Tidal River Management (TRM)-An Innovative Scientific Approach for Sustainable Sediment Management

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1 Paper ID: WRE International Conference on Recent Innovation in Civil Engineering for Sustainable Development () Department of Civil Engineering DUET - Gazipur, Bangladesh Tidal River Management (TRM)-An Innovative Scientific Approach for Sustainable Sediment Management M.Z. Rahman 1*, M.S. Islam 2 and Z.H. Khan 3 Institute of Water Modelling (IWM), House# 496, Road# 32, New DOHS, Mohakhali, Dhaka-1206, Bangladesh Abstract Drainage congestion occurs due to increase of sediment deposition on river bed. High tide brings huge sediment that deposits on river bed and ebb tide cannot erode that sediment. Gradually drainage capacity and tidal prism of the river decreases. TRM is an innovative method of allowing tidal movement from the peripheral river into an embanked low-lying area through a link canal. Overall mechanism of sediment management where incoming silt will be deposited in the TRM basin by reducing the tidal velocity instead of deposition in the canal bed on the other hand during ebb tide silt free water will pick silt from the river bed to reach in the equilibrium condition of sediment carrying capacity. Tidal prism (tidal volume in one tidal cycle) and tidal range is considered as an important hydraulic parameter to assess the effectiveness of tidal basin for tidal river management. Two potential beels, Sukdebpur Beel along the left bank of Betna River and Ticket Beel along the Right bank of Marirchap River have been selected to develop the Tidal River Management Model. One dimensional mathematical model has been used to calculate the tidal prism in the adjacent river and the link channel compared accordingly for assessing its effectiveness and sustainability by MIKE 11. The selected Sukdebpur tidal basin for the Betna River is more effective and can generate more tidal prism (11.10 Million-m 3 ) than required tidal volume (6.00 M- m 3 ). On the other hand the selected Ticket tidal basin for Marirchap River can generate a tidal prism (3.50 M-m 3 ) which is slightly greater than the required tidal prism (3.15 M-m 3 ) and can be considered as potential basin for maintaining the required drainage capacity of the river. The tidal range is 134 cm and 40cm in the Sukdebpur and ticket beel depending on the topography and location of the beels. Beels along the left bank and Right bank of Betna and Marirchap River will be utilized as tidal basin for long term sustainable drainage management. Keywords: Tidal River Management, Tidal Basin, Tidal Prism, Tidal Range, Sedimentation, Drainage 1. Introduction The study area is located in the Southwest Coastal Part of Bangladesh. The entire southwest coastal part of Bangladesh has been experiencing river sedimentation and drainage congestions over the last few decades. This region is characterized by a vast network of morphologically active tidal creeks, which are the main drainage route for coastal polders and low lying beels. Thus, the natural drainage patterns of the area is predominantly characterised by the influence of the incoming tide from the sea. Tidal flow brings huge quantity of silt from the sea into the river systems of the coastal area. Before polderisation in early sixties and seventies, major parts of this incoming silt deposited naturally on the low-lying land (beels). On the other hand, there was significant amount of fresh water flow from the Ganges helped to flush the incoming sediment with high tide from the sea and thus, the proper drainage capacity of these tidal creeks was maintained naturally. After polderisation and significant reduction of fresh water flow from the Ganges, this natural process seems to be hampered 1 Associate Specialist, Coast Port and Estuary Management Division, IWM, zia@iwmbd.org 2 Junior Specialist, Coast Port and Estuary Management Division, IWM, mds@iwmbd.org 3 Director, Coast Port and Estuary Management Division, IWM, zhk@iwmbd.org

2 955 significantly. The presence of coastal polders prevents the spreading of the natural tidal flows and restricts sedimentation on the low lying lands. This leads to large scale river bed sedimentation in the peripheral rivers of polders and reduced the drainage capacity. Consequently polder areas were suffering from water logging and drainage When the affect of devastating flood in the year 1998 was added with water logging and water could not be drained out in any way and became stagnant, local people of Keshabpur U.Z. had cut flood embankment at beel Bhaina to get rid of marooned situation. Due to having a high magnitude of head difference between C/S and R/S water levels, rapid drainage and recession of water took place. But after few days, tide started to enter in the beel as a natural phenomenon to create tidal flooding. To and fro (incoming and outgoing) tidal flow formed a wide channel which was beyond the capacity of the local people to close it when they desired. At the end of dry season the local people surprisingly noticed that land level of beel Bhaina had been raised significantly and the depth of Hari River had been increased. They correlated the phenomenon of deepening of the river and siltation in the beel to be the function of tidal movement to and from the beel. After maturity of siltation in the year 2001 and closing of the channel developed at the cut point, value of the land of beel Bhaina was approximately tripled. The local people became inclined and interested in this process and urged the KJDRP Consulting Team to apply the process sequentially in all the low lying beels along the both banks of Hari River in the project area. This was accepted by the KJDRP Consulting Team giving a new title as Tidal River Management (TRM) being this natural process environmental friendly, cost effective and socially acceptable. Under this study two potential beels, Sukdebpur Beel along the left bank of Betna River and Ticket Beel along the Right bank of Marirchap River have been selected to develop the Tidal River Management Model. Dredging of river would not be sustainable unless TRM operation is implemented along with the dredging activities. TRM needs to be operated at least 8 years or more in one beel depending on monitoring results. 2. Tidal River Management (TRM) A number of scientists opine that there is some relationship between the general dimensions of the entrance to a tidal estuary or tidal river in a sandy coast and volume of the tidal prism, but it appears that there was no previous attempt to achieve a definite correlation. Le Conte (1905) originated an equilibrium area concept for tidal inlets. O Brien (1931, 1969) examined field data from tidal inlets through sandy barriers in the 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 an Equation (1) in the form: A c = CP n...(1) Where, Ac is the minimum inlet cross-sectional area in the equilibrium condition, C is an empirically determined co-efficient, P is the tidal prism (typically during the spring tide/mean tide) and n is an exponent usually slightly less than unity. This type of analysis was carried out for the tidal rivers of southwest region in Bangladesh. Data of Kobadak River, Hari River, Pussur River and other rivers of this region has been used with a view to establishing a consistent relationship. In doing this, an excellent relationship was found between cross-sectional area and tidal prism expressed by the following Equation (2): Where, A c = 43.42P (2)

3 956 P = mean tidal prism (ebb + flood) in Mm 3 Ac = cross-sectional area below mid tide level in m 2 [Fig.1] shows the relationship of x-sectional area and tidal prism (O Brien Equilibrium relationship) Equation to line A = 43.42V Farleigh(1981, 1984), data of March-April 1980 ADC and ACE(1986), data of July 1981 River Cross Sectional Area(A) - m 2 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 Fig. 1. Relationship between Tidal Prism and cross-sectional area 3. Development of TRM Model A physically based TRM model has been developed for the assessment of effectiveness of the selected beels as a tidal basin to maintain the required drainage capacity of the Betna and Marirchap rivers using MIKE11 onedimensional river modelling system developed by DHI Water and Environment. In the model set up the downstream boundary is defined as tidal boundary providing half an hourly tidal water level time series for the dry season. The upstream boundary is defined as flow boundary. Typical schematized river networks and beels of the TRM model is shown in [Fig. 2]. 4. Effectiveness of Beels for Tidal River Management A low-lying area (beel) can be utilized as a tidal basin for trapping up the incoming sediment with tide for maintaining sustainable design dredged section of the silted up reaches of the rivers. It is important to assess the effectiveness of a beel for TRM before implementation since all beels may not be effective to generate required volume of tidal prism for sustaining the drainage capacity of the river and sedimentation inside the basin. The integrating one beel with the Betna Fig. 2. Typical schematized river and TRM basins of TRM model

4 957 and one beel with the Marirchap River has been simulated for dry seasons to obtain tidal volume in a tidal cycle, which is the indicator for the stability of a tidal river as mentioned earlier. The findings of the TRM model studies are given in Table-1.The selected tidal basin for the Betna River is more effective and can generate more tidal prism than required tidal volume. On the other hand the selected beels for Marirchap River can generate a tidal prism which is slightly greater than the required tidal prism and can be considered as potential basin for maintaining the required drainage capacity of the river. The tidal range in the selected beels varies from 23cm to 196 cm depending on the topography and location of the beels. The required and generated tidal volume is shown in [Fig. 3]. It has been found that TRM Fig.3. Comparison of required and generated tidal can be successfully operated in these two river basins using the selected beels for about 08 volume for different beels ( ) years for sediment and drainage management which is based on the performance of TRM basin depending on the location, area and bottom topography of the beel, length and location of the connecting channel and also the river bathymetry. Table-1. Effectiveness of Beels as Tidal River Management (TRM) Basin Name of Beels Maximum Tidal Range (m) inside the proposed Tidal Basin Generated Tidal Volume (Million-m 3 ) Required Tidal Volume (Million-m 3 ) Operation Period (Year) Remarks Sukdebpur Beel Eight (8) Effective Ticket Beel Eight (8) Effective 5. Design of TRM Infrastructure Selection of design parameters is important for successful and effective operation of beels as tidal basin. Following are the parameters that have been considered in design stages: The area of the Tidal basin and its suitability to act as an effective basin. The location of the link channel that will connect the basin to the adjacent river. The alignment of the peripheral embankment encircled the basin. The location and size of the outlet structures to ensure the drainage of the area hinders by the construction of Fig. 4. Details of Sukdebpur TRM Basin

5 958 peripheral embankment The provision of other additional structures requires for TRM operation. The crest level of the peripheral embankment for the TRM basin is fixed considering a design water level inside the beel and 0.90 m free board without the future impact of climate change. The design section of embankment shall have crest width of 3.0 m and side slopes of 1:3 in the basin side and 1:2 on the country side. The detailed list of activities to implement the Sukdebpur Beel and Ticket Beel as TRM basin is presented in Table-2. Details of TRM Basin of Sukdebpur Beel and Ticket Beel are shown in [Fig. 4] and [Fig.5] respectively. Table-2. Civil Works required for Proposed Tidal Basin at Sukdebpur Beel & Ticket Beel SI Features Quantity and Design Parameters Sukdebpur Beel A Effective area 526 ha Total Length : km B. Peripheral Embankment C. Link Channal Crest level : 4.0 mpwd Top width : 3.0 m Side slopes :1:3 for R/S (Beel side) ; 1:2 for C/S (Land Side) 1 st Link Channel 2 nd Link Channel Length : 536 m (approx.) Length : 235 m (approx.) Top width : 40 m Top width : 40 m Bottom width : 28 m Bottom width : 28 m Bottom level : mpwd Bottom level : mpwd Side slope : 1:2 (Both sides) Side slope : 1:2 (Both sides) D. Drainage Outlet : 3 No. each 1-Vent, 0.9 m diameter RCC pipe E. Baily Bridge Length : 40.0 m Ticket Beel A. Effective area 545 ha B. Peripheral New peripheral embankment in the beel : 7.98 km Embankment 1 st Link Channel 2 nd Link Channel Length : 240 m (approx.) Length : 300 m (approx.) C. Link Channal Top width : 30 m Top width : 30 m Bottom width : 12 m Bottom width : 12 m Bottom level : mpwd Bottom level : mpwd Side slope : 1:2 (Both sides) Side slope : 1:2 (Both sides) D. Baily Bridge Length : 30.0 m Fig. 5. Details of Ticket TRM Basin

6 Results and Discussions TRM basins for sediment management to increase tidal prism, which will result in increasing the drainage capacity of the rivers. The study indicates that beels adjacent to Betna River basin and Marirchap River basin are effective as tidal basins in terms of generating required tidal prism for Tidal River Management. The selected Sukdebpur tidal basin for the Betna River is more effective and can generate more tidal prism (11.10 Million-m3) than required tidal volume (6.00 Million-m 3 ). On the other hand the selected Ticket tidal basin for Marirchap River can generate a tidal prism (3.50 Million-m 3 ) which is slightly greater than the required tidal prism (3.15 Million-m 3 ). Model results shows that these beels generate tidal prism in the range of 3.50 Mm 3 to Mm 3 individually, which is higher, compared to the required tidal prism for maintaining the design/proper drainage capacity of the river. 7. Conclusion Restoration of Betna River, Marirchap River by dredging/excavation alone would not sustain since resiltation rate is almost 60-80% during dry season as virtually there is no upstream fresh water flow to flush the huge incoming sediment into the rivers with high tide. Thus, in addition to dredging/excavation, implementation of TRM, using operation of potential tidal basins is required to trap up the incoming sediment inside the basins for sustainable sediment and drainage management. Continuous TRM operation of all the potential beels along the Betna and Marirchap river basin would be effective. 8. Acknowledgement The authors express their gratitude to Institute of Water Modelling (IWM) and Bangladesh Water Development Board (BWDB) for the support to use their study findings in the paper. 9. References [1] EGIS, Final Report, Environmental Impact Assessment and Social Impact Assessment of Khulna-Jessore Drainage Rehabilitation Project, Water Resources Planning Organisation, [2] HASKONING, Second Coastal Embankment Rehabilitation Project (Main Report), [3] Halcorw, Southwest Area Water Resources Management Project (FAP 4), [4] IWM, Final Report, Feasibility Study for Drainage Improvement of Polder 1, 2, 6-8 and 6-8 (Ext.) by Mathematical Modelling under the Satkhira District, [5] IWM, Final Report, Engineering Survey, Morphological and Hydrological Study with Mathematical Modelling for Rehabilitation of Bhutiar Beel and Barnal-Salimpur- Kolabashukhali Flood Control and Drainage project in Khulna District, [6] IWM, Final Report, Feasibility Study and Detailed engineering Design for Long Term Solution of Drainage Problems in the Bhabodah Area, Bangladesh Water Development Board, [7] IWM, Final Report, Feasibility Study for sustainable drainage and flood management of Kobadak river basin under Jessore and Satkhira districts, Bangladesh Water Development Board, [8] IWM, Final Report, Mathematical Modelling Study for Planning and Design of Beel Kapalia Tidal Basin, Bangladesh Water Development Board, 2009.