Physical Scale Modelling of Urban Flood Systems

Urban flooding is defined as ‘an overflowing or irruption of water over urban pathways which are not usually submerged’. Current economic, climatic and social trends suggest that the frequency, magnitude and cost of flooding are likely to increase in the future. Hydraulic models are commonly used by engineers in order to predict and mitigate flood risk. However full scale calibration and validation datasets for these modelling tools are scarce. The main research objective of this thesis was to design and construct a physical model in order to provide datasets useful to verify, calibrate and validate computer model results in terms of energy losses in manholes. To address these issues, an experimental facility has been constructed to enable the investigation of energy losses under steady and unsteady flow conditions in a scaled sewer system. Originally the model was composed of six manholes and three main pipes and then it was modified into a single pipe linked to an urban surface through a single manhole. Experiments involved the measurement of flow rates, velocity, pressure and water depth within the physical models under different hydraulic scenarios. Steady flow tests were conducted to quantify energy losses though manhole structures with different inlet/outlet configurations under a range of hydraulic conditions. Unsteady flow tests were conducted to examine the performance of different computational hydraulic models. These tests have shown that the performance of the SWMM hydraulic model could be improved by including local losses in the calibration process. After modification the model was used to quantify sewer to surface and surface to sewer flow exchange through a single manhole during pluvial flooding. The work has demonstrated the feasibility of using weir and orifice equations within modelling tools to quantify this exchange under steady conditions. The model was used to empirically quantify discharge coefficients for energy loss equations which describe flow exchange for the first time

Interaction of above/below urban grounds: an experimental facility developed to analyse computer modelling results

10 p.International audienceThis research undertaken at the University of Sheffield aims to provide a better understanding of the interaction above/below ground urban floods. A newly constructed unique experimental facility has been developed in the water lab, and it includes a sewer system, composed by 3 main pipes, 6 manholes and a CSO (Combined Sewer Overflow), and a preliminary urban surface with the slope 1/1000. This paper describes the experimental facility that has been built, how the system is managed in real time control using Labview software, which methodology will be applied to increase the understanding of the exchange flow-rate between an urban surface and a below sewer system and it presents preliminary results obtained comparing physical results with computer software results such as InfoWorks, SWMM and SIPSON (NUNO MELO et al., 2012). Finally, the PIV system (Particle Image Velocimetry) that will be used for the acquisition of the images during the event of flooding is briefly explained. This last step will be useful for the comparison between 3D digital maps created by standard software against real physical results

Experimental Investigation Of Flow-Interactions Between Above And Below Ground Drainage Systems Through A Manhole

The frequency, magnitude and impact of pluvial flooding events both in the UK and worldwide is forecast to increase due to: climate change (Butler and Davies 2011), the effects of urbanisation and urban creep (Semadeni‐Davies et al 2008) as well as aging drainage infrastructure (Ashley et al. 2004). Pluvial flood models (e.g. Leandro et al. 2008) are increasingly being used to assess flood risk, develop asset investment strategies and develop surface water management plans. However due to the nature of urban flood events, it is very difficult to calibrate and validate such pluvial flood models. In particular the rate of exchange between above and below ground systems is a source of considerable uncertainty in urban flood modelling (Djordjević et al, 2005). This work utilises a unique surface/subsurface model (as described in Rubinato et al. 2012a, and Rubinato et al. 2012b) at the University of Sheffield. The model consists of a pipe drainage network linked via a manhole to an urban surface with a slope 1:1000. Inflow and outflow to the above and below ground systems can be controlled and monitored independently in real time. The specific objective of this work is: - Experimentally quantify the interaction of flow between the below and above ground systems for a range of pluvial flooding conditions. Water depth, flow rate will be measured in real time and initial results on exchange rate between the below system and the urban surface will be quantified and compared to commonly used energy loss equations with the determination of specific coefficients (free weir linkage; submerged weir linkage and orifice linkage). The overall aim of the research is to improve urban pluvial flood models and a more accurate understanding of the hydraulic characteristics of interaction points and to quantify surface flow paths

Numerical Modeling of Debris Flows Induced by Dam-Break Using the Smoothed Particle Hydrodynamics (SPH) Method

Dam-break flows may change into debris flows if certain conditions are satisfied, such as abundant loose material and steep slope. These debris flows are typically characterized by high density and can generate strong impact forces. Due to the complexity of the materials that they are made of, it has always been very challenging to numerically simulate these phenomena and accurately reproduce experimentally debris flows’ processes. Therefore, to fill this gap, the formation-movement processes of debris flows induced by dam-break were simulated numerically, modifying the existing smoothed particle hydrodynamics (SPH) method. By comparing the shape and the velocity of dam break debris flows under different configurations, it was found that when simulating the initiation process, the number of particles in the upstream section is overestimated while the number of particles in the downstream area is underestimated. Furthermore, the formation process of dam-break debris flow was simulated by three models which consider different combinations of the viscous force, the drag force and the virtual mass force. The method taking into account all these three kinds of interface forces produced the most accurate outcome for the numerical simulation of the formation process of dam-break debris flow. Finally, it was found that under different interface force models, the particle velocity distribution did not change significantly. However, the direction of the particle force changed, which is due to the fact that the SPH model considers generalized virtual mass forces, better replicating real case scenarios. The modalities of dam failures have significant impacts on the formation and development of debris flows. Therefore, the results of this study will help authorities to select safe sites for future rehabilitation and relocation projects and can also be used as an important basis for debris flow risk management. Future research will be necessary to understand more complex scenarios to investigate mechanisms of domino dam-failures and their effects on debris flows propagation

Modelling the Hydrological Effects of Woodland Planting on Infiltration and Peak Discharge Using HEC-HMS

Woodland planting is gaining momentum as a potential method of natural flood management (NFM), due to its ability to break up soil and increase infiltration and water storage. In this study, a 2.2 km2 area in Warwickshire, England, planted with woodland every year from 2006 to 2012, was sampled using a Mini Disk infiltrometer (MDI). Infiltration measurements were taken from 10 and 200 cm away from the trees, from November 2019 to August 2021. Two individual hydrological models were built using the US Hydraulic Engineering Center Hydrological Modelling System (HEC-HMS), to model the effects of infiltration change on peak flows from the site throughout the summer and winter. The models were calibrated and validated using empirical data; the Nash and Sutcliffe Efficiency (NSE) was used as an indicator of accuracy. Results from this study show that woodland planting reduced peak flow intensity compared to impermeable land cover by an average of 6%, 2%, and 1% for 6-h, 24-h, and 96-h winter storms, respectively, and 48%, 18%, and 3% for 6-h, 24-h, and 96-h summer storms, respectively. However, grassland simulations show the greatest reduction in peak flows, being 32%, 21%, and 10%, lower than woodland for 6-, 24-, and 96-h winter storms, respectively, and 6%, 3%, and 0.5% lower than woodland for 6-, 24-, and 96-h summer storms, respectively

The Impact of Tree Planting on Infiltration Dependent on Tree Proximity and Maturity at a Clay Site in Warwickshire, England

Urbanisation and the replacement of previously vegetated areas with impermeable surfaces reduces the lag times of overland flow and increases peak flows to receiving watercourses; the magnitude of this will increase as a result of climate change. Tree planting is gaining momentum as a potential method of natural flood management (NFM) due to its ability to break up soil and increase infiltration and water storage. In this study, a 2.2 km2 clay-textured area in Warwickshire, England, planted with trees every year from 2006 to 2012 was sampled to investigate how infiltration varies dependent on season and tree proximity and maturity. Infiltration data was collected from 10 and 200 cm away from selected sample trees from November 2019 to August 2021 using a Mini Disk infiltrometer (MDI). The results show that mean infiltration is higher at the 10 cm proximity compared with the 200 cm proximity by 75.87% in winter and 25.19% in summer. Further to this, mean 10 cm infiltration is 192% higher in summer compared with winter, and mean 200 cm infiltration is 310% higher in summer compared with winter. There is little evidence to suggest a relationship between infiltration and tree maturity at the study site