Laboratory Rainfall Simulation Experiments Research Articles

Roles of the stolon and erect grass species in surface–subsurface flow generation and red soil loss

Grass planting is an important and typical measure for soil and water conservation, while the roles of stolon and erect grass species in affecting surface–subsurface flow and soil erosion processes are little understood. Experimental plots (2 m × 0.5 m × 0.5 m) using red soil with two rainfall intensities (60 and 120 mm h−1) and grass coverage (30 % and 70 %) were conducted in laboratory rainfall simulation experiments. Two common and endemic stolon (Eremochloa ophiuroides) and erect (Lolium perenne) grass species were selected due to their fast growth, power adaptability, and wide application in reducing runoff and soil loss; and the bare soil plot was set up as the control. The results showed that bare plot had the highest mean surface flow coefficient (73.5 %), and then were L. perenne (57.1 %) and E. ophiuroides (44.9 %). The mean subsurface flow coefficients were found in order of E. ophiuroides (25.7 %), L. perenne (13.0 %) and bare plot (7.2 %). The role of stolon grass rather than erect grass in reducing surface flow but improving subsurface flow led to lower surface flow hydraulic characteristics such as flow velocity, Reynolds number, Darcy–Weisbach coefficient and Manning friction coefficient, and hydrodynamic characteristics like flow shear stress and stream power. Furthermore, the stolon grass generated the lowest soil loss rate (0.8 g m−2 min−1), while the values on erect grass and bare plot were 2.5 and 10.7 g m−2 min−1, respectively. The indicator of eroded sediment median size also proved the higher efficiency of stolon grass in soil loss control, with values of 31.8, 58.6 and 69.6 μm on stolon grass, erect grass and bare plots, respectively. The lower soil loss on stolon grass cover plot was attributed to the lower surface flow and hydraulic–hydrodynamic characteristics. The stolon grass was more sensitive to the coverage and rainfall intensity, which was explained by the higher flow and sediment trapping capability (2.2 ∼ 4.4 times) under lower coverage and rainfall intensity than erect grass, even under high rainfall intensity (1.1 ∼ 3.2 times). The stolon grass played more beneficial roles in soil and water conservation than erect grass, which was closely related to above-ground and under-ground parts of grass. This study can provide a better understanding of the roles of stolon and erect grass species for soil and water conservation and management practices.

Wide-Area Dynamic Sensing Method of Water Film Thickness on Asphalt Runway

Abstract Water film thickness is of great significance to the skid-resistant performance of an asphalt runway. Obtaining the precise information of water film thickness can help to assess the skid resistance with higher accuracy. This article proposes a wide-area dynamic sensing method of water film thickness on asphalt runway. The model, considering precipitation time-variability, runway geometry, and runway surface characteristics, is established to extract spatial temporal information of water film thickness dynamics on runway during intense rainfall. The model optimization is further conducted to deal with the problems of slope change, singularity issue, and time-spatial evolution of water film, which extends the single-point water film sensing to the large-area sensing. In order to evaluate the developed method, the calculations of water film thickness from the model are compared with the results obtained from a laboratory rainfall simulation experiment. The results show that the method has high accuracy in sensing the water film thickness and can realize a dynamic and whole-area acquisition of runway wet surface condition information. This method can provide data support for intelligent decision making of landing safety under rainfall conditions.

Impacts of distinct spatial arrangements of impervious surfaces on runoff and sediment fluxes from laboratory experiments

Urbanization affects runoff processes and sediment transport, but the magnitude of the impacts remains poorly understood. Different spatial patterns of pervious and impervious surfaces influence flow and sediment connectivity between hillslopes and stream networks. Following years of research on the peri-urbanizing Ribeira dos Covões catchment in Portugal, this study uses laboratory rainfall simulation experiments to better assess the impact of soil and pavement patterns on runoff (amount, runoff start and stop times) and sediment transport. Based on urban cores observed in the study catchment, the investigation focused on seven spatial patterns: bare soil (S), 100% pavement (P), and 60% pavement under continuous - C - surface placed upslope (CU) and downslope (CD), and dispersed - D - over the surface with regular (DR), irregular (DI) and linear (DL) distribution. A 1.00 m × 1.00 m flume, 0.05 m deep with a 9° slope, facilitated the experiments. The study used sandy-loam soil (1500 kg m−3) with concrete slabs representing pavement. Each experiment comprised a series of four rainfall simulations, each lasting 20 min (50 mm h−1), separated by 30-min intervals, to assess the impact of different initial soil moisture conditions. Results indicate that both spatial pattern and soil moisture drive runoff. Under dry conditions, CD provides runoff that is 7 times faster and about 4% higher than that for CU. Already wet conditions, however, produced 12% more runoff on CU than on CD. The greater runoff arose from faster soil saturation, driven by soil moisture increasing more quickly during the rainfall, as well as upstream runoff from paved surfaces, though runoff took longer to reach the outlet. The dispersed pavement pattern only affected runoff amount, with DL producing the highest coefficients (40–71%) and DI the lowest (25–55%), since longer flow paths increase the opportunities for water infiltration. Additionally, CU yielded 40% more sediment transport than CD, but the three dispersed patterns did not show a significant impact (p > 0.05). The results suggest that appropriate planning can reduce flood hazard and land degradation in urban areas, in particular by using dispersed patterns of sealed surfaces to enhance water infiltration and retention.

Influences of sand cover on erosion processes of loess slopes based on rainfall simulation experiments

Aeolian-fluvial interplay erosion regions are subject to intense soil erosion and are of particular concern in loess areas of northwestern China. Understanding the composition, distribution, and transport processes of eroded sediments in these regions is of considerable scientific significance for controlling soil erosion. In this study, based on laboratory rainfall simulation experiments, we analyzed rainfall-induced erosion processes on sand-covered loess slopes (SS) with different sand cover patterns (including length and thickness) and uncovered loess slopes (LS) to investigate the influences of sand cover on erosion processes of loess slopes in case regions of aeolian-fluvial erosion. The grain-size curves of eroded sediments were fitted using the Weibull function. Compositions of eroded sediments under different sand cover patterns and rainfall intensities were analyzed to explore sediment transport modes of SS. The influences of sand cover amount and pattern on erosion processes of loess slopes were also discussed. The results show that sand cover on loess slopes influences the proportion of loess erosion and that the compositions of eroded sediments vary between SS and LS. Sand cover on loess slopes transforms silt erosion into sand erosion by reducing splash erosion and changing the rainfall-induced erosion processes. The percentage of eroded sand from SS in the early stage of runoff and sediment generation is always higher than that in the late stage. Sand cover on loess slopes aggravates loess erosion, not only by adding sand as additional eroded sediments but also by increasing the amount of eroded loess, compared with the loess slopes without sand cover. The influence of sand cover pattern on runoff yield and the amount of eroded sediments is larger than that of sand cover amount. Furthermore, given the same sand cover pattern, a thicker sand cover could increase sand erosion while a thinner sand cover could aggravate loess erosion. This difference explains the existence of intense erosion on slopes that are thinly covered with sand in regions where aeolian erosion and fluvial erosion interact.

Upslope inflow, hillslope gradient and rainfall intensity impacts on ephemeral gully erosion

AbstractEphemeral gullies (EGs) are major contributors to sediment loss and land degradation on cultivated lands. However, the topography and rainfall impacts on EG development processes are still unclear, especially on steep loessial hillslopes such as the Loess Plateau. A series of laboratory rainfall simulation experiments were conducted to investigate the impacts of topographic characteristics (3 typical slope gradients (S): 26.8%, 36.4%, and 46.6%; and 5 upslope drainage areas (A): 16, 32, 64, 96, and 128 m2) and rainfall intensities (3 representative erosive rainfall intensities 50, 75, and 100 mm hr−1) on EG erosion on a steep loessial hillslope. A large slope adjustable soil pan (8 m‐long, 2 m‐wide, and 0.6 m‐deep) and a side‐sprinkler rainfall simulation system were used in this study. The results showed that soil loss increased when rainfall intensity, slope, and upslope drainage area increased. Upslope topography and inflow had great impacts on downslope EG erosion, and the contribution percentages ranged from 52.2% to 74.1%, from 48.3% to 71.4%, and from 29.5% to 66.7% for the 50, 75, and 100 mm hr−1 rainfall treatments, respectively. Runoff velocities with upslope inflow were 22.7% to 79.4% larger than those without inflow, and the upslope inflow was more effective than rainfall intensity in increasing runoff velocities in EG channels, thus caused more soil erosion. Soil loss equation based on rainfall intensity and AS2 (product of the upslope drainage area and the square of the local slope gradient) was established and validated. The determination coefficient (R2) and Nash–Sutcliffe simulation efficiency (ENS) were 0.80 and 0.87, which showed satisfactory accuracy. This equation can be used to predict the EG erosion in various topographic and rainfall conditions on steep loessial hillslopes.

Effects of urban grass coverage on rainfall-induced runoff in Xi'an loess region in China

In this study, laboratory rainfall simulation experiments were conducted to investigate the regulatory effects of grass coverage on rainfall-runoff processes. A total of 80 grass blocks planted with well-grown manilagrass, together with their root systems, were sampled from an eastern suburban area of Xi'an City in the northwest arid area of China and sent to a laboratory for rainfall simulation experiments. The runoff and infiltration processes of a slope with different grass coverage ratios and vegetation patterns were analyzed. The results show that the runoff coefficient decreases with the increase of the grass coverage ratio, and the influence of grass coverage on the reduction of runoff shows a high degree of spatial variation. At a constant grass coverage ratio, as the area of grass coverage moves downward, the runoff coefficient, total runoff, and flood peak discharge gradually decrease, and the flood peak occurs later. With the increase of the grass coverage ratio, the flood peak discharge gradually decreases, and the flood peak occurs later as well. In conclusion, a high grass coverage ratio with the area of grass coverage located at the lower part of the slope will lead to satisfactory regulatory effects on rainfall-induced runoff.

Quantifying the spatial variation of 7Be depth distributions towards improved erosion rate estimations

There is growing interest in the application of the natural fallout radionuclide 7Be as a soil erosion and sediment tracer. Development of robust datasets is, however, hampered by unquantified variability in its vertical distribution within surface soil. Models that convert 7Be inventory measurements to soil erosion estimates are all based on the observed depth distribution of 7Be, described by the relaxation mass depth (h0) parameter. Previous work, however, has not considered potential spatial variation in h0 linked to natural variability in soil physical properties, which could have major implications for the reliability of soil erosion estimates.Two complementary experiments were designed to study the variability in depth distribution within and between potential reference sites. First, a field sampling programme was carried out whereby two reference sites with variable degree of compaction were sampled using two different sectioning techniques, i.e. by use of a fine increment soil collector (FISC) and the scraping methodology. During a laboratory rainfall simulation experiment, water spiked with stable 9Be was used to study the variability in 9Be soil depth distribution within and between the two reference sites. In the field experiment, variations in the 7Be depth distribution, and thus in h0, were limited between both reference sites (13 to 16%). In contrast, the impact of the sectioning technique was remarkable, with scraping resulting in a higher h0 (up to 60%) compared to the estimates based on the use of a FISC. The rainfall simulation experiment offered the opportunity to study the variation in 9Be depth distribution in more detail. With an average h0 of 4.66kgm−2, 9Be penetrated deeper in the non-compacted (NC) reference site cores, while the compacted (C) cores showed an average h0 of 2.42kgm−2. The reported h0 values at the former site were also characterized by a larger coefficient of variation (24%) than those at the latter site (11%). Lower bulk density, higher infiltration rates and a pore network characterized by a higher macroporosity and connectivity, as revealed by the X-ray Computed Tomography (CT) scans, explained the deeper penetration of Be into the topsoil of reference site NC.The results indicate the importance of selecting appropriate reference sites and for ensuring an adequate sampling strategy to encompass local variability in soil physical properties. Hydraulic conductivity assessment could be a useful tool to properly assess suitable reference sites and the number of samples needed to assess the reference inventory.

English

Soils from semi-arid environments have traditionally been considered to have poorly stable aggregates. The use of aggregate stability methods for erosion research seems of particular interest. However, its use to predict crusting and erosion have yielded conflicting results. In this study, we investigate the possible relations between aggregate stability and soil physical properties, and soil water erosion parameters using the mean weight diameter concept, based on dry and wet sieving method. We introduced also a new aggregate stability parameter based on the stability quotient which takes into account the percentage of soil fraction larger than 2 mm. Twenty soil sites were sampled from a Mediterranean semi-arid watershed for aggregate stability test. This study is part of a research work about erosion processes in semi-arid regions of Tunisia conducted during the period 1999 to 2013. Results indicate a correlation between aggregate stability indices and quotients with organic matter content (r = 0.70), a positive correlation with clay content (r = 0.30), but a negative correlation with the amount of soil loss and splash (r = -0.49 and r = -0.40 respectively) as collected in a laboratory rainfall simulation experiment. The stability quotient is the best soil parameter that explains soil erosion. It shows that more than 80% of the soil samples have poor structure stability. The fractal dimension, a characteristic property of the number-size distribution of fragments as a mass of material is broken down, was calculated for all soil samples. After wet sieving all samples follow the law of fractal dimensions.   Key words: Aggregate stability, erosion, rainfall simulation, stability quotient, fractal analysis.

Integrated Modeling Approach to the Response of Soil Erosion and Sediment Export to Land-Use Change at the Basin Scale

AbstractLand use determines the erodibility and hydrological structure of land units. Grid-based modeling is an effective approach for handling the spatial heterogeneity of basin characteristics, such as land use, soil, rainfall, and topographical information. The reliability of simulation results from erosion models is circumscribed by considerable spatial variation of numerous parameters. In this study, a grid-based distributed soil erosion and sediment transport model was used in conjunction with a laboratory rainfall simulation experiment to determine the impact of land-use changes on soil erosion and sediment yield in a river basin during individual storms. Land-use changes in Lushi Basin, primarily in Henan, China, were analyzed by comparing historical land use maps from 1990 and 1995. By comparing the responses of four selected storms that were simulated under conditions indicated by these two land-use maps, the average erosion rates were found to increase from 1989 to 1996. The increase in the ave...

Soil erosion characteristics of ditch banks during reclamation of a saline/sodic soil in a coastal region of China: Field investigation and rainfall simulation

Understanding soil erosion processes is essential for assessing the extent and causes of soil erosion and for planning soil conservation including those on the banks of excavated ditches. Based on a field investigation and laboratory rainfall simulation experiments, the soil erosion characteristics of a saline/sodic soil undergoing reclamation that was exposed on irrigation/drainage ditch banks in a coastal region of China were investigated. Preliminary analyses of the data indicated that the soil erosion was more severe from the ditch banks in the coastal region than from those in other areas of China. The annual erosion loss from ditch banks in the most recently reclaimed farmland (2007) was 140kg/m2, but the erosion modulus decreased significantly (p<0.05) with increasing reclamation time. Increasing rainfall intensity and/or slope gradient increased runoff and sediment yield rates. However, there was a critical slope gradient of about 25°, above which no further increases occurred. As rainfall intensity and/or slope gradient increased, the overland flow velocity and the Froude number (Fr) increased notably. The Reynolds number (Re) increased with increasing rainfall intensity, but decreased and was more affected by increases in slope gradient. Positive parabolic relationships were found between sediment yield rates and Re. The resistance to flow decreased with increasing rainfall intensity and slope gradient. Negative correlations were found between the sediment yield rate and the resistance force (f and n). Covering the soil surface greatly decreased runoff and soil erosion by increasing the surface roughness and decreasing the runoff velocity. Compared with covers of stones or netting, and with no cover, the slope runoff coefficient and sediment yield rate of soil under straw mulch were the lowest with values of 0.043 and 0.14kg/m2min, respectively. Therefore, using straw mulch would be a way to control erosion until salt-tolerant plants colonized and protected the ditches.

Modeling the Impacts of Land Use Changes on Soil Erosion at the River Basin Scale

The potential for soil erosion is strongly affected by land use. A proper modeling approach should be selected to explicitly assess the impacts of land use changes on soil erosion at a river basin scale. The reliability of simulation results from erosion models is circumscribed by considerable spatial variation in many parameters. In this study, a grid–based distributed soil erosion and sediment transport model was used in conjunction with a laboratory rainfall simulation experiment to determine the impact of land use changes on soil erosion and sediment yield in a river basin during individual storms. Land use changes in the Lushi Basin, primarily in Henan, China, were analyzed by comparing the historical land use maps in 1990 and 1995. Through comparing the results for four selected storms that were compiled using these two land use maps, it was found that the average erosion rates increased from 1989 to 1996. The increase in average erosion rates followed the land use changes, especially the transformation of forest to farmland. The results indicated that even slight land use change, from forest to farmland or vice versa, had a significant effect on regional soil erosion rates and sediment supply to rivers.

Effect of Root Density on Erosion and Erodibility of a Loamy Soil Under Simulated Rain

Although both aboveground and belowground components of vegetation act together in reducing soil erosion, mainly the aboveground component has received attention in past research. The aim of this study was to evaluate the contribution of roots in soil erosion control and the effect of root density in soil erodibility and soil physical properties. Perennial ryegrass (Lolium perenne L. Hugo) was grown in soil pans, and laboratory rainfall simulation experiments were conducted after 4, 8, 12 weeks of their growth with seeding density of 50 kg ha-1, after 4 weeks for seeding density of 100 kg ha-1, and on a control. The experiments with ryegrass were done in the presence of complete plants and after clipping off the shoots. Roots of ryegrass grew rapidly, attaining densities of 0.614 kg m-2 and 2.280 kg m-2 in 4 and 12 weeks, respectively. With increasing root density, splash and wash decreased exponentially. There was positive correlation between soil shear strength and root density, but no influence of roots on bulk density and saturated hydraulic conductivity was observed.

Development of Soil Crusts Under Simulated Rainfall and Crust Formation on a Loess Soil as Influenced by Polyacrylamide

This study evaluated the morphological characteristics and dynamic variation in characteristics of soil crust and identified the relationships between soil crust and splash erosion under simulated rainfall. The effect of polyacrylamide (PAM) on soil aggregate stabilization and crust formation was also investigated. A laboratory rainfall simulation experiment was carried out using soil sample slices. The slices were examined under a polarized light microscopy and a scanning electron microscope (SEM). The results revealed that the soil crusts were thin and were characterized by a greater density, higher shear strength, finer porosity, and lower saturated hydraulic conductivity than the underlying soil. Two types of crusts, i.e., structural and depositional crusts, were observed. Soil texture was determined to be the most important soil variable influencing surface crust formation; depositional crust formation was primarily related to the skeleton characteristics of the soil and happened when the soil contained a high level of medium and large aggregates. The crust formation processes observed were as follows: 1) The fine particles on the soil surface became spattered, leached, and then rough in response to raindrop impact and 2) the fine particles were washed into the subsoil pores while a compact dense layer concurrently formed at soil surface due to the continual compaction by the raindrops. Therefore, the factors that influenced structural crust formation were a large amount of fine particles in the soil surface, continual impact of raindrops, dispersion of aggregates into fine particles, and the formation of a compact dense layer concurrently at the soil surface. It was concluded that the most important factor in the formation of soil crusts was raindrop impact. When polyacrylamide (PAM) was applied, it restored the soil structure and greatly increased soil aggregate stabilization. This effectively prevented crust formation. However, this function of PAM was not continuously effective and the crust reformed with long-term rainfall. In conclusion, this study showed that soil micromorphological studies were a useful method for evaluating soil crust formation.

Hydrological effects of a layer of vegetation ash on underlying wettable and water repellent soil

Hydrological processes after a wildfire may take place under soil conditions altered by heat and by the presence of ash. Soil and ash interact as a two-layer system with poorly understood hydrological properties, especially when ash covers water repellent soil. Here we quantify the effect of an ash layer (0, 5, 15 and 30mm depth) covering wettable and water repellent soil on (i) the hydrological response and the mechanism of runoff generation and (ii) the water repellency dynamics, for a rainfall event followed by different drying periods and a second rainfall event. Laboratory rainfall simulation experiments (82.5mmh−1 during 40min) at small plot-scale (0.09m2) were performed and surface and subsurface flow, sediment yield, splash detachment and moisture content evolution determined. Wettable soil without ash cover generated no surface runoff, but as a two-layer system temporary surface runoff was produced when ash became saturated, until water drained through the soil. Wetting and drying changed the hydrological properties of ash, increasing surface runoff for all ash depths. Over water repellent soil, the ash layer delayed and reduced surface runoff proportionally to ash depth (r=0.99), reduced soil water repellency and promoted fingered subsurface flow. Ash protected the soil from splash and sheet erosion, particularly for water repellent soil. The results demonstrate that (i) the presence of an ash layer can have contrasting effects on surface runoff, depending on the wettability of the underlying soil, and (ii) a single wetting and drying event can substantially modify ash hydrological properties.

Effects of Hydrodynamically Rough Grassed Waterways on Dissolved Reactive Phosphorus Loads Coming from Agricultural Watersheds

A modified type of grassed waterway (GWW) with large hydrodynamic roughness has proven ability to reduce sediment load and surface runoff under conditions where best management practices on the delivering fields reduce sediment inputs that could otherwise damage the grass cover. It is unknown how such a GWW affects the loading of surface runoff with dissolved reactive phosphorus (DRP). The effect on DRP was tested in a landscape-scale study where DRP concentrations and loads in surface runoff were measured in two watersheds in which GWWs were newly installed and increased in effectiveness over time. Both watersheds were compared with paired watersheds without GWW installation; all watersheds were continuously monitored over 5 yr (1993-1997). Additionally, DRP concentrations were measured in open field and throughfall precipitation under growing grass and crops in field experiments, and DRP concentrations in surface runoff from straw covered surfaces were determined with laboratory rainfall simulation experiments. Dissolved reactive P in throughfall for the different cover types was highly variable, and the highest concentrations (up to 2.8 mg L(-1)) occurred especially during flowering of the respective crop and after frost events. Dissolved reactive P concentrations in runoff from straw-covered surfaces were slightly higher compared with those from bare soil. On average, there was a small difference in DRP concentrations between throughfall under growing crops and grass and in runoff from bare or straw covered soil surfaces. Hence, the introduction of a relatively small grassed area has little effect on the DRP concentration in surface runoff from the total watershed. This finding was supported by the watershed data, where watersheds with and without GWW showed similar DRP concentrations. No change in DRP concentrations occurred over the 5-yr period. Such GWWs will thus reduce the DRP load analogously to the reduction in total surface runoff.

Storage of water on vegetation under simulated rainfall of varying intensity

Little is understood about how storage of water on forest canopies varies during rainfall, even though storage changes intensity of throughfall and thus affects a variety of hydrological processes. In this study, laboratory rainfall simulation experiments using varying intensities yielded a better understanding of dynamics of rainfall storage on woody vegetation. Branches of eight species generally retained more water at higher rainfall intensities than at lower intensities, but incremental storage gains decreased as rainfall intensity increased. Leaf area was the best predictor of storage, especially for broadleaved species. Stored water ranged from 0.05 to 1.1 mm effective depth on leaves, depending on species and rainfall intensity. Storage was generally about 0.2 mm greater at rainfall intensity 420 mm h −1 than at 20 mm h −1. Needle-leaved branches generally retained more water per leaf area than did branches from broadleaved species, but branches that stored most at lower rainfall intensities tended to accumulate less additional storage at higher intensities. A simple nonlinear model was capable of predicting both magnitude (good model performance) and temporal scale (fair model performance) of storage responses to varying rainfall intensities. We hypothesize a conceptual mechanical model of canopy storage during rainfall that includes the concepts of static and dynamic storage to account for intensity-driven changes in storage. Scaling up observations to the canopy scale using LAI resulted in an estimate of canopy storage that generally agrees with estimates by traditional methods.

The influence of seed size and shape on their removal by water erosion

Under semiarid environments where surface runoff takes place, the role of water erosion in the removal of seeds and then in the plant establishment and distribution is unknown. Within the scientific literature, data about the size and shape of seeds exist, but little information can be found about the susceptibility of seeds to be removed by water erosion processes. These data are important not only from an eco-geomorphological point of view, but also for afforestation strategies in order to improve the selection of species to develop the recovery of vegetation on degraded environments. Eighty-three seed species were selected from Southeastern Spain. Variables related to size, such as weight ( M), length ( L), width ( W) and height ( H) of these seeds, were measured. From this information, other variables such as the surface ( S), volume ( V), density ( D) and the ratio S/ M were calculated. Seed shape was characterised by the Flatness Index (F.I.= L+ W/2 H) and the Eccentricity Index (E.I.= L/ W). Laboratory rainfall simulation experiments at 55 mm h −1 during 25 min on 26×26 cm plots with slope angle equal to 11° were performed with five lots of 50 seeds of each species. Results demonstrated that size is the main factor explaining seed removal, whereas the shape becomes important only when the seeds are larger than 50 mg. Seeds for vegetation recovery planning should have a mass between 10 and 50 mg to avoid removal by water erosion.

Heavy Metal Transport from Dredged Sediment Derived Surface Soils in a Laboratory Rainfall Simulation Experiment

Heavy metals in dredged sedimentdisposal sites may be transported through runoff andpercolation. In the present study metal fluxes byrunoff and percolation were determined under simulatedrainfall at a slope of 19% and a rainfall intensityof approximately 40 mm h-1. These circumstancescorrespond to half the mean annual erosivity of rainunder Belgian weather conditions. Surface runoff andpercolating water samples were analysed for suspendedsolids, total dissolved carbon and Cd, Cr, Cu, Ni, Pb and Zn.Runoff rate and sediment yields were highest for asilt loam sediment, characterised by a low clay andorganic matter content. Metal concentrations in runoffand percolating water varied widely between thesediments studied and were related to the total metalcontent in the sediment. In runoff and percolatingwater from the contaminated sediments, metalconcentrations strongly exceeded the Netherlands Areference values for ground water quality. Very highmetal fluxes were observed for the recently oxidiseddredged sediment. Metal transport per unit surfacearea through percolating water was from two to morethan twenty times greater than that in surface runoff.

Laboratory experiments on sequential scour/deposition and their application to the development of banded vegetation

Alternating bands of vegetation and bare soil, reported from many dryland regions, have been identified as indicators of rangeland deterioration triggered by overgrazing, cattle trampling or climatic change. Banded vegetation occurs at a range of scales and although it has been reported from a number of different environments, it is not characteristic of all degraded rangelands. It does appear particularly frequently on low angle, smooth slopes over soils of high erodibility but low permeability, where scant rainfall is sporadic or highly seasonal. It has been attributed both to wind and to water erosion, but few data on the processes active or their specific response to limiting environmental variables are available. Small-scale banded vegetation associated with small scour steps occurs on low angle alluvio-lacustrine flats surrounding Lake Baringo in semi-arid northern Kenya. In this area of strong moisture deficit, the dominant factor controlling the incidence of ground vegetation is variation in near-surface moisture storage. The regular spacing of the small-scale vegetation bands reflects preferential moisture storage in regularly-spaced sediment deposits. Field and laboratory runon and rainfall simulation experiments, previously reported, provided some data on the processes and conditions involved in formation of these deposits. This paper describes more closely-controlled laboratory rainfall simulation and runon experiments, carried out to identify the sequential scour and deposition processes involved, which are ultimately responsible for the regular variations in moisture storage capacity. These experiments, carried out in an 8.5 m long flume, show critical stream power ( Ω) conditions in sheetwash and rain-impacted sheetflow required for sequential scour/deposition as 0.020–0.025 W m −2 and between 0.043 and 0.055 W m −2, respectively. Experimental results indicate that vegetation bands at the scale observed at Baringo are consistent with development by sequential scour and deposition, caused by some combination of sheetwash, rainsplash and rainflow. The vegetation bands are composed primarily of unpalatable low herbaceous plants, dominated by Trianthema triquetra. These plants colonize or survive better on deposition zones because of better soil moisture status. The vegetation bands can therefore indicate an initial stage in vegetation recovery rather than continuing rangeland deterioration, but at Baringo further development of vegetation appears to be restricted by high grazing intensities or by allelopathy.

Influence of rolled erosion control systems on temporal rainsplash response—a laboratory rainfall simulation experiment

Reduction of erosion and sediment-related pollution from urban construction sites or other degraded hillslopes often relies on the initial application of suitable rolled erosion control systems (RECS) before natural vegetation cover can be established. However, research has not clearly explained why some RECS perform better than others, or under what particular conditions one system is more suitable than another. An important link between the application of the most suitable RECS and better product design is process-based studies relating the physical properties of products to the reduction of erosion subprocesses. This study investigates time-varying reduction of rainsplash detachment and transport by 13 commonly used RECS. The results indicate that product diAerences in the protection they provide against splash processes vary over the duration of a rain event, and that this variation is related to individual product properties, especially surface coverage and thickness. These results should aid in the design of more eAective erosion control products and in the selection of the most suitable RECS for particular hillslope applications. #1997 by John Wiley & Sons, Ltd.