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Book Review

Working with the coast

Muthusankar, G., Jonathan, M. P., Lakshumanan, C., Roy, P. D., & Srinivasa-Raju, K. (2017). Coastal erosion vs man-made protective structures: evaluating a two-decade history from southeastern India. Natural Hazards, 85(1), 637-647.

BACKGROUND:

The edges of the continents are vulnerable to change. Their proximity to the ocean makes them susceptible to the forces of wave and tidal energy.   Beaches are especially susceptible to change because they are comprised of loose sediment, more commonly known as sand. The processes that result from the waves swashing on a beach are called erosion and accretion. Erosion is when waves crash on the beach with so much energy that the water picks up the sand and carries it along the beach.   Accretion is when the energy runs out and the sand falls back to the beach in a new location than it started in. The new location that the sand lands in may be down the beach horizontally from the original position.   This is because it sand particles follow the path of the wave, and the wave’s path on the beach is not symmetric. When a wave crashes on a beach it approaches at an angle, but when it reaches its turning point it follows the beach slope.   The term for this is called ‘longshore drift’ (figure 1).   Sand can also be transported offshore in rip currents, which are concentrated channels of flow parallel to the slope of the beach.

Figure 1: Longshore drift (source: https://commons.wikimedia.org/wiki/File:Longshoredrift.gif)

Figure 1: Longshore drift (source: https://commons.wikimedia.org/wiki/File:Longshoredrift.gif)

In attempts to control the dynamic coast humans install stationary structures like seawalls or groins. Seawalls are structures parallel to the coast and groins are structures perpendicular to the coast. These structures modify wave direction and impact erosion/accretion by mitigating wave energy. Although, moderately effective at controlling sediment transport for years to decades, they are not a permanent solution.   Oversight and maintenance is required to ensure that the erosion and accretion around the structures does not get out of hand. Every so often sediment that accumulates in a channel must be dredged (dug out) and placed somewhere else, such as on a beach that needs to be replenished. Monitoring stationary structures and the sediment dispersion they cause is also important because there can be dire consequences for the ecosystems in the area.

On the coast of two states in southeast India, Puducherry and Tamil Nadu (figure 2), construction of stationary structures has been used to inhibit shoreline modification by natural processes.   Construction on the coast was first implemented in 1735 when the French built a wall that was 2 kilometers long and towered 9 meters above sea level.   It was estimated that annually, between 1735 to 1986, wave, tidal, and current energy transported ~1 million cubic meters of sand from the southern beaches to the northern beaches and .~4 million cubic meters from the northern beaches to the southern beaches.   In other words, the northern beaches received double what they lost (net accretion), and the southern beaches only received half of what they lost (net erosion).

Figure 2: southeast India states of Tamil Nadu and Puducherry, harbor circled in red (source: google maps, modified)

Figure 2: southeast India states of Tamil Nadu and Puducherry, harbor circled in red (source: google maps, modified)

Between 1986 and 1989, a harbor was constructed in the estuarine region of Ariyankuppam River in Puducherry. Also built were a 350-meter long seawall and groins to prevent erosion and limit coastal drift. The structures reduced current speed by nearly half, subsequently preventing the transport of sediment from the south to the north.  In anticipation of a change in sediment transport and the eventual need to deliver sediment to the northern beaches, the project included a sand bypass system. Unfortunately, due to financial limitations, the system was only used between 2000 and 2001; during that time beaches in the north that had eroded started to reappear.

In 2002, more remedies to the shoreline were added to protect the northern beaches from erosion.   One method used was beach nourishment; sand that had accumulated in the harbor was dredged and placed in the north. This solution is only temporary, especially if a large storm with excess wave energy reaches the coast because it can easily displace all of the relocated sand.

In 2004, a 7-kilometer seawall and multiple groins were constructed to further mitigate the dynamic coast.  Even with the added structures, nearly .4 million cubic meters of sand was being deposited in the harbor annually. Another way to think of this is that .4 million cubic meters of sand was unable to reach the northern beaches each year. Not surprisingly, a 2010 report stated that the groins inhibited sediment transport and deposition.

METHODS:

Figure 3: Puducherry state and study area in yellow (source: google maps, modified)

Figure 3: Puducherry state and the study area (yellow line) (source: google maps, modified)

Scientists sought to quantify the impact of the man-made structures on the coast (figure 3). They used remote sensing, GIS imaging, and field observations from the past twenty years to assess movement of the shoreline and the rates of sediment erosion and accretion. The study focused on a twenty-year period split into three time frames, each bounded by modification actions taken by humans.   The first period is between 1991-2000, encompassing the period of time between when the harbor was completed to the implantation of the bypass system.   The second period spans from 2000-2005, to include the usage of the bypass system to when additional seawalls and groins were used. The third period is between 2005-2011, and represents the time following the construction.  Rates were calculated by dividing the distance that the shoreline moved by the time elapsed between the oldest and newest shorelines.

RESULTS:

Based on their calculations and observations scientists were able to confirm that remedial actions do have a role in coastal processes. They observed variations in erosion and accretion in between various steps of remediation, such as bypass system nourishment and structure erection (table 1).

At the northern beaches, erosion decreased with each step of remediation between 1991 and 2011. The total erosion to the shoreline each year decreased to one third of the 1991-2000 value of .238 square kilometers to .08 square kilometers.   The erosion rate decreased from .024 square kilometers per year to .019 square kilometers per year. Scientists speculate that the construction of a large seawall and groins in 2005 shifted erosion from north beaches into the adjacent Tamil Nadu State, which may account for the decrease in rates calculated.

In the southern beaches, accretion rates remained the same at .019 square kilometers per year, with each action taken.   The total accretion to the shoreline decreased by nearly half between the first two periods, and then increased slightly in the third period. The changes are attributed to the impacts of briefly using the bypass system.

Table 1: quantified estimates of erosion and accretion

(km^2/year) 1991-2000 2000-2005 2005-2011
Northern beach erosion rate -0.024 -0.02 -0.019
Southern beach accretion rate 0.019 0.019 0.019
Total Erosion Northern shoreline -0.238 -0.087 -0.08
total Accretion Southern shoreline 0.185 0.094 0.108

CONCLUSION:

All in all, researchers concluded that in the past twenty years, erosion has been the dominant process north of the harbor and accretion has been the dominant process south of harbor.   They found that since the construction of the harbor the area impacted by erosion was reduced at a rate of .24-.013 km^2/year). The area of accretion was found not have to changed in the 20 year period.

GREATER IMPORTANCE:

This projects and other projects similar to it are important for understanding how much humans can expect to mediate coastal processes. This study found that after a period of twenty years there were able to effectively reduce the area of erosion while maintaining accretion, but that they were not able to restore the system to its dynamics before the harbor was constructed.   Knowledge of the limited control humans have on coastal processes enables developers to make conscientious decisions about seaside construction. It also enables humans to speculate what changes will occur, as well as postulate and put into place cautionary remedies that help protect the environment.

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