GUWAHATI, June 26 � The Asian Development Bank (ADB) has maintained that geo-bag revetment is the only affordable anti-erosion technology to check erosion in Palasbari and Dibrugarh areas. This technology, sought to be used in these vulnerable reaches, is not at all aimed at imposing a mono-technology. It is rather aimed at exploring and developing alternative cost-effective and sustainable technologies compared to the costly boulder-based revetment or spurs.
These assertions were made by Kenichi Yokoyama, Mission Leader of Assam Integrated Flood and Riverbank Erosion Risk Management Project (AIFRERMP), proposed to be funded by the ADB.
However, he admitted that to remove the doubts of the local river engineers on the efficacy of the technology, more clarifications and demonstrations are needed.
The ADB official claimed that the geo-bags launched in Palasbari area were traced by the divers in April last. Around 20 people were engaged to pull one of these bags out of water. These people needed about ten minutes to pull out the bag, he said.
He also asserted that the report of the April, 2010 diving inspection at the site was supplied to the State Water Resources Department (WRD) on May 12 last.
Yokoyama was clarifying on behalf of the ADB on the issue of resentment expressed by the river engineers here over the alleged pressure mounted by a lobby in the ADB for using geo-bag revetments for protecting Palasbari-Gumi and Dibrugarh. He was contacted by this newspaper seeking clarification on the issues raised by the river engineers on the matter.
He said, the geo-bags were dumped in September 2009 by the Water Resources Department (WRD). During that time, the water level was at low stage, called low water level (LWL). However, at that time no coverage was provided to the slope between the LWL and the high water level (HWL), the river attains in the flood season.
The 20-metre erosion that has been reported, took
place along the slope between LWL and HWL where no coverage was provided, said the ADB official.
The bags were �dumped on a mass from the bank line,� and were expected to launch 35 metres when deep erosion takes place. However, between September and December 2009, there was only little erosion/scouring (of about 5 meters or so) in the upstream half of the 150-metre stretch. And in the downstream half, the rise in the overall bed level by one or two metres or so was observed. But the diving in December did not have sufficient record if the bags were launched or not.
WRD engaged IIT Guwahati to do a third party evaluation, which recently issued an interim report recognizing that geo-bags are an alternative technology that may need to be promoted given the increasing price of boulders and negative environmental implications of quarrying a large quantity of boulders. They also recommended that bags (and boulders) should be dumped in a systematic manner from a GPS-positioned pontoon.
ADB technical assistance team commented to WRD that silt deposit might have taken place in some sections of the 150 metre pilot dumping
stretch at the time of diving in December 2009, since the bathymetric survey indicated overall siltation activities in the lower half of the stretch
showing the bed level rising by one to two meters, said the ADB official.
In Dibrugarh, WRD has already adopted the same/similar geo-bag revetment
design in the Rohmaria areas in a large-scale Central Government-funded scheme, for which DPR was already cleared by the WRD/State and the Central Government.
Conventionally, WRD has pursued spurs to manage river erosion. The technology of porcupines may also be meant by the river engineers as �cheap technology,� which can be adopted in certain conditions. However, in the concerned sections of Palasbari and Dibrugarh (upstream areas), there is no space to construct spurs without possibly costing more than revetment. On the other hand, porcupines cannot control erosion in the deeper main channels in those areas, the ADB official claimed.
