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1.
Downstream hydrologic effects since the closure of Glenbawn Dam, a large dual-purpose storage for water conservation and flood mitigation, include: (i) a reduction in mean annual runoff of about 21 × 106 m3; (ii) a change in the probability distribution of mean daily flows involving a truncation of flows >;8 × 106 m3 d?1, a much reduced frequency of flows >7 × 105 m3 d?1 and an increased frequency of flows <7 × 105 m3 d?1; and (iii) a reduction in flood magnitude of at least 80 per cent for all probabilities of exceedance. From suspended sediment samples collected before and after dam closure, sediment trap efficiency has been estimated at 99 per cent.An accommodation adjustment of the channel has occurred upstream of the first unregulated tributary because the bed is armoured, the banks are well vegetated, some bedrock and concrete controls are present and all regulated releases are incompetent to transport the bed material. Immediately downstream of the first unregulated tributary, the channel has contracted by up to 45 per cent and degraded by up to 69 per cent during lateral migration. Further downstream no channel changes were recorded although the bed material has progressively coarsened over time.  相似文献   

2.
Expressing the rate of energy dissipation either within a basin or in a downstream manner is important in understanding occurrence and association between various morphological variables. This study focuses on the downstream variation in stream power and its relationship with morphological variates along the main channel of River Ona, Ibadan, Nigeria. River Ona stretches for 20.5 km downstream of Eleyele Dam. Twenty‐seven (27) points were sampled along this stretch. The 27 points were averaged into nine (9) reaches. At each cross‐section, channel characteristics were observed and measured. These include: width (W), depth (D), velocity (V), slope (S), Cross‐sectional area (A), discharge (Q), total stream power (TSP), specific stream power (SSP), width‐depth ratio (W/D), hydraulic radius (R) and wetted perimeter (P). This study confirmed that the influence of a dam present upstream of a river channel significantly alters the behaviour of slope and this in essence affects the distribution of energy along the river channel. Slope produces a logarithmic relationship with increasing distance downstream (Y = ‐32.79‐0.70InS, r = 0.70, r2 = 0.49, p < 0.05) while there was no significant relationship between Q and distance downstream.  相似文献   

3.
Although recent studies have quantified effects upon river channel morphology downstream of reservoirs, urban areas and channelization schemes, consideration has not been given to effects upon channels downstream of road bridges. Engineering literature focuses upon hydrologic considerations involving estimation of peak flow at the bridge site, and upon hydraulic aspects including backwater effects and scour around the bridge and piers. A reconnaissance survey at 19 bridges in the New Forest shows that channel shape changes below many bridges with width-depth ratios increasing substantially to double their value; that apparent channel capacities may increase below the bridge to twice their above-bridge value, due to incision and scour; and that such effects can be evident for distances downstream of the bridge of the order of 10 times channel width. Reference to large-scale topographic maps for one site, and to maps and large-scale aerial photographs for 15 sites in south central England, shows how these sources can be used to provide measurements of channel width above and below bridges and this indicates that the average downstream width is 1.99 times the upstream width. Such channel adjustments should be considered when measuring morphological and ecological characteristics of river channels and when explaining reaches of rapid channel adjustment.  相似文献   

4.
Drainage basin geometry was analyzed in the lower portion of the Big Black River, Mississippi. The study was centered on a reach of the Big Black River that encompasses an anomalous straight reach (ASR) and has morphometric characteristics that differ from those of upstream and downstream reaches. The study area was divided into three reaches, defined by alluvial valley and active floodplain width, sinuosity, and slope. Tributary streams with confluences in the three study reaches were investigated for evidence of surface tilting, and channel and valley slopes and sinuosity were measured. The average stream channel and valley slopes within the middle reach are nearly double those of the upper and lower reaches. Lateral stream migration within the tributary basins was quantitatively analyzed by measuring the asymmetry factor (AF) and transverse topographic symmetry factor (T) indices. While AF results suggest minimal to no lateral migration within the tributaries, the T results show some shifting. The results are inconclusive regarding the possible effect of neotectonic activity in the study area near the ASR. The mean southward migration may indicate a preferred migration direction relative to the general dip of the coastal plain and plunge of the Mississippi Embayment.  相似文献   

5.
Little is known about the downstream propagation of stream channel alterations initiated by urban flow regimes. In this study, freely available Light Detection and Ranging (LiDAR) data were combined with ground survey data to derive information on changes in stream cross sections in a North Carolina watershed, moving downstream across an urban-to-rural land-use boundary. Regression analyses relating LiDAR model and ground survey data exhibit poor relationships for all channel morphometric measurements except reach-average channel capacity. Application of the channel capacity regression reveals a negative power function pattern of downstream decline in channel enlargement. The largest declines occur before streams enter the rural landscape, with more gradual declines afterward. The largest declines in channel size might be attributable to reservoir effects along reaches with wide floodplains. Model results indicate no discernible enlargement once rural land covers and percentage impervious areas of about 60 percent and 16 percent of watershed area, respectively, are reached.  相似文献   

6.
Hack (1973) introduced the stream-gradient index as a measure to analyze longitudinal stream profiles. It is derived from the channel slope at a point and channel length as measured along the longest stream above that point. In the eastern United States, empirical evidence shows the stream-gradient index to be a rough approximation of stream power or competence. Validity of this relationship in other geomorphic and climatic regions was unproven. Eight canyon drainages in the Bear River Range, north-central Utah, provide a basis for testing the validity of stream-gradient index analysis in a subhumid montane area. These drainages are found to have a drainage area to discharge function, rate of increase in channel width downstream, and coverging stream network with an average length proportional to a power of the drainage similar to Hack's original study area. These similarities indicate that use of stream-gradient indices as approximations of stream power are valid in subhumid montane areas. Stream-gradient index analysis of canyon streams in the Bear River Range illustrates drainage adjustment to tectonic activity, bedrock lithology, and structural attitude.  相似文献   

7.
The Nord region of northern France, comprising the départements of the Nord and the Pas-de-Calais, epitomises the economic and social problems endured by many areas of the developed world which underwent rapid industrial growth and urbanisation in the nineteenth and early twentieth centuries but have experienced economic contraction and social depression since 1950. In 1968 the Nord region contained 3.8 million inhabitants (7.6 per cent of the French population) on 12,000 km2 (2.3 per cent of the national area) at an average density of 308 persons/km2, by comparison with the national average of 91/km2 (Roques 1969, p. 7). Eighty-one per cent of the regional population lived in urban areas, by contrast with 67 per cent nationally.  相似文献   

8.
The downstream fining of fluvial sediments is a fundamental tenet of drainage systems and, for decades, has been the subject of considerable research. Most of this research has focused on variability in channel-bed material. Other sedimentological components such as channel bars and banks, however, represent distinctively different processes occurring at various flow magnitudes and durations and thus provide an opportunity to examine a more comprehensive set of controls on the larger fluvial system. This study analyses downstream patterns of sediment size and composition for channel-bed material, bars, and banks in the Llano River watershed (11,568 km2) in central Texas, USA.Fluvial deposits in the study area were characterized through field, laboratory, and statistical analyses and standard sedimentary indices (d16, d50, d84, sorting) were computed. Two hundred thirty-eight sediment samples were collected at 15 sites along the main-stem channel with sampling occurring at the low-flow channel (thalweg), lateral bars, banks, and overbank locations. Channel-bar deposits are characterized by a downstream reduction in particle size, but low-flow-channel deposits have a substantially weaker trend, a discrepancy possibly attributed to uniformity and continuity of hydraulic sorting mechanisms during moderate and high flows. Channel-bar deposits reveal an abrupt downstream reduction in gravel size in the upper watershed, which is attributed to an increase in drainage area. Further, an abrupt gravel-to-sand transition occurs immediately downstream of a distinct lithologic change from mostly carbonate rocks to igneous and metamorphic rocks. The downstream decrease in channel-bar particle size occurs despite an increasingly constricted alluvial valley, commonly associated with greater unit stream power and relatively coarse sediment. Contrasting with channel-bed material, particle size of channel banks increases downstream, which is attributed to the addition of sand-sized sediment from igneous and metamorphic rocks. The consideration of distinctive sedimentological components of a dynamic fluvial system represents a more comprehensive and nuanced study of the topic of downstream sediment trends than prior studies, which is important to a range of engineering, biological, and planning issues at the watershed scale.  相似文献   

9.
黑河流域生态-水文过程集成研究是我国近年来干旱半干旱地区的研究热点,气候变化条件下流域生态-水文过程的响应机制是其中一个重要科学问题。基于1956-2015 年黑河流域气温、湿度、降水观测数据及多个大气环流特征量监测资料,采用小波分析方法研究了流域气象要素与大气环流特征量的周期性及遥相关。结果表明：（1）流域气温与大气环流特征量的遥相关关系最为明显,湿度、降水次之。（2）气温与Niño3.4 SSTA、SOI 的显著遥相关共同周期为12～16 a,与NAO、AO为9～11 a,与WP遥相关周期为5～9 a,与PDO为8～15 a。（3）湿度与NAO遥相关周期为4～5 a,与POL为3～6 a,与WP为9～15 a,与PNA为2～8 a。（4）降水与Niño3.4 SSTA遥相关周期为9～15 a,与NAO为2～4 a,与AO为3～7 a,与POL为11～19 a,与WP为9～14 a。（5）上述各遥相关周期的地域分布、显著时段与时滞特征各有差异。本文研究有助于加深流域气象要素对环流特征量响应机制的认识,为我国干旱半干旱地区内陆河流域严峻的水-生态问题的解决及水资源管理提供依据。  相似文献   

10.
Urban Heat Island (UHI) refers to a phenomenon whereby urban areas experience higher temperatures compared to the surrounding areas. Remote sensing-based Land Surface Temperature (LST) measurements can be utilized to measure UHI. This study emphasized on geostatistical remote sensing-based hot spot analysis ( G i * ) of UHI in Dhaka, Bangladesh as a way of examining the influences of Land Use Land Cover (LULC) on UHI from 1991 to 2015. Landsat 5 and 7 satellite-based remote sensing indices were used to explore LULC, UHI and environmental footprints during the study period. The Urban Compactness Ratio (CoR) was used to calculate the urban form and augmented characteristics. The Surface Urban Heat Island (SUHI) intensity (ΔT) was also used to explore the effects of UHI on the surrounding marginal area. Based on our investigations into LULC, we discovered that around 71.34 per cent of water bodies and 71.82 percent of vegetation cover decreased from 1991 to 2015 in Dhaka city. Contrastingly, according to CoR readings, 174.13 km2 of urban areas expanded by 249.77 per cent. Our hot spot analysis also revealed that there was a 93.73 per cent increase in hot concentration zones. Furthermore, the average temperature of the study area had increased by 3.26°C. We hope that the methods and results of this study can contribute to further research on urban climate.  相似文献   

11.
In summer 2005, a controlled flood of the Saskatchewan River (east-central Saskatchewan, Canada) resulted in general floodplain inundation and extensive natural levee deposition along a 60-km reach extending from 40 km below the E.B. Campbell dam to Cumberland Lake. Levee crests along channel banks were inundated for up to 7 weeks in some areas of the floodplain. New deposits on levee crests varied from 0 to 70 cm in thickness, displaying large variations both along reach and in opposing sites across channels. Mean grain sizes, mainly silt and very fine sand, likewise varied considerably among sample sites.Pre- and post-flood surveys of channel cross sections along the flooded reach permitted assessments of relationships between channel-area changes and patterns of levee sedimentation in this system in which virtually all new flood sediment was derived by channel scour. Results show that both net deposition and net erosion occurred within the channel cross sections, but that on average, net channel enlargement of 4.2% prevailed over the entire survey reach when weighted by cross-section size. Over the 60-km flooded reach, zones of thick levee deposition occur at or just downstream of two areas of major channel enlargement, and an intermediate zone of thin levee deposits is associated with an intermediate area of net channel aggradation. This bimodal distribution of flood-deposit thickness is inferred to have resulted from differences in sediment supply produced locally by the different extents of channel-perimeter erosion. Two other factors—(1) position of interfacial zones between clear floodbasin water and turbid channel water, and (2) difference in pre-flood levee heights—contributed to the poor correspondence in thickness and grain size between opposing levees at some sites. Additional features of the new levee deposits, including increases in transverse slopes, abrupt basinward fining, and paucity of deposition in distal areas due to clear floodbasin waters, are characteristic of strong front loading that results when suspended sediment production is restricted to channel erosion processes.  相似文献   

12.
An assessment is made of the geoecological functions of the green infrastructure for the cities of Canada by using landscape-basin approach at three spatial levels: regional, intra-urban and local. The study revealed regional and functional differences in the green infrastructure of Vancouver, Toronto and Ottawa, and determined the quantitative relationships between territories that have biospheric, recreational, foodproducing and environment-regulation functions. It is shown that the general urban (average according to the spatial scale) level develops many important attributes of configuration of the green infrastructure which come to be intimately linked to the history and modern tendencies of urban development. It is established that for the intra-urban level (Toronto), the large-area elements have the functions of a stabilization of the environment and sustainment of biodiversity, whereas area-limited fragments most often serve as recreational places of the public nearest access. We calculated the proportion of the urban area corresponding to different (according to the functions of runoff formation) types of catchment units; almost one-third of the area is represented by catchment units with no permanent streams or with a transformed drainage system. It is found that the highest percentage of forest land corresponds to the downstream portions of river basins and to areas along valleys, whereas these indicators are much lower in the upstream regions and near drainage divides. It is established that there is almost no correlation between the size of the runoff cell and its percentage of forest land and the degree of development.  相似文献   

13.
气候变化对黄河内蒙古段凌汛期的影响   总被引：2，自引：0，他引：2  
基于黄河内蒙古段1961-2009年气象与水文观测资料,选取黄河内蒙古段上游磴口站和下游托克托县作为研究对象,利用数理统计等方法,分析凌汛最危险地段上下游气候变化与凌汛期的关系。得出:①黄河内蒙古段的凌汛灾害是黄河上游的河道气温比下段高以及地形比降小等,形成黄河内蒙古段下游先封河、上游先开河的局面,导致磴口到托克托县段成为凌险最严重的地段。②随着气候变暖,近50 a黄河内蒙古段开河期提早、封河期推后,凌汛期缩短。尤其在1987年气温发生突变之后,开河期提早和推后趋势更为明显。同时气温突变之后,凌汛期平均温度变异系数增大,封、开河期冷暖变化剧烈,致使封开河期的波动性增大。③影响黄河内蒙古河段开、封河的关键因素是日平均气温小于-5 ℃的积温、日平均气温小于-5 ℃持续时间和封、开河期温度以及封、开河期流量大小等。极端温度事件如寒冷日数、冷夜指数和冷日指数的减少,以及暖夜指数和暖日指数的显著增加也是影响黄河内蒙古段凌汛期缩短的原因之一。  相似文献   

14.
Temperate Highland Peat Swamps on Sandstone (THPSS) and Coastal Upland Swamps in the Sydney Basin (CUSSB) are listed as endangered ecological communities under Australia's national and state legislation. They are threatened by a range of human and climate impacts. Across the region there are 3208 individual, valley-bottom, elongate-shaped, upland swamps that drain first- or second-order drainage lines and small catchment areas (mean?=?0.25?km2). They occur at a median distance of 57?km from the coast in areas with an average annual rainfall of 1505?mm/year and average annual temperature of 15°C. Those closer to the coast occur on elevations as low as 160?m a.s.l., and those further from the coast, on plateau country, occur up to 1172?m a.s.l. (median 634?m a.s.l.). The valleys that contain these swamps terminate downstream at a valley constriction or bedrock step, behind which alluvial materials have accumulated, and peat has formed to produce relatively steep swamps (median slope 6.2 per cent). Understanding the spatial distribution and physical attributes of these systems, and having the accompanying maps as a resource, is critical for development of sound, well-informed conservation, rehabilitation and monitoring programs, and for analysing the ecosystem services they provide.  相似文献   

15.
The planned construction of hundreds of hydroelectric dams in the Amazon basin has the potential to provide invaluable ‘clean’ energy resources for aiding in securing future regional energy needs and continued economic growth. These mega-structures, however, directly and indirectly interfere with natural ecosystem dynamics, and can cause noticeable tree loss. To improve our understanding of how hydroelectric dams affect the surrounding spatiotemporal patterns of forest disturbances, this case study integrated remote sensing spectral mixture analysis, GIS proximity analysis and statistical hypothesis testing to extract and evaluate spatially-explicit patterns of deforestation (clearing of entire forest patch) and forest degradation (reduced tree density) in the 80,000 km2 neighborhoods of the Brazil's Tucuruí Dam, the first large-scale hydroelectric project in the Amazon region, over a period of 25 years from 1988 to 2013. Results show that the average rates of deforestation were consistent during the first three time periods 1988–1995 (620 km2 per year), 1995–2001 (591 km2 per year), and 2001–2008 (660 km2 per year). However, such rate dramatically fell to half of historical levels after 2008, possibly reflecting the 2008 global economic crisis and enforcement of the Brazilian Law of Environmental Crimes. The rate of forest degradation was relatively stable from 1988 to 2013 and, on average, was 17.8% of the rate of deforestation. Deforestation and forest degradation were found to follow similar spatial patterns across the dam neighborhoods, upstream reaches or downstream reaches at the distances of 5 km–80 km, suggesting that small and large-scale forest disturbances may have been influencing each other in the vicinity of the dam. We further found that the neighborhoods of the Tucuruí Dam and the upstream region experienced similar degrees of canopy loss. Such loss was mainly attributed to the fast expansion of the Tucuruí town, and the intensive logging activities alongside major roads in the upstream reservoir region. In contrast, a significantly lower level of forest disturbance was discovered in the downstream region.  相似文献   

16.
Dams are a major source of fragmentation and degradation of rivers. Although substantial research has been conducted on the environmental impacts of large structures in the United States, smaller dams have received less attention. This study evaluated the impact of two dams of moderate size, the Elwha Dams, on the downstream channel system using field data collection at river cross‐sections. The relationship of average boundary shear stress (τo) to critical shear stress (τcr) served as the basis for determining channel bed material mobility under the two‐year and ten‐year flood events. The channel had the greatest channel bed mobility at the natural cross‐section upstream from the dams, low bed mobility between the structures, and an increase in channel bed mobility in the low gradient river segment near the mouth of the river. Low bed mobility tended to be associated with a lack of channel system complexity, including reduction or loss of bars and low alluvial terraces and their associated young riparian communities. Although these run‐of‐the‐river dams do not modify streamflow greatly, the loss of sediment from the channel system has had a substantial impact on bed mobility and geomorphic and biotic complexity of the Elwha River.  相似文献   

17.
R.M. Joeckel  G.M. Henebry   《Geomorphology》2008,102(3-4):407-418
The lower Platte River has undergone considerable change in channel and bar characteristics since the mid-1850s in four 20–25 km-long study stretches. The same net effect of historical channel shrinkage that was detected upstream from Grand Island, Nebraska, can also be detected in the lower river but differences in the behaviors of study stretches upstream and downstream from major tributaries are striking. The least relative decrease occurred downstream from the Loup River confluence, and the stretch downstream from the Elkhorn River confluence actually showed an increase in channel area during the 1940s. Bank erosion was also greater downstream of the tributaries between ca. 1860 and 1938/1941, particularly in stretch RG, which showed more lateral migration. The cumulative island area and the ratio of island area to channel area relative to the 1938/1941 baseline data showed comparatively great fluctuations in median island size in both downstream stretches. The erratic behavior of island size distributions over time indicates that large islands were accreted to the banks at different times, and that some small, newly-stabilized islands were episodically “flushed” out of the system. In the upstream stretches the stabilization of mobile bars to create new, small islands had a more consistent impact over time. Channel decrease by the abandonment of large, long-lived anabranches and by the in-place narrowing resulting from island accretion were more prominent in these upstream stretches. Across all of the study area, channel area appears to be stabilizing gradually as the rate of decrease lessens. This trend began earliest in stretch RG in the late 1950s and was accompanied by shifts in the size distributions of stabilized islands in that stretch into the 1960s. Elsewhere, even in the easternmost study stretch, stabilizing was occurring by the late 1960s, the same time frame documented by investigations of the Platte system upstream of the study area. Comprehensive management plans for the lower Platte River should account, at least in theory, for the observed differences in stream behavior upstream and downstream of the major eastern tributaries.  相似文献   

18.
Urban stream channel hazards   总被引：4，自引：0，他引：4  
Ken J Gregory  & Anne Chin 《Area》2002,34(3):312-321
In managing urban stream channels there are pressures to use soft engineering techniques to restore channels wherever possible, to undertake management within a drainage basin context, to produce sustainable solutions and to consider community views. However, specific methods for characterizing the channel network in terms of possible management options have not been developed explicitly for urban areas. A method of characterizing the entire drainage network of urban areas, based upon segmentation of the stream channel network according to the incidence of road crossings and stormwater outfalls, is proposed together with consideration of ways in which the segments can be characterized, including stream channel hazards as a means of providing one basis for urban channel management.  相似文献   

19.
Extensive valley fills have formed at the base of the escarpment in granitic catchments along the south coast of NSW. On the 1865 portion plan, the valley fill surface in the upper part of Wolumla Creek, in the Bega River catchment, was intact, but within a few decades of European settlement of the area the valley fill had been incised. Today the incised channel is up to 10 m deep and 100 m wide. The catchment drains an area of just 18.2 km2. Based on detailed field mapping, with extensive drilling and angering, the volume of the intact valley fill in upper Wolumla Creek in 1865 was approximately 5000 × 103 m3. Between 1865 and the present day, approximately 3500 × 103 m3 of this material has been removed, leaving roughly 1500× 103 m3 of material stored on the valley margins. During an initial period of discontinuous gullying, approximately 230 × 103 m3 of sand accumulated as a floodout. Subsequently, the incised channel became continuous, cutting through the floodout; over 50 per cent of floodout deposits were removed. Flushing of the materials released from upland valley fills has been very efficient in the Wolumla Creek catchment, with a sediment delivery ratio of around 70 per cent. The efficient downstream transfer of deposits reflects bedrock confinement in downstream reaches. Extensive volumes of material have accumulated along the lower reaches of the catchment, exacerbating the transformation to the geomorphic character of the lower Bega River.  相似文献   

20.
This study determines the spatial and temporal variability of in-channel storage within a small semiarid drainage basin in equatorial East Africa, and establishes a tentative sediment budget for coarse (>200 μm) in-channel sediments. Detailed measurements of in-channel sediment storage (mass) within third and fourth-order ephemeral channels were obtained using channel-pit excavations and probing with metal rods. Eighty-seven monumented cross-sections were established in February 1986 and resurveyed in December 1986, following the last runoff event of the year. These provided data on change in sediment storage on a 30-m channel reach basis. In addition, measurements of bankfull channel width, mean depth, cross-sectional area, wetted perimeter, hydraulic radius, channel slope and distance from the basin outlet were measured at each cross section. Total in-channel sediment storage was approximately 8640 t with 83% of this total stored within the Main (fourth-order) Channel. Stepwise multiple regression of In-transformed data indicated that bankfull channel width and distance from the outlet (which is strongly related with slope) were significantly related to in-channel storage. The variation in the ratio of stream power:critical power along the Main Channel may explain the distribution of in-channel sediments. Net aggradation of 50 to 60 t during 1986 was minor in relation to the total in-channel storage reservoir, but indicates that a static equilibrium condition cannot be assumed. Bedload output during 1986 was approximately 125 t, and the computed input of coarse sediments to the major channels within the basin was approximately 185 t. The sediment delivery ratio for the coarse material was approximately 68%, which indicates a relatively efficient transport system. [Key words: geomorphology, sediment budget, in-channel sediment storage, semiarid, drainage basin.]  相似文献   

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