Flood Risk Management
Integrated flood risk management is the combination of interventions that can be implemented in the short and long term. These interventions can be structural and nonstructural risk management activities which should be assessed on their cultural, environmental and cost-benefit relationship. These measures should also have tangible targets which can be measured, assessed and monitored over time.
Structural and Non-structural flood risk management
Measures to reduce flood risks will entail reducing the probability of flooding and/or reducing the adverse impacts of flooding. These measures are divided into structural measures and nonstructural flood risk management measures. Structural flood risk management measures aim to reduce flood risk by keeping floods away from people and key infrastructure for instance building dams, embankments etc are considered to be structural measures.
The use of structural flood risk management measures only reduces the risk of flooding. It cannot eliminate it. The flood risk reduction depends on the design life of the structure and the degree of flood risk that is acceptable. This is clearly demonstrated in the graph below where a scheme designed to cope with the 100-year event would only have a 45% probability of not failing within 100 years.
It is usually the case that investment in “protected” flood risk areas increases after flood defences (structural flood risk management measures) are in place; however, overtopping of flood defences can cause a significantly more economic impact. At the same time reduction in flood risk in one area may have significant environmental effects elsewhere.
Nonstructural flood risk management measures aim to keep properties from flooding through better planning and management of settlements, this includes building codes, use of specific types of building materials, flood forecasting and warning, improve institutions that manage flooding between others.
As a 100% level of protection can never be guaranteed, nonstructural flood risk management measures are necessary to reduce the consequences in case structural measures fail and a flood occurs.
Holistic Flood Risk Management approach
A more holistic approach to flooding is Integral Flood Risk Management initiatives which include a combination of structural and nonstructural measures. This figure below illustrates each category of measure (reduction of flood hazard, flood protection etc) and their corresponding policy and flood risk management field (top of the figure). The category of flood risk management measures is described in more detail below.
Reduction of flood hazard activities
Reduction of flood hazard in the flood risk management approach can be achieved by the implementation of the following initiatives.
Reduction of peak run-off by applying soil conservation measures that reduce sediment transport which in turn affect river capacity. Heavy rainfall that occurs after long periods of drought is more likely to run off the dry compacted soil and cause river levels to quickly rise risking flash flooding. Managing the water cycle as a whole will help to reduce the risks of both floods and drought
Provision of green Infrastructure by reducing the use of impermeable surfaces and reduce upstream basin drainage schemes
Regulation of river flows by water storage in reservoir or attenuation areas. Water from this areas can be re-used
Providing sustainable drainage systems in developments
Increase in river flow conveyance- Making "room for the river". Combine green infrastructure with urban development this allows to create waterfronts and add value to communities
Flood Protection
Flood protection within the flood risk management interventions includes physical barriers such as dykes, bypasses and other flood conveyance improvement interventions, storm surge barriers, coastal lagoons with sand barriers, reservoirs, and other flood retention facilities, raise local land levels and embankments.
Flood protection barriers can protect areas prone to flooding however they could cause high river-water levels, higher discharges downstream and high water levels upstream. They can also lead to river bank aggradation which leads to high levels of overtopping. For instance, embankments are vulnerable to failure on the outside of bends and vulnerable to breaching at crossings of abandoned channels. Nevertheless, embankments play an important role in flood protection and are part of well conceive flood risk management plans.
The use of flood protection barriers can be more flexible by using them to different levels of protection in different zones and by providing spillways to areas where flood water can be stored. Flood protection barriers such as embankments can also have multiple roles such as for transport purposes or they can be combined with housing and parking areas. These can become an integral part of the flood risk management interventions.
Land use regulation
Placing restrictions on land use and flood proofing reduce the flood impact. In the UK, an example of non-structural flood risk management is the Environment Agency floodplain zonification and National Planning Policy Framework on the land uses that are allowed within these flood zones. The land use can also be related to insurance premiums.
Adaptation of existing properties can reduce the impact of flooding. These include houses on mounds or poles, growing flood resistance crops. This flood risk management is not well developed in the UK as the Building Regulations does not have specific guidance regarding property adaptation to flooding.
Floodproofing includes measures to prevent flood water from reaching buildings and minimise its damage. The floodproofing methods are divided into elevation (raising the building), dry proofing (water does not enter the building) and wet-proofing (water enter the building but structural damage is minimised). These methods are discussed in detail in other blogs.
Raising Preparedness
Flood early warning systems can identify when a flood is imminent providing sufficient time to put temporary or emergency plan in place. These systems create a unique opportunity in the flood risk management plans to be able to better inform people by utilising flood risk maps that show the location of the flooding.
These systems, included in the flood risk management plans, should work together with disaster management programmes to increase communities resilience to flooding by assisting communities during the events and by improving the economic status of the communities. This allows them to recover faster.