geomorphic and hydrologic aspects of monsoon floods on the narmada and tapi rivers in central india abstract high magnitude floods during the monsoon season are considered to be india s recurring and leading natural disaster such large flood are extremely important events not only in terms of human impact but also from the standpoint of geomorphic effectiveness and geomorphic work the narmada and tapi rivers in central india are characterized by one of the most intense flood regimes in the seasonal tropics modern historical and palaeoflood records indicate that large floods are relatively common hydrologic and geomorphic events on the narmada and tapi rivers a catastrophic flood on tapi in july 1991 that occurred from heavy rains and a dam break represents one of the highest unit discharges yet recorded in any part of central india flood competence studies indicate that the flood power of the two rivers is comparable with some of the largest historic and prehistoric floods reported from other parts of the world the rivers have preserved geomorphic and sedimentologic records of palaeofloods for the last two millennium and in choral river a 5000 year old record has been located examination of the synoptic conditions associated with the flood generating low pressure systems reveals that all but a few of them are the result of bay of bengal depressions the flooding in such large rivers profoundly challenges flood hazard management because of limited instrumental records and large spatio temporal variation it is therefore imperative that the design engineering community in india incorporates the geomorphic sedimentologic and botanic information in the planning and design of water resource development projects

effects of bed discordance on flow dynamics at open channel confluences although a discordance in bed elevation between mainstream and tributary channels is a common feature of most river confluences this height differential is largely ignored in many models of junction flow dynamics here we present new laboratory flume data that demonstrate the significant influence of bed discordance on flow at an acute angled confluence a difference in bed height between mainstream and tributary channels is shown to obliterate flow deflection at the bed and create a distortion of the mixing layer between the flows resulting in fluid upwelling at the downstream junction corner this upwelling is responsible for the absence of a flow separation zone near the bed at the downstream junction corner and the lack of a zone of marked flow acceleration in the postconfluence channel these experiments reveal complex flow fields generated by bed discordance and suggest the need for major revisions to current models of river channel confluence flow dynamics and their application to sediment routing at these sites

dynamic evolution of riverbed potholes in the granitic bedrock of chota nagpur plateau middle part of subarnarekha river basin india abstract potholes are an extraordinary abiotic micro geomorphic feature on the granite and quartzite topography of the subarnarekha river basin these erosional features are carved by the running water actions of river subarnarekha and its tributaries the study has been conducted on 17 sites n 1707 which are exemplary for the development of this micro fluvial feature located on the rejuvenated middle part of subarnarekha river basin hydrological and geological factors are imperative for their formation joint length joint density joint orientation rock hardness variations of fluvial hydraulic bed roughness river bed slope and hydraulic gradient are conducive factors for the development of this geomorphic feature monsoon is the ideal season for carving these features by river turbulence when abrasive work of river is predominant on the river bed monsoon 3000 6500 k g m 2 s 3 non monsoon 0 50 3 0 k g m 2 s 3 the evolutionary sequence of potholes is from round to oval and then elongated with vertical scouring the relation between pothole radius and depth has been established to explain the process of its expansion joint length shows positive relation r2 0 71 with pothole area while joint density has a negative relation r2 0 29 as these are structurally weaker zones characterised by destruction of these features suspended load is also a hindrance in their formation sites with high suspended load are delineated as pothole destructive zones the mean dynamic evolution of circular oval and elongated shaped potholes occurs at spatio temporal level o e 0 005 0 60 m 3 c e 0 01 0 25 m 3 e e 0 005 0 30 m 3 and the initiation of this development depends on inner depth and radius of individual pothole

river incision into bedrock mechanics and relative efficacy of plucking abrasion and cavitation improved formulation of bedrock erosion laws requires knowledge of the actual processes operative at the bed we present qualitative field evidence from a wide range of settings that the relative efficacy of the various processes of fluvial erosion e g plucking abrasion cavitation solution is a strong function of substrate lithology and that joint spacing fractures and bedding planes exert the most direct control the relative importance of the various processes and the nature of the interplay between them are inferred from detailed observations of the morphology of erosional forms on channel bed and banks and their spatial distributions we find that plucking dominates wherever rocks are well jointed on a submeter scale hydraulic wedging of small clasts into cracks bashing and abrasion by bedload and chemical and physical weathering all contribute to the loosening and removal of joint blocks in more massive rocks abrasion by suspended sand appears to be rate limiting in the systems studied here concentration of erosion on downstream sides of obstacles and tight coupling between fluid flow patterns and fine scale morphology of erosion forms testify to the importance of abrasion by suspended load rather than bedload particles mechanical analyses indicate that erosion by suspended load abrasion is considerably more nonlinear in shear stress than erosion by plucking in addition a new analysis indicates that cavitation is more likely to occur in natural systems than previously argued cavitation must be considered a viable process in many actively incising bedrock channels and may contribute to the fluting and potholing of massive unjointed rocks that is otherwise attributed to suspended load abrasion direct field evidence of cavitation erosion is however lacking in terms of the well known shear stress or stream power erosion law erosion by plucking is consistent with a slope exponent n of 2 3 to 1 whereas erosion by suspended load abrasion is more consistent with a slope exponent of 5 3 given that substrate lithology appears to dictate the dominant erosion process this finding has important implications for long term landscape evolution and the models used to study it

dynamics of pothole growth as defined by field data and geometrical description 1 this paper examines cylindrical pothole growth on streambeds using empirical analyses of field data and geometric constraints pothole depths d and average radii at three localities have the relationship kd where k and are regression coefficients r2 0 72 observed 0 57 0 67 0 85 translate to d increasing faster than r at all localities the strong correlations and absence of potholes with very low or high ratios of depth diameter suggest that small concavities act as pothole seeds and enlargement is quasi systematic exploiting the power relationship growing potholes can be represented as deepening and radially expanding cylinders absolute and relative distributions of erosion can be calculated for floors and walls using this geometrical approach volumetrically more substrate is eroded from pothole walls than floors during growth for 0 5 among sample populations as much as 70 more material is eroded from walls than floors 0 85 wall and floor surface areas differ by 1 or more orders of magnitude for observed and as a result erosion rates are fastest atop floors differences in erosion rates may reflect the efficacy of erosion phenomena low angle impacts of tools on walls presumably have low erosion efficiencies efficacies are presumably influenced by substrate properties and floor and wall erosion rates are most comparable in the weakest observed strata although substantially more material is removed from walls at this locality 0 85 additional data is needed but quantifiable relationships may exist between geometries substrates and erosion phenomena

tectonics fracturing of rock and erosion 1 we argue that by fracturing rock not by raising it relative to base level tectonics plays its most important role in causing rapid incision of valleys and rapid erosion of hillslopes tectonic deformation riddles the upper crust with fractures which not only provide avenues for water flow and thus promote weathering and further disintegration of rock but also fragment bedrock into debris that is readily extracted and transported by surface processes bends in active faults require straining of adjacent rock masses aftershocks that occur subsequent to slip on primary faults reflect penetrative brittle deformation of the upper crust at least some aftershocks must nucleate or lengthen cracks which contribute to the comminution of these rock masses scaling rules suggest that dimensions of ruptures for very small m 2 earthquakes can be meters or less the gutenberg richter recurrence relationship implies that such earthquakes are common as high magnification seismographs in low noise environments confirm moreover large differences among fault plane solutions for aftershocks show that the small faults on which they occur are not parallel to one another some faults must intersect thus the upper crust in tectonically active regions should be fragmented into blocks down to the scale of boulders or smaller dismembered rock arrives at the earth s surface already prepared to be transported away as a corollary both deeply exhumed lower crust and posttectonic igneous rock never deformed under brittle conditions and not deformed recently should be less susceptible to detachment and subsequent transport than fractured rock

morphological and hydrological characteristics of some allochthonous river channels western deccan trap upland region india abstract relations are established between various channel morphologic variables for the upland rivers of western deccan trap region using seventy three sites three channel categories and ten parameters the level of explained variance differed for the three categories the link between bankfull width w and upstream length d s and between gradient g and d s suggests a general consistency of downstream adjsutment observations indicate that there is large variation in form ratio f sinuosity p and bed material size m g which cannot be statistically related to any of the measured morphologic variables an attempt to relate morphologic variables to discharge using limited data reveals that the maximum monsoon discharge q max has a very strong influence on the channel width and gradient rather than the mean monsoon discharge q mm the scatter in the dat has been attributed to the allochthonous nature of the drainage tributary influence perimeter lithology and to the geomorphic history of the area

fluvio sedimentary response of the monsoon fed indian rivers to late pleistocene holocene changes in monsoon strength reconstruction based on existing 14c dates abstract the primary objective of the present study is to identify major phases of alluviation in the indian region since the abrupt deglacial intensification of the monsoon 15 cal ka bp on the basis of analysis of 68 radiocarbon dates from two major hydro geomorphic regions of india the central ganga basin cgb and the deccan peninsula dp the recognition of main phases of alluviation and incision has been achieved by evaluating the temporal distribution and clustering of the radiocarbon dates from alluvial sequences the clusters were detected on the basis of the interpretation of the summed probability distribution plots derived by using oxcal version 4 0 1 and calpal version may 2006 software packages the summed probability plots reveal that periods of alluviation in the cgb represented by three clusters 13 9 12 3 11 9 11 2 and 9 8 9 0 cal ka bp occur roughly before the onset of early holocene monsoon optimum phase two other clusters occur in the intervals 3 6 2 8 and 1 1 0 9 cal ka bp the peak monsoon period generally lacks clusters of radiocarbon dates implying fluvial erosion and channel incision this period also shows clustering of radiocarbon dates of the abandoned channels in comparison 14c dates from dp alluvial units form clusters at 16 4 14 12 8 11 2 10 8 8 9 8 1 6 7 and 5 1 3 9 cal ka bp indicating an association with the deglacial early holocene humid phase alluviation in the dp appears to have continued more or less uninterrupted till the middle of the holocene epoch the beginning and end of the discernible gap in the radiocarbon dates of cgb 9 0 3 6 cal ka broadly corresponds with the two well established short term events of the holocene 8 2 and 4 2 ka cal bp in comparison the prominent gap of dp radiocarbon dates 3 9 2 1 ka cal bp approximately begins with the 4 2 ka cal bp short term event onset of aridity and ends with the 2 0 ka cal bp enhanced monsoon event notwithstanding the inter regional differences in the fluvio sedimentary response in the india region the clusters of radiocarbon dates indicate that the century to millennium scale variations in fluvial activity in the indian subcontinent were intimately linked to long term fluctuations in the monsoon strength during the late quaternary

empirical and theoretical investigations of sculpted forms in buckeye creek cave west virginia abstract sculpted forms play important roles in channel erosion but the controls on their growth or appearance are uncertain empirical evidence from sculpted forms eroded into limestones within buckeye creek cave west virginia shows that types of sculpture morphology are determined by the types of flow or vortex structures present within sculptures during floods small hemispherical sculptures maintain constant form as they enlarge and occupy fixed locations on channel walls because of their association with joints styolites and bedding planes taking advantage of the systematic growth and stability of the sculptures a geometrical model of sculpture growth is presented that relates the effects of sculpture size form and wall retreat to the relative erosion efficiencies and excesses required for sculpture growth numerical results obtained from the theoretical model reveal that small sculptures must erode more efficiently than large sculptures or they will be removed by wall retreat and that erosi

climate driven bedrock incision in an active mountain belt measurements of fluvial bedrock incision were made with submillimeter precision in the east central range of taiwan where long term exhumation rates and precipitation driven river discharge are independently known they indicate that valley lowering is driven by relatively frequent flows of moderate intensity abrasion by suspended sediment is an important fluvial wear process and channel bed geometry and the presence of widely spaced planes of weakness in the rock mass influence erosion rate and style

numerical modeling of fluvial strath terrace formation in response to oscillating climate many river systems in western north america retain a fluvial strath terrace rec ord of discontinuous downcutting into bedrock through the quaternary their importance lies in their use to interpret climatic events in the headwaters and to determine long term incision rates terrace formation has been ascribed to changes in sediment supply and or water discharge produced by late quaternary climatic fluctuations we use a one dimensional channel evolution model to explore whether temporal variations in sediment and water discharge can generate terrace sequences the model includes sediment transport vertical bedrock erosion limited by alluvial cover and lateral valley wall erosion we set limits on our modeling by using data collected from the terraced wind river basin two types of experiments were performed constant period sinusoidal input histories and variable period inputs scaled by the marine 18 o rec ord our simulations indicate that strath terrace formation requires input variability that produces a changing ratio of vertical to lateral erosion rates straths are cut when the channel floor is protected from erosion by sediment and are abandoned and terraces formed when incision can resume following sediment cover thinning high sediment supply promotes wide valley floors that are abandoned as sediment supply decreases in contrast wide valleys are promoted by low effective water discharge and are abandoned as discharge increases widening of the valley floors that become terraces occurs over many thousands of years the transition from valley widening to downcutting and terrace creation occurs in response to subtle input changes affecting local divergence of sediment transport capacity formation of terraces lags by several thousand years the input changes that cause their formation our results suggest that use of terrace ages to set limits on the timing of a specific event must be done with the knowledge that the system can take thousands of years to respond to a perturbation the incision rate calculated in the field from the lowest terrace in these systems will likely be higher than the rate calculated by using older terraces because the most recent fluvial response in the field is commonly downcutting associated with declining sediment input since the last glacial maximum this apparent increase in incision rates is observed in many river systems and should not necessarily be interpreted as a response to an increase in rock uplift rate

temporal variations in the specific stream power and total energy expenditure of a monsoonal river the tapi river india abstract in this study an attempt has been made to evaluate the temporal variations in specific stream power and the total energy available for geomorphic work during the monsoon season for the tapi river in central india continuous daily discharge data 1978 1990 hydraulic geometry equations and the relationship between discharge and water surface slope were used to compute the daily specific stream power for the savkheda gauging site in the lower tapi basin the total amount of energy generated by all the monsoon flows was estimated by integrating the area under the graph derived for the monsoon season the analyses of the 13 year daily discharge data reveal that the average and maximum values range from 4 20 w m 2 and 22 964 w m 2 respectively specific stream power duration curve derived for the site shows that for 25 of the time the power per unit area is 10 w m 2 furthermore unit stream power was found to be above the williams williams g p 1983 paleohydrological methods and some examples from swedish fluvial environments i cobble and boulder deposits geografiska annaler 65a 227 243 threshold of pebble movement 1 5 w m 2 cobble movement 16 w m 2 and boulder movement 90 w m 2 for 71 15 and 2 of the time respectively computations further indicate that the total amount of energy generated by the flows during the monsoon season is in the range of 37 mj deficit monsoon years to 256 mj excess monsoon and or flood years large floods have one third share in the total monsoon energy expenditure in the absence of appropriate data on the yearwise geomorphic effects the geomorphic work was evaluated in terms of the total suspended sediment load transported the total monsoon sediment load is strongly related to the total monsoon energy the results of the study indicate that the average flow competence and capacity are remarkably higher during wetter monsoon seasons and flood years than during the shorter and drier monsoon seasons the present analyses demonstrate that the flows are geomorphically effective for a greater part of the monsoon season except during the deficient monsoon years and there is little doubt that large magnitude floods are effective agents of geomorphic change in monsoonal rivers

controls on the geometry of potholes in bedrock channels potholes circular depressions carved into bedrock are the dominant roughness elements in many bedrock channels here we show using data from previous studies and new data from the smith river oregon that pothole depths increase in proportion to both the mean pothole radius such that the most common pothole depth to radius ratio is 2 and the diameter of the largest clasts episodically stored in potholes we present a theory for these observations based on computational fluid dynamics and sediment transport modeling of vortices in cylindrical cavities of different shapes and sizes we show that the shear stress at the bottom of a pothole which controls the rate of pothole growth is maximized for potholes with a depth to radius ratio of approximately 1 and decreases nonlinearly with increasing depth to radius ratio such that potholes with depth to radius ratios larger than 3 are uncommon our model provides a mechanistic explanation for pothole shapes and sizes

multi channel patterns of bedrock rivers an example from the central narmada basin india abstract an anomalous multiple channel pattern in bedrock is observed on a predominantly downcutting reach of the narmada river the multi channel reach 800 2750 m in width and 8500 m in length is bounded by major faults and is underlain by granite and gneiss bedrock geomorphological investigations reveal differences among the upstream middle and downstream sub reaches of the multi channel study area whereas the upstream sub reach is dominated by deep flows and fine sediments the lower sub reach is characterized by a steep gradient and rapids the middle sub reach is the widest and is marked by thickly forested islands and boulder berms the characteristics of the three different sub reaches suggest control by the interactions of lithology flood processes and tectonics estimations of hack s 1973 stream gradient index values indicate considerable variations for the sl values along the length of narmada river the highest value of gradient index sl 797 is associated with the multi channel reach implying lithologic or tectonic control given the dimensions of the reach and its channels it appears that the present hydrological regime is inadequate to produce the feature we hypothesize that the multi channel pattern development in bedrock was initiated by block or domal uplift enhanced gradients and extreme floods permitted the system to exploit linear weaknesses in the bedrock leading to the development of anabranches and establishment of multiple channels in bedrock abrupt changes in the channel planform and morphology at the study site indicate that the river is adjusting its channel geometry width depth gradient and plan configuration to a new equilibrium channel morphology through the action of the extreme floods characteristic of this fluvial environment

magnitude frequency distributions of boundary shear stress along a rapidly eroding bedrock river 1 the magnitude frequency distribution of boundary shear stress frames erosion rates in bedrock rivers but empirical constraints are rare particularly for extreme floods here we present measurements of mean stress b and its frequency distribution along a fast eroding river in taiwan we construct rating functions of discharge and hydraulic geometry using high resolution satellite images of flood patterns river stage time series topographic profiles and post flood field surveys the method allows us to assess the spatiotemporal variation in b along the channel the boundary shear stress pdf p b has a steep power law tail and includes semiannual floods generating b 100 200 pa and 50 year 3000 m3 s 1 events driving b 300 pa all such floods contribute to modification of the coarse alluvial cover and erosion of the bedrock bed given the steep tail decay in p b the rapid channel incision probably owes more to the exceptional frequency of moderate shear stresses than to the magnitude of the extremes

multiscale structural and lithologic controls in the development of stream potholes on granite bedrock rivers abstract jointing veins dikes and fracture patterns influence the genesis of potholes in bedrock rivers we use measures of surface rock strength schmidt hammer readings joint orientation and spacing and subsurface rock properties ultrasound velocity to analyze the spatial relationship between joints and potholes in three rivers of the spanish central system the tietar manzanares and alberche rivers at each site we measured the dimensions of between 45 and 77 potholes at least 50 randomly located schmidt hammer rebound values as well as schmidt hammer values in a 15 15 cm grid around each pothole and ultrasonic p wave velocities measured in a 15 15 cm grid around a subset of 12 of the potholes results support our hypotheses that most potholes correlate with joints 89 of potholes are related to joint sets potholes exhibit preferred orientations associated with dominant joints and pothole genesis correlates more strongly with variations in substrate resistance than with hydraulics we classify potholes by morphology and present a genetic sequence for specific morphologies in relation to substrate characteristics

early warning system for aftershocks a prototype early warning system to provide san francisco and oakland california a few tens of seconds warning of incoming strong ground shaking from already occurred m 3 7 aftershocks of the magnitude 7 1 17 october 1989 loma prieta earthquake was operational on 28 october 1989 the prototype system consisted of four components ground motion sensors in the epicentral area a central receiver a radio repeater and radio receivers one of the radio receivers was deployed at the california department of transportation caltrans headquarters at the damaged cypress street section of the i 880 freeway in oakland california on 28 october 1989 and provided about 20 sec of warning before shaking from the m 4 5 loma prieta aftershock that occurred on 2 november 1989 at 0550 utc in its first 6 months of operation the system generated triggers for all 12 m 3 7 aftershocks for which trigger documentation is preserved did not trigger on any m 3 6 aftershocks and produced one false trigger as a result of a now corrected single point of failure design flaw because the prototype system demonstrated that potentially useful warnings of strong shaking from aftershocks are feasible the usgs has completed a portable early warning system for aftershocks that can be deployed anywhere

geologic constraints on bedrock river incision using the stream power law denudation rate in unextended terranes is limited by the rate of bedrock channel incision often modeled as work rate on the channel bed by water and sediment or stream power the latter can be generalized as ka m s n where k represents the channel bed s resistance to lowenng whose variation with lithology is unknown a is drainage area a surrogate for discharge s is local slope and m and n are exponents whose values are debated we address these uncertainties by simulating the lowering of ancient river profiles using the finite difference method we vary m n and k to match the evolved profile as closely as possible to the corresponding modern river profile over a time period constrained by the age of the mapped paleoprofiles we find at least two end member incision laws ka 0 3 0 5 s for australian rivers with stable base levels and k f a 0 1 0 2 s n for rivers in kauai subject to abrupt base level change the long term lowering rate on the latter expression is a function of the frequency and magnitude of knickpoint erosion characterized by k f incision patterns from japan and california could follow either expression if they follow the first expression with m 0 4 k varies from 10 7 10 6 m 0 2 yr for granite and metamorphic rocks to 10 5 10 4 m 0 2 yr for volcaniclastic rocks and 10 4 10 2 m 0 2 yr for mudstones this potentially large variation in k with lithology could drive strong variability in the rate of long term landscape change including denudation rate and sediment yield

fundamental transitions in the geodynamo as suggested by paleomagnetic data abstract analysis of the reversal chronology shows that the reversal process effectively ceases during the cretaceous superchron the geodynamo appears to have two basic states a reversing states and a non reversing state this must reflect a fundamental transition in the geodynamo process at the onset and termination of the superchron we suggest that this transition and indeed the long term variations in reversal rate may reflect a change from a predominantly oscillatory dynamo to a steady one which might for example be caused by a change in the relative roles of the and effects in 2 dynamos if so this should probably have been reflected in reduced secular variation during the superchron unfortunately data are not available to test this prediction directly but we use the surface integral of the angular standard deviation of paleomagnetic field directions as a proxy this shows a significantly reduced secular variation during the superchron as predicted

sediment transport and bed morphology at river channel confluences river channel confluences form important morphological elements of every river system being points at which rapid changes in flow sediment discharge and hydraulic geometry must be accommodated this article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence bed morphology at channel confluences is characterized by three distinct elements avalanche faces at the mouth of each confluent channel a deep central scour and a bar within the separation zone formed at the downstream junction corner these elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels as confluence angle and discharge ratio increase the sediment contributions from the confluent channels are progressively segregated in their paths through the junction with sediment being transported around rather than through the centre of the confluence this segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence an increase in the scour depth a change in the orientation of the scour and an increase in the size of the separation zone bar field measurements closely replicate the flume simulation a model of sediment transport and bed morphology links these features to the fluid dynamics of these sites an understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record