Feb 2, 2012

Conservation of Water Resources in Little Feni River in Noakhali Coastal Region

Figure 1: Position of Musapur Regulator and Closure
Conventional source of water for drinking is either surface water or groundwater. Perennial rivers, reservoirs, lakes and ponds are the surface water source, while underground aquifer is the groundwater source. Both of the sources have limitations like water availability, water quality environment, physical locations, hydrogeology, salinity etc. Water availability, local bio diversity, flora fauna in the coastal region Noakhali district is now facing severe threat due to excessive salinity intrusion. There was a ragulator in ‘Kazirhat’ to confront the salinity intrusion. Due to this regulator, a huge land mass is reclaimed in the downstream of the Little Feni River. To ensure the security of this reclaimed region a new regulator is constructed in Musapur union. The main objective was to establish a control structure in a diversion channel of the Little Feni River and close Little Feni River. For this reason, after constructing a regulator on a diversion channel of the Little Feni River near Musapur Union, authority demolish the regulator at Kazirhat due to its poor condition. But the closure of the Little Feni River is yet to construct. For this reason, the local flood plain area namely Bashurhat, Sonagazi, Daganbhuiyan are very susceptible to tidal flow and salinity intrusion. This phenomenon makes ‘Musapur Regulator’ redundant.
The concerning authority is constructing the closure on an urgent basis. This closure and the regulator at Musapur will regain the robustness of local environment. 

The objectives this study is: 
  • To assess the feasibility of abstraction of water resources from Little Feni River when Musapur Regulator will be active. 
  • To calculate the storage of exploitable water resources from Little Feni rives 
  • To assess the effect of abstraction due to human intervention 

Study Area 

Location: The study area lies on the east and west bank of Little Feni River. It is bounded by Daganbhuiyan, Upazila on the north, Sonagazi Upazila on the East, Bashurhat Upazila on the west and Bay of Bengal on the South. 
Figure 2: Study area in the vicinity of the Little Feni River 
Climate and Hydrology: The project area experiences a typical monsoon climate, with hot wet summers from May to September and cooler dry winters. The mean annual rainfalls 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 crisscrossing the area generally prevents widespread erosion and, therefore, the low levels of sediment reported in the 1988 (HARZA, 1988) study are entirely consistent with the perceived rate of sedimentation in the khals of about 0.05 m/yr. 
Figure 3: Rainfall and Evaporation of the area adjacent to Bashurhat
Stored water in the Little Feni River is available for local drinking water supply, irrigation and other economical activities. Beside these use percolation, evaporation of the stored water will occur. In this study only evaporation will be considered. Rainfall data for 50 years (1961~2009) of Daganbhuiyan, Sonagazi and Bashurhat is analyzed. Similarly evaporation data of 3 years (2005~2007) is analyzed. It is observed that maximum rainfall occurs in July (660 mm). Evaporation is usually higher in dry season. Maximum evaporation occurs in April (91.5 mm). 

Little Feni River: Little Feni River one of the trans-boundary rivers of Bangladesh, originates from the Hill Tripura in India and enters Noakhali district near Gunabati after running over the south eastern part of Comilla district. Flowing further south, the Little Feni debouches into the Bamni River in the northeast of Sandwip Channel. The river contains a lot of meanders. A number of streams e.g. the Dakatia River and the Gumti River meets the river on its course.

Figure 4: Observed Water Level (m) in the Little Feni River 
Analysis 
Water availability assessment was carried out from long-term simulated discharge (comprising of 24-years model run from 1985 to 2009) of validated model available at Institute of Water Modelling (IWM). Data collection and updating / validation of models are carried out by IWM each year for various project purposes. However for each project the requirements of the generated model outputs are customized for specific purpose for which they are to be used as in case of this project. 

This model comprises of the entire greater Comilla and Noakhali Districts and includes an area of about 8,500 km2. It lies between 90o45′ and 91o31′ longitudes and 24o00′ and 22o30′ latitudes. The region is bounded on the north and east by the hilly areas of Tripura (India), on the west and northwest by the Meghna River, and on the south by the Meghna estuary. The total drainage basin of the region is 13,500 km2 out of which 5,000 km2 is located inside the Indian Territory; the territory within India is mostly hilly terrains. 

The overall performance of this model has remained consistently high over the years. The calibrated data at vicinity of Daganbhuiyan on the river Little Feni has been shown in Figure 3 
Figure 5: Calibrated Water Level at Gunabati on the Little Feni River
Surface water availability was assessed in terms of flow-duration curves using simulated data for year round discharge at the selected location of the Little Feni River near Daganbhuiyan. 

The demolished Kazirhat Regulator controlled the flow of the Little Feni River. The regulator had 20 vents and each vent consisted of a 12-ft diameter conduit pipe with flapgate at the downstream end. The sill level of the Regulator was at 0.00 mPWD. The river normally flows from north to south in the time of low tide throughout the year.

Figure 6: Musapur Regulator 



The regulator was closed at the end of October and remained closed up to April each year to store water in upstream for irrigation. During that time, the downstream channel got silted up, which clogged 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 was 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. Newly constructed ‘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. This regulator will be operated in almost same manner of ‘Kazirhat Regulator’. This regulator has maximum operating level of 5.00 mPWD. So, when water level rises above 5.00 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. 


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


Base Flow: After analyzing the discharge 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.

Table 1: Average monthly Flow (m3/s)
April
May
June
July
August
September
October
November
December
19.961
33.829
60.056
100.79
104.789
89.203
64.631
26.598
4.903

Results 
From above study, it is observed that flow of the month December can be refered as base flow. For use during driest period of year considering storage at 5.00 mPWD by Musapur Regulator, total storage is calculated 41,895,905 m3 (Annexure). This stored water would be available for the six dry month of year. Considering this fact we found a uniform discharge as 2.657 m3/s. From the above study we found minimum flow 4.903 m3/s. This ensures the sustainability of amount of water availability. There are losses in the water due to evaporation. Combined water demand for Daganbhuiyan, Sonagazi and Bashurhat can be safely extracted from this source. There is plenty of water remains in the channel after different extraction due to economical activities and losses. 

Limitation of the studyThe study is done by considering maximum operating level of Musapur regulator gate. There are various ways of losses from this water. The main problem for this amount of water is siltation. River bed is changing all the time, for this reason cross sectional properties also changes accordingly. If 50% water is available (1.329 m3/s) from total water, then it can also be considered as safe extractable water from the Little Feni River. 
Water use 

The closed water system is fresh water largely utilized for irrigation. The water downstream of the regulator is saline and under the influence of tides. The gates are used to drain out the upstream flow in the rainy season and later to store water in the upstream reservoir for use in the dry season. There is no mixing of sea water due to the flap gates which close automatically with the pressure of downstream water during tide. When the regulator gates are closed the reservoir water extends up to several kilometres upstream in the rivers facilitating reservoir storage. The greatest sediment inflow occurs during the month of June to September. Irrigation demand in these months is low, and operational policy should be to keep reservoir level as low as possible during these months, thereby minimizing loss of active storage. Such a policy is also advantageous from a flood control point of view. 

Land reclamation: Land reclamation will be happen as one of the after effects of closure of the tidal channel. However, a large tract of land regularly visited by floods, tidal surge and at other time by drought has been fully developed for round the year water management. Due to the closure, silt will be accumulating in the downstream of the regulator on a regular basis. 

Crop production: The farmers could not irrigate properly due to saline water intrusion into the lands and non availability of water in dry season. The regulator restricted entrance of saline water and provided round the year water availability thereby ensuring crop security. Because of the present availability of irrigation water, the local variants of paddy were substantially replaced by high yield paddy. Assurance of irrigation water also induced farmers to invest more of their resources on crop inputs. These two factors resulted in significant post project increases in the yields. 

Economic activities: The net increment of 1.2 million man-days of yearly incremental agricultural labor employment facilities have been created each year as a result of increased cropping intensity and production. With the introduction of improved agricultural practices, income and employment opportunity has also been increased to a greater extent. Improvement of socio-economic condition of people, agriculture based small cottage industries, sand mining related activities, housing, markets, mills, business, communication, transportation, employment in NGO’s, bank, educational institutions etc. have also been developed. Fish culture in the vicinity of the reservoir both upstream and downstream have contributed a lot to develop the economic condition of the people, though many of the ponds were established by encroachment into the reservoir area. In the downstream accreted area though, economic status of the poor has not changed where the rich are seen to utilize the resources and derive benefits. 

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Annexure 1: Calculation of volume considering maximum operating level of Musapur regulator
Chainage
(m)
Bed Level
(m2)
Area (m2)
Considering 5.0 m
operating level
Length
(m)
Average
Area (m2)
Volume
(m3)
104,700.00
0.60
914.25
1,500.00
1,679.45
2,519,171.25
103,200.00
-3.66
2,444.65
2,030.00
1,616.95
3,282,418.09
101,170.00
0.60
789.26
630.00
1,072.10
675,425.33
100,540.00
0.03
1,354.95
1,880.00
931.06
1,750,390.39
98,660.00
-1.48
507.17
560.00
762.49
426,997.01
98,100.00
0.01
1,017.82
600.00
929.33
557,600.48
97,500.00
0.19
840.85
700.00
903.21
632,244.64
96,800.00
0.02
965.56
500.00
918.54
459,269.21
96,300.00
0.54
871.51
1,200.00
723.78
868,537.26
95,100.00
-0.11
576.05
1,250.00
697.09
871,360.07
93,850.00
-0.68
818.13
1,050.00
622.18
653,285.53
92,800.00
1.45
426.23
1,500.00
408.73
613,101.29
91,300.00
1.11
391.24
800.00
327.76
262,205.73
90,500.00
1.62
264.27
800.00
516.44
413,149.33
89,700
-5.60
768.60
3,740.00
690.45
2,582,292.35
85,960
0.41
612.31
1,000.00
674.75
674,749.00
84,960
0.00
737.19
6,911.00
529.73
3,660,970.94
78,049
-0.37
322.27
10,082.00
370.48
3,735,169.28
67,967
-3.18
418.69
6,007.00
355.87
2,137,693.07
61,960
-1.26
293.05
4,800.00
300.25
1,441,214.40
57,160
-2.08
307.46
3,000.00
311.12
933,358.50
54,160
-2.31
314.78
3,000.00
308.23
924,676.50
51,160
0.45
301.67
3,000.00
230.02
690,072.00
48,160
0.75
158.38
3,000.00
236.27
708,820.50
45,160
0.81
314.17
3,000.00
674.71
2,024,133.00
42,160
1.35
1,035.25
3,000.00
714.34
2,143,017.00
39,160
1.01
393.43
2,900.00
305.16
884,956.75
36,260
1.51
216.89
3,078.00
197.35
607,434.07
33,182
1.97
177.81
3,152.00
271.82
856,764.03
30,030
1.80
365.83
3,010.00
325.86
980,826.56
27,020
1.82
285.89
1,990.00
270.14
537,586.56
25,030
1.70
254.40
2,730.00
238.70
651,657.83
22,300
2.13
223.00
2,650.00
169.84
450,068.05
19,650
2.13
116.67
2,970.00
148.71
441,673.16
16,680
1.90
180.75
3,070.00
132.93
408,085.89
13,610
2.44
85.10
3,040.00
79.99
243,183.28
10,570
3.22
74.89
2,800.00
46.74
130,858.00
7,770
4.03
18.58
2,870.00
14.76
42,374.12
4,900
3.61
10.95
2,990.00
6.16
18,425.88
1,910
4.52
1.38
1,000.00
0.69
689.00
910
5.57
0.00
Total Volume = 41,895,905 m3

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Reference:

  1. Rahman M.H., Ahmad E.; Zaman M., Management of Closed Off Tidal Basin- Muhuri Basin in Bangladesh
  2. BWDB, 2002. Feasibility Study of South Comilla North Noakhali Project
  3. GWMP Report, Surface Water Assessment for Daganbhuiyan, Bashurhat, Sonagazi, DPHE, 2011