A STUDY OF ASSESSMENT OF MINIMUM FLOW IN DOWNSTREAM OF A DAM FROM ECOLOGICAL CONSIDERATIONS

Abstract

River plays an important role in the social and economic development of the human resource sustaining in its basin providing the basic resource water for irrigation, drinking, and industrial use, waste assimilation, and recreation, etc. It is also the source of drinking water for wild and domestic animals and birds, as well as, the shelter place for aquatic lives. Any untoward deterioration of river water quality, which may cause due to the disposal of effluents from urban, industrial, and agricultural areas, would pose a threat to all those beneficial activities. Disposal of effluents into the rivers is a traditional concept. Rivers have a limiting cleansing capacity, of effluents- a known fact but seldom do we realize this important aspect. Pollution more than the self-cleaning capacity of the river water would deteriorate the water quality of the rivers. The health of a river would deteriorate more when the normal flow of the river gets restricted due to construction of a dam. The situation of the Brahmani River in Orissa particularly at the downstream of the Rengali dam is a witness of such problem. Due to the deteriorated water quality in the river the bio-diversity of the region is in danger. Down below the Rengali dam wastewaters generated from the industrial, mining and urban areas located on and around the river banks are discharged into the river Brahmani through three of its tributaries viz., Tikara, Nandira and Bangaru, which join the river at 25Km, 75 Km, and 95 Km respectively. During non-monsoon months, which occurs from November to May, these tributaries are fed mainly by the wastewaters but the main river course of the Brahmani is perennial. The normal lean period flows in the river are inadequate to maintain the requisite standards of water quality prescribed for different uses. BOD and DO are usually referred as the primary criteria of water quality index. The present study is aimed at: (I) simulation of the BOD and the DO of the river for a stretch of 310 Km for all low flow months, (ii) estimation of the self purification capacity of the river for different flow conditions, and (iii) assessment of the minimum flow requirement to achieve the water quality standards of category "B". QUAL2E, Enhanced stream water quality model developed by US-EPA based on the conceptual representation of pollutants transport in a river governed by the advection-dispersion, growth and decay, and sources and sinks, has been used as a tool for simulation of the BOD and the DO profiles of the River Brahmani, and for estimation of the minimum flow requirement. The input data, such as; river flow at the inlet and at the outlet and river water quality at different locations, point pollution flow rates and concentration of constituents, and ambient temperature etc. required for simulation of the BOD and DO were available from different Organizations. The other input data; coefficients and exponents of discharge for estimation of the flow velocity and the depth of flow and the longitudinal dispersion coefficients are computed utilizing the time series data of river observed at different locations. Using the observed BOD and DO data, the reaction rate coefficients , are estimated by trial and error method treating an inverse problem. The estimated model coefficients are: (i) de-oxygenation rate coefficient, K1 = 0.23 day 1, (ii) re-aeration rate coefficient, K2, the expression given by Owens et.al. (iii) Sediment Oxygen demand (SOD), K3 = 0.3 day"1, and Settling rate coefficient, K4 = 3.0 gm/sq./day. These calibrated values of reaction coefficients can be used for computation of BOD and DO profiles of the Brahmani river below the Rengali dam for any stress conditions. It is observed that during February through May, in a stretch measuring approximately 100 Km downstream of the Tikara river, i.e., first point load, the DO values reduce below the permissible limit of 5 mg/L. The critical month of DO deficit is May, which is also the minimum low flow month. vi The critical point of DO (=2.3 mg/I) is found to be about 115 Km below the Rengali Dam and it is near to the third point load, i.e., Bangaru river. Three alternatives; (I) augmentation of flow, (ii) control of pollution by treatment of effluents, and (iii) partial flow augmentation and partial treatment of effluents have been considered for analysis of minimum flow requirement in the river. For flow augmentation, it is assumed that quantity of flow required as augmented flow would be available from the upstream reach. Augmented flows show improvement in the DO concentration along the critical DO deficit zone, and follow a quadratic relationship with the DO concentration at a particular. This implies that the increase of DO at the initial stages of the augmented flow is more, and tends to a steady state of DO values as one increases the augmentation of flow after a certain value. Thus, flow augmentation alone is found not an effective method to recover the DO deficit. In the second alternative, different percentages of treatment to the effluents' BOD are considered to ascertain the assimilative capacity of the river water prevailing in different critical months. It is found that in different months the river requires different degree of treatment of effluents to achieve the target level of DO concentration. In the month of May, about 63% treatment of effluents' BOD over the existing pollution loads would be required to achieve the target DO value of 5 mg/L at the critical point. In the third alternative, i.e., partial treatment of effluents and partial flow augmentation, it is found that about 50% treatment to the effluents' BOD and minimum flow of about 300 cumecs maintain DO concentration of 5 mg/L along the river reach during all critical months. From ecological point of view, partial treatment and partial flow augmentation appears to be the best alternative.