Tidal River Management (TRM) for Selected Coastal Area of Bangladesh to Mitigate Drainage Congestion

Transcription

1 Tidal River Management (TRM) for Selected Coastal Area of Bangladesh to Mitigate Drainage Congestion Shampa, Md. Ibne Mayaz Pramanik Abstract The drainage congestion and sedimentation has been major issues over the years in Kobadak river basin, Bangladesh. Kobadak River has been experiencing huge siltation over a long reach that reduces the drainage capacity of river and causing prolonged drainage congestion. It affects every sphere of life including cultivable lands, employments, culture fish production, grazing land, bio-diversity and live-stocks. Human interventions such as construction of polders, bridges and encroachment into the river for cultivation deteriorated the condition of Kobadak River. The decrease of flushing flow from upstream and substantial reduction of tidal flooding area along the river by constructing polders caused s evere siltation over a long stretch of the river. Sedimentation mainly occurs during dry season since tidal flow is restricted over the flood plain by constructing polders and there is no fresh water flow into Kobadak River from the Ganges River to flush the sediment during low tide. This study focuses on prevailing problems and solutions of long standing drainage congestion by applying TRM in Kobadak River basin area under Satkhira district in Jalalpur beel area. In this study to solve the drainage congestion problem for a long period of time by TRM process two options are suggested eac h with two different scenarios. Each option is assessed with cross dam without cross dam scenarios using MIKE-11 hydrodynamic modeling system. Index Terms Beel, Drainage congestion, Kobadak river, MIKE-11, Sedimentation, Tidal basin, TRM. 1 INTRODUCTION Bangladesh is the biggest delta of the world formed by sediments carried by Ganges, Brahmaputra and Meghna river system. The rivers of southwestern region in Bangladesh are characterized by active deposition of sediment causing significant reduction in their drainage capacity [1]. Besides, construction of coastal polders that de-linked the flood plains from the rivers, and diminished upstream flow during the dry season deteriorated the sedimentation problem in the region[2]. Consequently, the area has been experiencing severe drainage congestion and water logging since the early eighties. The sedimentation effect of similar river and flood plain interventions were described by Sarker (2004)[3]. To solve these long standing problems, the Khulna-Jessore drainage rehabilitation project known as KJDRP was implemented during [4]. Later on, a popular concept based on generations of indigenous water management practices, formally known as a Tidal River Management (TRM), was adopted. TRM would allow natural movement of sediment-borne tidal water into a beel which is called tidal basin and allow deposition of sediment in the beel. During low tide the outgoing water would erode the river bed and increase the drainage capacity. After implementation of the project the prevailing drainage congestion was partially solved and agricultural, social and economic benefits were achieved[5]. Sediment management is the most challenging yet important aspect of TRM in the study area [6]. People allow their land to be used for tidal basin operation without any compensation, hoping that the land will rise after three or four years. However, monitoring results and community consultation reveal that almost in all cases sedimentation inside the tidal basin does not occur as expected. This result in people s a functional sediment management plan would be very helpful for successful TRM practice in future [7]. 1.1 Objectives Understand the drainage congestion problem in the coastal areas of Khulna region. Review of different approaches undertaken to mitigate the drainage congestion in that coastal region. Assess Tidal River Management (TRM) approach for alleviation of drainage congestion in selected area. 1.2 STUDY AREA Geographic Location The study area is located in the southwest region of Bangladesh within the Khulna division, Bangladesh and falls under the administrative jurisdiction of Tala upazila in Satkhira district. The study area lies in between 22044`40.3``N 23010`27.1``North Latitude and 88013`36.44``N 89050`00.00`` East Longitude (WGS 84). A location map for study area is shown in Figure 1 Shampa, currently working as professional at Centre for Environmental and Geographic Services, Bangladesh shampa_buet_05@yahoo.com or shampa@cegis.bd.com Md. Ibne Mayaz Pramanik, Bangladesh mayazpolok@yahoo.com Figure 1: Base map of the study area 1

2 1.2.4 Physical Setting The Kobadak River is situated in the southwest region of Bangladesh. The Kobadak River flows through Jessore, Satkhira and Khulna districts. Total length of the Kobadak River is 240 km (upper & lower) from Taherpur and Niamotkati. Due to the siltation of Kobadak River at Katakhali major flow of the river flows to Sibsa River through the Haria River and Minhaj-nadi. The study area is the basin area of Kobadak River under Jessore, Satkhira and Khulna districts. The study area covers mainly Chaugacha, Jhikargacha, Manirampur and Keshobpur of Jessore district and Koloroa, Tala, Asasuni and Syamnagar of Satkhira district including Paikgacha and Koira of Khulna district, which is shown in Figure 1. Total catchment area of Kobadak River is 1,02,000 ha (upper & lower Kobadak).The study area covers the upper part of Kobadak river from Taherpur to Katakhali which is about 75,000 ha. Figure 2: Beels in the Kobadak river basin area 2 TIDAL BASINS AND BEELS 2.1 Tidal Basins and Beels and Concepts of TRM A beel is a natural depression. Bangladesh being a deltaic country, land in the plains has been formed by sediments carried down by Ganges, Brahmaputra and Meghna river systems. Depressions are formed by numerous causes like subsidence of top soil caused by creation of a vacuum below by decomposition of organic substances mixed with silt, subsidence by tectonic movement, or degradable floods deposit sediment close to the river bank. But the land between two rivers remains low lying. Such a low lying land is also known as a beel. A tidal basin is a depressed low lying or beel adjacent to the sediment-laden tidal rivers. In the southwestern part of Bangladesh there are several tidal basins which are very useful for sediment management of sediment laden tidal rivers. Figure 2 shows the beels in Kobadak river basin. The concept of sediment management in tidal basin originated in the 1880s to serve both as visual centerpiece and as means for flushing the Washington channel, a harbor separated from the Potmic River by fill lands where East Potomic Park is situated[ 8]. Tidal basin of Potomac River. Shown in figure ORIGIN OF RIVER SEDIMENT IN THE SOUTHWESTERN AREA OF BANGLADESH Sedimentation in the tidal rivers of the southwestern area of Bangladesh is the main reason of the problem of water logging. These troublesome sediments have blocked the rivers and caused upstream drainage congestion and flooding. In most catchments where excessive sedimentation is taking place in the downstream river system, the origin of the problem is in the upstream catchment where land use changes have taken place in relatively steeply sloping areas. However, this is clearly not the case in the Satkhira area where upstream areas are very flat and excessive erosion is not occurring on these mainly agricultural upstream lands. It is clear from casual observation of the rivers in the study area during the dry season, when upstream flows are non-existent; the sediments are transported into these rivers by tidal flows. This movement of sediment is also confirmed by the field measurements of sediments concentrations and tidal flows by SWMC (June 1996 and October 1996) and other measurement programs. In the Satkhira area, the sediment derived from the downstream tidal rivers by natural erosion processes and transported upstream by tidal movements seems to be the most plausible explanation of their origin [6]. 2.3 Adverse Impacts of Coastal Polders In the early 1980s polders became a bane rather than a boon for the people, as rivers failed to maintain their natural courses. Tides deposited silt on the riverbeds rather than the floodplains for more than two decades halting the natural flow of the rivers. The consequent dearth of land formation left floodplains inside the polders lower than riverbanks outsides the polders. Rainwater, therefore, could not drain from the area s leading to chronic water logging. Adding to the tragedy, by construction of Farakka Barrage on the Ganges river and unilateral diversion of its water from 1975 started deteriorating the balanced (fresh water-tidal flow) ecosystem of the river region [7]. This was further aggravated by construction of reservoirs on the upper catchment of all trans-boundary rivers of the southwestern region. During the dry period of the year (January to April), the area receives almost no unplanned fresh water flow. Under the changed hydrological situation, many tidal channels outside the polders started experiencing abnormal sedimentation blocking the drainage paths of the polders. Prolonged water logging inside the polders were so severe in some cases the people of the area had the only option of migration. Bangladesh Water Development Board (BWDB) was fully aware and concerned about the drainage congestion of the polders of the Southwestern region. But the spatial extent of the problem and prevailing hydro-geomorphological conditions of the area are so complex that a holistic and well planned approach was needed. BWDB conducted six studies by engaging international and local consultants between 1986 to1998. Besides this planning exercise, BWDB dredged some badly silted up channels for immediate relief of the drainage congestion problems. From 1996 to 1998, about million cubic meters were reexcavated (manual and mechanical) to keep the main river system active. Dredging and re-excavation was done several times but it faced siltation every time. Figure 4 shows the Kobadak river with polders. Figure 3:Tidal basin of Potomac River 2

3 River Cross Sectional Area(A) - m 2 INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 1, ISSUE 5, JUNE 2012 ISSN A B Figure 4: (A).Kobadak River with 2010 Landsat Image (B) Kobadak river with coastal polders 2.4 THEORETICAL BASIS OF TIDAL BASIN SEDIMENTATION A number of scientists opine that there is a relationship between the general dimension of the entrance to tidal estuary or tidal river in a sandy coast and the tidal prism. But it appears that there was no previous attempt to determine a definite correlation until Le Conte (1905)[9] proposed an equilibrium area concept for tidal inlets. O Brien (1931, 1969)[10,11] examined field data inlets through sandy barriers in West Coast of the United States and determined a relationship between the minimum cross-sectional flow area of the entrance channel and the observed tidal prism, and established a relationship as: Ac=C.Pn (2.1) Where, Ac=the minimum inlet cross-sectional area in the equilibrium condition, C= an empirically determined coefficient, P=the tidal prism (typically during the spring tide/mean tide), and n=an exponent usually slightly less than unity. Similar analysis was carried out for the Hari River. With a view to establishing a consistent relation, data of the Pussur River were incorporated in developing the relationship, as it is also a highly tide dominated river in the South West Region. In doing this, a district relationship was found between cross-sectional area and tidal prism: Ac =43.42 P.9985 (2.2) Where, P= mean tidal prism (million m3), Ac =cross-sectional area (m2). The cross-sectional area and the corresponding tidal volume of Hari River generated by the East Beel Khuksia tidal basin at Ranai is fitted to established relationship of cross sectional area and tidal prism of the Hari River as shown in Figure Equation to line A = 43.42V Farleigh(1981, 1984), data of March-April 1980 ADC and ACE(1986), data of July 1981 KJDRP(1996), SWMC data of April 1996 Feasibility Study for Improvement of Navigability of Mongla Port, IWM data of March 2004 KJDRP (2004), IWM data of March 2004 KJDRP (2004), IWM data of May 2004 East Beel Khuksia TRM Basin (2007), IWM data of May Tidal Volume (V) -Million-m 3 Figure 5: O Brien equilibrium relationship for coastal river of Bangladesh Simple empirical equilibrium relationship shows that the crosssectional area of a tidal channel reduces in size if the tidal prism is reduced. Hence closing of a tidal river branch from the tidal system causes a reduction in the tidal prism in the whole tidal system downstream of the closure. The related reduction in the channel size, which will ultimately occur, can be estimated with the above mentioned relationship. The validity of the relationship also means that increasing a cross section by dredging without increasing the local tidal prism will result in an unsustainable solution. Natural forces will tend to restore the equilibrium condition by sedimentation in the dredged area. The reduced flow velocities in the enlarged river cross section will cause a gradient in the river velocity and consequently deposit sediment in the dredged river reach. This process stops when the equilibrium profile has been restored and the gradient in flow velocities has disappeared. 3

4 water Level(m) INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 1, ISSUE 5, JUNE 2012 ISSN Physical Method of Tidal River Management Tidal river management (TRM) involves taking full advantage of the natural tide movement in rivers. During flood tide, tide is allowed to enter into an embanked low-lying area (tidal basin) where the sediment carried in by flood tide is deposited. During ebb tide, water flows out of the tidal basin with greatly reduced sediment load and eventually erodes the downstream riverbed. The natural movement of flood and ebb tide along the tidal basin and along the downstream river maintains a proper drainage capacity in that river. Figure 6 shows the conceptual model of TRM Before TRM operation the critically silted stretches of the river needs to be de-silted by capital dredging and manual excavations to increase the drainage capacity otherwise TRM will not be effective. Tidal prism of the river is calculated. Then comparison is made between the existing tidal prism and the required tidal prism to maintain the design cross-section. A beel with adequate volume to accommodate the tidal prism is selected. Design and construction of link canal. Finally connecting the beel to the river through the link canal. 3.1 Data Collection and Processing Data (cross-section and bathymetric data, water level, discharge) are mainly collected from Institute of Water Modelling (IWM) Bangladesh Water Development Board (BWDB) 3.2 Model Setup The Tidal River Management model is done using MIKE-11 software. MIKE 11 is such a program that simulates flow and water level, water quality and sediment transport in rivers, flood plains, irrigation canals, reservoirs and other inland water bodies. MIKE 11 is a 1 dimensional river model developed by Danish Hydraulic Institute. Half hourly basis 24 hour s water level measurement data of river Haria and Minhaj in January has been used as downstream boundary of the model. The model has been used to simulate the dry flow such as February to May and hence the run-off from the catchment was not considered in the model setup. The upstream boundary is located near Mellekbari. The downstream boundary is located near Gadapur. Observed water level has been used as downstream boundary in the model. This TRM model consists of one upstream discharge boundary and two downstream water level boundaries. Constant zero flow has been used as dead end of the river and existing water levels in the beels. 3.3 Calibration of the Model The model has been calibrated to examine the degree of agreement between the model results and observed data. In the study the model was calibrated against water level. A sample plot of water level comparison at Niamotkati is shown in Figure Figure 6: Conceptual model of TRM 0 3 METHODOLOGY The methodologies followed in the study can be categorized in three groups: Input,Analysis, Output.The detailed methodology of the study are given in the chart Time(hr) Nimotkati_simulated Nimotkati_measured Input Literature review Secondary Data Collection Water level Discharge Cross-section Consultation with stake holders Analysis Understanding the main causes of drainage congestion Selection of beels for the study Data compilation and organization for Model input Model Development and simulation Chart 1: Methodology of the study Output To understand the drainage congestion problem in the coastal areas of Khulna region Assessment of Tidal River Management (TRM) for alleviation of drainage congestion in selected area. Figure 7: Calibration of the model 4 RESULTS AND DISCUSSION Before TRM operation the critical silted stretches of Kobadak River need to be de-silted by capital dredging and manual excavations to increase the drainage capacity otherwise TRM will not be effective. From hydrological data analysis the smallest design cross-section is taken Bed width, b= 25m Side slope, z= 1:3 Height h= 5m Bed slope= 1.25 cm/km Bed level= -1.0mPWD to -2mPWD So the cross-sectional area, Ac Ac = (b+z*h)*h 4

5 = (25+3*5)*5 =200m2 From equation Ac=43.42P Tidal prism, P=4.62 Million-m3 (for Ac =200 m2) This tidal prism is needed to maintain the required drainage capacity at chainage km but in the existing condition the tidal prism is 3.48Mm3.(from model result) It implies that if tidal prism is not increased then drainage capacity of the river will gradually shrink. Tidal prism can be increased in Kobadak River by allowing natural tidal movements into beels through link canal. The effectiveness of beel Jalalpur is assessed through model analysis. Based on number of link canal two different scenarios are assessed for beel Jalalpur. The results show that options with cross dam are more effective. But construction of seasonal cross dam is very costly and it needs to be removed before monsoon to drain excess water from upstream. It implies that without cross dam Jalalpur TRM is more effective. However cross dam is needed in the first year of operation to make the TRM more effective. Table 1 to5 show the detail results of the models. Table.1: Potential options for TRM in Kobadak River Option Description 1 One link canal: The only link canal is connected at 148 km chainage of Kobadak river 2 Two link canal: The first link canal is connected at 148 km chainage and the second link canal is connected at 150km chainage of Kobadak river Based on cross dam another two different scenarios are also assessed for beel Jalalpur Table 2: Potential scenarios for different options Scenario Description 1 Option 1 with cross dam 2 Option 2 with cross dam Based on these scenarios the tidal prism bellow of Kobadak River at chainage km is given in Table.3. Table.3: Tidal prisms for different options in Kobadak River Option1 Option2 Option Option with with Scenario 1 2 cross cross dam dam Tidal prism (Mm It implies Option 2 without cross dam is more effective. Here one link canal is connected to beel Jalalpur and another is connected to beel Jetua. Table.4: The tidal prism at link Jalalpur at 0.11 km chainage Option1 Scenario Option 1 Option 2 with cross Option2 with cross dam dam Tidal prism Mm Table.5: The tidal prism at link Jetua at km chainage Scenario Option 2 Option2 with cross dam Tidal prism 3 Mm Key Findings Two link canals need to be connected with the river. The first link canal is connected at 148 km chainage and the second link canal is connected at 150 km chainage of Kobadak River which gives the river total tidal prism 6.41 Mm3 in one tidal cycle throughout the entire TRM operation period. For more effective TRM operation a cross dam needs to be constructed during dry period in first year of TRM operation. Beel Jalalpur acts very effective as a tidal basin for Kobadak River. 5 CONCLUSIONS TRM is an eco-technical concept design to solve water logging problem in southwestern region of Bangladesh. This study focuses on prevailing problems and solutions of long standing drainage congestion by applying TRM in Kobadak River basin area under Satkhira district. A large area under Satkhira district has been experiencing drainage congestion over the years. Kobadak River lost its fresh water connection more than 100 years ago from Mathabanga River at the north of Taherpur and has become a mere seasonal and tidal river. Human interventions such as construction of polders, bridges and encroachment into the river for cultivation deteriorated the condition of Kobadak River. The decrease of flushing flow from upstream and substantial reduction of tidal flooding area along the river by constructing polders caused severe siltation over a long stretch of the river. Sedimentation mainly occurs during dry season since tidal flow is restricted over the flood plain by constructing polders and there is no fresh water flow into Kobadak River from the Ganges River to flush the sediment during low tide. Monsoon flow does not erode the deposited sediment on river bed because sediment concentration is also high during monsoon; that results huge sedimentation over a considerable reach of the river. Eventually the main Kobadak River, distributaries and tributaries lost drainage capacity causing long standing drainage congestion. Finally the concluding remarks are TRM operation is used to mitigate the drainage congestion in the study area. It is recommended that if the upland flow would increase in a certain amount by Ganges barrage or by any other means then no TRM operation is needed. As there are many beels in Kobadak River basin one at a time only one beel should be used as a tidal basin for TRM. Siltation of the whole beel is not desired though it is quite acceptable to local people who are engaged with agriculture. It destroys the biodiversity of the beel. So siltation should be allowed in a certain amount like 1m or 1.5m which also ensures the required environmental flow in the beel. Sediment inside the beel could be dredged yearly and by this way one beel can be act as tidal basin for many years. It saves the biodiversity of other beels in the study area at the same time keeps the river drainable and navigable for many years. So it is highly recommended from this study. 5

6 Appendix A ABBREVIATION ADB BWDB DEM DHI GOB IWM KJDRP LCS LGED LGI SSC SWMC TB WGS WMA WMO PWD Asian Development Bank Bangladesh Water Development Board Digital Elevation Model Danish Hydraulic Institute (now DHI Water and Environment) Government of Bangladesh Institute of Water Modelling (erstwhile SWMC) Khulna-Jessore Rehabilitation Project Labor Contracting Societies Local Government Engineering Department Local Government Institution Suspended Sediment Concentration Surface Water Modeling Centre (now named as IWM) Tidal Basin World Geodetic System Water Management Association Water Management Organization Public Works Department REFERENCES [1] IWM (2009), Mathematical Modeling Study for Planning and Design of Beel Kapalia Tidal Basin, Final Report, Bangladesh Water Development Board. [2] IWM (2005), Monitoring of Hydrological and Hydraulic Parameters on Tidal River Management under KJDRP Area, Final report, Bangladesh Water Development Board. [3] Sarker, M.H. (2004), Impact of Upstream Human Interventions on the Morphology of the Ganges-Gorai System, [4] IWM (2007), Monitoring the effects of Beel Khuksia TRM Basin and Dredging of Hari River for drainage improvement of Bhabodah area, final report, Bangladesh Water Development Board. [5] SMEC (2002), Khulna Jessore Drainage Rehabilitation Project, Project Completion Report (Final), Bangladesh Water Development Board. [6] SMEC (1997), Khulna-Jessore Drainage Rehabilitation Project, Feasibility Study Report for Overall Drainage Plan, Bangladesh Water Development Board. [7] Amir M S (2010), Socio-Technical Assessment of Sediment Management Options in Tidal Basins in Southwestern Bangladesh, Institute of Water and Flood Management (IWFM), Bangladesh University of Engineering and Technology (January 2010). [8] Green, s. (1974), Mills admits Being present during Tidal Basin Scuffle (http;// Post, Retrieved [9] LeConte, L.J., Discussion on the paper, Notes on the improvement of river and harbor outlets in the United States by D. A. Watt, paper no Trans. ASCE 55 (December): [10] O Brien, M.P.(1969) Equilibrium Flow areas of Inlets on sandy coasts, Proc ASCE, Journal, waterways and Harbors Div.,Vol.95, No. WWI, p [11] O Brien, M.P. (1931), Estuary Tidal Prism Relater to Entrance areas, Civil Engineering, Vol.1, No.8, p738 6