Floods are almost an offspring of climate change. Every time climate changes drastically, floods come next.
The first ever recorded climate change, so far, can be found in the bible. According to the book of Genesis, in the beginning there was no rain yet “for the Lord God had not caused it to rain on the earth . . . but a mist went up from the earth and watered the whole face of the ground (Genesis 2:5-6)”. Then, suddenly, in Noah’s time, it rained for the first time, and this was followed by a flood of an enormous magnitude.
Recent natural calamities have resulted in various occurrences of unprecedented flooding in places where flooding was not even considered a risk. To name some, the flood in Queensland which affected 70 towns, about 200,000 people and damaged properties worth one billion dollars; the Thailand flood in 2011 wherein 8.2 million people in 60 of Thailand's 77 provinces were affected, and economic losses were, so far, estimated to top $2 billion; the recent flood in the Philippines where, apart from property and material losses, thousands of lives were lost. The hardest hit in terms of casualties are the developing countries that do not have the technology and finances to handle such calamities. One classic example is the recent flood in Cagayan de Oro, Philippines, where the total death toll was about 3,000.
The adage, prevention is the best cure, is very much applicable to storm water management. Obviously, we cannot prevent massive outpouring of rain resulting from climate change, but designing a drainage system that can accommodate storm water can certainly prevent flood.
Understanding stormwater behaviour is key to preventing flooding during heavy rainfall. One thing worth noting is that flooding occurs where developed areas are, like residential and industrialized zones and regions that were environmentally molested by men, through the cutting or trees or removing vegetation. It is unlikely that flooding in greenfield regions or areas, which have not been developed yet, will occur. Thus, if we observe the stormwater behaviour in greenfield areas then we can find a solution to our flooding problem.
From the basic principle of ecological cycle, the Earth is capable of handling rain no matter how big the volume is. Therefore flooding should be out of the equation. When rain falls in greenfield areas, part of the rain water is absorbed by the soil, and then plants absorb it. When the soil becomes saturated, rain water will naturally flow into lower areas, then into a natural channel, into the river then into the sea. A small portion of it simply evaporates into the atmosphere. When this natural process is altered by man, due to development, flooding can happen. The reason is quite simple and straight forward. When an area is developed, new impermeable surfaces are created and plants are removed. This prevents rainwater from being absorbed by the soil in the new impermeable areas and those that were absorbed by the soil stay in the soil due to lack of plants to absorb them. When the next rain comes, the soil can no longer take more water, hence flooding occurs.
When you build a house, for example, the area that used to be permeable and filled with plants now becomes impermeable and existing plants obviously have to be removed. The more areas converted to impermeable areas such as a building’s footprint, roads, patios, impermeable footpaths and the like, the more stormwater will be flowing out from the new development. Ordinarily a drainage system is introduced to deal with this stormwater. The typical approach is to collect this water by pipes and channels and then let it drain into a catchment like river and the sea. Existing drainage systems, however, can only handle volumes of water relative to their capacity. When this limit is reached, flooding occurs.
Most of our drainage systems have been built a long time ago. Perhaps some of them have been design to give allowance to new developments, but some may not. Furthermore, they were designed relative to the current rainfall of their time, and now that rains are much heavier because of climate change, their design capacity may not be suitable anymore. And last of all, drainage systems must be properly maintained in order that its design capacity is likewise maintained. But in some cases, especially in poor countries, drainage systems are poorly maintained so when it rains, even if it isn’t exceptional rainfall, flooding occurs.
If we ask the question how we can prevent flooding resulting from developing existing greenfield areas, or redeveloping an already developed area (in other words, constructing more hard surface thus creating more impermeable area) the answer would simply be maintaining the rate of flow of water draining off the new developed area as it was before the development. From studies, it is an acceptable assumption that in greenfield areas the rate of flow of water draining out from it is approximately seven litres per hectare per hour. Therefore, in developing a greenfield area, the rate of flow water coming out from the new development should be limited to the total area (in hectare) developed multiplied by 7 litres per hour. Hence, to prevent flooding, new developed area should limit the outflow of stormwater to the obtained figure above. When redeveloping an area, the rate of flow resulting from the new development should be limited to what the existing outflow was. This can be obtained from the approved drainage design of the existing development or this can be modelled using drainage design software. In the absence of either the existing design or drainage software, a conservative computation of the existing impermeable area (roof and hard standing) multiplied by 50 millimetres of rain. The principle behind this approach is that the new development is not increasing the current outflow of storm water hence it has no impact on the current flood risk of the area. In the UK, a planning permit is normally granted if the developer can prove that the new development has maintained or lowered the current existing stormwater outflow.
Once the required stormwater outflow from the new development is known, the next question that needs to be answered is how this can be done. In other words, if there are new impermeable areas introduced then obviously the storm water outflow will increase. The simplest solution is using soakaways. This is simply a storage tank with perforated holes around it such that the stored water gradually soaks away into the ground and then eventually absorbed by plants or evaporates. This is ideal for sites where there is enough space and vegetation to accommodate a soakaway. Normally a five metre distance between a soakaway and the building foundations is required. This is to prevent foundations from sinking when the surrounding soil becomes saturated with water. The advantage of this system is that the amount of water flowing out from the site is actually zero. Alternatively, the stormwater can be split into two. One part goes to the public drainage system, rated to be equal or less than the existing flow and the other part can go to a soakaway.
If a soakaway is not feasible the outflow can be limited by introducing a storm water attenuation system. This limits the outflow to the desired rate by storing the water in an attenuation tank and then gradually releasing it to the public drainage system using an outflow reducing feature like a hydro-brake or vortex or a rated pump. In both cases the key point is determining the size of the storage tank. Using a drainage design software storage volume can be computed relative to the impermeable areas, the desired outflow and the worst possible rainfall that can happen in the site. Sometimes a combination of a soakaway and attenuation could be the ideal solution to manage stormwater.
If we do things right, flood will not always come next when climate changes.
The more commonly used soakaways in storm drainage management are ring soakaways, trench type, hydrocell and permeable pavement soakaways.
A ring type soakaway is built from precast concrete ring manholes with at least a 2.1 metre internal diameter and effective height, and where part of the manhole is below 300 mm from the invert level and has perforated holes of about 25 mm diameter, spaced at about 150 mm centres and surrounded with gravel.
A trench type soakaway is made of gravel placed into a trench of at least 450 wide with a distributor pipe within it. The water that gets into the pipe is eventually distributed to the surrounding gravel and then soaks away to the surrounding soil.
A hydro-cell soakaway is made from plastic crates placed together to form a storage tank. It is wrapped with a permeable textile all around so that the stored water can soak away to the surrounding soil.
Permeable pavers are sub-base of a pavement of reasonable depth constructed from selected gravel and wrapped around with permeable textile as the others. It acts as a foundation for the pavement and at the same time as storage tank.
The most common type of attenuation system is one that uses a hydro-brake manhole at the end of the drainage system just before connecting to public drainage or to a watercourse. The hydro-brake attenuates the flow forcing storm water to be stored in a storage tank that could either be made of a reinforced concrete material, hydro-cells, permeable pavements or within oversized pipes and manholes of the drainage system.