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Etude et gestion des impacts hydrogéomorphologiques de la périurbanisation. L'exemple du bassin de l'Yzeron dans l'Ouest lyonnais

Abstract : The Yzeron basin (147 km²) consists of three major units: 1) in the west, the Monts du Lyonnais, a subdued mountain whose land is predominantly rural; 2) in the center, the Plateau Lyonnais, a plateau over a large part of which extends the current periurban belt of Lyon; 3) in the east, the paleovalley of the Rhône and Saône Rivers, most of which is occupied by a dense urbanization. Many incisions have been identified using field indicators and downstream hydraulic geometry relationships (space-time substitution technique). All incised reaches are located in headwater streams (1st and 2nd Strahler order). Incisions are more important on the Plateau Lyonnais where they were mainly caused by impervious surfaces and storm sewer outlets. The effect of urbanization is also emphasized by dendrochronological analyses of riparian trees showing that these incisions mostly began between 1970 and 1990. Indeed, urban areas have doubled during this period over the entire Yzeron basin, rising from 15 to 30% (Radojevic et al. 2002), and this average increase was probably higher on the plateau which had in the same time the highest relative population growth. On the plateau, no incision was identified in the basins which impervious cover is less than 10%, threshold already observed by several authors in different regions (Schueler 1995 after Doll et al. 2002, Booth et Jackson 1997, Bledsoe et al. 2001a, Coleman et al. 2005). Nevertheless, one third of the incisions (as a percentage of total length of incised reaches) were also identified in the Monts du Lyonnais where urbanization is poorly developed. In this area, riparian trees growth was not affected by bed lowering so we cannot date the incisions. Nevertheless, small dimensions of incised channels and their stage of evolution suggest that it is also relatively recent incisions, although their origins can back several decades, even centuries. Indeed, the study of historical land use evolution shows that croplands, especially arable land (cereal crops mainly) and vines, were much more developed in the 19th and early 20th centuries. Moreover, the age of human settlements in the Lyon region, as well as some historical archives (Privolt 2010; N. Jacob, personal communication), suggest that these croplands were extensive well before the 19th century. This may have contributed for centuries to soil erosion on the slopes and sedimentary fill of the valley bottoms. This is even more likely that croplands were mainly on slopes while valley bottoms, wetter, were generally in meadows. Then, higher roughness valley bottom was favorable to the deposit of particles eroded by runoff on cultivated slopes (colluviums) or carried by overflows during flood (alluviums). Land use data indicates a significant decrease in agricultural land, especially vineyards and plowed fields, during the 20th century. In the Monts du Lyonnais, these have mainly been replaced by grasslands and, more recently, by afforestation. This change has greatly reduced runoff and soil erosion, leading to a relative sediment deficit that contributes to disrupt channel stability. In addition, bed aggradation in valley bottoms which experienced initially sedimentary fill (headwater basins, with a gentle slope and a greater width) has resulted in a local slope increase with downstream reaches whose energy is relatively stronger (no deposits), which ultimately promotes regressive incisions. In the Plateau Lyonnais, croplands have also been replaced by urban areas. Then, when urbanization extends after long-term effects of cropland, the latter two disturbances are cumulative and may result in incisions highly developed, as for example in the Chaudanne River. Furthermore, whatever the land use, artificial drainage networks (roads, trails, storm sewers) can lead to incisions. These are all longer than drains length and valleybottom slope increases. This trend could be related to the low availability of mobile sediment in the bed and banks: as water inputs increases with drain length, more distance is necessary to satisfy sediment transport capacity because it cannot be fully satisfied by erosion in the vertical and lateral dimensions; when slope increases, channels have more coarse material resistant to erosion and lower lateral dynamics, which promotes further adjustments in longitudinal dimension. The secondary influence of slope may be related to soil thickness and basin size: in general, soil thickness is lower in steep valley bottoms so that artificial drains are more able to intercept subsurface runoff; steep valley bottoms are also associated with smaller basins where subsurface runoff is usually dominant so that surface runoff increase may be higher after roads or trails development. Moreover, influence of basin size was clearly highlighted even among low order streams: there is no incision beyond 4 km², and this threshold drops to 2 km² for most streams in the Monts du Lyonnais. Small streams are thus more sensitive to hydrologic disturbances, i.e. all human interventions that help to reduce infiltration and to intercept subsurface runoff. Geomorphic sensitivity to incision depends also to valley bottom morphology that can be described by a typological approach (Schmitt et al. 2004). Our results highlight the usefulness of this hydrogeomorphic typology, combined with the drainage area, to quickly identify stream reaches potentially sensitive to incision. Hydrological effects of combined sewer overflows on streambed stability were studied in the Chaudanne River where the largest incision was identified. Combined sewer overflows increase frequency and intensity of most morphogenic floods (above the critical discharge) but it is not accompanied by an equivalent increase of their annual duration. It also tends to reduce discharges at low flow and the annual duration of low intensity floods, at least compared to largest floods. This causes a change in the relative geomorphic effectiveness of discharges of different intensities. Channels must therefore adjust to significantly higher discharges, but short, delaying the recovery of a new morphodynamic equilibrium (longer relaxation time). This may also explain the nature of the channel response, i.e. a large incision rather than a gradual adjustment of its size. Although incised reaches extend only over a relatively small part of the Yzeron hydrographic network (6%), channel enlargement ratios are significant (11,7 on average) and eroded banks are mainly composed of sand (53% on average). Headwater channel incisions are therefore an important source of fine sediment for the main branches of the river system (3rd to 5th Strahler order), with sandy sedimentation problems. According to fluorescent sand tracking, sediment delivery from upstream incised reaches represents from 27 to 62% of the annual sediment transport of the Yzeron River for the fraction 0,5-4 mm (66% of downstream sand deposits). Despite the uncertainties in sediment transport estimates, this result suggests that headwater channel incisions are a major problem for instream physical habitat given the low capacity of the system to remove these sediments. Transport capacity of the main branches is also greatly reduced by a large number of weirs. When the bed slope is low, these structures systematically result in a sandy bed of several tens or even hundreds of meters. Finally, we have formulated various operational recommendations to limit hydrogeomorphic impacts due to anthropization, and especially urbanization. These recommendations are based on three scales of intervention: hillslopes, headwater stream channels and main branches of the river network. The proposed measures aim in particular to slow down or temporarily store waterflows on hillslopes, to control and adjust the long profile of incised reaches in case of local issues, and to trap some of the sand load upstream reaches most sensitive to sand deposits.
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Submitted on : Wednesday, October 3, 2012 - 11:39:18 PM
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Loic Grospretre. Etude et gestion des impacts hydrogéomorphologiques de la périurbanisation. L'exemple du bassin de l'Yzeron dans l'Ouest lyonnais. Géographie. Université Lumière - Lyon II, 2011. Français. ⟨tel-00738293⟩

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