MORPHOLOGICAL PROCESSES AND EFFECTIVE RIVER EROSION MANAGEMENT: A CASE STUDY OF THE ARIAL KHAN RIVER IN BANGLADESH

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1 MORPHOLOGICAL PROCESSES AND EFFECTIVE RIVER EROSION MANAGEMENT: A CASE STUDY OF THE ARIAL KHAN RIVER IN BANGLADESH Jakia Akter* 1, Maminul Haque Sarker, Preetha Haque 1 Center for environmental and Geographic Information Services (CEGIS), House # 6, Road # 23/C, Gulshan-1, Dhaka-1212, Bangladesh. jakter@cegisbd.com ABSTRACT Every river is unique. Therefore, effective erosion management in rivers should be based on their morphological processes. The Arial Khan, a major and very dynamic river in Bangladesh, creates huge bank erosion which is one of the main sources of the sufferings of the local people. This article proposes a cost effective and efficient method of erosion management based on the morphological process of the Arial Khan River. Satellite images, old maps and hydromorphological data have been used to understand the morphology and planform of the river. Erosion in this river does not continue at a particular location or for more than 5 years. A quicker and cheaper option for bank protection would be suited for local scale erosion management. On the other hand, planform analysis of the Arial Khan River shows that the meandering length and amplitude are four and one kilometers respectively. In addition, as the river in an active delta, it should also be allowed the flexibility for land formation and development. For long term erosion management, fixing the nodal sections might be more efficient and cost-effective as well as environmental friendly than a rigid and costly solution. Keywords: Meandering bend development, riverbank erosion, morphological process, nodal section, erosion management. 1. INTRODUCTION Rivers in an active delta continuously respond to and adjust with the delta development process. A high rate of riverbank erosion and subsequent accretion are commonly observed morphological processes in the rivers of active deltas. Every river is unique (Schumm and Winkley, 1994); the type and extent of bankline migration largely depend on the characteristics of the physiographical setting where the river is placed and the morphological dynamics of the river itself. Measures for effective and efficient management of the rivers in an active delta should be based on the morphodynamics of those rivers. Thus the interventions should be based on proper understanding of river morphology. The Arial Khan River is a sweet water source for this region (Figure 1.1). The dynamic behavior of its off-take and the erosion processes of this river cause sufferings to several thousands of people. Navigation in this river has also deteriorated drastically during the last two decades. This article reports the historical development and morphodynamics of the Arial Khan River in different time-scales based on which effective and efficient interventions are outlined for sustainable river erosion management.

2 Figure 1: Study area 2. APPROACH Historical maps and time-series satellite images from 1973 to 2011 are the key data that have been used for preparing this article. Other than these, hydrological data such as on water level and discharge collected by the Bangladesh Water Development Board (BWDB), and hydrography survey charts prepared by the Bangladesh Inland Water Transport Authority (BIWTA) have been used along with some primary data collected during field visits. Historical maps, time-series satellite images as well as hydrological, hydro-morphological and planform data of the river have been used to understand its geomorphological development. The erosion prone area and their extent have been identified from planform analysis and subsequently, the future extent of erosion has been predicted. Based on the understanding of the hydromorphological characteristics of the river, its future morphological developments have been assessed and erosion management initiatives have been outlined.

3 3. HYDRO-MORPHOLOGICAL CHARACTERISTICS Discharge Presently, Chowdhury Char is the perennial off-take of the Arial Khan River. The seasonal and annual variability of discharge in this river is very high. The hydro-morphological condition of the river depends on the position of the off-take and the deviation of flow direction to the off-take from the parent river, the Padma. During the last few decades, the location and geometry of the off-take have been changed several times (CEGIS, 2010). Figure 2 shows the annual maximum flow of Chowdhury Char and the present off-take of the Arial Khan River (Figure 1) with respect to the annual maximum flow of the Padma River at Mawa. Maximum discharge data from 1965 to 2009 show that the trend is increasing in the Arial Khan River. However, data indicate that the flow volume of the Arial Khan was reducing rapidly during the first decade of the current century. The flow in the Arial Khan River is highly characterized by seasonal variation, as shown in Figure 3. The average flow during February and March is less than 50 m 3 /s, whereas the average flow of August and September is more than 2000 m 3 /s, indicating a very high variability of mean monthly flow. Figure 2: Annual maximum flow of the Arial Khan and Upper Kumar rivers The average discharge in February, representing the yearly minimum flow (Figure 4), shows that it was increasing up to 1973 and later dropped to almost zero. However, in the mid-1980s minimum flow started to rise and increased rapidly until the mid 1990s. After that period, it started to plummet again. On the other hand the average flow in August, representing the flood flow of the river, shows a flow pattern almost similar to that of February. CEGIS (2011 & 2012) has shown a strong co-relation with the planform of the Padma River to the discharge in the Arial Khan River. When the off-take of the Arial Khan was at the outer bank of the main channel of the Padma, the flow in the river was found to have increased. Until the 1970s, there were several off-takes of the Arial Khan River other than the present off-take at Chowdhury Char. Later, only the present offtake of the Arial Khan at Chowdhury Char became the only perennial source and hence the flow increased up to the late 1990s.

4 Figure 3: Annual stream flow of the Arial Khan River ( ) Figure 4: Yearly minimum flow in February and maximum flow in the Arial Khan A study conducted by CEGIS (2010) predicted the development of the Padma River at the Arial Khan off-take at Chowdhury Char. The prediction indicated that within the following decades, the location of the Arial Khan off-take would remain at the outer bend of the main channel of the Padma. This prediction suggests that the flow into the Arial Khan River would increase both in dry and wet seasons. Subsequently, the increase of monsoon discharge would increase river bank erosion. Water level The maximum annual water levels in the Arial Khan River show no significant changes over time, whereas annual minimum water levels show a noticeable decreasing trend (Figure 5). Water levels at off-take and Madaripur Town show that, after the late 1970s water level dropped about a meter when the main flow of the Padma was at its left bank and the off-take of the Arial Khan River was almost disconnected during dry season. As shown in Figure 4, the discharge decreased after the 1970s and the water level also showed a similar decreasing trend after the 1970s.

5 Figure 5: Annual maximum and minimum water levels Sediment Bed materials in the Arial Khan River consist of fine sand. Most of the bed material transportation occurs in suspension mode. A few sediment datasets were available under the FAP 24 study during Figure 6 shows that the sediment load in the Arial Khan River increased more substantially during the late 1980s, which was highly related to the discharge. The increase of bed material load was probably due to the favorable position of the off-take of the Arial Khan River. The decadal average bed material load in the Arial Khan River was about six million tonne per year in the 1990s, when the average annual volume of discharge was about thirty billion cubic meter. Figure 6: Yearly variation of discharge and bed material load 4. PLANFORM CHARACTERISTICS The planforms of the Arial Khan and the Upper Kumar have been analyzed based on some reaches shown in Figure 7. Reach A, Reach B and Reach C are identified from the Arial Khan off-take to the bridge, from the bridge to the off-take of the Upper Kumar River and from the off-take of the Upper Kumar river to Madaripur Town. The planform characteristics of the reaches are the base for reaching demarcation.

6 Width Reach-wise wetted surface area has been calculated from satellite images and the distance along the center line has been estimated. The length averaged width has been computed from the ratio of those two parameters. In the 1970s, there were more than one off-take of the Arial Khan and they were about 300 m wide. With the abandonment of the other off-takes, the width of the existing channel was increased. The 1943 Topo-maps show that the Arial Khan River had four off-takes from the Padma River. The width has started to increase since the early 2000s, whereas the average width of the Upper Kumar River is 135 m which shows it to be very steady in nature. Sinuosity The sinuosity of a river reach is the ratio of meandering length and the straight chord length. The Arial Khan River has a tendency to be straightened at a high rate. The Upper Kumar River is sinuous and during the last four decades its sinuosity was highest in the early 2000s. Reach A as shown in Figure 9, has a decreasing trend and presently the sinuosity has dropped below 1.5. The lower part of Reach A is bounded by the bridge and guide bunds of the bridge and hence this part has become more or less controlled. On the other hand, the upstream part of Reach B is also confined by the bridge and guide bunds. Before the construction of the bridge, the sinuosity of Reach B was increasing. However, there was a sudden drop after the constructing of the bridge. The guide bunds and associated structures are determining the river direction and so Reach B is about to become straight and then the sinuosity will become close to one. Probably the river response to the guide bunds is still continuing and spatial coverage may be up to the end of this reach. Reach C has a decreasing sinuosity trend. The figure for this reach is not representative as a huge loop cut occurred within this reach. In addition, the very dynamic off-take of the Upper Kumar River is placed within this reach. The straightening behaviours formed in the upstream reaches are propagating downward and causing Reach C to become less sinuous.

7 Figure 7: Planform changes in different reaches Figure 8: Changes in width over time

8 Figure 9: Changes in sinuosity over time Cut-off A number of cut-offs have occurred in the Arial Khan River during the last three decades, but the Upper Kumar River did not have a similar experience in that period. In fact, the underlying soil of the Upper Kumar River is of the low Ganges Floodplain, which is relatively less susceptible to erosion and has less stream power. The erodible material of the active Ganges Floodplain makes the Arial Khan River very vulnerable to erosion and cut-off. A cut-off occurred between 1973 and 1984 as shown in Figure 10. Another cut-off occurred at the same place after the construction of the Arial Khan Bridge between 1999 and The cut-off ratio of this location was about 3. Another great cut-off occurred close to the off-take of the Upper Kumar River and hence the offtake of the Upper Kumar River shifted by several kilometers.

9 5. EROSION PROCESS Bend development The riverbed of the Arial Khan River mainly lies on the old bed of the Padma River. The Padma (Ganges) flowed along the present course of the Arial Khan River even two hundred years ago as found in Rennel s map (1776) and Tassin s map (1840). Therefore, the bank and bed materials of the Arial Khan River are very erodible and susceptible to erosion. Its planform shows a meandering nature and a loop cut (Figure 10). The meandering to straightened planform and shifting of the river show the dynamism of the river. After straightening of the channel, another meander develops very rapidly and erosion occurs at that location, generally along the outer bend. As a result, many scrawl bars appear in satellite images. Studies conducted by CEGIS (2012) have shown the bend development in different reaches. Figure 10: Development of eroding bends Figure 10 shows the development of eroding bends at every reach over time and progression of bank erosion. Overland flow and sand spreading at the outer bank of the Arial Khan River are regular phenomena. This breaching causes sedimentation in the floodplain that makes land unsuitable for agricultural practices. All the eroding bends in the Arial Khan River are found to be translated downward and very often change their locations of erosion within 2-5 years.

10 The bend development in Reach A is classified mainly rotation with expansion, whereas translation with expansion is the bend development process in Reach B. On the other hand, only expansion is the development process in Reach C. The bend development process in the Upper Kumar River is very slow, as the river is located on the Low Ganges Floodplain whose materials are less erosiove as compared to the active Ganges Floodplain. From the bend development analysis, it is found that the river reaches, from the Arial Khan Bridge to Madaripur Town, maintain a four kilometer wave length with one kilometer meandering amplitude. Rate and erosion extent The rate of erosion and accretion along the banks of the Arial Khan River is measured as several hundred sq. meter every year. Analysis of banklines derived from time series satellite images of 1973 to 2010 reveals that erosion and accretion had occurred simultaneously. Erosion in settlement areas causes much damage than erosion occurring along cultivated or fallow lands. Figure 11 shows the erosion and accretion in the Arial Khan River in two different periods. Figure 11: Rate of erosion and accretion (km 2 y -1 ) in different reaches of the Arial Khan River In Reach A the erosion is found to be higher as it covers the off-take of the Arial Khan River, which is very dynamic. Moreover, in recent years this single off take of the Arial Khan has become active perennially and hence it has become more dynamic than in the past resulting in more erosion in the reach. Although accretion is also a governing issue in Reach A, these reclaimed lands are of no use initially due to huge sand deposition. However, erosion and accretion are not of much concern in Reach A, as production and settlement density are much less there. In the last two decades, erosion in Reach B has become almost half of the previous two decades and accretion has also reduced more than half probably as an effect of the Arial Khan Bridge. Ever since a cut-off has occurred in this reach, the river has been in the straightening process. The guide bunds of the Arial Khan Bridge have directed the river flow to a straight path. Huge erosion has been occurring in the last few years at the lower part of Reach B where the first bend developed after the Bridge location. In the lower part of the river the erosion and accretion processes are more or less balanced, although bend erosion at some outer river bends is causing large scale family migration. The figure reveals that the erosion rates of two of the three reaches (reaches B and C) were reduced, whereas the accretion rate has reduced in all the reaches. It is found that from 1993 to 2010, erosion was the governing process for all reaches.

11 6. MEASURES Erosion management in local scale The Arial Khan River is very dynamic and its eroding bend may not remain at a specific location for more than 5 years. In Bangladesh, it usually takes 3-4 years to implement bank protection measures as it involves time required for planning, approval for fund, design, tendering, awarding of the contract and implementation. Hence, permanent structures like revetments may not be useful for erosion mitigation because while preparatory activities are underway the river would likely have changed its erosion activity to another location, resulting in huge socio-economic loss. Therefore, this study suggests carrying out some emergency work, such as sand-filled geo-bag dumping at erosion-prone areas along the Arial Khan River, which is less costly and the money can be obtained from the emergency fund of the local government. Figure 12: Short-term erosion mitigation measures at erosion vulnerable locations A few locations have been suggested for interventions to achieve a short-term solution (Figure 10). An example of erosion prone area is shown in Figure 10. The figure illustrates the erosion vulnerable areas with settlements along the river bank in the next five years. Urgent bankline erosion protection measures can only resist damages. However even though the erosion extent in the Upper Kumar River is not much, it exists for a longer time scale. Hence, for protecting the land and settlement along the Upper Kumar, permanent structures like revetments may be a good solution. Long-term river management Effective solution for long-term river management would be based on the river morphology and planform characteristics. The flow regime of the Arial Khan River is changing with the changes in the local morphology of the Padma River at the Arial Khan off-take. As a result, the river is becoming unstable, and therefore requires training works to be carried out. If the off-take of the Arial Khan is fixed, a stable flow regime can be ensured and the river training works can become effective.

12 During the preparation of the outline of the river training works for arriving at a long-term solution, the river training works should have a close resemblance to the preferred forms of natural rivers. Training of the whole river may restrict the riverbank erosion completely, but this may not be economically viable or environment friendly. The Arial Khan is a dynamic river in an active delta and it has to be allowed flexibility for land formation and development. An attempt has been made to fix the river at an interval closely resembling the meandering wavelength as a means of long-term solution. Within two successive fixed points the river would have the freedom to move, but the spatial extent of oscillation over time would be much less than that of the present river (Figure 11). Based on the present morphological study, fixing of nodal sections has been suggested at a few locations from the Arial Khan River Bridge to Madaripur Town. About 4 km of meandering wave length has been considered for fixing the interval of the successive nodal sections. In addition, meandering amplitude has been estimated at about one km using time-series satellite images. After fixing the nodal sections, it is assumed that the extent of meandering oscillation would be limited within the meandering amplitude. The proposed outline of the river training works, however, will be required to be modified during implementation of the works to fit with the changing situation. Figure 13: Trained Arial Khan River For assessing the benefits from this type of intervention, a river envelope from the off-take to Madaripur Town has been drawn using several maps and time-series satellite images from 1943 to It is likely that the river may encroach into any area within this envelope at any time in the coming decades. Comparing the nodal oscillation extent with the river envelope, it is expected that fixing the nodal sections would help to reclaim about 52 sq km of land within 30 km of river

13 reaches from the bridge to Madaripur Town. Similar measures may not be applicable for the river reaches where meandering amplitude and sinuosity are very high. 7. CONCLUSIONS AND RECCOMENDATIONS The study demonstrates that the flow regime in the Arial Khan River varies over time depending on the morphology of the Padma River at the off-take, which makes the river very unstable. The meandering bend development processes of the Arial Khan River are mainly dominated by translation and expansion and erosion does not exist in one location for more than five years. However, the situation is different in the Upper Kumar River where erosion continues along its banks for a longer period. Local erosion management in the Arial Khan should be temporary and cheaper in nature, which may not be applicable for the Upper Kumar River. For long term erosion management in the Arial Khan River, fixing the nodal sections is proposed, which might be an efficient and cost-effective as well as environmental friendly solution. ACKNOWLEDGEMENTS The results of this article are mainkly based on studies carried out by CEGIS for the BIWTA and the BWDB. The authors are thankful to the Executive Director of CEGIS, Mr. Giasuddin Ahmed Choudhury, and their colleagues in CEGIS. REFERENCES CEGIS, Padma Multipurpose Bridge Design Project. River Training Works, Updated Scheme Design Report, Annex C: Morphological Analysis of Padma River, Volume II, prepared for Bangladesh Bridge Authority. CEGIS, Bank erosion and navigability problem in the river Arial Khan from Madaripur launch ghat to Kalikapur-Habiganj-Rajarhat-Srinadi via Tekerhat river route. Final report prepared for BIWTA, Ministry of Shipping, Government of Bangladesh. CEGIS, Morphological Processes of the Arial Khan River and Outline of the River Training Works Technical Report: 4, prepared for BWDB under the project of Capital Dredging, Ministry of Water Resources, Government of Bangladesh. Schumm, S.A. and Winkley, B.R., 1994; The Character of large Alluvial Rivers, In The Variability of the Large Alluvial Rivers, Schumm, S.A. and Winkley B.R. (eds.), pp. 1-9, Publ. by ASCE press, NY