Showing papers on "Stream power published in 2003"

Valley incision by debris flows: Evidence of a topographic signature

Abstract: [1] The sculpture of valleys by flowing water is widely recognized, and simplified models of incision by this process (e.g., the stream power law) are the basis for most recent landscape evolution models. Under steady state conditions a stream power law predicts that channel slope varies as an inverse power law of drainage area. Using both contour maps and laser altimetry, we find that this inverse power law rarely extends to slopes greater than ∼0.03 to 0.10, values below which debris flows rarely travel. Instead, with decreasing drainage area the rate of increase in slope declines, leading to a curved relationship on a log-log plot of slope against drainage area. Fieldwork in the western United States and Taiwan indicates that debris flow incision of bedrock valley floors tends to terminate upstream of where strath terraces begin and where area-slope data follow fluvial power laws. These observations lead us to propose that the steeper portions of unglaciated valley networks of landscapes steep enough to produce mass failures are predominately cut by debris flows, whose topographic signature is an area-slope plot that curves in log-log space. This matters greatly as valleys with curved area-slope plots are both extensive by length (>80% of large steepland basins) and comprise large fractions of main stem valley relief (25–100%). As a consequence, valleys carved by debris flows, not rivers, bound most hillslopes in unglaciated steeplands. Debris flow scour of these valleys appears to limit the height of some mountains to substantially lower elevations than river incision laws would predict, an effect absent in current landscape evolution models. We anticipate that an understanding of debris flow incision, for which we currently lack even an empirical expression, would substantially change model results and inferences drawn about linkages between landscape morphology and tectonics, climate, and geology.

Detachment of Undisturbed Soil by Shallow Flow

Abstract: Quantification of soil detachment rates is necessary to establish a basic understanding of soil erosion processes and to develop fundamental-based erosion models. Many studies have been conducted on the detachment rates of disturbed soils, but very little has been done to quantify the rates of detachment for natural soil conditions. This study was conducted to evaluate the influence of flow discharge, slope gradient, flow velocity, shear stress, stream power, and unit stream power on detachment rates of natural, undisturbed, mixed mesic typical Udorthent soil. Flow rates ranged from 0.25 to 2.0 L s -1 and slope gradient ranged from 8.8 to 46.6%. This study was compared with a previous study that used disturbed soil prepared by static compression. The results indicated that the detachment rates of disturbed soil were 1 to 23 times greater than the ones of natural undisturbed soil. It was necessary to use natural undisturbed soil samples to simulate the detachment process and to evaluate the influence of hydraulic parameter on detachment rate. Along with flow rate increasing, detachment rate increased as a linear function. Detachment rate also increased with slope gradient, but the functional relationship between the two variables depended on flow rate. Stepwise regression analysis indicated that detachment rate could be well predicted by a power function of flow rate and slope gradient (R 2 = 0.96). Mean flow velocity was closely correlated to detachment rate (r 2 = 0.91). Flow detachment rate was better correlated to a power function of stream power (r 2 = 0.95) than to functions of either shear stress or unit stream power.

Flow hydraulics and geomorphic effects of glacial-lake outburst floods in the Mount Everest region, Nepal

Abstract: Glacial-lake outburst floods (GLOFs) on 3 September 1977 and 4 August 1985 dramatically modified channels and valleys in the Mount Everest region of Nepal by eroding, transporting, and depositing large quantities of sediment for tens of kilometres along the flood routes. The GLOF discharges were 7 to 60 times greater than normal floods derived from snowmelt runoff, glacier meltwater, and monsoonal precipitation (referred to as seasonal high flow floods, SHFFs). Specific stream power values ranged from as low as 1900 W m−2 in wide, low-gradient valley segments to as high as 51 700 W m−2 in narrow, high-gradient valley segments bounded by bedrock. Along the upper 16 km of the GLOF routes, the reach-averaged specific stream power of the GLOFs was 3·2 to 8·0 times greater than the reach-averaged specific stream power of the SHFFs. The greatest geomorphic change occurred along the upper 10 to 16 km of the GLOF routes, where the ratio between the GLOF specific stream power and the SHFF specific stream power was the greatest, there was an abundant supply of sediment, and channel/valley boundaries consisted primarily of unconsolidated sediment. Below 11 to 16 km from the source area, the geomorphic effects of the GLOFs were reduced because of the lower specific stream power ratio between the GLOFs and SHFFs, more resistant bedrock flow boundaries, reduced sediment supply, and the occurrence of past GLOFs. Copyright © 2003 John Wiley & Sons, Ltd.

Cenozoic river profile development in the Upper Lachlan catchment (SE Australia) as a test of quantitative fluvial incision models

Abstract: [1] We have used early Miocene valley-filling basalts to reconstruct fluvial long profiles in the Upper Lachlan catchment, SE Australia, in order to use these as well-constrained initial conditions in a forward model of fluvial incision. Many different fluvial incision algorithms have been proposed, and it is not clear at present which one of these best captures the behavior of bedrock rivers. We test five different formulations; the ability of these models to reproduce the observed present-day stream profiles and amounts of incision is assessed using a weighted-mean misfit criterion as well as the structure of the misfit function. The results show that for all models, parameter combinations can be found that reproduce the amounts of incision reasonably well. However, for some models, these best fit parameter combinations do not seem to have a physical significance, whereas for some others, best fit parameter combinations are such that the models tend to mimic the behavior of other models. Overall, best fit model predictions are obtained for a detachment-limited stream power model or an “undercapacity” model that includes a river width term that varies as a function of drainage area. The uncertainty in initial conditions does not have a strong impact on model outcomes. The model results suggest, however, that lithological variation may be responsible for variations in parameter values of a factor of 3–5.

River channel changes in the Rhone Delta (France) since the end of the Little Ice Age: geomorphological adjustment to hydroclimatic change and natural resource management

Abstract: This study provides a representative example of a river affected by artificial width contraction and consequent bed incision in a context of sea-level rise Geomorphological adjustment in the Rhone catchment since the final stage of the Little Ice Age has been induced by hydroclimatic change and human disturbances These adjustments are examined to highlight the response of two sand-bed distributary channels of the Rhone Delta By using methods such as sedimentological evidence, historical sources, planimetric resurvey, repeated longitudinal and cross-profiling and hydraulic data, the geomorphological and hydraulic responses to channel changes are characterised and their consequences assessed It is shown that the reduction of sediment yield is a combined effect of a decrease of flood frequency, sediment dredging and building of dams on the Rhone River and its tributaries The combination of decreased catchment sediment yield, artificial width contraction, increased stream power and boundary shear stress is the cause of channel incision of the Rhone River In the coastal zone, the increase of the Rhone River's stream power has facilitated the transport of sediment down to the subdeltas Engineering works, designed to stabilise the planform and to limit overbank flooding, caused (1) channel entrenchment, thus (2) the destabilization of infrastructure (eg bridges, quays) along the river, (3) the lowering of the phreatic water table in the floodplain, which in turn induced (4) soil salinization (change in salt wedge position); eventually, the channelization led to (5) an increase of the flood hazard and risks because, although more rare, the inundations have become more destructive However, the contribution of the large flood events is negligible in terms of progressive channel in-filling because of their low frequency Finally, no relation exists between the vertical adjustment of the Rhone channel through entrenchment and relative sea level, which rose at a rate of up to 2 mm year−1 during the 20th century Clearly, natural resource management, in particular engineering activities, were the main cause of the river channel changes that occurred during the last century

Downstream coarsening in headwater channels

Abstract: [1] Field data from four mountain drainage basins in western Washington document systematic downstream coarsening of median bed surface grain size (D50) and a subsequent shift to downstream fining at a drainage area of about 10 km2. Analyses of network-wide patterns of unit stream power derived from both channel surveys and digital elevation models reveal maximum unit stream power that in all four study areas roughly corresponds with both the grain size maxima and an inflection in the drainage area-slope relation thought to represent the transition from debris flow-dominated channels to fluvially dominated channels. Our results support the hypothesis that basin-wide trends in D50 are hydraulically controlled by systematic variations in unit stream power in addition to lag deposits forced by mass-wasting processes. The similar relations found in our four study areas suggest that the tendency for downstream coarsening may be ubiquitous in headwater reaches of mountain drainage basins where debris flow processes set the channel gradient.

Equation for high-rate gully erosion

Abstract: An approach to gully erosion is presented in this paper. The approach is based on general equations derived from theoretical considerations. The equations apply to a situation of intense erosion rate, such as at peak discharge during the few critical rainstorms, able to generate or to widen gullies. Equations linking gully widening to gully deepening are derived. They do not depend on the way in which concentrated flow aggressiveness is estimated. The equation expressing gully width/depth relationship was successfully compared with data from the literature. When runoff aggressiveness was estimated through unit stream power and bottom flow shear stress, the width/discharge relationships found were similar to those expected on the basis of previous studies (e.g., Leopold and Maddock [U. S. Geol. Surv. Prof. Pap. 252 (1953) 57 pp.]) even if slope gradient explicitly appears in contrast with empirical evidence. Only threshold conditions for gullies indicate that flow shear stress (for laminar flow conditions) can explain the observed trends. This astonishing result most probably indicates that gully initiation needs more complex contexts to be explained than the one here used (based on a Montgomery and Dietrich [1994. Landscape dissection and drainage area–slope thresholds. In: M.J. Kirkby (ed.), Process Models and Theoretical Geomorphology. Wiley, 221–246] approach). A selection of the proposed equations have been arranged into a research model and an example of the outcome has been given for two situations typical of cropland in southern Tuscany (Italy). The results indicate that the spatial distribution of soil characteristics and of land use influences significantly gully generation and evolution. This further confirms that gully morphological thresholds cannot be explained by simple approaches.

Geomorphic and Hydrologic Assessment of Erosion Hazards at the Norman Municipal Landfill, Canadian River Floodplain, Central Oklahoma

Abstract: The Norman, Oklahoma, municipal landfill closed in 1985 after 63 years of operation, because it was identified as a point source of hazardous leachate composed of organic and inorganic compounds. The landfill is located on the floodplain of the Canadian River, a sand-bed river characterized by erodible channel boundaries and by large variation in mean monthly discharges. In 1986, floodwaters eroded riprap protection at the southern end of the landfill and penetrated the landfill's clay cap, thereby exposing the landfill contents. The impact of this moderate-magnitude flood event (Q12) was the catalyst to investigate erosion hazards at the Norman landfill. This geomorphic investigation analyzed floodplain geomorphology and historical channel changes, flood-frequency distributions, an erosion threshold, the geomorphic effectiveness of discharge events, and other factors that influence erosion hazards at the landfill site. The erosion hazard at the Norman landfill is a function of the location of the landfill with respect to the channel thalweg, erosional resistance of the channel margins, magnitude and duration of discrete discharge events, channel form and hydraulic geometry, and cumulative effects related to a series of discharge events. Based on current climatic conditions and historical channel changes, a minimum erosion threshold is set at bankfull discharge (Q = 572 m3/s). The annual probability of exceeding this threshold is 0.53. In addition, this analysis indicates that peak stream power is less informative than total energy expenditures when estimating the erosion potential or geomorphic effectiveness of discrete discharge events. On the Canadian River, long-duration, moderate-magnitude floods can have larger total energy expenditures than shorter-duration, high-magnitude floods and therefore represent the most serious erosion hazard to floodplain structures.

Sediment budget for cane land on the Lower Herbert River floodplain, North Queensland, Australia

Abstract: Land use in the river catchments of tropical North Queensland appears to have increased the export of sediment and nutrients to the coast. Although evidence of harmful effect of sediment on coastal and riverine ecosystems is limited, there is a growing concern about its possible negative impacts. Sugarcane cultivation on the floodplains of the tropical North Queensland river catchments is thought to be an important source of excess sediment in the river drainage systems. Minimum-tillage, trash blanket harvesting has been shown to reduce erosion from sloping sugarcane fields, but in the strongly modified floodplain landscape other elements (e.g. drains, water furrows and headlands) could still be important sediment sources. The main objectives of this thesis are to quantify the amount of sediment coming from low-lying cane land and identify the important sediment sources in the landscape. The results of this thesis enable sugarcane farmers to take targeted measures for further reduction of the export of sediment and nutrients. Sediment budgets provide a useful approach to identify and quantify potential sediment sources. For this study a sediment budget is calculated for a part of the Ripple Creek catchment, which is a sub-catchment of the Lower Herbert River. The input of sediment from all potential sources in cane land and the storage of sediment within the catchment have been quantified and compared with the output of sediment from the catchment. Input from, and storage on headlands, main drains, minor drains and water furrows, was estimated from erosion pin and surface profile measurements. Input from forested upland, input from fields and the output at the outlet of the catchment was estimated with discharge data from gauged streams and flumes. Data for the sediment budget were collected during two ‘wet’-seasons: 1999-2000 and 2000-2001. The results of the sediment budget indicate that this tropical floodplain area is a net source of sediment. Plant cane fields, which do not have a protective trash cover, were the largest net source of sediment during the 1999-2000 season. Sediment input from water furrows was higher, but there was also considerable storage of sediment in this landscape element. Headlands tend to act as sinks. The source or sink function of drains is less clear, but seems to depend on their shape and vegetation cover. An important problem in this study is the high uncertainty in the estimates of the sediment budget components and is, for example, likely to be the cause of the imbalance in the sediment budget. High uncertainties have particularly affected the results from the 20002001 season. The main source of uncertainty is spatial variation in the erosion and deposition processes. Uncertainty has to be taken into consideration when interpreting the budget results. The observation of a floodplain as sediment source contradicts the general understanding that floodplains are areas of sediment storage within river catchments. A second objective of this thesis was therefore to provide an answer to the question: how can floodplains in the tropical North Queensland catchments can be a source of sediment? In geomorphic literature various factors have been pointed out, that could control floodplain erosion processes. However, their importance is not 'uniquely identified'. Among the most apparent factors are the stream power of the floodwater and the resistance of the floodplain surface both through its sedimentary composition and the vegetation cover. If the cultivated floodplains of the North Queensland catchments are considered in the light of these factors, there is a justified reason to expect them to be a sediment source. Cultivation has lowered the resistance of their surface; increased drainage has increased the drainage velocity and flood control structures have altered flooding patterns. For the Ripple Creek floodplain four qualitative scenarios have been developed that describe erosion and deposition under different flow conditions. Two of these scenarios were experienced during the budget study, involving runoff from local hillslopes and heavy rainfall, which caused floodplain erosion. In the longer term larger flood events, involving floodwater from the Herbert River, may lead to different erosion and deposition processes. The present study has shown that the tropical floodplain of the Herbert River catchment can be a source of sediment under particular flow conditions. It has also shown which elements in the sugarcane landscape are the most important sediment sources under these conditions. This understanding will enable sugarcane farmers to further reduce sediment export from cane land and prevent the negative impact this may have on the North Queensland coastal ecosystems.

Targets for ecological rehabilitation of the lower Rhine and Meuse based on a historic-geomorphologic reference

Abstract: Ecological restoration of the lower Rhine and Meuse floodplains aims at improvement of biodiversity and enhancement of natural hydro-morphodynamics, this within the constraints of safety and navigability. Restoration plans, the choice of landscaping measures, and the eco-morphological targets should account for the natural differences in morphodynamics and ecological patterns along the rivers. To achieve this, the hydro- and morphodynamics as well as the landscape pattern that existed in historic times - before the main regulation works in 1850 - served as a reference. Bio-geomorphological characteristics were determined of 20 to 30-km river reaches along the lower Rhine and Meuse, including the estuary. Using old river maps and historic water level data the 19 th century pattern of geomorphology, natural vegetation, and land use was reconstructed. Quantitative hydro-morphological parameters (width-depth ratio, specific stream power, Shields parameter, water level fluctuations) were determined for 12 river reaches. From these, the downstream charges along the river reaches were assessed, landscape types were related to hydro-morph/)dynamic processes, and characteristic processes and landscape patterns for different reaches were identified. Using these results, guidelines for floodplain restoration targets and landscaping measures were given at the scale of the river reaches.

Minimising sediment delivery to rivers: a spatial modelling approach developed for commercial forestry

Abstract: Minimising sediment delivery to rivers is a significant objective in achieving sustainable land-use, especially in forestry and agriculture. In KwaZulu -Natal, South Africa, this is a particular challenge due to the frequent occurrence of erodible soils, steep slopes, and high rainfall intensities. Consequently, loss of topsoil is generally high, and results in turbid rivers with excessive deposition of sediment in lowland rivers. While the use of riparian buffers has been shown to be an effective strategy in trapping sediment, the extensive landholdings of forestry companies requires that sites should be prioritised for management intervention. Furthermore, site-specific guidelines are required that are compatible with the environmental management systems of forestry companies. This paper presents an approach that establishes priorities for management intervention based on the development of three spatial indices. While the method has been tested for a forestry land-use, the principles and approach are transferable to other agricultural landscapes. Establishing the erosion potential for commercial forestry areas is readily accomplished through the application of the USLE-based models . The principles of the RUSLE model were applied in association with the Unit Stream Power (Moore & Burch 1986) method. This modified, spatial application of the RUSLE model results in a relative indication of soil erosion potential (Sediment Supply Index). The second index (Delivery Risk Index) provides an indication of risk across the landscape during the sediment transport phase. It is based on the following road, stream and topographic factors, roadstream adjacency, road-stream intersections, road surface erodibility, road surface condition, and terrain morphology. The final index (Sediment Attenuation Index) identifies areas likely to act as sediment traps, based on the topography in which they occur. These three indices are used conjunctively to devel op integrated sediment management strategies, addressing the problem at source and during transport. However, prioritisation of areas for management intervention using continuous surface, landscape-based indices is difficult. It is necessary to relate sediment production to the stream to which it will be delivered. To accomplish this the study area was divided into micro-catchments. The mean Sediment Supply Index per catchment was divided by the stream length draining each of the micro-catchments to derive an index of sediment loading to streams. This index is useful in identifying priorities for management intervention across the landscape. To satisfy practical application requirements, specific recommendations regarding the required buffer widths to act as an efficient, final defence against sediment delivery to the watercourses are required. The micro-catchments form the basis of the analysis for developing buffer-width recommendations. The mean slope and sediment supply for each catchment is used to deve lop buffer width recommendations for the streams draining the catchments, using the method proposed by Karssies and Prosser (2001). The application of the modelling results at both landscape and compartment levels enables tactical and operational management strategies to be developed.

Critical unit stream power for sediment transport

Abstract: Yang's (1996) sediment transport theory based on unit stream power is one of the most accurate theories, but in his equations the use of product of slope and critical velocity instead for critical unit stream power is not suitable. Dimensionless critical unit stream power required at incipient motion can be derived from the principle of conservation of power as a function of dimensionless particle diameter and relative roughness. Based on a lot of data sets, this new criterion was developed. By use of this new criteria, Yang's (1973) sand transport formula and his 1984 gravel transport formula could be improved when sediment concentration is less than about 100 ppm by weight.

Architectures,Relationships between Discharges and Width/depth Ratios of Stream Cross Profiles, and Stream Powers of Anastomosing Rrivers

Abstract: As a new type of rivers, anastomosing river has been concerned by researchers although some questions about it was not clear for understanding it This paper discusses the river architectures, relationships between width/depth ratios of stream cross sections and discharges, and stream powers of anastomosing rivers according to references and obtained data recently It is helpful to researchers who is interested in this river pattern to develop the theory for it Many aspects of anastomosing river appear particular characteristics compared with other river patterns In the planform architectures, the multiple channels joining each other enclose flood basins on which vegetation, swamps and lakes develop basically Longitudinal gradients of the channels are very low while the channel width/depth ratios are smaller than 40 In the depositional architectures of cross profiles, some isolated sand bodies of channel deposits are "floating" in the mud bodies of flood basins In the half logarithm diagram of width/depth ratios of stream cross profiles vs discharges, the scatters of anastomosing rivers are below compared with that of other river patterns The stream powers of anastomosing rivers are very low compared with the old trunk channel from which it diverted because the channel gradients and discharges of every anastomosing channel are smaller than that of the old trunk channel The specific stream power of the anastomosing channels: eastern Songzi River, western Songzi River, Hudu River, Ouchi River, northern Ouchi River and Songliheliu River are 30 W/m 2, 55 W/m 2, 28 W/m 2, 64 W/m 2, 37 W/m 2 and 27 W/m 2, respectively Obviously, all of them are smaller than 10 W/m 2 But the specific stream power of anabranched Changjiang trunk channel is 140 W/m 2 All of the characteristics of anastomosing rivers indicate that this river pattern is different from the anabranched rivers represented by the lower Changjiang River, especially from other river patterns

Role of Resistance Coefficient in Seasonal Adjustments in Alluvial Rivers

Abstract: Using the river, canal, and flume data, it is found that the role played by the seasonal variation in the Manning rugosity coefficient, n, equalizes the sediment transporting capacity with sediment supply in the dune bed-form regime. Aggradation/degradation in a sand-bed river is thus avoided on a seasonal basis. A relation between the dimensionless rugosity coefficient and the dimensionless ratio of stream power to sediment concentration exists in sand-bed rivers for any specific gauge height. Once field measurements of the sediment transport rate are made and such a relationship is worked out, thereafter, it may be possible to estimate the sediment concentration for each observed flow discharge merely by applying this relationship adopting the data of velocity, depth, slope, and bed material size which are routinely observed at a stream-gauging site.

Grain-Size Analysis of Maumee and Vedra Channel Sediments (Mars) Using Equilibrium Sediment Transport Theory

Abstract: Introduction: Catastrophic outflows are one agent responsible for the redistribution of martian surface material The late Hesperian, Maja Valles outflow stretches 1200 km from Juvenate Chasma into western Chryse Planitia [1] The initial outflow ponded on Lunae Planum before spilling across Xanthe Terra highlands to debouche into the Chryse basin The material moved by the outflow is dependent upon the sediment transport rate and the flow rate This paper examines the probable grain-size moved in channels across Xanthe Terra Method: Equilibrium Sediment Transport Theory [2-4] postulates that an equilibrium stream achieves the minimum ratio of stream power to sediment load, ie, that the greatest possible proportion of stream energy is employed in the transport of available sediment load and that the maximum sediment transport rate is achieved for the flow rate In achieving this equilibrium, the stream adjusts its channel geometry until the channel hydraulics achieve the ideal balance between stream power and bed mobility to maximize the transport of the available sediment One of the most critical factors in this balance is the grain-size of the bed material Although this theory was developed for channel design in engineered systems [4], it can be applied to the analysis of natural channels created by high paleoflows assuming that the channel hydraulics can be sufficiently constrained The acquisition of the MOLA data has provided the needed data to constrain the channel hydraulics for the Maumee and Vedra channels Using equations for sediment transport [5, 6], White et al [4] showed that sediment concentration, X, could be expressed as

Application Of Unit Stream Power Theory To Stable Meandering And BraidedReaches Of Sefidroud River

Abstract: It is important to understand how channel geometry relates to discharge, and to be able to predict how the channel will adjust for a given discharge. This is significant both in a theoretical way, especially in relating channel geometry to given discharges in semi-arid and arid areas, and also in apractical way, for instance in river training and for a variety of instream and external uses. This needs to be able to predict channel geometry led to the development of the concept of regime, which was initially developed with respect to irrigation canals. This concept has since been applied to natural rivers, with the idea of "dominant discharge" being used to represent what discharge the channel is in regime for. Empirical regime equations for stable gravel-bed Sefidroud River were obtained based on data from 32 cross-sections. The stage discharge-rating curve for each cross-section was computed using one-dimensional ISIS hydraulic model. Morphological and sedimentological data were also obtained from field survey to determine stable width and depth from non-dimensional unit stream power theory for braided reaches of Sefidroud River.