In view of the changes required to be achieved being very drastic and cover a long reach, a long term river training project divided into four phases have been suggested as below:

Phase - I : River training measures to achieve shifting of channels of river Dibang to its alignment of 1990. 

Closing of newly developing bypass/offtaking channels in the vicinity of the main offtake at Anant Nalla. 

Phase - II : River training measures to achieve shifting of channels of river Lohit to its alignment of 1990. 

Closing of newly developing bypass/offtaking channels in the vicinity of the main offtake at Anant Nalla. It is expected that new channels would continue to develop for quite some time.

Training measures for stabilization of the newly shifted river course of Dibang river. 

Phase - III : Traing measures to induce siltation in Anant Nalla. 

Phase – IV :  Construction of cross embankment at the mouth of Anant Nalla along with suitable training measures to stop the offtake completely.

EXECUTION OF THE PROJECT

After a detailed study of the river configuration, the Brahmaputra Board took up the diversion of the river Dibang at Dholla from its north-south course to a course parallel to the flow direction of the Lohit and thus reduce the pressure on the Lohit river at Hatighuli. It was decided that RCC porcupines and locally available material like bamboos and wood will be used instead of costly techniques.

PHASE-I

The first phase of the work was taken up in January, 2003 and completed in mid-April at a cost of Rs.6.5 crore. The flow of water was adequately controlled.

The reinforced concrete RCC porcupines were interspersed with bamboo porcupines to reduce the cost. The turfing work was also initiated alongside the bunds. All this resulted in checking the soil erosion. As a result, the overflow of water in the monsoon of 2003, instead of causing any further erosion had in fact, reversed the process. It was found that the porcupines across the Dibang river have induced heavy siltation to an average depth of 3 metres. It was also observed that 80 per cent of the flow of the Dibang river had been diverted through a channel, upstream of the porcupines, flowing in an east-west direction, as planned. On the other hand, there was heavy erosion in the north bank of the new channel indicating increased flow. All this resulted in reduced pressure on the flow of the Lohit river along its south bank. Consequently, there was no flood havoc in more than 14 villages that year in Hatighuli and the entire bund constructed by the Board from Dholla to Hathiguli did not experience any severe flood and remained intact . This also reduced the water pressure in the Rahmoria area for the first time in the last two decades, minimizing the erosion.

An expert team consisting of officials of the Brahmaputra Board, CWPRS, CWC and Govt of Assam again visited the area to observe the post flood effects of the works executed under Phase-I and recommended further measures to be taken up for diversion of balance flow of river Dibang and diversion of river Lohit to its original course.These works were taken up as Phase-I(B). Under this phase, the Tie-bund constructed in Phase-I was repaired, permeable RCC porcupine screens were constructed across the old silted up channel of river Dibang, and protection works were taken up in the left bank of Lohit. The works of Phase-I(B) were completed in July 2004 within the sanctioned amount of Phase-I, i.e. Rs. 10.47 crores. As a result of these works, the balance flow of river Dibang was completely diverted towards the northern side. Further, siltation took place on the left bank of Lohit river in the  Dhollaghat area.

PHASE-II

The works taken up under this phase was aimed to divert the flow of Lohit towards further north to meet Dibang.

The components of Phase-II works are as follows :

1. Construction of permeable of RCC porcupine   screens across the river Lohit from down stream of Dhollaghat to new channel of Dibang.

2.      Construction of pilot channel to guide the flow of Lohit to meet new Dibang channel.

Due to the Phase-II works, considerable amount of flow of Lohit river during flood period could be diverted towards the north of Saikhowa Reserve Forest. In the reach from Sisini to down stream of Dhollaghat, the deeper channel of the river shifted towards north. Siltation took place near the Dhollaghat area and the flood channel off taking from Sisini almost silted up.

Works under Phase-II was started in Feb'2004 and completed in July'2004 at a cost of Rs. 4.16 Crore.

PHASE-III

For further improvement of the diversion of the rivers towards north, proposals under  Phase-III in the line of recommendation of the Joint Inspection Team after their visit to site on 10th to 13th of  December, 2004 were prepared.

The proposals included the following provisions :

A.  For diversion of Lohit river :

·         Construction of 7 rows RCC porcupine screens across the river Lohit for a length of 5500m ;

·         Construction of 3 rows of RCC porcupine along the pilot channel for length of 4000 m ;

·         Construction of 3 rows of 3m RCC porcupine along the left bank of river Lohit from tying point of the RCC screen across the Lohit for a length of 2000m and a screen of length 600m at off-take of the spill channel at Sisini for protection of left bank of Lohit river ;

·         Construction/ Improvement of existing pilot channel by excavation at the entry and the exit portions and improvement of the channel in middle portions wherever necessary by mechanical means (Barge mounted excavators, Draglines etc.) to guide the flow of Lohit to meet the new channel of Dibang near Saikhowa Reserve Forest.

B. Along the left bank of Dibang river at Bahbari  :

·         Construction of 3 rows of 3m RCC porcupine for a length of 150m at u/s of existing Tie Bund to close the spill channel.

·         Construction of 4 rows of 3 m RCC porcupine in the vulnerable portion at the D/S of the Tie Bund ( across the off-take of old Dibang) for a length of 1200m.

The works of Phase-III were taken up by the Brahmaputra Board in March 2007 and were completed by July 2007 at a cost of Rs. 7.66 crores. On implementation of the Phase-III works, the main channel of river Lohit was shifted for about  300 m to 500 m towards the north-west direction, close to the original channel. At the same time, significant siltation was observed along the erosion prone river bank near Hatighuli.

PHASE-IV

For further diversion of river Lohit to its original course and to reduce/stop flow along the offtake channels (Anant Nalla, Balu Nalla), it was now proposed to take up the Phase-IV works during 2008-09. In this regard an estimate of Rs. 37.31 crores was prepared.

The following works are proposed for diversion of Lohit River under this phase.

i.                    Construction of pilot channel.

ii.                  Construction of coffer dam across Lohit River with sand filled cement and Geo bags.

iii.                R.C.C. porcupine works in the form of Screen and Spurs.

iv.                Bank stabilization with sand filled cement bags in the most vulnerable locations in the reach from Sisini to Kalaighat.

v.                  Construction of embankment from Kalaighat to Saikhowa R.F. for 3650 m.

On left bank of Dibang river, the following works are proposed. 

i.                    Extension, strengthening and construction of Tie bund along left bank of river Dibang at Bahbari and across spill channel downstream of Bahbari.

ii.                  R.C.C. porcupine wok in the form of construction of short porcupine spurs to protect the left bank of river Dibang.

The following works were completed upto May'2011

i.   Construction of pilot channel.

ii.   Part of R.C.C. porcupine works in the form of Screens and spurs.

iii.   Extension, strengthening and construction of Tie-bund along left bank of rover Dibang at bahbari and across spill channel downstream of Bahbari.

An amount of Rs. 23.67 Crore is utilised upto May'2011.