Environ Earth Sci (2015) 73:6613–6623 DOI 10.1007/s12665-014-3884-3

ORIGINAL ARTICLE

Planform channel dynamics and bank migration hazard assessment of a highly sinuous river in the north-eastern zone of

Mithun Deb • Celso Ferreira

Received: 7 February 2014 / Accepted: 15 November 2014 / Published online: 23 November 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract Rivers or watercourses in Sylhet plain of Keywords River planform Á Regression analysis Á Bangladesh are very dynamic over time. The aim of the Hazard assessment Á Remote sensing and GIS present study is to investigate the channel planform dynamics, the prediction of future lateral migration of the bankline and assessment of the hazards associated with Introduction Manu River, Bangladesh. The name Manu River represents the river study reach from , to , The erosion and accretion process in river bends is a Bangladesh of 41.6 km. Channel contraction and expan- recurrent problem of great significance from an engineer- sion are very frequent in this river. Studies had shown that ing point of view. River erosion severely affects many the sinuosity of the river varies from 1.36 to 2.48. Also the human activities and generates countless sufferings to the meandering ratio of the river ranges from 0.32 to 1.67. people. Bank stability more often gets threatened by nat- Satellite data were collected and analyzed to determine the ural calamities like flash floods. Modifications in normal planform characteristics, meandering parameters and inputs result in transformations in the planform shape, bankline migration rates between 1975 and 2013. The sinuosity, and braiding index (Knighton 1984). Winter- study is based on the Landsat satellite imagery of 1975, bottom (2000) summaries the significance of studying 1988, 1999, 2003 and 2005 and aerial photograph of 2009 historical channel changes that can ascertain to be vital in and 2013 which were accumulated from United States understanding the revised channel dynamics. River channel Geological Survey. Specific critical bends have been changes can result in various environmental and social elected where bank erosion and accretion rates were severe. consequences such as difficulties in navigation manage- The purpose of this paper is to show the erosion rate, ment, flood hazard and the alteration of aquatic and behavior and pattern of upstream side of Manu River, riparian ecosystems (Li et al. 2007). Many large engi- Bangladesh through the study period of 38 years from 1975 neering projects such as the routing of highways, railways, to 2013 by comparing its different geomorphological as well as siting of bridges and buildings along rivers need parameters. This study has contributed to develop a model to take full attention of the impending migration of river for bankline migration prediction of the forthcoming years bank channel in future (Xu et al. 2011; Crosato 2009; Heo and address the physical hazard to provide a risk surface. et al. 2009). Present assessment has revealed that, there are some crit- Bangladesh is predominantly a riverine country where ical regions where erosion would be up to 29.18 hectares in river bank erosion is a yearly devastating phenomenon. 2025. Bangladesh is located in a large floodplain–delta complex of three mighty rivers: the , the Brahmaputra and the Meghna (GBM). Consequently, an enormous amount of M. Deb (&) Á C. Ferreira water flows through Bangladesh every year. Rahman Department of Civil, Environmental and Infrastructure (2010) estimated that every year an average of 870 Million Engineering, George Mason University, Fairfax, VA 22030, USA Acre Feet (MAF) of water flows into the country from e-mail: [email protected] India. Throughout the peak flow season (July–September), 123 6614 Environ Earth Sci (2015) 73:6613–6623 most of the rivers usually overflow their banks onto the location it is the essential source of livelihood to the people low-lying adjacent flat land which inundates large areas of Moulvibazar, Bangladesh from a long period. Heavy and causes widespread damage to crops and property. Elahi rainfall in the mountainous reach as well as in the plains, (1991) evaluated that about one million people are directly lack of adequate slope to drain out the extraordinary dis- affected each year by River bank erosion in Bangladesh charge of the rivers and accumulative silt load due to and most of them writhe from its consequence and cul- amplified deforestation are the main causes of flood and minate as landless displaces. Elahi (1991) and Islam (2000) river bank migration in this basin. The soil type of Manu concluded that river bank erosion and population dis- River embankment is sandy-clay (Hossain and Sakai placements are the recurring social and economic problems 2011). The mean rainfall in the area is about 2,800 mm and in Bangladesh. Population transposition due to flood and 65 % of this is concentrated in the month of May–Sep- bank erosion is considered as one of the major contributors tember (Saleh 1996). The mean highest and lowest water to landlessness and destitution of rustic population of levels of Manu River are 18.88 and 12.77 m, respectively. Bangladesh. Plentiful sites through this river basin have writhed from The extensive bank erosion in the Manu River basin had channel migration as a result of erosion that threatened led to numerous public and economic consequences. The bordering settlements and substructures. Studies have analysis of Manu River is of valuable importance to the shown that, the sinuosity of the river varies from 1.36 to people living in the north-eastern zone of Bangladesh 2.48 and the meandering ratio of the river ranges from 0.32 because of its unstable and highly sinuous nature. The to 1.67 (Deb et al. 2012). To evaluate the river bank ero- purpose of this work is to create a future risk surface sur- sion hazard, 13 vulnerable sites (Fig. 1) were assessed rounding the river by continuously assessing previous along the study reach of 41.6 km which are migrating bankline erosions. Furthermore, primary benefit of this enormously with the time. work will be to help the civilizations living near the shoreline to shield their resources. The novel data pre- sented here are evidently of scientific awareness to those Methods pursuing to understand the hazards of river bankline shifting and are also important to pragmatic issues such as In this study, Landsat MSS, TM and ETM images of the predicting channel migration rates for engineering and study area were collected to develop the georeferenced planning resolutions. Kinoshita (1957, 1962) and Leopold data. In the first step, the satellite images were georefer- et al. (1957) had contributed the basic idea of the rapid enced using ArcGIS 10.1. Consequently, banklines were development of the meandering channel migrations. River digitized from the georeferenced satellite imageries using erosion and accretion are natural processes in an alluvial the same software and then overlapped. The superimposed river, though alluvial rivers are fundamentally dynamic in banklines gives the inclusive channel migration configu- nature, reacting to the disparity in water and sediment ration of the Manu River from 1975 to 2013 and the rate of inputs. However, local developments such as sand exca- erosion and deposition (Fig. 2). Linear regression method vating, bank revetment and land use modifications can is used to determine the midline position of the river in change the normal geomorphologic dynamics of rivers 2025 using the historical values. Then finally, buffer lines (Batalla et al. 2004; Surian 1999; Kesel 2003; Surian and were created surrounding the vulnerable sites for the year Rinaldi 2003). of 2025 to predict and generate some ideas about the hazard which is associated with erosion and accretion in the river. Study area Data processing The Manu River is one of the most dynamic and unstable rivers in Bangladesh originating in the Tripura region. It A number of datasets were used to calculate the riverbank flows from the Kahoisib peak of the Sakkanklang range transformations. Selecting an appropriate cell size is not located in the Hill Tripura (India) and passes through always simple. Basically, in GIS, accuracy of the results various narrow gorges with escarpments of naked rock completely depends on precision of the datasets. The main rising often 30 m and more, and cutting into deep and clear datasets used in this study were pulled together from dif- pools. As it descends into the more level plain, it becomes a ferent types Landsats, mostly based on the availability. broad inactive stream with a meander course. The river Landsat MSS images from 1975, TM images from 1988 enters Bangladesh through in Moulvibazar and ETM images from 1999, 2003, 2005, 2009 and 2013 district, when it changes to northwest and north to meet the were collected from United States Geological Survey Kushiyara at Manumukh. Because of its geographic (USGS) (http://landsat.usgs.gov/). Time series of historical 123 Environ Earth Sci (2015) 73:6613–6623 6615 datasets were inconsistent and resolution of these rasters application of GIS provides a spatial dimension to mor- varied with time also. Like, Landsat MSS image of 1975 phometry, which helps to understand the variation of has 80 m resolution, while all other TM and ETM images quantitative morphometric parameters (Thomas et al. were collected with 40 m resolution based on the available 2012). Essential revisions by Gurnell et al. (1994) and resource. Therefore, we expect to have some distortions Gurnell (1997) have delivered a valuable perception into and errors up to 10–15 m from 1975 imagery due to its the possibilities that GIS proposes for river channel vari- larger cell sizes. Those images were typically attained in ation investigation. One of the main difficulties in digitiz- the month of February–April. The assessment of the riv- ing the outline of gravel-bed rivers from aerial photography erbank location was carried out in six phases, 1975–1988 is the channel boundaries, which are not as well described (13 years), 1988–1999 (11 years), 1999–2003 (4 years), as those for rivers with a more meandering planform. River 2003–2005 (2 years), 2005–2009 (4 years) and 2009–2013 channel was defined as an elongated area where streamflow (4 years) for both the riverbanks (Fig. 3). This investiga- occurred with sufficient frequency, force, and duration to tion was completed to measure the alterations in riverbank preclude the presence of vegetation such that 90 % or hazardous location that have followed over time and to greater of the area is bare ground or water (Gurnell 1997; isolate the places where erosion and accretion occurred in Winterbottom 2000; Tiegs and Pohl 2005). Nicoll and every particular time period. Hickin (2010) digitized riverbank outlines using the water periphery to represent the verge of the channel as this Data analysis boundary is clearly distinct in Landsat images. Using ArcGIS 10.1, incessant polygons were created to epitomize GIS proved to be an effective and accurate tool for quan- the river channel in each year using the Landsat images. tifying and analyzing channel change along the study Consequently, these polygons were superimposed using reach, both at the medium-term time scale through the use Geographic coordinate system and WGS 1984 datum of a variety of map data sources (Winterbottom 2000). The (Fig. 2). The riverbank erosion and accretion were

Fig. 1 Study area and cross-sectional locations 123 6616 Environ Earth Sci (2015) 73:6613–6623

Fig. 2 Super-imposed shape files of different years calculated separately for each side of the riverbanks and migration of channel midline from it. Finally, regression time phases. Along the study reach of 41.6 km from India method was used to compute the location of midline in border to Moulvibazar, Bangladesh, 13 critical areas have 2025 at different cross-sections (Table 2). been identified where channel migration is threatening the surrounding populations and their assets. Hence, using the discovered erosion and accretion values, linear regression Results and discussion method was practiced to find out the probable shifting location of the mean channel midline in 2025. It was The landsat images quantified the significant change necessary to identify the probable location of the midline to occurred in Manu River between 1975 and 2013. Several create the buffer surrounding the vulnerable area and to river morphological parameters underwent critical changes discover the future hazards. by causing enormous loss to the river bank sides. The area of the study reaches and mean channel width fluctuated Midline prediction using linear regression method abruptly with the time. Nevertheless, sinuosity values in the study range showed that, it is consistently a highly Das et al. (2012) and Richard et al. (2005) had extensively sinuous river (Table 1). Moreover, channel midline shift used the linear regression method to predict the bankline was also determined for hazard analysis for the year of position for computing the future erosion around the river 2025 using linear regression method. bank. For Manu River future hazard analysis, we applied the linear regression method to estimate the midline posi- Planform dynamics tion of the channel for 2025 using historical values from the digitized shape files. Initially, superimposed shape file The Manu River reaches from India border to Moulvibazar, of 1975 was designated as a base year for the study. Bangladesh have experienced major changes along its way Consequently, positions of the following shape files were through the entire study period of 38 years. The actual measured from the base year of 1975 to evaluate the lateral length of the study reach varied from 41.6 to 46.2 km, 123 Environ Earth Sci (2015) 73:6613–6623 6617

Fig. 3 Bend migration analysis at cross-section 06

Table 1 Planform 1975 1988 1999 2003 2005 2009 2013 characteristics of the study reach Actual length (km) 44.36 43.67 46.2 44.8 42.05 42.1 41.6 Axial length (km) 20.43 20.42 20.9 20.79 20.74 20.94 20.68 Area (sq. km) 5.03 3.8 3.55 5.08 3.66 4.55 5.38 Mean channel width (m) 113.39 87.02 76.84 113.39 87.04 108.08 129.33 Sinuosity 2.17 2.14 2.21 2.15 2.03 2.01 2.01 where the axial length remained the same. The area and (leftward direction) at section 11 and 10, respectively width of the river fluctuated with time. Table 1 shows that, (Fig. 4). Erosion and accretion rate varied throughout the mean channel area and width endured to 3.55 sq. km and whole study period on a large scale. Figure 5 illustrates 76.84 m in 1999. Meandering ratio of Manu River varied that, erosion rate peaked to 15 m/year at left bank and from 0.32 to 1.67 at some specific locations (Deb et al. accretion went up to 20.55 m/year at right bank of sec- 2012). Sinuosity of the entire study reach climbed to 2.21 tion 10. Channel mean width varied at cross-section 07, 08 in 1999 and decreased through the time (Table 1). and 10 where it increased in the first two but got narrowed at the last one. Bankline change and erosion Evaluation period: 1988–1999 Evaluation period: 1975–1988 The study reach suffered from more vulnerable effects of River bankline changes during the period 1975–1988 are erosion during 1988–1999 than the previous one. During presented in Fig. 4 for different sections in the study reach. this study period, section 04, 08, 09 and 13 of the study The maximum left bank migration found is of 238.6 m reach were affected severely because of the bankline (rightward direction) and right bank shifting of 267.13 m movement. The left bank and right bank of section 04

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Fig. 4 River bank shifting assessment for the whole study reach, (?) value indicates movement of the bank to the right side and (-) indicates to the left side

Fig. 5 River bank erosion/accretion rate for the whole study reach, (?) value indicates accretion to the right side and (-) indicates erosion reached to 279.36 m and 286.72 m simultaneously Evaluation period: 2003–2005 (Fig. 4). As can be observed from the Fig. 5, the erosion and accretion rate at section 04 and 13 were also of sig- The overall change indicated a leftward movement of the nificant value of 25 and 16 m/year, respectively. channel planform in the study time. The maximum river bank shifting was registered by cross-section 08 and 09. Evaluation period: 1999–2003 Right side of the section moved up to 61.66 m to the left direction during this interval by narrowing the channel There was no significant change in the banklines during the width (Fig. 4). Erosion rate peaked to 30 m/year at time period of 1999–2003 except section 12. In cross- location 09 toward the left direction. Meanwhile at section 12, the left and right side of the river migrated location 07, it decreased the channel width by accreting at leftward in a large scale (Fig. 4). Consequently, the erosion both sides. and accretion rate were found to be 59.14 and 53.09 m/year, which caused serious erosion and damage sur- Evaluation period: 2005–2009 rounding the bankside. Mean channel width of the river increased to a large scale at section 01 as it got eroded in There was a substantial shift of river bank at locations 01 both sides. Moreover, because of the large amount of and 06 during 2005–2009 (Fig. 4). Apparently, maximum erosion in the right bank also of cross-section 07, width got bank migration occurred at location 01 where both banks increased there. moved to the right side by 100 m. In addition, left bank of

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Fig. 6 Channel mean width variation

Table 2 Channel midline shift analysis and prediction using regression, (?) value indicates midline shifting to the right side and (-) indicates to the left side Cross-sections 1975–1988 1975–1999 1975–2003 1975–2005 1975–2009 1975–2013 1975–2025 Shift R2

1 -42.07 10.55 16.032 20.06 186.93 166.24 173.62 0.49 2 115.88 255.19 247.21 232.74 236.77 231.07 360.94 0.83 3 -163.86 -183.44 -230.36 -255.97 -252.03 -262.53 -368.92 0.92 4 -134.9 -447.39 -545.65 -537.67 -661.38 -708.37 -956.83 0.98 5 -14.97 139.7 391.46 415.25 457.98 466.34 655.15 0.83 6 -85.08 -306.42 -392.81 -429 -395.44 -411.3 -618.075 0.92 7 119.8 224.36 298.07 309.51 362.35 347.99 499.34 0.98 8 -17.58 -138.33 -176.87 -214.09 -283.53 -362.47 -417.97 0.89 9 21.64 184.88 168.67 226.28 207.85 187.76 308.37 0.83 10 -240.39 -380.19 -440.86 -502.19 -508.2 -478.42 -722.6 0.94 11 238.84 413.65 436.17 418.39 440.92 437.39 654.13 0.90 12 -13.58 -11.69 -262.13 -258.76 -290.03 -322.02 -416.48 0.73 13 68.92 252.1 304.22 277.91 322.18 345.29 484.39 0.95 cross-section 08 migrated to the left direction by 90 m and Midline prediction in 2025 using linear regression caused channel widening. Erosion and accretion rate at method section 01 during this study time reached up to 38 m/year by causing hazards surrounding the bank side (Fig. 5). The midline of Manu River was determined for different years to document the trend of the channel shifting. It was Evaluation period: 2009–2013 observed from the assessment that, midline of the river is migrating linearly in most of the sections (Table 2). During During 2009–2013, there were instances of leftward shift- assessment at cross-section 01 and 12, midline shifting was ing of both banklines in the whole study reach. Right bank found to be inconsistent as it changed abruptly to the left movement of the river observed to be maximum at cross- and right direction. Channel movement to the right side section 08, where it peaked to 100 m to the leftward was found as a priority among cross-sections through the direction with an erosion rate of 25 m/year. Width of the whole site. Assessment of R2 showed a good linear relation channel increased during this study time at section 01, 03, between the values while it lowered to 0.49 and 0.73 at 06 and 12, while it narrowed at locations 10 and 13 cross-section 01 and 12, respectively. Figure 7 shows the (Fig. 6). linear regression analysis at cross-section 07 and 13.

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Fig. 7 Prediction for midline of the river in 2025 using linear regression method

Fig. 8 Hazard assessment at cross-section 04 and 06

Table 3 Future hazard assessment for different locations at Manu River Cross-sections Location Position Erosion (hec) Accretion (hec)

1 Dighirpar Bazar 91°59020.27500E–24°23032.02800N 1.33 0.4 2 Sydal Bazar 91°58035.36900E–24°25013.19800N 17 11.02 3 Sydal Bazar 91°57046.28200E–24°25016.18900N 11.72 10.98 4 Pirer Bazar 91°56059.25500E–24°25020.03600N 25.44 19.02 5 Hajipur 91°5708.49600E–24°25037.86100N 15.4 12.6 6 Tilagaon 91°55047.09500E–24°26013.2600N 29.18 22.2 7 Kuokapun Bazar 91°55021.34900E–24°26059.07800N 11.7 9.78 8 Tilagaon 91°55015.09700E–24°27053.03800N 5.63 4.58 9 Noya Bazar 91°55026.87100E–24°27034.59200N 4.13 3.14 10 Bhoter Bazar 91°54046.27900E–24°2801.63300N 27 27.058 11 Kamar Chak 91°52045.85900E–24°29037.53400N 19.09 16.18 12 Kamar Chak 91°52029.91200E–24°29026.06600N 8.77 9.26 13 Dorji Mahal Bazar 91°5106.06500E–24°29051.68600N 11.44 10.29

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Hazard assessment their sustainability. Table 3 illustrates that, the maxi- mum erosion of 29.18 hectares will occur at cross- Sighting the midline position of the river in 2025 in section 06 by increasing the mean river width. More- different cross-sections, buffer lines were created by over, it will erode and accrede 27 hec via shifting the using ArcGIS 10.1 with average width of diverse years river in cross-section 10 in 2025. Hazard assessment (Fig. 8). Reason behind this noble work was to estimate has included only land area damage and erosion theerosionandaccretionthatwilloccurin2025at detection for this study. different critical areas. The resulting channel changes cause various environmental and socioeconomic conse- Impacts of hydrologic regime changes quences in terms of navigation, loss of riparian land and infrastructure, flood hazards and the alteration of Hydrologic regime variation is a vital contributing factor of aquatic and riparian ecosystems (Yao et al. 2013). This hydraulic geometry change (Leopold et al. 1964; Knighton outcome has addressed the physical hazards which are 1984; Petts and Amoros 1984; Merritt and Wohl 2003). certain in the near future and surely it is going to help The behavior of the selected reach of the Manu River in the people living near the shoreline to make a plan for response to confined and widespread flooding has been scrutinized over a period of 38 years. Precipitation data were collected for the study duration to show the monsoon (June–September) and post-monsoon (October–January) rainfall variation in the project area (Fig. 9). Discharge and water level were collected from Bangladesh Water Development Board (BWDB) of 38 years from 1975 to 2013 to analyze the hydrologic conditions of Manu River. Figure 10 shows the discharge variation in the river, where it went to 1,000 m3/s during the high flood condition and became 3.0 m3/s at the dry period. This vital oscillation of discharge is one of the main reasons of vulnerability beside the river side. Each year during dry period, sediments were getting deposited in the river bed to a huge amount which had constantly reduced the channel cross-sectional depth. Meanwhile, when flash flood approaches from the hilly region, it creates pressure to the river concave bank which is of clay soil and erodes the channel. River water level Fig. 9 Precipitation values during monsoon and post-monsoon data at two different stations were also assembled for the period study (Fig. 11).

Fig. 10 Maximum and minimum discharge at Manu River 123 6622 Environ Earth Sci (2015) 73:6613–6623

changes, detailed river cross-sectional data and recent hydrologic data. Improvements to the hazard prediction in these areas can be further developed with these studies. In summary, it is obvious that the riverbanks had experienced a moderate to high erosion rate in Manu River basin which resulted in hazards in the surrounding bankline areas and will continue to devastate in future unless proper protective measures are taken into consideration.

References

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