Movies taken by witnesses of extreme ood events are increasingly available on video sharing websites. They potentially provide highly valuable information on ow velocities and hydraulic processes that can help improve the post-ood determination of discharges in streams and ooded areas. We investigated the troubles and potential of applying the now mature LSPIV technique to such ood movies that are recorded under non-ideal conditions. Processing was performed using user-friendly, free software only, such as Fudaa-LSPIV. Typical issues related to the image processing and to the hydrological analysis are illustrated using a selected example of a pulsed ash-ood ow lmed in a mountainous torrent. Simple corrections for lens distortion (sheye) and limited incoherent camera movement (shake) were successfully applied and the related errors were reduced to a few percents. Testing the dierent image resolution levels oered by YouTube showed that the dierence in time-averaged longitudinal velocity was less than 5% compared to full resolution. A limited number of GRPs, typically 10, is required but they must be adequately distributed around the area of interest. The indirect determination of the water level is the main source of uncertainty in the results, usually much more than errors due to the longitudinal slope and waviness of the ow free-surface. The image-based method yielded direct discharge estimates of the base ow between pulses, of the pulse waves, and of the time-averaged ow over a movie sequence including a series of 5 pulses. A comparison with traditional indirect determination methods showed that the criticaldepth method may produce signicantly biased results for such a fast, unsteady ow, while the 1 Author-produced version of the article published in Hydrological Processes (2016), Volume 30, Issue 1, p 90-105The original publication is available at http://onlinelibrary.wiley.com DOI: 10.1002/hyp.10532 slope-area method seems to be more robust but would overestimate the time-averaged ow rate if applied to the high-water marks of a pulsed ow.

International audienceIn this paper we explore several indicators to evidence the impact of land use change, and particularly of urbanization/artificialization on discharge series of periurban catchments. A first set of indicators is derived from the literature and describes the monthly and annual hydrological regime, low flows and high flows, and flow components. Statistical tests are also applied to assess the existence of trends/rup-tures on the longest time series. In addition, new indicators, especially built to show the impact of sewer overflow devices (SODs) and infiltration into sewer networks are proposed. The method is applied to the Yzeron (150 km 2) catchment, located close to Lyon city (France) where various discharge gauges with a variable time step are available on sub-catchments ranging from a few to 130 km 2 (some of them nested), with a large variety of land uses (forest, agricultural land, artificialized areas). In addition, discharge is also measured in a SOD and a combined sewer network so that the relevance of the new proposed indicators can be assessed. In the largest sub-catchments, the results show a decrease of specific discharge from upstream to downstream corresponding to an increase of artificialized areas, except for high flows. When a SOD is present, the specific discharge is increased for frequencies larger than 50%, and the frequency of zero daily discharge is decreased. Waste water can be the only source of water in autumn month in a 4.1 km 2 sub-catchment. Base flow is also decreased for the most urbanized catchments. Our results confirm the impact of SODs on the modification of the flood regime, with an increase of frequent floods, but a marginal impact on the largest floods, mainly governed by saturation of the rural parts of the catchments. The decomposition of the sewer discharge shows that, on an annual basis, infiltration in the sewer network accounts for 30% of the total discharge and runoff due to rainwater to about 40% (the remaining being composed of the waste water discharge). It can explain the decrease of base flow. Our analysis shows that, for periurban catchments, a long term monitoring of nested sub-catchments and infrastructures (SODs, sewer networks) with a small time step, is very valuable and provides data allowing a quantitative assessment of the impact of urbanization on the whole hydrological regime

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