FLOOD DISASTER MANAGEMENT

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FLOOD DISASTER MANAGEMENT

In the management of flood disasters, flood hazard/risk mapping is one of the vital steps undertaken to prepare for and mitigate the effects of a flooding event. Some of the other vital steps may include vulnerability analysis, climate forecasting, flood plain management and enforcement of standards and codes [1] [2] [20].

VULNERABILITY ANALYSIS

The population and structures within areas delineated as flood-prone are looked at during a vulnerability analysis. During the vulnerability analysis, the potential costs of flooding are evaluated as pertaining to damage of critical infrastructure such as utilities, bridges roads, buildings and crops. Because vulnerability analysis detects the population at utmost risk, it can also be used to determine the emergency responses that may be essential such as temporary shelters and evacuation parameters [10] [20].

CLIMATE FORECASTING

Based on identified changes in the patterns of ocean and atmospheric circulation, the magnitude of a storm can be forecasted and this information can help in emergency response preparedness. This information can be used to reduce the severity of flooding when it occurs by creating awareness, increasing food storage and management of fresh water [5] [20].

FLOOD PLAIN MANAGEMENT

Activities within areas identified as flood prone can be managed so as to minimize flood damage to existing infrastructure. The measures undertaken to manage activities can be grouped into two; structural and nonstructural measures. Structural measures deals with the construction of protective works such as flood storage reservoirs, storm channels and embankments to carry water away from the flood area. Nonstructural measures help in controlling development in flood prone areas at a low cost. Land-use planning, Zoning of flood-prone lands, Redevelopment of flood-prone areas, Compensation, incentives and Insurance are some of the nonstructural measures that can applied [7] [20].

ENFORCEMENT OF STANDARDS AND CODES

Standards and codes for flood-prone areas should be enforced to help minimize the impact of flood events. Enforcement procedures should be simple enough to aid the implementation of penalties with regards to noncompliance to the standards and codes. Regular emergency response drills should be undertaken to ensure that flood prevention measures still work [20].

TYPES OF FLOODS

Several types of floods occur but based on the geomorphology of a location, only a few may be experienced in that particular location. The following are the classifications that floods are generally grouped into and some of these flood classifications encompasses several flood sub types [12];
1. Pluvial (Surface) Flooding
2. Riverine flooding
3. Groundwater flooding (Ground failures)
4. Coastal flooding
[12] Pluvial, riverine flooding and ground failure are usually affected by surface water runoff. When rain falls onto the surface of the earth, the following happen concurrently; the water either permeates the soil, evaporate or runs over the surface i.e. the hydrological cycle. The magnitudes of each of these actions is greatly dependent on the ground cover which can be built up areas (residential areas, offices and business areas), pavements (parking lots, roads) and open space (grassland, agricultural lands etcetera). When the rate of evaporation and the permeation capacity of the soil is exceeded by the intensity of rainfall or the ground cover is impervious, surface runoff occurs [21].
As population density increases and more areas become urbanized, the amount of impervious areas increases whiles the amount of natural terrain that can absorb rainfall decreases and this in turn leads to an increase in flooding associated with surface water runoff.

PLUVIAL (SURFACE) FLOODING

When heavy rainfall produces a flooding event exclusive of an overflowing water body, it is referred to as pluvial flooding [21]. The most common type of pluvial flooding is urban drainage.
In undeveloped locations, nature provides the drainage system but once an area is built up, it becomes necessary to find ways and means of eliminating excess water which cannot infiltrate the ground due to impervious surfaces [12]. The idea of urban drainage is the use of a closed passage system to contain and dispose of excess surface runoff water after rainfall. This philosophy implies that no matter how heavy the rainfall or how long it last, the drainage system should be able to contain and get rid of the runoff [21].
This system works very well in developed countries where all the drains are enclosed and there are gullies under every street constructed to collect water from road surfaces. The only concern is the prevention of downstream flooding and this is done through the regulating and controlling of the runoff water known as storm water management [20]. In developing countries such as Ghana, most of the urban cities have open drains which makes it very easy for rubbish and waste to get inside the drains and choke them as shown in figure 2 below. As these drains are choked, any amount of rainfall lasting any duration results in some form of flooding with heavy downpours resulting in floods whose aftermath is the loss of life and property. This is the most prevalent type of flood in Ghana.

RIVERINE FLOODING

Riverine flooding occurs as a consequence of runoff water exceeding the capacity of channels, either natural or man-made, and overflowing to adjacent low lying areas [21]. Riverine flooding dynamics vary with terrain. Runoff in mountainous regions may occur minutes after heavy rainfall but in flat and low lying areas, water may cover the land for days or weeks. There are two different types of riverine flooding and these are overbank flooding and flash flooding.
Overbank flooding: This occurs when the volume of water in a river or stream increases and exceeds its capacity and overflows onto adjacent floodplains due to surface water runoff after a heavy downpour, the spill of a dam, melting of snow or ice jams [21]. This is the second most common flood (heavy downpour) event in Ghana.
Flash flood is a fast and dangerous flow of high water into a usually dry area, or a swift rise in a stream or river above a predetermined flood level [6]. It is characterized by a high velocity, intense gush of water that ensues in an existing river channel with little or no notice. Flash floods are more dangerous and destructive to life and property than overbank flooding because of the speed with which flooding occurs and large amounts of debris carried with the flow [6] [21].

GROUNDWATER FLOODING (GROUND FAILURE)

The onslaught of some floods are from below ground. As the water table rises to the surface due to prolonged periods of rainfall, it can wash away portions of the topsoil [7]. This can cause an array of ground failures which includes sinking soil (subsidence) and liquefaction; a development in which water-soaked silt loses stability and acts like a liquid [21]. Subsidence and liquefaction may lead to mud floods and mudflows. Mud flood implies a flood in which the water conveys, about as much as fifty percent (50%) by volume, heavy masses of silt which may include coarse debris [7] [21]. Mud flow refers to a flood which is made up of mud and water: the make-up of the mud is a flowing mass of soft wet unconsolidated earth and fine grained debris.

COASTAL FLOODING

Coastal flooding is caused by the combination of heavy storms or other extreme weather conditions together with high tides which causes sea levels to rise above normal and force sea water on to land [21]. The causal agents for coastal flooding are storm surges and earthquakes. A storm surge is the rise in sea water above normal tide levels due mainly to low atmospheric pressure and wind action over a long expanse of open water. When there is a storm or hurricane, suction is created by the low pressure inside the eye of the storm and this creates a dome of water [5]. If the storm is near land, strong winds in the storm pushes the dome on to land as a surge. Underwater earthquakes, caused by the movement of tectonic plates, offsets large extents of the ocean floor [5]. The abrupt vertical shifts over such large extents displaces large amounts of water, generating long-range and destructive waves known as tsunami [5] [21].
Each type of flooding type and its resulting hazard can be modelled by the different methodologies for flood hazard mapping but some methods are better suited to some forms of flooding depending on the objective and purpose of the map and the availability of data. This thesis focuses on the perennial pluvial flooding (urban drainage) of the city of Accra Ghana. The GIS and remote sensing approach is the methodology that will be utilized due to the unavailability of data for other methodologies.

METHODOLOGIES FOR FLOOD RISK MAPPING

Flood risk maps are very useful in preparing for and mitigation of floods by minimizing the exposure and vulnerability of human lives, property and infrastructure
In recent time, it is becoming increasingly common to use GIS and Remote Sensing for flood delimitation and its attendant risks and hazards [21]. The usage of GIS and Remote Sensing for flood delimitation is usually carried out conjointly with computer–automated models which combine digital terrain models, hydraulic models and the hydrology of flood plains [21]. There are three different approaches to developing a flood hazard map;

 HISTORIC APPROACH

This is based on past flood events. Pertinent information to delineate flood regions may be gathered from old maps, photographs, satellite images, written reports, or any other document. The historic approach is generally used for broad purpose flood assessments maps and initial flood assessment maps. This approach affords data on areas noted to have been inundated in the past by flood waters. The data to be derived from maps and satellite images can be in the form of dates on which flood events occur, the specific location of the affected areas and the extent of damage to human lives, property and infrastructure. Data from written reports make available information about the causes for the floods, the areas affected and the magnitude of the flood. Photographs help to compare current physical conditions of a location with the conditions existing when the reports were written [21]. The disadvantage of the historic approach is that for a particular flood event, there may not be enough information or data covering the whole event and hence resulting flood maps are incomplete [22].
Wagemaker and Jjemba [23] generated a flood map for Uganda based on extreme weather events recorded in local newspapers, to aid in the disaster preparedness in flood prone Uganda. Newspaper repositories of the Daily Monitor and the New Vision, both national newspapers, were used as a data source. A database of 3726 news articles between 2001 and 2015 from the two newspapers were used. Using similar features as a base, sentences were clustered s and annotated into four classes: 1. Current flood event 2. Past event or flood warning 3. Mixed and 4. Unrelated. The results yielded a total of 1173 of news articles with flood sentences and geographical reference. These articles were then used to generate a flood map for interested districts.
Boudou et al [24] assessed the temporal evolution of flood vulnerability of two French cities, Besançon and Moissac, through mapping of land use from historical information. The aim of the research was to focus on the two cities that have been significantly flooded in the past and to understand how their vulnerability to flooding had changed up to the present day. An initial total of 176 major floods in France since 1770 were selected based on the following considerations: diversity of flood types, strong flood hazard or spatial extent and important socio-economic impacts. The 176 floods were evaluated and cut down to focus on 9 based on three main features; flood intensity, flood severity and spatial extent of damage. Historical land use data was analyzed to allow the mapping of land use and occupation within the areas affected by the selected floods, both in past and present contexts, to provide an insight of the complexity of flood risk evolution at a local scale.

GEOMORPHOLOGIC APPROACH

This approach involves the interpretation of distinct marks left in the landscape by past Floods and flows. The interpretation can be used to derive flood extents and other parameters such as magnitude of flood can be derived to a certain degree. This approach can also be used for broad purpose flood assessments maps and initial flood assessment maps but it is frequently used for validation during the stage of detailed mapping. The geomorphologic approach maps the geomorphologic indications related to a river or stream. It is based on floodplain analysis, features associated with erosion and sedimentation development, river channels etcetera. The analysis consist of the classification and interpretation of changes detected on the river bed and all observable morphological features [21]. Geomorphologic approach is most advantageous when there is the need to determine the effect of erosion and deposition in the flood plain but the method is rather restraining because it is mostly applied over small areas such as river basins and streams and it is time intensive, in that, it requires data over a long stretch of time of how the landscape has changed with past floods and flows; that is, geological time scale. This method does not also provide any indication of the probability of flood event occurrence [25].
Fernandez-Lavado et al [26] developed a hazard map for flash-floods in the municipality of Jucuaran, El Salvador by mapping the geomorphological evidence such as alluvial fans, preferential stream channels, erosion and sedimentation. The geomorphological effects caused by hurricane Mitch in 1998 were regarded as a reference event. The process for developing the hazard map involved three complementary techniques; vertical aerial photointerpretation of the river basins, Fieldwork and eyewitness reports and Community workshop. Evidence from aerial photographs were corroborated from local people during fieldwork and the community workshop was used to obtain a historical perspective and specific information about the reference event (Hurricane Mitch) to complement eyewitness reports obtained during field work.
Muianga [27] developed a flood hazard zonation map for lower Limpopo, Mozambique using a combination of geomorphological features extracted from aerial photographs together with reports and local knowledge. A process known as ”terrain mapping unit” was used to divide lower Limpopo into 32 units of fluvial, marine, Aeolian and denudational. Four levels of flood hazard were then defined; High, moderate, low and no flood. The units that showed high hazard levels were those formed by recent alluvial deposits and located along the Limpopo River.

Table of contents :

1. INTRODUCTION
1.1 BACKGROUND
1.2 FLOOD RISK
1.3 REMOTE SENSING AND GEOGRAPHIC INFORMATION SYSTEM FOR FLOOD RISK MAPPING AND FLOODING EXTENT ASSESSMENT
1.4 RESEARCH OBJECTIVE
1.5 STRUCTURE OF THESIS
2. LITERATURE REVIEW
2.1 FLOOD DISASTER MANAGEMENT
2.1.1 VULNERABILITY ANALYSIS
2.1.2 CLIMATE FORECASTING
2.1.4 FLOOD PLAIN MANAGEMENT
2.1.5 ENFORCEMENT OF STANDARDS AND CODES
2.2 TYPES OF FLOODS
2.2.1 PLUVIAL (SURFACE) FLOODING
2.2.2 RIVERINE FLOODING
2.2.3 GROUNDWATER FLOODING (GROUND FAILURE)
2.2.4 COASTAL FLOODING
2.3 METHODOLOGIES FOR FLOOD RISK MAPPING
2.3.1 HISTORIC APPROACH
2.3.2 GEOMORPHOLOGIC APPROACH
2.3.3 MODELLING APPROACH
2.3.3.1 GIS WITH HYDRAULIC/HYDROLOGICAL APPROACH
2.3.3.2 GIS WITH REMOTE SENSING APPROACH
2.4 REMOTE SENSING FOR FLOOD DAMAGE ASSESSMENT
3. STUDY AREA AND DATA DESCRIPTION
3.1 GEOGRAPHY
3.2 GEOLOGY AND SOILS
3.2.1 GEOLOGY
3.2.2 SOIL
3.3 CLIMATE
3.4 DRAINAGE
3.5 DATA SOURCE
4. METHODOLOGY
4.1 SPATIAL DATA PREPROCESSING
4.1.1 THE STREAM NETWORK (FLOW ACCUMULATION)
4.1.2 ELEVATION AND SLOPE
4.1.3 LAND USE/COVER
4.1.4 RAINFALL (PRECIPITATION)
4.1.5 GEOLOGY (SOIL TYPE)
4.2 MULTI CRITERIA ANALYSIS
4.2.1 THE ANALYTIC HIERARCHY PROCESS
4.2.1.1 PAIRWISE COMPARISON METHOD
4.2.2 THE WEIGHTED OVERLAY PROCESS
4.2.2.1 WEIGHTED OVERLAY ANALYSIS
4.3 SENTINEL-1A SAR IMAGERY FOR FLOOD DAMAGE ASSESSMENT
4.3.1 IMAGE PRE-PROCESSING
4.3.1.1 IMAGE CALIBRATION
4.3.1.2 GEOMETRIC CORRECTION
4.3.1.3 SPECKLE FILTERING
4.3.2 BINARIZATION
5. RESULTS AND ANALYSIS
5.1 MULTI CRITERIA ANALYSIS RESULTS
5.2 SAR-BASED FLOODING EXTENT MAPPING
5.3 DISCUSSION
6. CONCLUSION
REFERENCES

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