Redd (2012) identified the following factors as the causes of flooding:
Flooding occurs when a river bursts its banks and overflows onto the surrounding land. There are many factors which can cause a flood. There are natural phenomenon and human activities in his environment (Redd 2012):.
Prolonged rainfall – if it rains for a long time, the land around a river can become saturated (it\'s holding as much water or moisture as can be absorbed).
If there is more rainfall it cannot be soaked up, so it runs along the surface – this is known as surface run-off.
Heavy rainfall – if there is heavy rainfall there is less chance of it being soaked up by the soil (infiltration) so it runs off into the river. The faster the water reaches the river, the more likely it will flood.
Geology – permeable rocks allow water to pass through pores and cracks, whereas impermeable rocks do not.
If a valley is made up of impermeable rocks, there is a higher chance of flooding as there is an increase in surface run-off.
The variation in steepness of the slope is responsible for water accumulation as runoff moves slowly in low lying areas creating problems in parts of the city. A steep valley is more likely to flood than a flatter valley because the rainfall will run off into the river more quickly.
Increasing in runoff volume
Runoff is generated due to two reasons, first increase in rainfall and second, land use changes due to high urbanization.
This resulted in more built up areas and reduced permeable surfaces and open spaces. Thus the surface runoff due to rain has increased considerably as the impervious layer has increased. Inabilities to deal with increased runoff have been one of the causes of increasing urban floods.
Encroachments on river beds. The encroachments along rivers and drains are common in urban areas. These areas are easily available to slums..
Vegetation – trees and plants absorb water, this is known as interception. Lots of vegetation reduces flood risk. Sometimes people cut down trees (deforestation). This increases the flood risk, as the water is not intercepted as it flows into the river.
Urban land use – when an area surrounding a river is built on, there is an increase in the amount of tarmac and concrete, which are impermeable surfaces. Drains and sewers take water directly to the river which increases flood risk.
Benin city is visibly drained by two rivers, Ikpoba and Ogba flowing from the northeast and northwest to the southern part of the city respectively. The soil in Benin City is composed of sands, clayey sands and discontinuous clay sequences of the Niger delta Basin (Ogbonna, Amangabara, Itulua 2011). The Benin sands are generally loose poorly sorted reddish clayey sands lacking proper bedding structures). In Benin City, the sand thickness is on average of about 800m, with high water bearing capacity. The sand is generally believed to be highly permeable, porous and prolific in water yields.
Benin City being one of the major cities in Nigeria has witnessed population explosion with over 1,531555 million in population in last decade with little or no infrastructural developmental planning. This has resulted to increase in solid and liquid waste generation, indiscriminate refuse disposal, machinery and vehicular exhausts that come from fuel combustion and emission, particulates from wear and tear of tires, automobile bodies and break lining.
Ikhile, (2012)used factorial and indicator krigging methods with GIS package examine the variations and pollution sources of soils metallic ions in the urban cities. From the result of his studies, the short and long-range variations and soil pollution by Cd, Pb, Zn, Cu, Mo, Ni, As Mn Cr and B was as a result of the land use pattern of the area studied.
The high concentration of Zn, Pb, Cu, and Mn metals in several locations could be linked to several human activities which include indiscriminate dumping of solid and liquid waste, and vehicular activities. High Zn concentration occurs around areas very close to waste dump site, abattoir, and metal scraps dump site as well as erosion prone areas. These areas include Upper Lawani/ Okhoro areas, Oko/ Airport road area, Etete/ Ogba areas, Oregbeni Barack and Uwelu market areas (Ikhile, 2012). While high concentration of Cu and Pb are found to be high in Ago Osakwe and New Benin areas, characterized by mechanic activities and major markets. Ni, Co and Mn metals are found to have high concentration around the city center. Hot spots of metal concentration were observed from the composite metal geochemical map, mostly in industrial areas and on areas with high population density and vehicular activities. Human activities contributed to the concentration of these metals in Benin City. These activities include commercial activities, domestic waste, population influence, wear and tear of vulcanized rubber tires as well as settlement pattern. All these can block the drainage system as the concentration increases to intolerable limit.
Omiunu, (2008) from his studies revealed that the use of Geographic Information System (GIS) such as computer system package that can capture, store, check, integrate, manipulate analyses as well as help to display information geographically with references, are now being used to mark out metallic ions concentration and distribution in an environment,
Areas that are at risk of floods are: