Slobodan Djordjević

Comparison of 1D/1D and 1D/2D Coupled (Sewer/Surface) Hydraulic Models for Urban Flood Simulation

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.

Urban flood impact assessment: A state-of-the-art review

Flooding can cause major disruptions in cities, and lead to significant impacts on people, the economy and on the environment. These impacts may be exacerbated by climate and socio-economic changes. Resilience thinking has become an important way for city planners and decision makers to manage flood risks. Despite different definitions of resilience, a consistent theme is that flood resilient cities are impacted less by extreme flood events. Therefore, flood risk professionals and planners need to understand flood impacts to build flood resilient cities. This paper presents a state-of-the-art literature review on flood impact assessment in urban areas, detailing their application, and their limitations. It describes both techniques for dealing with individual categories of impacts, as well as methodologies for integrating them. The paper will also identify future avenues for progress in improving the techniques.

Experimental and numerical investigation of interactions between above and below ground drainage systems

This paper presents the results of the experimental and numerical investigation of interactions between surface flood flow in urban areas and the flow in below ground drainage systems (sewer pipes and manholes). An experimental rig has been set up at the Water Engineering Laboratory at the University of Sheffield. It consists of a full scale gully structure with inlet grating, which connects the 8 m(2) surface area with the pipe underneath that can function as an outfall and is also further connected to a tank so that it can come under surcharging conditions and cause outflow from the gully. A three-dimensional CFD (Computational Fluid Dynamics) model has been set up to investigate the hydraulic performance of this type of gully inlet during the interactions between surface flood flow and surcharged pipe flow. Preliminary results show that the numerical model can replicate various complex 3D flow features observed in laboratory conditions. This agreement is overall better in the case of water entering the gully than for the outflow conditions. The influence of the surface transverse slope on flow characteristics has been demonstrated. It is shown that re-circulation zones can form downstream from the gully. The number and size of these zones is influenced by the transverse terrain slope.

A general framework for flood risk-based storm sewer network design

Storm sewer networks are usually designed under a certain predefined design storm. This paper proposes a general framework for a risk-based storm sewer network design, which is capable of taking future risk into account. In the framework, the flood risk evaluation is located within an optimisation loop. The flood risk is obtained by integrating the area under the probabilistic flood consequence curve, which is identified by simulations of sewer system performance under several design storms. The genetic algorithm is adapted for this specific optimisation problem in order to make the procedure computationally efficient. The framework is applied to the designs of a synthetic network and a real-world network. The results show that the general framework is able to identify the network design balancing the construction cost and flood risk.

An analysis of the combined consequences of pluvial and fluvial flooding

Intense rainfall in urban areas often generates both pluvial flooding due to the limited capacity of drainage systems, as well as fluvial flooding caused by deluges from river channels. The concurrence of pluvial and fluvial flooding can aggravate their (individual) potential damages. To analyse the impact caused by individual and composite type of flooding, the SIPSON/UIM model, an integrated 1D sewer and 2D overland flow was applied to numerical modelling. An event matrix of possible pluvial scenarios was combined with hypothetic overtopping and breaching situations to estimate the surface flooding consequences in the Stockbridge area, Keighley (Bradford, UK). The modelling results identified different flooding drivers in different parts of the study area and showed that the worst scenarios resulted from synthesised events.

Modelling sewer discharge via displacement of manhole covers during flood events using 1D/2D SIPSON/P-DWave dual drainage simulations

In urban areas, overloaded sewers may result in surcharge that causes surface flooding. The overflow from sewer systems mainly starts at the inlets until the pressure head in the manhole is high enough to lift up its cover, at which stage the surcharged flow may be discharged via the gap between the bottom of the manhole cover and the ground surface. In this paper, we propose a new approach to simulate such a dynamic between the sewer and the surface flow in coupled surface and sewer flow modelling. Two case studies are employed to demonstrate the differences between the new linking model and the traditional model that simplifies the process. The results show that the new approach is capable of describing the physical phenomena when manhole covers restrict the drainage flow from the surface to the sewer network and reduce the surcharge flow and vice versa.

From hazard to impact: flood damage assessment tools for mega cities

In this paper, a set of GIS-based tools is presented that combines information from hydraulic modelling results, spatially varied object attributes and damage functions to assess flood damage. They can directly process the outputs of hydraulic modelling packages to calculate the direct tangible damage, the risk to life, and the health impact of individual flood events. The tools also combine information from multiple events to calculate the expected annual damage. The land cover classes from urban growth models can be also used in the tools to assess flood damage under future conditions. This paper describes the algorithms implemented, and the results of their application in the mega city of Dhaka in Bangladesh. Complications and technical issues in real-world applications are discussed, and their solutions are also presented. Although it is difficult to obtain reliable data for model validation, the sensitivity of the results to spatial resolution and input parameters is investigated to demonstrate that the tools can provide robust estimations even with coarse data resolution, when a fine masking cell size is used. The tools were designed to be flexible, so that they can also be used to evaluate different hazard impacts, and adopted in various GIS platforms easily.

Stability criteria for flooded vehicles: a state‐of‐the‐art review

Hazard conditions related to vehicular circulation are important in flood risk management. The knowledge of vehicles stability when those are exposed to flooding is crucial for an informed flood risk management in urban areas. After losing stability, the vehicle becomes buoyant and may be washed away with potential injuries and fatalities. Therefore, the analysis of the stability of vehicles exposed to flooding is important in order to make decisions to reduce the damages and hazards. Herein a comprehensive state-of-the-art on stability of vehicles exposed to flooding is presented. The different studies have been gathered in experimental, theoretical and guidelines proposals and all of them focusing on parked vehicles. There is a clear need to conduct more research in this field by testing a greater variety of models in order to offer a more general methodology to define stability threshold for any vehicle exposed to flooding. Nevertheless, in this work, it has been demonstrate that the most safety stability criterion for vehicles exposed to flooding up to now is the proposed in the Guide AR&R.

Mathematical model of unsteady transport and its experimental verification in a compound open channel flow

Development of a mathematical model of transport processes in open channel flow and the experimental verification of the model are described in this paper. The original numerical procedure for solving the unsteady 2D dispersion equation, based on an operator-splitting approach, has been developed and validated by comparison with an analytical solution. The model was used for the interpretation of dynamic measurements of tracer concentration after instantaneous injection in a laboratory channel containing a compound cross section. Two conditions were examined (the water flows in the deeper rectangular part of the channel only, and the flow over the entire width). Very good agreement between measurements and computational results was obtained.

Influence of sewer network models on urban flood damage assessment based on coupled 1D/2D models

One of the main steps in assessing flood risk in urbanised areas is the quantification of damage costs. Damage is often estimated based on depth-damage curves for which depth maps are obtained (ideally) from coupled flood models. While the comparative analysis of flood damage models has been extensively researched in the literature, the influence of the underlying sewer network model has not been investigated. In this study, a 2D locally inertial equations flood risk analysis model (GWM) is coupled with two stormwater models (SWMM, tested with three different configurations, and SIPSON). The assessment of the network performance is made through the total exchanged volume between the surface and stormwater system, the surcharged conduits and manholes, maximum overland inundation and costs using a stepped version of the multi-coloured manual depth-damage curves for continuous urban fabric. The models behave similarly; however, they do show differences in the head pressures, number of surcharged manholes and maximum depth in some locations. The case study results show that despite the good agreement in damage between the four configurations (≈6% maximum disagreement), some localised high differences in maximum depth observed [0.25 (m)] exist. It was also shown that SWMM needs to be calibrated in order to perform similarly to Preissmann slot models.