Browsing by Author "Morgan, R. P. C."
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Item Open Access The effect of grass cover on bank erosion.(Cranfield University, 1989-09) Tengbeh, G. Tamba; Morgan, R. P. C.The scour experiments on the bare root-free and root-permeated soils indicated that for each soil, critical tractive force (CTF) linearly increases with both root density and vane shear strength. However, for both soilsq CTF was mainly related to vaneýshear strengthp indicating the potential importance of soil shear strength as an index of scour erodibility of cohesive channel bank materials. The analysis of the relative effects of the grass vegetation parameters on scour resistance confirmed the dominance of vegetation shoots relative to the roots in resisting scour in-non-bending vege- tation. The results showed that it is the initial introduction of vegetation into bare (root-free) bank conditions that produces the greatest increase in scour resistance and that subsequent increases in vegetation density bring about relatively lower increases in scour resistance. However, in all the vegetation densities studied, root- permeated so-ils contributed significantly to scour resistance in low flows especially through low vegetation densities. Compared to root- free soil conditions, sandy clay loam soils permeated with 1.8 g/CM3 of roots increased their scour resistance by more than 400%. Althgouh these results may only be indicative of the low flow depths as would exist in shallow grassed channels commonly used for agricultural run- off drainage, they nevertheless highlight the importance of root density in contributing to the total flow resistance of grassed channel banks. The bank stability analysis indicated that for low channel banks (1.5m high), grass roots can stabilise banks with even vertical slopes against toe and slope vailures. For high (2.25m) and vertical bank conditions, the results indi- cate that the effects of increases in root density may need to be com- plemented by bank shaping in order to achieve stability. The scour and bank stability findings indicate that the three most important charac- teristics for the selection of grass vegetation for bank protection are quick establishmentg the development of a stiff shoot system and a strong root mat.Item Open Access Erosion hazard assessment in the upper Ewaso Ng’iro basin of Kenya: application of GIS, USLE and EUROSEM(1999-09-15) Mati, Bancy Mbura; Morgan, R. P. C.; Gichuki, F. N.; Quinton, J. N.; Brewer, Timothy R.; Liniger, H. P.A methodology was developed for assessing soil erosion hazard in the Upper Ewaso Ng’iro basin of Kenya, using Geographic Information Systems (GIS), the Universal Soil Loss Equation (USLE) and the European Soil Erosion Model (EUROSEM). The USLE was used in a GIS environment by creating thematic maps of R, K, L, S, C and P and then calculating soil loss by raster-grid modelling with Arc/Info GRID. The rainfall erosivity factor (R) was derived from relationships between rainfall amount and erosivity using erosion plot data from within the catchment. The nature of the relationship was found to be a function of agro-climatic zones of the region. Mean annual erosivities ranged from 145 to 990 J m'2 hr'1. For a given amount of rainfall, erosivity was higher in zone IV than in the wetter zones II-III. The soil erodibility factor (K) was estimated using the USLE nomograph and data from laboratory analysis of field samples collected from representative major soil mapping units. The K-values were low to medium, ranging from 0.10 to 0.25 over 84 percent of the basin. The topographic factor (LS) was obtained by creating Digital Elevation Models (DEMs) of the basin with TOPOGRIDTOOL of Arc/Info. These were then used to determine the slope steepness and length factor values, calculated with raster-grid modelling. Although DEMs proved a useful tool, maximum values of both steepness and length had to be set in this reconnaissance study to achieve reasonable results. A finer resolution of input data and a smaller grid cell size are needed for accurate determination. The cover and management factors (C) were obtained by determining the land cover types within the basin using remotely sensed data (SPOT 1 colour composite prints) and ground truthing studies. The factor values were estimated from USLE guide tables and measurements of cover from plots and test sites. Some 70 percent of the basin is covered by rangelands. The conservation practice (P) factor values were estimated from USLE guide tables and then applied to areas where soil conservation had been introduced according to maps obtained from the Ministry of Agriculture. The USLE was validated using data from erosion plots. A value of R2 = 0.645 was obtained between predicted and measured values but the standard error was rather high (e = 5.745 t ha’1 yr'1). Using an annual soil loss of 9.0 t ha'1 yr'1 as tolerance level, some 36 percent of the basin was found to experience unacceptably high erosion rates. Most of this area was communal grazing land and cropland where soil conservation measures had not been applied. A critical land cover type within the grazing land is shrubland, where vegetation cover is less than 40 percent and high erosion risk was predicted and confirmed by field surveys. EUROSEM could not be integrated within a GIS in the time available for research. It was therefore simulated outside GIS environment, where it was applied to Embori and Mukogodo plot data using separate data sets for calibration and validation. Calibration was used to obtain input parameters for saturated hydraulic conductivity, cohesion and Manning’s roughness coefficients. Validation gave correlation coefficients of 0.907 and 0.840 for predictions of storm runoff and soil loss respectively at Embori; the corresponding values for bare soil plots at Mukogodo were 0.895 and 0.577. However, EUROSEM predicted runoff poorly (R2 = 0.570) and failed to predict soil loss at all the vegetated plots at Mukogodo. The model was applied to simulated vegetation covers of barley, maize, grass and forest for a 36.7 mm rainstorm at Embori. The simulated soil losses showed an exponential decrease with increasing cover. At a threshold cover of 70 percent, soil loss diminished to zero under grass and forest and decreased to a minimum value under barley and maize. These results support the USLE simulations, which showed that areas with more than 70 percent cover (such as forest) had a low erosion hazard, even with steep slopes and high rainfall erosivities. This research has demonstrated that GIS can be used with the USLE to assess and quantify erosion hazard, giving results that can be used for conservation planning. EUROSEM can be applied successfully to bare soil and cropland, but application to other land covers requires further investigation. Land cover and topography are the main factors controlling the spatial distribution of soil loss in the Upper Ewaso Ng’iro basin. Future conservation activities should be concentrated on the rangelands.Item Open Access Extent, causes and rates of upland soil erosion in England and Wales(Cranfield University, 2000-08) McHugh, Marianne; Harrod, T. R.; Morgan, R. P. C.The uplands of England and Wales are internationally important for nature conservation and are prized nationally for their landscape, recreational and cultural value. Reflecting this status, recent appeals, have prompted fundamental research into degradation of the upland habitat. This work was instigated as part of that research to determine the current extent, causes and rates of upland degraded soil. The research was based principally upon a statistically robust and objective selection of 399 field sites. Erosion extent and condition were recorded and related to morphology, environment and management conditions within both field sites and sub-catchments. The short-term rate of soil loss was determined through the completion of cross-sectional traverses on erosion gullies in 1997 and in 1999. Longer-term variations in the extent and causes of upland erosion were established through the interpretation of aerial photographs taken between 1946 and 1989. Erosion measured on field sites in 1999 was estimated to represent 24 566 ha and 0.284 km3 in upland England and Wales. Of this, 18 025 ha and 0.242 km3was attributed to water erosion, which included large-scale blanket peat degradation. Biotic factors accounted for 6 541 ha and 0.041 km3 of erosion, evident on 68% of eroded field sites. Wind was a negligible contributor to upland erosion. Upland eroded area in England and Wales increased by over 518 ha between 1997 and 1999: humans and animals were responsible for 99.9% of this increase. Within the last half-century, the creation and perpetuation of erosion was also principally due to humans and animals. Water-eroded features on both peat and mineral soils showed stabilisation and revegetation throughout the same period. These results expose the highly degraded state of the upland environment and the alarming rate at which erosion is proceeding. The implications of this erosion and proposals for mitigation policies are discussed.Item Open Access Laboratory experimentation for the statistical derivation of equations for soil erosion modelling and soil conservation design.(Cranfield University, 1982-04) Quansah, C.; Morgan, R. P. C.Since Ellison (1947) described the process of erosion as comprising a) the detachment of soil particles from the soil mass by raindrop impact, b) detachment by runoff, c) the transport of the detached particles by raindrop impact, and d) transport by runoff, research has been directed into the mechanics of each of these four phases and how the phases might be linked together in the form of a soil erosion model, such as the Meyer-Wischmeier (1969) model. From a literature review, it became evident that in spite of this work, gaps in knowledge still exist and that i) most studies on soil erosion tend to lump the processes together; ii) whilst a considerable amount of investigation has been carried out on splash erosion, the other processes have received very little attention; iii) there is no explicit study on the effects of factor-interactions on the processes and the role of the laboratory as a place for studying interactions by controlling factors has not attracted much attention; iv) equipment and techniques for the separate evaluation of the detachment and transport of soil particles by overland flow are not available; and v) studies on the hydraulic characteristics of overland flow in relation to the detachment and transport of soil particles in such flows are scarce. This study was therefore specifically aimed at establishing a sounder research base for modelling the subprocesses and ultimate~ for soil conservation design b,y: i) evaluating separate~ each of the above subprocesses; ii) assessing the influence of the factors affecting the processes, particular~ their interactioDS; and iii) examining the hydraulics of soil particle detachment and transport by overland flow with and without rain. As a means to achieve these objectives, a factorial experiment vas set up in the laboratory to examine both the individual effects of rainfall intensity (50, 80, 110, 140 mm h- 1 ) , soil ~ (standard sand, ISIUld, clay loam and clay) and slope steep:1.8Ss (3.5, 7.0, 10.5 and 14.0 per cent) and their interactions on each of the above subprocesses. Additionally, the effects of four rates of runoff (1.0, 1.6, 2.2 and 2.8 ~min) on the hydraulic characteristics of flow such as velocity, depth, Reyuolds number, Froude number and friction factor were examined and used in characterizing the detachment and transport of soil particles in these flows. For each subprocess, these variables were replicated four times. Splash detachment and transport were determined by simulating rainfall from a nozzle simulator over a target soil placed in a rectangular soil tray (10 x 20 x 4 cm) which being set in the centre of a catching tray (90 x 80 x 30 cm) allows for the separate determination of ups lope and downslope splash. The separate measurement of the detachment and transport of soil particles by overland flow with and without rain was carried out b,y using a specially designed rainfall simulator - bed flume facility with runoff and sediment input and measuring devices. The results were analysed by analysis of variance to show the Significance of soil type, rainfall in tensi ty, flow rate t slope steepness and their first and second order interactions in influencing the processes studied. Multiple correlation techniques were used to search for the best associations between the erosion influencing variables and soil loss. RegreSSion analySis was used for establishing predictive equations for detachment and transport rates. Detachment of the test soils by splash can be placed in rank order of standard sand, sand, clay and clay loam with increasing resistance. For splash transport the order is standard sand ) clay > sand > clay loam. For each soil type there are significant increases in splash detachment and transport with increasing rain intensity and slope steepness. The most significant interactions influencing the two splash processes are soil x intensity and slope x intensity for detachment and transport respectivel,J. Significant interactions show that the factors are not independent of each other; the simple effects of a factor differ, and the magnitude of any simple effect varies according to the level of the other factors of the interaction term. The factors influencing detachment by flow without rain rank in ~ order of importance as soil type, slope steepness and discharge. The corresponding order for flow with rain is discharge, slope steepness and soil type. The order of soil detachability for both flow with and without rain is standard sand , sand ~ clay loam> clay. There are also significant increases in detachment rate as slope steepness and flow rate increase. It is further shown that the first and second order interactions of the above factors Significantly influence detachment by flow. On a relative basis, the second order interaction is small and the importance of the first order interactions can be placed in an increasing order of slope x soil, slope x discharge t and discharge x soil for flow without rain. For flow with rain, they rank as slope x soil, discharge x soil, and slope x discharge. The slope x soil interaction showed that as slope steepens the influence of each Boil on detachment rates increases with the proportionate increase being greater for sand and standard sand than for clay and clay loam. The slope x discharge interaction revealed significant increases in detachment rate for all slopes as discharge increased. The magnitude of the response is however greater at the lower than higher slopes. As slope steepness increases, detachment rates by flow with and without rain are also enhanced. The increase was proportionately more for the 1.0 and 1.6 J/min than 2.2 and 2.8 J/min flows. The Boil x discharge interactiC?n also indicated that, for flow without rain, detachability increases more for clay and clay loam than for the sand and standal'd sand as discharge increases. In the presence of rain however, the response of the soils did not differ much. Detachment by flow without rain is predominantly by rilling. In the presence of rain, detacbment rates by flow are increased about three fold and relatively even removal of soil particles from the eroding bed is characteristic. Raindrop impact thus appears to inhibit rill formation by overland flow especially on small slope steepnesses. There is a critical slope steepness at which both raindrop impact and overland flow contribute equally to total detachment. At slopes lower than the critical value, raindrop impact is the main detaching agent whilst flow predominates the detachment process at steeper slopes. The critical slope steepness is soil specific and decreases in the order of clay ~ clay loam ) sand ~ standard sand. The transport of soil particles by combined flow and rain is significantly influenced by soil type, slope steepness, flow rate and their first and second order interactions. Transport rates decreased in the order of sand > standard sand ) clay ) clay loam. Increases in discharge and slope steepness significantly increased transport capacity. For a discharge range of 1.0 - 2.8 l/min, transport capacity increased four fold. The most significant interaction that influences transport capacity is slope x soil. Where factors interact significantly, interpretation of results based solely on the main effects of the influencing factors m&1 result in loss of vital information and lead to wrong conclusions. For example, examination of the slope x soil interaction showed that at lower slopes (3.5 and 7.0 per cent) combined flow and rain has a greater transport capacity for the larger clay and clay loam aggregates than for the fine grains of sand and standard sand. This is obscured when effects are averaged over all the slopes as is the case when only main effects are considered.Item Open Access Paulownia agroforestry in China - a contribution to adaptive research.(Cranfield University, 1996-10) Wu, Yunying; Morgan, R. P. C.Nearly 2 million hectares of farmland have been intercropped with rows of paulownia (Paulownia elongata) trees in the North Central Plain of China. Paulownia provides an important timber source and long term income as a "saving bank". It is in harmony both ecologically and economically with the understorey winter wheat (Triticum spp.), the farmers' "survival food". Funded by ODA (Overseas Development Administration, UK), the present study investigates the role of the paulownia-crop intercropping (PCI) system in rural socio-economic development. It also identifies the socio-economic and technical factors that affect the promotion and success of PCI and examines the needs and problems encountered in the development of PCI under the present "Individual Responsibility System" (IRS). A socio-economic survey on PCI in Chengwu, Shandong Province, as a case study, shows that paulownia intercropping is an important component of socio-economic development in the poor areas. Existing policy on paulownia intercropping and land distribution causes some problems for farmers, and technical guidance on optimum tree spacing, management and maximum production of PCI with emphasis on wheat yield is eagerly awaited. An adaptive research approach to address the farmers' needs was applied in this study by interpreting scientific research to produce a useful, simple and practical model to optimise tree spacing for a range of scenarios. Data used for analysis were collected during 1983 - 1992 from an experiment station at Dangshan, Anhui Province, on a project supported by The International Development Research Centre (IDRC) of Canada. The important components of the PCI system including tree growth, growth and yield of understorey crops, and light, as a key microclimate factor affecting understorey crops, were analysed to further understanding on the natural functioning of the PCI system. The denser the spacing, the earlier the fast-growth stage of the trees occurs. However, there was no significant difference among the spacings in any of the growth variables at the end of the study in the 10th intercropping year. There are always reductions in light transmissivity (LT) and Photosynthesis Active Radiation (PAR) and the proportion of PAR to total solar radiation in the intercropped fields due to light interception by the trees. These effects are more apparent in areas near the trees, in denser spacings, in older paulownia, and when the paulownia have fully developed leaves. Intercropped wheat benefits from the modified microclimate, such as reduced temperature and increased soil water contents and relative humidity. Air temperature is the only microclimatic factor determining wheat leaf conductance, which gives a negative effect. Wheat yields start to decrease when LT is reduced to 81.8% as an average and about 10% higher for dense spacing and older trees and 10% lower for larger spacing and younger trees. Leaf conductance, biomass and yield of the intercropped wheat are slightly above those in open fields when the trees are young and widely spaced, but lower otherwise. The distribution of wheat yield across the alley in the intercropped field was similar to that of light, apart from in the dense spacing of older age trees where the competition of tree roots for nutrients probably also reduces yields near the tree rows. The reduction of leaf conductance and yield of summer crops by paulownia is greater than for the winter crop due to shading by the trees and different response of the crops. The yield reduction of cotton and maize is 15% at the edge of the tree canopy and as much as 50% near the tree rows. Regression models were developed to express DBH (used as an index of tree growth) as a function of age; light transmissivity as a function of distance across the alley for different spacings and ages of trees; understorey wheat yield as a function of distance across the alley for different spacings and ages of trees; and understorey wheat yield as a function of light transmissivity. The models can yield information on rates of tree growth and changes in understorey wheat yield and LT, by inputting tree age, and distance from west row under a given tree spacing. The output will help farmers optimise the tree-row spacing depending on whether their objective is to maximise timber production, crop production or the economic return from both. The models could also provide useful and practical technical guidance to help decision-making and settle disputes among farmers and between farmers and the local authority. Models were validated by the data collected during 1993 - 1994 from farmers' fields in Chengwu, Shandong Province. All the models perform well in these conditions and it is recommended that they be applied to PCI in all parts of North Central Plain of China due to the similarity of climate and soil conditions. Optimum tree spacings under different scenarios were also recommended. The present study demonstrated the approach of adaptive research in PCI agroforestry by linking scientific findings to rural socio-economics and farmers' needs. It indicated the equal importance of three components: socio-economic survey, experimental data analysis and interpretation, and application of scientific findings. Socio-economic survey at the village or farm level is essential for identifying the needs of farmers from PCI under IRS conditions. Data analysis helps scientists and extension officers to understand and interpret the dynamic eco-biological interactions of PCI systems . Regression modelling is a practical, efficient, simple and easy way of interpreting and applying scientific findings to a realistic PCI system.Item Open Access The use of geotextiles for soil erosion control(Cranfield University, 2000-07) Rickson, R. Jane; Morgan, R. P. C.Geotextiles are used by engineers for a variety of applications, such as filtration, separation, slope stabilisation, drainage and soil erosion control. At present, there is little research on geotextiles for erosion control, despite the increase in their use in the field. There are no guidelines for soil conservationists or field/civil engineers to indicate which product will be most effective for any given site. Studies that do exist tend to be qualitative and descriptive, rather than quantitative and scientific. There is also very limited identification and understanding of the salient physical properties of erosion control geotextiles. The present laboratory experiments aim to contribute objective, scientific data. These experiments evaluate the performance of seven different erosion control geotextiles. The products are selected to be representative of the types of erosion control geotextile currently on the market. The products tested are four natural fibred products, including three woven bionets and one biomat. Three synthetic geomats are also tested: two are buried and one is installed on the soil surface. A bare soil plot is used as the control in all the tests. The experiments are designed to simulate erosion processes at the sub-process level. This is achieved by simulating rainsplash and overland flow, both separately, and in combination. Experimental variables used include rainfall intensity (35 mm/hr, 95 mm/hr and 115 mm/hr), overland flow rate (40 ml/sec) and soil type (sandy loam and clay loam). Runoff volume, infiltration volume and soil loss are collected for each experimental run. From the results of these tests, it is possible to indicate how geotextiles modify incoming rainfall and surface hydrology, and therefore affect rates of soil detachment and transport. The results show that erosion control effectiveness is influenced by the physical characteristics of the geotextiles tested, soil type and rainfall intensity. The products tested have insignificant effect on runoff volumes generated, but soil loss varies considerably for the different treatments. Overall, the natural, woven products Rickson, R.J. 2000 Cranfield UNIVERSITY perform most effectively, reducing soil loss significantly when compared with the bare soil control, for all experimental conditions tested. The buried, synthetic products were not as efficient at controlling soil loss: under some experimental conditions soil loss from these treatments was even greater than that observed for the bare soil control. The results are analysed in terms of the salient properties of the geotextiles, which explain their performance. The salient properties identified were: percentage ground cover provided by the geotextile, water holding capacity, Geotextile Induced Roughness, wet weight of geotextile and ability to increase overland flow depth. The limitations of the laboratory-based research are indicated, such as the problems of extrapolation from small test plots up to field scale applications, and the difficulties of controlling the interactions between the geotextiles and the experimental variables. Implications of the research to the erosion control industry are made. For existing and potential end-users of erosion control geotextiles, the decision to specify these products is not only related to technical performance (as quantified in the laboratory studies), but also to the assessment of erosion risk, costs and compliance criteria. For manufacturers, identification of the salient properties of effective erosion control geotextiles helps in the development and design of improved products. Recommendations for future research include study of the variability in geotextile performance as related to external factors such as soil type, rainfall intensity, slope steepness and slope length. Incorporating the effect of erosion control geotextiles into physically based erosion prediction models such as EUROSEM and WEPP has great potential. Identification and quantification of critical values of the salient properties of erosion control geotextiles has considerable scope. The performance of geotextiles at controlling erosion over longer time periods (greater than one storm event) requires further investigation. Finally, research into the synergistic relationships between geotextiles and vegetation also warrants further research.