# Delineation of the Flooded Areas in Urban Environments Based on a Simplified Approach
## 2 Materials and Methods  
The modelling approach for the urban pluvial flooding (as proposed in this study) is based on a lumped hydrological model coupled with GIS analysis. In our simplistic case, no overland flow was simulated, and the flooded urban areas were obtained using GIS processing of the simulated runoff volumes. As per simulating the rainfall-generated flooding surface extent and surface flooding depths, the approach is quite similar to other volume-based methods such as RFSM (Rapid Flood Spreading Model) [30]. The volume-based approach proposed in this study uses the runoff volumes simulated using hydrological modelling, which are distributed over the urban areas based on local topography and land depressions, as inputs. Subsequently, the flood extent and depth grids can be generated for the urban areas that can be further used by decision-making stakeholders. For the volume-based approach to be implemented, a pre-process of the terrain is required. In this phase, a number of potential flooded areas (cells) corresponding to terrain depressions and areas where the water accumulates during urban flooding are delineated. For the urban areas, the delineation of the flood cell should also consider the street profile and the way the streets connect to each other, since they can convey or store the runoff volumes. For each flood cell, the level-volume curve, named also capacity curve, is determined. In the case of adjacent flood cells, the ridge elevation can be also determined. Its levels must be exceeded in order for the cells to become intercommunicating. Based on the simulated volumes from the hydrological model, the flood areas are filled using the capacity curve. If the maximum level of a flood area is exceeded (over the ridge level), part of the excess volume can be transferred to the neighbouring cell. The key steps used for delineating the flood cells are presented in the following chapters. The volume-based approached was also used for calibrating the hydrological model. Based on the pictures or previous flooding marks, the flood elevation could be determined, and using the capacity curve, the volume of a flood event could be identified. This volume can be further used for model calibration for a certain urban pluvial flood event, in case no other data are available. Regarding the second component of the approach, a lumped hydrological model was built using the software package MIKE by DHI, namely the UHM-MIKE11 hydrological module. Studies undertaken at the event scale have suggested the NRCS-CN method would underestimate the peak discharge due to an inappropriate effect of the initial abstraction on the hyteograph peaks [25]. A procedure which combines the Soil Conservation Service-Curve Number (SCS-CN) method and the Green-Ampt (GA) infiltration equation was developed by [31], aiming at an improved estimation of the surface runoff volumes, with a sub-daily time resolution. Yet, this approach has introduced two new parameters: \(K_{s}\), the saturated hydraulic conductivity (mm/h), and \(N_{s}\), the soil moisture-tension parameter (mm). Considering the scarcity of the calibration data (only one value of the volume stored in the depression areas for each rainfall event) and the danger of over-parameterization, the classic SCS-CN method was herein applied. Two loss models were tested during the development of the study, namely they proportional loss (based on runoff coefficient) and SCS-CN loss model. Upon their comparison, the SCS-CN was adopted, as it provided better results for the observed events. CN represents the curve number and is based on the basin's hydrological soil group (A, B, C and D), land use, and antecedent moisture condition (AMC) of the basin at the beginning of the rainfall event. The SCS-CN model works with three levels of AMC based on the antecedent global rainfall in the previous 5-day period. The three AMC levels are defined as follows: AMC-I, dry soil conditions(close to wilting point); AMC-II, average moisture conditions; and AMC-III, saturated soil conditions [32].