Erosion Control: Riverbank protection solutions for Subansiri Lower HEP

Biswajit Basu, Director (Projects); Miren Kr Verma, General Manager, Subansiri, Lower Project; A. N. Mohammed, Part-time Consultant, Subansiri Lower HE Project, NHPC Limited

The river Subansiri, the largest tributary of the river Brahmaputra, has a total length of 375 km, draining over a basin of 37,000 square km. The river remains stable while flowing through the mountainous portion of the basin, but becomes unsteady upon entering the alluvial plains of Assam. The reason for the river becoming unstable may be attributed to a dramatic reduction in the slope as it enters the plain portion of the basin, which is mostly confined to the state of Assam. The construction of the 2,000 MW Subansiri Lower hydroelectric project (HEP) on the Arunachal Pradesh-Assam border at Gerukamukh in Dhemaji district has raised concerns about the adverse impact on the flora and fauna in the downstream portion, with serious concerns about the need to address the endemic problem of erosion of the alluvial regime of the river Subansiri. With the objective of protecting the downstr – eam of the Subansiri HEP, various measures have been suggested. These include provision of elevated embankments with proper protection measures from the dam site to about 15 km downstream to control the river discharge; river bank protection works up to 30 km downstream of the Subansiri Lower HEP; and strengthening and raising the embankments of the Subansiri river.

Design of bank protection structures

A river passing through populated/agricultural areas necessitates the protection of adjacent lands and properties threatened by erosion. In order to limit the movement of the bank of a meandering river, certain structures are constructed on the riverbank, which are called riverbank protection works. The purpose of bank protection may be training of the river, protection of adjacent land and properties, protection of nearby hydraulic structures such as embankments, among other things. Different measures have been adopted to reduce the flood/erosion losses and protect the floodplains. Some of the structural measures for flood management/erosion control to bring relief to the flood-prone areas by managing floods and thereby the flood levels include construction of flood embankments; channel improvement; watershed management; construction of spurs, groynes, etc.; construction of bank revetment along with a launching apron; and RCC porcupines and vetiver grass, geocells and geobags.

Riverbank/erosion control measures

Riverbanks when normally subjected to the direct attack of the river cause bank failures, leading to consequences such as washing away of soil particles from the bank by strong currents; undermining the toe of the bank by eddies, currents, etc.

The riverbank, therefore, requires protective measures against these failures and the commonly employed protective measures can be classified into:

Revetment mattresses to protect against erosive action of the river
Spurs/Groynes to deflect/dampen high velocity attacks on the embankment
Different grade control measures to tame a river flowing in steep terrain
Improving shear strength of embankment soil by growing shallow rooted vegetation.

Erosion protection measures for the Subansiri river: Design solutions

According to the comprehensive study of river morphology, erosion pattern and channel migration pattern of the river Subansiri by a team from IIT Roorkee, the total land lost due to erosion, excluding forest land, during the period 2008-10 till 2010-14 was found to be 3.382 km2 per year and 2.07 km2 per year respectively. For the years 2008 and 2010, the total land lost to erosion was found to be 6.764 km2 and channel abandonment/ deposition stood at 2.021 km2; the maximum value of bank retreat on the left bank was 634.799 m and the maximum value of channel deposition on the left bank stood at 666.102 m; the maximum value of bank retreat on the right bank was 741.5 m and the maximum value of channel deposition on the right bank stood at 789.599 m. Based on the analysis of several alternatives carried out with the objective to provide a solid protection system that can withstand critical hydraulic forces, including high velocity of the order of 5 metres per second or more, the IIT Roorkee 2013 Report suggested sack gabions filled with stones. – Other alternatives were sack gabions filled with stones; sack gabions lined with geotextile and filled with sand; geotextile bags filled with sand; and geotextile tubes filled with local material. These alternatives were analysed based on ease of installation, economics and stability.

For execution in dry conditions during the lean season

Provision of spurs at 250-300 m c/c either with boulder filled gabion boxes of size 3 m x 1 m x 1 m with sand filled geobags of size 1 m x 0.7 m placed in wire crates. The length of spurs is kept as 27-30 m into the river portion to divert the river flow away from banks and create pockets for siltation;
0.5 m thick and 7-10 m wide launching apron with boulder-filled gabion mattresses of 3 m x 1 m x 0.5 m;
1.0 m depth toe key with boulder filled gabion boxes of size 3 m x 1 m x 1 m;
0.5 m thick slop revetment with boulder filled gabion mattresses of size 3 m x 1 m x 0.5 m;
Placement of RCC jack jetties in two layers of three rows and two rows each along the launching apron to break the erosive flow of vortices and induce sedimentation along riverbanks;
1 m depth surface key with boulder filled gabion boxes of size 3 m x 1 m x 1 m.

For execution underwater with river depth at 2-3 m during the lean season

Provision of spurs at 250-300 m c/c either with boulder filled gabion boxes of size 3 m x 1 m x 1 m or with sand filled geobags of size 1 m x 0.7 m placed in wire crates. Length of spurs kept as 27-30 m in river portion to divert river flow away from the banks and create pockets for siltation;
0.96 m thick and 7-10 m wide launching apron with boulder filled sack gabions of size 2 m long x 0.96 m diameter;
1 m depth toe key with boulder filled gabion boxes of size 3 m x 1 m x 1 m;
0.5 m thick slope revetment with boulder filled gabion mattresses of size 3 m x 1 m x 0.5 m;
Placement of RCC jack jetties in two layers of three rows and two rows each along the launching apron to break the erosive flow of vortices and induce sedimentation along riverbanks;
1 m depth surface key with boulder filled gabion boxes of size 3 m x 1 m x 1 m.

For execution underwater with river depth at 3-4 m during the lean season

A launching apron is developed in layers with sand filled in geobags of size 2 m x 0.92 m diameter placed inside sack gabions 2 m long x 0.96 m in diameter, first layer laid is 14 m wide, second layer is 5 m wide whereas the third/fourth layer is developed as per gabion wall design and lowest water level (LWL);
1 m depth toe key with sand filled gabion boxes of size 1 m x 0.7 m placed inside gabion boxes of size 3 m x 1 m x 1 m;
0.5 m thick slope revetment with sand filled geobags of size 1 m x 0.7 m placed inside gabion mattress of size 3 m x 1 m x 0.5 m;
0.5 m thick surface key with sand filled geobags of size 1 m x 0.7 m placed inside gabion mattresses of size 3 m x 1 m x 0.5 m.

For execution underwater with river depth at 4-5 m during the lean season

A launching apron is developed in layers with sand filled in geobags of size 2 m x 0.92 m diameter placed inside sack gabions 2 m long x 0.96 m diameter;
1 m depth toe key with sand filled geo bags of size 1 m x 0.7 m placed inside gabion boxes of size 3 m x 1 m x 1 m;
0.5 m thick slope revetment with sand filled geobags of size 1 m x 0.7 m placed inside gabion mattresses of size 3 m x 1 m x 0.5 m;
0.5 m thick surface key with sand filled geobags of size 1 m x 0.7 m placed inside gabion mattresses of size 3 m x 1 m x 0.5 m.

For execution underwater with river depth at 5-6 m during the lean season

A launching apron is developed in layers with sand filled in geobags of size 2 m x 0.92 m diameter placed inside sack gabions of size 2 m x 0.96 m diameter, the first layer laid is 18 m wide, the second layer is 7 m wide whereas the third/fourth layer is developed as per gabion wall design and LWL;
1 m depth toe key with sand filled geobags of size 1 m x 0.7 m placed inside gabion boxes of size 3 m x 1 m x 1 m;
0.5 m thick slope revetment with sand filled geobags of size 1 m x 0.7 m placed inside gabion mattresses of size 3 m x 1 m x 0.5 m;
0.5 m thick surface key with sand filled geobags of size 1 m x 0.7 m placed inside gabion mattresses of size 3 m x 1 m x 0.5 m.

Material specifications

Sack gabion (size 2 m long x 0.96 m diameter): Mechanically woven hexagonal shaped doubly twisted (DT) galvanised and PVC coated wire mesh with mesh size 100 x 120 mm conforming to IS 16014:2012;
Gabion box (size 3 m x 1 m x 1 m): Mechanically woven hexagonal shaped doubly twisted (DT) galvanised and PVC coated wire mesh with mesh size 100 mm x 120 mm conforming to IS 16014:2012;
Gabion mattress (size 3 m x 1 m x 0.5 m): Mechanically woven hexagonal shaped DT galvanised and PVC coated wire mesh with mesh size 100 x 120 mm conforming to IS 16014:2012;
Geobag (size 2 m long x 0.92 m diameter): 700 GSM non-woven needle punched UV stabilised polyester geobag
Geo bag (size 1.0 m x 0.7 m diameter): 300 GSM non-woven needle punched UV stabilised polyester geobag;
Filter media: 300 GSM non-woven needle punched UV stabilised polyester geotextile fabric.

Results and conclusion

According to the Subansiri riverbank erosion study, the total land lost to erosion was 6.764 km2 and deposition stood at 2.021 km2 during the period 2008-10. The maximum value of bank retreat on the left bank was 634.8 m and the maximum value of channel deposition on the left bank stood at 666.1 m. The maximum value of bank retreat on the right bank was recorded as 741.5 m and the maximum value of channel deposition on the right bank was 789.6 m during the period. A total of 3.382 km2 of prime inhabited and agricultural land resources in the study area were eroded by the Subansiri river annually during 2008-10. The implementation of bank protection measures in identified vulnerable reaches has resulted in erosion control and moderate reclaim of land to the extent of 20-25 m in the first three years of implementation in the protected reach. A reduction in flow velocity along the river bank has been achieved in the erosion-prone areas with the development of submerged jack jetty fields. The use of jack jetty fields, along with spurs, trail dykes, slope revetment and sack gabion/gabion box toe wall, has proved to be an effective and robust bank protection measure and should be adopted on a large scale for effective flood management and erosion control.