Showing papers on "Stream power published in 2000"

Hydraulic geometry and maximum flow efficiency as products of the principle of least action

Abstract: Basic flow relationships have previously been seen to be insufficient to explain the self-adjusting mechanism of alluvial channels and as a consequence extremal hypotheses have been incorporated into the analyses. In contrast, this study finds that by introducing a channel form factor (width/depth ratio), the self-adjusting mechanism of alluvial channels can be illustrated directly with the basic flow relations of continuity, resistance and sediment transport. Natural channel flow is able to reach an optimum state (Maximum Flow Efficiency (MFE), defined as the maximum sediment transporting capacity per unit available stream power) with regard to the adjustment of channel form such that rivers exhibit regular hydraulic geometry relations at dominant or bankfull stage. Within the context of MFE, this study offers support for the use of the concepts of maximum sediment transporting capacity (MSTC) and minimum stream power (MSP). Furthermore, this study indicates that the principle of least action is able to provide a physical explanation for the existence of MFE, MSTC and MSP. Potential energy is minimized and consequently sediment transport is maximized in alluvial channels. Copyright © 2000 John Wiley & Sons, Ltd.

Large wood retention in river channels: the case of the Fiume Tagliamento, Italy

Abstract: After more than 300 years of widespread and intensive river management, few examples of complex, unmanaged river systems remain within Europe. An exception is the Fiume Tagliamento, Italy, which retains a riparian woodland margin and unconfined river channel system throughout almost the entire 170 km length of its river corridor. A research programme is underway focusing on a range of related aspects of the hydrology, fluvial geomorphology and ecology of the Tagliamento. This paper contributes to that programme by focusing on large wood retention. The paper adopts a simple force:resistance approach at the scale of the entire river corridor in order to identify reaches of the river with a high wood retention potential. Information on the character of the river corridor is derived from 1:10 000 scale topographic maps. A range of indices measured at 330 transects across the river corridor supports a classification of the geomorphological style of the river which reflects the presence and abundance of properties previously identified in the literature as large wood retention sites. This classification provides a qualitative representation of the resistance of the corridor to wood movement and thus its overall wood-retention potential. The map-derived indices are also used to extrapolate estimates of the ten year return period flood to each of the 330 transects so that the downstream pattern of unit stream power can be quantified as an index representing force in the analysis. Although input of wood is an important factor in many river systems, it is assumed not to be a limiting factor along the Tagliamento, where riparian woodland is abundant. Field observations of large wood storage illustrate that wood retention at eight sites along the river reflects the presence and abundance of the features incorporated in the classification of geomorphological style, including the complexity of the channel network, the availability of exposed gravel areas, and the presence of islands. In general at the time of survey in August 1998, open gravel areas were estimated to store approximately 1 t ha-1 of wood in single-thread reaches and 6 t ha-1 in multiple-thread reaches. Established islands were estimated to store an average of 80 t ha-1 of wood. Nevertheless, there was considerable variability between sites, and pioneer islands, which are not represented on maps or readily identified from air photographs because of their small size, were estimated to store an order of magnitude more wood than established islands. Furthermore, the wood storage from this sample of eight sites did not reflect variability in estimated unit stream power. A series of areas for further research are identified, which can be explored using field data, and which will throw more light on the processes of wood retention in this extremely dynamic fluvial environment.

River incision or diversion in response to bedrock uplift

Abstract: INTRODUCTIONMany researchers have examined the long-term behavior of alluvialrivers (i.e., Schumm et al., 1987; Snow and Slingerland, 1990; Paola et al.,1992). On long time scales, alluvial rivers act as sediment-transport systemsthat broadly deposit sediment over low areas in their basins (i.e.,Tucker andSlingerland, 1996). The behavior of bedrock rivers is less well understood,although most workers agree that bedrock erosion is some function of streampower (Howard and Kerby, 1983; Seidl and Dietrich, 1992; Howard et al.,1994). Accordingly,researchers addressing a river’s response to bedrock up-lift equate incision with relatively high stream power and diversion with rela-tively low stream power (e.g., Burbank et al., 1996). However, these studiesassume that the erosion rate of the bedrock channel across the mountainrange controls the eventual outcome; they have not examined the behavior ofthe alluvial river upstream and downstream of the uplift. We propose to lookat the entire erosional and depositional river system and examine the long-term response of a coupled alluvial-bedrock river system to tectonic uplift.Our motivation is general and aims at a greater understanding offluvial and tectonic interactions, but our model is necessarily both specificand simplified:the tectonic uplift of a bedrock massif into the path of a later-ally unconstrained alluvial river. Such a scenario has two possible out-comes: either the river maintains its original course and incises through theuplift, or the river diverts and forms a new course around the uplift. A sig-nificant complication in the analysis of this scenario is that the evolvingriver system is partial alluvial and partial bedrock channeled. Because thebehaviors of these two river types are different, modeling requires couplingdiverse systems. We construct an analytical model to perform a first-orderanalysis of this coupled system. Our objective is to identify the specific fac-tors that determine the final outcome of river diversion or incision.Our model focuses on long time scales (10

Re-examination of Bagnold's empirical bedload formulae

Abstract: Bagnold developed his formula for bedload transport over several decades, with the final form of the relation given in his 1980 paper. In this formula, bedload transport rate is a function of stream power above some threshold value, depth and grain size. In 1986, he presented a graph which illustrated the strength of his relation. A double-log graph of bedload transport rate, adjusted for depth and grain size, versus excess stream power was shown to collapse along a line having a slope of 1·5. However, Bagnold based his analyses on limited data. In this paper, the formula is re-examined using a large data set in order to define the most consistent empirical representation, and dimensional analysis is performed to seek a rationalization of the formula. Functional analysis is performed for the final version of the equation defined by Bagnold to determine if the slope of 1·5 is preserved and to assess the strength of the relation. Finally, relations between excess stream power and bedload transport are examined for a fixed slope of 1·5 to assess the performance of various depth and grain size adjustment factors. The rational scaling is found to provide the best result. Copyright © 2000 John Wiley & Sons, Ltd.

Prediction of Downstream Geomorphological Changes after Dam Construction: A Stream Power Approach

Abstract: A literature survey on methods of computing stable river-channel geometry, demanding a small amount of work effort and few input data, has been made and is presented. Besides the use of empirical regime equations and the use of an extremal hypothesis in conjunction with a sediment-transport and a flow-friction theory, new regression equations have been formulated which are used together with a sedimenttransport equation. These methods may prove efficient when predicting changes, such as after dam and reservoir construction, on an alluvial river. Calculations using the different methods have been exemplified on a natural river.

Physical and Human Factors Influencing Potential Fish Habitat Distribution along a Mountain River, France

Abstract: Potential fish habitat along the Drome River, France, is a function of the distribution of large woody debris, boulders, undercut banks, gravel substrate, and pools. The distribution of these features is, in turn, a function of channel geomorphology, watershed and riparian forest characteristics. We conducted field work and analysed aerial photographs for 190 elementary segments of 500 m length along the Drome River's 95 km course from the Alps westward to its confluence with the Rhone River near Loriol. The Drome River does not follow the classic pattern of a monotone downstream decrease in gradient and change in channel characteristics. Although channel gradient, braided index and channel incision all decrease downstream, stream power is independent of longitudinal distance. These variables are largely controlled by geomorphic, human or hydrologic factors at the reach scale. Potential fish habitat richness decreases downstream, but individual habitat variables affecting habitat richness do not necessarily decrease downstream, many being controlled by local factors rather than by position along the continuum. Large woody debris is more abundant in braided reaches located directly downstream of confluences with main tributaries or downstream input sites. Boulders are most abundant downstream of failed bank protection works or in gorges. To improve fish habitat in the Drome River, we recommend taking a long-term and large-scale perspective. Because structures placed in this unstable channel are likely to be washed downstream, we propose to emulate natural river dynamics and to permit large woody debris to enter the channel in unstable reaches via bank erosion, and that this debris not be removed (as is routinely done now) but permitted to migrate downstream through the system, creating fish habitat en route.

Tidal Stream Renewable Offshore Power Generation (TS-Ropg)

Abstract: Publisher Summary The Engineering Business is developing a number of practical tidal stream power generation schemes. They are all based on rotating hydrofoils up and down to generate oscillatory motion from flowing water. Two schemes are described in the chapter: the Active Water Column Generator (AWGC) and the Stingray tidal stream generator. The AWCG development has been partly funded by a DTI Smart Award. With a growing market for renewable energy several technologies are becoming established. EB believes that tidal stream energy can compete with offshore wind developments. It depends on the available energy at the proposed site and the economics of installing the additional generating capacity. The predictable nature of tidal stream energy can complement the variability of wind power by providing a regular base load supply.

Hurricane floods as extreme geomorphic events

Abstract: Tropical cyclones (also known as hurricanes or typhoons) generate extreme floods in parts of the tropics and subtropics between about 10° and 30° of latitude. Accounts of the resulting high shear stress and unit stream power, enhanced stream competence, sediment transport and storage, and channel forms are now available for a limited number of streams in Australia, South Asia, and the Caribbean. Not only are these very large floods but they also tend to occur at an interval of decades, rather than as 50 year or 100 year floods. This paper discusses the hydrology of these extreme floods, areas of their occurrence, their effect on channel size and form, and the flushing of sediment downchannel. The recovery of the channel from its response to such floods is indirectly dependent on: (a) the size difference between the hurricane floods and floods that occur at smaller intervals such as 1-2 years, and (b) the texture of the sediment in the channel. Streams draining basins where tracks of tropical cyclones cross tectonically active terrains tend to be strongly controlled by extreme events.

The hydraulic resistance of sand streambeds under steady flow conditions

Abstract: Laboratory and river data has been analyzed in terms of applied power principles to determine the relationships between absolute roughness and bed deformation under steady state sediment transport conditions. It has been found that the full spectrum of bed conditions can be represented by simple relationships within a single system. As the bed forms grow, the unit stream power being applied along the bed decreases. The process of deformation continues until equilibrium is reached. Dynamic equilibrium is reached with the average rate of deposition of particles equal to the rate of re-suspension. By expressing the applied power in terms of the size of the boundary eddies i.e. the bed form sizes and comparing this with the power required to suspend the sediment particles, the link is obtained between absolute roughness and particle characteristics. Transition from lower regime conditions to upper regime conditions is initiated when the applied (laminar) power along the bed becomes greater than that which is ...

Sediment Transport in the Lower Mississippi River

Abstract: : The Lower Mississippi River, extending from Cairo, Illinois to the Gulf of Mexico, annually transports approximately 170 million tonnes of sediment. Historically, the quantity and calibre of sediment derived from catchment erosion have been affected by changes in land-use and management. For example, soil erosion increased during the 19th and early 20th centuries due to settlement by Europeans and this may have elevated catchment sediment supply to the Mississippi River, while more recently the supply of sediment from tributaries is known to have decreased markedly as a result of river engineering and management. Specifically, the construction of large dams as part of the Mississippi River and Tributaries (MRT Dardeau and Causey, 1990). However, a case can be made that the bed material load must have increased since the 1940s. This argument is based on analysis of morphological changes observed along the river that have led to an overall increase in slope and available stream power, coupled with the fact that bed material sizes along the river have remained almost constant.

Episodes of flash floods and boulder transport in the Upper Jordan River

Abstract: Because direct measurement of boulder transport in natural rivers is difficult, there are very few field studies on the subject Long-term field studies 1969-1999, were conducted in the Upper Jordan channel to evaluate the geomorphic impact and persistence of a 1:100 year catastrophic flood, and to analyse the relationship between the river's hydrological regime and boulder transport behaviour A channel reach of 210 m length and 25-35 m width, with 260 boulders larger than 500 mm b-axis, was resurveyed periodically over 30 years Detailed field surveys, photographs, and painted rocks were used to assess the bed load transport During the study period, four major flood events had the ability to transport boulders of sizes from D 50 1000 mm up to the largest, namely 1700 mm b-axis size Stream power values increase where the depth-slope product is maximized In the wide sections of the channel with lower slope values, the deposition of big boulders determines the formation of bar structures The catastrophic flood has a decisive effect on the modification of the fluvial landform, being a geomorphologically effective event