Oct 19, 2011

Assessment of a Closed Tidal River: Case Study- Little Feni River in Bangladesh

South Noakhali is a tidal area. Salinity is its nature. But this is very harmful for local flora and fauna. For that reason a closure is to be constructed on Little Feni River. Here a study is commenced to assess the current situation. The salinity attacked area lies on the east and west bank of Little Feni River. It is bounded by Feni Sadar, Daganbhuiyan, Senbagh Upazila on the north, Mirsharai, Sonagazi Upazila on the East, Noakhali Sadar, Bashurhat Upazila on the west and Bay of Bengal on the South. There is a regulator in Musapur Union to control the flow of Little Feni River.

Study area in the vicinity of Little Feni River 

The project area experiences a typical monsoon climate, with hot wet summers from May to September and cooler dry winters. The mean annual rainfall at Noakhali, Lakshmipur and Hajiganj are about 3,200 mm, 2,600 mm and 2,000 mm respectively. Evapotranspiration exceeds rainfall for the months of November to April, and boro rice generally requires irrigation. Several major cyclones have crossed the area, but damage is mostly limited to that caused by high winds, with the newly accreted land to the south of the project area bearing the brunt of the tidal surges. 



With the accretion of this new land, drainage of the project area has been severely affected, due both to the lengthening and siltation of drainage paths to the south, and also because the new land itself is in places a little higher in elevation. The result is the Begumganj depression, which, although not in fact remarkably low (above 3.5 m above PWD), is almost completely encircled by higher ground, and tends to accumulate runoff from surrounding areas. As floodwaters rise, the Begumganj depression tends to merge with the Laksham depression to the north. The original natural drainage channel for the Begumganj depression area was to the south, via Noakhali Khal, but the deposition of sediment in this channel means that it is now ineffective. 


The Lower Meghna carries the combined flows of the Ganges, Jamuna and Upper Meghna Rivers. The coincidence of seasonal rainfall with peak flood discharges in the Lower Meghna exacerbates internal drainage problems. The Lower Meghna is the outfall water level control on drainage for most of the project area. The seasonal range in Lower Meghna water levels reduces in a southerly direction towards the Bay of Bengal. At Chandpur, the monsoon seasonal range of water levels is in the order of 2.0 m and there is a tidal range of 0.8 m in the dry season. At Rahmatkhali, the monsoon seasonal range in water levels is of the order of 1.0 m, and the dry season tidal range is of the order of 2.0 m. There is a notable change in water surface slope in the Meghna between wet and dry season. 

Sedimentation in the main khals has been relatively small over the years and some of this has been caused by excavated material being washed back into the khal during rainstorms. In the Begumganj depression where flow velocities are very low, the siltation of the bed might be expected to be at its highest and yet no re-excavation of this section has occurred in the last 20 years and the bed has silted by less than 1.0 m in this period. Further downstream, there is no evidence of accretion and as stated previously the problems are more concerned with erosion. The large number of existing embankments criss-crossing the area generally prevents widespread erosion. 

Little Feni River
It originates from the Kakri River, in the west of Kashinagar Bazar of Comilla District, and falls into the Bay of Bengal through the Sandwip Channel. The river creates a number of loops passing through Gunabati and down of Selonia. The Kazirhat Regulator controls its flow at a chainage of 98.938 km of its total length of 120km. The river was surveyed by SWMC in November 1999 (from chainage 8.50 km at Kashinagar bridge to 112 km at Chardarbesh/Mushapur). The Kazirhat Regulator has twenty vents and each vent consists of a 12-ft diameter conduit pipe with flapgate at the downstream end. The sill level of the Regulator is at 0.00m (PWD). The river normally flows from north to south throughout the year. The regulator is closed at the end of October, and remains closed up to April each year to store water in upstream for irrigation. During this time, the downstream channel gets silted up, which clogs the structure completely. Heavy rainfall in early June of each year creates a high water level in the upstream. Thus, water with a relatively high velocity is managed to pass through a manmade water route downstream of one gate of the regulator by which the deposited sediment gets washed out and situation for normal operation of the structure is restored. 
Cross section (top) and conveyance (bottom) near Companiganj 

There is a regulator at the closure of Little Feni river named ‘Musapur Regulator’ is going to function at the end of year 2011. This regulator is under BWDB jurisdiction. This regulator has a purpose to ensure sufficient water in Little Feni River, blockage of salinity intrusion and drainage of the vicinity. ‘Musapur Regulator’ has maximum operating level of 5 mPWD. So, when water level rises above 5 m, the gate would control the level by opening the gate. Moreover, the gate would be closed for the time of daily high tide. This action is necessary to prevent salinity intrusion. From different studies, water level is assumed 2 m for total length of about 12 km from Kazirhat to upstream of little Feni. 
Musapur Regulator 

Vent No.
23
Vent Size
Width (m)
3
Height (m)
3
Sill Level (m, PWD)
-0.5
Water Level (m, PWD)
C/S
Max.
5
Min.
0
R/S
Max.
6.2
Min.
-1.5

After analyzing the discharge near Companiganj from 1985~2008, it is observed the average minimum flow occurs in December. In that time of year, no other flow contributed the channel flow rather than base flow. 

April 19.961 
m3/s
May 33.829 
m3/s
June 60.056 
m3/s
July 100.79 
m3/s
August 104.789 
m3/s
September 89.203 
m3/s
October 64.631 
m3/s
November 26.598 
m3/s
December 4.903 
m3/s



Bed slope from Kazirhat to Companiganj
Flow of the month December can be refered as base flow. For use during driest period of year considering storage at 5 mPWD by Musapur Regulator, total storage is calculated 27,910,750 m3. This stored water would be available for the six dry month of year (excluding base flow). There are losses in the water due to evaporation. Combined water demand for Daganbhuiyan (0.1589 m3/s), Sonagazi (0.0639 m3/s) and Bashurhat (0.1104 m3/s) is 0.3332 m3/s can be safely extracted from this sources; there is plenty of water remains in the channel after different extraction due to economical activities and losses.