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1.
This study evaluated the use by fish of restored tidal wetlands and identified links between fish species composition and habitat characteristics. We compared the attributes of natural and constructed channel habitats in Sweetwater Marsh National Wildlife Refuge, San Diego Bay, California, by using fish monitoring data to explore the relationships between channel environmental characteristics and fish species composition. Fishes were sampled annually for 8 yr (1989–1996) at eight sampling sites, four in constructed marshes and four in natural marshes, using beach seines and blocking nets. We also measured channel habitat characteristics, including channel hydrology (stream order), width and maximum depth, bank slope, water quality (DO, temperature, salinity), and sediment composition. Fish colonization was rapid in constructed channels, and there was no obvious relationship between channel age and species richness or density. Total richness and total density did not differ significantly between constructed and natural channels, although California killifish (Fundulus parvipinnis) were found in significantly higher densities in constructed channels. Multivariate analyses showed fish assemblage composition was related to channel habitat characteristics, suggesting a channel’s physical properties were more important in determining fish use than its restoration status. This relationship highlights the importance of designing restoration projects with natural hydrologic features and choosing proper assessment criteria in order to avoid misleading interpretations of constructed channel success. We recommend that future projects be designed to mimic natural marsh hydrogeomorphology and diversity more closely, the assessment process utilize better estimates of fish habitat function (e.g., individual and community-based species trends, residence time, feeding, growth) and reference site choice, and experimental research be further incorporated into the restoration process.  相似文献   

2.
The pervious lateral bars (parafluvial zone) and beds (hyporheic zone), where stream water and groundwater exchange, are dynamic sites of hydrological and biological retention. The significance of these biogeochemical ‘hotspots’ to stream and groundwater metabolism is largely controlled by filtration capacity, defined as the extent to which subsurface flowpaths and matrix hydraulic conductivity modify water characteristics. Where hydraulic conductivity is high, gradients in biogeochemistry and microbial activity along subsurface flowpaths were hypothesized to be less marked than where hydraulic conductivity is low. This hypothesis was tested in two riffles and gravel bars in an Australian subtropical stream. At one site, gradients in chemical and microbial variables along flowpaths were associated with reduced hydraulic conductivity, longer water residence time and reduced filtration capacity compared with the second site where filtration capacity was greater and longitudinal biogeochemical trends were dampened. These results imply that factors affecting the sediment matrix in this subtropical stream can alter filtration capacity, interstitial microbial activity and biogeochemical gradients along subsurface flowpaths. This hydroecological approach also indicates potential for a simple field technique to estimate filtration capacity and predict the prevailing hyporheic gradients in microbial activity and biogeochemical processing efficiency, with significant implications for stream ecosystem function.  相似文献   

3.
In-stream structures including cross-vanes, J-hooks, rock vanes, and W-weirs are widely used in river restoration to limit bank erosion, prevent changes in channel gradient, and improve aquatic habitat. During this investigation, a rapid assessment protocol was combined with post-project monitoring data to assess factors influencing the performance of more than 558 in-stream structures and rootwads in North Carolina. Cross-sectional survey data examined for 221 cross sections from 26 sites showed that channel adjustments were highly variable from site to site, but approximately 60 % of the sites underwent at least a 20 % net change in channel capacity. Evaluation of in-stream structures ranging from 1 to 8 years in age showed that about half of the structures were impaired at 10 of the 26 sites. Major structural damage was often associated with floods of low to moderate frequency and magnitude. Failure mechanisms varied between sites and structure types, but included: (1) erosion of the channel bed and banks (outflanking); (2) movement of rock materials during floods; and (3) burial of the structures in the channel bed. Sites with reconstructed channels that exhibited large changes in channel capacity possessed the highest rates of structural impairment, suggesting that channel adjustments between structures led to their degradation of function. The data question whether currently used in-stream structures are capable of stabilizing reconfigured channels for even short periods when applied to dynamic rivers.  相似文献   

4.
In Scandinavia, most fluvial erosion takes place in the Quaternary glacial overburden at a restricted number of small source areas along individual drainage channels. As a consequence, a sample of active stream sediment is representative of only a very limited portion of the drainage area. This restriction makes stream sediment less reliable for regional exploration than generally expected. Overbank (levee or river-plain) sediment produced during large floods is an alternate more representative sampling medium. The sediment suspended during a flood has a much more widespread origin, and when the load is deposited upon the flood plain, nearly horizontal strata are formed and preserved at levels above the ordinary stream channel. A composite sample through a vertical section of such strata represents a great number of sediment sources that have been active at different times and forms an integrated sample of the entire catchment area. Because young sediments overlay older, the uppermost layers will be contaminated by pollutants in industrialized regions, but those at depth may remain pristine and will to a greater extent reflect the natural pre-industrial environment. In regional geochemical mapping, overbank sediment can be sampled at widely spaced sites, keeping costs per unit area low. Examples from Norway (1 sample station per 500 km2) show that overbank sediment produces broad geochemical patterns with high contrasts reflecting the bedrock geochemistry. Some patterns agree with known geological units and metallogenic provinces, but hitherto unknown major structures have also been indicated. A large Mo-deposit missed by a traditional stream survey is readily detected in the overbank sediment. It is concluded that overbank sediment is a promising alternate sample medium that should be tested in other physiographic regions.  相似文献   

5.
 A significant proportion of stream sediment yield in North America comes from stream channel and bank erosion. One method used for stream stabilization is the bank installation of timber and stone fish-shelter structures, but there is little evidence for their potential effectiveness. Nine to nineteen years of precise survey data from Coon Creek, Wisconsin, however, show that fish structures enhance sediment deposition along the stream and may retard lateral migration of channels. Such structures have greater utility for sediment control when streams are eroding away a high bank and replacing it with a lower bank. Received: 18 October 1996 · Accepted: 4 February 1997  相似文献   

6.
Average relative stream channel migration rates of .21 meters per year (.72 feet per year) for saline tidal wetland stream channels, and .32 meters per year (1.04 feet per year) for freshwater tidal wetland channels were calculated for a 32 year period (1940 to 1972) using photogrammetric techniques. Saline wetland stream channels averaged higher indices of sinuosity, i.e., the ratio of total channel length to linear downstream distance (1.95), when compared with sinuosities of freshwater tidal channels (1.46). The difference is attributed to differences in vegetation types and consequent soil holding capacity between saline and freshwater tidal wetland environments. Saline channels become entrenched because the banks are supported by dense root systems, while freshwater tidal channels flow through a more homogenous substrate and behave much like channels which cross mud-flats in the intertidal zone. Higher average meander amplitudes (one-half the peak to trough distance of a given meander wave) for saline channels (171 meters) versus lower amplitudes for freshwater channels (114 meters) suggest that meander loops for saline channels are determined primarily by the erosional characteristics of stream banks and by other local factors rather than by hydrodynamic factors such as flow velocity or discharge. It has been stated that meander migration features do not occur in homogenous soil materials (Leopold, et al. 1964); the tendency of saline channels to form these features is attributed to differential erosion caused by variations in root system density. Conversely, the morphology of freshwater tidal channels is influenced by hydrodynamic factors including discharge, and is due to the existence of more homogenous materials, i.e., muddy soils devoid of extensive root systems. An analysis of ebb and flood discharge data arrived at for each tidal channel using existing tidal current velocity and upland discharge records supports the fact that relatively greater erosive forces occur in salt marsh than in fresh tidal marsh areas. A poor statistical correlation between rates of stream channel migration and hydraulic stream flow data such as velocity and discharge must be accepted with caution due to the method of approximating tidal discharge values. The correlation suggests that under normal tidal conditions both saline and freshwater tidal channels migrate little, if any, and thus represent an apparently balanced relatively low energy system. For this reason it is believed that most stream channel migration in both saline and freshwater wetlands occurs as a result of increased forces due to storms.  相似文献   

7.
Some drainage ditches in the intensively managed row-crop agricultural region of southern Minnesota evolved from a trapezoidal form to multi-staged channel forms similar to natural streams. Older ditches constructed in cohesive sediment of the Des Moines Lobe till tend to follow a channel evolution model developed by Simon and Hupp. Site cross sections, longitudinal water and bed profiles and bed material particle size were determined according to Harrelson and others at 24 older ditch reaches, 5 newly constructed ditch reaches and 13 natural stream reaches. Morphological features were hypothesized to change from trapezoidal form to flat bench banks, similar to benches found in natural stream channels. All data were statistically analyzed with respect to drainage area using regression, because channel form is directly related to drainage area for a given climate, geology and land use. Results show similar regression slope and intercept for bankfull channel width and bankfull cross-sectional area (CSA) of older ditches and natural streams compared to typical trapezoidal designed ditches. Evolved ditches developed a small floodplain bench above the ditch bed and adjusted their bankfull widths similar to natural stream channels with respect to drainage area. Old ditches showed a relatively strong R 2 (0.82, 0.68) for channel CSA and width, a weaker R 2 (0.45) for water surface slope, and little to no correlation with bed particle size. Channel form appears to have adjusted more quickly than bed facets and/or bed particle size distribution. However, stepwise regression determined that D84, width/depth ratio and mean bankfull depth explained 83?% of the variability of channel features across varying drainage areas. Findings suggest a possible reduction of long-term maintenance costs if older ditches are allowed to evolve over time. A stable ditch form similar to natural streams is typically self-sustaining, suggesting that prior to a scheduled clean-out, the ditch should be examined for hydraulic capacity, sediment transport and bank stability.  相似文献   

8.
The USA Clean Water Act requires the development of a total maximum daily load (TMDL) when Minnesota’s water quality standard for turbidity is exceeded; however, regions underlain with fine-grained lacustrine deposits yield large natural background loads of suspended inorganic sediment. A review of hydrogeologic pathways was conducted along with the statistical analysis of geomorphic metrics, collected at 15 sites with varying drainage areas in the upper Nemadji River basin, northeastern Minnesota. Regression analysis indicated a strong linkage between bankfull cross-sectional area and drainage area. Dimensionless geomorphic metric ratios were developed to predict channel evolution potential and associated channel erosion risk. Sites located in drainage areas less than 2 km2 had low erosion risk and showed a correlation between channel slope and relative roughness (D 84/mean bankfull channel depth, 88%). A principal components analysis explained over 98% of the variance between sites and indicated five important channel shape metrics to predict channel erosion: bankfull width, bankfull depth, maximum depth, cross-sectional area, and valley beltwidth. Mass wasting of cohesive stream channel sediment was influenced by groundwater discharge and produced turbid waters in the upper Nemadji River.  相似文献   

9.
The nature of the drainage system beneath ice sheets is crucial to their dynamic behaviour but remains problematic. An experimentally based theory of coupling between groundwater and major channel systems is applied to the esker systems in the area occupied the last ice sheet in Europe, which we regard as a fossil imprint of major longitudinal drainage channels. We conclude that the large-scale distribution and spacing of major eskers is consistent with the theory of groundwater control, in which esker spacing is partly controlled by the transmissivity of the bed.It is concluded that esker patterns reflect the large-scale organisation of the subglacial drainage pattern in which channel development is coupled to groundwater flow and to the ice sheet's dynamic regime. The theory is then used to deduce: basal meltwater recharge rates and their spatial variability from esker spacing in an area in which the ice sheet was actively streaming during its final retreat; patterns of palaeo-groundwater flow and head distribution; and the seasonally varying magnitude of discharge from stream tunnels at the retreating ice sheet margin. Major channel/esker systems appear to have been stable at least over several hundred of years during the retreat of the ice sheet, although major dynamic events are demonstrably associated with major shifts in the hydraulic regime.Modelling suggests: that glaciation can stimulate deep groundwater circulation cells that are spatially linked to channel locations, with groundwater flow predominantly transverse to ice flow; that the circulation pattern has the potential to create large-scale anomalies in groundwater chemistry; and that the spacing of channels will change through the glacial cycle, influencing water pressures in stream tunnels, subglacial hydraulic gradients and effective pressure. If the latter is reduced sufficiently, it could trigger enhanced bed deformation, thus coupling drainage to ice sheet movement. It suggests the possibility of distinctive phases of sediment deformation and drumlin mobilisation during a glacial cycle.  相似文献   

10.
《Sedimentology》2018,65(3):931-951
Submarine leveed channels are sculpted by turbidity currents that are commonly highly stratified. Both the concentration and the grain size decrease upward in the flow, and this is a fundamental factor that affects the location and grain size of deposits around a channel. This study presents laboratory experiments that link the morphological evolution of a progressively developing leveed channel to the suspended sediment structure of the turbidity currents. Previously, it was difficult to link turbidity current structure to channel–levee development because observations from natural systems were limited to the depositional products while experiments did not show realistic morphodynamics due to scaling issues related to the sediment transport. This study uses a novel experimental approach to overcome scaling issues, which results in channel inception and evolution on an initially featureless slope. Depth of the channel increased continuously as a result of levee aggradation combined with varying rates of channel floor aggradation and degradation. The resulting levees are fining upward and the grain‐size trend in the levee matches the upward decrease in grain size in the flow. It is shown that such deposit trends can result from internal channel dynamics and do not have to reflect upstream forcing. The suspended sediment structure can also be linked to the lateral transition from sediment bypass in the channel thalweg to sediment deposition on the levees. The transition occurs because the sediment concentration is below the flow capacity in the channel thalweg, while higher up on the channel walls the concentration exceeds capacity resulting in deposition of the inner levee. Thus, a framework is provided to predict the growth pattern and facies of a levee from the suspended sediment structure in a turbidity current.  相似文献   

11.
Overbank sedimentation rates were studied in former channels of three rivers in south-eastern France. Depth and spatial distribution of sediment, as well as geometry, hydrological connectivity and age of 39 lakes, were both measured and calculated. The mean sedimentation rate of lakes varied between 0 and 2·57 cm year−1. Sedimentation rates are linked to water depth and often undergo a decreasing gradient from the downstream outlet to the inner part of the lake. Multiple regression modelling demonstrates that sediment depth is essentially a function of overbank flow frequency. The greater the difference between upstream and downstream overbank flow frequency, the faster the sedimentation rate. These differences in sedimentation rates also correspond to different former channel geometry: the rates are slower in narrow and straight channels (former braided, and point-bar backwater channels), and faster in large and sinuous channels (exhibiting meanders, anastomosing channels and coves). The suspended sediment flux is variable from one reach to another, the middle reach of the Rhône conveying more sediment than the upper reaches, the Doubs or the Ain reaches. The suspended sediment flux does not explain a statistical difference in lake sedimentation rates between the reaches, which also provide clear evidence of the importance of local connectivity controls. Sedimentation patterns were also complicated by temporal changes in lake connectivity associated with geomorphological or anthropogenic changes operating within the main channel.  相似文献   

12.
This paper uses the results of landscape evolution models and morphometric data from the Andes of northern Peru and the eastern Swiss Alps to illustrate how the ratio between sediment transport on hillslopes and in channels influences landscape and channel network morphologies and dynamics. The headwaters of fluvial- and debris-flow-dominated systems (channelized processes) are characterized by rough, high-relief, highly incised surfaces which contain a dense and hence a closely spaced channel network. Also, these systems tend to respond rapidly to modifications in external forcing (e.g., rock uplift and/or precipitation). This is the case because the high channel density results in a high bulk diffusivity. In contrast, headwaters where landsliding is an important sediment source are characterized by a low channel density and by rather straight and unstable channels. In addition, the topographies are generally smooth. The low channel density then results in a relatively low bulk diffusivity. As a consequence, response times are greater in headwaters of landslide-dominated systems than in highly dissected drainages. The Peruvian and Swiss case studies show how regional differences in climate and the litho-tectonic architecture potentially exert contrasting controls on the relative importance of channelized versus hillslope processes and thus on the overall geomorphometry. Specifically, the Peruvian example illustrates to what extent the storminess of climate has influenced production and transport of sediment on hillslopes and in channels, and how these differences are seen in the morphometry of the landscape. The Swiss example shows how the bedding orientation of the bedrock drives channelized and hillslope processes to contrasting extents, and how these differences are mirrored in the landscape. An erratum to this article can be found at  相似文献   

13.
本文主要采取历史水下地形和水位数据分析、干流河槽现场测量、室内测试和综合评价等方法对重大水利工程影响长江中下游干流河槽和岸线进行了分析和研究,取得如下新进展：（1）创新构建了一套多模态传感器系统,实现陆上和水下一体化水动力、沉积和地貌特征测量与数据采集。（2）调查研究发现,长江干流河槽冲刷强烈,岸线窝崩、条崩发育。（3）悬沙和床沙粗化,河床阻力下降,发育侵蚀型链珠状沙波,长江大桥主桥墩冲刷严重。（4）潮区界显著上移,潮区界变动河段地貌发生重要变化。在此基础上,研究认为应该加强长江中下游干流河槽、沿岸高陡岸坡、支流入汇干流河口、崩岸以及跨江大桥桥墩冲刷等调查、监测和成因机理分析。上述研究成果对长江岸滩防护和修复、航道整治、沿岸防洪、长江大桥桥墩维护等具有重要意义。  相似文献   

14.
《Applied Geochemistry》2003,18(2):195-220
Samples of fine-grained channel bed sediment and overbank floodplain deposits were collected along the main channels of the Rivers Aire (and its main tributary, the River Calder) and Swale, in Yorkshire, UK, in order to investigate downstream changes in the storage and deposition of heavy metals (Cr, Cu, Pb, Zn), total P and the sum of selected PCB congeners, and to estimate the total storage of these contaminants within the main channels and floodplains of these river systems. Downstream trends in the contaminant content of the <63 μm fraction of channel bed and floodplain sediment in the study rivers are controlled mainly by the location of the main sources of the contaminants, which varies between rivers. In the Rivers Aire and Calder, the contaminant content of the <63 μm fraction of channel bed and floodplain sediment generally increases in a downstream direction, reflecting the location of the main urban and industrialized areas in the middle and lower parts of the basin. In the River Swale, the concentrations of most of the contaminants examined are approximately constant along the length of the river, due to the relatively unpolluted nature of this river. However, the Pb and Zn content of fine channel bed sediment decreases downstream, due to the location of historic metal mines in the headwaters of this river, and the effect of downstream dilution with uncontaminated sediment. The magnitude and spatial variation of contaminant storage and deposition on channel beds and floodplains are also controlled by the amount of <63 μm sediment stored on the channel bed and deposited on the floodplain during overbank events. Consequently, contaminant deposition and storage are strongly influenced by the surface area of the floodplain and channel bed. Contaminant storage on the channel beds of the study rivers is, therefore, generally greatest in the middle and lower reaches of the rivers, since channel width increases downstream. Comparisons of the estimates of total storage of specific contaminants on the channel beds of the main channel systems of the study rivers with the annual contaminant flux at the catchment outlets indicate that channel storage represents <3% of the outlet flux and is, therefore, of limited importance in regulating that flux. Similar comparisons between the annual deposition flux of specific contaminants to the floodplains of the study rivers and the annual contaminant flux at the catchment outlet, emphasise the potential importance of floodplain deposition as a conveyance loss. In the case of the River Aire the floodplain deposition flux is equivalent to between ca. 2% (PCBs) and 36% (Pb) of the outlet flux. With the exception of PCBs, for which the value is ≌0, the equivalent values for the River Swale range between 18% (P) and 95% (Pb). The study emphasises that knowledge of the fine-grained sediment delivery system operating in a river basin is an essential prerequisite for understanding the transport and storage of sediment-associated contaminants in river systems and that conveyance losses associated with floodplain deposition exert an important control on downstream contaminant fluxes and the fate of such contaminants.  相似文献   

15.
A hierarchical typology for the channels and bars within aggradational wandering gravel-bed rivers is developed from an examination of a 50 km reach of lower Fraser River, British Columbia, Canada. Unit bars, built by stacking of gravelly bedload sheets, are the key dynamic element of the sediment transfer system, linking sediment transport during individual freshets to the creation, development and remoulding of compound bar platforms that have either a lateral or medial style. Primary and secondary unit bars are identified, respectively, as those that deliver sediment to compound bars from the principal channel and those that redistribute sediment across the compound bar via seasonal anabranches and smaller channels. The record of bar accretion evident in ground-penetrating radar sequences is consistent with the long-term development of bar complexes derived from historical aerial photographs. For two compound bars, inter-annual changes associated with individual sediment transport episodes are measured using detailed topographic surveys and longer-term changes are quantified using sediment budgets derived for individual bars from periodic channel surveys. Annual sediment turnover on the bars is comparable with the bed material transfer rate along the channel, indicating that relatively little bed material bypasses the bars. Bar construction and change are accomplished mainly by lateral accretion as the river has limited capacity to raise bed load onto higher surfaces. Styles of accretion and erosion and, therefore, the major bar form morphologies on Fraser River are familiar and consistent with those in gravelly braided channels but the wandering style does exhibit some distinctive features. For example, 65-year histories reveal the potential for long sequences of uninterrupted accretion in relatively stable wandering rivers that are unlikely in braided rivers.  相似文献   

16.
The Bosphorus Strait accommodates two‐way flow between the Aegean and Black Seas. The Aegean (Mediterranean) inflow has speeds of 5 to 15 cm sec?1 in the strait and a salinity contrast of ～12‰ to 16‰ with the Black Sea surface waters on the shelf. An anastomosed channel network crosses the shelf and in water deeper than 70 m is characterized by first‐order channels 5 to 10 m deep, local lateral accretion bedding, muddy in‐channel barforms, and a variety of sediment waves both on channel floors and bar crests, crevasse channels entering the overbank area and levée/overbank deposits which are radiocarbon‐dated in cores to be younger than ～7·5 to 8·0 ka. This channel network accommodates the saline density current formed by the Mediterranean inflow. The density contrast between the density underflow and the ambient water mass is ～0·01 g cm?3, similar to the density contrast ascribed to low‐concentration turbidity currents in the deep sea. Channel‐floor deposits are sandy to gravelly with local shell concentrations. Low‐relief bedforms on the channel floor have relatively straight crests, upflow‐dipping cross‐stratification, heights 1 to 1·5 m and wavelengths 85 to 155 m. Bankfull flows are subcritical, so these probably are not antidunes. Bar tops are ornamented locally with mudwaves having heights 1 to 2 m and wavelengths ～20 to 100 m; these are potentially antidunes formed under shallow overbank flows. Towards the shelf edge, the degree of channel bifurcation increases dramatically and bar tops are dissected locally by secondary channels, some of which terminate in hanging valleys. Conical mounds on the shelf (possibly mud volcanoes or sites of fluid seepage) interact with the channel network by promoting accretion of muddy streamlined macroforms in their lee. This channel network may be one of the largest and most accessible natural laboratories on Earth for the study of continuously flowing density currents. Although the driver is salinity contrast, the underflow transports sufficient sediment to form levée wedges and large streamlined barforms, and presumably transports sediment into deep water.  相似文献   

17.
Multicriteria decision analysis tool is used in many water resources and environmental management projects. The Malnichara is one of the natural channels in Sylhet city (Bangladesh) responsible for storm runoff conveyance to the downstream Surma river. The channel is found to be encroached at many locations of the city and found to be very vulnerable. The authority has taken decision to improve natural channels by a traditional approach, e.g. constructing box culvert. In most cases, stakeholders’ participation is ignored in such type of decision making. Hence, efforts were made to evaluate three common alternatives viz. sodding natural channel, lined natural channel and box culvert for the channel improvement. The channel is hydrologically divided into two parts: the upper portion (Choukidekhi-Kanishail) and the lower portion (Kanishail-Topoban). Both parts were separately analyzed. Small groups of stakeholders were interviewed for the selection of criteria and for the assignment of weighed factor and scores. Experts’ opinions were also taken through consultation. Nine criteria from four categories such as technical, economic, environment and social aspects were selected. The relative performances of alternatives were evaluated using the weighed sum technique of multicriteria decision analysis. It was found that the sodding natural channel is the best alternative for both portions of the channel. However, the choice is very sensitive to the social criteria.  相似文献   

18.
Engelund's (1974a, b) theoretical bed topography model for curved channels is modified and shown to fit natural channels when using values for the dynamic bed load friction coefficient of about 0·4–0·5. The dependence of this coefficient on grain size for flow in the lower flow regime is discussed. Using the bed topography model simultaneously, Allen's (1970a, b) theory for grain size variation over point bar profiles is shown to be broadly applicable. The interaction between sediment, flow and bed topography in bends is therefore adequately described.  相似文献   

19.
Pools,riffles, and channelization   总被引：2，自引：0，他引：2  
The addition of regularly spaced deeps (pools) and shallows (riffles) that provide a variety of flow conditions, areal sorting of stream-bed material, cover for wildlife, and a positive aesthetic experience, may be desirable in many channel projects. Such designs will reduce adverse environmental impacts of stream channel modifications. Analysis of variance for pool-to-pool spacing data suggests that there is no significant difference with respect to channel width between pools that form in natural streams and those in streams affected by a variety of human uses. Short of channelization, which changes the channel width, pools and riffles, within limits, are not particularly sensitive to environmental stress. Experiments in Gum Branch near Charlotte, North Carolina, support the hypothesis that channel form and process evolve in harmony and that manipulation of cross-channel morphology can influence the development of desired channel processes. Planned manipulation of its channel form induced Gum Branch to develop as desired. Morphologic stability consisting of incipient point bars, pools, and riffles was maintained over a period of high magnitude flood events, only to be degraded later by a wave of sediment derived from upstream construction and stream-bank failures. Thus, environmentally desirable channel morphology in urban streams cannot remain stable if changes in the sediment load or storm-water runoff exceed the limits of the stream's ability to make internal adjustments while maintaining morphologic stability.  相似文献   

20.
沙洲是塑造分汊型河道最重要的形态因子,其发育与蚀退由于上游来水来沙变化呈现冲淤交替,从而影响分汊河道输水输沙平衡.通过单个卵石沙洲的淤积和冲刷试验,揭示不同加沙速率、粒径和来流量条件下,沙洲淤积和冲刷规律,并建立简化理论模型分析沙洲淤积速率.结果表明,4组加沙试验中,分流点后出现明显淤积下延至洲头,左汊和右汊成为输沙通道,洲尾中心线两侧的左右汊道有泥沙淤积,洲尾未出现淤积.7组清水冲刷试验中,洲头最先承受冲刷和蚀退,并沿洲体冲刷延伸,洲头冲刷的泥沙沿左右汊水流带到下游,洲尾未出现明显冲刷.卵石沙洲以洲头淤积为主导发育模式,泥沙粒径、洲头坡角和分流角是决定淤积速率的关键因子.  相似文献   

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