Hydrological variability and channel changes in upper Ganga plains

Spatial variations in channel patterns as well as coexistence of stable confined and dynamic unconfined channel reaches are typical of the channel system in upper Ganga plains. Water discharge, suspended sediment load and channel morphological parameters for the period 1970-2000 were analysed to understand their controls on fluvial dynamics in a 200 km long reach of the Ganga river between Farrukhabad and Kanpur. A comparison of river plan forms since 1871 reveals that the river channel is confined in 1-2 kilometres long reaches at nine different places, where no significant movement has occurred. The rest of the reaches have witnessed frequent channel switching up to 4 km across the flow. Field observation and multi-date image analysis suggest that the channel movements are due to formation of channel bars, channel bifurcation and meandering shift. Study of channel cross profile from digital elevation model revealed that the Ganga river is flowing along a broad, incised valley, where active channel is flowing almost along right edge of the valley created by the river itself. In above mentioned reaches channel is highly entrenched (bordered by 10-12m high cliff) and alternating with relatively shallow, multiple dynamic channel reaches. Large cross sectional area due to high bank relief created accommodation space for conveyance of high discharge at several places. Flood frequency analysis shows no floods with stages high enough to inundate the interfluve area separated by cliff on river's right bank side. Flow duration analyses show that there is only 2% probability that the flow volume can exceed its bankfull stage and 90% probability that mean annual discharge is less than mean annual flood, it also shows 70% of the time flow volume is lower than mean annual flow. Flood intensity and discharge variability plot of all rivers shows that no river is flood-prone in the upper Ganga plains unlike the lower Ganga plains, where river floods almost every year. The basic control of channel movement and entrenchment is envisaged as the balance between stream power and critical power (needed to transport sediment load). The exceedence of stream power from critical stream power would trigger incision and would create stable confined reaches. However, at other places where stream power is very close to critical power, high discharges (bankfull discharge) cause lateral erosion which actually forces the channel to move sideways. Flow duration analysis of discharge and suspended sediment at different gauging station shows that changes in daily mean sediment transport with time are caused by duration of flow. Therefore, a sub-bankfull discharge persisting for long time can also produce an effect similar to that of the bankfull discharge. Although we do not have quantitative measurement on critical stream power, we observe that the stream power at the entrenched reaches is much higher than the average stream power in comparison to the wide aggrading reaches. Where stream power is less than critical power, the channel loses a part of its sediment load and causes aggradation. The aggradation within the channel causes reduction in the accommodation space and, thus the main channel gets bifurcated into numerous shallow channels. We conclude that the spatial and temporal variation of channel planform characteristics and channel dynamics are related to stream power variations, flow characteristics and aggradation-degradation regimes. Key words: Planform, flow duration, stream power, channel switching.