Effect of upstream withdrawal on river wetland connectivity in Surma-Kushiyara basin

Abstract

This research was undertaken to investigate the probable impact of upstream storage and diversion works on Barak river by India on a selected wetland area in the Surma and Kushiyara basin. Three scenarios of flow regulation chosen for analysis include: (i) average year flow without Tipaimukh dam, (ii) average year flow with Tipaimukh dam, and (iii) average year flow with Tipaimukh dam and Fulertal barrage. The Chunia beel wetland area and the Kakura kIwi, providing one major connectivity with the Surma river were chosen for detailed analysis. Through field visits on-site knowledge about the existing connectivity between the river system and wetland has been gathered. Also information about agriculture and fisheries were obtained. One-dimensional model simulation was performed for the average hydrologic year with the above three scenarios. The effect of upstream dam and barrage on annual peak flows and seasonal hydro graphs was analyzed. Two-dimensional model simulation was used to determine the changes in the extent and depth of flooding in the river basin. Changes in land type, that is how the inundation area of larger depths changes to a shallowcr depths and the consequent effect on cropping pattern and fisheries were studied for the selected bee I wet land area Due to the Tipaimukh Dam the peak flow in Surma river is likely to decrease by about 15% with about 3% decrease in the peak water level. The corresponding decrease .in the peak flow and water level in Kakura khal connecting the Surma and Chaunia beel will be about 32% and 3%, respectively. This causes reduction in the ,extent of flooding in the river basin as discussed below. In the pre-monsoon season, for Kakura khal water level shows an increase of 0.73 m in the first week of April causing a reversal of flow from the river to the khal. Similar reversal of now is observed in the second and fourth week of May. This may be due to pre-monsoon release from the reservoir to make room for flood storage. In the post-monsoon discharge decreases in a fluctuating manner in the first two weeks of October, then increases in the third week of the month, and thereafter a slight increase in discharge is observed for the rest of the period. In the dry season discharge increases slightly probably due to release for hydropower generation showing an increase in water level ranging from 0.7 m to 0.69 m. As to the extent of flooding in the river basin, it is seen that in the post-dam situation, the extent of inundation shows a decrease by about 60% in pre-monsoon, 22.5% in IV monsoon and 63% in post-monsoon season. For the barrage in addition to the dam no further change in inundation extent was evident from the simulation results. Also there will be some redistribution of Fa, F I, etc. land types. In the pre-monsoon season area of Fa land type shows an increase of about 18% whereas F I, F2, F3 areas decrease by 46, 76, 93% respectively and F4 type disappears. This means that the deeply flooded area decreases. Some additional changes in land types due to the effect of barrage in addition to the dam were seen to occur. Fa and FI land areas show a slight increase, F2 shows a slight decrease, and F3 and F4 types do not, change. In monsoon Fa and F2 land areas increase by about 11a and 39%, respectively, but F2, F3 and F4 areas decrease by 29, 49, and 97%, respectively due to dam. On the other hand areas under all land types exhibit decrease by amounts ranging from 54 to 82%. For the barrage in addition to the dam, Fa area slightly decreases, Fl type slightly increases and the other land classes remain unchanged. Finally, the changes in land type, cropping pattern and fisheries in the selected bee I wetland were analyzed. Results of simulation indicate that some modification in land type in the selected Clmnia bee I wetland area in monsoon is likely to take place due to dam. Fa land type remains unchanged, F2 type decreases by about 17%, F3 type disappears, and FI appears which was nonexistent before. However in pre- and postmonsoon seasons, no change in land type is indicated. Obviously due to change in land type there will be some change in cropping pattern. Area under local Boro may reduce, some areas may become suitable for broadcast and HYV Aus and Rabi crop. This provides some indication about the qualitative changes in cropping pattern but evaluation of the changes in the amount of production could not be done within the scope of this study. As the deep water area is likely to be reduced, survival of fish requiring such habitat will be difficult. Habitat type for carp, catfish, and eel fish species may be reduced. River and flood plain flow simulation performed in this study were based on an assumed inflow hydro graph at Amalshid based on preliminary information available from some reports. No statement was available as to how much water is proposed to be stored or diverted by the Tipaimukh dam and/or the Fulertal barrage. There is much scope to improve this type of analysis if information about the operating rules and flow regulation become available.