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Bank instability and erosion problems of Bishnumati River, Kathmandu, Nepal
Abstract
The Bishnumati River, a major tributary of the Bagmati River in the Kathmandu basin, suffers from bank erosion, river instability, and environmental degradation. These processes are responsible for the loss of sediments, modification of river morphology, loss of vegetative buffer zone, and deterioration of stream environment. Bank erosion hazard in the Bishnumati River was assessed at ten different reaches using bank height ratio, ratio of riparian vegetation rooting depth to bank height, rooting density percentage, bank slope, and bank surface protection. Vertical and lateral stability of the river was assessed at four reference segments, namely at Bishnumatigaun, Okhaltar, Mahadevtar, and Tamsipakha, respectively from upstream to downstream. The riverbanks in the Bishnumatigaun segment show fluctuating trends of bank erosion potential from upstream to downstream. All the four segments of the Bishnumati River are vertically unstable and all the segments except the Okhaltar are entrenched. The Tamsipakha and Okhaltar segments have a high risk of lateral shifting, as they possess high values of bank erosion hazard index, meandering width ratio, and width-depth ratio. The Bishnumati river is in a degrading condition and capable of eroding sediments. The main causes of river instability are the excavation of river sediments, encroachment on riverbanks, clearing of riparian vegetation, and canalization.
... Most of the rivers of the Kathmandu valley like the Bagmati River, the Bishnumati River, the Manahara River and the Nakhu Khola have been experiencing stream bank erosion hazard and bank instability problems in recent year (Tamrakar 2004b;Adhikari and Tamrakar 2005;Tamrakar et al. 2011 andMaharjan andTamrakar 2010). The natural state of these river are deteriorated due to bank erosion, mining of construction materials, encroachment of the channel area, deposing of municipal and industrial wastes and artificial modification of channel. ...
... Erosion of stream beds and banks is an essential, natural and continuing process. Rutherfurd et al. (1999) Stability conditions of the Bishnumati and the Manahara Rivers were evaluated by (Tamrakar 2004a;2004b;Adhikari and Tamrakar 2005;Bajracharya and Tamrakar 2007;Bajracharya 2009). Tamrakar (2004) studied degradation problems of the Bishnumati River and concluded that River channel scouring, bank erosion; solutions of riparian vegetation, contamination of river by disposal are the major problems in the Bishumati River corridor. ...
... Tamrakar (2004) studied degradation problems of the Bishnumati River and concluded that River channel scouring, bank erosion; solutions of riparian vegetation, contamination of river by disposal are the major problems in the Bishumati River corridor. Adhikari and Tamrakar (2005) studied bank instability and erosion problems in Bisnumati River, Kathmandu. They found high bank erodability and channel instability in this river. ...
This is My M.Sc. thesis Pdf Abstract The fifth order Kodku River, one of the tributary of the Manahara River, is being studied to identify streambank erosion processes and bank stability condition. Quantitative measurement of stability condition and erosion scenario of the streambank is more rigorous and significant during river restoration and management in the urban area. The river is suffering from streambank erosion hazards and bank instability causing great threat to the infrastructures, land and settlement areas.In order to understand the present status of the river, planform geometry, cross-sectional characteristics and hydrologic parameters were calculated and analysis from the desk study and field survey dividing the entire river into five segments namely Badikhel, Taukhel, Arubot, Thaiba and Harisiddhi Segments. Major streambank erosion processes were (i) sub aerial erosion which includes sheet erosion, rill erosions & gully erosions, (ii) sub- aqueous erosion which includes impinging flow erosion and parallel flow erosion and (ii) mass failure mechanism that includes shallow slip, slab failure, rotational sip and cantilever failure. Streambank erosion map was also prepared in 1:10,000 scales. Landuse change, vegetation clearance, meander migration, unconsolidated and non cohesive bank materials, human encroachment are some causes of the bank erosion. Riparian vegetation zone along the river were analyzed and documented in map of scale 1:10,000. At upper segment near the Badikhel bank erosion is less documented because of proper and dense riparian vegetation but in other downstream segments bank erosion is more frequent and rigorous where spare and lack of vegetation is observed. In order to identified the bank stability condition and toe erosion scenario of the Kodku River, the Bank Stability and Toe Erosion Model (BSTEM) was used for eight different sites using the BSTEM version 5.4 that calculates a factor of safety (Fs) for multilayer streambank, based on limit equilibrium-method. Streambanks of the uppermost segments at the transects BK1(Lower Badikhel) and BK2 (Upper Taukhel) are stable, where Fs exceeds 1.3 and maximum lateral retreat of channel ranges from 30.59 to 116.007 cm with 0.290-0.482 sq. m of the total eroded area of the bank-toe resulting less bank toe erosion. Presences of cohesive material at bank top with Canopy and understorey cover are the causes of stable bank. Streambank of the transects BK3 (Arubot) and the BK4 (Thaiba) are unstable as Fs ranges from 0.75 to 0.92, and the maximum lateral retreat of channel ranges from 70.83 to 208.81 cm with total eroded bank toe area of 0.117–1.695 sq. m. resulting excessive bank toe erosion problems. Major causes of instability are the presence of unconsolidated non cohesive bank and bank toe material, high scouring, and sparse riparian vegetation. Streambank of transect BK5 (Lower Thaiba), BK6 (Upper Harisiddhi) and BK7 (Lower Harisiddhi) are conditionally stable to stable. Low bank height and bank angle, low gradient with some river training activities are the causes of the bank stability. Streambanks at transect BK8 (Upper Imadol) are unstable. In this segment river stretches are encroached by human and impaired badly by direct disposal of solid and liquid wastes which causes streambanks unstable. Transect BK4 (Thaiba) is found most unstable and more hazardous to bank toe erosion than other segment because it has maximum failure volume of 165 cu. m, maximum sediment loading of 99.97 ton, maximum lateral retreat of 208.81 cm and maximum eroded area of 1.695 sq. m when comparing with other segments. Where the Fs are low and banks are disturbed by encroachment, suitable bioengineering measures can be implemented to mitigate excessive bank toe erosion and failure.
... Stability conditions of the Bishnumati and the Manahara Rivers were evaluated by Tamrakar (2004aTamrakar ( , 2004b Adhikari and Tamrakar (2005), Bajracharya and Tamrakar (2007) Shrestha and Tamrakar, (2007) and Tamrakar and Bajracharya (2009). Tamrakar (2004a and2004b) studied disturbing factors influencing the Bishnumati River degradation problem and concluded that human-induced as well as natural factors were responsible for river environment degradation. ...
... The studies evaluated that the upstream segments of the Bishnumati River is laterally unstable while the downstream segments is vertically unstable. Adhikari and Tamrakar (2005) studied bank erosion hazard along the Bishnumati River corridor, and concluded that the mid to the lower segments of the river fall on high hazard zone. Bajracharya and Tamrakar (2007) studied environmental status of the Manahara River. ...
The Kodku River is a southern tributary of the Manahara River and extends for about 15.86 km with 35.67 sq. km of
watershed area. It is quite a potential linkage between the hilly, southern Kathmandu and the urban, inner Kathmandu. The
river corridors are frequently subject to bank erosion, slope movements and flash flooding. Riverbank erosion is an important
cause of toe erosion of slopes causing landslides and also posing threat on the infrastructures. Stream channel stability is crucial
to understand overall river stability. Recognition of existing stability condition of river is to understand nature and behavior of
the river, and is important in many ways: (a) to recognize the bank erosion and lateral instability hazard, (b) to develop
infrastructure along or nearby the river corridor, (c) to start on where to restore the river, (d) to develop reservoir and exploit
natural resources, and (e) to develop safe settlement areas. The Kodku River is a gravelly mixed-load meandering river. Level
II classification distinguishes the Badikhel Segment as a ‘B4c’ type stream, the Taukhel Segment as a ‘C6c’ type, and the fifth
order segments such as the Arubot, Thaiba and Harisidhi Segments as ‘C4c’ type streams. The ‘B4c’ type stream is entrenched
and somewhat laterally confined by steep valley slopes and terrace landforms. It has the highest unit stream power (16.64 Nm/s/m2),
high potential of bed material scouring and tendency of vertical instability. The ‘C6c’ type stream is a meandering
stream with shallow channel and wide valley. The ‘C4c’ type streams have shallow and wide meandering channels with well
developed flood plains and lateral bars, and have the least unit stream power (in Harishiddi Segment 0.11N-m/s/m2), low
potential of river bed material erosion but have tendency of lateral instabilities. The bank erosion hazard map indicates that
the upper third order stretch and few downstream stretches lie in low hazard zone, but the overall areas of the Harisidhi
Segment, Gwarko, Imadol and some other areas lie in high to very high hazard zone because of devegetation, modification of
channels and other anthropogenic activities in addition to the weak nature of the bank materials.
... Unsystematic mining has caused many problems such as erosion, river bank instability and river pollution (Tamrakar, 2004;Adhikari and Tamrakar, 2005). For Kathmandu, the growth of infrastructure and population has demanded about 3100 m 3 of sand per day in 2001, while only 35% of the terrace mining is registered (Sayami and Tamrakar, 2007), and that number excludes the illegal river mining supply. ...
Fluvial terraces, floodplains, and alluvial fans are the main landforms to store sediments and to decouple hillslopes from eroding mountain rivers. Such low-relief landforms are also preferred locations for humans to settle in otherwise steep and poorly accessible terrain. Abundant water and sediment as essential sources for buildings and infrastructure make these areas amenable places to live at. Yet valley floors are also prone to rare and catastrophic sedimentation that can overload river systems by abruptly increasing the volume of sediment supply, thus causing massive floodplain aggradation, lateral channel instability, and increased flooding. Some valley-fill sediments should thus record these catastrophic sediment pulses, allowing insights into their timing, magnitude, and consequences. This thesis pursues this theme and focuses on a prominent ~150 km2 valley fill in the Pokhara Valley just south of the Annapurna Massif in central Nepal. The Pokhara Valley is conspicuously broad and gentle compared to the surrounding dissected mountain terrain, and is filled with locally more than 70 m of clastic debris. The area’s main river, Seti Khola, descends from the Annapurna Sabche Cirque at 3500-4500 m asl down to 900 m asl where it incises into this valley fill. Humans began to settle on this extensive fan surface in the 1750’s when the Trans-Himalayan trade route connected the Higher Himalayas, passing Pokhara city, with the subtropical lowlands of the Terai. High and unstable river terraces and steep gorges undermined by fast flowing rivers with highly seasonal (monsoon-driven) discharge, a high earthquake risk, and a growing population make the Pokhara Valley an ideal place to study the recent geological and geomorphic history of its sediments and the implication for natural hazard appraisals. The objective of this thesis is to quantify the timing, the sedimentologic and geomorphic processes as well as the fluvial response to a series of strong sediment pulses. I report diagnostic sedimentary archives, lithofacies of the fan terraces, their geochemical provenance, radiocarbon-age dating and the stratigraphic relationship between them. All these various and independent lines of evidence show consistently that multiple sediment pulses filled the Pokhara Valley in medieval times, most likely in connection with, if not triggered by, strong seismic ground shaking. The geomorphic and sedimentary evidence is consistent with catastrophic fluvial aggradation tied to the timing of three medieval Himalayan earthquakes in ~1100, 1255, and 1344 AD. Sediment provenance and calibrated radiocarbon-age data are the key to distinguish three individual sediment pulses, as these are not evident from their sedimentology alone. I explore various measures of adjustment and fluvial response of the river system following these massive aggradation pulses. By using proxies such as net volumetric erosion, incision and erosion rates, clast provenance on active river banks, geomorphic markers such as re-exhumed tree trunks in growth position, and knickpoint locations in tributary valleys, I estimate the response of the river network in the Pokhara Valley to earthquake disturbance over several centuries. Estimates of the removed volumes since catastrophic valley filling began, require average net sediment yields of up to 4200 t km−2 yr−1 since, rates that are consistent with those reported for Himalayan rivers. The lithological composition of active channel-bed load differs from that of local bedrock material, confirming that rivers have adjusted 30-50% depending on data of different tributary catchments, locally incising with rates of 160-220 mm yr−1. In many tributaries to the Seti Khola, most of the contemporary river loads come from a Higher Himalayan source, thus excluding local hillslopes as sources. This imbalance in sediment provenance emphasizes how the medieval sediment pulses must have rapidly traversed up to 70 km downstream to invade the downstream reaches of the tributaries up to 8 km upstream, thereby blocking the local drainage and thus reinforcing, or locally creating new, floodplain lakes still visible in the landscape today. Understanding the formation, origin, mechanism and geomorphic processes of this valley fill is crucial to understand the landscape evolution and response to catastrophic sediment pulses. Several earthquake-triggered long-runout rock-ice avalanches or catastrophic dam burst in the Higher Himalayas are the only plausible mechanisms to explain both the geomorphic and sedimentary legacy that I document here. In any case, the Pokhara Valley was most likely hit by a cascade of extremely rare processes over some two centuries starting in the early 11th century. Nowhere in the Himalayas do we find valley fills of comparable size and equally well documented depositional history, making the Pokhara Valley one of the most extensively dated valley fill in the Himalayas to date. Judging from the growing record of historic Himalayan earthquakes in Nepal that were traced and dated in fault trenches, this thesis shows that sedimentary archives can be used to directly aid reconstructions and predictions of both earthquake triggers and impacts from a sedimentary-response perspective. The knowledge about the timing, evolution, and response of the Pokhara Valley and its river system to earthquake triggered sediment pulses is important to address the seismic and geomorphic risk for the city of Pokhara. This thesis demonstrates how geomorphic evidence on catastrophic valley infill can help to independently verify paleoseismological fault-trench records and may initiate re-thinking on post-seismic hazard assessments in active mountain regions.
... Excavation of riverbed sand has invited various environmental problems in river. Unsystematic mining has caused erosion and instability of the riverbanks, and pollution of rivers (Tamrakar 2004;Adhikari and Tamrakar 2005;Bajracharya 2006). Several workers studied on sand of the Kathmandu Valley with different aspects (Kharel et al. 1992;Sadaula 1993;Paudel et al. 1995;Kharel et al. 2002). ...
Growing trend of urbanization in Kathmandu has increased the demands of sand for building materials of concrete. Demand of sand has been fulfilled from terrace and riverbed mining in northern region of Kathmandu and by importing from west of Kathmandu. Riverbed excavation and some quarries in terrace deposits are illegally operated. Although riverbed excavation is prohibited, majority of the sand comes from such mining. Questionnaire analysis, analysis of secondary data and sand samples were carried out in laboratory to obtain (a) the existing status of sand mining and (b) the quality of sand supplied to the market. About 60% demand of sands have been fulfilled through riverbeds while 40% have been fulfilled through terrace deposits. The Sanla and the Manahara Rivers have been the most efficiently excavated rivers. About 1865 m3 sands from river and 1238 m3 from terraces have been excavated per day. Future demand of sand may increase considering 60% increase of household in the valley. Out of total excavation sites in the valley, 40% illegal sites are located in rivers and 5% in terrace deposits. Mining policy and laws are weak to control illegal mining and to bring miners and dwellers into the framework of taxation. The river or terrace sands do not differ much in major constituents, but the former sands are more matured. Both sands are hazardous in terms of mica content that approaches 10 to 32% lying beyond the acceptable limit of 8%. Other deleterious materials are quite low in percentage and do not exceed 3%. If processing can be applied to reduce mica content at a commercial-scale, the sands in the Kathmandu Valley will be probably of good quality. doi: 10.3126/bdg.v10i0.1424 Bulletin of the Department of Geology, Tribhuvan University, Kathmandu, Nepal, Vol. 10, 2007, pp.89-98
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