The Use of Dam Environmental Vulnerability Index (DEVI) for Assessing Vulnerability of Bengawan Solo Watershed, Indonesia

Bengawan Solo is the longest river in Java, but current conditions show that its watershed is in a critical condition. Deforestation was very intensive in the last three decades that contributed to degradation of the watershed. Other factor contributing to the degradation is dam construction. However, our knowledge on the impact of dam construction on the environment and its vulnerability is poorly understood. Here, we assessed vulnerability of the watershed based on physical properties such as existing dams, morpho-dynamic activities, and deforested area. The study aims to identify the vulnerability of the Bengawan Solo watershed based on dam environmental vulnerability index (DEVI) approach, and to analyse the dominant variable contributing to DEVI. For calculating DEVI, several data were needed including land cover, rainfall, stream water stage, soil type, stream network, and dams. The results showed that Bengawan Solo watershed had moderate to high vulnerability (60%). Moderate level was identified for Madiun and Wonogiri sub-watershed, while high level was in Cepu and Babat sub-watershed. Our findings revealed that morpho-dynamic activities as represented by sediment rate and stream water stage had contributed to the high DEVI value as in Cepu and Babat sub-watershed. Further, influence of dams in this research was not dominant implying that any improvement to the DEVI approach remains research challenges. The improvement of the approach is expected to better identify the impact of dam construction on environment, situated in other regions than Amazon, where it was firstly developed.

CATASTROPHIC ICE-STONE INERTIAL GAS-DYNAMIC AVALANCHE-TYPE STREAMS AS AN OCCURRENCE OF PAROXYSMAL AIR-BLAST DIRECTIONAL GAS-DYNAMIC SURGES OF GLACIERS. ABOUT CORRELATION OF THE CONCEPTS «AVALANCHE-TYPE STREAM» AND «MUDFLOW»

Рассмотрены особенности катастрофических гляциальных лавинообразных потоков. Выделен особый тип этих потоков, особенности возникновения и движения которых указывают на то, что эти потоки являются проявлениями пароксизмальных взрывоподобных направленных газодинамических выбросов ледников. Дано объяснение механизма и экстраординарных параметров кинематики движения таких потоков. Подчеркнуто принципиальное отличие этих потоков от селей. Соответственно, селевая трактовка Колкинской катастрофы 2002 г. в ее эпицентральной и сопредельной с ней зоне, включающей зону транзита гляциального лавинообразного потока, является необоснованной и ошибочной, с чем согласуется безуспешность предпринятых попыток гидравлического моделирования этой катастрофы. Показано, что эти потоки не относятся к числу склоново-гравитационных, не представляют собой ни обычный, ни особый тип подобных явлений The features of catastrophic glacial avalanche-type streams are considered in the article. A special type of these streams is singled out. The features of origination and motion of such streams point at the fact that they are the evidence of paroxysmal air-blast directional gas-dynamic surges of glaciers. The mechanism and extraordinary parameters of motion kinematics of these streams are explained. Accordingly a mudflow version of the Kolka catastrophe of 2002 in its epicentral and adjoining zone including the motion zone of glacial avalanche-type stream is unreasonable and false what confirms unsuccessful efforts of hydraulic modelling of this catastrophe. It is shown that such streams are referred to the slope-gravity phenomena and are either ordinary no special type of such phenomena

Riverbank erosion potential and channel stability status of the Kodku River, southern Kathmandu Basin, Central Nepal

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.DOI: http://dx.doi.org/10.3126/bdg.v17i0.12723Bulletin of the Department of Geology, Vol. 17, 2014, pp. 1-41

Morpho-hydrologic parameters and classification of the Kodku River for stream stability assessment, southern Kathmandu, Central Nepal

The Kodku River Corridor is one of the most potential corridors for future development of roads that would link the southern remote areas of the Kathmandu Valley to the inner core areas. River stability is of great concern as the unstable segment of river may pose threat on infrastructures, and adjacent cultivated lands and settlement areas. In this light, the preliminary assessment of the Kodku River as a part of the stability assessment was undertaken. The broad level geomorphic and hydrologic parameters, and Level I and II classifications of the river were made to assess for stability condition.The Kodku River is a fifth order stream, extending for about 15.86 km and its watershed covering an area of 35.67 sq. km. The relative relief is extremely high to low, and diminishes with change of landforms from steep terrain in the southern part to the gentle sloped terraces in the northern part of the watershed. Drainage texture is fine to very coarse, from the southern to the norther parts of the watershed. All the stream segments are sinuous (K = 1.2) whereas the Arubot Segment is the highly meandering (1.7). Both meander wavelength and belt width increase with increasing stream order. Five types of valleys have been distinguished; I, II, IV, VI, and VIII type valleys. Based on valley type distribution, channel pattern, shape and channel slope, the stream segments have been classified using Level I assessment as ‘B’ type, ‘C’ type and ‘Aa+’ type streams. The ‘B’ type streams are all located in the third order segment. Except the ‘Aa+’ type stream, all the fifth and fourth order stream segments belong to ‘C’ type streams. Width/Depth ratio varying between 10.5 and 29.5 indicates laterally unstable channel segments. The bank height ratio, which varies between 1.6 and 2.4, indicates moderate incision and shows vertical instability of streams. The Badikhel Segment is relatively more entrenched (1.7) while the Taukhel Segment is the least entrenched (7.1). Since Entrenchment Ratio exceeds 1.6, the stream segments are considered to have moderate to low entrenchment. Based on the bed material load, the Kodku River is a gravelly mixed-load river, in which pebbles to silt/clay occur. Level II classification distinguishes three-types of streams; ‘B4c” type (Badikhel Segment), ‘C6c’ type (Taukhel Segment), and ‘C4c” type (Arubot, Thaiba and Harisidhi Segments). The ‘B4c’ type stream has tendency of vertical instability. The ‘C6c” and ‘C4c” type streams have shallow and wide meandering channels with well developed flood plains and lateral bars, and reflect tendency of lateral instability.DOI: http://dx.doi.org/10.3126/bdg.v16i0.8880 Bulletin of the Department of Geology Vol. 16, 2013, pp. 1–20

Morphology and classification of the main stem Bagmati River, Central Nepal

Rivers are significant geomorphic agents which can carry huge amounts of sediments and water from their catchments to depositional basins. Due course of flow they can bring various hazards such as flooding, inundation of flood plains, erosion of banks and trigger various slope movements. To understand nature and behavior of the Bagmati River, which is a perennial, storm and spring-fed, eight order river of central Nepal, the planiform morphological parameters of this river were analysed and river segments along the main stem stretches were classified into various stream-types, using topographic maps, aerial photographs, and satellite imageries. The Bagmati River is an 8th order perennial river. The Bagmati River basin comprises 39 sub-basins of 4th order and higher. The averages of meander wavelengths (Lm), meander belt width (Wblt), radius of curvature (Rc) and sinuosity (K) of the 7th order main stem Bagmati River are 598.27 m, 164.07 m, 252.35 m and 1.38, respectively. Similarly, the averages of Lm, Wblt, Rc, and K of the 8th order main stem rivers are respectively, 496.77, 126.58 m, 286.13 m, and 1.55. All these parameters reflect fluctuating trends of variation along the downstream stretch. However, the portion of the 7th order river shows abrupt increase of Lm after the Bagmati River crosses the Main Boundary Thrust. The Rc also indicates slight increase in this region. The sinuosity reflects overall increase from the 7th to the 8th order rivers. This increase in sinuosity is attributed to the decrease in slope and competency of the river. There exists a moderate degree of correlation between K and Wblt showing that the Wblt of the Bagmati River tends to increase as the sinuosity increases. The stream segments of the Bagmati River fall on A-, B-, C-, G- and D-type rivers. All the segments of the Bagmati River, lying within the Kathmandu Valley and having substrate of the fluvio-lacustrine valley-fill sediments, fall on C-type stream because of low entrenchment and high width/depth ratio. Similarly, A-type streams are developed at segments which flow through the Tistung Formation of the granite terrain of the Lesser Himalaya most probably due to entrenched and gorged valley and high slope. Likewise, B- and G-type streams are developed in terrains of the Lesser Himalaya and the Sub-Himalaya. The B-type streams are moderately entrenched with sinuous to meandering and wide channels. The D-type stream segment which is a multi-thread channel is characteristically developed within the terrain of the Sub-Himalaya presumably due to the change in slope caused by thrusting, and greater input of sediments from the Siwaliks in relation to weak and loosely consolidated lithology. DOI: http://dx.doi.org/10.3126/bdg.v15i0.7415 Bulletin of the Department of Geology, Vol. 15, 2012, pp. 23-34