Assessment of morphological aspects and bank protection works of the river Surma

Abstract

Realizing the gravity of the changed hydro morphological characteristics of the hilly river, Surma, this study has been conducted to investigate the morphological characteristics, sediment transport phenomena and also to analyze the design parameters of the implemented bank protection works of this river at Sylhet. Mainly, cross sectional properties at 24 selected sections of the river Surma for the years 1970, 1989, 1991 and 1999 were analyzed. It is observed that sedimentation is portrayed in the period 1972-73 to 1981-82 whereas erosion can be illustrated in the period from 1981-82 to 1991-92. Sedimentation dominates over a period often years before reaching a peak of erosive activity in 1999. In addition, it has been found that the river had turned into wider and shallower up to 1989. After that, it changed into comparatively narrower and shallower for the last decade (I989 to 1999). Variation of width in successive years shows that the section no. S-18 and S-38 experienced the sudden rise and fall during the 30 years period. In addition, the maximum rise in calculated mean bed level is happened at section no. S-42 (I 3.13 m) whereas the maximum fall in mean bed is found at section no. S- 18 (0.82m in 1991) and S-3 (0.46m in 1989). Due to the deposition of huge sediment load (about 3.00 MT/year), the river has lost its normal carrying capacity about 27% and flash flood visits almost every year inundating the surrounding lands and towns. Discharge carrying capacity and sediment concentration of 14 cross sections of the river Surma starting from S-29 (Sylhet) to S-42 (Lubachara) have been estimated under bankful conditions, using a simplified resistance equation and a sediment predictor. Results indicate that the section no. S-31 has the highest discharge carrying capacity (973.22 cumecs) whereas section no. S-41 has the lowest of that (133.55 cumecs) among all the selected sections. In this regard, it can be mentioned that the bankful discharge of Surma at Sylhet is 1373.33 cumecs. Superimposition of satellite images shows that the change of planform of this river is not significant over the last five years period (I 997 -2002). Sediment load of the Surma river has been computed with the help of three well known sediment predictors such as Engelund-Hansen's equation, Ackers-White's equation and van Rijn's equation for the years 1975, 1989 and 1999 at the station Sylhet and were compared with the values of the measured sediment load of those years. The observed annual sediment load at Sylhet was 2.77 6MT, 3.121 MT and 3.0971 MT for the year 1975,1989 and 1999 respectively. However, the average annual sediment load predicted from Engelund-Hansen's equation is 0.727 MT, from Ackers White's equation is 1.818 MT and from van Rijn's formula is 0.828 MT. Comparison shows that out the three sediment predictors Ackers White's formula is found to be well suited against the observed data of the river, Surma. The implemented bank protection works starting from Kazirbazar to Mollapara at Sylhet City has been also considered for this present study. Depending on the assembled field data and information, hydraulic design parameters of the bank protection works were evaluated in terms of effectiveness, suitability and sustainability.