Historical Soil Erosion

Levee overtopping creates large scoured region at the landside, which greatly affects the levee performance. Both 3D and 2D experiments were conducted to analyze the downstream scour profile after levee overtopping. For elucidating the difference of scour pattern under various overtopping water depth and width conditions, 3D scour profiles are investigated. In addition, vertical 2D scour profiles are also investigated to compare the 3D scour profiles with 2D experiment results. From the analysis, it has been observed that for overtopping width (W bank) to levee height (h bank) ratio of 6 to 8, multiple scour holes appear with overtopping height (d bank) to levee height (h bank) ratio of 0.1 to 0.2, while for same W bank /h bank , two scour holes are created when d bank /h bank =0.25 to 0.3. Comparison of 2D and 3D scour data sets at downstream scour profiles shows that 3D experiments tend to create larger scour depth and shorter scour length than 2D cases. This result indicates the importance of 3D experimental analysis for levee design. .

Olive orchards are an important agro-ecosystem in the Mediterranean. Soil erosion is a widely recognized threat to their sustainability. However, the variability of short-term soil erosion measurements and the limited understanding of driving processes result in a considerable uncertainty over the long-term effects of soil erosion. This study aims at measuring and modelling soil erosion rates in olive orchards over a 250-year period, and relating these to changes in management practices and yield, as documented from historical sources. In three study areas in S-Spain, the height of relic tree mounds was measured in olive orchards dated between 153 and 291 years old to determine soil profile truncation. Measured average soil erosion rates were between 29 and 47 t ha -1 year -1 . Historical documents allowed characterizing land management since 1752 in eight distinct periods. This information was then used to calibrate a soil erosion model, combining water and tillage erosion. The model reproduced the temporal patterns in soil erosion rates and showed considerable historical variation: between 8 and 124 t ha -1 year -1 for water and between 3 and 42 t ha -1 year -1 for tillage. Mainly due to improved agronomic practices, yield was not affected by soil erosion and has continuously increased over time.

We present a computer simulation of hydraulic erosion on levees, dams, and earth embankments, with emphasis on rill and gully initiation and propagation. We focus on erosion features that occur after an earthen structure is overtopped. We have developed a 3D fluid and hydraulic erosion simulation engine using Smoothed Particle Hydrodynamics (SPH). We present the results of digital simulations for different soil types. Furthermore, small-scale physical models of levees composed of different soils were constructed and tested experimentally. The digital simulations are compared to physical experimental results to validate the computer models.

Research is being done to study the details and progress of soil erosion on levees and dams, including the formation and progression of rills and gullies on the slopes, and eventually to final breaching. These detailed observations of erosion differ from the typical predictions of only the maximum erosion or scour depths, for example around submerged bridge piers. Computer simulations and geotechnical centrifuge modelling will, in the future, be validated using these observations. For testing, single layer sand models were utilized, and will be followed by clayey and mixed soils, and increased number of layers. The computer simulations will incorporate 3-D Navier-Stokes fluid simulations, and a novel segmented height field extended to allow soil undercuts was developed. The primary intent of the research is to study small-scale erosion on earthen embankments and, ultimately, develop novel and robust erosion software, validated by physical modelling. Figure 1: A levee that was overtopped for several hours during the Katrina hurricane but did not fail. The dramatic gouging/scooping on the lower portions of the levee is due to increased water flow at the base of the levee and the nonhomogeneous nature of the embankment.

ABSTRACT: Research is being done to study the details and progress of soil erosion on levees and dams, including the formation and progression of rills and gullies on the slopes, and eventually to final breaching. These detailed observations of erosion differ from the typical predictions of only the maximum erosion or scour depths, for example around submerged bridge piers. Computer simulations and geotechnical centrifuge modelling will, in the future, be validated using these observations. For testing, single layer sand models were utilized, and will be followed by clayey and mixed soils, and increased number of layers. The computer simulations will incorporate 3-D Navier-Stokes fluid simulations, and a novel segmented height field extended to allow soil undercuts was developed. The primary intent of the research is to study small-scale erosion on earthen embankments and, ultimately, develop novel and robust erosion software, validated by physical modelling. Figure 2: Rills form in...

Improved computer models of erosion have been developed, considering soil hydraulic conductivity. The models deal with erosion of levees, dams and embankments due to overtopping. The simulations trace the formation of rills and gullies, beginning with initial overtopping and continuing to final breaching. Physical models performed at 1-g and high g using a geotechnical centrifuge have been used to calibrate the models. Previous models did not consider soil hydraulic conductivity, and although results were quite good for the formation of rills and gullies and sediment quantities, breach times were underestimated. Essentially the water flow was treated as if passing over a solid surface, not entering the soil, and the total water flow was available for erosion. Thus, breach times were underestimated. Soil erodibility parameters had to be adjusted in order to achieve good agreement with breach times. The new models developed consider soil hydraulic conductivity, and produce good agreem...

Three dimensional experiments are conducted to analyze the downstream scour dimensions after levee overtopping. For elucidating the difference of scour dimension under various hydraulic (overtopping flow height at the levee top, d bank) and geometric parameters (levee height, h bank , overtopping flow width along the levee, W bank), 3D scour profiles are investigated. From the analysis, it has been observed that at equilibrium conditions, for the same d bank cases scour depth (S d) is higher for greater h bank (10cm) compared with lower levee height (6cm). In contrast, scour length (S l) is longer in case of lower h bank compared to higher h bank for same d bank. The stronger hydraulic jump that took place in case of smaller levee height cases appears to play a significant role in developing greater scour length along with shorter scour depth. Three separate equations for S d , S l and S w (scour width) have been proposed from multiple regression analysis. These results can be useful for the study of levee design.

Levee overtopping creates large scoured region at the landside, which greatly affects the levee performance. Both 3D and 2D experiments were conducted to analyze the downstream scour profile after levee overtopping. For elucidating the difference of scour pattern under various overtopping water depth and width conditions, 3D scour profiles are investigated. In addition, vertical 2D scour profiles are also investigated to compare the 3D scour profiles with 2D experiment results. From the analysis, it has been observed that for overtopping width (W bank) to levee height (h bank) ratio of 6 to 8, multiple scour holes appear with overtopping height (d bank) to levee height (h bank) ratio of 0.1 to 0.2, while for same W bank /h bank , two scour holes are created when d bank /h bank =0.25 to 0.3. Comparison of 2D and 3D scour data sets at downstream scour profiles shows that 3D experiments tend to create larger scour depth and shorter scour length than 2D cases. This result indicates the importance of 3D experimental analysis for levee design. .

Levee overtopping creates large scoured region at the landside, which greatly affects the levee performance. Both 3D and 2D experiments were conducted to analyze the downstream scour profile after levee overtopping. For elucidating the difference of scour pattern under various overtopping water depth and width conditions, 3D scour profiles are investigated. In addition, vertical 2D scour profiles are also investigated to compare the 3D scour profiles with 2D experiment results. From the analysis, it has been observed that for overtopping width (W bank) to levee height (h bank) ratio of 6 to 8, multiple scour holes appear with overtopping height (d bank) to levee height (h bank) ratio of 0.1 to 0.2, while for same W bank /h bank , two scour holes are created when d bank /h bank =0.25 to 0.3. Comparison of 2D and 3D scour data sets at downstream scour profiles shows that 3D experiments tend to create larger scour depth and shorter scour length than 2D cases. This result indicates the importance of 3D experimental analysis for levee design. .

Soil translocation due to tillage by the ox-drawn ard plough appears to be an important source of colluviation behind stone bunds and lynchets in the Ethiopian highlands. To quantify erosion rates caused by this plough in Ethiopia, painted and numbered rock fragments, 3±5 cm in intermediate diameter, were used as tracers to monitor soil movement on 16 sites, each having a different slope gradient, in the district of Dogu'a Tembien, Tigray Region, Ethiopia. Average tillage depth was 8.1 cm and the net mean downslope displacement distance per tillage operation ranged from 4.7 cm for a 0.03 m m À1 slope to 34.4 cm for a ®eld with a gradient of 0.48 m m À1 . There was a strong correlation (R 2 0X84, P`0X001) between slope gradient and downslope displacement. Where present, large rock fragments (>15 cm intermediate diameter) are obstacles to the downslope movement of tilled soil. The unit soil transport rate (Qs) per tillage operation ranged from 4.8 kg m À1 on the 0.03 m m À1 slope to 38.7 kg m À1 on the 0.48 m m À1 slope. These values represent the mass of soil deposited by tillage behind 1 m of lynchet or stone bund. During each tillage operation the same mass of soil is also removed from the foot of the upper stone bund or lynchet. For the ®rst tillage operation, before the onset of the rainy season, the tillage transport coef®cient (K) was 68 kg m À1 . As farmers till 1±4 times per year, annual K values can be assessed to range from 68 to 272 kg m À1 . These values are less than those observed for mechanised tillage, which however, is usually conducted on less steep slopes. On average, tillage erosion can be held responsible for half of the sediment deposited behind newly constructed stone bunds in the Tigray highlands. #

Soil translocation due to tillage by the ox-drawn ard plough appears to be an important source of colluviation behind stone bunds and lynchets in the Ethiopian highlands. To quantify erosion rates caused by this plough in Ethiopia, painted and numbered rock fragments, 3±5 cm in intermediate diameter, were used as tracers to monitor soil movement on 16 sites, each having a different slope gradient, in the district of Dogu'a Tembien, Tigray Region, Ethiopia. Average tillage depth was 8.1 cm and the net mean downslope displacement distance per tillage operation ranged from 4.7 cm for a 0.03 m m À1 slope to 34.4 cm for a ®eld with a gradient of 0.48 m m À1 . There was a strong correlation (R 2 0X84, P`0X001) between slope gradient and downslope displacement. Where present, large rock fragments (>15 cm intermediate diameter) are obstacles to the downslope movement of tilled soil. The unit soil transport rate (Qs) per tillage operation ranged from 4.8 kg m À1 on the 0.03 m m À1 slope to 38.7 kg m À1 on the 0.48 m m À1 slope. These values represent the mass of soil deposited by tillage behind 1 m of lynchet or stone bund. During each tillage operation the same mass of soil is also removed from the foot of the upper stone bund or lynchet. For the ®rst tillage operation, before the onset of the rainy season, the tillage transport coef®cient (K) was 68 kg m À1 . As farmers till 1±4 times per year, annual K values can be assessed to range from 68 to 272 kg m À1 . These values are less than those observed for mechanised tillage, which however, is usually conducted on less steep slopes. On average, tillage erosion can be held responsible for half of the sediment deposited behind newly constructed stone bunds in the Tigray highlands. #

The two most significant parameters which affect the erodibility of soils are soil plasticity and soil compaction during the levee construction. The erodibility potential of levee material has an influence on scour generation due to overtopping from floodwalls. In this investigation, 1/20 scaled physical models with different soil plasticity indexes and compaction levels are considered to evaluate levee performance against scouring. Typical levee construction methods for Mississippi river levees were followed for construction of physical models. In each test, the soil mass was compacted in several thin layers to reach the targeted field compaction. During the lab tests, different geometric and hydraulic parameters of levee and flow were monitored to identify the scour development. The Froude number of the flow was measured in each test to ensure a reasonable comparison between the flow in laboratory-scaled test and typical overtopping in the field. The results of physical modeling tests indicate how the scour potential for levees vary with different soil plasticity and compaction levels during the construction.

This paper describes the SOTREDI project, "Soil TREatment for Dikes", undertaken by Lhoist Group, a lime producer, since 2005. This project was led in partnership with research centres and universities, and presents some potential applications. The objective was to demonstrate that soil treatment with lime, besides enhancing the materials workability, confers high levels of mechanical properties, low permeability values, and is likely to reduce piping and internal erosion hazards, and to significantly increase the erosion resistance against overtopping and overflowing. This paper is a first step toward the application of lime-treated soils in earthen hydraulic structures (namely canals, dams, dikes and levees).

The evolution of agricultural landscapes is the result of soil redistribution processes caused by water erosion and human activity. Some agricultural areas, well-documented by historical and archaeological data, are useful to understand the evolution of the landscape structures and soil erosion dynamics. This research has been developed in one of the most relevant archaeological areas of East Africa, the site of Aksum (Ethiopia), where the flourishing and decay of the Aksumite Kingdom (400BC 800AD) took place. This site provided favorable conditions for a long-term approach to the study of soil conservation techniques, since such techniques have been maintained from ancient times to the present. In the current study, the surfaces of three terrace systems have been surveyed and analyzed in terms of soil loss, with the plough mark methodology (Ciampalini et al., 2008(Ciampalini et al., , 2012, providing the amount of soil loss within the last two millennia. In this context, we performed long-term modelling for tillage and water erosion with the expert model, LandSoil (Ciampalini et al., in press), which is based on a raster distributed approach and accounts for the principal components in soil erosion-deposition processes.

From the antiquity, the hilly landscape around the city of Axum in northern Ethiopia has been terraced and extensively used for agriculture purposes. Impacts of the “ard” plough on protruding and/or buried boulder at a depth less than the tillage depth produce scratches on their sides or their top. The spatial relationship among plough marks and rock surface orientations gives information on the ploughing direction, whereas the relative height from the present ground surface to the top of the marks and information on the age of cultivation can be used to assess soil loss rates. Parameters to define plough mark orientation were defined and measured in the field on boulders with plough marks within a study terrace located in an archaeological area with an approx of 2040 year age. These parameters are height from the ground, dip, slope and frequency.The measurements also show that the initial terrace slope was steeper than the present one confirming the effectiveness of the traditional terracing practice in soil conservation because of the low soil erosion rates assessed over a so large time interval. For the main time intervals of the Axumite history, corresponding to the rise and fall of the Axumite civilization and to variable conditions of human pressure on the land, the maximum erosion rates inferred are of the order of 3.4 Mg ha− 1 y− 1. This result is compared with erosion rates calculated with other methods reported in literature (e.g. PSIAC) and field measurement on terraces in the same region leading to values coinciding with those obtained by the plough marks method.