What You Need To Know About Flood Risk Planning

Flooding situation in Accra, the capital of Ghana is increasingly becoming a ritual, my heart bleeds over deaths emanating from flooding which could have possibly be prevented. This calls for national concern and immediate re- planning of the Accra city and other flood zones. It has been noted, that among the natural hazards that do occur in the world, flooding is probably, the most devastating, wide spread and frequent. Flooding is a natural phenomenon; whereas it may, in itself not pose problems, human activities however, have in many circumstances change flood behaviour. Activities in the flood plains and catchments such as construction of infrastructures e;g highways, roads, bridges, markets and shopping centres, may increase the magnitude of flood, which increases the damage to properties and life. With current trends of changes in weather and climatic conditions, rain flooding has even become more pronounced and presents a more threatening concern than before. In Ghana it has been observed that since independence, new maximum flows levels have been recorded on several areas in our only capital city (Accra) and villages, with implication for revised flood risk analysis and large economic impacts. The most recent is the 26th OCTOBER 2011 flood around the Kwame Nkrumah circle and other parts in Accra which claimed at least five human lives and destruction of properties. Flooding fatalities in Ghana have been rampant in recent times, the financial damage to property, disruption to communications and business losses amount to millions of cedis and have all featured regularly in appraisals of flood impacts by Nadmo and City authorities. The substantial loss of property and life to flood therefore calls for effective and resilient sustainable flood risk management system. Effective management of flood risk, involves collating of data from different sources (i.e hydrology, geology, watershed, soils, rainfall, drainage system, demographic data, land use, etc) in one platform for better analysis and visualization. Geographic Information Systems (GIS) technologies have been recognized as a powerful set of tools to integrate, capture, store, manipulate analyse and visualized geo-reference data from diverse sources. Over the last two decades advancement in the field of remote sensing and geographic information system (GIS) has greatly facilitated the operation of flood mapping and flood risk assessment. Earth observation technologies (Remote Sensing) and Geographical Information Systems (GIS) are playing a large role in help helping to understand flood behavior by generating flood extend or flood risk maps. The main advantage of using GIS for flood management is that it does not only generate a visualization of flooding but also creates potentials to further analyze this product in order to estimate probable damage due to flood. Developing flood management strategy, involves integration of GIS and Remote Sensing (RS) applications with hydrologic and hydraulic models. A hydraulic modelling especially computer model is required to carry out the flood simulation to produce flood level at various locations in the Country. The output of the hydraulic simulation can be transferred to GIS software to generate flood layer for various scenarios. The combination of GIS software and hydraulic software help to speed up the process of producing flood risk map. In orders to carry out flood risks or hazard mappings, it is essential to investigate the extent of previous flood events to form the basis for planning and prediction of future floods. Whereas, it is very difficult to map the extent of flood as they occur, remotely sense data (satellite imagery) provides such data from which extent and damage of previous flood can be extracted. As part of providing the flood management plans for the city of Accra, remote sensing data could be analyzed to estimate and map the extent of previous flood to provide the basis for development of flood risk areas. Apart from using the RS to delineate inundation areas, it can also be use to monitor flooding. In the last decades earth observation systems such as Landsat Thematic Mapper, SPOT Imagery and Advanced Very High Resolution Radiometer (AVHRR) data have also been found useful for floods delineation. Flood hazard mapping and flood inundation modelling are the vital components in flood mitigation measures and land use planning, and are prerequisites for the flood insurance schemes. One of the most primary issues for flood management is to identify the area having higher hazard potential. Flooding is a natural hazard and flood risk is defined in terms of occurrence in certain year usually hundred-year flood. Flood hazard zones could be derived from a series of spatial analysis involving overlay, buffering analysis, use of land use / land cover data, topographic data, satellite imagery, DEM, soil map, demographic map, and flood water depths and flood extent maps derived from previous flood. Three buffer zones could be created to determine the distances at which flood risk within the flood hazard zones can be considered as (High, Medium or Low). Buffer zones distances could be determined using the flood extent map of previous flood.
Evacuation planning is very important in dealing with hazards when they occur. It is therefore very important to plan evacuation route to reduce risk levels that will occur during flood. In planning evacuation routes for the Accra city, two sets of data are
necessary, population density map and street map. This could be combined with flood depths map and flood extent maps, to identity streets and roads that will be inundated and those that lies within high flood risk zones. To aid evacuation, road/street density for the city could be calculated or mapped (using spatial analysis by calculating the total length of road divide by the total area). Road densities could be mapped for all the various hazard zones using the flood extent as the limit boundary. This will help determine the street/road area in terms of the inundated area. To determine evacuation vulnerability all nodes within the road or street data must have a representative population and all street segments have the number of lanes in each direction. This involves transferring data from the population dataset to the nodes of the roads/street data. One approached to achieve this will involve the use of Thiessen polygons model to generated polygon around the nodes of the street coverage and overlaid with the population. The polygon fragments are then each assigned to a node based on their population. This will help calculate the number of residents or population per lane and thereby determine network connectivity with population. It is my prayer and wish that one day the adverse impacts of flooding situation in Ghana would be reduced. WE MUST STOP THE BLAME GAME AND WORK TOGETHER AS PEOPLE WITH ONE DESTINY.

My sincerest condolence goes to the bereaved families and those who have lost their properties. I see a change Ghana, and those who are prepared to face the odds, with God as their anchor knows no defeat. Long live Ghana....

Stephen S kansuk
Energy and Environmental Mgt Professional
[email protected]