RIVER BANK PROTECTION USING POLYMER GABIONS – A CASE STUDY ON THE RIVER TAPI, INDIA

Rivers, with their dynamic nature, often pose challenges to infrastructure along their banks. The case of the Tapi River in Gujarat, India, brought to light the need for a robust solution to counter erosion and flooding, especially in an industrial complex. This article delves into the problem faced, the design and mitigation approach adopted, challenges encountered, and the innovative solution implemented for river bank protection using polymer gabions.

The Problem

The entire complex HMD, Gujrat is on the banks of river Tapi which rests on a stratum of deep black soils. Due to tidal variations and flood during monsoons, around 1 km long water front in the area has been observed under constant erosion. Inspite of temporary protection works, a permanent protection measure was needed to prevent erosion and restrict ingress of flood water into the plant area.

Design & Mitigation Approach

A comprehensive site analysis was done. Studies were carried out by sinking 7 boreholes to a depth of 15 m and 7 driven boreholes at depth for a depth of 2 m deep from the existing ground at selected locations. As per the new survey report, the maximum current velocity is in the order of 5 m/s. After studying the GI report, the boreholes which were found critical having layered soils were considered for PLAXIS analysis.

A comparison study between PP rope gabion and metal gabion under water and marine constructions concluded that since polymer is inert to corrosive marine environment and thus preferred for coastal protection works. Gabion ropes were included because of their robustness and high performance under water and wherever there is no direct contact with UV rays.

To protect the toe of the embankment, 3 rows of 150 mm hollow circular pipes of length 7 meters are placed along the toe gabions at 500 mm c/c. The steel pipes were used as anchorage and polypropylene tarred rope gabions filled with boulders/geobags shall be placed between the steel pipe.

Since this comes in seismic zone 3, several stability analyses were conducted and has high tidal conditions as well, a suitable design was prepared. GTFL’s composite solution comprised of PVC coated metal gabion revetments, geobags, polymer gabions for this project. The pre-filled gabions were placed in position under water with the help of boats and hydra of suitable boom length. PP gabions used were manufactured using ropes of diameter ranging from 9 to 16 mm having adequately stabilized for degradation against ultraviolet radiation.

Challenges encountered

The uneven river bed profile and high tidal conditions were some of the major challenges to overcome. One of the biggest challenges in the construction was to deal with tidal variation, as the site is in tidal zone, as well as a complete offshore application. Our team would get only 3 to 4 hours of effective working every day and it required detailed planning of resources to execute on time.

It was the team of highly skilled divers that helped in the placement. Flexible nature of the polymer gabion boxes could stabilize smoothly on a highly uneven surface, thus proving the choice of this material for apron construction was appropriate. The pre-filled gabions were placed in position under water with the help of a hydra and boats of suitable boom length.

GTFL team had also installed an Acoustic Doppler Current Profiler to measure the velocity of water and to monitor the performance of the river bank protection installed. An acoustic doppler current profiler is a device that uses sound waves to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. It can be used for oceanography, river discharge, wave and turbulence measurements, and submarine navigation. It works by transmitting and receiving pings of sound that reflect off the moving particles and calculating the frequency shift of the echo.

By analyzing the frequency shift of the returned echoes, the ADCP can calculate the speed and direction of the water flow at different depths.

On a final note:

The design of Tapi river bank protection works was novel in its preference for a “soft” approach in contrast to the traditional solution using sheet piles. This novel design was backed up with proper execution methodology to create a “bio-engineered” structure, withstanding operational challenges, having paramount construction standard which requires very low maintenance and blends over time with its surroundings leaving very little environmental impact.

About Garware Technical Fibres Limited:

Garware Technical Fibres Ltd. is one of India’s leading players in the technical textiles sector. Established in 1976, the company today is a multi-divisional, multi-geographical technical textiles company and is known for providing world class innovative solutions in high performance aquaculture cage nets, fishing nets, sports nets, safety nets, agricultural nets, coated fabrics, polymer ropes and geosynthetics.

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