Detailed uncertainty analysis of urban hydraulic structures in large catchments
Creators: name: Beg, Md Nazmul Azim
Publisher: name: Zenodo
Year: 2018
Subjects: Manhole
Gully,
PIV
Head loss coefficients
Discharge Coefficients
Uncertainty
OpenFOAM®
LARSIM
Flood Forecasting
Monte Carlo

description: Quantifying and reducing uncertainty in a numerical model improve model reliability and enhance model performance. For an urban drainage flow, the state-of-the-art modelling method is dual drainage modelling where the surface drainage system is represented as two dimensional, and the buried piped network is represented as one dimensional. Most of these models use the St. Venant equation to calculate the flow, which does not help with the modelling of drainage structures such as manholes and gullies as they have complex three-dimensional flow. These structures are represented as point entities using head loss coefficients and discharge coefficients. This involves uncertainty in results as the values of these coefficients are highly case dependent. Urban drainage models also require hydrological input boundaries such as discharge and water levels, for which they are dependent on measurements or outputs of a hydrological model. A major part of this thesis focuses on finding the uncertainty of manhole head loss coefficients and gully discharge coefficients (Chapter 3 to Chapter 6). The last part of the thesis (Chapter 7) focuses on quantifying uncertainties in rainfall-runoff hydrological model results to ensure efficient flood forecasting.

In analysing the uncertainty of the manhole and gully modelling, the primary focus was given to understanding the hydraulics and the flow through these structures correctly. Three different sets of laboratory experiments were conducted in characterising the manhole and the gully flow. In the first measurement set, velocity measurements were done in a small-scale manhole. Stereoscopic Particle Image Velocimetry was utilised to record the three velocity components in three different two-dimensional planes at various hydraulic conditions. In the second experimental measurement set, a prototype scale manhole was utilised to measure its flow and surcharge depths at different surcharges. In the third experimental work, velocity measurements were conducted in a prototype gully using ADV.

Computational fluid dynamics (CFD) was used to investigate the flow behaviour and inspect the manhole head loss coefficients and gully discharge coefficients at distinct conditions. The scaled manhole model was replicated primarily utilising OpenFOAM® CFD modelling tools. A multiphase Volume of Fluid (VOF) model was constructed by applying four different Reynolds Averaged Navier Stokes (RANS) turbulence models. The PIV measurement data was used to validate the CFD modelling procedure. The comparison showed that among four different RANS models the RNG k-ε model is the best choice considering model result quality of velocity and pressure as well as time requirement in model simulation. Later, RNG k-ε model was applied in replication of the prototype scale manhole used in the laboratory. The model also showed good performance when model data were compared with the experimental measurement of discharge and pressure. Lastly, the validated CFD model was used to find uncertainty of manhole head loss coefficients in prototype scale manhole models. Three different manhole moulds found in the drainage systems were analysed numerically: Type A with sump zone, Type B without a sump and Type C with benching. The effect of small bending angles and manhole of inlet diameter ratios were further checked for Type A manhole. Results showed that at very small bending angles, jet flow core region can still dissipate through the outlet pipe and causes a minimal increase of head loss at the post threshold surcharge. At higher bending angles, the core jet flow disappears in the manhole, and higher head loss coefficients are observed for all surcharge. The Type A manhole showed an alternation of the hydraulic regime at manhole diameter ratio of 3.0. Over this ratio, manholes showed indication of threshold surcharge zone, which was absent in lower ratios.

CFD model of prototype gully showed good agreement when compared to ADV velocity data. The validated gully model was used to check different gully discharge coefficients for different gully outlet water levels. Three different downstream surcharge levels were identified for which the gully flows show different discharge coefficients.

The last part of the dissertation focuses on quantifying uncertainty in a rainfall-runoff model. A tool was developed which can be used for automated flood forecasting including estimation of input and parameter uncertainties. It can also forecast flood discharge using ensemble-based Monte Carlo method. The tool was successfully applied at the city of Kulmbach located within Upper Main river catchment in Germany. A new methodology is proposed for flood forecasting named ‘Discharge Interval Method’. When compared with the traditional Monte Carlo method, the Discharge Interval Method was also found to efficiently hindcast historical flood events in a fraction of time without downgrading forecast quality.