All the factors contributing the causes of flooding were delineated & examined, drainage, soil types, slope, landuser/land cover, and geomorphology, their results are discussed intensively as follows:
Soil mapping: Soil texture is the "feel" of the soil when a moist quantity is manipulated between thumb and forefinger. It is one of the more useful tests in evaluating soil. Sands because of their large grain size allow faster permeability of water than clays. Sands hold very little water. Loam soils contain sand, silt and clay in such proportions that stickiness and non-adhesiveness is in balance - so the soils are mouldable but not sticky. The infiltration rate is the velocity or speed at which water enters into the soil. It is usually measured by the depth (in mm) of the water layer that can enter the soil in one hour. Loam soil has an infiltration rate of 10–20 mm/hr, while Sandy-loam soil has infiltration rate of 20–30 mm/hr. these are the two soil texture in the study area while the third is creeks alone the river banks.
Digital Elevation Model: The geomorphologic relief and elevation analysis is an important factor and central to this study. The Digital Elevation Model (DEM) used for this work, was 15 m spatial resolution. It was generated from digitized contour with 50 feet interval extracted the SRTM data. Digital Elevation Model and the output revealed relief to varying in height from less than 15.57 m to 89.64 m. From analysis, 15 settlements ‟ lies within 15.57–25.58 meters above mean sea level, 5 settlements are within 26.59–46.60 meter above sea level, and 11 settlements are above 47.61–89.64 meters above sea level.
Slope and Elevation: Slope discovers the steepest downhill slope for a position on a surface. The Slope command captures an input surface raster as well as calculates an output raster containing the slope at every cell. The decrease in the slope value, the flatter the terrain, and the increase in the slope value, the steeper the terrain the output slope raster may be calculated as percentage slope or degree of slope. Flooding occurs due to slope failure, a stable slope is one whose resisting forces are more than the downslope driving forces, Flood occurs when gravity and other driving forces exceed the strength of slope materials (rocks and soil) in slope, majority of the slides happen in the category Moderate, Moderately Steep and steep. High slopes are often associated with low vulnerability while low slopes have an increased level of flood vulnerability. Slope length and steepness influence the movement/velocity and speed of water down the slope of the settlement are measured in percentage Unit: (%) < 0–2.0, 2.0–3.4, 3.4–5.1, 5.1–7.4, and 7.4–8.6.
The factor in deriving hydrologic characteristics of a surface is the ability to establish the trend of flow from each cell in the raster. This is done with the Flow Direction tool. This tool captures a surface in the form of input and outputs raster showing the trend of flow in every cell. If the Output drop raster option is selected, an output raster is formed showing a ratio of the highest alteration in elevation from every cell along with the trend of flow to the path length amid centers of cells which is shown in percentages. If the Force the entire edge cells to flow outward option is selected, the entire cells along the edge of the surface raster will flow outward from the surface raster. Figure 4.4 is the flow direction grid model of the research area created using DEM. The flow direction map shows variations in the slope of the research area, with values ranging from 1-128 indicated with shades of colors from blue to yellow. The blue color region shows areas of very low elevation while the blue is the area of fairly high altitude.
Land use/land cover: Each composed image was ordered into 4 classes: built-up areas, vegetation, Roads, and water body. The outcome of the classifications of land cover is found in Fig. 4.5. The classification and quantification of images of the study zone (which covers an aggregate of land area of 759.46 square kilometres (km2) were necessary for the detection of changes in various LULC observed within the study area and over the study period. Thus, the fixed LULC dispersal for the study year was in 2014. The table reveals that, Vegetation occupies the largest area with 355.74 km2of the entire study area, while built-up covered 275.94 km2, water body covered 125.47 km2 and roads occupied the least area of 2.31 km2.
Flood risk mapping: In 2007 the Intergovernmental Panel on Climate Change (IPCC) projected a high end estimate of 60 cm (2 ft) through 2099, but their 2014 report raised the high-end estimate to about 90 cm (3 ft). A number of later studies have concluded that a global sea level rise of 200 to 270 cm (6.6 to 8.9 ft) this century is "physically plausible". A conservative estimate of the long-term projections is that each Celsius degree of temperature rise triggers a sea level rise of approximately 2.3 meters. Elevation of raster is essentially the grid of cells, each cell containing an elevation above mean sea level, so we want to know what will be flooded, so if we assume that the sea level will rise by 15ft in future for a variety of reason like increase in temperature, more than 50% of some of the parts of Okrika will experience frequent flooding in years to come. While less than 50% of parts of Bonny and Ogu/Bolo will experience flooding in years to come.