Collaborative Development of High Resolution Pluvial Flood Maps for Flanders

Abstract

As part of the low countries and with one of the highest population densities worldwide, the Flemish region has experienced a long history of flooding causing tens of millions euro damage each year. In response to this, water managers invested over the past decade in flood modelling and mapping with a fluvial origin. In recent years, pluvial flooding has also occurred numerous times in Flanders, but a region-wide map describing these processes more in detail in terms of extent, depth and probability was lacking. Following a pilot-study in 2016, the VMM undertook in 2017 the VLAGG1- project to develop a region-wide, high-resolution pluvial flood map for Flanders. Via a combination of state-of-the art methodologies and web technologies, a draft flood map was presented to a broad reviewing community across Flanders, who were then able to improve it further by adding local knowledge on known flooding and more detailed data on key hydraulic structures. In a three month period, over 7000 additions were made by 370 delegates from 165 organizations that have been incorporated into, and significantly improved the quality of the final flood maps which are due to be published in 2019.

Assessing the Governance Context Support for Creating a Pluvial Flood Risk Map with Climate Change Scenarios: The Flemish Subnational Case

“…For each cell, we analyzed first the individual answer and then, we compared it with the rest of the stakeholders' answers to reach a general conclusion. Secondary sources, such as previous research on the topic [7,28] as well as European [79] and legal documents [80], were used to confirm or complement the information gathered during the interviews. The results are considered reliable, since the interviewees reported in similar and complementary ways.…”

“…Flood mapping is part of the measures taken by the Flemish government to meet the requirements of the European Flood and Water Framework Directives. The Directive requires the member states to provide information about the source of flooding to the European Commission [7]. It also requires the members' states "to assess its territory for significant risk from flooding, to map the flood extent, identify the potential adverse consequences of future floods for human health, the environment, cultural heritage and economic activity in these areas, and to take adequate and coordinated measures to reduce this flood risk" [79].…”

“…The new maps have been the replacement of the first-generation flood maps from 2000, which had important shortcomings. There was a lack of calculation of overland flow paths, the resolution of 5m was limiting the use of a parcel-scale, and there was a lack of regional information regarding the flood extent [7].…”

“…In Europe, 'fluvial floods', caused by overflowing rivers, have received important attention, while 'pluvial floods', caused in urban areas by extreme rainfall events, have been rarely studied [6]. In the case of Belgium, for example, by 2016, Flanders had only been reporting fluvial floods to the European Commission [7]. Although fluvial floods tend to be more devastating and impressive, their frequency is lower than pluvial floods [6].…”

“…Actually, cities in Belgium have already struggled more with floods caused by pluvial events rather than tidal causes [16]. Specifically in Flanders, when considering both fluvial and pluvial floods with climate change effects, pluvial floods have a dominant impact and by 2100, it is expected that pluvial floods will damage 2.5 times more buildings, among the most affected being the cities of Antwerp, Gent, Brugge, Leuven, and Kortrijk [7]. These negative consequences of pluvial floods have favored the development of new strategies from the government.…”