首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
Quantitative regional assessments of streambed sedimentation and its likely causes are hampered because field investigations typically lack the requisite sample size, measurements, or precision for sound geomorphic and statistical interpretation. We adapted an index of relative bed stability (RBS) for data calculated from a national stream survey field protocol to enable general evaluation of bed stability and anthropogenic sedimentation in synoptic ecological surveys. RBS is the ratio of bed surface geometric mean particle diameter (Dgm) divided by estimated critical diameter (Dcbf) at bankfull flow, based on a modified Shield's criterion for incipient motion. Application of RBS to adequately depict bed stability in complex natural streams, however, has been limited because typical calculations of RBS do not explicitly account for reductions in bed shear stress that result from channel form roughness. We modified the index (RBS) to incorporate the reduction in bed shear stress available for sediment transport that results from the hydraulic resistance of large wood and longitudinal irregularities in channel dimensions (“form roughness”). Based on dimensional analysis, we derived an adjustment to bankfull shear stress by multiplying the bankfull hydraulic radius (Rbf) by the one-third power of the ratio of particle-derived resistance to total hydraulic resistance (Cp/Ct)1/3, where both resistances are empirically based calculations. We computed Cp using a Keulegan equation relating resistance to relative submergence of bed particles. We then derived an empirical equation to predict reach-scale hydraulic resistance Ct from thalweg mean depth, thalweg mean residual depth, and large wood volume based on field dye transit studies, in which total hydraulic resistance Ct was measured over a wide range of natural stream channel complexity, including manipulation of large wood volumes. We tested our estimates of Ct and RBS by applying them to data from a summer low flow probability sample of 104 wadeable stream reaches in the Coastal Ecoregion of Oregon and Washington, USA. Stream discharges calculated using these Ct estimates compared favorably with velocity–area measurements of discharge during summer low flow, and with the range of 1 to 2-year recurrence floods (scaled by drainage area) at U.S.Geological Survey gauged sites in the same region. Log [RBS] ranged from − 4.2 to + 0.98 in the survey region. Dgm ranged from silt to boulders, while estimated bankfull critical diameter, Dcbf, ranged from very fine gravel to large boulders. The median value of Dcbf (adjusted for form roughness influences) averaged 40% (inter quartile range 28 to 59%) of the unadjusted estimate Dcbf. Log[RBS] was consistently negatively related to human disturbances likely to produce excess sediment inputs or hydrologic alteration. Log [RBS] ranged from − 1.9 to + 0.5 in the streams within the lower quartile of human disturbance in their basin and riparian areas and was substantially lower (− 4.2 to − 1.1) in streams within the upper quartile of human disturbance. The synoptic survey methods and designs we used appear adequate to evaluate regional patterns in bed stability and sedimentation and their general relationship to human disturbances. Although the RBS concept also shows promise for evaluating sediment and bed stability in individual streams, our approach is relatively coarse, so site-specific assessments using these rapid field methods might prudently be confined to identifying severe cases of sedimentation or channel alteration. Greater confidence to discern subtle differences in site-specific assessments could be gained by calculating RBS using more precise field measurements of channel slope, bed particle size and bankfull dimensions, and by refining our adjustments for energy loss from channel form roughness.  相似文献   

2.
Land use practices in Colorado during the last two centuries altered the supply of sediment and water to many channels in the upper South Platte Basin. As a result of increased supply of sediment and mobility and reduced peak flows, the characteristics of pools associated with channel constrictions, referred to as forced pools, may have been altered. Increased supply of sediment and reduced transport capacity of high flows could lead to aggradation in forced pools. Channel confined by road corridors could lead to high velocities at normal flows, increased energy dissipation from riprap, or even increased pool frequency resulting from failed riprap. To assess potential alterations, four hypotheses were tested: (1) impacted streams will show significantly different mean volume of pools than a control stream; (2) mean volume of pools on streams where land-use activities increased the supply of sediment will be significantly different from streams solely affected by flow regulation; (3) the strongest change in characteristics of pools of impacted streams will be a reduced volume of pools; (4) streams affected by road corridors will show statistically lower spacing of pools than streams unaffected by roads. The downstream spacing and residual volume of twenty consecutive forced pools were surveyed on five streams in the Colorado Front Range that varied from no contemporary impact to multiple historical and contemporary impacts. ANCOVA with stepwise model selection indicated that degree of land-use (categorical), bankfull spacing of pools, upstream riffle slope and expansion ratio were all significant (α = 0.1) predictors of volume of pools (R2 = 0.73). Simple linear regression of mean volume of pools and stream specific variables (gradient, drainage area and discharge) and least square means comparison of mean volume of pools indicated a need to standardize volumes of pools by slope and discharge so that the volumes of pools could be compared among different levels of land-use. Significant correlations between drainage area and volume of pools allowed volume of pools to be standardized by drainage area and thereby discharge. This dimensionless variable was also significantly correlated with channel slope, which permitted the construction of a new variable, PVQS (volume of pools standardized by discharge and slope). Least square means comparison of mean PVQS revealed that the control reach was significantly different from road-impacted reaches. Mean volume of pools was significantly larger in the control reach compared to all but one road-impacted stream. This was likely a function of higher wood loading in the control reach and the competence of high flows in the road-impacted reach. Streams affected by road corridors did not have significantly different bankfull spacing of pools from streams not impacted by roads. The multiple interactions among control and response variables explored in this study indicate the need to identify the most constrained and sensitive response variables when attempting to assess channel response to land use.  相似文献   

3.
This research examines variations in bankfull cross-sections along a steep stream of the Dolomites (Cordevole stream, Belluno, Northern Italy). Field measurements were conducted to determine variations in the channel top-width at bankfull stage in relation to the drainage area and to the length of the flow path. After grouping the bed morphologies according to the Montgomery and Buffington [Montgomery, D.R., Buffington, J.M., 1997. Channel-reach morphology in mountain drainage basins. Geol. Soc. of Am. Bull. 109 (5) 596–611.] classification, we analyzed the increase in bankfull width for the dominant stream units (cascades, step pools, isolated pools and colluvial reaches at the head of the basin). We observed that the morphologies more related to the drainage area are colluvial reaches and pools; the less adaptable are steps and cascades. These differences likely result from the absence of lateral constriction in the colluvial reaches and pools, whereas the presence of coarser sediments in the bed can affect the transverse adjustments in steps and reaches dominated by cascades. Linkages between cross-section geometry and parameters related to flow (i.e. drainage area and stream power) have been analyzed together with the distribution of surface grain sizes and its coarsening pattern. The existence of distinctive bankfull widths between different morphological units points out the degree of susceptibility to be modeled according to the channel slope, reference diameter (D90), and contributing area.  相似文献   

4.
《自然地理学》2013,34(6):492-510
Coarse woody debris (CWD) is an important component of headwater streams, however, few studies have investigated the geomorphic effects of CWD in the southern Appalachians. In the Great Smoky Mountains, debris slides supply large volumes of CWD and sediment to low-order streams. This study investigates the effect of CWD on bankfull channel dimensions and in-channel sediment storage along second-order streams. Comparisons are made between streams that have experienced recent debris slides and those that have not. CWD channel obstructions are larger but less frequent along debris-slide-affected streams. Dendrochronological evidence indicates that CWD can remain in channels for over 100 yr. Relatively short residence times of CWD along debris-slide-affected streams suggest that logs are frequently flushed through these streams. CWD causes channel widening along all study streams, but the volume of sediment stored in the channel behind CWD obstructions is up to four times greater than the volume of sediment represented by bank erosion associated with CWD. Two large log jams formed by debris slides at tributary junctions stored approximately 4000 m3 of sediment. Sediment stored by CWD was finer than mean bed particle size, and thus represents a significant sediment source when CWD obstructions are breached.  相似文献   

5.
This research evaluates the impact of rural-to-urban land use conversion on channel morphology and riparian vegetation for three streams in the Central Redbed Plains geomorphic province (central Great Plains ecoregion) of Oklahoma. The Deep Fork Creek watershed is largely urbanized; the Skeleton Creek watershed is largely rural; and the Stillwater Creek watershed is experiencing a rapid transition from rural to urban land cover. Each channel was divided into reaches based on tributary junctions, sinuosity, and slope. Field surveys were conducted at transects in a total of 90 reaches, including measurements of channel units, channel cross-section at bankfull stage, and riparian vegetation. Historical aerial photographs were available for only Stillwater Creek watershed, which were used to document land cover in this watershed, especially changes in the extent of urban areas (impervious cover).The three streams have very low gradients (< 0.001), width-to-depth ratios < 10, and cohesive channel banks, but have incised into red Permian shales and sandstone. The riparian vegetation is dominated by cottonwoods, ash, and elm trees that provide a dense root mat on stream banks where the riparian vegetation is intact. Channels increased in width and depth in the downstream direction as is normally expected, but the substrate materials and channel units remained unchanged. Statistical analyses demonstrated that urbanization did not explain spatial patterns of changes in any variables. These three channels in the central Redbed Plains are responding as flumes during peak flows, funneling runoff and the wash-load sediment downstream in major runoff events without any effect on channel dimensions. Therefore, local geological conditions (similar bedrock, cohesive substrates and similar riparian vegetation) are mitigating the effects of urbanization.  相似文献   

6.
Experiments with marked pebbles were carried out on different sized rivers of the Belgian Ardenne (catchment areas varying from less than 1 km2 to 2700 km2). Specific stream power required to cause bedload movement was evaluated and critical values were obtained. Three types of relationship between critical specific stream power (ω0) and grain size (D) were established. The values for ω0 in the largest river (the Ourthe) were the lowest and were close to the values obtained for mountainous rivers carrying large boulders. In medium sized rivers (catchment area between 40 and 500 km2), the critical unit stream power was higher. It is likely that it is due to the bedform's greater resistance. This resistance would use up some of the energy that can cause movement and transport of bedload. The amount of resistance of the bedform can be expressed as bedform shear stress (τ″), determined by the relationship between grain shear stress (τ′—that determines movement and transport of the bedload) and the total shear stress (τ). This ratio varies between 0.4 and 0.5 in the medium sized rivers, compared to 0.7 in the Ourthe. In headwater streams (less than 20 km2), there is greater loss of energy due to bedform resistance (τ′/τ<0.3). Critical specific stream power is higher in this third type of river than in the other two.  相似文献   

7.
Identification of steps and pools from stream longitudinal profile data   总被引：1，自引：0，他引：1  
Field research on step–pool channels has largely focused on the dimensions and sequence of steps and pools and how these features vary with slope, grain sizes and other governing variables. Measurements by different investigators are frequently compared, yet no means to identify steps and pools objectively have been used. Automated surveying instruments record the morphology of streams in unprecedented detail making it possible to objectively identify steps and pools, provided an appropriate classification procedure can be developed.To achieve objective identification of steps and pools from long profile survey data, we applied a series of scale-free geometric rules that include minimum step length (2.25% of bankfull width (Wb)), minimum pool length (10% of Wb), minimum residual depth (0.23% of Wb), minimum drop height (3.3% of Wb), and minimum step slope (10° greater than the mean slope). The rules perform as well as the mean response of 11 step–pool researchers who were asked to classify four long profiles, and the results correspond well with the channel morphologies identified during the field surveys from which the long profiles were generated. The method outperforms four other techniques that have been proposed. Sensitivity analysis shows that the method is most sensitive to the choice of minimum pool length and minimum drop height.Detailed bed scans of a step–pool channel created in a flume reveal that a single long profile with a fixed sampling interval poorly characterizes the steps and pools; five or more long profiles spread across the channel are required if a fixed sampling interval is used and the data suggest that survey points should be located more frequently than the diameter of the step-forming material. A single long profile collected by a surveyor who chooses breaks in slope and representative survey points was found to adequately characterize the mean bed profile.  相似文献   

8.
This article evaluates the potential of 1-m resolution, 128-band hyperspectral imagery for mapping in-stream habitats, depths, and woody debris in third- to fifth-order streams in the northern Yellowstone region. Maximum likelihood supervised classification using principal component images provided overall classification accuracies for in-stream habitats (glides, riffles, pools, and eddy drop zones) ranging from 69% for third-order streams to 86% for fifth-order streams. This scale dependency of classification accuracy was probably driven by the greater proportion of transitional boundary areas in the smaller streams. Multiple regressions of measured depths (y) versus principal component scores (x1, x2,…, xn) generated R2 values ranging from 67% for high-gradient riffles to 99% for glides in a fifth-order reach. R2 values were lower in third-order reaches, ranging from 28% for runs and glides to 94% for pools. The less accurate depth estimates obtained for smaller streams probably resulted from the relative increase in the number of mixed pixels, where a wide range of depths and surface turbulence occurred within a single pixel. Matched filter (MF) mapping of woody debris generated overall accuracies of 83% in the fifth-order Lamar River. Accuracy figures for the in-stream habitat and wood mapping may have been misleadingly low because the fine-resolution imagery captured fine-scale variations not mapped by field teams, which in turn generated false “misclassifications” when the image and field maps were compared.The use of high spatial resolution hyperspectral (HSRH) imagery for stream mapping is limited by the need for clear water to measure depth, by any tree cover obscuring the stream, and by the limited availability of airborne hyperspectral sensors. Nonetheless, the high accuracies achieved in northern Yellowstone streams indicate that HSRH imagery can be a powerful tool for watershed-wide mapping, monitoring, and modeling of streams.  相似文献   

9.
Stream temperatures are critical to coldwater fish and vary with microclimate, geomorphology, and hydrology, including influx of groundwater. Spatial variability of stream temperatures was examined at reach and watershed scales within the 816 km2 Navarro River watershed in California. Field monitoring and numerical modeling illustrate that stream temperatures were highest at sites with high solar incidence (low shading and wide streams), long travel times, and low discharge. Microclimate helps explain deviation from the general pattern of streams warming with increasing drainage area. Reach-scale field observations of channel width and groundwater influx explain variation in stream temperatures not captured by watershed-scale models.  相似文献   

10.
A detailed understanding of channel forming and maintenance processes in mountain streams requires some measurement and/or prediction of bed load transport and sediment mobility. Traditional field based measurements of such processes are problematic because of the high formative discharges characteristic of such streams. The application of Optically Stimulated Luminescence (OSL) dating is proposed here as a new way of determining actual residency times of fine sediments and consequently validating selected predictions for the entrainment of sediment in these streams. Model predictions of sediment mobility for selected step-pool and plane-bed channels in a mountain catchment in south eastern Australia are initially calculated using equations of hydraulic competence and the one-dimensional HEC-RAS model. Results indicate that floods exceeding bankfull with recurrence intervals up to 13 years are competent to mobilise the maximum overlying surface grain sizes at both sites. OSL minimum age model results from 7 samples of well bleached quartz in the fine matrix particles indicate general agreement with selected competence equations. The apparent long (100–1400 y) burial age of most of the mineral quartz, however, suggests that competent flows are not able to flush all subsurface fine-bed material. The depth of maximum bed load exchange (flushing) was limited to ≤ twice the depth of the overlying D90 grain size. Application of OSL in this study provides important insight into the nature of storage and flushing of matrix material in mountain streams.  相似文献   

11.
The mechanisms which control the formation and maintenance of pool–riffles are fundamental aspects of channel form and process. Most of the previous investigations on pool–riffle sequences have focused on alluvial rivers, and relatively few exist on the maintenance of these bedforms in boulder-bed channels. Here, we use a high-resolution two-dimensional flow model to investigate the interactions among large roughness elements, channel hydraulics, and the maintenance of a forced pool–riffle sequence in a boulder-bed stream. Model output indicates that at low discharge, a peak zone of shear stress and velocity occurs over the riffle. At or near bankfull discharge, the peak in velocity and shear stress is found at the pool head because of strong flow convergence created by large roughness elements. The strength of flow convergence is enhanced during model simulations of bankfull flow, resulting in a narrow, high velocity core that is translated through the pool head and pool center. The jet is strengthened by a backwater effect upstream of the constriction and the development of an eddy zone on the lee side of the boulder. The extent of flow convergence and divergence is quantified by identifying the effective width, defined here as the width which conveys 90% of the highest modeled velocities. At low flow, the ratio of effective width between the pool and riffle is roughly 1:1, indicating little flow convergence or divergence. At bankfull discharge, the ratio of effective width is approximately 1:3 between the pool and downstream riffle, illustrating the strong flow convergence at the pool head. The effective width tends to equalize again with a ratio of 1:1 between the pool and riffle during a modeled discharge of a five-year flood, as the large roughness elements above the pool become drowned out. Results suggest that forced pool–riffle sequences in boulder-bed streams are maintained by flows at or near bankfull discharge because of stage-dependent variability in depth-averaged velocity and tractive force.  相似文献   

12.
Ronald B. Zelt  Ellen E. Wohl   《Geomorphology》2004,57(3-4):217-233
Large variability in responses of stream sediment and large woody debris (LWD) to severe fire has limited the accurate prediction of the magnitude and duration of fire effects on streams. Conditions in one Absaroka Range stream that was severely burned in 1988 were compared to those in an adjacent, undisturbed stream to improve understanding of fire effects on channel and LWD characteristics beyond the first few years. Ten reaches of each stream were sampled during summer 1999.Average bankfull channel width was greater in burned Jones Creek than in unburned Crow Creek. LWD frequency and overall frequency of LWD accumulations were greater in Crow Creek than Jones Creek. Debris-jam frequency was greater in Jones Creek after accounting for differences in the frequency of pieces with length greater than channel width. Larger piece size and better anchoring contributed to more frequent, small accumulations of LWD in Crow Creek. Differences between streams in LWD frequency are consistent with greater mobility of debris in burned Jones Creek. LWD-associated fine-sediment deposits were thicker but less frequent along Jones Creek than Crow Creek.  相似文献   

13.
The Kunlun fault is one of the largest strike-slip faults in northern Tibet, China. In this paper, we focus upon the Kusai Lake–Kunlun Pass segment of the fault to understand the geomorphic development of offset streams caused by repeated large seismic events, based on tectono-geomorphic analysis of high-resolution satellite remote sensing images combined with field studies. The results indicate that systematic left-lateral stream offsets appear at various scales across the fault zone: Lateral offsets of small gullies caused by the 2001 Mw 7.8 Kunlun earthquake vary typically from 3 m to 6 m, meanwhile streams with cumulative offsets of 10 m, 25–30 m, 50–70 m, 250–300 m and 750–1400 m have resulted from repeated large seismic events during the late Quaternary. An average slip rate of 10 ± 1 mm/year has been estimated from the lateral stream offsets and 14C ages of alluvial fan surfaces incised by the streams. A three-dimensional model showing tectono-geomorphic features along a left-lateral strike-slip fault is also presented. The Kusai Lake–Kunlun Pass segment provides an opportunity to understand the relationship between geomorphic features produced by individual large seismic events and long-term geomorphic development caused by repeated large seismic events along a major strike-slip fault.  相似文献   

14.
The impact of large twentieth century floods on the riparian vegetation and channel morphology of the relatively wide anabranching and braided Nahal Arava, southern Israel, was documented as part of developing tools to (a) identify recent large floods, (b) determine these flood's respective magnitudes in alluvial ungauged streams, and (c) determine long-term upper bounds to flood stages and magnitudes. Along most of its course Nahal Paran, a major tributary that impacts the morphology, floods and sediments of Nahal Arava at the study reach, is a coarse-gravel, braided ephemeral stream. Downstream of the Arava–Paran confluence, aeolian and fluvial sand delivered from eastern Arava valley alters the channel morphology. The sand has accreted up to 2.5 m above the distinct current channels, facilitating the recording of large floods. This sand enhances the establishment of denser riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum) that interacts with floods and affects stream morphology. A temporal association was found between specific floods recorded upstream and tree-ring ages of re-growth of flood-damaged tamarix trees (‘Sigafoos trees’) in the past 30 years. This association can be utilized for developing a twentieth century flood chronology in hyperarid ungauged basins in the region. The minimum magnitude of the largest flood that covered the entire channel width, estimated from flood deposits, is approximately 1700–1800 m3s− 1. This is a larger magnitude than the largest gauged flood of 1150 m3s− 1 that occurred in 1970 about 30 km upstream in Nahal Paran. Our estimation agrees with flood magnitude estimated from the regional envelope curve of the largest floods. Based on Holocene alluvial stratigraphy and OSL dating in the study reach we also conclude that flood stages did not reach the late Holocene ( 2.2 ka) surface and therefore we estimate a non-exceedance upper bound of  2000 m3s− 1 flood magnitudes for Nahal Arava during that interval. This study indicates that in unfavorable areas the combination of hydrology, fluvial morphology and botanic evidence can increase our understanding of ungauged basins and give information crucial for hydrology planning.  相似文献   

15.
Alpa Sridhar   《Geomorphology》2007,88(3-4):285-297
This paper attempts to quantify contemporary and palaeo-discharges and changes in the hydrologic regime through the mid–late Holocene in the alluvial reach of the arid Mahi River basin in western India. The occurrence of terraces and pointbars high above active river levels and change in the width/depth ratio can be regarded as geomorphic responses to changes in discharge. Discharge estimates are made based on the channel dimensions and established empirical relations for the three types of channels: mid–late Holocene, historic (the channel that deposited extensive pointbars above the present-day average flow level) and the present ones. The bankfull discharge of the mid–late Holocene channel was  55 000 m3 s− 1 and that of the historic channel was  9500 m3 s− 1, some  25 times and  5 times greater than that of the present river (2000 m3 s− 1), respectively. Since the mid–late Holocene, the channel form has changed from wide, large-amplitude meanders to smaller meanders, and decreases in the width/depth ratio, unit stream power and the bed shear stresses have occurred. It can be inferred that there has been a trend of decreasing precipitation since the mid–late Holocene.  相似文献   

16.
Ecoregions are generally homogenous areas that comprise similar associations of geology, climate, soils, land use, and potential natural vegetation. However, a certain amount of physical variability exists in ecoregions which makes them spatially diverse in their characteristics. Euclidean Sum of Squares was used to calculate the amount of stream reach dissimilarity within and between three prominent eastern Oklahoma ecoregions: Boston Mountains, Ozark Highlands, and Ouachita Mountains. Dissimilarity proximity coefficients were used to cluster stream reaches and create exemplars for each cluster. Streams were classified using the Rosgen classification to evaluate whether dissimilarity values could be applied to the classification. Metrics used to calculate stream reach dissimilarity were median particle size, bankfull width, width–depth ratio, gradient, and sinuosity. Maximum stream reach dissimilarity between two sites within the same ecoregion is highest in the Ouachita Mountains (18.59) and followed by the Ozark Highlands (13.12) and Boston Mountains (9.58). Removal of ecoregion boundaries increased maximum proximity coefficient dissimilarity between two sites in the Ozark Highlands and Ouachita Mountains to 34.26. Utilizing the dissimilarity of channel morphology for the purpose of regional classification helps in the understanding of how broad-scale processes influence the fluvial system.  相似文献   

17.
Reach-scale channel geometry of mountain streams   总被引：3，自引：0，他引：3  
Ellen Wohl  David M. Merritt   《Geomorphology》2008,93(3-4):168-185
The basic patterns and processes of steep channels remain poorly known relative to lower-gradient channels. In this analysis, characteristics of step-pool, plane-bed, and pool-riffle channels are examined using a data set of 335 channel reaches from the western United States, Nepal, New Zealand, and Panama. We analyzed differences among the three channel types with respect to hydraulics, channel geometry, boundary roughness, and bedforms. Step-pool channels have significantly steeper gradients, coarser substrate, higher values of shear stress and stream power for a given discharge, and larger ratios of bedform amplitude/wavelength (H/L). Pool-riffle channels have greater width/depth ratios and relative grain submergence (R/D84) than the other channel types. Plane-bed channels tend to have intermediate values for most variables examined. Relative form submergence (R/H) is statistically similar for step-pool and pool-riffle channels. Despite the lesser relative grain submergence and greater bedform amplitude of step-pool channels, mean values of Darcy–Weisbach friction factor do not change in response to changes in relative grain submergence. These patterns suggest that adjustments along mountain streams effectively maximize resistance to flow and minimize downstream variability in resistance among the different channel types.  相似文献   

18.
Predicting channel patterns   总被引：1，自引：0，他引：1  
The proposed distinction between meandering and braided river channel patterns, on the basis of bankfull specific stream power and bed material size, is analysed and rejected. Only by using regime-based estimates of channel widths (rather than actual widths) has discrimination been achieved, and it is argued that this procedure is unacceptable.An alternative is to explore the patterning processes underlying the marked pattern scatter on bankfull stream power/bed material size plots. Of the five sets of patterning processes, large-scale bedform development and stability is seen as especially important for meandering and braiding. For gravel-bed rivers, bedforms developed at around or above bankfull stage appear important for pattern generation, with braiding relating to higher excess shear stress and Froude number. There seems to be an upper threshold to both meandering and braiding which is achieved at extreme discharges and steep gradients, as on steep alluvial fans, rather than for the rivers with available flow data here considered. For sand-bed rivers with greater excess shear stress, the equivalent upper plane bed threshold may occur below bankfull, with bed material mobility and bedform modification occurring over a wider range of sub-bankfull discharges. Sand-bed channel margin outlines appear to be less perturbed by bedform effects than gravel bed planforms, and they may have naturally straight or sinuous planforms. Bedform relief may nevertheless lead to some being designated as braided when viewed at low flows.It is concluded that the use of a single-stage stream power measure and bed material size alone is unlikely to achieve meandering/braiding discrimination.  相似文献   

19.
Despite more than 40 yr of research attributing temporal changes in streambank erosion rates to subaerial processes, little quantitative information is available on the relationships between streambank erodibility (kd) and critical shear stress (τc) and the environmental conditions and processes that enhance streambank erosion potential. The study goal was to evaluate temporal changes in kd and τc from soil desiccation and freeze–thaw cycling. Soil erodibility and τc were measured monthly in situ using a multiangle, submerged jet test device. Soil moisture, temperature, and bulk density as well as precipitation, air temperature, and stream stage were measured continuously to determine changes in soil moisture content and state. Pairwise Mann–Whitney tests indicted kd was 2.9 and 2.1 times higher (p < 0.0065) during the winter (December–March) than in the spring/fall (April–May, October–November) and the summer (June–September), respectively. Regression analysis showed 80% of the variability in kd was explained by freeze–thaw cycling alone. Study results also indicated soil bulk density was highly influenced by winter weather conditions (r2 = 0.86): bulk density was inversely related to both soil water content and freeze–thaw cycling. Results showed that significant changes in the resistance of streambank soils to fluvial erosion can be attributed to subaerial processes. Water resource professionals should consider the implications of increased soil erodibility during the winter in the development of channel erosion models and stream restoration designs.  相似文献   

20.
Prediction of alluvial channel pattern of perennial rivers   总被引：2，自引：0，他引：2  
Purely braided, meandering and straight channels can be considered as end-members of a continuum of alluvial channel patterns. Several researchers have succeeded in separating channel patterns in fields defined by flow related parameters. However, the discriminators of the principal channel patterns derived from these diagrams all require some a priori knowledge of the channel geometry. In this paper a method is presented which enables prediction of the equilibrium conditions for the occurrence of braided and high sinuosity meandering rivers in unconfined alluvial floodplains. The method is based on two, almost channel pattern independent, boundary conditions: median grain size of the river bed material, and a potential specific stream power parameter related to bankfull discharge or mean annual flood and valley gradient. This can be regarded as a potential maximum of the available flow energy corresponding to the minimum sinuosity condition, P = 1. Based on an analysis of 228 datasets of measurement sites along rivers from many parts of the world an independent discriminating function was found that separates the occurrence of braided rivers and meandering rivers with P > 1.5. The function applies to equilibrium conditions of rivers that neither incise nor show rapid aggradation, with a bankfull or mean annual flood discharge above 10 m3/s and a median bed material grain size between 0.1 and 100 mm.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号