Bank erosion management
8.2 Promising erosion management options for Brahmaputra
Specific bank protection measures in Brahmaputra have yielded satisfactory results by solving erosion problem of particular reaches. But one measure cannot be considered as a complete solution to the problem of the whole river because causes and mechanisms of erosion differ at different parts of the river due to variations in flow direction of the main channel, water velocity, topography and bank material properties. It is necessary to identify the underlying causes and erosion mechanisms in different sites. Common erosion mitigation method for all effected sites is ineffective and this is the main reason of failure of many existing erosion protection measures. A field survey conducted among 80 families of erosion effected areas of Sonitpur, Barpeta, Goalpara and Dhubri districts has revealed that there was no consultation with effected people regarding erosion protection works.
Again, there was no compensation for land lost in erosion. It is essential to include public consultation component in policy for better understanding of the river and the erosion problem. Rehabilitation of erosion effected people is another important issue which is lacking in current erosion management policies.
It is essential to study the whole river to identify effected and erosion prone areas based on severity of the problem and probability of occurrence. Site specific erosion protection measures considering causes and mechanisms of erosion and impact assessment studies for protection of other vulnerable reaches is important. Loss in erosion can be reduced if protective measures are taken up immediately after signs of erosion of the banks.
Different reaches of the river can be categorized into two major groups based on braiding, shifting of the main channel and bank material properties to select appropriate river training works. Some river training works for two types of reaches are highlighted below:
a) Reach with main channel(s) towards the bank(s)
For places eroded by direct attack of the main channel, e.g., Rohmoria at upstream, revetments along with flow deflectors at immediate upstream areas can be suitable option.
Revetment is a protective structure with stony material, usually natural stone or concrete.
The negative effect of concrete blocks is very limited and they have almost no considerable effect on river hydro-system (Maritime Navigation Commission, 2011).
Articulated concrete block mattress has an appropriate flexibility that can be installed on different conditions of slope revetments (Yamini et al., 2017).
b) Braided reach with secondary channels towards the bank(s)
Bank revetment, spurs etc. have proved to be highly capital intensive and hence can’t be applied to long reaches seeking erosion protection. In erosion sites of highly braided area where secondary channels attack the banks, pro-siltation devices can be applied to induce siltation in banks and channelization of the main channel. RCC Jack Jetty and bamboo submerged vanes can be considered as a semi-permanent cost effective approach of river management for sedimentation along with arresting erosion (Sharma and Nayak, 2015).
Due to abundance of bamboo in Assam, bamboo porcupines are cost effective option with less environmental impacts. Low slope banks with loose structure having relatively more organic content, e.g., erosion site of Nematighat (from Chapter 5), can be managed with geo bags/ tubes coupled with bioengineering tools, e.g., vetiver plantation for its effective bank stabilization with cost effective nature, low maintenance, longevity and little environmental impacts. The vetiver is a special type of grass having longer roots of length up to 3 m. These roots have an average tensile strength of 75MPa (CWC, 2012). Massive plantation with vetiver, bamboo etc. along the banks of vulnerable reaches can provide protection from threat of erosion.
A braided reach can be narrowed by using submerged vanes which are small flow-training structures designed and installed on the riverbed to modify the near-bed flow pattern and redistribute flow and sediment transport within the channel cross section (Odgaard, 2017).
Vanes have been used in the past to successfully close off secondary branches of a river (Chabert et al. 1961). Odgaard (2017) have showed that submerged vanes are effective in managing braided rivers like Brahmaputra to close off selected secondary branches and reduce the lateral extent.
Existing embankments under direct attack of the river need erosion protection. Erosion is one cause of embankment breach in many cases along with other factors, e.g., sudden settlement, overtopping, seepage and leakage (WRD, 2016). Erosion protection measures for embankments may be revetment/ mattressing, spurs/ groynes, grade control measures or improvement of shear strength (by growing shallow rooted vegetation). Recently in 2016, state government of Assam has decided to start dredging of Brahmaputra with objectives of erosion control, sediment management and flood control (NDTV, 2016).
Although, dredging is practiced in different rivers of the world for navigation, remediation and flood protection; there are evidences of negative impacts from dredging, e.g., increasing risk downstream, ecological risks affecting physical habitat, disrupting riverine processes and reduced connectivity with the floodplain (Barbe et al., 2000; Gob et al., 2005; Freedman and Stauffer, 2013). Channels which have been artificially deepened by dredging silt-up more frequently as they return to their pre-dredged state. In these situations, dredging will be an unsustainable activity since it needs to be repeated regularly (Environment Agency, 2013). Sediment budget of Brahmaputra River is likely to be changed after dredging operation since 21% suspended sediment of the river is contributed from scouring (from Chapter 4). Dredging will alter this figure and it may have other implications including increased aggradation and changed river morphology at downstream.
Land use and land cover study of the river will be helpful in site/ route selection for dredging of the river bed. Dredged material, i.e., sands, can be used to fill geo-tubes and to strengthen existing embankments to super levee as developed in Japan (Takahasi and
Uitto, 2004). Super levee is a high standard river embankment with a broad width (Stalenberg and Kikumori, 2008) having mild slope of 1:30 (Arakawa – Karyu River Office and MLIT, 2006) and is resistant to earthquake, overflow and seepage. The mild slope of the super levee prevents sliding of the top layer. The great width of the super levee also reduces seepage (Arakawa – Karyu River Office and MLIT, 2006). Super levee concept will overcome the drawbacks of existing embankment system.
From above discussions, it can be concluded that understanding of river processes and underlying causes are necessary for solution of erosion problem of Brahmaputra and river training works should be site specific with little environmental impacts. All approaches related to bank erosion management should be carried out in line with the hydrological character of Brahmaputra.