Jun 18, 2012

Water Resources Management in Coastal Belt of Bangladesh: A Case Study of Chalna, Khulna


Bangladesh is facing the threat of global warming and sea level rise. This impact is severe in the coastal region. Proper water resources management is must in this problematic area. The coastal belt of Bangladesh is under a polder system. This system protects the mass people from great threats of natural and man maid calamities over the time. Human requirement of water is increasing day by day as life is getting better and better. So, a good management is required to augment the water resources.
River System around Chalna
Chalna area has potential water sources of nearby river i.e. Posur River and Chunkuri River. Posur River flows in the eastern periphery while Chunkuri River flows in the southern periphery. Posur River is an important navigation channel and ‘Mongla’ sea port is located on the bank of this river. This river is about 142 km long and it is one of the major rivers in the Sunderban. Posur River is a tidal river and water remains saline about 6 months in a year. So an alternate source is required for these 6 saline months. Chalna is situated under polder 31. There are few fresh water khal within this polder i.e. Chalna khal, Bhadra River. These river and khal can be preserved as fresh water source after some development work. There are few large ponds in Pourashava boundary. These ponds retain water round the water. This water may not be feasible for long term period, but they are suitable for short period of time. A map showing the river system around Chalna is shown in Figure 1.

Considering the special importance of Chalna and availability of fresh water for drinking water (the region has large number of rivers which are highly saline in most part of the year). Rainfall runoff trapped inside the coastal polders also get saline due to shrimp culture. The GW has been found to be saline at all shallow to deep tubewells.

Water Resources can be managed in different feasible ways. Some of them are stated below:
1. Use of the Posur river water for the 6 non-saline months and conservation of water at protected areas for the remaining part of the year. Mongla has such experience.
2. Explore the possibilities of using water of internal reservoirs.
3. Exploration with deep drilling and making search for fresh water.

Chalna is lying within polder 31 which has no adverse impact of shrimp culture on agriculture. This is a classic example of local awareness. Shrimp culture requires saline water. As a reason, shrimp farmers allow saline water in the channel. But salinity has other adverse impact over local flora, fauna, life style, food culture everything. This adverse impact is realized by the local people. Huge resistance against shrimp cultivation was generated. For this reason, the channels inside polder 31 is free from salinity and Rain water has flushed away most of the residual salinity.

Rainfall and Percolation
After analyzing 23 year data of rainfall, Average yearly net rainfall (after percolation) has been found 2,270 mm. After forming a reservoir, some water will percolated into the ground. It is been realized from experience and observation that Chalna has a soil pattern of sandy loam. A percolation rate of 3.0 mm/day is assumed for this type of soil condition. An average monthly rainfall is presented graphically below in Figure 2.
Monthly average Effective Rainfall Distribution
Evaporation: After analyzing 25 years of evaporation data of Khulna, it is found that average dry season (September~February) evaporation loss (83 mm) is lesser than average wet season (March~August) evaporation loss (125 mm) in that southern part of Bangladesh. Annual evaporation losses are shown below in Figure 3. Evaporation data does not differ much in a short distance. Khulna and Chalna is not very far away from each other. So, it can be assumed that evaporation remains almost same in both of this region.

Monthly average Evaporation
Water Quality Analysis

Table 1: Available WQ data is near Lobonchora at the Rupsha River in 2009.

Standard
6.5 - 8.5
600
-
200-500
10
-
6
0.2
1000
10
4
Month
pH
Chloride
Alkalinity
Hardness
Turbidity
EC
DO
BOD
TDS
SS
COD
mg/L
mg/L
mg/L
NT U
μS/cm
mg/L
mg/L
mg/L
mg/L
mg/L
January
7.6
247
34
298
80.8
628
5.1
1
314
125
56
February
7.71
1610
32
489
105.2
1240
4.9
1.1
620
138
66
March
7.65
3946
36
789
180
1760
4.9
1.1
880
260
122
April
7.51
8950
34
2312
188
14800
4.7
1.1
7.4
210
180
May
7.65
11473
34
2867
198
17200
4.8
0.9
8600
211
230
June
7.58
5639
36
876
156.8
560
4.9
0.8
280
109
126
July
7.68
1153
30
167
145.8
280
5.1
0.6
140
78
68
August
7.75
194
32
1846
112.6
340
5.1
0.8
170
101
34
September
7.67
57
36
1246
782
310
5.3
0.6
155
90
24
October
7.71
39
34
124
74.8
310
5
0.6
155
90
24
November
7.67
58
38
-
79
340
5.1
0.8
170
50
24
December
7.67
58
38
-
79
340
5.1
0.8
170
50
24

*Reference- Department of Environment


Salinity is the main problem in Chalna area. Other parameters are not so threatening here. WQ data is available near ‘Lobonchora’ in Rupsha River. It is assumed that WQ will not differ much from Chalna.
Salinity intensity near Dacope at Posur River.
After analyzing recent salinity data from Rupsha and Mongla, it is observed that the river is saline free from August to January.
Probable Reservoir in Chalna
There are two feasible channels which has the potential of being reservoir. One is ‘Chalna Khal’. This khal is formed from Habrakhali River, flows through the Chalna Pourashava and drains into Posur River. This khal has about 20 m width and 5 m depth and both the intake and outfall point is controlled by regulator. So this is a feasible reservoir for the vicinity. There is another possible reservoir. In the south side, there is a river named ‘Bhadra River’. This is a huge river with a width of about 100 m.and depth of 6~7 m. Both of the intake and outfall is also controlled here. This river connects ‘Habrakhali River’ and ‘Dhaki River’. A part of this channel may be preserved by cutting a bypass channel. The preserved part can be able to supply sufficient water. As the whole area is under a polder, the entire area (0.05 km2) should possess a huge amount of runoff, which will ultimately store in the reservoirs. It is presumed that about 109,612 m3 can be available to use in the dry 6 months. Furthermore, if the regulators is maintained properly and efficiently, this water can be augmented more but allowing fresh water in the channel and blocking saline water.
Chalna Khal


Special Thanks to these peoples, without them this research might not be completed-
1. Mr. Emaduddin Ahmed
2. Mr. Saad Siddique
3. Mr. Rafiqul Islam