Flooding, a Consequence of Anthropogenic Neglect

PAPER PRESENTED AT THE 201 3 WORLD ENVIRONMENT DAY

CELEBRATION IN FUTO

BY

ENGR. PROF C. O. OWUAMA & ENGR. UJA, E.

Institute of Erosion Studies Federal University of

Technology, Owerri.

1 INTRODUCTION:

Flooding can be defined as an overflow of huge amount of water onto a normally dry land. It occurs when the overflowing water submerges land and causes deluge. In this case the underlying soil may be supersaturated if the overlying soil is permeable or relatively dry if impermeable. It shall be noted when rain falls onto surface, the water partially loss is either through surface runoff and /or evapotranspiration and / or infiltration. Where the surface is impermeable the entire precipitation flows as runoff but the surface is fully vegetated on a gentle slope the water entirely dissipates through evapotranspiration and infiltration.

2 CAUSES OF FLOODING [1]

Weather events

Flooding is normally caused by natural weather events such as: Heavy rainfall and thunderstorms over a short period Prolonged, extensive rainfall High tide combined with stormy conditions.

It is also predicted that climate change_ will increase the risk of flooding in all parts of the world. The following factors can also cause or contribute to flooding.

Poor Maintenance

-     Faulty sewer networks

-     Poor or insufficient drainage networks

-     Inadequate maintenance of watercourses, i.e., non desilting of drainage channels

Development / planning issues

Inappropriate development in flood plains Building on land in a way that prevents rainfall from draining away naturally this includes roads, built environment and car parks that are impermeable to water, increasing the risk of flooding from rainwater runoff. Flood defence schemes that are defective or badly designed

3 FORMS OF FLOOD [2]

I] Naturally Induced Scenario

Tidal Flooding: This results from a combination of low pressure weather systems and peak high tides resulting to overtopping or breaching of sea and river defences. Both sea and river defences may be overtopped or breached by a combination of low pressure weather systems and peak high tides. Storms with high wind speeds cause tall and powerful waves and low pressure fronts cause sea levels to rise above normal levels. High tide levels vary through the lunar and solar cycle and when superimposed upon other tidal variations exceptionally high tides result.

The onset of flooding from the sea and tidal rivers is often sudden and the extreme forces driving it present a significant danger to life. The east coast storm surge of 1 953 claimed 307 people's lives in the UK and 1,835 in Holland. A similar storm surge tide in September 2007 came within a few centimeters of breaching a number of the UK'S coastal defences.

It is often possible to forecast, with reasonable accuracy, this type of flooding due to the predictability of the tide and trackability of low pressure systems. The duration of this type of flooding is also limited by the cycle of the tides where drainage is available.

Fluvial Flooding: Flooding occurs in the floodplains of rivers when the capacity of water courses is exceeded as a result of rainfall or snow and ice melts within catchment areas further upstream. Blockages of water courses and flood channels or tide locking may also lead to ponding and rising water levels. Rivers defences may then be overtopped due to increase water levels, or breached by large objects of debris carried at high water velocities. Flooding from rivers has in recent years been experienced in the Niger Delta region and Niger/Benue River flood plains of Nigeria. The onset may be quite slow in some catchments with steadily rising levels. However, flash can occur in steep catchments and is far more immediate. Flooding from rivers particularly in recognized floodplains can usually be predicted with good accuracy.

However flash floods from sudden downpours such continue to challenge the capability of detection and forecasting systems. Water over about 250mm in depth may carry debris particularly in urban locations and can also be cold. Even travelling at low speeds this can make it extremely hazardous to people caught in it.

Ground Water: Low lying areas sitting over aquifers may periodically flood as ground water levels rise. This type of flooding is often seasonal and therefore can be forecasted with good accuracy. It is often slow in its onset.

II] Anthropogenic Induced Scenario

Pluvial Flooding: Surface water flooding is caused by rainwater run-off from urban and land with low absorbency. Increased intensity of development in urban areas has given rise to land with a large proportion of non-permeable surfaces, a problem often exacerbated by overloaded and out-dated drainage infrastructure. These circumstances, combined with intense rainfall, can give rise to localised flooding.

This sort of flooding often occurs outside of recognised floodplains and because it is caused by quite localised weather conditions it is very difficult to forecast. Its onset can also be very rapid, and the level of flooding very severe. In each rainy season flooding experienced in most urban cities is not directly caused by rivers but by surface water. Large volumes of rainfall early in the season usually saturate the ground and intense rainfall later causes both urban and rural areas to flood.

Flooding from sewers: Flooding from sewers can occur where there are combined storm and foul sewers and their capacity is exceeded due to large amounts of surface water run-off in a short time. Poor cleaning and maintenance can lead to blockages that can also cause local flooding. This type of flooding is hard to predict, has significant sanitary consequences for those affected, and can occur very rapidly.

Flooding from Man Made Infrastructure; Canals, reservoirs and other man-made structures can fail causing flooding to areas downstream. Industrial activities, water mains and pumping stations can also give rise to flooding due to failure.

4 The Effects of Flooding

The effects of flooding from the sources outlined above are felt by various ‘receptors’. These include, people, buildings, infrastructure, agriculture, open recreational space and the natural world. In extreme cases flooding may cause a loss of life. However, the social and emotional costs from flooding can also be significant and are often widespread and indiscriminate in flooded areas. These costs include displacement from homes, loss of personal valuables, and the ongoing fear and insecurity caused by the experience. Potable water supplies may be lost or contaminated in a flood and this can have immediate health effects upon people and animals.

The economy can also be severely affected by flooding. Businesses may lose stock, patronage, data and productivity, and disruption to utilities and transport infrastructure can have knock-on effects to a wider area. Tourism, farming and livestock can equally be affected.

The built environment may be damaged or destroyed as a result of flooding with high repair costs and long period required for reinstatement. The public realm is often badly affected through
damage and the deposit of potentially large quantities of debris. Land contamination may also be transported and during flooding.

Vital infrastructure may also be damage or disrupted. Electricity and gas supplies can be interrupted to individual properties but also to wider communities if sub stations and transformers themselves and flooded. Road links, railways, canals etc. may be blocked causing disruption to the wider transport network, and accessibility severely disrupted for local inhabitants, especially amongst those considered most vulnerable.

Over 20,000 lives per year perish in flood and 75 million people adversely affected worldwide, according to a study by Smith in 1 996 [3]. The effects of flooding and managing the flood risk in UK costs around #2.2 billion GBP each year (2).

5 Factors which Determine the Effects of Flooding

-     The level of predictability- this affects the timing, accuracy and communication of warnings given before a flood.

-     The rate on onset of the flood - how quickly the water arrives and speed at which it rises will govern the opportunity for people to prepare and respond effectively for a flood.

-     The speed and depth of the water- this dictates the level of exposure of people and property to a flood. It is difficult to stand or wade through even relatively shallow water that is moving. Flood water often carries debris, including trees, and water over 1 m in depth can carry objects the size of cars. Fast flowing water can apply devastating force to property and other receptors.

-     The duration of the flood- this is another important factor in determining the extent of its impact, particularly on individuals and affected communities.

5 Anthropogenic Neglect

Human neglect is often responsible for urban flood. Therefore, to minimize the flood, reduce its impact and safeguard the environment Government must think, Space developers must think, Academia must think and Commoners must think.

That is, think while:

i.      Developing a road network.

a.  Carry out an Environmental impact assessment for large projects.

b.  Provide suitable and adequate drainage system on both sides of the road. Terminate the road drains safely.

ii.     Developing a space for housing accommodation.

a.  Provide good drainage system.

b.  Allow green beds within the premises to aid in absorbing surface runoff.

iii.   Erecting infrastructures.

a.  Avoid building on or across drainage channels.

b.  Avoid dumping construction waste on drainage channels.

iv.   Disposing of wastes.

a.  Do not dump wastes in drainage channels.

v.    Installing electricity substations.

a.  Avoid low lying areas.

Human neglects can also aggravate problems of flooding by:

·        Inaccurate prediction or forecast of impending chaotic weather condition and tidal surge.

·        Unavailability of enabling instrumentation for weather forecast and tidal observation. Loss of concentration on the job.

Conclusion.

If we have to SAVE or sustain the environment from the devastating effect of flood, Government, the business community and academia must THINK and then

·        Encourage researchers to adapt or adopt or improve on the existing technologies in flood control [4].

·        Modify regulatory laws on development of open spaces Create awareness in the educational curriculum.

·        Enforce regulation especially in the design, construction and maintenance of water regulatory systems.

It is only after then we can relax, EAT and enjoy the human environment.