FLUVIAL REGIME OF THE MANIPUR RIVER BASIN AND LOKTAK LAKE WITH STUDY OF BACKFLOW

Abstract

The effect of fluvial changes in an intricate river basin is taken up for study. Though observed from decades ago, no system based studies as regards to the sediment-water behaviour has been done in the Manipur River Basin till date. With the availability of coherent data in the last few years, base time scale is adopted as March 2000 to April 2002 for the formulation of parameters in the numerical HEC-6 model An extensive regressionapproach to multi-variate neuro-computations are used to check the adequacy as well as the sensitivity of the data variables (viz.,Stage, Discharge, Sediment Discharge,Flow Velocity ,etc). Appropriate response parameters to the fluvial system as Stage- Discharge, Sediment Discharge Rating, Frequencies of Flow and Sediment ,etc, are then developed on the best tested dataset and fit. Spatial data is used to obtain the morphological values of the rivers and Loktak Iake.A method to implement the discharge at which it carries the maximum sediment load -'Effective Discharge' in each of the rivers under consideration ( Imphal / Manipur, Nambul, Nambol, Khordak and Ungamel rivers) in the Basin is designed based on the frequency scales and flow values. An effort to establish the non-equilibrium sediment dynamics between the Inflow and Outflow in the major natural reservoir - Loktak Lake,is presented by blending the boundary conditions on the lines of backflow (inflow) and Outflow or bi-directional flows taking place between the main storage — Loktak lake and the main artery channel — Imphal or Manipur river. Some of the sediment properties are prepared on measured records and conventional methods. The effective discharge values and response factors are used as the determining parameter in the model. A study of peculiar case of backflow (inflow) - Outflow directions over relatively short time periods due to rapidly varying flood events and barrage operations in the two interlink channels between the Lake and the main river is also taken as part and partial of the study. The hydraulic transients in the two 11 Khordak and Ungamel channels, between the lake body and the Imphal / Manipur River is also modelled with an integrated neurocomputing based multi-variate approach and HEC-6 methodology to coherent the transient behaviour. A built-in procedure is adopted for obtaining separate `Inflow' and 'Outflow' Stage-Discharges for each of the two channels within the same yearly periods, on the basis of daily measured data and with reference to the cross-section of consideration. The mean discharges of tributaries are also accorded in the major river G-Q relations during the HEC-6 formulations. The genre output is then linked to a common database for respective sediment load transport comparisons and validation at selected gauging sites such as in the periphery of the Lake and at the main outflow point of the Basin (Ithai). The other geomorphological attributes and the impact of a natural control section in the form of a "submerged rock impediment" (average height of 7 m and extending to about 2.5 kms) along the bed of the main Imphal / Manipur river , 26.6 kms downstream of the barrage is highlighted in a longitudinal representation ,before it traverses into the Chindwin - Irrawady Basin. A spatial interpretation of the Basin refecting the sub-dendritic to sub-parallel drainage network are done along with spatial data abstraction using GIS methodology.