Human Impact and Channel Hydrological Modification: A Study on Dwarka River of Eastern India

Transcription

1 Human Impact and Channel Hydrological Modification: A Study on Dwarka River of Eastern India Surajit Let and Dr. Swades Pal Abstract With course of time the physical landscape of the river is getting changed and at the same time human interference also has massively topsy-turvied the normalcy of the river. Dwarka River is one of the Tributary of Bhagirathi River in eastern India. This river basin covers densely populated tracts of Jharkhand and West Bengal state. Now due to the desperate intervention of human being through construction of barrage across river, embankment along river banks, lodge construction along river banks etc. to draw immediate profit from river and river command area, river has changed its hydrological character in diversified ways. One of the most important modified hydrological phenomena in Dwarka river basin is flood. If morphodynamics of the flood is possible to manage with very cooperative way the execution of wise resource use of the basin will be made possible. Keywords Spatial flood dynamics, Flood frequency, Flood water stagnation period, human interference, Hydrological modification. R I. INTRODUCTION IVER basin is one of the most important and lucrative habitat of human living. Within a basin a large set of hydrological phenomena has been working on in its own way. Over pressure of population and their behavioral change has been altering the normal pattern of hydrological phenomena. As per normal law people again has been getting back its negative result. One of the most important modified hydrological phenomena in Dwarka river basin is flood. Flood is simply defined as a state of high water level along the river channel or on coast that leads to inundation of land which is normally submerged. Once a time flood was seemed to be beneficial for human civilization when fertile silt carried out by the rivers enriched the production ability of an agriculture land and flood phenomena was fruitfully rhythmic in manner. But now a day due to desperate invention of human being through construction of dam, across river, embankment along river to draw immediate profit from river the intensity of flood has increased many times and it carries huge sand instead of fertile silt and flood is considered as human fears. As a result problem of flood today become great challenge to our society, to our economy as well as sustain the status of development. In this present study author would like to show how flood characters have been proliferating over time and what the main causes of such extreme hydrological modification are. II. ABOUT THE STUDY AREA The Dwarka River - a tributary of Mayurakshi is a well known name in the river atlas of Rarh Bengal including parts of West Bengal and Jharkhand states of India. Originating at Kushpahari of SanthalParganas of Jharkhand it has been flowing through Mayureswar and Rampurhat police station areas of Birbhum district and ultimately joins the Mayurakshi near Hizole wetland in Kandi Police station of Murshidabad district, West Bengal. Its basin area can be delimited by 23 58' to 24 29' North latitudes and 87 18' to 88 12' East longitudes covering an area of km2. Total length of the river is km. Administratively, the study region includes Pakur and Dumka districts of Jharkhand and Birbhum and Murshidabad districts of West Bengal and 12 CD blocks of the same districts. Surajit Let is a Research Scholar with Department of Geography, Visva Bharati, Santiniketan, India. ( surajitgeo@gmail.com). Dr. Swades Pal is working as a lecturer in Deptt. Of Geography Rampurhat College, Rampurhat, W.B. India. ( swadeshpal82@yahoo.in) Fig. 1 Reference map of the study area

2 III. PHYSICAL BACKDROP Geologically the upper part of the basin dates back to the deposition of Dharwanian sedimentary followed by Hercinian orogeny from Cambrian to Silurian period. The extreme eastern part of upper basin is characterized by unclassified Ggranitic Gneiss with enclaves of metamorphic geomaterials. The two major deposition is divided by a narrow stretch (N.E to S.W) of Silt stone, Sand stone and Shales with coal deposition. There are some small patches of Hornblende, Schist and Amphibolites, Charnockite/acid granulite and Augen gneiss and Migmatite, Sandstones and Shales etc. The middle catchment is mostly characterized by the deposition of Lateritic soil and hard clays impregnated with Caliche nodules. The platform of this region was set through the tectonic activities associated with tertiary epoch followed by lying down of the recent (late Pleistocene) and successive southward march of the delta plain. The lower catchment nearer to the confluence is mostly characterized by recent alluvial deposition of alternative layers of sand silt and clay attributed by alleviation river diversion, flooding and consequent shaping by twin action of Bramhani and Dwarka. The deposition of hard clays impregnated with Cliché nodules of the middle continued in the lower catchment also up to the meeting place of Bramhani and Dwarka as well as at a distance from the river course in the confluence. Physiographically, the upper Dwarka consisting of SanthalPargana highland of Chotonagpur plateau fringe is characterized by numerous rounded flat topped isolated hillocks with an average elevation about 271 m. The surface is rugged and stony due to the effect of continuous action of various agents of denudation. The Lower Dwarka is consisting of Birbhum Rarh and Murshidabad Rarh Plain specifically relatively low lying region. The Birbhum Rarh is actually the eastward extension of Chotonagpur plateau fringe. It is slightly elevated lateritic region with exposures of Gondwana and high ravine land. The Murshidabad Rarh plain is the marginal land between delta proper in the east and lateritic Rarh in the west. Murshidabad plain is characterized by low lying topography with numerous rivers and gully segments, wetlands, swamps, chars etc. Annual rainfall in this region is about 145cm. Out of total 80% rainfall happens during monsoon period (June to October). IV. MATERIAL AND METHODS The present study is based on primary and secondary data. A detail field survey has been conducted in different parts of Dwarka River basin. The primary data are collected through questionnaires among local people and using dumpy level survey (for cross profile of river). The secondary data like flood level height, flood frequency, flood stagnation period etc. have collected from irrigation departments, block offices and statistic Hand Book of Murshidabad, Birbhum. Pakur and Dumka districts published by Government of west Bengal and Government of Jharkhand. The secondary data has also been collected from Survey of India toposheetand Google image. The flood intensity character of individual year(1978, 2000 and 2007) for Dwarka River basin have been estimated using two parameters namely flood heightand flood stagnation period. In this study weighted score method has used for integrated flood intensity map. Integrated flood map is prepared using three parameters flood level high, flood frequency and flood stagnation period. Where M = Maximum value of column, N = variable Weighted values are added together to show the integrated data. On the basis of composite weighted score the entire basin area has been divided into four broad flood namely Intensive flood (>240) Moderate flood (80-240) Low flood (<80) (iv) no flood (0). V. RESULT AND DISCUSSION Table I clearly shows that flood affected areas have been increasing over time. In last century 1978 flood year was the greatest in the sense of flood extension and damages. But flood year 2000 has broken down all the previous record and account the most extensive flood character. Figure 2 and 3 respectively show the status of most intensive flood extension and maximum flood affected areas. It is noticed that flood 2000 not only submerged the lower basin but also extensive part of upper basin area. TABLE I DISTRIBUTION OF FLOOD AFFECTED AREA Flood Intensive flood Moderate flood Low flood No flood (1)

3 Fig. 4 Integrated Flood Map Fig. 2 Intensive flood affected areas within basin area Flood TABLE II DISTRIBUTION OF FLOOD AFFECTED AREA Area % of area to total Intensive flood Moderate flood km km Low flood km No flood km A. Flood Zoning Fig. 3 Maximum flood affected areas On the basis of said methodology, integration of flood parameters has done and resulted score has been classified into three s like: 1. Intensive flood Basically the confluence catchment area of Dwarka River basin counts under intensive flood. Almost all the mouzas of Kandi block are experienced by intensive flood. About 32 mouzas of lower catchment isghastly lashed by flood damage. Total area of intense flood is about km2. 2. Moderate flood Upper part of Dwarka River basin specially the eastern part of Murshidabad district, eastern part of Birbhum district come under this category. Total area of moderate flood coverage is sq.km. 3. Low flood In relatively upper part of this river basin specially the western part of Birbhum of West Bengal and eastern part of Dumka district of Jharkhand come under this flood. Total areal coverage is sq.km. 4. No flood In extreme upper portion of the catchment area where there is no broad river, where surface flow is more common than any channel flow the occurrences of flood is almost nil. Total coverage of this kind of no flood is sq.km. B. Causes of Hydrological Modification 1. Barrage Construction Man-made barriers like dams and storage reservoirs, leads to excessive deposition both in upstream and downstream below the structure, and consequent increase of flood levels and extent [2]. Deucha barrage across barrage Dwarka have irrefutable effect onmainly lower part of Dwarka river basin. These projects bear some boon for the upstream habitants because it supplies irrigation water but it is really devastating for downstream residents. During monsoon period due to heavy rain these barrages release huge water volume at a time beyond the retaining capacity of the rivers cause massive flood in the western portion of Birbhum and entire Murshidabad district within Dwarka catchment. Flood-2000, flood-2007 is two immemorable flood of this current century is because of sudden outbreak of barrage water. For example, during 2000, within 5 days (18th to 22nd Sept.) Deucha barrage had released cusec.waterin additions with short period outburst of rain and as consequences massive inundation was happened. During September, 2007 flood devastation this region has experienced due to sudden water discharge from Deucha barrage. Figure 8 shows the peak discharge in different periods. During flood 2000, flood water level is far above the extreme danger level. During flood 2007, sudden release of barrage water raises the level of water far above the extreme danger level. It causes break down of embankment and consequent flood (vide 6&7).

4 Fig. 5 Water discharge pattern from Deucha dam When the brick factory at the river bank sands are carried out to those points digging the embankment points. These points are the most fragile and vulnerable for embankment breaching during flood. In downstream area about 17 such points are found where this type of activity has been going on. In 2007 flood, out of these 17 points 11 points have breached. 4. Channel Capturing by Building Construction Tarapith is now one of the most important pilgrim centre. To accommodate mass of pilgrims the sky scrapper lodge has been raising up rapidly. They have been constructing lodge capturing the existing river channel. It has constricted the channel tightly, so the width of the channel has reduced to 37.8 m. but 250 m. upstream the width is 57 m. So during monsoon, water is not freely escaped downstream and few km. upstream areas have been inundated. Such phenomena was totally absent before lodge building in this river. Figure 8 to 10 show the real configuration of river channel just upstream, downstream and in Tarapith. Fig. 6 Discharge level, Sept, 2000 and Discharge level, Oct., Poor Embankment Present day s river is mostly guided by human beings along the guidelines demarcated by the arrogant signature of human engineering e.g. embankment. In lower part of Dwarka all the rivers are regulated utmost by river embankment of about greater than km. length, most of the embankments are 4.26 m. to >6.40 m. high. Almost every 2-3 years interval, elevation of the embankment is getting rise in parity with rising flood height. During 2007 centre pool of the Dwarka- Babla system have been raised up almost about m. Sir William Wilcox (1930) [3] accused the river side embankment as satanic chains and proposed canal for draining out the flood water. Prof. M.N. Saha (1935) [4] was also concerned about the effect of embankments on the hydrological characters of the rivers while explaining the causes of the flood of Bengal. Mr. S.C. Majumdar (1945) [5] warned about the long term evil effect of the embankment. Noteworthy, (a) material of the river embankment oftenly used for construction are very fragile loose sand or sandy soil. High mobility of sand within channel bed, in cohesive bank materials often responsible for embankment breaching and flood. Moreover, soil used for embankment is just collected from base of the bundh as a result a deep scour has been developed alongside the outer margin of embankment basement. These scours contain water like marshy land almost all through the years as well as weaken the basement. If embankment is the ultimatum its base should be much wider than apex to resist huge water pressure. 3. Brick Kiln Industry within River or River Bank Brick factory within the river or river bank facilitate easy access of sand and water. So, they have tendency to locate on those places. When they are within the river hindering the flow pattern and when final products have brought to target points, the embankment cuts and make accessible for transportation. Fig. 7 Channel Configuration at the just upstream of Tarapith Fig. 8 Channel Configuration at the just upstream of Tarapith Fig. 9 Channel Configuration at the just downstream of Tarapith VI. CONCLUSION So, along with natural causes, acceleration of hydrological

5 modification and flood accumulation was supported by human made causes as mentioned above. Immediate steps regarding scientific regulation of barrage water, either to make river embankment free or to make it broad based and narrow apex, and bank material should be much cohesive. River path should not be obstructed at any cost. It is in fact true that as much we will able to keep ourselves away from interfering on river, river will produce blissful benefits for human being. REFERENCES [1] Rasmussen, J.L. (1994): Flood Plain Management into the 21st Century: A Blueprint for Change-Sharing the Challenge. Water International, 19(4), pp [2] Z.W. Kundzewicz, Coping with Hydrological Extremes, Water International.vol. 25(1) pp , [3] W. William, Lectures on the Ancient System of Irrigation in Bengal, University of Calcutta [4] M.N. Saha, Collected works of M.N. Saha, University of Calcutta, vol.i, [5] S.C. Majumdar, Rivers of Bengal Delta, Department of Irrigation, Govt. of West Bengal, 1941.