therefore changes in temperatures in the sea (of around 1-2’c) therefore creating sea level rise as the sea expands. This inturn creates more storms as the oceans are deeper. These storms create violent waves which remove sand from the beaches and destroy the beaches, due to there being less sand the waves become even stronger and more violent as the sea is even deeper, creating positive feedback. A clear example of this once again is Santa Barbara harbour where positive feedback involving intense waves removing the beach sand let to 13 million dollar damages to the beach and many tonnes of sand had to be driven in on lorrys in order to renourish the beaches. Added to this the El nino events further impact the beach system as many fish cannot survive in the warmer temperatures and so die. This disrupts the food chain as there is less food for the other marine animals and so they die also, creating positive feedback and destruction of the marine ecosystem.
Another factor that links to those above is strong onshore winds and destructive plunging waves. These both create loss of sand on the beaches as the wind blows away the sand and so there is less of it and also the destructive waves create erosion of the beach sand due to there violent backwash which takes sand off of the beaches. Less sand means that there will be even stronger waves as the water is deeper and there is less resistance to the waves, inturn removing further amounts of sand and therefore creating positive feedback.
A fourth factor that creates positive feedback and damages californias coasts is the knock on effect of coastal management north of the area interrupt the littoral cell. This littoral cell is a self contained area that consist of a range of sand sources (cliffs rivers etc. ) that provide sand nourishment to the beach. Coastal management north of these littoral cells such as groins mean that less sand nourishes the beach as less gets to it from long shore drift as the coastal management in the north is designed to stop that beach losing sand. But this has a knock on effect on the beaches south of the area, causing them to narrow. If these narrow then there will be less sand taken by longshore drift and deposited on the next southernly beach as either the wind doesn’t get to the newly narrowed beaches, or the waves only get a little bit of the sand as there isn’t much. This can create positive feedback in two ways. The first is that every beach south of the coastal management narrows as beach A doesn’t get the normal amount of nourishment from longshore drift and so A narrows, leading to less sand being transported to B so B narrows, causing less to be transported to C etc. the second is that the constant erosion by the waves means that there will be more intense waves as they have deeper water and less resistance against the sand and so more sand is eroded, causing more intense waves and creating positive feedback.
The last factor is linked to the sand sources mentioned above which are key to the beach and the open coastal system. Rivers are one of these and they account for around 90% of beach nourishment. Therefore when these rivers are dammed up vastly less sediment gets to the beach ( shown by in Santa Barbara where dams such as lake hodges meant that a fall of 29 thousand cubic metres in sand from rivers occurred falling from 32 thousand cubic meters to just 3 thousand another example is due to the completion of construction of two large dams on the Luan River in 1979, its annual sediment delivery to the Bohai Sea has been cut from 20.2 million tons to an annual sediment flux of only 1.9 million tons) and so the beaches shrink as they have less sand. Once again this causes long shore drift and waves to take away sand without any sand renourishing the beach, this inturn means more sand is taken from higher intensity waves creating positive feedback.