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排序方式: 共有970条查询结果,搜索用时 15 毫秒
821.
Edward B. Rashin Casey J. Clishe Andrew T. Loch Johanna M. Bell 《Journal of the American Water Resources Association》2006,42(5):1307-1327
ABSTRACT: Timber harvest best management practices (BMPs) in Washington State were evaluated to determine their effectiveness at achieving water quality standards pertaining to sediment related effects. A weight‐of‐evidence approach was used to determine BMP effectiveness based on assessment of erosion with sediment delivery to streams, physical disturbance of stream channels, and aquatic habitat conditions during the first two years following harvest. Stream buffers were effective at preventing chronic sediment delivery to streams and physical disturbance of stream channels. Practices for ground‐based harvest and cable yarding in the vicinity of small streams without buffers were ineffective or only partially effective at preventing water quality impacts. The primary operational factors influencing BMP effectiveness were: the proximity of ground disturbing activities to streams; presence or absence of designated stream buffers; the use of special timber falling and yarding practices intended to minimize physical disturbance of stream channels; and timing of harvest to occur during snow cover or frozen ground conditions. Important site factors included the density of small streams at harvest sites and the steepness of inner stream valley slopes. Recommendations are given for practices that provide a high confidence of achieving water quality standards by preventing chronic sediment delivery and avoiding direct stream channel disturbance. 相似文献
822.
823.
Highland catchments in tropical regions are frequently subjected to soil erosion and the transport of chemicals downstream.
Any drastic changes in land use will increase the severity of these processes of land degradation. A simulation study using
GLEAMS (Groundwater Loading Effects of Agricultural Management Systems) was conducted at a catchment presently under tea farming
in Cameron Highlands, Malaysia. Soil, water and nutrient transport associated with several alternative land uses was studied.
In addition, the effect of a disruptive form of land clearing on soil, water and nutrient losses was also investigated. Modelling
with GLEAMS required information from field measurements and observations, laboratory analyses, guide tables, industry records,
maps and reports published by soil survey and meteorological departments.
The most critical step in simulating soil and water movement using GLEAMS is the identification of a representative flow sequence.
In the catchment under study, the representative flow sequence was overland flow–channel 1–channel 2. Input data on soil erodibility,
porosity and surface roughness were manipulated to represent various degrees and forms of disturbance to the surface soil
layer. For all land uses studied, the highest soil loss was predicted for the overland flow area where slope gradient is high
and the soil friable. The variations in soil loss, runoff and nutrient loss between landscape elements and between land uses
were consistent with soil erosion features observed in the field. Soil and nutrient losses were substantial for crops such
as cabbage that required land shaping activities and frequent ploughing of the soil. Predicted data on enrichment ratio of
specific surface (ERSS) are consistent with nutrient enrichment processes in the field and could prove to be useful in studies
on chemical transport in highland catchments. 相似文献
824.
Jose de Anda Harvey Shear Ulrich Maniak Pedro F. Zdrate‐del Valle 《Journal of the American Water Resources Association》2004,40(1):97-109
ABSTRACT: Lake Chapala is the largest natural water body in Mexico and also one of the most important shallow lakes in Latin America. For the past several years it has suffered various environmental problems such as the upstream overuse of water, contamination, and sedimentation. For the past 10 years the lake has had less than 50 percent of its historical water level over the past century. No criteria are reported in the literature that establish a water storage volume that will guarantee water quality conditions necessary for the survival of the lake. After determining the behavior of total solids concentrations in relation to the variations in the lake's depth, we proposed a minimum water column height of 5.0 m, representing a storage volume of about 5,000 Mm3. This volume would result in the recommended water quality standards for total dissolved solids. Calculated distribution maps show that the total solids concentration in the lake has increased since the end of the 1970s. The solids are primarily concentrated in the eastern part of the lake, as a consequence of the high solids discharged from the Lerma River, its main tributary. 相似文献
825.
William J. Elliot 《Journal of the American Water Resources Association》2004,40(2):299-309
ABSTRACT: The Water Erosion Prediction Project (WEPP) is a physically based erosion model for applications to dryland and irrigated agriculture, rangeland, and forests. U.S. Forest Service (USFS) experience showed that WEPP was not being adapted because of the difficulty in building files describing the input conditions in the existing interfaces. To address this difficulty, a suite of Internet interfaces with a database was developed to more easily predict soil erosion for a wide range of climatic and forest conditions, including roads, fires, and timber harvest. The database included a much larger climate database than was previously available for applications in remote forest and rangeland areas. Validation results showed reasonable agreement between erosion values reported in the literature and values predicted by the interfaces to the WEPP model. 相似文献
826.
H. Evan Cornfield Vicente L. Lopes 《Journal of the American Water Resources Association》2004,40(2):321-332
ABSTRACT: A process based, distributed runoff erosion model (KINEROS2) was used to examine problems of parameter identification of sediment entrainment equations for small watersheds. Two multipliers were used to reflect the distributed nature of the sediment entrainment parameters: one multiplier for a raindrop induced entrainment parameter, and one multiplier for a flow induced entrainment parameter. The study was conducted in three parts. First, parameter identification was studied for simulated error free data sets where the parameter values were known. Second, the number of data points in the simulated sedigraphs was reduced to reflect the effect of temporal sampling frequency on parameter identification. Finally, event data from a small range‐land watershed were used to examine parameter identifiability when the parameter values are unknown. Results demonstrated that whereas unique multiplier values can be obtained for simulated error free data, unique parameter values could not be obtained for some event data. Unique multiplier values for raindrop induced entrainment and flow induced entrainment were found for events with greater than a two‐year return period (~25 mm) that also had at least 10 mm of rain in ten minutes. It was also found that the three‐minute sampling frequency used for the sediment sampler might be inadequate to identify parameters in some cases. 相似文献
827.
Ebrahim Amiri‐Tokaldany Stephen E. Darby Paul Tosswell 《Journal of the American Water Resources Association》2003,39(4):897-909
ABSTRACT: When extensive lengths of river become stabilized (e.g., by channel incision), riverbank erosion can result in considerable riparian land loss and the delivery of large volumes of sediment downstream. The ability to predict the stability and failure geometry of eroding riverbanks is therefore an important prerequisite in estimating the rate of bank erosion and sediment yield associated with bank erosion. In this paper, a new stability analysis for layered river banks is introduced. The new analysis differs from many previous analyses in that it takes into consideration the effects of positive pore water pressure in the saturated portion, and negative pore water pressure in the unsaturated portion, of the bank as well as the influence of hydrostatic confining pressure due to the water level in the river. In addition, the failure plane is not constrained to pass through the toe of the bank and the bank profile geometry is not restricted to an idealized special case. The predictive ability of the new bank stability analysis is assessed using data from two field sites. Subsequently, a methodology for applying the bank stability analysis at the scale of the river reach (0.1 to 10 km) is discussed. This method involves the use of empirical models of bed level adjustment to estimate the magnitude of incision at specific locations along the reach, with these estimates used to drive the stability analysis. Application of the new method is demonstrated with an example. 相似文献
828.
829.
Bruce P. Hayden Robert Dolan Scott Hoffman Alan Robinson 《Environmental management》1978,2(3):209-218
Beach erosion on Cinnamon Bay in the U.S. Virgin Islands results from storm waves in the North Atlantic. The net trend of erosion, however, is due to local environmental degradation. Reef die-back, initiated during the period of high sedimentation associated with 18th and 19th century sugar cane cropping, has reduced the rate of generation of new coral sands. Sand losses to deep water during periods of storm waves are not replenished. Engineering measures designed to check erosion at Cinnamon Bay have had little effect in this environment of low wave energies, but sand deficiency. The appropriate environmental focus for management of reef-beach systems is reef health and thus, water quality. 相似文献
830.
Nani G. Bhowmik 《Journal of the American Water Resources Association》1978,14(5):1064-1079
ABSTRACT: A method is reported for estimating the height of wind waves in any lake for a given wind condition. Maximum wind speeds from five climatological stations in and around Ilinois for the period of 1950–1972 were analyzed and the maximum wind speed for various durations and return periods were presented. Statistical analysis of wind wave data collected from Carlyle Lake indicated the Rayleigh distribution fitted the wave height distribution reasonably well and that the nondimensional energy spectra followed the (f/fm)-5 rule in the equilibrium range of frequencies. From a consideration of various forces and physical properties of riprap particles and water, a relationship was developed to estimate the stable weight of riprap particles. A practical design criteria is proposed to stabilize lake shores against wind waves. 相似文献