Choose one (1) of the following geologic hazards: earthquake, landslide, or floo
ID: 231331 • Letter: C
Question
Choose one (1) of the following geologic hazards: earthquake, landslide, or flood. Next, determine the key factors that influence the occurrence of your chosen hazard. Then, analyze the human role in elevating the risks of occurrence, as well as mitigation strategies to minimize damage and loss of life.
Describe three (3) ways that Wegener's continental drift hypothesis has helped to shape modern plate tectonic theory, and then explain why his hypothesis was not widely accepted by his peers when first proposed. Next, analyze at least two (2) types of evidence used to support plate tectonic theory
Explanation / Answer
A)
Flooding occurs when a river’s discharge exceeds its channel’s volume causing the river to overflow onto the area surrounding the channel known as the floodplain. Flooding is not a normal condition for the river, but it is an extreme situation due to high levels of flow. The extent to which the river exceeds the flow that can be contained in its banks decided the severity of the flood and is sometimes related to how often flooding occurs. The increase in discharge can be triggered by several factors. The most general cause of flooding is prolonged rainfall. If there is a continuous rain, the ground will become saturated and the soil will no longer be able to store water leading to increased surface runoff. Rainwater will enter the river much faster than it would if the ground was not saturated leading to higher discharge levels and floods. Short period of heavy rain can also lead to floods. If there’s a sudden “burst” of heavy rain, the rainwater won’t be able to penetrate through the ground fast enough and the water will instead enter the river via surface runoff. This leads to a sudden and large increase in the river’s discharge which can result in a flash flood.
Although many floods are triggered directly by precipitation just a few hours after it falls some floods can be triggered by precipitation that fell many months ago. Precipitation that falls as snow can remain as snow on the ground until it melts. When the snow melts, large volumes of meltwater will enter the river increasing its discharge and triggering floods. These floods are often annual, occurring every year when snow melts in the spring.For example In Bangladesh, melting snow in the Himalayas triggers annual floods in the summer.
Flash floods can also be triggered by slightly more catastrophic events. Erupting volcanoes can trigger very large flash floods called jökulhlaups when glaciers are partially or even fully melted by an erupting volcano or some other form of geothermal activity. The meltwater can enter rivers and greatly increase the river’s discharge leading to a flood.
There are many factors affecting the frequency of flood. The size and shape of a river’s drainage basin dictates how much precipitation the river can receive and how quickly it will arrive. The permeability of the soil and rock in a drainage basin is a big factor in flooding. If the basin’s soil is impermeable, maybe because it has been saturated by previous rainfall or has been baked by prolonged heating, then any precipitation that falls won’t infiltrate and will instead run straight into the river, increasing the river’s discharge and triggering floods. Similarly, if the rocks in the area are non-porous or impermeable (such as granite or clay) then water won’t be able to infiltrate into the rocks and will, again, run straight off into the river increasing its discharge. The vegetation cover in a basin will affect flooding. If a basin has very dense vegetation cover, the vegetation will intercept precipitation and store it, reducing the volume of water entering a river. The relief and steepness of the basin affects how quickly water enters a river and so how likely a river is to flood. If the river’s valley has steep sides, water will quickly enter a river increasing the river’s discharge. The number of tributaries flowing into a river affects the likelihood of floods. If a river has a lot of tributaries, the river’s discharge will be much higher because lots of water will be entering it from its tributaries.
The impact of human activities on flooding
Urbanisation
Urban areas have less permeable surfaces leading to infiltration excess overland flow. They often also have a rapid drainage system delivering more flood water in a faster time (Davie, 2008).
Deforestation
In poor countries rapid deforetation has taken place. Deforestation can intensify river flooding by affecting the soil structure, reducing infiltration rates and reducing water storage. Evidence has shown that forests provide an intercepting layer for rainfall, which reduces the rate at which water reaches the surface. Forest soils also have a high organic matter which can absorb high amounts of water reducing overland flow
Agricultural Drainage
The growth in drained agricultural land provides rapid removal of surplus water, so therefore less storm runoff might occur. However, it can also lead to greater flooding on more permeable soils, where the speed at which water follows subsurface flow paths is increased
Channel alterations
When channelisation occurs, which is ‘the confinement of a river into a permanent rigid, channel structure’, flood risk can be increased even though it is supposed to offer flood protection. This is because it often causes problems further downstream, as water travels downstream at a faster than normal rate.
Climate Change
With the expected increases in temperatures due to the ‘greenhouse effect’, more melting of glaciers, thermal expansion of the oceans and a decrease in the volume of ice sheets is likely to occur (Figure 2), which will lead to a rise in sea-level. In low lying areas, local subsidence will also have an impact. It is estimated due to thermal expansion of the oceans in the last century, sea levels rose by 2-8cm, with another 20-40cm increase expected this century. Furthermore, it is expected that precipitation will increase by 3-15% globally, with wetter winters in the UK, with studies showing if CO2 concentrations double, average precipitation will increase.
Mitigation refers to any structural or non-structural measures undertaken to limit the adverse impact of hazards.
Flood Mitigation Measures:- It can be divided into three (3) main areas:
Control Over the River
In order to achieve control over the flow and height of the water carried by the river, the channel, flood plain or watershed must undergo some physical alterations.
These include:
Control Over the Land
To protect against flooding, certain Land Use Policies were developed. These are
Designated Floodways and encroachment lines
These are the lateral boundaries of the floodway where no construction or land filling should be permitted. This is done to ensure that the flow of water is not obstructed.
Zoning
This is a legal tool used by governments to control development in areas which are or are likely to become prone to flooding;
Subdivision Regulations
These specify the manner in which land may be divided. Typical provisions show the extent of the flood plain on maps. Floodway limits or encroachment lines prohibit filling in channels and floodways that restrict flow and require that each lot contain a building site with an elevation above the flood level.
Building Codes
These are standards for construction of buildings and other structures and, if enforced, can reduce damages to buildings in flood-prone areas. Some requirements include, the establishment of basement elevations and first flood elevations consistent with potential flood levels, ensuring that buildings have adequate structural strength which would likely withstand water pressure or the high velocity of flowing water, prohibiting the use of equipment that might be hazardous to life when submerged and installing proper anchorage to prevent the floatation of buildings.
Other Measures
These include flood proofing, flood forecasting, warning and evacuation systems.
Flood Proofing
This is a combination of structural changes and adjustment to properties which can be used in new or existing construction. Action includes seepage control, protective coverings, elevation or raising anchorage and under pinning.
Flood Forecasting
This is reliable, accurate and timely forecasting of floods, coupled with timely evacuation to save lives and reduce property losses.
Temporary Evacuation
This removes persons and property from the path of flood waters.
Permanent Evacuation
This removes an affected population from areas subject to inundation. This involves the acquisition of lands and the removal of developments. The acquired lands can be used for agriculture, parks or other purposes that would not interfere with flood flows or result in material damage.
Flood Insurance
This assists by compensating for flood damage. Insurance rates should realistically reflect the flood risk in order to avoid encouragement of improper development of flood plains.
B)
Continental drift, in the context of the modern theory of plate tectonics , is explained as the movement of lithospheric plates over the asthenosphere (the molten, ductile, upper portion of the earth's mantle). Modern geophyicists and geologists explain the drift of the continents within the context of plate tectonic theory. The visible continents, a part of the lithospheric plates upon which they ride, shift slowly over time as a result of the forces driving plate tectonics. Moreover, plate tectonic theory is so robust in its ability to explain and predict geological processes. According to this theory, scientists believe that Earth's surface is broken into a number of shifting slabs or plates, which average about 50 miles in thickness. These plates move relative to one another above a hotter, deeper, more mobile zone at average rates as great as a few inches per year. Most of the world's active volcanoes are located along or near the boundaries between shifting plates and are called plate-boundary volcanoes.
In 1915, the German geologist and meteorologist Alfred Wegener first proposed the theory of continental drift, which states that parts of the Earth's crust slowly drift over a liquid core. The fossil record supports and gives acceptance to the theories of continental drift and plate tectonics. Wegener hypothesized that there was a gigantic supercontinent 200 million years ago, named Pangaea, meaning "All-earth". Pangaea started to break up into two smaller supercontinents, called Laurasia and Gondwanaland, during the Jurassic period. By the end of the Cretaceous period, the continents were separating into land masses that look like our modern-day continents. Wegener published this theory in his 1915 book, On the Origin of Continents and Oceans.
One of the main problems with his theory was that he did not propose a driving mechanism for the motion of the continents. What was the force that moved the continents? Where did it come from? How much force was needed to move a continent? The driving mechanism, an important key to the continental drift theory, lay out of reach until the 1960s. Wegener had made his claims based on data from the continents, but the oceans cover 70 percent of Earth's surface – a vast area hidden from his view under kilometers of water. But the first and second World Wars brought major technical and scientific developments that allowed scientists to (1) map the ocean floor and (2) measure the magnetism of seafloor rocks in detail. These two sets of data provided geologists with additional evidence for the process of continental drift.
Evidence used to support plate tectonic theory