GEO-ENVIRONMENTAL STUDIES IN PARTS OF A MOUNTAINOUS WATERSHED, KUMAUN HIMALAYA, INDIA

Abstract

The inherent nature ofthe Himalaya ,s well recognized. I, ,s character.sed by a variety of rock types includmg highly weathered, fractured and folded rocks andforms, steep slopes, rugged relief, high intensity of rainfall, actively eroding hydraulic regime ofriver system and frequently varying bydro.ogica, conditions Th cumulative impact of these variables often results ,„ landslides. Moreover, there has been aspurt in development activities in the las, rtvo decades mamly related ,„ road constiuction, hydropower generation, agroforestry, tourism and small scale industries Development activities when implemented a, a rapid pace may increase the instability of hi,, slopes. The resulting landslides often modify the drainage pattern hydrological conditions and ecological balance in the region. The present study of the hydrological environment ,„ parts of Ladh.ya watershed falling ,„ ,he Dlstt, Plthoragarh was mj .^ ^ ^ ^ environmental aspects. The Ladhiya over rising ,„ the Kumaun Lesser Himalaya ,s a major perennial tributary of the nver Kali. The mam thrust of the study has been given on the following aspects: ♦ To study the morphometry analysis using Horton-Strahler approach. ♦ Hydrological studies including rainfall- runoff relationship using SCS-CN model. ♦ Landslide Hazard Zonation (LHZ) mapping of the study area. The investigations are based on the study of hydrometeorological records (of rainfall- runoff), topographic sheets, field work and other published data and maps. The geology of the Ladhiya watershed has been studied following the terminology and description use by Valdiya (1980). The study area lies over two Vll major lithotectonic zones viz., the Outer Himalaya and the Outer Lesser Himalaya. The Outer Himalaya comprises the rocks of the Siwaliks of Mid-Miocene to Pleistocene age. The Outer Lesser Himalaya is dominantaly composed of unfossiliferous sequence of low to medium grade metasediments and crystallines of palaeozoic to mesozoic age. These are separated in nappe sheets and exhibit evidence ofmultiple thrusting and fracturing. Quantitative morphometry analysis of the study area has been carried out using the Horton-Strahler approach. Various types of drainage patterns and drainage anomalies have been identified. The morphometric parameters measured and computed are stream order, stream number, stream length, mean stream length, bifurcation ratio, drainage dens.ty, stream frequency, and drainage texture and hypsometric analysis. The bifurcation ratio and drainage density should normally reflect the hydrological behaviour of a watershed but the findings of this study seem to suggest the morphometric immaturity of this 6th order basin as manifested by the rugged relief and the youthful topography. However, from the analysis it is observed that some of the parameters are also dependent on the bedrock lithology. Further,the study of the hypsometric curves relationship with instability of hill slopes has indicated that right bank of Ladhiya river is more unstable instead of left bank.. The SCS-CN model has been used for rainfall-runoff modelling in the study area. The calibration efficiency of the model is 80.081% and on validation it is 92.127%. The observed and computed hydrograph shows the peakflows and time to peaks are closely matching. It reveals that SCS model has considerable potential in simulating continuous long term daily flows in and has good validity in the Himalayan watersheds. The knowledge of landslide potential is very important for sustainable development of the mountainous terrains. Therefore, astudy is made to delineate the landslide hazard zones. For preparation of such a map aLandslide Hazard Evaluation ' Factor (LHEF) rating scheme has been adopted. This technique is based on an empirical approach and includes the major inherent causative factors of slope instability such as lithology, structure, slope morphometry, relative relief, land use Vlll and land cover and hydrogeologioa. (surface moisture) conditions. The external contnbutory factors such as rainfall and seismicty are not included since they are erratic ,„ nature and their impact on landshde potential cemno, be estimated with particular reference to a slope facet. VVeerrvy HHTighHhTHazar7d (VdHePH'C),'SH"i"gh"H^aza"rd"(H°Hf)*,*M>od^erat*e H-ahza»r<dd(M»H«) an"d™Leolwy Hazar (LH). Atotal of four hundred and thirty five slope facets spanning over an area of about ,56km' were delineated, on, ofwhich th.rty four facets ofHigh Hazard zones, two hundred nineteen of Moderate Hazard zones and sixty two of Low Hazard zones are found to be present ,„ «he area of study The area covered by VHH, HH MH, LH zones are 14.14, 54.52,26.42 and 5.12 per oen.e respectively The Slope Facets fallmg under low hazard zones are considered stable Though moderate hazard slope facets are also considered stable ,„ general, bu, they may have some local pockets of unstable zones wh.ch are to be investigated during project implementation. The High Hazard and Very High Hazard (VHH) slope facets represent high potential unstable slopes. These facets are found mostly along the water courses indicating tha, they are actively eroding the bed side slopes. However most ofthe latter type slope facets are located along the streams joining the Ladhiya nver on the right and left bank ,„ the study area ,„ fact these streams account fir nearly mne VHH slope facets Since these slides are s,,„ active in nature,, is essentia, to have slope stabilization measures such as check dams and biotechnica, slope stabilization. In order to alleviate ,he problems of slope instability and undesirable hydrologica changes ,suitable remedial measures have been suggested for the study *ea The above studies have demonstrated the applicability of ajudicious approach w1^ "8 md reS,0nn8 ^ — -nario of „,ma,ayan