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1.
In a mountain environment, the transport of coarse material is a key factor for many fields such as geomorphology, ecology, hazard assessment, and reservoir management. Despite this, there have been only a few field investigations of bedload, in particular using multiple monitoring methods. In this sense, attention has frequently focused on the effects of “high magnitude/low frequency floods” rather than on “ordinary events”. This study aims to analyze the sediment dynamics triggered by three high-frequency floods (recurrence interval “RI” between 1.1 and 1.7 yr) that occurred in the Rio Cordon basin during 2014. The flood events were investigated in terms of both sediment mobility and bedload magnitude. The Rio Cordon is an Alpine basin located in northeastern Italy. The catchment has a surface area of 5 km2, ranging between 1763 and 2763 m above sea level. The Rio Cordon flows on an armored streambed layer, with a stable step-pool configuration and large boulders. Since 1986, the basin has been equipped with a permanent station to continuously monitor water discharge and sediment flux. To investigate sediment mobility, 250 PIT-tags were installed in the streambed in 2012. The 2014 floods showed a clear difference in terms of tracer displacement. The near-bankfull events showed equal mobility conditions, with mean travel distance one order of magnitude higher than the below-bankfull event. Furthermore, only the near-bankfull events transported coarse material to the monitoring station. Both events had a peak discharge up to 2.06 m3 s-1, but the bedload transport rates differed by more than one order of magnitude, proving that under the current supply-limited condition, the bedload appears more related to the sediment supply than to the magnitude of the hydrological features. In this sense, the results demonstrated that near-bankfull events can mobilize large amounts of material for long distances, and that floods of apparently similar magnitude may lead to different sediment dynamics, depending on the type and amount of sediment supply.  相似文献   

2.
In a small experimental catchment of the Dolomites (Rio Cordon, 5 km2) field observations have been carried out on the movement of various sized bed material particles. Displacement length of 860 marked pebbles, cobbles and boulders (0·032 < D < 0·512 m) has been measured along the river bed during individual snowmelt and flood events in the periods 1993–1994 and 1996–1998. Floods were grouped into two categories. The first includes ‘ordinary’ events, which are characterized by peak discharges with a return period of 1–5 years and by an hourly bedload rate not exceeding 20 m3 h?1. The second refers to ‘exceptional’ events with a return period of 50–60 years. A flood of this latter type occurred on 14 September 1994, with a peak discharge of 10·4 m3 s?1 and average hourly bedload rate of 324 m3 h?1. The variation according to grain size of total displacement length Li depends on the degree of mobilization of the individual fractions of the bed surface: Li is independent of Di for smaller, fully mobile grain sizes and decreases rapidly with Di for larger fractions in a state of partial transport. Sustained selective transport without a supply of sediment from upstream leads to the development of a stable coarse armoured surface through progressive winnowing of finer material from the bed surface. With supply unlimited conditions for transport, both the occurrence of extreme events and the duration of a sequences of ‘ordinary’ floods play an important role in the degree of mobilization of the individual fractions of the bed. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

3.
The mobility conditions of bedload transport in an alpine high‐gradient step–pool stream (Rio Cordon) are analysed. Since 1986, a device system at the downstream end section of the stream has been operating in order to monitor the water discharge, suspended sediment and bedload transport. Sediment distribution of bedload transported by various floods has been analysed, and equal‐mobility evidence is recognized only for the high‐magnitude flows ever recorded (RI > 50 years). The thresholds for size‐selective and equal‐mobility transport conditions are identified and quantified by using both data provided by the fractional transport rate and by length displacements of marked particles. Size‐selective bedload transport seems to dominate when the critical shear stress of the size fractions τci considered is exceeded, whereas the equal‐mobility condition is approached as levels of excess shear stress become higher (τeqi = 1·45τci). Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

4.
This paper provides comprehensive evidence that sediment routing around pools is a key mechanism for pool‐riffle maintenance in sinuous upland gravel‐bed streams. The findings suggest that pools do not require a reversal in energy for them to scour out any accumulated sediments, if little or no sediments are fed into them. A combination of clast tracing using passive integrated transponder (PIT) tagging and bedload traps (positioned along the thalweg on the upstream riffle, pool entrance, pool exit and downstream riffle) are used to provide information on clast pathways and sediment sorting through a single pool‐riffle unit. Computational fluid dynamics (CFD) is also used to explore hydraulic variability and flow pathways. Clast tracing results provide a strong indication that clasts are not fed through pools, rather they are transported across point bar surfaces, or around bar edges (depending upon previous clast position, clast size, and event magnitude). Spatial variations in bedload transport were found throughout the pool‐riffle unit. The pool entrance bedload trap was often found to be empty, when the others had filled, further supporting the notion that little or no sediment was fed into the pool. The pool exit slope trap would occasionally fill with sediment, thought to be sourced from the eroding outer bank. CFD results demonstrate higher pool shear stresses (τ ≈ 140 N m–2) in a localized zone adjacent to an eroding outer bank, compared to the upstream and downstream riffles (τ ≈ 60 N m–2) at flows of 6 · 2 m3 s–1 (≈ 60% of the bankfull discharge) and above. There was marginal evidence for near‐bed velocity reversal. Near‐bed streamlines, produced from velocity vectors indicate that flow paths are diverted over the bar top rather than being fed through the thalweg. Some streamlines appear to brush the outer edge of the pool for the 4 · 9 m3 s–1 to 7 · 8 m3 s–1 (between 50 and 80% of the bankfull discharge) simulations, however complete avoidance was found for discharges greater than this. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

5.
The transfer of sediment through a highly regulated large fluvial system (lower Ebro River) was analysed during two consecutive floods by means of sediment sampling. Suspended sediment and bedload transport were measured upstream and downstream of large reservoirs. The dams substantially altered flood timing, particularly the peaks, which were advanced downstream from the dams for flood control purposes. The suspended sediment yield upstream from the dams was 1 700 000 tonnes, which represented nearly 99 per cent of the total solid yield. The mean concentrations were close to 0·5 g l?1. The sediment yield downstream from the dams was an order of magnitude lower (173 000 tonnes), showing a mean concentration of 0·05 g l?1. The dams captured up to 95 per cent of the fine sediment carried in suspension in the river channel, preventing it from reaching the lowermost reaches of the river and the delta plain. Total bedload transport upstream from the dams was estimated to be about 25 000 tonnes, only 1·5 per cent of the total load. The median bedload rate was 100 gms?1. Below the dams, the river carried 178 000 tonnes, around 51 per cent of the total load, at a mean rate of 250 g ms?1. The results of sediment transport upstream and downstream from the large dams illustrate the magnitude of the sediment deficit in the lower Ebro River. The river mobilized a total of 350 000 tonnes in the downstream reaches, which were not replaced by sediment from upstream. Therefore, sediment was necessarily entrained from the riverbed and channel banks, causing a mean incision of 33 mm over the 27 km long study reach, altogether a significant step towards the long‐term degradation of the lower Ebro River. Copyright © 2005 John Wiley & Sons, Ltd.  相似文献   

6.
Sediment loads have been measured in six Swiss mountain torrents over several decades. Most of these torrent catchments are situated in the prealpine belt. They have catchment areas of between 0·5 and 1·7 km2. Bedslopes at the measuring sites vary between 5 and 17 per cent, and peak discharges up to 12 m3 s−1 have been recorded. Geophone sensors installed in the Erlenbach stream allow bedload transport activity to be monitored and sediment volumes associated with each flood event to be determined. A detailed analysis of the measurements in this stream results in an empirical equation in which the sediment load per flood event is expressed as a function of the effective runoff volume (discharges above the threshold for bedload motion) and of the normalized peak discharge. For the total of 143 investigated flood events in the Erlenbach stream, the deviation of the predicted from the measured value is within a factor of two for more than two-thirds of all events. A distinction can be made between summer and winter events in analysing the bedload transport events. The summer events, mainly caused by thunderstorms, transport comparatively larger sediment loads than the winter events. For the other investigated streams, the periods of the deposited sediment volume surveys cover in general several flood events. An analysis is performed analogous to that for the Erlenbach stream. The sediment loads show a similar dependency on the two factors effective runoff volume and normalized peak discharge. However, the exponents of these factors in the power law expressions differ from stream to stream. A comparison of the investigated stream shows that some of the variation can be explained by considering the bedslope above the measuring site. The inclusion of a bedslope factor is in agreement with laboratory investigations on bedload transport. © 1997 John Wiley & Sons, Ltd.  相似文献   

7.
Raise Beck is a mountain torrent located in the central Lake District fells, northern England (drainage area of 1·27 km2). The torrent shows evidence of several major flood events, the most recent of which was in January 1995. This event caused a major channel avulsion at the fan apex diverting the main flood flow to the south, blocking the A591 trunk road and causing local flooding. The meteorological conditions associated with this event are described using local rainfall records and climatic data. Records show 164 mm of rainfall in the 24 hours preceding the flood. The peak flood discharge is reconstructed using palaeohydrological and rainfall–runoff methods, which provide discharge values of 27–74 m3 s?1, and 4–6 m3 s?1, respectively. The flood transported boulders with b‐axes up to 1400 mm. These results raise some important general questions about flood estimation in steep mountain catchments. The geomorphological impact of the event is evaluated by comparing aerial photographs from before and after the flood, along with direct field observations. Over the historical timescale the impact and occurrence of flooding is investigated using lichenometry, long‐term rainfall data, and documentary records. Two major historical floods events are identified in the middle of the nineteenth century. The deposits of the recent and historical flood events dominate the sedimentological evidence of flooding at Raise Beck, therefore the catchment is sensitive to high magnitude, low frequency events. Following the 1995 flood much of the lower catchment was channelized using rip‐rap bank protection, re‐establishing flow north towards Thirlmere. The likely success of this management strategy in containing future floods is considered, based on an analysis of channel capacities. It is concluded that the channelization scheme is only a short‐term solution, which would fail to contain the discharge of an event equivalent to the January 1995 flood. Copyright © 2002 John Wiley & Sons, Ltd.  相似文献   

8.
On 29 August, 2003, an intense convective storm system affected the Fella River basin, in the eastern Italian Alps, producing rainfall peaks of approximately 390 mm in 12 h. The storm triggered an unusually large debris flow in the ungauged Rio Cucco basin (0·65 km2), with a volume of approximately 78 000 m3. The analysis of the time evolution of the rainstorm over the basin has been based on rainfall estimates from radar observations and data recorded by a raingauge network. Detailed geomorphological field surveys, carried out both before and after the flood of August 2003, and the application of a distributed hydrological model have enabled assessment of flood response, estimation of erosion volumes and sediment supply to the channel network. The accounts of two eyewitnesses have provided useful elements for reconstructing the time evolution and the flow processes involved in the event. Liquid peak discharge estimates cluster around 20 m3 s?1 km?2, placing this event on the flood envelope curve for the eastern Italian Alps. The hydrological analysis has shown that the major controls of the flood response were the exceptional cumulated rainfall amount, required to exceed the large initial losses, and the large rainfall intensities at hourly temporal scales, required to generate high flood response at the considered basin scale. Observations on the deposits accumulated on the alluvial fan indicate that, although the dominant flow process was a debris flow, sheetflood also contributed to fan aggradation and fluvial reworking had an important role in winnowing debris‐flow lobes and redistributing sediment on the fan surface. This points out to the large discharge values during the recession phase of the flood, implying an important role for subsurface flow on runoff generation of this extreme flash flood event. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

9.
Effects of sediment load on hydraulics of overland flow on steep slopes   总被引：6，自引：0，他引：6  
Eroded sediment may have significant effects on the hydraulics of overland flow, but few studies have been performed to quantify these effects on steep slopes. This study investigated the potential effects of sediment load on Reynolds number, Froude number, flow depth, mean velocity, Darcy–Weisbach friction coefficient, shear stress, stream power, and unit stream power of overland flow in a sand‐glued hydraulic flume under a wide range of hydraulic conditions and sediment loads. Slope gradients were varied from 8·7 to 34·2%, unit flow rates from 0·66 to 5·26×10?3 m2 s?1, and sediment loads from 0 to 6·95 kg m?1 s?1. Both Reynolds number (Re) and Froude number (Fr) decreased as sediment load increased, implying a decrease in flow turbulence. This inverse relationship should be considered in modeling soil erosion processes. Flow depth increased as sediment load increased with a mean value of 1·227 mm, caused by an increase in volume of sediment‐laden flow (contribution 62·4%) and a decrease in mean flow velocity (contribution 37·6%). The mean flow velocity decreased by up to 0·071 m s?1 as sediment load increased. The Darcy–Weisbach friction coefficient (f) increased with sediment load, showing that the total energy consumption increased with sediment load. The effects of sediment load on f depended on flow discharge: as flow discharge increased, the influence of sediment load on f decreased due to increased flow depth and reduced relative roughness. Flow shear stress and stream power increased with sediment load, on average, by 80·5% and 60·2%, respectively; however, unit stream power decreased by an average of 11·1% as sediment load increased. Further studies are needed to extend and apply the insights obtained under these controlled conditions to real‐world overland flow conditions. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

10.
This paper describes and analyses a hillslope–channel slope failure event that occurred at Wet Swine Gill, Lake District, northern England. This comprised a hillslope slide (180 m3, c. 203 ± 36 t), which coupled with the adjacent stream, resulting in a channelized debris flow and fluvial flood. The timing of the event is constrained between January and March 2002. The hillslope failure occurred in response to a rainfall/snowmelt trigger, on ground recently disturbed by a heather moorland fire and modified by artificial drainage. Slide and flow dynamics are estimated using reconstructed velocity and discharge values along the sediment transfer path. There is a rapid downstream reduction in both maximum velocity, from 9·8 to 1·3 m s?1; and maximum discharge, ranging from 33·5 to 2·4 m3 s?1. A volumetric sediment budget quantified a high degree of coupling between the hillslope and immediate channel (～92%: 167 m3), but virtually all of the sediment was retained in the first‐order tributary channel. Approximately 44% (81 m3) of the slide volume was retained in the run‐up deposit, and termination of the debris flow prior to the main river meant that the remainder did not discharge into the fluvial system downstream. These results suggest poor transmission of sediment to the main river at the time of the event, but importantly an increase in available material for post‐event sediment transfer processes within the small upland tributary. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

11.
Comparatively little is known about the hydrology of desert flash-floods despite the extent of the world's drylands. There is even less known about their sedimentary behaviour and particularly about the movement of coarse material as bedload. The results of an intense field monitoring programme carried out on an ephemeral gravel-bed stream in the northern Negev Desert are presented. In this semi-arid setting, flow duration analysis indicates that the channel is hydrologically active for 2% of the time, or about seven days per year, and that overbank flow can be expected for only 0·03% of the time—about three hours per year. Multipeaked flood hydrographs are the norm, reflecting many factors including the arrival of separate slugs of discharge from contributing subcatchments. The passage of the initial flood bore is surprisingly slow, but the rising limb of the flood hydrograph is rapid with a median time of rise of 10 minutes, in keeping with expected flash-flood behaviour. Bedload flux is high, averaging 2·67 kg s−1 m−1 during the period that the channel carries flow. This gives very high bedload sediment yield despite the infrequent and short duration of flood flows and matches the high yield of suspended sediment. The relationship between bedload flux and boundary shear stress is simple, in contrast with perennial gravel-bed streams, and the exponent of the log–log relationship is 1·52. Of great value is that the behaviour of the Nahal Eshtemoa corroborates a pattern established by the authors previously in a smaller tributary stream. © 1998 John Wiley & Sons, Ltd.  相似文献   

12.
Geomorphological evidence and recent trash lines were used as stage indicators in a step-backwater computer model of high discharges through an ungauged bedrock channel. The simulation indicated that the peak discharge from the 26.7 m2 catchment was close to 150m3s?1 during the passage of Hurricane Charlie in August 1986. This estimate can be compared with an estimate of 130–160 m3s?1 obtained using the Flood Studies Report (FSR) unit hydrograph methodology. Other palaeostage marks indicate that higher stages have occurred at an earlier time associated with a discharge of 200 m3s?1. However, consideration of both the geometry of a plunge pool and transport criteria for bedrock blocks in the channel indicates that floods since 1986 have not exceeded 150 m3s?1. Given that the estimated probable maximum flood (PMF) calculated from revised FSR procedure is at least 240 m3s?1, it is concluded that compelling evidence for floods equal to the PMF is lacking. Taking into consideration the uncertainty of the discharge estimation, the 1986 flood computed using field evidence has a minimum return period of 100 years using the FSR procedure. This may be compared with a return period for the same event in the neighbouring gauged River Greta of > 100 years and a rainfall return period of 190 years. In as much as discharges of similar order to FSR estimates are indicated, it is concluded (a) that regional geomorphological evidence and flood simulation within ungauged catchments may be useful as a verification for hydrological estimates of recent widespread flood magnitude and (b) that palaeohydraulic computation can be useful in determining the magnitude of the local maximum [historic] flood when determining design discharges for hydraulic structures within specific catchments.  相似文献   

13.
Several methods were employed in the Ardennian rivers (Belgium) to determine the depth of the active layer mobilized during floods and to evaluate the bedload discharge associated with these events. The use of scour chains has shown that the depth of the active layer is systematically less than the b‐axis of the average particle size (D50) of the elements which compose the surface layer of the riffles. This indicates that only a partial transport exists during low magnitude floods. The bedload discharge has been evaluated by combining data obtained using the scour chains technique and the distance covered by tracers. Quantities of sediment transported during frequent floods are relatively low (0·02 t km–2) due to the armour layer which protects the subsurface material. These low values are also related to the fact that the distance calculated for mobilized bedload only applies to tracers fitted with PIT (passive integrated transponder)‐tags (diameter > 20 mm), whereas part of the bedload discharge is composed of sand and fine gravel transported over greater distances than the pebbles. The break‐up of the armour layer was observed only once, for a decennial discharge. During this event, the bedload discharge increased considerably (2 t km–2). The use of sediment traps, data from dredging and a Helley–Smith sampler confirm the low bedload transport in Ardennian rivers in comparison to the bedload transport in other geomorphological contexts. This difference is explained by the presence of an armoured layer but also by the imbricated structures of flat bed elements which increase the resistance to the flow. Finally, the use of the old iron industry wastes allowed to quantify the thickness of the bed reworked over the past centuries. In the Lembrée River, the river‐bed contains slag elements up to a depth of about 50 cm, indicating that exceptional floods may rework the bed to a considerable depth. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

14.
In this paper, we present a methodology to construct a sediment budget for meso‐scale catchments. We combine extensive field surveys and expert knowledge of the catchment with a sediment delivery model. The meso‐scale Mediterranean drainage basin of the Dragonja (91 km2), southwest Slovenia, was chosen as case study area. During the field surveys, sheet wash was observed on sloping agricultural fields during numerous rainfall events, which was found to be the main source of sediment. With the sediment yield model WATEM/SEDEM the estimated net erosion on the hillslopes 4·1 t ha–1 y–1 (91% of inputs). The second source, bank erosion (4·2%; 0·25 t ha–1 y–1) was monitored during several years with erosion pins and photogrammetric techniques. The last source, channel incision, was derived from geomorphological mapping and lichenomery and provided 3·8% (0·17 t ha–1 y–1) of the sediment input. The river transports its suspended sediment mainly during high‐flow events (sampled with automated water samplers). About 27% (1·2 t ha–1 y–1) of the sediment delivered to the channel is deposited on floodplains and low terraces downstream (estimated with geomorphological mapping, coring and cesium‐137 measurements). The sediment transported as bedload disintegrates during transport to the outlet due to the softness of the bedrock material. As a result, the river carries no bedload when it reaches the sea. The results imply a build‐up of sediment in the valleys catchment. However, extreme flood events may flush large amounts of sediment stored in the lower parts of the system. Geomorphological evidence exists in the catchment that such high magnitude, low frequency events have happened in the past. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

15.
Floods have become increasingly important in fluvial export of water, sediment and carbon (C). Using high-frequency sampling, the export of water, sediment and C was examined in the Wuding River catchment on the Chinese Loess Plateau. With groundwater as an important contributor to runoff all year round, floods were relatively less important in the export of water. However, large floods were disproportionately important in exporting sediment and inorganic C (DIC) and organic C (DOC and POC). The three largest floods in each year transported 53.6–97.3 and 41.4–77% of the annual sediment and C fluxes, respectively. An extreme flood in 2017 alone contributed 94.6 and 73.1% of the annual sediment and C fluxes, respectively, in just 7 days, which included 20.3, 92.1 and 35.7% of the annual DOC, POC and DIC fluxes, respectively. A stable carbon isotope (δ13C) analysis of POC indicated that modern soils and C3 plants were its primary source. Furthermore, floods greatly accelerated CO2 degassing due to elevated gas transfer velocity, although stream water CO2 partial pressure (pCO2) exhibited a decreasing trend with flow discharge. Although these results illustrated that increasing runoff diluted pCO2, the timing and magnitude of floods were found to be critical in determining the response of pCO2 to flow dynamics. Low-magnitude floods in the early wet season increased pCO2 because of enhanced organic matter input, while subsequent large floods caused a lower pCO2 due to greatly reduced organic matter supply. Finally, continuous monitoring of a complete flood event showed that the CO2 efflux during the flood (2348 ± 664 mg C m–2 day–1) was three times that under low-flow conditions (808 ± 98 mg C m–2 day–1). Our study suggests that infrequent, heavy storm events, which are predicted to increase under climate change, will greatly alter the transport regimes of sediment and C. © 2020 John Wiley & Sons, Ltd.  相似文献   

16.
This study concerns the problem of water erosion in the Sahel. Surface water and sediment yields (suspended matter and bedload) were monitored for 3 years (1998–2000) at the outlet of a small grazed catchment (1·4 ha) in the northern part of Burkina Faso. The catchment consists of about 64% sandy deposits (DRY soil surface type), which support most of the vegetation, and about 34% of crusted bare soils (ERO soil surface type). The annual solid‐matter export is more than 90% suspended sediment, varying between 4·0 and 8·4 t ha?1. The bedload represents less than 10% of soil losses. In a single flood event (10 year return period), the sediment yield can reach 4·2 t ha?1. During the period studied, a small proportion (20 to 32%) of the floods was thus responsible for a large proportion (80%) of the solid transport. Seasonal variation of the suspended‐matter content was also observed: high mean values (9 g l?1) in June, decreasing in July and stabilizing in August (between 2 and 4 g l?1). This behaviour may be a consequence of a reorganization of the soil surfaces that have been destroyed by trampling animals during the previous long dry season, vegetation growth (increase in the protecting effect of the herbaceous cover) and, to a lesser extent, particle‐supply limitation (exhaustion of dust deposits during July). The particle‐size distribution in the suspended matter collected at the catchment outlet is 60% made up of clay: fraction ≤2 µ m. The contribution of this clay is maximum when the water rises and its kaolinite/quartz ratio is then close to that of the ERO‐type surfaces. This indicates that these surfaces are the main source of clay within the catchment. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

17.
《水文科学杂志》2013,58(6):899-915
Abstract

The results are described of 16 years operation of a measuring station for the automatic recording of water discharge, bed load and suspended sediment transport in the Rio Cordon catchment, a small alpine basin (5 km2) located in northeastern Italy. Hillslope erosion processes were investigated by surveying individual sediment sources repeatedly. Annual and seasonal variations of suspended sediment load during the period 1986–2001 are analysed along with their contribution to the total sediment yield. The results show that suspended load accounted for 76% of total load and that most of the suspended sediment transport occurred during two flood events: an extreme summer flash flood in September 1994 (27% of the 16-years total suspended load) and a snowmelt-induced event in May 2001 accompanied by a mud flow which fed the stream with sediments. The role of active sediment source areas is discussed in relation to the changes in flood peak—suspended load trends which became apparent after both the 1994 and the 2001 events.  相似文献   

18.
The Manning equation is one of the most widely used formulae for calculating the velocity of shallow overland flow in hydrological and erosion models. Precise estimation of the Manning's friction coefficient (n) is critical to determining overland flow and soil erosion processes. Few studies have been conducted to quantify the effects of sediment load on Manning's n on steep slopes. This study was conducted to investigate the potential effects of sediment load on Manning's n in a flume with a fixed bed, under wide ranges of hydraulics and sediment loads. Slope gradient varied from 8·7 to 34·2%, unit flow rate from 0·66 to 5·26 × 10?3 m2 s?1, and sediment load from 0 to 6·95 kg m?1 s?1. The Reynolds number ranged from 350 to 5899. Results showed that Manning's n varied in both sediment‐free and sediment‐laden flows ranging from 0·012 to 0·055. The apparent Manning's coefficients of sediment‐laden flow were much greater than those of sediment‐free flow. The mean Manning coefficient of sediment‐laden flow was 51·27% greater than the mean value of sediment‐free flow. For sediment‐laden flow, Manning's n could be estimated with a power function of unit flow discharge and sediment content. Further studies are needed to quantify the potential effects of sediment load on the Manning's n on erodible beds and in fields. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

19.
Discharge characteristics in six adjacent mountainous watersheds in northern New Mexico, U.S.A., vary substantially between basins underlain by different lithologies. Relatively resistant gneisses and granites underlie two basins (drainage areas: 43 and 94 km2) that have high unit discharge (0·010 to 0·14 m3s?1 km?2), high bankfull discharge, and sustained high discharge. Less resistant sandstones and shales underlie four basins (drainage areas: 96 to 215 km2) that have relatively low unit discharge (0·001 to 0·005 m3s?1 km?2), relatively low bankfull discharge, and peak discharges that are not sustained as long as those in the crystalline terrane. Analysis of snowmelt-runoff water budgets suggests that three factors control hydrologic conditions in the basins. First, area-elevation distributions appear to control the timing and amounts of water input. These distributions probably reflect the erosional resistance of the different lithologies. Second, lithology appears to control runoff production in areas having minor amounts of storage. Third, glacial deposits in headwater regions control discharge duration and timing via storage and return flow releases. The amount of return flow released by glacial deposits, however, is probably controlled by the permeability of underlying bedrock. Therefore it appears that the duration, timing, and magnitude of discharge events in the study area are controlled both directly and indirectly by lithology. Stream power and shear stress estimates derived from bankfull discharge and bed-material size data suggest that higher bedload transport rates and larger bedload particle sizes exist in streams draining crystalline rocks than in streams draining sedimentary terrane. It appears that source-area lithology, by controlling discharge production, also influences stream power, bedload transport capabilities, and therefore total amounts of bedload transport.  相似文献   

20.
In much of western United States destructive floods after wildfire are frequently caused by localized, short‐duration convective thunderstorms; however, little is known about post‐fire flooding from longer‐duration, low‐intensity mesoscale storms. In this study we estimate and compare peak flows from convective and mesoscale floods following the 2012 High Park Fire in the ungaged 15.5 km2 Skin Gulch basin in the northcentral Colorado Front Range. The convective storm on 6 July 2012 came just days after the wildfire was contained. Radar data indicated that the total rainfall was 20–47 mm, and the maximum rainfall intensities (upwards of 50 mm h?1) were concentrated over portions of the watershed that burned at high severity. The mesoscale storm on 9–15 September 2013 produced 220–240 mm of rain but had maximum 15‐min intensities of only 25–32 mm h?1. Peak flows for each flood were estimated using three independent techniques. Our best estimate using a 2D hydraulic model was 28 m3 s?1 km?2 for the flood following the convective storm, placing it among the largest rainfall‐runoff floods per unit area in the United States. In contrast, the flood associated with the mesoscale flood was only 6 m3 s?1 km?2, but the long‐duration flood caused extensive channel incision and widening, indicating that this storm was much more geomorphically effective. The peak flow estimates for the 2013 flood had a higher relative uncertainty and this stemmed from whether we used pre‐ or post‐flood channel topography. The results document the extent to which a high and moderate severity forest fire can greatly increase peak flows and alter channel morphology, illustrate how indirect peak flow estimates have larger errors than is generally assumed, and indicate that the magnitude of post‐fire floods and geomorphic change can be affected by the timing, magnitude, duration, and sequence of rainstorms. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

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