Gestion des inondations | EPFL Graph Search

vignette|Exemple de gestion des inondations : La conservation de zones d'expansion des crues suffisantes réduit considérablement les dommages érosifs causés par les inondations. La gestion des inondations désigne l'ensemble des méthodes utilisées pour contrôler la submersion temporaire naturelle ou artificielle d'un espace par de l’eau liquide. Ces méthodes sont principalement utilisées pour réduire ou prévenir les effets néfastes des eaux de crue et protéger les espaces naturels ou urbains de ces effets. Définition Méthodes Barrages barrage Impacts environnementaux Les barrages modifient la géomorphologie du lit, ce qui peut dans certains cas augmenter le risque de nouvelles inondations. Digues digues Végétalisation Modification du lit Canaux Canal de contrôle des inondations Polders Polder Impacts environnementaux La gestion des inondations, entraînant des aménag

Modelling the potential of Natural Flood Management practices in an upland peatland catchment with the distributed TOPMODEL

Hélène Amélie Madeleine Boisgontier

In order to reduce flood risk, there is a growing interest in more nature-based solutions, such as Natural Flood Management (NFM), rather than hard-engineering ones. To help build the evidence towards these solutions, we analysed the influence of different land cover change and land management in an upland peatland catchment in the Yorkshire Dales, UK. Using the distributed TOPMODEL from Gao et al., we first modelled the impact of revegetation, grazing and prescribed burning on the flood hydrograph. Then, tree planting as woodland and hedgerow, was implemented in the model and its influence on the flood peak was assessed. Revegetation of the whole catchment, from bare peat to \textit{Eriophorum} grasses, reduced the flood peak by 47% and delayed it by an hour and twenty minutes for an 18mm/hour rainfall. Taking a baseline \textit{Eriophorum} cover of the catchment, prescribed burning did not have a significant impact and grazing gave a 45-minute earlier and 31% higher flood peak. Woodland and hedgerow planting on grazed areas both had a positive influence on the flood peak. Planting hedgerows on every field edges in the catchment reduced the flood peak by 14% and delayed it by 15 minutes for the same above rainfall. Different scenarios of woodland planting were analysed: existing, floodplain, riparian and wider catchment woodlands. The most efficient scenarios were floodplain and riparian planting which gave a similar 3-hour late and 59% lower flood peak. The existing woodland scenario was the least efficient (12.5% reduction and 6-minute delay) and the wider catchment scenario was in between (40% reduction and 1-hour delay). Location of planting in the catchment was found to be very important and planting is near-stream zones is more efficient than in the headwaters or midslope of the catchment. Tree type and age at the catchment scale did not have a strong influence. Efficiency of tree planting on flood peak reduction and delay decreases with increasing rainfall intensity and storm duration. Floodplain woodland was found to be the most interesting scenario as flood risk was significantly reduced while only changing the land cover of a small portion of the catchment area (3%).

2018

Ecological response of a multi-purpose river development project using macro-invertebrates richness and fish habitat value

Marc Pellaud

It has been acknowledged that river morphology and hydrology have been intensively altered due to the anthropic demands in floodplain land use and management, flood protection, promotion of navigability or energy production. Rivers were transformed in water highways, having lost contact with their surrounding floodplain as well as the plethora of ecological processes and occupants once thriving in these ecotonal zones. The identification of this emerging threat of morphological and hydrological alteration on ecological integrity adds further complexity in the exploitation of hydrosystem resources. These resources are heavily coveted and guarded by different lobbies each having strategic views on future project development. Stakeholders may want to promote hydro-electricity, ecologists a natural reserve, communes may wish to have an increased flood protection and leisure promoters a nautical center. As a result, the proposition of a river development project is certain to face opposition of one party or the other. The motivations of this dissertation are anchored in this context, where various and sometimes conflicting potentials for hydrosystem exploitation remain. This works aims at contributing scientifically to an innovative approach at the conception phase of a multi-purpose river development project by developing the ecological module to be implemented in the general project's optimizer. The SYNERGIE project hypothesis is that it should be possible to identify a synergetic pattern joining the interests of ecological integrity, flood safety, energy production and leisure development. Such a multi-objective river development project would stand more chance of acceptance. This dissertation focuses on the ecological aspects of such a river development project and an application on the regulated Swiss Upper Rhone River. Is expected an ecological answer to a river development project design / management which has to be compatible with Heller's Heller (2007) general SYNERGIE project optimizer taking into account all the project poles. The system of interest is composed of a buffering reservoir of ca. 1 km2, a run-of-the-river dam, a hydro power-plant, and an artificial river ensuring longitudinal continuum. The primary part of the work consisted in an extensive literature review on system understanding, anthropic alterations and quality assessment / prediction tool available. The approach consisted of two levels (1) the general ecological considerations to be followed at the project reservoir scale and (2) the measure of the downstream ecological response through modeling. General ecological considerations at the reservoir scale were the implementation of an artificial river ensuring longitudinal connectivity, implementation of artificial ecotonal boosters and the allocation of a sanctuary zone with limited public access. The downstream measure of ecological integrity was based on the choice of three taxonomic groups of macro-invertebrates and four ecological guilds (groups) of fish. Mayflies (Ephemeroptera), stoneflies (Plecoptera) and caddisflies (Trichoptera) richness were predicted using simple hydrological and morphological covariates (i.e. substrate, current speed,...) coupled to system specific faunistic surveys. Bank, riffle, pool and midstream fish guilds habitat values were determined using existing methods. By using the simulation results of river development project scenarios as inputs, the ecological response (i.e. the measure of ecological integrity) was computed following the assumptions that high predicted macro-invertebrate richness and high guilds habitat values were linked to a high ecological integrity. An emphasis on the hydropeaking effect in relation with river morphology was performed on macro-invertebrates. They were found to respond well to hydrological and morphological changes induced by river development projects while the approach by fish habitat value encountered limitations in its applicability. Four multi-objective project scenarios were analyzed, (1) the actual state of the Swiss Upper Rhone River at the Riddes site (VS), (2) a hypothetical hydropeaking mitigation project, (3) a hypothetical bed widening project and (4) a hypothetical bed widening coupled to hydropeakaing mitigation project. The actual state resulted in the worst measure of ecological integrity, with comparable results for hydropeaking mitigation project or the bed widening project. The highest measure of ecological integrity was observed for the coupling of hydropeaking mitigation and bed widening. These results showed that a multi-purpose project can increase the ecological integrity of the Swiss Upper Rhone River, produce electricity, provide protection from floods and develop local leisure activities. The synergetic effect of the project could be measured by project acceptance. Nevertheless, our knowledge on the hydropeaking effect on hydrosystem should still be completed by more research in order to give more weight to the ecological implication of hydropeaking.

EPFL2007

Design of a bed load and driftwood filtering dam, analysis of the phenomena and hydraulic design

Giovanni De Cesare

Flood protection often calls on to the realization of retention works for bed load as well as wood and debris flow. Certain relatively recent arrangements did not perform according to their intended function, what shows the complexity of the design and the implementation of such works. Adaptations were necessary to reach the security objectives. The design of a retention dam for solid materials and floating driftwood requires the consideration of numerous hydraulic and material transport processes. The analyses and design validation can be made with two approaches: physical modelling by the construction of a reduced scale model and the test realization or numerical simulation, by means of software packages such as GESMAT (1D) or TOPOFLOW (2D). The present work consists in implementing both approaches, in estimating and in comparing the answers which could be given for a bed load and debris flow filtering dam on a river with a slope of the order of 10%. Thanks to water level gauges and visual observations during tests on the physical model, the progression of the obstructions by driftwood and bed load is well understood, and the effectiveness of these obstructions proven. The tested work plays at first a role of filtering and retention and secondly a role of side overflow towards a zone with low damage potential, when the capacity of the in-stream retention space is reached. The performed numerical simulations, essentially in 1D, reproduce well the phenomena of bed load aggradation. Moreover, the potential obstruction by floating wood is considered and influences the behavior of the structure. By putting in parallel physical and numerical models, it was possible thanks to the results from the physical scale model to refine the numerical simulation tools taking into consideration additional components and behavior-type rules. These further established rules can now be used for other cases where physical modelling is not foreseen.

CRC Press/Balkema2014