Himalayan Piedmont Research Articles

Reach scale channel planform dynamics of transboundary River Jaldhaka within Himalayan Foreland Basin

ABSTRACT The slopes of Himalayan piedmont and foothill plain are being modified by the Himalayan origin river systems. In this study, spatiotemporal dynamics of channel planform of braided River Jaldhaka have been estimated using United States Army Map (1956) and landsat satellite images (1980-MSS, 1990-TM, 2000-TM, 2010-ETM+ and 2022-OLI) for the delineation of banklines, channel length, sinuosity index, braiding index, channel widths, identification of partial channel avulsion sites and bankline migration using Digital Shoreline Assessment System (DSAS) toolbar. Linear Regression Rate, Net Bankline Movement and End Point Rate were also calculated. Results revealed that the lower reach is more sinuous (maximum average channel sinuosity 1.65) compared with reach-I (1.29) and II (1.11)). The maximum channel width was in the year 2022 (968.6 m) and minimum average channel width was in 1956 (239.87 m). The maximum BI value (3.55) has been calculated in the sub reach-2 (SR-2, reach-I). Similarly, DSAS output shows maximum average EPR values (6.43 m/year) in lower reach which was most erosion prone bank. Bank facies study showed that non-cohesive bank materials accelerate rapid rate of channel avulsion and bank erosion. The variation of discharge, velocity, presence of vegetation and anthropogenic interventions controls the nature of planform changes of Jaldhaka River.

Preliminary assessment of the suspended sediment dynamics in the Sikkim–Darjeeling Himalayan river

The Sikkim–Darjeeling Himalaya region receives the highest amount of rainfall along the whole southern Himalayan margin and is known for the occurrence of extreme hydrometeorological and geomorphological events. The massive amounts of water and sediment transported each year through the mountain part of the Teesta River drainage system (∼8,150 km2)––the largest river in the region––have been severely impacted by dam construction in recent decades. The aim of the current study was to determine, for the first time in this part of the Himalaya region, the dynamics of suspended sediment transfer at a number of points distributed through the mountainous part of the Teesta River catchment prior to dam construction and preliminarily assess the impact of dam operations on the suspended sediment. Sediment sources were identified using a database of landslide inventories from 1965 to 2019, combined with visual interpretation of satellite imagery from the U.S. Corona programme and Google Earth. Hydrological and sediment data up to the second half of the 1990s were used to reconstruct the discharge and suspended sediment dynamics before direct human intervention in the river channels. The beginning and end of the construction of the reservoirs was determined by analyzing satellite images. The impact of dam operations on the suspended sediment was compiled from the available literature. The results of the current study indicate that the primary sources of sediment are landslides caused by the interaction of rainfall and road undercutting of slopes as well as channel erosion. During extreme rainfall events, the influence of deforested areas in the mobilization and delivery of sediment to the river network increases. The current analysis reveals that reconstruction of the suspended sediment dynamics should take into account the course of extreme events responsible for supplying material to the river network, as well as the long-term remobilization of already deposited sediment in the river channel. It was found that the mean suspended sediment load (SSL) following extreme rainfall, flooding, and landslides in the Teesta River catchment can be up to four times higher than its average values for the same catchment unaffected by such an event, and the effects can be observed for more than a decade afterwards. Under these conditions, the mean suspended sediment yield can reach 12,000 and up to 20,000 t/(km2·y) in individual years, which is among the highest in the Himalaya region and, indeed, the world. The construction of 13 dams in the last 30 years has disrupted the hydrological regime and sediment transport in the Teesta River catchment along 70% of its main course and largest tributaries, and this has resulted in the selective retention of coarser material in the reservoirs and a reduction in the SSL in the Himalayan piedmont. The high density of the dams suggests that further transport of suspended sediment will depend on the efficiency of the water and sediment management at the reservoirs, which may be affected by irregular natural extreme events.

Groundwater over-exploitation driven ground subsidence in the himalayan piedmont zone: Implication for aquifer health due to urbanization

Unsustainable groundwater exploitation often triggers inelastic aquifer compaction, endangering future groundwater potential. The Himalayan piedmont fan regions are undergoing fast urbanization and experiencing rapid groundwater depletion and water table drop. The piedmont fans have laterally varying aquifer geometry with slope-dominated aquifer water flow and experience variable spatiotemporal changes in response to groundwater depletion. We estimated the ground deformation over piedmont fans around Chandigarh (India), a growing urban centre, during 2014–2022 using satellite-based Interferometric Point Target Analysis (IPTA). We used both ascending and descending Differential Interferometric Synthetic Aperture Radar (DInSAR) acquisition modes to derive the vertical ground deformation by combining their line of sight (LOS) velocities. The region is experiencing vertical subsidence up to ∼60 mm/yr with prominent hotspots, where the temporal subsidence is compared with the groundwater cone of depression and urbanization over decades. Comparing the subsidence with the aquifer parameters from the bore wells suggests that the clay confining aquifer level III and semiconfined level II in the distal fan are experiencing the highest subsidence. The stress-strain relationship of these hotspot regions reveals the inelastic compaction of the aquifers producing severe subsidence. The piedmont zones of the Himalayas with identical aquifer geometry and population growth are facing similar challenges, and the combined DInSAR-IPTA and observational groundwater data modeling could provide a robust assessment for effective groundwater-aquifer health monitoring and management.

An introspect of neotectonic response on channel bars morphodynamic of the rivers in Darjeeling Himalayan piedmont zone, India

ABSTRACT The present research focuses on the recent tectonic activities and their effect on quaternary deposition such as channel bars in the 80 km × 50 km river corridor of the Darjeeling Himalayan Foothill Region (DHFR). The active orogeny belt has direct control over the fluvial deposition in the master river systems of Tista, Jaldhaka, and Torsa and their sub-systems of Neora, Murti, Kurti, Lish, Gish, Chel, and Mal. The present research has used some indices like the stream-length gradient index , mountain front sinuosity , valley-floor width to valley height ratio , and especially longitudinal profiles of the river. Channel bar area, longest length, width of channel bar, elongation ratio, relative relief, and relief ratio were calculated to get the introspect of neotectonic response on channel bars morphodynamic. The results on selected 10 channel bars of the upper DHFR depict that they are highly affected by neotectonic deformation. The channel bars of the middle part of the DHFR also have significant morphological signatures of tectonic movements. The reflection of these active tectonics is visible in the channel bars of the lower DHFR. These channel bars show a high rate of aggradation responsible for continuously changing the morphology and morphometry of these bars.

Distribution of active tectonics in the Himalayan piedmont (Darjeeling, eastern India) inferred from Horizontal‐to‐Vertical spectral ratio analysis of passive seismic records

AbstractThe pattern of active deformation of frontal structures in Darjeeling Himalaya is complex with out‐of‐sequence reactivations in the chain and development of scarps associated with earthquake ruptures reaching the surface in the piedmont. To clarify the distribution of active deformation in this area, we analyse passive seismic records by the Horizontal‐to‐Vertical Spectral Ratio method along three NS trending profiles. We image the Siwalik sedimentary rocks / recent deposits interface under the piedmont and show folded and faulted geometries. Two of these faults are located under scarps of about 10 m affecting the 3.7 ± 0.7 ka old surface of the Tista megafan. Such features imply that about half of the convergence is expressed south of the Himalayan front while the other part occurs out‐of‐sequence in the chain, suggesting a very limited activity of the Main Frontal Thrust itself.

Suitability assessment of soils of Panchagarh and Thakurgaon for tea (Camellia sinensis L.) and orange (Citrus aurantium L.) cultivation

Experiments were conducted in soils of the five Upazilas, namely Atwari, Tetulia, Boda, Ranisankail and Panchagarh Sadar of Panchagarh and Thakurgaon districts of Bangladesh to assess their potentialities for tea and orange cultivation. Geographic Information Systems (GIS) based soil suitability map was prepared depending on the hydrological, land and soil characteristics. It was observed that about 86, 76 and 50% soils in Atwari, Tetulia and Panchagarh Sadar Upazilas, respectively are suitable for tea and orange cultivation. But in Boda and Ranisankail Upazilas above 60% soils are suitable for cultivation of tea and orange. Besides, five representative soil pedons, namely Bhajanpur, Ranisankail, Pirgachha, Domar and Dimla were studied and compared with the standard suitability formats. The results revealed that soils of the Thakurgaon and Panchagarh districts under the Old Himalayan piedmont plain are suitable for tea and orange cultivation. There is a vast scope of utilizing the land and soil resources of the above sites for tea and orange farming.

Formation, Migration, and Morphodynamic Alteration of 50 Channel Bars in Darjeeling Himalayan Piedmont Zone, India

Channel bars are common, striking fluvio-geomorphic depositional features of alluvial rivers. The study of this article has aimed to investigate the formation, migration, and morphodynamic alteration of channel bars (n = 50) in gravel braided alluvial rivers in Darjeeling Himalayan Piedmont zone. Dynamics of micro to meso bar deposition is mainly accomplished by the channel gradient, huge upstream landslide, and variation of discharge. Multi-criteria analysis method has been used to explain the variation of principal component analysis (PCA1, PCA2, and PCA3), both monsoon to post-monsoon. Bridge scouring, empirical aggradation, and degradation values are 0.1 to 1.05 m and [Formula: see text]. Migration, re-generation, and degradation of the micro bars with their optimum morphology show changes during the rainy season. In the upper ([Formula: see text]), middle ([Formula: see text]), and lower ([Formula: see text]) piedmont region, the bar dynamic area are 0.12 to 0.71, 0.12 to 2.83, and 0.10 to 5.43 km2, respectively. In total, 58% to 72% stability of the channel bars is observed in the upper piedmont region. The coefficient of determination ( R²) of channel bar area and width shows positive (0.63) relation.

Response of sediment flux, bridge scouring on river bed morphology and geomorphic resilience in middle-lower part of river Chel, Eastern Himalayan foothills zone, India

Dynamism of river bed morphology is continuous and an integrated process of mountain's natural outflow in Himalayan Foothill Rivers. In-stream seasonal sediment flux, bridge establishments, bridge scouring and railway-road construction have detrimental influence over the natural flow regime of the river thereby causing channel degradation in the piedmont zone of eastern Himalaya of India, specifically near the urban areas where river management strategies are very weak. Such extreme anthropogenic events have a distinctive influence in changing river bed morphology in the middle-lower part of river Chel in Darjeeling Himalayan piedmont zone of India. Huge sediment aggradation and transportation networks historically have affected River Chel and have caused channel avulsion which poses a serious threat to the physical and ecological environment of Chel. Aggradation and lifting of sediments from the river bed are fatal for the geomorphologic character of the river, though no studies have been undertaken yet, to analyse the situation quantitatively in this regard. This paper presents evidence that suggests the changing river bed morphology, the dynamic nature of the river, the poor health of the river which is principally the impact of the seasonal sediment flux and bridge construction and thus some methods of river restoration in the middle-lower part of the river have been explained. Agglomeration of sediment, flow bifurcation, the volume of seasonal water mass, bed elevation changes and scouring are a major dynamic disposition for change of river bed morphology. Point bar, sidebar, braided active channel, ripples, and rise in the bed elevation are rapid changes in upper to lower part of bridges that are examined in the present study. In the dry season (December to February), a huge amount of sediment lifting makes incommodious man-made holes and a number of emergent channels which principally control the formation of a new active channel in the next monsoon season. The avulsion of major intra-active channels and morphological changes of the river bed for the last 85 years have also been observed in this study.

Amendment of Acidic Soil with Lime and Manure for Enhancing Fertility, Nutrient Uptake and Yield of Wheat-Mungbean-Monsoon Rice in the Old Himalayan Piedmont Plain

Soil acidic conditions and the decline in soil fertility are among the critical factors that constraint higher crop productivity in the Old Himalayan Piedmont Plain (OHPP), Bangladesh. The study was conducted to evaluate the effect of lime and manure on soil fertility, nutrients and yields of wheat, mungbean and rice. Experiments were done at Agricultural Research Station (ARS), Bangladesh Agricultural Research Institute (BARI) farm and farmer field over two consecutive years with the cropping pattern, namely wheat-mungbean-transplanted (T.) aman rice/monsoon rice. The varieties used were Bijoy for wheat, BARI mung6 for mungbean and Bina dhan7 for T. aman rice. There were nine treatment combinations with three lime levels (0, 1 and 2 ton dololime ha-1) and three manure treatments (poultry manure, farmyard manure and no manure) with three replications. The rate of poultry manure was 3 t ha-1 and that of farmyard manure was 5 t ha-1. Nutrients from manure sources were supplemented with chemical fertilizers to adjust recommended dose. Lime was added to the first crop for entire two crop cycles and manures were applied to the first crop of each crop cycle. Soil pH increased by 0.5-1.11 units, the higher values were observed with higher rates of lime application. Soil organic matter (SOM) increased slightly due to manure treatment. Soil phosphorus availability increased, zinc and boron availability decreased, but the potassium and sulphur availability remained almost unchanged after liming. Application of lime and manure had significant positive effect on the yield of wheat, and their positive residual effects on mungbean and T. aman rice. The effect of 1 t lime ha-1 was comparable with that of 2 t lime ha-1. Between two manures, poultry manure performed better than FYM on crop yields. The trend of plant nutrient uptake by wheat, mungbean and rice followed the trend of these crops yield increase, i.e., crops that were able to uptake more nutrients shown higher yields. The treatment combinations with 1 t ha-1 lime and 3 t ha-1 poultry manure produced an average 35-55% yield benefit over control for the first crop (wheat) and 41-43% yield benefit for the third crop (T. aman rice). This study suggests that dololime @ 1 t ha-1 coupled with poultry manure @ 3 t ha-1 or FYM @ 5 t ha-1 would be an efficient practice for better soil acidic condition, soil fertility and productivity of crops in the Himalayan piedmont soil of Bangladesh.

Neoglacial climate anomalies and the Harappan metamorphosis

Abstract. Climate exerted constraints on the growth and decline of past human societies but our knowledge of temporal and spatial climatic patterns is often too restricted to address causal connections. At a global scale, the inter-hemispheric thermal balance provides an emergent framework for understanding regional Holocene climate variability. As the thermal balance adjusted to gradual changes in the seasonality of insolation, the Intertropical Convergence Zone migrated southward accompanied by a weakening of the Indian summer monsoon. Superimposed on this trend, anomalies such as the Little Ice Age point to asymmetric changes in the extratropics of either hemisphere. Here we present a reconstruction of the Indian winter monsoon in the Arabian Sea for the last 6000 years based on paleobiological records in sediments from the continental margin of Pakistan at two levels of ecological complexity: sedimentary ancient DNA reflecting water column environmental states and planktonic foraminifers sensitive to winter conditions. We show that strong winter monsoons between ca. 4500 and 3000 years ago occurred during a period characterized by a series of weak interhemispheric temperature contrast intervals, which we identify as the early neoglacial anomalies (ENA). The strong winter monsoons during ENA were accompanied by changes in wind and precipitation patterns that are particularly evident across the eastern Northern Hemisphere and tropics. This coordinated climate reorganization may have helped trigger the metamorphosis of the urban Harappan civilization into a rural society through a push–pull migration from summer flood-deficient river valleys to the Himalayan piedmont plains with augmented winter rains. The decline in the winter monsoon between 3300 and 3000 years ago at the end of ENA could have played a role in the demise of the rural late Harappans during that time as the first Iron Age culture established itself on the Ghaggar-Hakra interfluve. Finally, we speculate that time-transgressive land cover changes due to aridification of the tropics may have led to a generalized instability of the global climate during ENA at the transition from the warmer Holocene thermal maximum to the cooler Neoglacial.

Socioenvironmental issues of river bed material extraction in the Himalayan piedmont (India)

The present study focuses on the Balason river running through the Himalayan piedmont zone (near Siliguri, India). The objective of the study is an assessment of the environmental effects of river bed material extraction by humans and the dependence of indigenous people on the river and its ecosystem services. The analysis is based on results of field work consisting of geodetic measurements of the river channel and interviews among the local community from the Nimtijot village. Historical hydrological data were also used for the study. The results of the investigation show that the Balason river is heavily affected by excessive exploitation of river bed material during dry season and the replenishment of extracted material in a monsoon season is not always sufficient. It leads to channel deepening. The local community working in the river heavily depends for its livelihood on continuing this activity. A decreasing amount of bed material to be extracted may lead to degradation of the strong relationship between the local community and their natural environment (river).

Tea plantations as a driving force of long-term land use and population changes in the Eastern Himalayan piedmont

Reconstructing the long-term impacts of tea plantations on land use and cover (LULC) change over time is important for making policy decisions on management and conservation. This can be very effectively done by integrating historical and remote sensing data; however, such approach has rarely been used to date. The present study analysed the role of the large tea plantation system in the forest cover change in 2105.9 km2 of the Himalayan piedmont in India’s West Bengal state, from the first settled agriculture in the mid-19th century to present day. The impact of domestic and international markets together with local factors such as floods on tea plantation development were also investigated to identify potential scenarios of LULC change in the next few decades. Analysis of historical and remote sensing data as well as census reports revealed that between 1874 and 2010, forest area decreased by 69.5% and land under tea cultivation increased to 30.7%. Population density grew as much as 5122.2% to 470 people km−2 between 1872 and 2011. Piedmont development was not uniform and denoted two periods: new land use structure formation (1874–1930) and its further stabilization (1930–2010). The first period showed rapid deforestation at an annual rate of 16–30 km2, with an increase of tea production areas up to 16 km2 per year. The second period saw slower deforestation rates (2–4 km2 annually) and steady growth of tea production areas (2 km2 annually). Population growth changed from workforce immigration to settled families. Limited space for settlement expansion favoured population concentration and initiated urbanization processes. Despite the initiation of deforestation and migration through the location of tea gardens, the close relationship between deforestation and population density has not been found in piedmont. At the early stages of plantation development, the greatest deforestation occurred at the lowest population density in the piedmont history. Later, a significant decline in the deforestation rate was accompanied by a demographic explosion. At the same time, reserved forest played the very important role in preserving the remnants of forest cover. Features of the natural environment and land use policy led to tea cultivation approaching maximum expansion in the studied area.

Reconstruction of Late Quaternary Neotectonic Movements and Fluvial Activity in Sikkimese-Bhutanese Himalayan Piedmont

AbstractIn the part of Sikkimese-Bhutanese Himalaya the youngest Siwalik overthrust had not developed, and the piedmont zone with extensive fans forms a semicircular gulf dismembered by several faults and minor overthrusts. Some uplifted parts of older deformed alluvial fans contain the lenses of organic clays which were earlier dated at 22–34 ka BP byTo distinguish various alluvial formations and phases of tectonic activity the authors dated older levels in several localities by OSL method as well as investigated soil profiles. Most mature soils over elevated blocks built of coarse alluvia were dated between 50 and 60 ka BP. Probably the previous

Study of some soils of the Himalayan piedmont plain of Bangladesh

Results of four representative pedons of the Old Himalayan piedmont, collected horizonwise from Thakurgaon district, revealed that the color of the topsoil was dark grayish brown to dark yellowish brown and that of the subsoils was dark yellowish brown to light olive-brown and light brownish grey when moist. The soils have moderately well drained conditions except Baliadangi soils which are imperfectly drained. Prismatic to subangular blocky structure has been developed in subsurface horizons of all the profiles, whereas massive structure was found in the surface horizons. Sand was by far the dominant fraction in the soils. The texture of the soils varied from clay loam to sandy loam. The irregular vertical distribution patterns of sand/silt ratio in the profiles indicate the heterogeneous nature of the parent materials. The pH of the soils ranged from 5.4 to 6.26. The organic matter contents as well as total nitrogen of these soils were, in general, low. The C/N ratios of the soils varied from 4.3 to 10.3. The cation exchange capacity (CEC) in most of the studied soils were low. Calcium and magnesium were the dominant cations in all the soils. DOI: http://dx.doi.org/10.3329/jasbs.v38i2.15610 J. Asiat. Soc. Bangladesh, Sci. 38(2): 199-206, December 2012

Natural and human impact on land use change of the Sikkimese-Bhutanese Himalayan Piedmont, India

Abstract . Natural and human causes of land use transformation were analysed in the Sikkimese-Bhutanese Himalayan piedmont over last 150 years with special emphasize on period 1930-2010. A hydrologic and geomorphic approach was employed to delineate three study areas along river courses on alluvial fans stretching 10 km from the mountain front. The visual interpretation of topographic maps and satellite images combined with logical rules in GIS were used for determination of stable and dynamic areas from the viewpoint of land use changes. Analysis indicates rapid replacement of natural forest through location of tea plantations, crop cultivation and development of settlement in the late 19th century. The shift from natural to human dominated landscape caused increase land use stability through enlargement of areas with monoculture cultivation of tea and paddy between 1930 and 2010. The natural fluvial activity intensified by human induced deforestation of Himalayan margin and clearance of riparian vegetation along foothill channels became the most important factors of present-day land use changes of piedmont.

Tectonic control on the drainage system in a piedmont region in tectonically active eastern Himalayas

The impact of neotectonic activity on drainage system has been studied in a large alluvial fan in the eastern Himalayan piedmont area between the Mal River and the Murti River. Two distinct E-Wlineaments passing through this area had been identified by Nakata (1972, 1989) as active faults. The northern lineament manifested as Matiali scarp and the southern one manifested as Chalsa scarp represent the ramp anticlines over two blind faults, probably the Main Boundary Thrust (MBT) and the Himalayan Frontal Thrust (HFT), respectively. The fan surface is folded into two antiforms with a synform in between. These folds are interpreted as fault propagation folds over the two north dipping blind thrusts. Two lineaments trending NNE-SSW and nearly N-S, respectively, are identified, and parts of present day courses of the Murti and Neora Rivers follow them. These lineaments are named as Murti and Neora lineaments and are interpreted to represent a conjugate set of normal faults. The rivers have changed their courses by the influence of these normal faults along the Murti and Neora lineaments and their profiles show knick points where they cross E-W thrusts. The overall drainage pattern is changed from radial pattern in north of the Matiali scarp to a subparallel one in south due to these conjugate normal faults. The interfluve area between these two rivers is uplifted as a result of vertical movements on the above mentioned faults. Four major terraces and some minor terraces are present along the major river valleys and these are formed due to episodic upliftment of the ground and subsequent down-cutting of the rivers. The uppermost terrace shows a northerly slope north of the Chalsa scarp as a result of folding mentioned above. But rivers on this terrace form incised channels keeping their flow southerly suggesting that they are antecedent to the folding and their downcutting kept pace with the tectonism.

Plate Motion of India and Interseismic Strain in the Nepal Himalaya from GPS and DORIS Measurements

We analyse geodetically estimated deformation across the Nepal Himalaya in order to determine the geodetic rate of shortening between Southern Tibet and India, previously proposed to range from 12 to 21 mm yr^(−1). The dataset includes spirit-levelling data along a road going from the Indian to the Tibetan border across Central Nepal, data from the DORIS station on Everest, which has been analysed since 1993, GPS campaign measurements from surveys carried on between 1995 and 2001, as well as data from continuous GPS stations along a transect at the logitude of Kathmandu operated continuously since 1997. The GPS data were processed in International Terrestrial Reference Frame 2000 (ITRF2000), together with the data from 20 International GNSS Service (IGS) stations and then combined using quasi observation combination analysis (QOCA). Finally, spatially complementary velocities at stations in Southern Tibet, initially determined in ITRF97, were expressed in ITRF2000. After analysing previous studies by different authors, we determined the pole of rotation of the Indian tectonic plate to be located in ITRF2000 at 51.409±1.560°N and−10.915± 5.556°E, with an angular velocity of 0.483±0.015°. Myr^(−1). Internal deformation of India is found to be small, corresponding to less than about 2 mm yr^(−1) of baseline change between Southern India and the Himalayan piedmont. Based on an elastic dislocation model of interseismic strain and taking into account the uncertainty on India plate motion, the mean convergence rate across Central and Eastern Nepal is estimated to 19 ± 2.5 mm yr^(−1), (at the 67% confidence level). The main himalayan thrust (MHT) fault was found to be locked from the surface to a depth of about 20km over a width of about 115 km. In these regions, the model parameters are well constrained, thanks to the long and continuous time-series from the permanent GPS as well as DORIS data. Further west, a convergence rate of 13.4 ± 5 mm yr^(−1), as well as a fault zone, locked over 150 km, are proposed. The slight discrepancy between the geologically estimated deformation rate of 21 ± 1.5 mm yr^(−1) and the 19 ± 2.5 mm yr^(−1) geodetic rate in Central and Eastern Nepal, as well as the lower geodetic rate in Western Nepal compared to Eastern Nepal, places bounds on possible temporal variations of the pattern and rate of strain in the period between large earthquakes in this region.

Fertility Status of Bangladesh Soils -A Review

Fertility status of Bangladesh soils was critically evaluated by reviewing the studies which have been carried out in universities and research institutes of Bangladesh. Almost all upland soils are low in organic matter and deficient in N. Availability o9f P to the crops is a problem mainly in calcareous coils of Ganges floodplain and acidic soils of terrace and hill areas. Status of K is not a great problem in floodplain areas, but terrace and piedmont soils are not capable of supplying enough K to the crops. Although P and K deficiencies are not severe, addition of these two nutrients is a must for getting higher yield. Paddy soils remain under water during growing period which causes unavailability of S and Zn fro rice crops. Other dryland crops also face the problem of S and Zn deficiencies in many areas of Bangladesh. Boron was reported to be deficient in some regions. Magnesium deficiency was found in the coarse-textured soils of Old Himalayan piedmont plain, Brown hill soils and Grey floodplain soils of the northern part of the country. The Ca reserve of many floodplain soils is depleting due to decalcification process and deficiency of Ca is probable in near future. Deficiencies of Cu and Mn were reported but very rate. The overall fertility status is not satisfactory, need for fertility conservation is stressed

バングラデシュ北西部の地形・洪水特性からみた水害対策計画の問題点

The World Bank has made “Bangladesh Action Plan for Flood Control” which consists of 26 components after severe floods of 1987 and 1988. Japanese and UK governments donate the Component No.2, for Northwest Region. The author attended the preliminary study in 1990 as a geomorphologist and found some problems on flood mitigation projects related with characteristics of landform and flood feature.The northwest region is divided into the following four geomorphological units: 1) Barind Tract, 2) the Himalayan piedmont plains, 3) the Alluvial lowland along the Brahmaputra-Jamuna and 4) the Alluvial lowland along the Ganges. The embankments are frequently broken by floods because river beds are unstable in the Himalayan piedmont plains. In the alluvial lowland along the B.-Jamuna, the river channels are unstable and change their courses frequently during annual floods with breaching embankments, since the B.-Jamuna changed its course in the 18th century. In the alluvial lowland along the Ganges, distinct large natural levees have developed along the Ganges and a broad back-marsh area lies along the lower Atrai Basin. The “Polder Project” for flood protection is progressing in the lower Atrai Basin. However, each polder is too broad to protect the whole area. In order to protect their houses, people often cut embankments for draining inundation water not only from inside of the polder to river, but also from outside of the polder into the protected area.In the Flood Action Plan, the World Bank proposed two major projects in this area, namely, an interceptor drain from the Upper Atrai into the B.-Jamuna, and a diversion channel from the Lower Atrai into the Ganges. The main technical problem on both the Interceptor and the Diversion channel is the water level at the outfalls. The Interceptor is proposed along the Karatoya-Atrai, a heavy free-meandered river. The Interceptor needs high embankments whole from the Atrai to the B.-Jamuna. The Diversion channel is less feasible ; since the river bed of the Atrai is lower than that of the Ganges.Apart from these major projects, the author suggests smaller but reasonable projects for this area. In the southern part, the Polder Project is to be improved: smaller units of polders with consideration of geomorphological setting and local land-use conditions. A comprehensive project which contains both structural and non-structural methods for flood control and rural development programs as waterside district will support the development of this area.