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:

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

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

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

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

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

Material specifications

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.