The inundation of the coastal area along the Chellanam hamlet on the southern coast of India during the Tauktae cyclone was in the limelight as several households, roads and public facilities were severely affected. XBeach model in surfbeat mode is simulated from May 13 to May 17, 2021. Figure 2 shows the validation of significant wave height (Hs) at an offshore and coastal buoy location during Tauktae which ensure the correctness of the wave boundary conditions given to Xbeach model. Wave model was able to simulate the cyclone-induced high waves in the AS
The significant wave height of the longwave component (Hln) is estimated as described in section 4.2.2 for the point locations shown in Fig. 1. Figure 3 (b and c) show the time series Hln estimated for the point locations at regions A and B and the offshore wave condition is plotted in Fig. 3a. A notable increase in the Hln can be observed from 14:00 hrs on May 14 that continued till 10:00 hrs on May 15, 2021 specifically at point locations near the coast. In both regions, Hln peaks to about 0.3 m near the coast. The relative increase in Hln corresponds to the time when high waves (Hsh, Fig. 3a) generated by cyclone Tauktae reached the coast of Chellanam. The amplitude of the long wave is approximately proportional to the height of the incident short wave and independent of the period[22]. Figure 4 shows the change in the significant wave height of Hsh and Hln from the offshore boundary to the coast during the highest wave event of the cyclone impact. While approaching the coast, the energy of the short wave gets dissipated, and the wave height is reduced, whereas the wave height of the longwave component increased from negligible height at the boundary to more than 0.3 m towards the coast. The coastward increase in infragravity waves is due to the shoaling and superposition of incoming and reflected infragravity waves[23] and the energy of gravity waves is significantly reduced due to refraction, bottom friction and breaking.
The momentum of the waves is transferred to the water column at the surf zone, which leads to an increase in the water level called the wave setup. The maximum water level due to the wave setup from May 14, 14:00 hrs to May 15, 10:00 hrs is shown in Fig. 5a, and the spatial distribution of maximum significant wave height during the same period is shown in Fig. 5b. Along the coastal zone, the significant wave shows spatial variability, where the coastline in region A is impacted with high waves compared to the region marked as B. Spatial variation in the water level due to wave setup (Fig. 5a) is prominent along the coast. The water levels are high on the northern coast (location marked as A) and at the location marked as B, the highest water level falls at a distance away from the coast. The maximum water level averaged over ten cross-shore profiles at locations A and B are shown in Fig. 6. At location A, the water level forms a steep slope in the coastal region with a maximum elevation of over 0.6 m reaching close to the shoreline. Even though the maximum elevation of wave setup at location B is around 0.6 meters, the elevation near the shoreline is about 0.4 m. It can be observed that after reaching the maximum elevation at a distance of about 1 km from the shoreline, the water elevation gradually drops out to around 0.4 meters at the shoreline. [24] established an empirical relationship for wave setup that is proportional to the beach slope. The cross-shore bathymetry profile steeps at location A and the bathymetry profile is gently slopping at B, with respective slopes of 0.22 and 0.08.
The impact of the Tauktae cyclone at the Chellanam coastal region occurred during the low tidal condition and might have probably elevated the wave setup. During high tide, large waves reach the shore without breaking, resulting in a reduced height of wave setup, and during low to mid-tide, the wave setup gets pronounced[25]. Figure 7 shows a similar result, where the predicted tidal condition at Chellanam is overlaid with the hourly averaged surface elevation near the shoreline of the region marked as A. The peaks in the wave setup coincide with the low tidal or mid-tidal conditions at Chellanam. During high tides, the waves at Chellanam probably reach the coastal protection measures without breaking and low tide favours the wave breaking near the shoreline inducing wave setup and elevated water level at the coast.
Figure 8 shows the maximum inundation extent during the period overlaid on Google Earth. Even though the waves overtopped and inundated the entire coastline, the landward inundation is maximum to the northern part of the domain. The inland inundation reached about 300 meters in the northern part. Reports during the Tauktae cyclone have confirmed the inundation of the coastal road around 300 meters away from the shoreline at places.
The wave overtopping at Chellanam is inferred to have a critical dependency on the coastal bathymetry slope, formation of infra gravity waves and the elevation of coastal waters due to the wave setup. In the surfbeat mode, XBeach has simulated the long period infragravity waves that peaked its significant wave height to over 0.3 meters. The significant wave height of the long wave components increases as the waves propagate to the shoreline. The time-averaged surface elevation of coastal waters or the wave setup and the varying elevation over the mean surface due to the components of short waves and long waves contribute towards the maximum wave runup that leads the storm waves to move up the beaches or structures. The maximum elevation of coastal waters at the shoreline reached about 0.7 m, and the spatial variation observed is critically controlled by the bathymetric profile. The temporal variability is influenced by the incoming short and long waves along with the tidal conditions. Coastal regions with steep bathymetry have elevated wave setup at the shoreline and have facilitated the inundation of low-lying regions of the coastal area. In coastal regions with a steep slope, the coupled interaction of longwave and short waves modulated the fluctuations above the high wave setup at the shoreline that created extreme wave runup, flooding the inland regions of the coastal area. In coastal regions with a gentle slope, the storm wave energy is dissipated significantly, and the longwave component is dominated. With reduced wave setup in these regions, the infragravity waves overtopped the coastal protection structures with runup along the narrow band of coastal stretch, which induced damage to the properties.