01. THE PROBLEM:
With effect from activation of the southwest monsoonal wind, there rises the cry of coastal fisheries communities for their houses being washed off by the sea. This is a natural happening and a land of about 3-4 square kilometers from the island is annually claimed by the sea.
The reservation of 100 meters from the water periphery is just a measure of saving lives and properties of the coastal communities but the land is so simply neglected to be washed off.
Therefore it has to be recognized as a National Need of the country to protect the coastal belt at least by the name of conservation of the map of Sri Lanka.
02. FAILURES IN GENERAL PRACTICES:
Dumping of rock boulders upon the sandy bank lined with a porous membrane is the common engineering practice but there are some disadvantageous effects which stand against sustainability of the process such as;
I. Less durability of the application: It has been observed moreover that boulders are shifted in to the sea due to active earth pressure from bank side under surcharge loading conditions
II. The difficulty of howling and placing boulders on sea banks which are not easily accessible
03. THE WALL RAKE TECHNIQUE FOR SEA BANK PROTECTION:
Economy durability and environment friendliness are the main organs of sustainability of a system and planers have to look for alternative scenarios when techniques in general practices are failing. This letter is aimed therefore to introduce a novel technique of sea and river bank preservation.
Sea bank protection is very difficult because any linear application along the periphery could be failed due to thunder blows of the impulsive wave strokes during activation of the monsoonal wind. Boulders could be rolled in to the sea and any retaining type of structures could be fallen.
Similarly a green protection alone cannot sustain because plants are uprooted by the impulsive wave strokes. Therefore a combination of both techniques such as a mangrove growth in a structural frame is introduced herein by the name WALL RAKE THECHNIQUE for sea bank protection.
FIGURE-1
As shown in the figure-1, walls in the rake are built perpendicular to the bank and momentum of wave strokes is dissipated by a short cutoff wall at the bottom. A rubble buffer is applied as a protection against erosion and the green application above is supported by the buffer and walls in the rake.
FIGURE-2
Construction could not be difficult because concrete is just to dump into deep trenches cut by machinery or manually in the existing bank. The cutoff wall has to be anchored to the wall rake and holes have to be provided as shown in the figure-2, to release seepage from the landside.
FIGURE-3
The technique is more environment friendly than other techniques because it creates an aquatic ecosystem for plant and animal species to build their habitats through which the resilience of the system is improved.
04. ADVANTAGEOUS EFFECTS:
1. The multi-purpose applicability of the technique is the advantage in the first place. It can be applied as a protection upon earth banks at sea beaches, river banks at bends, and steep sloppy earth cuts in constructions etc.
2. The application is everlasting and no operation & maintenance cost required at all.
3. Stability against earth slips and landslides is the other advantageous effect by the technique.
4. Environment friendliness of the application is the biggest advantage and it creates a series of pocket aquatic ecosystems along the bank to improve resilience of the system.
Cyril H Thalpe Gamage
BSc (Eng) Sri Lanka, MSc (Env Eng)IHE-Netherlands, MIESL