Sediment budgeting: A case study in the Katiorin drainage basin,Kenya

The primary objective of this study was to compute a detailed budget for a small semiarid tropical drainage basin in Kenya. Results indicated that transfer of sediments (‘inputs’) from primary source areas was minor in comparison to changes in storage. The major sediment source area within the Katiorin drainage basin was the colluvial hillslope zone. The net change in storage within this zone was approximately 2100 Mg yr^?1. Surface wash and rilling were the dominant transport processes responsible for the remobilization of colluvial sediments. Sediment storage within the in-channel reservoir increased by 60 Mg yr^?1, which was minor when compared to the total store of sediment in this reservoir. During 1986, the channel network stored only a small fraction ( < 3 per cent) of the sediment delivered from the hillslope subsystem. Therefore, the in-channel reservoir had limited influence on sediment conveyance to the basin outlet. These data indicate that a static equilibrium condition cannot be assumed within the Katiorin drainage basin. Such an assumption would result in erosion estimates of approximately 5.5 mm yr^?1 for the entire basin (based on a sediment output of 7430 Mg km^?2 yr^?1 and a measured bulk density of 1.35 Mg m^?3). However, this masked the actual rates of 1.2 to 7.1 mm yr^?1 in subbasin primary source areas, and rates of 0.6 to 17 mm yr^?1 for colluvial material in the various subbasins. The extreme accelerated erosion rates resulted from minimal ground vegetation, steep slopes, soil crust formation, an erodible substrate, and a well-integrated drainage network for rapid conveyance of sediments from the hillslope subsystem to the basin outlet.     

Micropiping processes and biancana evolution in southeast Tuscany, Italy

Biancane badlands consisting of small domes dissected by rills and micropipes, with rough disordered microrehef, can be found along the Apennines in Italy. The dominant processes forming biancane differ from those of badlands formed on smectite-rich mudrocks, as micropipes associated with pseudokarstic enlargement of pores and cracks predominate and form the main routes for evacuation of eroded material.Biancana evolution is controlled by water infiltration into intact bedrock, producing an erodible weathering ‘rind’ which is more porous than intact rock. This rind is easily removed by rill or micropipe flow, and erosion is therefore ‘weathering-controlled’, depending on rind production by infiltrating water. Infiltration is initially slow and stepped, due to slow water movement through very small capillary pores in intact rock alternating with rapid filling of macropores and cracks. This occurs due to rapid matrix pore enlargement by dispersion and/or dissolution. The infiltration pattern is accurately reproduced by a model built on progressive development of weathering layers by moisture penetration. Model results are consistent with weathering rind depths and erosion observed in the field, and show that a pipe network can be generated on newly exposed rock by the rainfall of one year.Propagation of the pipe network diverts a progressively larger proportion of runoff into micropipes, expanding weathering rind production within the biancana as well as on the surface. Internal weathering and flow progressively dominate with few unweathered corestones, and the biancana gradually collapses into a penultimate “soufflé-like” form.     

Flow energy and channel adjustments in rills developed in loamy sand and sandy loam soils

The storms usually associated with rill development in nature are seldom prolonged, so development is often interrupted by interstorm disturbances, e.g. weathering or tillage. In laboratory simulated rainfall experiments, active rill development can be prolonged, and under these conditions typically passes through a period of intense incision, channel extension and bifurcation before reaching quasi‐stable conditions in which little form change occurs. This paper presents laboratory experiments with coarse textured soils under simulated rainfall which show how channel adjustment processes contribute to the evolution of quasi‐stability. Newly incised rills were stabilized for detailed study of links between rill configuration and flow energy. On a loamy sand, adjustment towards equilibrium occurred due to channel widening and meandering, whereas on a sandy loam, mobile knickpoints and chutes, pulsations in flow width and flow depth and changes in stream power and sediment discharge occurred as the channel adjusted towards equilibrium. The tendency of rill systems towards quasi‐stability is shown by changes in stream power values which show short‐lived minima. Differences in energy dissipation in stabilized rills indicate that minimization of energy dissipation was reached locally between knickpoints and at the downstream ends of rills. In the absence of energy gradients in knickpoints and chutes, stabilized rill sections tended toward equilibrium by establishing uniform energy expenditure. The study confirmed that energy dissipation increased with flow aspect ratio. In stabilized rills, flow acceleration reduced energy dissipation on the loamy sand but not on the sandy loam. On both soils flow deceleration tended to increase energy dissipation. Understanding how rill systems evolve towards stability is essential in order to predict how interruptions between storms may affect long‐term rill dynamics. This is essential if event‐based physical models are to become effective in predicting sediment transport on rilled hillslopes under changing weather and climatic conditions. Copyright © 2008 John Wiley and Sons, Ltd.     

Hydraulic conditions for rill incision under simulated rainfall: A laboratory experiment

A series of controlled laboratory experiments were conducted in order to obtain precise data on the hydraulic and sediment transport conditions during rill formation. Tests were carried out using a crusting-prone binary mixed soil in a 15 m long flume at an average slope of 0·087 under simulated rainfall. Rainfall intensities varied from 30–35 mm h^?1 and developed about 70 per cent of the kinetic energy of natural rainfall of similar intensity. Runoff and sediment discharge measured at the downstream weir were strongly influenced by rill forming processes. Essentially, rill incision reduced runoff discharge as a result of increased percolation in rill channels but increased sediment discharge. Secondary entrainment processes, such as bank collapse, also increased sediment discharge at the weir. Knickpoint bifurcation and colluvial deposition, however, decreased sediment discharge. Rills always developed through the formation of a knickpoint. The critical condition for knickpoint initiation was the development of supercritical flow and waves which mould and incise the bed. Prior smoothing of the soil surface by entrainment and redistribution of sediment facilitated supercritical flow. Statistical analysis showed that hydraulic and sediment transport conditions differed significantly in rilled and unrilled flows. The relationship between sediment discharge, rill erosion, and flow hydraulics was found to be nonlinear, conforming to a standard power function in the form y = ax^b. Rills were also associated with significantly increased sediment transport capacities. However, rill initiation was not clearly defined by any specific hydraulic threshold. Instead, rilled and unrilled flows were separated by zones of transition within which both types of flow occur.     

The influence of slope angle on soil entrainment by sheetwash and rainsplash

Although a number of studies of the variation of soil transport with increase in slope angle have appeared, few include an information on the interaction of sheetwash and rainsplash on high slope angles, which is necessary to test Horton's proposed polynomial relationship. Virtually no studies are available which compare the influence of changes in soil type or antecedent moisture on established relationships. This paper reports the testing of eight soils from Alberta, Canada, under simulated rainfall on ten slopes from 3° to 30°. Material eroded was separated into that transported by rainsplash and that by sheetwash. In general, it was found that the influence of changes in slope angle on soil transport is best described by polynomial relationships, but these are shown to vary considerably between rainspash and sheetwash, between different soil types and for different antecedent moisture conditions. Despite careful control of all factors other than slope angle very high variability of results was experienced. Causes of variability are examined and the need for evaluation of the effect of test plot size on variability of results is suggested.     

Estimating parameters of the channel width–flow discharge relation using rill and gully channel junction data

Eroding channels can usually be characterized by a power relationship between channel width (W) and channel discharge (Q). This paper examines the WQ relation using a recently developed channel junction approach to extend the validity of the WQ relation and to develop a procedure for estimating the WQ exponent and proportionality coefficient. Rill and gully channel data from the literature, and new data collected in different badland areas and in a few forest mountain streams, are analysed. Analysis shows that the WQ relation for channel width collected in badlands and forests agrees with trends observed for cropland. The exponent increases with increasing channel width in a continuous fashion rather than in a step‐like way and tends to a maximum whose value ranges between 0·5 and 0·6. The proportionality coefficient can be split into two terms, one expressing the case in which an eroding channel can broaden, the other reflecting the difficulties in removing the less erodible clods or rock fragments from the channel bed. Its splitting allows the development of a more correct form of the WQ relation in agreement with modern approaches of channel geometry: one part has the dimension of a discharge and makes the power base dimensionless, while the other brings the dimension of a length, needed for the channel width, into the WQ relation. The interpretation of the two constants is supported by data collected in rainfall‐runoff simulation experiments conducted in the field. Values characterizing the two constants in some environments are also given. Nevertheless the approach is not sufficiently parameterized yet to be of practical use (e.g. in models or for estimating peak discharge in areas where rill channels have formed). Copyright © 2009 John Wiley & Sons, Ltd.     

Experimental study of rill bank collapse

Rill bank collapse is an important component in the adjustment of channel morphology to changes in discharge and sediment flux. Sediment inputs from bank collapse cause abrupt changes in flow resistance, flow patterns and downstream sediment concentrations. Generally, bank retreat involves gradual lateral erosion, caused by flow shear stress, and sudden bank collapse, triggered by complex interactions between channel flow and bank and soil water conditions. Collapse occurs when bank height exceeds the critical height where gravitational forces overcome soil shear strength. An experimental study examined conditions for collapse in eroding rill channels. Experiments with and without a deep water table were carried out on a meandering rill channel in a loamy sand and sandy loam in a laboratory flume under simulated rainfall and controlled runon. Different discharges were used to initiate knickpoint and rill incision. Soil water dynamics were monitored using microstandpipes, tensiometers and time domain reflectometer probes (TDR probes). Bank collapse occurred with newly developed or rising pre‐existing water tables near rill banks, associated with knickpoint migration. Knickpoint scour increased effective bank height, caused positive pore water pressure in the bank toe and reduced negative pore pressures in the unsaturated zone to near zero. Matric tension in unsaturated parts of the bank and a surface seal on the ‘interrill’ zone behind the bank enhanced stability, while increased effective bank height and positive pore water pressure at the bank toe caused instability. With soil water contents >35 per cent (sandy loam) and >23 per cent (loamy sand), critical bank heights were 0·11–0·12 m and 0·06–0·07 m, respectively. Bank toe undercutting at the outside of the rill bends also triggered instability. Bank displacement was quite different on the two soils. On the loamy sand, the failed block slid to the channel bed, revealing only the upper half of the failure plane, while on the sandy loam the failed block toppled forwards, exposing the failure plane for the complete bank height. This study has shown that it is possible to predict location, frequency and magnitude of the rill bank collapse, providing a basis for incorporation into predictive models for hillslope soil loss or rill network development. Copyright © 2006 John Wiley & Sons, Ltd.     

Soil erodibility and processes of water erosion on hillslope

The importance of the inherent resistance of soil to erosional processes, or soil erodibility, is generally recognized in hillslope and fluvial geomorphology, but the full implications of the dynamic soil properties that affect erodibility are seldom considered. In Canada, a wide spectrum of soils and erosional processes has stimulated much research related to soil erodibility. This paper aims to place this work in an international framework of research on water erosion processes, and to identify critical emerging research questions. It focuses particularly on experimental research on rill and interrill erosion using simulated rainfall and recently developed techniques that provide data at appropriate temporal and spatial scales, essential for event-based soil erosion prediction. Results show that many components of erosional response, such as partitioning between rill and interrill or surface and subsurface processes, threshold hydraulic conditions for rill incision, rill network configuration and hillslope sediment delivery, are strongly affected by spatially variable and temporally dynamic soil properties. This agrees with other recent studies, but contrasts markedly with long-held concepts of soil credibility as an essentially constant property for any soil type. Properties that determine erodibility, such as soil aggregation and shear strength, are strongly affected by climatic factors such as rainfall distribution and frost action, and show systematic seasonal variation. They can also change significantly over much shorter time scales with subtle variations in soil water conditions, organic composition, microbiological activity, age-hardening and the structural effect of applied stresses. Property changes between and during rainstorms can dramatically affect the incidence and intensity of rill and interrill erosion and, therefore, both short and long-term hillslope erosional response. Similar property changes, linked to climatic conditions, may also significantly influence the stability and resilience of plant species and vegetation systems. Full understanding of such changes is essential if current event-based soil erosion models such as WEPP and EUROSEM are to attain their full potential predictive precision. The complexity of the interacting processes involved may, however, ultimately make stochastic modelling more effective than physically based modelling in predicting hillslope response to erodibility dynamics.