CHAPTER 8 ASSESSMENT OF COASTAL VULNERABILITY INDEX

Transcription

1 124 CHAPTER 8 ASSESSMENT OF COASTAL VULNERABILITY INDEX 8.1 INTRODUCTION In order to assess the vulnerability of the shoreline considered under this study against the changing environmental conditions, an index called Coastal Vulnerability Index (CVI) has been adopted. CVI was also used to map relative vulnerability of the coast to future sea-level rise The CVI ranks the following in terms of their physical contribution to shoreline change: geomorphology, regional coastal slope, shoreline change rates, mean tidal range and mean wave height. Rankings for each variable were combined and an index value calculated for 1-minute grid cells covering the entire shoreline of study area. This approach combines the coastal system's susceptibility to change with its natural ability to adapt to changing environmental conditions, yielding a quantitative, although relative, measure of the shoreline s natural vulnerability to the effects of coastal dynamic eco system. The CVI provides an objective technique for evaluation and longterm planning by scientists and administrators. An accurate and quantitative approach to predict coastal change is difficult. Even the kinds of data necessary to make shoreline response predictions are the subject of scientific debate. A number of predictive approaches have been proposed (National Research Council, 1990), including: 1) extrapolation of historical data (e.g., coastal erosion rates), 2) static inundation modeling, 3) application of a

2 125 simple geometric model (e.g., the Bruun Rule), 4) application of a sediment dynamics/budget model, or 5) Monte Carlo (probabilistic) simulation based on parameterized physical forcing variables. However, each of these approaches has inadequacies or can be invalid for certain applications (National Research Council, 1990). Additionally, shoreline response to ecosystem change is further complicated by human modification of the natural coast such as beach nourishment projects, and engineered structures such as seawalls, revetments, groins, and jetties. Understanding how a natural or modified coast will respond to dynamic ecosystem change is essential in preserving vulnerable coastal resources. The primary challenge in predicting shoreline response is quantifying the important variables that contribute to coastal evolution in a given area. To address the multi-faceted task of predicting shoreline change impact, the USGS has implemented a methodology to identify areas that may be most vulnerable (Hammar-Klose and Thieler 2001). This technique uses different ranges of vulnerability (low to very high) to describe a coast s susceptibility to physical changes in coastal eco system. The vulnerability determined here focuses on six variables which strongly influence coastal evolution: 1. Geomorphology 2. Historical shoreline change rate 3. Regional coastal slope 4. Mean significant wave height 5. Mean tidal range 6. Average sea level rise These variables can be divided into two groups: 1) geologic variables and 2) physical process variables. The geologic variables are geomorphology, historic shoreline change rate, and coastal slope; they account for a shoreline's

3 126 relative resistance to erosion, long-term erosion/accretion trend, and its susceptibility to flooding, respectively. The physical process variables are significant wave height and tidal range, all of which contribute to the inundation hazards of a particular section of coastline over time scales from hours to centuries. A relatively simple vulnerability ranking system (Table 8.1) allows these variables to be incorporated into an equation that produces a coastal vulnerability index (CVI). The CVI can be used by scientists and decision makers to evaluate the likelihood that physical change may occur along a shoreline. Additionally, CVI enables the incorporation of information provided by this vulnerability assessment technique into general management plans. Table 8.1 Vulnerability Ranking Methodology by USGS (2004) Vulnerability Ranking Variable Geomorphology Shoreline Erosion/ Accretion m/year Very Low 1 Rocky Shore Low 2 Medium cliffs, Indented coasts Moderate 3 Low cliffs, Glacial drift, Alluvial plains High 4 Cobble Beaches, Estuary, Lagoon Very High 5 Barrier beaches, Sand beaches, Salt marsh, Mud flats, Deltas, Mangrove, Coral > < -2.0 Near Shore Slope > < 0.30 Mean Wave Height < > 1.25 Mean Tide Range > < 1.0 Relative Sea level Change mm/ year > 3.4

4 CVI FOR THE PRESENT STUDY USGS has used this CVI for assessing coastal vulnerability against future sea level rise. But for the present study, the assessment of Coastal Vulnerability Index based on Coastal processes, the following modifications have been done from the CVI assessment done by the USGS (2004). 1. Since the CVI is assessed with respect to coastal processes and the relative sea level rise per year is insignificant in the present study area, the parameter sea level rise has not been considered for the assessment of CVI for the present study. 2. For the present study, near shore coastal slope has perfect negative correlation with shoreline oscillation; the CVI assessment methodology with respect to near shore slope has been modified. 3. CVI assessment methodology with respect to mean tide range has been modified. The coastal Vulnerability Index based on shoreline oscillation depends on five variables namely, 1. Geomorphology, 2. Rate of shoreline erosion and accretion (m/year), 3. Near shore slope 4. Mean wave height 5. Mean Tidal range. The selected ranges of coastal vulnerability Index based on shoreline oscillation for the present study has been illustrated in the Table 8.2.

5 128 The coastal vulnerability index (CVI) provides an insight into the relative response of shoreline to its surrounding dynamic system. The data presented here can be viewed in at least two ways: 1) as an example of where physical changes are most likely to occur as shoreline oscillation; and 2) as a planning tool for the Cauvery Delta shoreline. As ranked in this study, geomorphology, shoreline change, and significant wave height are the most important variables in determining the CVI for the Cauvery Delta coastline. Wave height, tide range, does not contribute to the spatial variability in the coastal vulnerability index. The shoreline under study preserves a dynamic natural environment, which must be understood in order to be managed properly. The CVI is one way that a shoreline can be assessed objectively the natural factors that contribute to the evolution of the coastal zone, and thus how the same may evolve in the future. 8.3 DATA RANKING Table 8.3 shows the five variables described in the above and include both quantitative and qualitative information. Actual variable values are assigned a vulnerability ranking based on value ranges, whereas the nonnumerical geomorphology variable is ranked qualitatively according to the relative resistance of a given landform to erosion. Shorelines with erosion/accretion rates between -2.0 and +2.0 m/yr are ranked as moderately vulnerable. Increasingly higher erosion or accretion rates are ranked as correspondingly higher or lower vulnerability. Regional coastal slopes range from very high risk (>1.2 percent) to very low risk (<0.3 percent). Mean wave height rankings range from very low (<1.0 m) to very high (>1.75 m). Tidal range is ranked such that 4 micro tidal (> 0.95 m) coasts are very high vulnerability and micro tidal (< 0.50 m) coasts are very low vulnerability. For the present study, CVI ranking for shoreline oscillation, near shoreslope, mean wave height and mean tidal ranges has been modified from

6 129 that of USGS as per field data range. For all the parameters, the respective data range has been divided into five ranges and ranked as very low, low moderate, high and very high vulnerability and assigned weightages as 1, 2, 3, 4 & 5 respectively. This modification helps to assess the vulnerability within the data range obtained in the field work and ensures the computation of weighted CVI for the study area. Table 8.2 Modified Vulnerability Ranking Methodology based on Coastal processes for the present study Vulnerability Ranking Variable Very Low 1 Low 2 Moderate 3 High 4 Very High 5 Geomorphology Barrier beaches, Medium Cobble Low cliffs, Sand beaches, Rocky cliffs, Beaches, Glacial drift, Salt marsh, Mud Shore Indented Estuary, Alluvial plains flats, Deltas, coasts Lagoon Mangrove, Coral Shoreline Erosion/ Accretion m/year > < Nearshore Slope < > 1.20 Mean Wave Height < > 1.75 Mean Tide Range < > 0.95 The coastal stretch selected for the present study has been divided in to 1 interval grids and each grid has been assigned the vulnerability rank for each parameters based on above table. Then the coastal vulnerability index for each grid (Table 8.4) has been obtained as, CVI a * b * c * d * e 5 (8.1) where, a = Rank of geomorphology, b = Rank of shoreline erosion/accretion rate, c = Rank of coastal slope, d = Rank of mean wave height, and e = Rank of mean tide range

7 130 Table 8.3 Coastal Parameters of Study area in 1 interval grid Latitude Geomorphology Coastal Parameters Rate of shoreline Oscillation (m/year) Nearshore Slope (%) Sandy Beach Sandy Beach Sandy Beach Estuary Estuary Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Estuary Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Estuary Sandy Beach Sandy Beach Sandy Beach Estuary Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Estuary Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Sandy Beach Mean Wave Height (m) Mean Tidal range (m)

8 131 Table 8.4 Evaluation of Coastal Vulnerability Index Latitude Geomorphology Assignment of Rank for Each Parameters Rate of Mean shoreline Nearshore Wave Oscillation Slope (%) Height (m/year) (m) Mean Tidal range (m) CVI

9 132 (a) For the stretch from Kollidam to Poompuhar Figure 8.1 (Continued)

10 133 (b) Stretch from Poompuhar to Nagapattinam Figure 8.1 Coastal Vulnerability Index of Study Area

11 Percentage Low Vulnerable Moderately Vulnerable High Vulnerable Very High Vulnerable Figure 8.2 Histogram of Percentage of Cauvery Delta Shoreline in each CVI Vulnerability Category Table 8.3 illustrates various coastal parameters of study area in 1 interval grid which are required to estimate the CVI and Table 8.4 shows the assignment of rank for each parameter and computation of CVI for the study area in the same interval a mentioned earlier. The calculated CVI values for the study area from Kollidam rivermouth to Nagapattinam Port range from to The mean CVI value is 4.484; the mode and the median are and 4.64 respectively. The standard deviation is The 25th, 50th, and 75th percentiles are 3.464, and 5.367, respectively. Figure 8.1 shows a map of the CVI divided into ranges (low very high) for study area from Kollidam river mouth to Nagapattinam Port. CVI values were divided into low, moderate, high, and very high-vulnerability categories based on the quartile ranges and visual inspection of the data. CVI values below 4.00 are assigned to the low vulnerability category. Values from

12 to 4.50 are considered moderate vulnerability. High-vulnerability values lie between 4.51 and CVI values above 5.00 are classified as very high vulnerability. Figure 8.2 shows a histogram of the percentage of shoreline in each vulnerability category. Nearly 71 km (45 miles) of shoreline is evaluated along the Cauvery delta seashore. Of this total, 27 percent of the mapped shoreline is classified as being at very high vulnerability to shoreline oscillation. Twenty seven is classified as high vulnerability, seven percent as moderate vulnerability, and thirty nine percent as low vulnerability. Further it is suggested that for the shoreline stretches having very high and high coastal vulnerability, provision of rubble mound sea walls or tetrapods or development of thick vegetative cover may protect the coast from adverse oscillation and thereby ensuring safe living of the surrounding habitants.