Arunachal | Subansiri Lower H E Project: Downstream River Bank Protection Solution– Part-3: Execution
Subansiri River Bank Protection Works Execution:
For execution in dry conditions during the lean season
a) Provision of Spurs at 250-300m c/c either with boulder-filled Gabion Box of size 3m x 1m x 1m with sand-filled Geo Bag of size 1m x 0.7m placed in wire crates. Length of spurs kept as 27-30m into river portion to divert river flow away from the banks and create pockets for siltation.
b) 0.5m thick & 7-10m wide launching apron with boulder-filled Gabion Mattress of size 3m x 1m x 0.5m.
c) 1.0m depth Toe Key with boulder-filled Gabion Box of size 3m x 1m x 1m.
d) 0.5m thick Slop Revetment with boulder-filled Gabion Mattress of size 3m x 1m x 0.5m.
e) Placement of RCC Jack Jetty in 2 layers of 3 rows & 2 rows each along the launching apron to break the erosive flow of vortices and induce sedimentation along river banks.
f) 1.0m depth Surface Key with boulder-filled Gabion Box of size 3m x 1m x 1m.
g) Geotextile fabric laid all along as filter media.
For execution underwater with a depth of river as 2-3m during the lean season
a) Provision of Spurs at 250-300m c/c either with boulder-filled Gabion Box of size 3m x 1m x 1m with sand-filled Geo Bag of size 1m x 0.7m placed in wire crates. Length of spurs kept as 27-30m into river portion to divert river flow away from the banks and create pockets for siltation.
b) 0.96m thick & 7-10m wide launching apron with boulder filled Sack Gabion of size 2m long x 0.96m dia.
c) 1.0m depth Toe Key with boulder-filled Gabion Box of size 3m x 1m x 1m.
d) 0.5m thick Slop Revetment with boulder-filled Gabion Mattress of size 3m x 1m x 0.5m.
e) Placement of RCC Jack Jetty in 2 layers of 3 rows & 2 rows each along the launching apron to break the erosive flow of vortices and induce sedimentation along river banks.
f) 1.0m depth Surface Key with boulder-filled Gabion Box of size 3m x 1m x 1m.
g) Geotextile fabric laid all along as filter media.
For execution underwater with a depth of river as 3-4 m during the lean season
a) The launching apron is developed in layers with sand filled in Geo Bags of size 2m x 0.92m dia. placed inside Sack Gabion of the size of each unit is 2m long x 0.96m dia. 1st layer laid is 14m wide, 2nd layer is 5m wide whereas 3rd/4th layer to be developed as per gabion wall design & LWL.
b) 1.0m depth Toe Key with sand-filled Gabion Bags of size 1m x 0.7m placed inside Gabion Box of size 3m x 1m x 1m.
c) 0.5m thick Slop Revetment with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size 3m x 1m x 0.5m.
d) 0.5m thick Surface Key with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size 3m x 1m x 0.5m.
e) Geo-textile fabric laid all along as filter media.
For execution underwater with a depth of river as 4-5 m during the lean season
a) The launching apron is developed in layers with sand filled in Geo Bags of size 2m x 0.92m dia. placed inside Sack Gabion of the size of each unit is 2m long x 0.96m dia. 1st layer laid is 16m wide, 2nd layer is 6m wide whereas 3rd/4th layer to be developed as per gabion wall design & LWL.
b) 1.0m depth Toe Key with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Box of size 3m x 1m x 1m.
c) 0.5m thick Slop Revetment with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size 3m x 1m x 0.5m.
d) 0.5m thick Surface Key with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size 3m x 1m x 0.5m.
e) Geotextile fabric laid all along as filter media.
For execution underwater with a depth of river as 5-6 m during the lean season
a) The launching apron is developed in layers with sand filled in Geo Bags of size 2m x 0.92m dia. placed inside Sack Gabion of the size of each unit is 2m long x 0.96m dia. 1st layer laid is 18m wide, 2nd layer is 7m wide whereas 3rd/4th layer to be developed as per gabion wall design & LWL.
b) 1.0m depth Toe Key with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Box of size 3m x 1m x 1m.
c) 0.5m thick Slop Revetment with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size 3m x 1m x 0.5m.
d) 0.5m thick Surface Key with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size 3m x 1m x 0.5m.
e) Geotextile fabric laid all along as filter media.
For execution underwater with varying depth (as above) with the provision of the embankment where bank height is low and is submerged during flood season
a) Launching apron is developed in layers with sand filled in Geo Bags of size 2m x 0.92m dia. placed inside Sack Gabion of size of each unit is 2m long x 0.96m dia. 1st layer laid is 18m wide, 2nd layer is 7m wide whereas 3rd/4th layer to be developed as per gabion wall design & LWL.
b) 1.0m depth Toe Key with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Box of size 3m x 1m x 1m above LWL.
c) 0.5m thick Slop Revetment with sand-filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size 3m x 1m x 0.5mupto bank height and along river side embankment slope.
d) 0.5m thick& 1m/2m wide Surface Key with sand filled Geo Bags of size 1m x 0.7m placed inside Gabion Mattress of size of each unit is 3m x 1m x 0.5m on embankment top.
e) Embankment bottom width to be designed as per height of embankment & allowable hydraulic gradient for locally available material.
f) Geotextile fabric laid all along as filter media.
Material Specifications :
a) The adopted material specifications conform to the specifications as per the relevant Codal Provisions:
b) Sack Gabion (size 2m long x 0.96m dia.): Mechanically woven hexagonal shaped doubly twisted (DT) galvanized & PVC coated wire mesh with mesh size as 100 x 120 mm conforming to IS 16014:2012
c) Gabion Box (size 3m x 1m x 1m): Mechanically woven hexagonal shaped doubly twisted (DT) galvanized & PVC coated wire mesh with mesh size as 100 x 120 mm conforming to IS 16014:2012
d) Gabion Mattress (size 3m x 1m x 0.5m): Mechanically woven hexagonal shaped doubly twisted (DT) galvanized & PVC coated wire mesh with mesh size as 100 x 120 mm conforming to IS 16014:2012
e) Geo Bag (size 2m long x 0.92m dia.): 700 GSM Non-Woven Needle Punched UV Stablized Polyester Geo Bag.
f) Geo Bag (size 1.0m x 0.7m dia.): 300 GSM Non-Woven Needle Punched UV Stabilized Polyester Geo Bag.
g) Filter Media: 300 GSM Non-Woven Needle Punched UV Stabilized Polyester Geo-textile fabric.
Design Drawings:
The design drawings depict the design solutions suggested for the project and these drawings were found to be in accordance with relevant Codes and guidelines.
Completed Protection Works of RCC Jack Jetty
RESULTS & CONCLUSION:
Subansiri river bank erosion study revealed
- Total land lost to erosion is found to be 6.764 km2 and deposition stands at 2.021 km2 during the period (2008-2010).
- The maximum value of bank retreat on the left bank comes to be 634.8 m and the maximum value of channel deposition on the left bank stands at 666.1 m (Period 2008-2010).
- The maximum value of bank retreat on the Right bank comes to be 741.5 m and the maximum value of channel deposition on the Right bank stands at 789.6 m (Period 2008-2010).
- A total of 3.382 sq. Km of prime inhabited and agricultural land resources in the study area has been eroded by the Subansiri River annually (Period 2008-2010).
Results of implementation of bank protection measures
- The application 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.
- Reduction in flow velocity along river bank has been achieved in erosion-prone areas with the development of submerged jack jetty fields.
The use of jack jetty field along with spurs, trail dykes, slope revetment and sack gabion/ gabion box toe wall etc. have proved to be an effective and robust bank protection measures which should be adopted at a large scale for effective flood management and erosion control measures.
