[en] Highlights: • The number of small dams is still increasing and is approaching 39 dams per square kilometre. • Small dams lead to a decrease in annual stream discharge of 13% ± 8%. • Cumulative impacts cannot be estimated using simple indicators. • Cumulative impacts are difficult to estimate and are most often quantified from modelling. • The lack of information on small reservoir characteristics is a real shortcoming for properly estimating their cumulative impacts. The number of small reservoirs has increased due to their reduced cost, the availability of many favourable locations, and their easy access due to proximity. The cumulative impacts of such small reservoirs are not easy to estimate, even when solely considering hydrology, which is partially due to the difficulty in collecting data on the functioning of such reservoirs. However, there is evidence indicating that the cumulative impacts of such reservoirs are significant. The aim of this article is to present a review of the studies that address the cumulative impacts of small reservoirs on hydrology, focusing on the methodology and on the way in which these impacts are assessed. Most of the studies addressing the hydrological cumulative impacts focused on the annual stream discharge, with decreases ranging from 0.2% to 36% with a mean value of 13.4% ± 8% over approximately 30 references. However, it is shown that similar densities of small reservoirs can lead to different impacts on stream discharge in different regions. This result is probably due to the hydro-climatic conditions and makes defining simple indicators to provide a first guess of the cumulative impacts difficult. The impacts also vary in time, with a more intense reduction in the river discharge during the dry years than during the wet years. This finding is certainly an important point to take into consideration in the context of climate change. Two methods are mostly used to estimate cumulative impacts: i) exclusively data-based methods and ii) models. The assumptions, interests and shortcomings of these methods are presented. Scientific tracks are proposed to address the four main shortcomings, namely the estimation of the associated uncertainties, the lack of knowledge on reservoir characteristics and water abstraction and the accuracy of the impact indicators.

[en] This report treats of the link between climate change and sea level. It was written in complement to volume 4 of the Jouzel's mission report 'Regionalized scenarios. France's climate in the 21. Century' published in August 2014. It updates volume 3 of this report already devoted to the sea level issue and published in February 2012. It aims at taking stocks of the knowledge about past and future sea level changes at the world and French coast scales, and about the main physical impacts of sea level rise (marine submersion, coastal erosion, saline influx into coastal aquifers and impacts on coastal and harbour infrastructures). However, the socio-economic impact and related risks management are not approached. It is based on the IPCC expertise and on results of works carried out by the French scientific community

[en] The coastline of France faces a range of highly specific challenges, in terms of demographics, urbanisation, socio-economic activities and the environment. The attractiveness of coastlines, and the ever-growing trend for soil sealing, is making these territories increasingly vulnerable to the impacts of climate change. The main impacts of climate change are rising temperatures, changing rainfall and cyclone patterns, the acidification of water and rising sea levels - an impact that is addressed in detail in this work. Data gathered since the early 20. century reveal a significant trend, namely that sea levels rose substantially and quickly during the last century, at a rate five times faster than in previous millennia. In statistical terms, the average sea level rose by 1.7 ± 0.2 mm per year between 1901 and 2010, and by 3.2 ± 0.4 mm per year between 1993 in 2014. All current climate models predict that average global sea levels will rise at a rate faster than that observed between 1971 and 2010 during the 21. century. This means that, in addition to the 20 cm rise in sea levels observed in the last 150 years, the sea is expected to rise by a further 26 to 82 cm by the end of this century. The exact rise will depend on the success of efforts to reduce greenhouse gas emissions. Although the rise in average sea levels in the last 150 years may seem insignificant when compared with the height of waves and tides, the IPCC's summary report (2014) suggests that an average rise in sea levels of 0.5 m would, without suitable adaptation and protection measures, result in flooding occurring between 10 and 100 times more often. This report presents: 1 - The main impacts of climate change on the coastline 2 - Climate change and sea level: from the planet to the French coastline 3 - Adaptation - from crisis management to planning: 'Waves and flooding' warnings; Cost of coastal natural disasters and the insurance mechanism; Regulating territories exposed to coastal flooding risks; A national coastal erosion anticipation strategy; Experimental relocation of activities and property: towards spatial reconfiguration of territories threatened by coastal risks; Scientific knowledge, today and tomorrow; The LiCCo project and the perspectives of elected officials on both sides of the Channel