Bangladesh is situated in the downstream position of few major rivers. As a reason, River borne problem are very frequent here. Flood, drought, salinity intrusion in coastal region and climate change impact is seriously threatening the water resources of Bangladesh. Economic activity of Bangladesh is vastly based on whether surface water or ground water. Surface water is mainly used as fish farming, navigation etc. Surface water as drinking water is begun to be used in limited scale due to the increasing rate of pollution. Ground water is hugely used in irrigation, drinking water supply etc. But due to excessive use of ground water and (in some region) slow recharge rate, ground water is depleting drastically. Impounding rain water in those areas are useful. Impounding Reservoir is a reservoir with outlets controlled by gates that release stored surface water as needed in a dry season; may also store water for domestic or industrial use or for flood control. It is also known as storage reservoir.
The factors behind the feasibility of an impounding reservoir are stated below:
1. Top soil and top clay layer thickness. Clay is considered as impenetrable to water. So, thicker clay layer represents good water holder.
2. Impounding water in the reservoir is greatly affected by evaporation and pollution. In this case, impounding water in sub-surface is more preferable rather than impounding in super surface, if aquifer characteristics are favorable. This ‘artificially recharge’ technology is new in Bangladesh, but for some areas i.e. coastal belt, Dhaka city (where depletion rate of GW is great) this technology is very useful.
3. The bottom of an impounding reservoir should not be exposed in the aquifer, in fact a clear clay thickness say 2~5 m should present above the aquifer.
4. When an impounding reservoir is formed or dug, it cannot contain expected amount of water readily. Because the exposed soil is vulnerable to infiltration of water whether it is clayey or not. But few years of time, an impervious layer of clay is formed at the bottom of the reservoir and it helps to store more water.
Case Study: Senbagh (Noakhali District)
River System: Senbagh is in an area bounded by numerous khals and the Little Feni River. The Little Feni is a secondary river with origin in India. The river flows from the foot hills of the Tripura hills with contribution from numerous tributaries. Near the border area, it has connections with two other secondary rivers in the region as tributaries, the Kakri River and the Suagazi Khal. The combined flow of these flows in a southward direction receiving flows from the surrounding watershed and ultimately flows into the Bay of Bengal. The Little Feni River is controlled at its downstream by a regulator. The Little Feni River passes through an area which is morphologically active. The Little Feni River is a perennial river and always has significant flows to be exploited as a sustainable surface water source.
The Begumganj Khal is a connecting Khal between Senbagh and the Little Feni River. It flows in an eastwardly direction draining the watershed to the Little Feni River which has flown southwardly to the sea. The Begumganj khal passes through an area which is morphologically active. Begumganj khal is not a perennial source and always insignificant flows except during monsoon it can to be exploited as a sustainable surface water source for water supply to Senbagh Pourashava. The aquifer system in the area in general arsenic contaminated in the upper layers and significant Iron contamination as has been reported by the people in the area.
|Figure 1: Rivers System around Senbagh|
Software used: The software used for this study is developed by Mr. Md. Zoinal Abedin. He collects GW related from different sources. Then he developed software based on ‘Visual Basic’ platform.
Analysis: Hydrogeological investigation for the Senbagh has been carried out to define the hydrostratigraphic layers and aquifer properties. About 13 borelog is available from different sources for hydrogeological investigation for this area. Figure 2 shows the distributions of borelogs location and hydrostratigraphic cross section alignment A-A/ of Senbagh.
|Figure 2: Borelog location and Hydrostratigraphic section alignment A-A/ of Senbagh area|
Subsurface lithological characterization of Senbagh area has been analysed from lithoilogy and depth location of different aquifer layers. Spatial distribution of hydrostratigraphy evolved from the striplogs represents the nature and distribution of the productive aquifer, which is composed of medium to coarse sand. It confirms the lateral extent of aquifer layers for considerable distance.
|Figure 3: Striplogs and distribution borelog location in and around Senbagh|
In Figure 4 columnar sections of three borelogs are shown which indicate top most clay layer varying from place to place. A major aquifer at depth location ranges from 70 m to 100 m exists roughly throughout the area. Specific yield of this aquifer varies from 0.18 to 0.20 indicating that the aquifer consists of fine to medium sand.
|Figure 4: Three Columnar Section and Specific Yield of Borelogs in and around Senbagh|
Findings: Hydrostratigraphic section A-A΄ illustrates very thick clay layers of depth varying from approximately 05 m to 65 m throughout area. Within the top clay layer a composite aquifer lens exists at Kabirpur and Arjuntala. Another composite aquifer lens also exists at North Baliakandi. One productive aquifer is evidenced at depth from 65 m to 100 m in the area. Hydrostratigraphic section represents that the composite aquifer layer is not fully penetrated.
|Figure 5: Hydrostratigraphic section along A-A/ of Senbagh Pourashava and surrounding area|
It is obvious from the hydrostratigraphic section that impounding reservoir in Dumuria mouza and Kabilpur mouza, because about 20~60 m clay layer prevails there. If we consider about 5 m clear clay layer from bottom aquifer just to ensure that no water leaks into the aquifer, then about 15 m depth of reservoir is sufficient for reserving water.
Conclusion: In real case, Noakhali district is covered by numerous ponds, khals and rivers. Some ponds carry water round the year, some don’t. Main reason as we can see is the shallow top clay layer. This method of assessing impounding reservoir is very useful for arid regions to store water.
(Special thanks to Mr. Md. Zoinal Abedin, Mr. Rafiqul Islam and Mr. Ibrahim Adham.)
GWMP Report on Hydrogeological Investigation (DPHE), IWM, 2011
National Survey Data, BGS, DFID, DPHE, 2000