Among the natural phenomena, flood is one of the biggest destroying factors in developing countries which always bring numerous people’s life, property and assets to the hazard. The growing population, urban development and industrialization are the main factors that have undesirable consequences in the hydrology of watershed region in the cities. developing countries experience urban flood during rain seasons due to the limited water percolation to the earth’s surface as a result of impermeable surface which a has been introduced by man. It is therefore tangible that we require some construction of transition and collection of surface runoff and integrated management to reduce the flood (Ngigi, 2003:944).
Water-related disasters such as floods, droughts and water- borne diseases affect more than 200 million people each year and claim more lives than war. The damage done by water-related disasters thwarts sustainable development and perpetuates poverty. Over the last 10 years, disasters of hydrological, meteorological and climatic origin have been responsible for over 90% of all deaths due to natural disasters. More than 2,000 water-related disasters on all scales occurred during the last decade. Asia and Africa were the most affected continents, with floods accounting for half of these disasters and water-borne and vector-disease outbreaks accounted for a significant fraction of remaining disasters. In term of lives claimed, floods accounted for 15% of all deaths related to natural disasters.
According to RELMA, (2007), Africa is considered a water-scarce continent with most of countries regularly experiencing extreme water shortage during periodic dry spells. Rapid population growth and inefficient use of resources increases the deficit between available supplies of water and the needs of the people.
As a resource dwindle and water demand increases, large scale water supply projects become unviable. There is need to decentralize water supply to household and small community level. There is great potential to make better use of water resources by harvesting rainwater and storing it locally for household and productive purposes (ibid, 2007).
The lack of water is the largest constraints to sustainable livelihoods in many parts of Africa. Rapid runoff during the rainy season frequently results in a high proportion going to waste or even becoming destructive. Harvesting rainwater where and when it falls presents opportunities to address both water scarcity and floods at local level (Vohland & Barry, 2009: 120).
Roof catchment has been practiced over the past 2000 years. The earliest evidence in Africa relates to cisterns in the western Mediterranean Coastal Desert in Egypt which date back to roman times. At present 2000-3000 cisterns in the region are still operational with their sizes ranging from 200-300 cubic meters. Traditionally, people have collected and stored rainwater running from eaves of their thatched roofs in earthenware pots.
Along the east coast of Africa, rainwater collection systems known locally as djabias have been constructed in several countries and are still in used until today. These consist of rectangular tanks made of coral blocks and were traditionally plastered with local cement made from burning and crushing coral. These systems often had a small purpose-built catchment area to complement roof run off. By the first half of 20th century, roof catchments were often build at missions, churches and in schools.
Causes of Floods
There are numerous types and causes of floods which include; urban flooding is caused by rainfall overwhelming drainage capacity in urban areas, the impact can be very high because the areas affected are densely populated and contain vital infrastructure. Continuing development in flood-prone areas increases the risk.
The cost of urban flooding could rise to between 1-10 billion pounds a year by 2080s if no action is taken to reduce the risks. The following factors tend to be the cause of increased urban flood; Ageing drainage infrastructure- where by a lot of sewerage and drainage network is old and in poor condition, more buildings- the development of new structures cover previously permeable ground increasing the amount of surface water running off into drainages and sewers, Increased paving- increasing proportions of impermeable ground in existing developments as people build patios and pave over front gardens, and lastly, climate change- wetter winter and heavier summer shower are expected to put more pressure on urban drainage. Climate models predict that winter rainfall will increase by 20-30% by 2080s.
Natural floods: This type of floods is caused naturally by overflow of huge volume of water from rivers, lakes, coastal or heavy rains or downpours, tsunamis and hurricanes. They could be riverine floods caused by rivers. Estuarine floods caused by a combination of sea tidal surges and storm-forces winds or coastal floods caused by tsunamis and cyclones.
Catastrophic floods: This type of floods is caused by significant or unexpected events such as dam breakages. Heavy rains causes rise of water levels in dams, rivers, lakes and water starts to overflow to the adjoining areas causing deluge.
Snow floods: floods caused by snow melting as global temperature rises as a result of global warming making the snow caps to melt faster. Continuous and faster snow melting and rises the level of oceanic water which consequently raises the level of water in rivers causing floods.
The satellite towns majorly experience urban floods due to the increased paved areas, natural floods due to reduced vegetation cover and increased amount of rainfall over the years and catastrophic floods as a result of bursting of Athi-River when heavy rains occur.
The Consequences of Floods
Flooding, when the soil percolation capacity is overwhelmed and cannot drain effectively, the surface run-off the overflow travels down roads paths and floods low lying areas causing damage distress and loss of life.
Extreme floods have serious impacts on social, economic, environmental, political, and cultural factors of a given area, therefore, calling for a need of more coordinated post-audits following extreme floods to provide possible guidelines for communities to implement to limit long term of these impacts. (Friesema et al, 1979).
Economic effect, Economic hardships due to a temporary decline in tourism, rebuilding costs, or food shortages leading to price increases which is a common effect after severe floods. Chronically wet houses, high linked to increased respiratory problems such as asthma and other illnesses which results to increase spending on medical services. In developing countries floods have distinctive long-term effects. They can be divided into three categories: first, the consequences for human health which include death, physical injury, disease transmission, malnutrition and loss of morale; secondly, the consequences for agriculture which include destruction of farm products and reduced agricultural productivity; and finally, the impacts on housing and infrastructure which include damage of settlements and utilities such as schools, health facilities and churches. The duration and significance of the impacts depend on the levels of resources available to easy recovery and on the scope of the devastation.
In evaluating how extreme floods affect real estate prices, experts argue that residents with the most severe flooding do experience long lasting impacts on the house price. Estates within unaffected areas and with limited damage seem to be unaffected, however, the prices go up for some estates in affected areas because following the flood, estate owners replace all the appliances, paint and carpets generally increasing the houses in value to reflect the sprucing up.
After a disaster a community goes through four phases that overlap: the emergency, restoration, replacement reconstruction, and commemorative betterment period. Each of these phases takes about ten times longer than the previous one. The rate of recovery is related to the extent of damage, community capacity in terms of the available recovery resources, the prevailing pre-disaster trends, and community leadership and planning (Haas, Kates and Bowden, 1977). Losses of floods are considered to be direct or indirect. Direct flood losses are difficult to quantify and it is even more difficult to evaluate indirect flood losses. Direct losses are the number of business and homes destroyed, while indirect losses include migration from the area as a result of the flood, tax losses as consumers shop outside the damaged area and what are the costs of outbreaks of waterborne diseases (Hanchett, Akhter and Akhter, 1998:226-7).
Lastly, pollution, Surface run-off can be a major source of pollution. It picks up potential harmful substances from surface including; oil, house-hold chemicals, fecal materials among others and transfer them to watercourses. When combined sewer overflows in time of heavy rainfall, excess foul water is discharged directly into water bodies. Untreated discharges pose risks to human health as they may contain toxins and pathogens such as virus that causes hepatitis A and bacterial infection related diseases.
The Potential of Roof Catchment
Rainwater harvesting is mostly referred to as an “emerging technology”; despite the fact that rainwater cisterns are not a new concept. In the Middle East in 2000 B.C, middle-class dwellings stored rainwater in cisterns for use as a domestic supply as well as private bathing facilities for the wealthy.
Rainwater catchment can be done at a domestic level for household uses, industrial level for use in factories or at agricultural level for irrigation purposes. The rainwater can be stored differently for all these uses though, the way it is collected is always the same.
Water is one of those resources which have to be used with caution. This is true in countries which have a tropical climate. In particular reference to Kenya, where dry condition prevails, and scarcity of water during dry season is more rampant. The scarcity of water is more felt by those who have livestock and crops. But, this can be overcome by harvesting the rain water during the rainy season. A rainwater collection system can be an excellent alternative source for constant supply of good quality water. Like all other sources of water, rainwater harvesting has advantages.
The advantages are:
- Water Quality: Rainwater is purer than the water treated with chlorine as rainwater is generally one of the better sources of an alternate water supply when compared with other sources of water that may be available this is because; rain water is created through the natural process of evaporation, contains minerals and compounds necessary for healthy growth of life of both livestock and crops, Savings in terms of water bill. Rainwater is a source of water that can provide a cost effective and alternative source of good quality water.
- Simple Construction: The construction of roof catchment systems is not complicated and most people can easily build their own system with readily available materials and local people can easily be trained to build one, minimizing its cost.
- Ease to operate and maintain: The operation and maintenance of a household rainwater collection system is controlled by the individual without having to rely upon the maintenance practices of a municipally controlled water system.
- Convenience: Rainwater collection provides a convenient source of water at the immediate place where it will be used or consumed. It also provides an essential reserve in times of emergency or breakdown of public water supply systems, particularly during natural disasters.
- The technology is flexible and adaptable: The systems can be built to meet almost any requirements. Poor households can start with a single small tank and add more when they can afford them. It is also adapted to suit most individual circumstances and to fit most any household’s budget. It can improve the engineering of building foundations when cisterns are built as part of the substructure of the buildings, as in the case of mandatory cisterns. And lastly, the physical and chemical properties of rainwater may be superior to those of groundwater or surface waters that may have been subjected to pollution, sometimes from unknown sources.
Flood management is an evolving field of science. Hence, it is possible to study the evolutionary possibilities by observing different evolutionary paths, and thus, better understand the evolutionary forces or phase shifts. The natural phase is the phase in which no form of flood management policy exists. Due to shifting anthropogenic pressures, climate, the failure of current flood management policy, or the emergence of a dominant worldview, society will evolve or modify a system to cope with floods. Thus, society will adopt one or another evolutionary path, the traditional path or the alternative path.
It is assumed that more productivity, resulting from farming and industry, can occur when floods are less frequent. This assumption is based on research conducted in Bangladesh, where it was shown that more productivity occurs in poldered areas. Hence, a scenario in which floods are controlled will lead to greater productivity increases. However, this assumption is only true if flood management infrastructure is adequately operated and maintained.
The central concept of a system of dike compartments is controlled flooding, limiting the affected area and minimising the flood damage. This suggests that even though flooding is allowed the floods are still totally controlled by humans. The principle behind a system of dike compartments is that dikes with an inflow gate and an outflow gate surround each compartment. Compartment has different flooding probabilities based on the value of the land within the compartment. Moreover, considering that such a strategy is utilizing current systems of dikes, the restoration or improvement of the ecology is unlikely to occur. Hence, this strategy aims to: reduce flood risk, both by keeping flood probability low and by reducing the flood damage; maintain economic productivity in the floodplain areas; and minimize social impacts that may arise from more radical strategies.
Living with floods as a flood management policy focuses entirely on a society’s ability to live or co-exist with floods rather than fight against floods; any damage that arises from a flood is eliminated. Primarily, society adapts to the natural flood conditions and develops the land in accordance with the prevailing natural conditions. A society that co-exists with floods will; minimize risk by reducing the damage inflicted by floods to nearly zero; be totally aware of floods as they form a part of the society’s daily life; and aim to minimize environmental degradation by maintaining the natural system.
Capturing water from the roof tops is an empowering method of bringing greater ecological balance and local resilience to dense cities. This rooftop technology is a sure mark of humanity’s evolution towards more alternatives and sustainable water sources. Several actions such as flood mitigation strategy have been taken worldwide to address floods but more actions which puts community participation into consideration needs to be put in place. Anthropogenic activities such as poor farming methods, poor disposal of solid waste, and urbanization have highly contributed to frequent occurrence of floods in this area.
- Establishment of Sustainable Drainage Systems (SUDs): This is an alternative to conventional drainage is to mimic natural drainage with an aim to reduce floods and improve the quality of water draining from urban surface run-off. This will be able to solve the problem of clogged drainage systems and increase the surface area of ground water percolation. This can be achieved through developing areas of vegetation like grassy banks or green roofs or natural water storage features like ponds. Also use of engineered components such as porous paving which will reduce peak flow rate of run-off, encourage uptake of water by the ground, transfer of run-off in a controlled manner to the other sites and lastly, capturing water directly on site for controlled storage or discharge.
- Technological improvement: This is an essential aspect in rainwater harvesting which needs to be better addressed. This can be achieved through: Development of first-flush bypass devices which are more effective and easier to maintain and operate than those currently available and in use; involvement of the public health department in the monitoring of water quality to ensure that the harvested water is standard for drinking and to be used in other domestic chores especially during floods; monitoring of building constructions to ensure that roof catchment is integrated throughout the construction process; provision of assistance from governmental sources to ensure that the appropriate cistern sizes which are able to contain the amount of water from the provided catchment are built; provision of assistance to the public in sizing, locating, and selecting roofing and cisterns materials and constructing cisterns and storage tanks, and development of a standardized plumbing and monitoring code and preparation of guidance materials for inclusion of rainwater harvesting in a multi-sourced water reserve sources management environment.
- There is need to strengthen linkage between different environment management and planning agencies to develop an integrated disaster management system, flood forecasting and warning system at the community level and put in place clear definition of roles of various institutions; planning of flood mitigation measures should be forced into the different government planning schemes which can provide a forum for the effective participation of the communities in planning of flood mitigation measures.
- Integration of roof catchment in to planning: By integrating rain catchment system into the design of a home, it will create a cost effective, convenient system where all the components can be located in one area for easy monitoring and maintenance. Integrated home systems are excellent for new construction and a perfect way to save money on municipal or well water costs.
Friesema et.al. (1979). Aftermath: Communities after natural disasters, Sage, Beverly Hills.
Haas, J.E., Kates, R.W., Bowden, M.J. (1977). Reconstruction following disaster, MIT Press, Cambridge.
Hanchett, S., Akhter, J. and Akhter, K.R. (1998). Gender and society in Bangladesh’s flood action plan, in Water, culture and power, J.M. Donahue and B.R. Johnston (eds), Island Press, Washington, DC pp. 209-237.
Ngigi S.N (2003). What is the limit of up-scaling rainwater harvesting in a river basin? Physics and Chemistry of the Earth, Vol.28, Pp. 943-958
Relma (Regional Land Management Unit). 2007. Good to the last drop, Capturing Africa’s potential for rainwater harvesting. PDF document. http://www.relma.org/PDFs/Issue%202%20‐%20Rainwater%20Harvesting.pdf. Accessed 2 December 2013.
Vohland, K & Barry, B (2009) A Review of In situ rainwater harvesting (RWH) practices modifying landscape functions in African drylands, Agricultural Ecosystems and Environment, Vol.131, Pp.119-127