Flood Hydrology, Hydraulics and Hydrodynamics of the Tapi River, Western India

It is evident that most of the geomorphic work in seasonal tropics is accomplished by individual flood events. Studies on some large Indian rivers indicate that the channel forms and processes are related to very large but relatively infrequent flood events (Goswami DC, Water Resour Res 21:959–978, 1985; Kale VS, Ely LL, Enzel Y, Baker VR, Geomorphology 10:157–168, 1994; Gupta A, Natural and anthropogenic influences in the fluvial geomorphology. American Geophysical Union, Washington DC, 1995; Gupta A, Kale VS, Rajaguru SN, Varieties of Fluvial Form. Wiley, New York, 1999). The impact of floods depends not so much on the amount of water as on the energy exerted by it. The adjustments in the width-depth ratio and hydraulic variables with discharge have been shown to very useful concepts in evaluating the potential of flows to be geomorphologically effective (Kale VS, Ely LL, Enzel Y, Baker VR, Geomorphology 10:157–168, 1994; Gupta A, Natural and anthropogenic influences in the fluvial geomorphology. American Geophysical Union, Washington DC, 1995). Baker and Costa (Catastrophic flooding. Allen and Unwin, London, 1987) suggested that geomorphic effectiveness of floods is related to the flood power defined in terms of channel boundary shear stress and power per unit area of bed. Moreover, Baker (Flood geomorphology. Wiley Interscience, New York, 1988) and Wohl (J Geol 101:749–761, 1993) noted that the regime conditions of the flows and the degree of turbulence also play a role of considerable importance in the erosion and transport of coarse sediment. Therefore, in the present chapter an attempt has been made to understand flood hydrology, hydraulics, and hydrodynamic characteristics of the Tapi River and its tributaries, on the basis of available annual peak discharge data and cross-sectional data collected during the field surveys. The analysis reveals that even during the monsoon, the discharges fluctuate by several orders of magnitude. The episodic high flow events are sharp and distinct. The time series plots of annual maximum series data reflect considerable interannual variability. High variability is also indicated by the values of coefficient of variation and the flash flood magnitude index. In general, the maximum annual peak discharges are 2–4 times higher than the mean annual maximum discharges. All these indices indicate that the possibility of the river experiencing significant geomorphic work during large floods is higher. High unit discharges in the upper reaches indicate the high potential of floods. The channel of the Tapi River is box-shaped, with more or less flat channel floor and high banks. Therefore, during the dry season and during low flows, the water spreads, and the width is high and depth is low. Consequently, the width-depth ratio is high and the channel reflects all the characteristics of a shallow, wide channel. However, in response to heavy rainfall as the stage and discharge increases, there is an increase only in the depth of flow. As a result, the width-depth ratio decreases, and the hydraulic efficiency increases dramatically. The results of hydraulic geometry analysis suggest that the behavior of the alluvial Tapi River is not truly alluvial but quasi-bedrock. The channel geometry of the Tapi River plays a significant role in efficient conveyance of monsoon floods through the changes in the hydraulic variables with increasing discharge. The unit stream power and bed shear stress range between 27 and 1518 Wm-2 and 10 and 263 Nm-2, respectively. Although the estimated values of stream power are not unusually high, hydraulic parameters suggest that if the duration is long, large floods (Qmax) are capable of perhaps eroding alluvial banks and moving cobbles and pebbles in temporary suspension and boulders as a bed load (Baker VR, Costa JE, Catastrophic flooding. Allen and Unwin, London, 1987). The estimated Froude numbers for different sites, as expected, are less than 1, indicating that the flows are dominantly subcritical. However, the occurrence of impressive scablands and other erosional features in bedrock channels, particularly between Dhanora and Burhanpur, suggests that Froude numbers close to 1 are reached from time to time (Kale VS, Ely LL, Enzel Y, Baker VR, Geomorphology 10:157–168, 1994). High values of Reynolds number indicate that the flood discharges could be extremely turbulent and, thus, are capable of accomplishing a variety of geomorphic activities. The bedrock reaches at Dhanora produce the highest Reynolds number, and it is likely that this may be the reach of very high and intense bedrock erosion. Estimates of the values of critical velocity for inception of cavitation indicate that none of the powerful floods on the Tapi River, for which some data are available, exceed the conditions expressed by the equation. This is in spite of the fact that the maximum surface velocities in the alluvial sections range between 3.7 and 5.5 m/s during large floods. This, therefore, suggests that the process of cavitation is confined only to very narrow, steep reaches during extraordinary floods, which occur at a much longer interval.