Environmental Status of Manahara River, Kathmandu, Nepal
Total Page：16
File Type：pdf, Size：1020Kb
Bulletin of the Department of Geology C Bulletin of the Department of Geology, Tribhuvan University, Kathmandu, Nepal, Vol. 10, 2007, pp. 21–32 e y n g t o ra l l eo De G partment of Kirtipur Environmental status of Manahara River, Kathmandu, Nepal *Ramita Bajracharya and Naresh Kazi Tamrakar Central Department of Geology, Tribhuvan University, Kirtipur, Kathmandu, Nepal ABSTRACT The Manahara River located in northeast part of the Kathmandu Valley has been disturbed for last one decade by several anthropogenic activities and natural causes thereby deteriorating its recreational functions and stream habitat. To obtain an existing environmental condition and disturbances of the river, the river was surveyed for its habitat, pollution level and surface water quality. Among the five representative segments of the river, the downstream segment (Sano Thimi) was scored into intermediate category showing more pollution and environmental deterioration compared to the upstream segments. Turbidity, electrical conductivity, chemical oxygen demand, biological oxygen demand and ammonia increase, whereas dissolved oxygen decreases from upstream to downstream with exponential functions. Aquatic lives like Garra sp. (Buduna), Schizothorax sp. (Asala), Channa sp. (Hiele) and Heteropneustes sp. (Singe) were observed except in downstream of the Jadibuti Bridge situated downstream from Sano Thimi stretch. Fish species were rare from the Jadibuti area most probably due to reduced dissoved oxygen (5 mg/l). Coliform bacteria ranged from 3000 to 4000 in the Manahara River showing high amount of bacterial contamination. Major disturbances, which affect river habitat and surface water quality of the Manahara River were destruction of riparian buffer zones, excavation excessive amount of sand from the river, encroachment of floodplains and bars, solid waste and sewer effluent, and tendency of landuse change. To retard environmental degradation of the Manahara River from the human-induced activities, local government needs to take immediate action. INTRODUCTION The Manahara River as a whole has been in Rivers are natural resources which have ecological deteriorating condition from last one decade. Growing and recreational functions. People mostly depend on number of population and shelters, and building of rivers for agricultural and domestic purposes. Many structures and factories on the river banks are probably temples and crematories located around the river major causes for environmental deterioration have increased cultural values of the rivers. But with (reduction of stream bank stability, cultural, ecological rapid growing population and urbanization, different and recreational functions and surface water quality activities like unplanned building and encroachment, of rivers). Some of the major disturbances such as clearing of riparian vegetation along the river banks, landuse change, bank erosion, scoring, encroachment, disposal of waste materials in river and unwise mining dilution of riparian vegetation, contamination by of construction materials from the rivers are effluents and solid waste and shifting of the river commonly observed in rivers of the Kathmandu channel are quite evident along the Manahara River. Valley. Human as well as natural phenomena are Due to lack of awareness among people and concerned responsible for bringing disturbances in the river authorities, such disturbances are unmanageable at system. The Bishnumati River, one of the biggest present. If the disturbances in the river continue in tributaries of the Bagmati River, is suffering from the current trend, they will aggravate system instability these types of activities (Tamrakar 2004a and b). and will bring unrecoverable deterioration to the river environment. Therefore, it is important to identify *Corresponding author: present environmental condition and major E-mail address: [email protected] disturbances to rehabilitate the river in the future R. Bajracharya and N. K. Tamrakar/ Bulletin of the Department of Geology, Vol. 10, 2007, pp. 21–32 because many people depend on the water of the 80 Manahara River. UNESCO world heritage site, 30 CHINA Changunarayan Temple and several other places such as International Airport, Sali Nadi Temple and NEPAL Nilbarahi Temple are located around this river. Therefore, this paper aims in identifying environmental condition of the river and potential INDIAKathmandu disturbing factors. 26 MANAHARA RIVER BASIN 86 The Manahara River is located in the northeast of 85o 15’ 85o 30’ the Kathmandu Valley (Fig. 1). It is elongated in Shivapuri Range r shape having 28 km length from NE to SW covering e iv i R 83 sq. km of area. This river is straight and narrow at 27 m in upstream stretch and is meandered and wide in o u n 45’ o 45’ h r downstream stretch. The northern and eastern is ive B R 27 a catchments, composed of gneiss, granite, quartzite, ha r na metasandstone, phyllite and schist possess fine Ma drainage texture, high relative relief and high drainage r e frequency. The western catchment, composed of soft v i KATHMANDU fluvio-lacustrine deposits, possesses coarse drainage R i t texture, low relative relief and low dranage frequency a m g N (Bajracharya 2006). 27 a o B 35’ o The maximum average monthly rainfall received 35’ was 567 mm in July and minimum was nil in 0 2 4 Km 27 November (Sankhu Station) during 1980–2004 (DHM Manahara River Watershed 2004). The total monsoon rainfall (1980-2004) is 85o 15’ slowly increasing. Generally, the Manahara River Basin is presently covered by cultivated, settlement Fig. 1 Location map of study area and forest lands. Cultivation and settlement areas have been increasing against forest area. About 77% are characteristics of depositional pattern. Pebbles of landscape were occupied by cultivation, 17% by and sand with silt are main dominated channel forest and 6% by urban areas in 2002 (Shrestha 2007). materials. The middle segment (Mulpani) is highly sinuous FLUVIAL GEOMORPHOLOGY (1.81) with less entrenchement ratio (12). The material The Manahara River is a perennial stream fed by is composed of sandy pebble. Large point bar, side storm flow and spring. The mainstem of the river is bar and mid bar characterize depositional pattern. a fifth order stream and it’s major tributaries such as Very gentle slope (0.0056 m/m), high width/depth the Sali Nadi River, the Ghatte Khola and the Mahadev ratio (70) and low BHR (1.3) indicates flooding Khola are forth order streams (Fig. 2). The fifth order during high rainfall and channel shifting. stream extends for 19 km showing highly meandering The upper stretch (Kurthali) also shows highly pattern. The lowest stretch (Sano Thimi) of this sinuous (1.5) pattern and less entrenched nature mainstem shows entrenched nature (Entrenchment (7.71). River channel has alternating riffles and pools, ratio, ER = 1.39) with high bank height ratio (BHR and has very gentle slope (0.011 m/m). Large point = 1.63) and gentle slope (0.0235 m/m). River cross- bar and side bar contains cobble, pebble and sand. sectional area, discharge and velocity are highest. Erosional scars and landslides are common of the Point bar, side bar and point bar with few mid bar river banks. 22 Environmental status of Manahara River, Kathmandu, Nepal 85o22’30” 85o25’00” 85o27’30” 85o30’00” N 2395 Gagalphedi 1925 2096 5 0 1 2 3 4 5 Km 1730 Dandakateri 1508 o 45’00” a l o a h l 27 27 K o o h 1861 v Lapsephedi K 45’00” d e a h a a h i M g l d a a N 1861 S i 1390 Manahara l Kurthali 1402 a S M Sankhu Thali ana ha 4 ra River la 3 MN1 atte Kho 1635 1326 Gh 1814 Mulpani 1541 Telkot Nagarkot o 2 Changu Narayan 1632 1832 42’30” 1658 1318 MN2 27 27 o 1358 42’30” Legend 1 Gothatar Sano Thimi stretch Somathali River 1 MN3 Bodegau Watershed boundary 2 Mulpani stretch Tribhuvan International Airport Sinamangal 1658 Elevation (m) Kurthali stretch Sano Thimi 3 1301 MN1 Sample 4 Sankhu stretch Jadibuti te uman 5 Dandakateri stretch MN4 an H o er 40’00” Riv 85o22’30” 85o25’00” 85o27’30” 85o30’00” 27 Fig. 2 Location of study stretches and water samples The forth order stream extends for 5.9 km showing from each of the five stretches were measured for moderately entrenched segment (ER = 1.43). The longitudinal profile, parameters of water quality and channel has alternating riffles and pools, and has riparian vegetation. higher slope (0.14 m/m) compared to the fifth order stream. The river bed material consists of cobble, River habitat assessment pebble and sand with silt. Landslide and bank erosion River habitat assessment was made in three parts are common on steep slope river bank. according to the procedure of Texas Natural Resource Conservation, TNRCC (2001). In first part, three The third order stream stretch (Dandakateri) attributes, i. e. primary, secondary and tertiary were extends for 1.1 km. It is straight, narrow and measured during field survey (Table 1). Primary data moderately entrenched (ER = 1.45) with slope of included (a) habitat type, (b) number of riffles, (c) 0.07 m/m. Riverbed material is dominantly gravel dominant substrate type, (d) percent gravel and (e) (boulder, cobble and pebble) and minorly sand. instream cover type, which were used for evaluating the shelter and food quality for aquatic organisms. METHODS Secondary attributes focused the structures of the River habitat assessment, pollution assessment stream channel and included (a) channel flow status, and river water quality assessment were used to (b) stream width and (c) stream depth. The tertiary evaluate stream habitat condition, riparian vegetation atributes included data for (a) riparian buffer zone, and water quality condition of the river. Four locations (b) aesthetics of reach, (c) bank angle, (d) tree canopy 23 R. Bajracharya and N. K. Tamrakar/ Bulletin of the Department of Geology, Vol. 10, 2007, pp. 21–32 Table 1: Summary of the river habitat assessment of the Manahara River Segments Parameters Sano Thimi Mulpani Kurthali Sankhu Dandakateri Stream order 5th 5th 5th 4th 3rd Streambed slope (m/m) 0.02 0.0056 0.011 0.014 0.07 Length of stream evaluated (m) 500 760 550 400 160 No.