SURFACE ROUGHNESS EVOLUTION OF SOILS CONTAINING ROCK FRAGMENTS

Soil surface roughness is a dynamic property which determines, to a large extent, erosion and infiltration rates. Although soils containing rock fragments are widespread in the Mediterranean region, the effect of the latter on surface roughness evolution is yet poorly understood. Therefore, laboratory experiments were conducted in order to investigate the effect of rock fragment content, rock fragment size and initial moisture content of the fine earth on the evolution of interrill surface roughness during simulated rainfall. Surface elevations of simulated plough layers along transects of 50 cm length were measured before and after simulated rainfall (totalling 192.5 mm, I = 70 mm h⁻¹) with a laser microreliefmeter. The results were used to investigate whether systematic variations in interrill surface roughness along stony hillslopes in southeastern Spain could be attributed to rock fragment cover and rock fragment size. Soil surface elevations were measured along the contour lines (50 cm long transects) with a contact microreliefmeter. Roughness was expressed by two parameters related to the height and frequency of roughness elements, respectively: standard deviation of de-trended surface elevations (random roughness: RR), and correlation length (L) derived from exponential fits of the autocorrelation functions. The frequently used assumption that surface roughness (RR) of cultivated topsoils decreases exponentially with cumulative rain is not valid for soil surfaces covered by rock fragments. The RR of soils containing small rock fragments (1.7–2.7 cm) increased with cumulative rainfall after an initial decrease during the first 17.5 mm of rainfall. For soils containing large rock fragments (7.7 cm), RR increased with rainfall above a threshold rock fragment content by mass of 52 per cent. For a given rainfall application, RR increased non-linearly with rock fragment content. The correlation length for soils containing small rock fragments decreases with rock fragment content and is significantly lower than for soils with large rock fragments. Soils covered with small rock fragments (large RR and small L) are thus well protected against raindrop impact by a water film in the depressions between the rock fragments. On abandoned agricultural fields along hillslopes in southeastern Spain, rock fragments cover increases non-linearly with slope owing to selective erosion of finer particles on steep slopes. The increase of surface cover by large rock fragments (>25 mm) is even more pronounced. The simultaneous increase of rock fragment cover and rock fragment size with slope explains the non-linear increase of RR with slope. These relationships differ for soils covered by platy misaschists and those covered with cubic andesites. The variations in correlation length along the hillslopes are not clear, probably owing to a simultaneous increase in rock fragment cover and rock fragment size. These findings may provide a better prediction of soil surface roughness of interrill areas covered by rock fragments using slope angle and lithology.     

Terrestrial laser scanning soil surfaces: a field methodology to examine soil surface roughness and overland flow hydraulics

Conventional roughness–resistance relationships developed for pipe and open‐channel flows cannot accurately describe shallow overland flows over natural rough surfaces. This paper develops a new field methodology combining terrestrial laser scanning (TLS) and overland flow simulation to provide a high‐resolution dataset of surface roughness and overland flow hydraulics as simulated on natural bare soil surfaces. This method permits a close examination of the factors controlling flow velocity and a re‐evaluation of the relationship between surface roughness and flow resistance. The aggregate effect of flow dynamics, infiltration and depression storage on retarding the passage of water over a surface is important where runoff‐generating areas are distant from well‐defined channels. Experiments to separate these effects show that this ‘effective resistance’ is dominated by surface roughness. Eight measurements of surface roughness are found to be related to flow resistance: standard deviation of elevations, inundation ratio, pit density (measured both perpendicular and parallel to the flow direction), slope, median depth, skewness of the depth distribution and frontal area. Hillslope position is found to affect the significant roughness measures. In contrast, infiltration rate has little effect on the velocity of water fronts advancing over the soil surfaces examined here and the effect of depression storage is limited. Overland flow resistance is depth dependent where complex microtopographic structures are progressively inundated. Copyright © 2010 John Wiley & Sons, Ltd.     

Topographic controls upon soil macropore flow

Macropores are important components of soil hydrology. The spatial distribution of macropore flow as a proportion of saturated hydraulic conductivity was tested on six humid–temperate slopes using transects of tension infiltrometer measurements. Automated water table and overland flow monitoring allowed the timing of, and differentiation between, saturation‐excess overland flow and infiltration‐excess overland flow occurrence on the slopes to be determined and related to tension‐infiltrometer measurements. Two slopes were covered with blanket peat, two with stagnohumic gleys and two with brown earth soils. None of the slopes had been disturbed by agricultural activity within the last 20 years. This controlled the potential for tillage impacts on macropores. The proportion of near‐surface macropore flow to saturated hydraulic conductivity was found to vary according to slope position. The spatial patterns were not the same for all hillslopes. On the four non‐peat slopes there was a relationship between locations of overland flow occurrence and reduced macroporosity. This relationship did not exist for the peat slopes investigated because they experienced overland flow across their whole slope surfaces. Nevertheless, they still had a distinctive spatial pattern of macropore flow according to slope position. For the other soils tested, parts of slopes that were susceptible to saturation‐excess overland flow (e.g. hilltoes or flat hilltops) tended to have least macropore flow. To a lesser extent, for the parts of slopes susceptible to infiltration‐excess overland flow, the proportion of macropore flow as a component of infiltration was also smaller compared with the rest of the slope. The roles of macropore creation and macropore infilling by sheet wash are discussed, and it is noted that the combination of these may result in distinctive topographically controlled spatial patterns of macropore flow. Copyright © 2008 John Wiley & Sons, Ltd.     

Characteristics of stemflow for tall stewartia (Stewartia monadelpha) growing on a hillslope

The characteristics of stemflow were observed in a tall stewartia (Stewartia monadelpha) deciduous forest on a hillslope in central Japan, revealing new findings for a previously unreported type of deciduous forest. Using 2-year observations of 250 rainfall events, we analyzed seasonal and spatial variations in stemflow for several trees, and applied additional data sets of throughfall and plant area index (PAI) to produce a rough estimate of seasonal variations in rainfall redistribution processes and canopy architecture for a single tree. Compared to previous findings for other deciduous tree species, the ratios of throughfall, stemflow, and interception to open-area rainfall obviously varied with PAI changes for tall stewartia. Meteorological conditions of rainfall amount, rainfall intensity, wind speed, and wind direction had little effect on stemflow generation, which was mainly affected by variation in canopy architecture. Three novel characteristics of stemflow were identified for several tall stewartia trees. First, the yearly stemflow ratio at the forest-stand level for tall stewartia (12%) was high compared to previous findings on beech and oak stands, indicating tall stewartia has considerably high potential to generate a great amount of stemflow. Second, stemflow tended to be 1.3–2.0 times greater in the leafed period than in the leafless period. Third, the amount of stemflow was 12–132 times greater on the downslope side of the stem than on the upslope side. It likely caused by the uneven area between the upslope and downslope sides of the canopy and by asymmetrical stemflow pathways between the upslope and downslope sides of the trunk due to downslope tilting of the tree trunk.     

Ice storms,tree throw,and hillslope sediment transport in northern hardwood forests

In December 2008, 694 trees uprooted within a 108 ha (1·08 km²) watershed in central Massachusetts due to a severe ice storm, resulting in the displacement of ~1300 m³ of root material, unconsolidated sediment, and fractured bedrock. Overall, we find that uprooting and tree throw is often grouped in clusters and cascades; conifers displace more material than deciduous trees; areas with abundant mature hemlock and steep slopes are more susceptible to tree throw, with clusters as dense as 125 per hectare; and failure is predominantly downhill, suggesting that ice storms promote efficient downslope hillslope sediment transport in northern hardwood forests. Combining the recurrence interval of severe storms in New England (20–75 years) with the forest response presented here, we calculate a sediment transport rate of 2–5 × 10^?5 m³ m^?1 a^?1 averaged over the entire watershed. Forest susceptibility to tree throw differed based on location in the watershed; some areas experienced up to ~30× higher than average sediment transport rates, while others experienced no tree throw. Two severe storms following the 2008 ice storm (hurricane in 2011; snow storm in October 2012) did not result in significant tree throw within the study area, highlighting that the coupling of storm severity and forest susceptibility controls the amount of tree throw during a given forest disturbance. In addition to recent tree throw from the 2008 ice storm, widespread pit and mound microtopography in the study area indicates that tree throw is a recurrent process in this landscape. Two factors emerge that will influence future ice storms related hillslope sediment transport in the steep forested hillslopes of New England: regional climate gradients and changing climate determine the size, intensity and recurrence of ice storms; forest management practices and health control the tree age and type. Copyright © 2015 John Wiley & Sons, Ltd.     

Parametric study of a physically-based,plot-scale hillslope hydrological model through virtual experiments

Abstract

A physically-based hillslope hydrological model with shallow overland flow and rapid subsurface stormflow components was developed and calibrated using field experiments conducted on a preferential path nested hillslope in northeast India. Virtual experiments were carried out to perform sensitivity analysis of the model using the automated parameter estimation (PEST) algorithm. Different physical parameters of the model were varied to study the resulting effects on overland flow and subsurface stormflow responses from the theoretical hillslopes. It was observed that topographical shapes had significant effects on overland flow hydrographs. The slope profiles, surface storage, relief, rainfall intensity and infiltration rates primarily controlled the overland flow response of the hillslopes. Prompt subsurface stormflow responses were mainly dominated by lateral preferential flow, as soil matrix flow rates were very slow. Rainfall intensity and soil macropore structures were the most influential parameters on subsurface stormflow. The number of connected soil macropores was a more sensitive parameter than the size of macropores. In hillslopes with highly active vertical and lateral preferential pathways, saturation excess overland flow was not evident. However, saturation excess overland flow was generated if the lateral macropores were disconnected. Under such conditions, rainfall intensity, duration and preferential flow rate governed the process of saturation excess overland flow generation from hillslopes.

Editor D. Koutsoyiannis; Associate editor C. Perrin     

Physically based modelling of sheet erosion (detachment and deposition processes) in complex hillslopes

A one‐dimensional uncoupled model governed by this research is a physics‐based modelling of the rainfall‐runoff induced erosion process. The presented model is composed of three parts of a three‐dimensional (3D) hillslope geometry, a nonlinear storage (kinematic wave) model for hillslope hydrological response, and an unsteady physically based surface erosion model. The 3D hillslope geometry model allows describing of the hillslope morphology by defining their plan shape and profile curvature. By changing these two topographic parameters, nine basic hillslope types are derived. The modelling of hillslope hydrological response is based on a flow continuity equation as the relation of discharge and flow depth is passed on kinematic wave approximation. The erosion model is based on a mass conservation equation for unsteady flow. The model assumes that suspended sediment does not affect flow dynamics. The model also accounts for the effect of flow depth plus loose soil depth on soil detachment. The presented model was run for two different precipitations, slope content, and length, and results were plotted for sediment detachment/deposition rate. Based on the obtained results, in hillslopes with convex and straight profile curvatures, sediment detachment only occurred in the whole length of the hillslope. However, in concave ones, sediment detachment and deposition only occurred together in hillslope. The hillslopes with straight profiles and convergent plans have the highest rate of detachment. Also, results show that most detachment rates occur in convex profile curvatures, which are about 15 times more than in straight profiles. Copyright © 2015 John Wiley & Sons, Ltd.     

Landslide inventory in a rugged forested watershed: a comparison between air-photo and field survey data

Landslide inventories are routinely compiled by means of aerial photo interpretation (API). When examining photo pairs, the forest canopy (notably in old-growth forest) hides a population of “not visible” landslides. In the present study, we attempt to estimate how important is the contribution of landslides not detectable from aerial photographs to the global mass of sediment production from mass failures on forested terrain of the Capilano basin, coastal British Columbia. API was coupled with intensive fieldwork for identification and measurement of all landslides. A 30-year framework was adopted. We show that “not visible” landslides can represent up to 85% of the total number of failures and account for 30% of the volume of debris mobilised. Such percentages display high sub-basin variability with rates of sediment production varying by one order of magnitude between two sub-basins of the study area. This is explained qualitatively by GIS-based analysis of slope frequency distributions, drainage density, and spatial distribution of surficial materials. Such observations find further support in the definitions of transport-limited and supply-limited basins. As a practical consideration to land managers, we envisage that supplementary fieldwork for landslide identification is mandatory in transport-limited systems only. Fieldwork has demonstrated that gully-related failures have a greater importance than one could expect from API.     

Spatial structures of stream and hillslope drainage networks following gully erosion after wildfire

The drainage networks of catchment areas burned by wildfire were analysed at several scales. The smallest scale (1–1000 m²) representative of hillslopes, and the small scale (1000 m² to 1 km²), representative of small catchments, were characterized by the analysis of field measurements. The large scale (1–1000 km²), representative of perennial stream networks, was derived from a 30‐m digital elevation model and analysed by computer analysis. Scaling laws used to describe large‐scale drainage networks could be extrapolated to the small scale but could not describe the smallest scale of drainage structures observed in the hillslope region. The hillslope drainage network appears to have a second‐order effect that reduces the number of order 1 and order 2 streams predicted by the large‐scale channel structure. This network comprises two spatial patterns of rills with width‐to‐depth ratios typically less than 10. One pattern is parallel rills draining nearly planar hillslope surfaces, and the other pattern is three to six converging rills draining the critical source area uphill from an order 1 channel head. The magnitude of this critical area depends on infiltration, hillslope roughness and critical shear stress for erosion of sediment, all of which can be substantially altered by wildfire. Order 1 and 2 streams were found to constitute the interface region, which is altered by a disturbance, like wildfire, from subtle unchannelized drainages in unburned catchments to incised drainages. These drainages are characterized by gullies also with width‐to‐depth ratios typically less than 10 in burned catchments. The regions (hillslope, interface and channel) had different drainage network structures to collect and transfer water and sediment. Copyright © 2005 John Wiley & Sons, Ltd.