Recent developments in flood modeling have led to the concept of coupled (sewer/surface) hydraulic models. In this paper two coupled models are examined; a one-dimensional (1D) sewer model coupled with a 1D surface network model (1D/1D) and a 1D sewer model coupled with a two-dimensional (2D) surface flow model (1D/2D). Flow over the terrain is better modeled by 2D models, whereas in confined channels 1D models provide a good approximation with less computational effort. This paper presents a comparison of the simulation results of 1D/1D model and a 1D/2D model. The methodology adopted for setting up the models is outlined and explained in detail as well as the 1D/1D modeling techniques used for reproducing the 1D/2D model results. The surface flow comparison clarifies the limitations of the 1D/1D model and indicates that the definition of the surface pathways, the linking elements sewer/surface, and inclusion of virtual manholes are key factors for setting up a more accurate 1D/1D model.

Pluvial flooding in urban areas is characterized by a gradually varying inundation process caused by surcharge of the sewer manholes. Therefore urban flood models need to simulate the interaction between the sewer network and the overland flow in order to accurately predict the flood inundation extents. In this work we present a methodology for linking 2D overland flow models with the storm sewer model SWMM 5. SWMM 5 is a well-known free open-source code originally developed in 1971. The latest major release saw its structure re-written in C ++ allowing it to be compiled as a command line executable or through a series of calls made to function inside a dynamic link library (DLL). The methodology developed herein is written inside the same DLL in C + +, and is able to simulate the bi-directional interaction between both models during simulation. Validation is done in a real case study with an existing urban flood coupled model. The novelty herein is that the new methodology can be added to SWMM without the need for editing SWMM's original code. Furthermore, it is directly applicable to other coupled overland flow models aiming to use SWMM 5 as the sewer network model.

Flooding in urban areas represents a particular challenge to modellers and flood risk managers because of the complex interactions of surface and sewer flows. Quantified flood risk estimates provide a common metric that can be used to compare risks from different sources. In situations where there are several organisations responsible for flood risk management we wish to be able to disaggregate the total risk and attribute it to different components in the system and/or agents with responsibility for risk reduction in order to target management actions. Two approaches to risk attribution are discussed: Standards-based attribution, which is a deterministic approach, based upon the performance of different engineering components in the system at their “design standard”. Sensitivity-based attribution, which apportions risk between the variables that influence the total flood risk. Whilst both these approaches are feasible for the small system considered here, in practice urban flooding systems involve tens of thousands of variables. The only feasible approach to tackling this problem for large urban systems is therefore by hierarchical simplification of the system, with the attribution analysis being applied in several tiers of detail. In this paper, the applicability of a hierarchical approach is demonstrated in the context of sewer pipe blockages. The results demonstrate the potential of attribution methods to support the development of integrated urban flood risk management strategies, as they can identify the forcing variables and infrastructure components that have the most influence upon flood risk.