Rajiv Sinha

Braiding and meandering parameters

Abstract Modifications to standard definitions of braiding and meandering are proposed to indicate the morphology of every river channel reach quantitatively, whether it has single channel or multiple channels. Sinuosity (P) is defined as, P = Lcmax/LR, where Lcmax is the length of the midline of the channel (in single-channel rivers), or the widest channel (in multi-channel rivers), and LR is the overall length of the reach. Braiding is a measure of channel multiplicity and a new term ‘braid-channel ratio’ (B) has been defined as, B = Lctot/Lcmax, where Lctot is the total of the mid-channel lengths of all the channels in a reach. Another expression for the braid-channel ratio which gives the same numerical result is, B = Pctot/P, where the total sinuosity, Pctot = Lctot/LR, and P has been defined above. Scatter plots on braid-channel ratio/sinuosity axes show a negative correlation between these parameters, as would be expected from the above relationships. Single-channel rivers (B = 1) have relatively higher sinuosities and the upper limit of these is the point at which cut-off becomes highly probable. For multi-channel rivers (B > 1), sinuosity remains low, reflecting the limiting effect of braid bars on the development of fully developed spiral secondary flow. Data analyses showing that increases in channel slope and bankfull discharge are associated with changes from meandering to braided morphology are diverting attention from the importance of the increased availability of bed-load-grade sediment as a control.

Climate control on erosion distribution over the Himalaya during the past ~100 ka

Sediment samples from a 50-m-long core representing ~100 ka of deposition, taken from the Ganga Plain on the campus of the Indian Institute of Technology, Kanpur, were analyzed for Sr and Nd isotope compositions. Both 87 Sr/ 86 Sr and ϵ Nd vary significantly with depth in the core, 0.72701–0.76708 and −14.4 to −16.6, respectively, within the range for silicate rocks of the Higher and the Lesser Himalaya. The variations in the isotope compositions reflect variations in the mixing proportion of sediments from the Higher and Lesser Himalaya, the two major sediment sources to the Ganga. The opposite trends in 87 Sr/ 86 Sr and ϵ Nd depth profiles further confirm this hypothesis. The isotope profiles exhibit two major excursions, ca. 20 ka and ca. 70 ka ago, coinciding with periods of precipitation minima and larger glacial cover. These excursions are the result of a decrease in the proportion of sediment from the Higher Himalaya due to a decrease in monsoon precipitation and an increase in glacial cover that are in turn caused by lower solar insolation. This study highlights the significant influence of climate on erosion in the Himalaya.

Craton-derived alluvium as a major sediment source in the Himalayan Foreland Basin of India

Within the Himalayan Foreland Basin, the axial Yamuna River with Himalayan headwaters lies along the northern margin of the Indian Craton, giving the impression that cratonic rivers have contributed little to the basin compared with Himalayan drainages. However, the Betwa, Chambal, and other rivers, which drain northward into the Yamuna, are vigorous monsoonal rivers with large catchments. Stratigraphic and petrographic evidence shows that sediment derived largely from these rivers extends north of the axial Yamuna River. Red feldspathic sand and gravel underlie much of the southern foreland basin at shallow depth (>25 m), where its topmost strata are dated at ca. 119 ka ago, and extend at deeper levels (>500 m) to about one-third of the distance across the foreland basin. Petrographic analysis confirms a match with modern Betwa River sands, which derive their feldspar from granitic gneisses of the Bundelkhand Complex. Along the Yamuna Valley the red alluvium is overlain by gray alluvium dated at 82–35 ka ago, which also yields a cratonic signature, with large amounts of smectite derived from the Deccan Traps. Cratonic contributions are evident in alluvium as young as 9 ka ago in a section 25 km north of the Yamuna. This gray cratonic sediment was probably deposited in part by the Chambal River, which transports high-grade metamorphic minerals from the Banded Gneiss Complex of the Aravalli belt. Cratonic sediment appears to interfinger with Himalayan detritus farther north below the Ganga-Yamuna Interfluve. With its headwaters in the tectonically unstable Indus-Ganga watershed area, the Yamuna River may have occupied its present course late in the Quaternary, and if so, cratonic rivers may have provided the basin9s axial drainage for prolonged periods. The penetration of Himalayan sediment to the distal foreland basin may reflect avulsion of orogenic rivers along the craton margin, in addition to dynamic transverse drainage systems from the Himalaya that pushed the axial drainage to the basin9s feather edge. The wide spread of cratonic sediment would have been enhanced by slow subsidence in the distal foreland basin and focusing of rivers into a basin reentrant.

Derivation of Unit Hydrograph from GIUH Analysis for a Himalayan River

Unit Hydrograph (UH) is the most popular and widely used method for predicting flood hydrograph resulting from a known storm in a basin area. However, the non-availability of UH due to poor network of raingauge stations in flood prone Indian river basins is a major concern. The computation of Hortons ratios and their application in generating the Geomorphological Instantaneous Unit Hydrograph (GIUH) can provide a solution for ungauged rivers. A detailed drainage network analysis was carried out for a 5th order flood- prone Himalayan river system in order to highlight its significance in flood management program. The equations for GIUH of 5th order stream were derived through Markov Chain analysis. The GIUH model for the 5th order stream was used to derive the first ever analytical UH of the river. Further, it was applied to determine the 50-yr return period flood. The 50-yr return period flood matches with the result of flood frequency analysis based on observed peak discharge data. This drainage network analysis and application of GIUH can provide a significant contribution towards flood management program.

Geomorphological Manifestations of the Flood Hazard: A Remote Sensing Based Approach

Abstract Flood hazard is one of the most severe problems in the Himalayan river basins. Although floods are essentially hydrological phenomenon, the uneven distribution of floods in the river basin highlights the control of geomorphological and geological factors. A proper understanding of these factors is critical for a successful flood management programme. Remote sensing data is of immense value in evaluating the geomorphological and geological controls in flooding. The present paper highlights the control of geomorphology and neotectonics on flood hazard in north Bihar Plains, eastern India. The Indian Remote Sensing (IRS) data has been used and a variety of image processing techniques have been employed.

Understanding dynamics of large rivers aided by satellite remote sensing: a case study from Lower Ganga plains, India

The advent of satellite remote sensing has provided a huge opportunity to geomorphologists to study the temporal dynamics of large rivers. The repetitive coverage of satellite data is an important archive to reconstruct historical-scale dynamics of large rivers and to understand the causal factors. In the lower reaches of the Ganga River around Farakka, natural as well as anthropogenic factors have influenced large-scale dynamics of the river during the last 234 years. The construction of the Farakka barrage was started in 1961 and was completed in 1971 except the feeder canal. The barrage was finally commissioned in 1975 but the serious problems of aggradation both upstream and downstream of the barrage had started much earlier resulting in significant changes in channel morphology and position. The channel upstream of the Farakka barrage has moved towards the east but channel shifting downstream of the Farakka barrage has been erratic. We argue that sedimentological readjustments due to aggradation and bar growth are the major factors influencing the river dynamics in this region.

Evolution of the Indian summer monsoon: synthesis of continental records

Abstract Fluvial sediments of the Siwalik succession in the Himalayan Foreland Basin form the most important continental archive for reconstructing monsoonal fluctuations during the Late Miocene to Late Pleistocene. A number of proxy records suggest multiple phases of monsoonal intensification with peaks at 10.5, 5.5 and 3 Ma after which the strength of the monsoon decreased to modern day values with minor fluctuations. Detailed evaluation of Late Quaternary interfluve stratigraphic development in the Ganga plains shows that interfluve areas near the major rivers aggraded periodically between 27 and 90 ka. They subsequently degraded or accumulated sediment only locally, probably reflecting decreased monsoonal precipitation around the Last Glacial Maximum. Increased precipitation during the 15 to 5 ka period of monsoon recovery probably increased discharge and promoted incision and widespread badland formation. In western India, the fluvial records back to c. 128 ka suggest a stronger monsoon around 80 ka followed by periods of weakened monsoon around 70 to 30 ka and progressive desiccation in the glacial period. Holocene lacustrine records from western Rajasthan suggest maximum lake levels at c. 6 ka and complete desiccation between c. 3 and 4 ka.

On the controls of fluvial hazards in the north Bihar plains, eastern India

Abstract The rivers of the north Bihar plains, eastern India, pose three major fluvial hazards: rapid lateral migration, frequent flooding and extensive bank erosion. Lateral shifting of the Kosi, Gandak and several other rivers in the area has been attributed mainly to neotectonic tilting and subsidence of the area, and to some extent, local topography and sedimentological readjustments in the basin. Overbank flooding is a perennial problem, with most of the rivers of the north Bihar plains causing enormous damage to life and property. The construction of embankments along major portions of the rivers is only a short-term solution to mitigate floods not only because of frequent breaches in the embankment due to extremely high discharges during high flows but also because of the fact that these rivers carry a high sediment load causing rapid siltation and thereby raising the water level in a few years’ time. Severe bank erosion takes place during the lateral shifting of rivers as well as during high flows. Efforts to prevent these fluvial hazards in the area have largely failed as the geological and geomorphological considerations have not been taken into account.

Exploring the channel connectivity structure of the August 2008 avulsion belt of the Kosi River, India: Application to flood risk assessment

The August 2008 avulsion of the Kosi River, northern India, resulted in a maximum eastward shift of >100 km and created an avulsion belt of 2722 km 2 . Based on A.D. 2000 Shuttle Radar Topography Mission data and on 2005 Landsat Thematic Mapper satellite image–derived channel network (pre-avulsion), we use a topography-driven connectivity model to simulate the avulsion pathway, which corresponds, to a large extent, to that observed in the post-avulsion period. We then use this model to postulate the avulsive course of the river from another upstream point based on avulsion threshold analysis. Our results demonstrate that this model has the potential for postulating the path of an avulsive channel, and can provide a priori information on the areas likely to be flooded following an embankment breach.

Linking the morphology of fluvial fan systems to aquifer stratigraphy in the Sutlej‐Yamuna plain of northwest India

The Indo-Gangetic foreland basin has some of the highest rates of groundwater extraction in the world, focused in the states of Punjab and Haryana in northwest India. Any assessment of the effects of extraction on groundwater variation requires understanding of the geometry and sedimentary architecture of the alluvial aquifers, which in turn are set by their geomorphic and depositional setting. To assess the overall architecture of the aquifer system, we used satellite imagery and digital elevation models to map the geomorphology of the Sutlej and Yamuna fan systems, while aquifer geometry was assessed using 243 wells that extend to ∼200 m depth. Aquifers formed by sandy channel bodies in the subsurface of the Sutlej and Yamuna fans have a median thickness of 7 and 6 m, respectively, and follow heavy-tailed thickness distributions. These distributions, along with evidence of persistence in aquifer fractions as determined from compensation analysis, indicate persistent reoccupation of channel positions and suggest that the major aquifers consist of stacked, multistoried channel bodies. The percentage of aquifer material in individual boreholes decreases down fan, although the exponent on the aquifer body thickness distribution remains similar, indicating that the total number of aquifer bodies decreases down fan but that individual bodies do not thin appreciably, particularly on the Yamuna fan. The interfan area and the fan marginal zone have thinner aquifers and a lower proportion of aquifer material, even in proximal locations. We conclude that geomorphic setting provides a first-order control on the thickness, geometry, and stacking pattern of aquifer bodies across this critical region.