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Introduction
The geological processes such erosion and landslides are common in the youth stage of geomorphic development of an area. In the upstream direction denudation is intense, and is triggered by intense rainfall and flooding, thereby accelerating the processes of erosion and consequent deposition of sediment load in low lying areas and reservoirs. The area with high rising hills, waterfalls and coffee plantations is also suffering from losses due to the effects of land degradation by floods and mass movement of the earth.
A natural hazard is a damaging phenomenon within a specified period and region due to a set of existing or predicted conditions that can cause loss of life as well as property. Floods are the overflow of water beyond its natural limits, mainly due to heavy rainfall or snowmelt and sometimes because of collapsing of the dam. Other reasons include deforestation, climate change, etc. Kodagu district saw devastating floods and landslides in 2018-2019 including Madikeri, which were not only influenced by rain but also anthropogenic activities, which disturbed the equilibrium of the catchment.
Causes and type of Flood
Different types of flood have different effects in terms of their impact, damage, and cost, both economically and to the public. Floods are usually caused by heavy rainfall, snowmelt, when ocean waves come on shore, or when dams or embankments break. The different types of flooding include Fluvial flooding, Coastal flooding and Pluvial flooding.
Fluvial flooding occurs when rivers burst their banks as a result of sustained or intense rainfall Overbank flooding occurs when water rises overflows over the edges of a river or stream. Flash flooding is characterized by an intense, high velocity torrent of water that occurs in an existing river channel with little to no notice Coastal flooding is caused by extreme tidal conditions including high tides, storm flows and tsunamis.
Coastal flooding is categorized in three levels, minor where erosion takes place, moderate where erosion and public life is affected, and major where erosion and threat to life and property is seen.
Pluvial flooding occurs when an extremely heavy downpour of rain saturates drainage systems and the excess water cannot be absorbed. Intense rain saturates an urban drainage system, water flows out into the streets. Run-off flowing water from rain falling on hillsides that are unable to absorb water.
Study Area
Madikeri taluk is thickly wooded grandeur on the Western Ghats, is the most beautiful hill station of Karnataka. It occupies an area of 1,434.4 km² in the Western Ghats of south western Karnataka. It consists of forest and coffee plantations and agro-forestry is also seen. The main river in Kodagu is the Cauvery, which originates at talakaveri, located on the eastern side of the Western Ghats, and its tributaries drain the greater part of Kodagu district. In July and August rainfall is intense, and there are often showers into November. Yearly rainfall may exceed 4,000 mm in some areas. Average temperature of 15° C, ranging from 11 to 28° C, with highest temperature occurring in April and May.
MATERIALS AND METHODOLOGY
NDVI
The Normalized Difference Vegetation Index (NDVI) is a numerical indicator that uses the visible and near-infrared bands of the
electromagnetic spectrum, and is adopted to analyze remote sensing measurements and assess whether the target being observed contains live green vegetation or not. The NDVI algorithm subtracts the red reflectance values from the near-infrared and divides it by the sum of near-infrared and red bands.
DEM
Digital Elevation Model (DEM) is the digital representation of the land surface elevation with respect to any reference datum. Any digital representation of a topographic surface can refer through DEM data. It represents the simplest form of topography digitally. It is also used in determining attributes such as elevation at any point, slope, and aspect. Drainage basins and channel networks are also identified from DEMs
DRAINAGE DENSITY
Drainage density is defined as the ratio of the total length of streams in a watershed over its contributing area. The drainage density exerts on flood peaks significant controls which can be broadly divided between direct and indirect effects. The most significant direct effect there is the control associated with the length of the stream network and hillslope paths. If there is an increasing drainage density it implies increasing flood peaks. Moreover, a long concentration time implies more opportunities for water to infiltrate. Therefore ,decreasing drainage density generally implies decreasing flood volumes.
TOPOGRAPHIC WETNESS INDEX (TWI)
The locations with large upslope area receive a high index value and are expected to have relatively higher water availability than locations with a small upslope area that are assumed to have relatively lower water availability and therefore receive a small index value. Steep locations receive a small index value and are expected to be better drained than gently sloped locations, which receive a high index value.
The TWI is defined as ,
TWI = A/tanβ
where tanβ is the local slope of the ground surface and A - is the upslope area per unit contour length, a is also called specific upslope area and computed as a = A/L, where A [m2 ] is the upslope area and L [m] is the contour length. Factors contributing to TWI are fill, flow direction, flow accumulation, slope in degree, Radians of slope = (Slope in degree*1.570796)/90, Tan slope =con (slope>0, tan(slope), 0.001), Flow accumulation scaled= (flow accumulation+1) *cell size ,TWI=Ln(Flow accumulation scaled/Tan slope).
LAND USE AND LAND COVER (LULC)
Land use and land cover changes play an essential role in the studies of regional, local and global environments. Land cover refers to how the earth's surface is covered by forests, wetlands, impervious surfaces, agricultural and other types of land and water. Land use refers to how humans use the landscape, whether for development, conservation, or mixed uses. Land use includes recreation areas, wildlife habitats, agricultural land, and built-up land. Biophysical attributes, the global climate and ecosystem activities are associated with significant changes in the land cover.
DISTANCE FROM ROAD AND RIVERS
Construction of roads beside a slope results in some tensions, load decreasing on the slope heels and landslides happening eventually. Therefore, this is taken as a proximity factor. Hydrological condition of the area or the saturation degree of the soil on slopes has been defined as a stability contributing parameter. Water infiltration in soil, runoff and groundwater flows all happened under the hydrological circumstance. Closeness of the slopes to rivers lines may affect its stability, because the proximity to rivers would activate the erosion process.
GIS-AHP MULTI-CRITERIA ANALYSIS
Flood hazard can be assessed using multi-criteria analysis methods. Analytic hierarchy process is the most widely used multi-criteria analysis method and has been applied to wide range of scientific fields. In order to calculate the relative weight of each factor, correlation among factors influencing the flood hazard must be considered. Relative weight estimation of each factor is a direct method and pairwise comparison. After calculating weights, it is necessary to calculate consistency ratio (CR) to check the degree of consistency of judgements that has been made by any expert.
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The formula of getting CR value: CR = CI / RI * 100
where,
>> CR is the consistency ratio
>> CI is the ratio of consistency index
>> RI is the random inconsistency index of a randomly generated pairwise comparison matrix of order 1 to 10.
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CI = (λ – η) / (n - 1)
>> CI is the consistency index,
>> λ is simply the average of consistency vector
>> "n" refers to the number of total criteria
The above analysis has focused on obtaining the risk values for the region of Madikeri town. Identifying flood prone areas has helped decision makers to reduce flood related damages when occurred and take necessary steps to act on during the disaster. The above analysis was carried out through an analytical hierarchy method, which included drainage density (16%), distance from rivers (14%) and TWI (14%) as its main factors with high weighting percentage. The regions having high drainage density were observed to have very high risk of floods, this is usually along the path of the river. Therefore, regions with a close distance to the river also seem to have very high risk values. Even the elevation along this regions are very low, which is susceptible to floods.
REFERENCES
B, Pallard & Castellarin, Attilio & Montanari, Alberto. (2008). A look at the links between drainage density and flood statistics. Hydrology and Earth System Sciences Discussions. 5. 10.5194/hess-13-1019-2009.
Dikpal, Ramesh. (2022). Landslide Susceptibility Zonation mapping of Madikeri Taluk, Kodagu district of Karnataka, India using Remote Sensing and Geographic Information System Techniques. INTERNATIONAL JOURNAL OF SCIENTIFIC RESEARCH IN ENGINEERING AND MANAGEMENT.
06. 10.55041/IJSREM11464.
Gn, Vivekananda & Swathi, R & Sujith, AVLN. (2020). Multi-temporal image analysis for LULC classification and change detection. European Journal of Remote Sensing.
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Guha, S., & Govil, H. (2020). Land surface temperature and normalized difference vegetation index relationship: a seasonal study on a tropical city. SN Applied Sciences, 2(10), 1661.