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Hydrogeomorphic adjustments of stream channels disturbed by urban runoff (Yzeron River basin, France)

Navratil, O., Breil, P., Schmitt, L., GrosprĂȘtre, L., Albert, M.B.
Journal of hydrology 2013 v.485 pp. 24-36
basins, drainage, environmental impact, floods, guidelines, monitoring, normal values, rivers, roads, sediment yield, stream channels, urban areas, urban runoff, watersheds, France
This paper presents a field investigation on hydrogeomorphic adjustments of small streams in a 147km2 periurban catchment, the Yzeron River catchment located in France. The rapid development of periurban areas in the world is now considered as one of the main factor impacting river systems. Urban disturbances are most of the time associated with irreversible alterations of the hydrological regime, the sediment yields, with major ecological impacts and additional socio-economical costs. Nineteen stream reaches have been considered in this study, with drainage areas ranging from 0.2 to 33.9km2 and total impervious areas ranging from 1% to 52% of the basin surface. A regional analysis was led in order (i) to quantify the hydrogeomorphic adjustments of stream channels in this periurban context, i.e. the ratios between observed values and reference/rural values; and (ii) to identify the main anthropogenic controlling factors of these adjustments. Results show that urban river channels experience a global enlargement, with a mean bankfull discharge ratio of 1.8, bankfull width and depth ratios of 1.3 and a bankfull area ratio of 1.8. This study also outlines the global increase of hydrogeomorphic adjustments with the increase of the fraction of impervious area and the level of disturbance of the flood regime. However, local anthropogenic factors seem to be much more relevant to explain the highest adjustment ratios at several river reaches (enlargement ratio up to 55). The vicinity of a river reach with road sewers and/or the urban areas is identified to be a very important factor that affects significantly the smallest streams (drainage area less than 5km2). On the contrary, at several reaches no significant deepening or widening was observed although roads/urban sewers and urban areas were identified in their catchment. Several hypotheses are proposed, but additional works with new data (river monitoring) would be needed to propose management and/or restoration guidelines in periurban catchments.