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1.
Erosion from logging road surfaces, cut slopes, banks, and ditches represents a chronic source of sediment input to streams that can degrade aquatic habitats. Road surface erosion is of particular concern because the magnitude of sediment generation when traffic levels are high can be large. Current models for predicting sediment production from roads require information on area‐specific sediment delivery, which is not often available. Here, we developed a model to quantify suspended sediment concentrations (SSC) generated by forest roads surfaces under different conditions of use and density. This model is designed for a typical medium‐size coastal watershed of British Columbia or the American Pacific Northwest, and was applied to the Chilliwack River watershed as a case study. The results illustrate that intensive use of forest roads combined with high road density can increase the number of extreme sedimentation events over a predetermined threshold. A comparison of the effects of road density and the level of road use suggests that the level of road use is more important than the road density for the generation of fine sediment from road surfaces. However, the model omits the impact of roads on mass movements in a watershed, which represent a major source of sediment in steep watersheds, so the effect of road density is likely more substantial than the model predicts. The model is an attempt to overcome field data limitations by using an empirical relation between SSC and traffic variables, and presents a starting point for more intensive field studies that could be used to validate it. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

2.
Approximately 80% of the road network in Brazil is unpaved and shows evidences of a high erosion potential. In the semi-arid Caatinga Biome in the northeast of the country, a monitoring programme has been done for two years in order to analyze runoff and sediment production from unpaved rural road-ways and from embankments. Sediment production ranged from 0.30 to 0.92 Mg/ha yr, higher than in undisturbed areas, but generally lower than that reported for unpaved roads in other regions. However, this is a semi-arid area with low rainfall and runoff and, hence, with a limited hydrological connectivity and sediment production. Sediment production on an embankment with no vegetation was around ten times higher than on an embankment with vegetation. On the road surface, annual sediment production (normalized for gradient) in a section with traffic was three times higher than for a road surface without traffic. In addition, events that occurred after roadway maintenance activities generated peaks of sedi-ment concentration of over 5000 mg/L. These results suggest that sediment production from roads and embankments with bare surfaces is at least one order of magnitude higher than in undisturbed catch-ment areas. Maintenance activity and vehicle traffic contribute to an increase in sediment availability and impact on the sediment concentration, but less intensely on sediment loads, which depend on the runoff magnitude of the events occurring after roadway maintenance. It was also found that the natural vegetation of the semi-arid region potentially captures sediment on roadway embankments;thereby playing an important role in breaking connectivity between the sediment flow from unpaved roads and the natural drainage system of the catchment.  相似文献   

3.
Previous studies have identified unpaved roads as the primary source of erosion on St John in the US Virgin Islands, but these studies estimated road erosion rates only as annual averages based primarily on road rill measurements. The goal of this project was to quantify the effect of unpaved roads on runoff and sediment production on St John, and to better understand the key controlling factors. To this end runoff and sediment yields were measured from July 1996 to March 1997 from three plots on naturally vegetated hillslopes, four plots on unpaved road surfaces and two cutslope plots. Sediment yields were also measured from seven road segments with contributing areas ranging from 90 to 700 m2. With respect to the vegetated plots, only the two largest storm events generated runoff and there was no measurable sediment yield. Runoff from the road surface plots generally occurred when storm precipitation exceeded 6 mm. Sediment yields from the four road surface plots ranged from 0·9 to 15 kg m−2 a−1, and sediment concentrations were typically 20–80 kg m−3. Differences in runoff between the two cutslope plots were consistent with the difference in upslope contributing area. A sprinkler experiment confirmed that cross‐slope roads intercept shallow subsurface stormflow and convert this into surface runoff. At the road segment scale the estimated sediment yields were 0·1 to 7·4 kg m−2 a−1. Road surface runoff was best predicted by storm precipitation, while sediment yields for at least three of the four road surface plots were significantly correlated with storm rainfall, storm intensity and storm runoff. Sediment yields at the road segment scale were best predicted by road surface area, and sediment yields per unit area were most strongly correlated with road segment slope. The one road segment subjected to heavy traffic and more frequent regrading produced more than twice as much sediment per unit area than comparable segments with no truck traffic. Particle‐size analyses indicate a preferential erosion of fine particles from the road surface and a rapid surface coarsening of new roads. Published in 2001 by John Wiley & Sons, Ltd.  相似文献   

4.
The relative contribution of forest roads to total catchment exports of suspended sediment, phosphorus, and nitrogen was estimated for a 13 451 ha forested catchment in southeastern Australia. Instrumentation was installed for 1 year to quantify total in‐stream exports of suspended sediment, phosphorus, and nitrogen. In addition, a total of 101 road–stream crossings were mapped and characterized in detail within the catchment to identify the properties of the road section where the road network and the stream network intersect. Sediment and nutrient generation rates from different forest road types within the catchment were quantified using permanent instrumentation and rainfall simulation. Sediment and nutrient generation rates, mapped stream crossing information, traffic data and annual rainfall data were used to estimate annual loads of sediment, phosphorus, and nitrogen from each stream crossing in the catchment. The annual sum of these loads was compared with the measured total catchment exports to estimate the proportional contribution of loads from roads within the catchment. The results indicated that 3·15 ha of near‐stream unsealed road surface with an average slope of 8·4% delivered an estimated 50 t of the 1142 t of total suspended sediment exported from the catchment, or about 4·4% of the total sediment load from the forest. Stream discharge over this period was 69 573 Ml. The unsealed road network delivered an estimated maximum of 22 kg of the 1244 kg of total phosphorus from the catchment, or less than 1·8% of the total load from the forest. The average sediment and phosphorous load per crossing was estimated at 0·5 t (standard deviation 1·0 t) and 0·22 kg (standard deviation 0·30 kg) respectively. The lower proportional contribution of total phosphorus resulted from a low ratio of total phosphorus to total suspended sediment for the road‐derived sediment. The unsealed road network delivered approximately 33 kg of the 20 163 kg of total nitrogen, about 0·16% of the total load of nitrogen from the forest. The data indicate that, in this catchment, improvement of stream crossings would yield only small benefits in terms of net catchment exports of total suspended sediment and total phosphorus, and no benefit in terms of total nitrogen. These results are for a catchment with minimal road‐related mass movement, and extrapolation of these findings to the broader forested estate requires further research. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

5.
Roads have been widely studied as sources of runoff and sediment and identified as pollutant production sources to receiving waters. Despite the wealth of research on logging roads in forested, upland settings, little work has been conducted to examine the role of extensive networks of rural, low‐volume, unpaved roads on water quality degradation at the catchment scale. We studied a network of municipal unpaved roads in the northeastern US to identify the type and spatial extent of ‘hydro‐geomorphic impairments’ to water quality. We mapped erosional and depositional features on roads to develop an estimate of pollutant production. We also mapped the type and location of design interventions or best management practices (BMPs) used to improve road drainage and mitigate water quality impairment. We used statistical analyses to identify key controls on the frequency and magnitude of erosional features on the road network, and GIS to scale up from the survey results to the catchment scale to identify the likely importance of unpaved roads as a pollutant source in this setting. An average of 21 hydro‐geomorphic impairments were mapped per kilometer of road, averaging 0.3 m3 in volume. Road gradient and slope position were key controls on the occurrence of these features. The presence of BMPs effectively reduced erosion frequency. Scaled up to the watershed and using a conservative estimate of road–stream connectivity, our results for the Winooski River watershed in the northeastern US suggest that roughly 16% and 6% of the average annual sediment and phosphorus flux, respectively, of the Winooski River may be derived from unpaved roads. Our study identifies an under‐appreciated source of water quality degradation in rural watersheds, provides insights into identifying ‘hot spots’ of pollutant production associated with these networks, and points to effectiveness of design interventions in mitigating these adverse impacts on water quality. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

6.
High and moderate severity wildfires should increase sediment production from unpaved roads due to the increased surface runoff from upslope, and increase road–stream connectivity due to the decrease in downslope surface roughness as well as the increase in surface runoff and erosion. Because no study has documented these effects, we surveyed road surface erosion features and quantified road–stream connectivity as a function of fire severity and road segment characteristics. The data were collected one year after the High Park wildfire from 141 hydrologically distinct road segments along 6.8 km of an unpaved road west of Fort Collins, Colorado. Road segments below areas burned at high and moderate severity had significantly more rills than road segments below areas that burned at low severity. Road segment slope was an important control on the proportion of segment length with rills, and the strength of the relationship between road segment slope and the amount of rilling increased with burn severity. Flatter road segments tended to capture the sediment eroded from upslope burned areas. In areas burned at high and moderate severity all of the road segments had drainage features extending to a stream, and 78% of the segments in areas burned at low severity also were connected. These exceptionally high rates of road–stream connectivity are attributed to the increased runoff from upslope, the segment‐scale collection and funneling of hillslope and road surface runoff to a single drainage point, and the reduced infiltration and trapping capacity of the burned area below the road. The results show the need to either outslope the roads or increase the frequency of constructed drainage features after wildfires, particularly for steeper road segments in areas burned at high or moderate severity. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

7.
Post-fire catchment and water utility managers throughout the world use predictive models to estimate potential erosion risks to aid in evaluating downstream impacts of increased runoff and erosion, and to target critical areas within a fire for applying mitigation practices. Erosion prediction can be complicated by forest road networks. Using novel GIS technology and soil erosion modelling, this study evaluated the effect of roads on surface runoff, erosion and sediment yields following a wildfire and determined that the predictive models were providing reasonable results. The GeoWEPP model was used to simulate onsite erosion and offsite sediment delivery before and after fire disturbance using a 2-m resolution DEM as the terrain layer. Erosion rates in excess of 4 Mg ha−1 year−1 were predicted mainly from steep moderate and high severity burn areas. Roads influenced surface runoff flow path distributions and sub-catchment delineations, affecting the spatial distribution of sediment detachment and transport. Roads tended to reduce estimated erosion on slopes below the roads but increases in erosion rates were estimated for road fillslopes. Estimated deposition amounts on roads and in sediment basins were similar to measured amounts. The results confirm that road prisms, culverts and road ditches influence sedimentation processes after wildfire, and they present opportunities to detain eroded sediments.  相似文献   

8.
Model predictions concerning the endangerment of on‐site and off‐site damages due to runoff, soil erosion and sedimentation under alternative design and operation policies are of particular importance in recent catchment planning and management. By using the raster‐based model approach, linear landscape elements, such as streets and roads, and their impacts on flow paths are often neglected. Therefore, the aim of this study was to analyse the effects of linear landscape elements on patterns of soil erosion, sediment transport and sedimentation. To accomplish this, roads are considered while determining flow paths. Simulations in the well‐investigated catchment of the Wahnbach River (54 km²) in a low mountain range in Germany were carried out using a combination of different models for hydrology and soil erosion. Although the study focuses on the catchment scale of the Wahnbach River, detailed investigations concerning the sub‐catchment scale (21 ha) were also conducted. The simulation results show that these spatial structures mainly affect the pattern of soil erosion and sedimentation. On the sub‐catchment scale, improved identification of active zones for sediment dynamic becomes possible. On the catchment scale, the predicted runoff is about 20% higher, and sediment outputs were four times larger than predicted when roads were considered. Soil erosion increases by 37% whereas sedimentation is reduced by 29%. The model improvement could not be evaluated on the catchment scale because of the high variability and heterogeneity of land use and soils, but road impacts could be explained by simulations on the sub‐catchment scale. It can be concluded that runoff concentration due to rerouted flow paths leads to lower non‐concentrated and higher concentric‐linear surface runoff. Thus, infiltration losses decline and surface runoff and soil erosion increase because sedimentation is reduced. Further, runoff concentration can cause soil erosion hot spots. In the model concept used in this study, buffering of runoff and sediments on the upslope side of roads and in local depressions adjacent to roads cannot be simulated. Flow paths will only be rerouted because of road impacts, but the temporal ponding of water is not simulated. Therefore, the drastic increase of predicted sediment output due to road impact does not seem to be reliable. However, results indicate that the consideration of roads when determining flow paths enabled more detailed simulations of surface runoff, soil erosion and sedimentation. Thus, progress in model‐based decision‐making support for river catchment planning and management can be achieved. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

9.
Hydrological and sediment fluxes were monitored for a 1 yr period in a tropical headwater catchment where a 3 yr old logging road caused substantial Hortonian overland flow (HOF) and intercepted subsurface flow (ISSF). On a 51·5 m road section, ISSF became an increasingly important component of total road runoff, up to more than 90% for large storms. The proportion of ISSF contributed by road cuts along more or less planar slopes compared with ISSF from a zero‐order basin (convergent slopes) truncated by the road declined with increasing rainfall. During the monitored storms that generated ISSF along the road, on average, 28% of sediment export and 79% of runoff from the road section were directly attributable to ISSF. Estimates of total sediment export from the road surface (170 t ha?1 yr?1) and suspended sediment export from the logging‐disturbed catchment (4 t ha?1 yr?1) were exceptionally high despite 3 yr of recovery. ISSF caused not only additional road‐generated sediment export, but also exacerbated HOF‐driven erosion by creating a poor foundation for vegetation recovery on the road surface. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

10.
Hydrometric and isotopic (oxygen-18) observations were used to delineate the runoff processes operating in several headwater catchments on the Precambrian Shield of Canada. The catchments comprise patches of conifer forest situated on thin soils among areas of lichen-covered granitic bedrock. Horton overland flow occurred from the lichen-bedrock areas in all precipitation events that exceeded 4–6 mm. Runoff from the forest stands occurred mainly as subsurface stormflow, but in some instances saturation overland flow was observed. The occurrence of saturation overland flow was controlled by the topography of the bedrock beneath the forest soils. The area contributing runoff and the pathway by which water was conveyed to the catchment outflow switched from the open lichen-bedrock areas producing overland flow on the rising limb of the storm hydrograph to the forest stands contributing subsurface stormflow on the recession limb of the hydrograph. The areal extent and position of the landscape units in the basin were important to the rate and magnitude of stormflow production. Runoff was generated from the catchments only during and immediately after snowmelt and/or rainfall events. The catchments were dry and/or frozen for about 70% of the year.  相似文献   

11.
The objective of this paper is to quantify, and enable the prediction of, sediment delivery and water pollution impacts from a spectrum of forest roads. Ten 100–200 m long sections of forest road were selected to incorporate a wide range of the key physical site factors that are likely to affect the rate of sediment generation. Each road section was permanently instrumented for 1 year to measure rainfall and runoff continuously. Suspended load, bedload, and traffic were integrated measurements over 2‐ to 3‐week site‐service intervals. Total annual sediment load (normalized for slope) varied about 25‐fold, from 216 mg m?2 per millimetre of rain for a high‐quality gravel surfaced road with minimal traffic to 5373 mg m?2 per millimetre of rain for an unsurfaced road on an erodible subsoil with moderate light‐vehicle traffic. For the seven gravel‐surfaced roads in this study, truck traffic (axles/week) explained 97% of the variation in annual sediment delivery (per unit of rainfall) from the road. Equations are proposed that allow annual sediment delivery rates to be estimated when net rainfall, road slope, road area, and truck traffic are known. Roads produce runoff rapidly and were found to deliver sediment for about the same duration as rainfall is falling, in this study varying between 5 and 10% of the time. The patterns of sediment delivery measured from the experimental roads (frequency, duration, and intensity) in this study are similar to levels that have been shown to alter the composition of in‐stream macroinvertebrate communities in small (e.g. <10 l s?1), clean, mountain streams. However, in larger well‐mixed streams (e.g. >500 l s?1), dilution is sufficient to prevent concentrations reaching critical levels that are likely to result in biological impacts. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

12.
Unpaved roads are believed to be the primary source of terrigenous sediments being delivered to marine ecosystems around the island of St John in the eastern Caribbean. The objectives of this study were to: (1) measure runoff and suspended sediment yields from a road segment; (2) develop and test two event‐based runoff and sediment prediction models; and (3) compare the predicted sediment yields against measured values from an empirical road erosion model and from a sediment trap. The runoff models use the Green–Ampt infiltration equation to predict excess precipitation and then use either an empirically derived unit hydrograph or a kinematic wave to generate runoff hydrographs. Precipitation, runoff, and suspended sediment data were collected from a 230 m long, mostly unpaved road segment over an 8‐month period. Only 3–5 mm of rainfall was sufficient to initiate runoff from the road surface. Both models simulated similar hydrographs. Model performance was poor for storms with less than 1 cm of rainfall, but improved for larger events. The largest source of error was the inability to predict initial infiltration rates. The two runoff models were coupled with empirical sediment rating curves, and the predicted sediment yields were approximately 0·11 kg per square meter of road surface per centimetre of precipitation. The sediment trap data indicated a road erosion rate of 0·27 kg m?2 cm?1. The difference in sediment production between these two methods can be attributed to the fact that the suspended sediment samples were predominantly sand and silt, whereas the sediment trap yielded mostly sand and gravel. The combination of these data sets yields a road surface erosion rate of 0·31 kg m?2 cm?1, or approximately 36 kg m?2 year?1. This is four orders of magnitude higher than the measured erosion rate from undisturbed hillslopes. The results confirm the importance of unpaved roads in altering runoff and erosion rates in a tropical setting, provide insights into the controlling processes, and provide guidance for predicting runoff and sediment yields at the road‐segment scale. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

13.
This study examines runoff and sediment generation rates within the road prism on unsealed road segments in the Cuttagee Creek catchment near Bermagui in New South Wales, Australia. A large (600 m2) rainfall simulator was used to measure runoff and sediment yields from each of the potential sediment and runoff sources and pathways. These included the road surface, table‐drain, upslope contributing area and cutslope face, and the entire road segment as measured at the drain outlet. Experiments were conducted on two major types of road (ridge‐top and cut‐and‐fill) of varying traffic usage and maintenance standard for two 30‐minute simulations of increasing rainfall intensity. From the range of possible sources within the road prism, the road surface produced the dominant source of excess runoff and sediment at each site with limited contributions from the table‐drain, cutslope face or contributing hillslope. Sediment generation varied significantly with road usage and traffic intensity. Road usage was strongly related to the amount of loose available sediment as measured prior to the experiments. Table‐drains acted primarily as sediment traps during the low rainfall event but changes in sediment concentration within the drains were observed as runoff volumes increased during the higher rainfall event of 110 mm h?1, releasing sediment previously stored in litter and organic dams. The experiments demonstrate the potential roles of various features of the road prism in the generation and movement of sediment and water. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

14.
Diffuse sediment pollution impairs water quality, exerts a key control on the transfer and fate of nutrients and contaminants and causes deleterious impacts on freshwater ecology. A variety of catchment sediment sources can contribute to such problems. Sediment control strategies and effective targeting of mitigation options therefore require robust quantitative information on the key sources of the sediment problem at catchment scale. Recent observations by Catchment Sensitive Farming Officers (CSFO's) in England have highlighted road verges damaged and eroded by passing vehicles, particularly large farm machinery, and livestock herd movement as visually important potential sources of local sediment problems. A study was therefore undertaken to assess the relative importance of damaged road verges as a suspended sediment source in three sub‐catchments of the Hampshire Avon drainage basin, southern UK. Road verge sediment contributions were apportioned in conjunction with those from agricultural topsoils and channel banks/subsurface sources. Time‐integrating isokinetic samplers were deployed to sample suspended sediment fluxes at the outlets of two control sub‐catchments drained by the Rivers Chitterne and Till selected to characterize areas with a low road network density and limited visual evidence of verge damage, as well as the River Sem sub‐catchment used to represent areas where road verge damage is more prevalent. The findings of a sediment source fingerprinting investigation based on a combination of intermittent sampling campaigns spanning the period 22/5/02–27/4/08 suggested that the respective overall mean relative sediment contributions from damaged road verges were 5 ± 3%, 4 ± 2% and 20 ± 2%. Relative inputs from damaged road verges for any specific sampling period in the River Sem sub‐catchment were as high as 33 ± 2%. Reconstruction of historical sources in the same sub‐catchment, based on the geochemical record stored in a floodplain depth profile, suggested that the significance of damaged road verges as a sediment source has increased over the past 15–20 years. The findings provide important information on damaged road verges as a primary source of suspended sediment and imply that catchment sediment control strategies and mitigation plans should consider such verges in addition to those agricultural and channel sources traditionally taken into account when attempting to reduce sediment pressures on aquatic resources. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

15.
Post‐logging changes in catchment sediment yield have traditionally been attributed to increases in hillslope erosion and delivery rates as a result of forest harvesting activities. Linking hillslope erosion to catchment yield in forestry environments remains difficult, however, primarily because of the scarcity of data on the nature of hillslope sediment storage and delivery processes. A large rainfall simulator (350 m2) was used to apply rainstorms to a logged hillslope containing a snig track (skid trail) and a general logging or harvesting area (GHA) on 10 forest compartments in south‐eastern Australia. The experiments confirmed that the compacted, disturbed surfaces, such as roads and tracks, are the dominant sources of sediment in forestry areas. Sediment transport rates were limited by available sediment supply on both the snig track and the GHA, introducing important implications for the modelling of these surfaces using sediment transport capacity theories. Sediment delivery from the snig track to the adjacent GHA, via a cross‐bank (drainage diversion), was strongly influenced by the percentage fine fraction in the eroded sediment. Preferential deposition of coarse aggregates was measured at erosion control structures and on the adjacent GHA. Over 50% of fine‐grained material were deposited on the hillslope over a relatively short, flow path length of <5 m, highlighting the effectiveness of runoff diversion as a practice in reducing sediment flux. The transfer of water and sediment from disturbed to less disturbed parts of the landscape, and the associated potential for sediment storage, needs to be considered as part of any catchment impact assessment. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

16.
Many forested steeplands in the western United States display a legacy of disturbances due to timber harvest, mining or wildfires, for example. Such disturbances have caused accelerated hillslope erosion, leading to increased sedimentation in fish‐bearing streams. Several restoration techniques have been implemented to address these problems in mountain catchments, many of which involve the removal of abandoned roads and re‐establishing drainage networks across road prisms. With limited restoration funds to be applied across large catchments, land managers are faced with deciding which areas and problems should be treated first, and by which technique, in order to design the most effective and cost‐effective sediment reduction strategy. Currently most restoration is conducted on a site‐specific scale according to uniform treatment policies. To create catchment‐scale policies for restoration, we developed two optimization models – dynamic programming and genetic algorithms – to determine the most cost‐effective treatment level for roads and stream crossings in a pilot study basin with approximately 700 road segments and crossings. These models considered the trade‐offs between the cost and effectiveness of different restoration strategies to minimize the predicted erosion from all forest roads within a catchment, while meeting a specified budget constraint. The optimal sediment reduction strategies developed by these models performed much better than two strategies of uniform erosion control which are commonly applied to road erosion problems by land managers, with sediment savings increased by an additional 48 to 80 per cent. These optimization models can be used to formulate the most cost‐effective restoration policy for sediment reduction on a catchment scale. Thus, cost savings can be applied to further restoration work within the catchment. Nevertheless, the models are based on erosion rates measured on past restoration sites, and need to be updated as additional monitoring studies evaluate long‐term basin response to erosion control treatments. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

17.
Gravel-filled traps were buried in the beds of streams draining steep logged and unlogged catchments of the Dazzler Range in northern Tasmania, Australia, and removed after storm events, to assess infiltration of fine (less than 1 mm) material into the bed. All stream catchments were geomorphically similar, over similar altitude ranges and had moderately erodible sandy-clay soils on 25–35° slopes. Study catchments were selected to control for aspect, logging treatment and coupe age. Fine sediment infiltration into the stream bed was assessed for 15 tributary ephemeral streams in logged areas and 11 streams in unlogged areas. The logged catchments had been clearfelled in three time periods — 1990–1991, 1988–1989 and 1986–1987 — all by skyline cable logging. Trap yield was also assessed in riffles of the perennial valley floor streams upstream and downstream of the junction of six logged and six unlogged tributaries and upstream and downstream of four old but actively used road crossings. Trap yield was significantly higher in logged than in unlogged ephemeral streams for size fractions ranging from less than 125 to 500 μm, by factors ranging from two to three, but not for sediment between 0.5 and 1.0 mm. Trap yield of organic sediment of less than 125 μm declined with time after logging and burning, whereas inorganic sediment yield showed no clear trend with coupe age. Trap yield of 0.5–1.0 mm organic sediment was also significantly enhanced by logging and by burning. Sediment yield of streams logged in 1986–1987 was not significantly higher than for control streams, whereas inorganic sediment and 0.5–1.0 mm organic sediment yields were highest for recently burnt coupes. A significantly greater number of increases in trap yield occurred between riffle pairs of valley floor streams adjacent to junctions of logged tributaries, when compared with control riffle pairs. Logged tributary junctions were associated with an increase in the organic content of sediment. Road crossings were associated with large increases in infiltration in adjacent riffle pairs, 30–50 years after construction. Current forest practices do not protect ephemeral headwater streams from enhanced sediment inputs, the long-term significance of which is unknown. Recovery of sediment fluxes in these streams to background levels appears to take 5 years or longer.  相似文献   

18.
Anders Malmer 《水文研究》2004,18(5):853-864
In 1998 a wild fire struck a paired catchment research area under long‐term monitoring of hydrological and nutrient budgets. Streamwater quality as concentrations of dissolved and suspended particulate matter was monitored during 1·5–2·5 years after the fire in streams from seven different catchments. As the catchments, due to earlier experimental treatments, had different vegetations, varying effects related to different fire intensities were observed. The highest, mean stormflow, suspended sediment concentrations resulted from intensive fire in secondary vegetation that had experienced severe soil disturbance in previous treatments (crawler tractor timber extraction 10 years earlier). Stormflow concentrations were typically still about 400 mg l?1 in 1999 (10–21 months after the fire), which was about the maximum recorded concentration in streams during initial soil disturbance in 1988. Forest fire in natural forest resulted in less than half as high stormflow concentrations. For dissolved elements in streamwater there was a positive relation between fuel load (and fire intensity) and concentration and longevity of effects. Stream baseflow dissolved nutrient concentrations were high in the months following the fire. Mean baseflow K concentrations were 8–15 mg l?1 in streams draining catchments with intensive fire in secondary vegetation with large amounts of fuel. After controlled fire for forest plantation establishment in 1988 corresponding concentrations were 3–5 mg l?1, and after forest fire in natural forest in this study about 2 mg l?1. This study shows differences in response from controlled fire for land management, forest fire in natural forests and wild fires in manmade vegetations. These differences relate to resistance and resilience to fire for the involved ecosystems. There is reason to believe that wild fires and repeated wild fires during or after droughts, in successions caused by human influence, may lead to larger losses of ecosystem nutrient capital from sites compared with forest fires in natural forests. As fire in the humid tropics becomes more common, in an increasingly spatially fragmented landscape, it will be important to be aware of these differences. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

19.
The suspended sediment response of a small catchment subjected to farmland abandonment and subsequent plant recolonization was studied in relation to its hydrological functioning. The analysis of data over a seven‐year period demonstrated that suspended sediment yield was greatly influenced by the occurrence of intense, low‐frequency events. Greater amounts of suspended sediment were exported during spring, when the catchment was hydrologically more active. Rainfall intensity and baseflow at the start of a flood event had a strong influence on the sediment response, suggesting that several hydrological processes were active within the catchment. SSC (suspended sediment concentration)‐Q hysteretic loop analysis at the event scale aided understanding of the sedimentological and hydrological behaviour of the catchment. During the study period the SSC‐Q loops showed a high degree of seasonality and two main patterns strongly related to catchment wetness were distinguished. When the catchment was dry (mainly during summer and the beginning of autumn) the predominant process was infiltration excess runoff over sparsely vegetated areas close to the main channel. Under these conditions, floods exhibited a counter‐clockwise hysteretic loop and were characterized by a small streamflow response, short duration and high SSC. Under wet conditions (mainly during winter and spring), saturation excess runoff was increasingly dominant over vegetated areas. Under these conditions, floods exhibited a clockwise hysteretic loop, and were characterized by a larger streamflow response, longer duration and higher suspended sediment yield. The lower SSC during the falling stage of the hydrograph is likely to be due to dilution effects related to the contribution of clean water resulting from enlargement of the saturated areas, together with an increase in the baseflow discharge. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

20.
Although unpaved roads are well‐recognized as important sources of Hortonian overland flow and sediment in forested areas, their role in agriculturally‐active rural settings still lacks adequate documentation. In this study, we assessed the effect of micro‐catchment size, slope, and ground cover on runoff and sediment generation from graveled roadbeds servicing a rural area in southern Brazil. Fifteen replications based on 30‐min‐long simulated rainfall experiments were performed at constant rainfall intensities of 22–58 mm h?1 on roadbeds with varying characteristics including ~3–7 m2 micro‐catchment areas, 2–11° slopes, 2–9.7‐m‐long shallow rill features, and 30–100% gravel cover. The contributions of micro‐catchment size and rill length were the most important physical characteristics affecting runoff response and sediment production; both the size of the micro‐catchment and the length of the rills were inversely related to sediment loss and this contradicts most of the rill erosion literature. The effect of micro‐catchment size on runoff and sediment response suggests a potentially problematic spatial‐scale subjectivity of experimental plot results. The inverse relationship between rill length and sediment generation is interpreted here as related to the predominance of coarse fragments within rills, the inability of the shallow flows generated during the simulations to erode this sediment, and their role as zones of net sediment storage. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

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