Much of the water and sediment fluxes in semi-arid catchments are found to be highly concentrated in localized pathways. Identifying the location of these pathways in the landscape is important for management and restoration. Measures can then be targeted so as to minimize the potential for erosion and sediment flux along these pathways. A method of repeat field mapping of flow and sediment pathways suitable for Mediterranean catchments is presented. Several small catchments in Cárcavo basin, SE Spain, differing in topographic and land use characteristics, were monitored under several events. Morphometric properties of pathways were analysed and compared with rainfall characteristics. Number and length of pathways varied with rainfall characteristics and also antecedent conditions. In low rainfall events, runoff sources and main pathways were disconnected, but in a larger event, the network of pathways became fully connected. The pathway patterns showed that man-made lines such as terrace embankments and tracks have a major influence on sediment connectivity. Microtopographic factors, soil moisture and the presence of vegetation are highly influential on pathways and the frequency of water and sediment fluxes. Runoff and erosion hotpots for the development of pathways were identified, which should be targeted for mitigation and restoration measures using vegetation. The relevance of local scale factors emphasizes the importance of repeat field observations to understand connectivity and pathways development in the landscape.

Vegetation forms a major component of the channel environment of dryland rivers in Mediterranean Europe yet the interactions between vegetation and fluvial processes in dryland river channels have received relatively little attention. Characteristic of dryland channels is a degree of complexity and irregularity in morphology coupled with abrupt changes in patterns of riparian vegetation along their course. This is in contrast to more temperate and humid channels, which show strong downstream trends in morphology and a regularity in the distribution of vegetation across the valley floor. The general effect of vegetation in these channels appears to be to enhance the processes of sedimentation and increase resistance to erosion. However, at high stress levels vegetation may be removed. The limited work that has been undertaken indicates that there is considerable variability in the effect that vegetation has on channel processes, depending on both the magnitude of the flow and characteristics of the vegetation, such as their distribution and positioning within the channel and factors such as plant flexibility, cross-sectional area (blockage ratio), number of stems. Studies of sediment connectivity are beginning to yield new insights into the dynamics of dryland rivers, and in particular the role of vegetation in reducing the connectivity of sediment transfers downstream. Detailed surveying at monitored sites such as is being carried out within the EU-funded project RECONDES is required to study the interactions between vegetation, sediment and flow in dryland rivers of the Mediterranean region.

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Abstract:The interaction of vegetation and flow in channels is important for understanding the influences of forces in channels and effects on erosion, sediment flux and deposition; it has implications for channel habitats, channel instability and restoration schemes. Methods are needed for calculating forces on plants and data are required on thresholds for plant destruction and survival. A simple method of calculating the effect of hydraulics on vegetation and its zonation within ephemeral channels is described. Detailed cross section surveys of channel morphology, vegetation and estimates of Manning's n are input into the software program WinXSPRO to calculate the hydraulics of flows across the channel for a given event or flow level, incorporating subdivision into zones of differing morphology and vegetation across the section. This was applied to a number of cross sections on ephemeral channels in SE Spain and typical roughness values for Mediterranean vegetation types in channels were assessed. The method is demonstrated with reference to two well-documented floods in SE Spain, in September 1997 on the Torrealvilla and in October 2003 along the Salada. These flows led to the mortality of herbs, reed and smaller shrub species. Some damage to larger shrubs and trees occurred, but trees such as Tamarisk (Tamarix canariensis) were shown to withstand high forces. Some grasses were highly resistant to removal and induced sedimentation. Significant erosion was limited to areas with little vegetation covering the channel floor. Further quantification of resistance of vegetation to flows and upper threshold values for removal is continuing by relating calculated hydraulic conditions using the methods outlined to measurements of vegetation responses in events at monitoring sites.