In what ways may the risk of flooding in a drainage basin be increased by human activities? (16)
//Floods occur during periods of high discharge, when a stream runs out of its confines and submerges surrounding areas. A flood is an unusual high-water stage in which water overflows its natural or artificial banks onto normally dry land. Floods are not only be caused by natural factors, high precipitation intensity but also human activities. Hence, this essay seeks to explain how the risk of flooding in a drainage basin is increased by human activities.
//Firstly, urbanization can increase the risk of flooding in a drainage basin. Urbanization is defined as the increase in the proportion of people living in towns and cities. Urbanization drastically alters the drainage traits of natural catchments, or drainage areas, by increasing the volume and rate of surface runoff. While the impact of on river system may be minimal, the carrying capacity of small streams may be quickly exceeded, causing flooding and erosion issues. Often, the runoff from intense rainfall exceeds the carrying capacity of the sewer system, creating a backup in the system and hence the flooding of basements and roads are aggravated. Hence, this highlights the human activities such as urbanization increases the risk of flooding in a drainage basin.
//Secondly, the risk of flooding in a drainage basin can be increased by human activities such as dam failure. Flooding can result from dam failure or other hydraulic structures, when a veritable of water can go hurtling down the river channel. As might be expected, the suddenness and magnitude of such an event, which is due to human failure in design, can have severe repercussions. One such example is the St Francis Dam, near Saugus, California in 1929, killing 450 people. Another example to illustrate dam failure is The Johnstown in Pennsylvania dam-built of earthen material collapsed after a period of heavy rainfall in 1989 where an astounding 2200 lives were lost. Hence, this highlights that dam failure increased the risk of flooding in a drainage basin.
//Thirdly, deforestation as a result of human activities can increase the risk of flooding in a drainage basin. Deforestation is defined as clearing of the earth's forests on a massive scale, often resulting in damage to the quality of the land. Deforestation has a massive impact on flooding such as the release of sediment due to an increase erosion with the removal of vegetation. Also, vast amount of eroded soil wind up in river beds, shrinking channels and the river’s ability to carry water without flooding. One example to illustrate the extensiveness of the deforestation is in the Himalayas in Nepal, is said to have contributed to the sedimentation problems in rivers draining the region, reducing river capacity and increases the susceptibility of flooding. Hence, deforestation as a result of human activities can increase the risk of flooding in a drainage basin.
//However, it is wrong to presume that the risk of flooding in a drainage basin is only caused by human activities as the natural and physical factors also increase the risk of flooding in a drainage basin.

//Firstly, 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 lag time). A large drainage basin means that the river’s catchment area is large so it will collect a lot of water, increasing discharge. If the basin is circular in shape, the precipitation will enter the river at roughly the same time because all points in the basin are equidistant from one another. This will produce a high peak discharge and can lead to flash floods. Hence, the size and shape of a drainage basin can increase the risk of flooding in a drainage basin.
//Secondly, 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. Conversely, if a basin is sparsely vegetated then there will be no interception and so more water will enter a river. Vegetation helps bind soil too. With no vegetation, the soil is highly susceptible to mass wasting which can cause large volumes of soil to enter a river and reduce the river’s capacity. Therefore, the amount of vegetation present will also increase the risk of flooding in a drainage basin.
//In summary, the risk of flooding in a drainage basin is caused mostly by human activities as human modifications tend to have a undesirable impact on both the environment as well as the human population.