# Delineation of the Flooded Areas in Urban Environments Based on a Simplified Approach
## 1 Introduction  
The frequency and the magnitude of extreme rainfalls have increased over many regions of the Earth [1] Consequently, an increased hazard of flooding has been observed, accompanied by a vulnerability increase heavily related to the economic and social development in the floodplains [2]. In order to provide a standard legal framework for the flood risk management within the European Union, the European Commission issued the Floods Directive 2007/60/EC, which aims at mitigating and managing the risks that floods pose to human health, the environment, cultural heritage, and economic activities. According to the Directive, all Member States must develop flood hazard and flooding risk mappings. These maps should represent the integrated knowledge base for further elaborating appropriate objectives and suitable measures that are normally dealt with in the Flood Risk Management Plans of the river basins [3]. Urban areas can be flooded by riverine floods and by pluvial runoff [4]. The pluvial floods in urban areas are generally produced by uncontrolled urban development accompanied by the increase of the urban impervious surfaces (when not accompanied by corresponding extensions of the drainage systems). Furthermore, climate changes are expected to worsen the flooding control within the cities by bringing in supplementaryloads to existing urban storm drainage systems. After exceeding the carrying capacity of the existing urban drainage system, water flows directly along the streets (urban waterlogging), interacting with the overland flow [5; 6]. Water that flows on the terrain can either fill the small natural retention areas, re-enter the sewer system, or flow across the terrain surface through preferential pathways [7]. Flood prevention in urban areas comprises various floodproof measures, as issued from structural and non-structural concepts of preventing or reducing potential flood damages. The traditional urban solution is represented by sewage systems (grey solution), to which underground storage cisterns and pumping stations can be added; the surface water is drained into underground collectors, being subsequently evacuated into the outdoor environment. If the local morphologic conditions are favourable, detention ponds or storage tanks can also be used to cut the peak discharge and to reduce the pressure on the urban rainwater drainage network or on the receptor rivers [8; 9]. As previously mentioned, classic stormwater management is based on the opportune off-flow of excess water towards neighbouring water bodies. However, the urban areas represent subsystems of river basins, and their water management is expected to support the overall water management of the entire basin [10]. The drainage of large cities during strong rainfalls could bring significant flooding to their downstream areas. Sustainable Urban Drainage Systems (SUDS), known also as flood-planned city, Low Impact Development (LID), and Sponge City, represents an alternative to classic urban water management. The main goal of such systems is to transform the urban hydrological cycle, bringing it as close as possible to the natural hydrological cycle by properly addressing the source control, permeable conveyance, storage, and pollution management [11]. Within this context, the spatial planning concepts must promote a more holistic, sustainable, and strategic view [11]. Green infrastructure measures, as an alternative or complementary approach to SUDS, are used to augment the capacity of the urban stormwater management. A mixed approach of structural and non-structural flooding control, based on detention tanks (DTs) and LID facilities is currently widely adopted [12]. While the DTs are meant to mainly cut the peak of the urban floods, the LID reduces and delays the flood peak, while having beneficial effects on water quality [12]. Although they are different, Urban Drainage Systems (UDS), DTs, and LID are interconnected systems, working together for flood mitigation [13].