Environment and Computer Laboratory HS 2014 Water Resources Management I Hydraulic Modelling of Open Channel Flows Supervisor: Assistants: Time & Place: Ellen Cerwinka ([email protected]) Nina and Simona 07. November – 12. December 2014 Friday from 12:45 – 16:30 HIL E 15.2 IfU Institute of Environmental Engineering Prof. Dr. Paolo Burlando Chair of Hydrology and Water Resources Management Organisational Issues  Schedule Date Topic Room Friday, 07. Nov Open Channel Flow – hydrologic flood routing Introduction into Exercise 1 Start work on hydraulic flood routing in HEC-RAS HIL E 15.2 Friday, 14. Nov Individual work on Exercise 1 Hydraulic flood routing – HEC-RAS HIL E 15.2 Friday, 21. Nov … … Friday, 28. Nov Individual work on Exercise 1 Hydraulic flood routing – HEC-RAS Probabilistic inundation mapping … Friday, 05. Dec … … Friday, 12. Dec … … Organisational Issues  1 Exercise + Paper Assignment    Building up the model Hydraulic Modelling of Open Channel Flows using HEC-RAS Transferring the results of HEC-RAS into ArcGIS  Exercise carried out in groups of 2 students  Hand in one report for the exercise and the paper assignment   Grading of WRM I   Report around 18 pages HEC-RAS 90%, Paper Assignment 10% Deadline for submission: 17. December 2014 Organisational Issues  Literature on hydraulic flood routing and HEC-RAS     Lecture notes „Hydraulik 1“ Prof. Kinzelbach (German) Bollrich: Technische Hydromechanik Band 1 (German) User Manual HEC-RAS, Hydraulic Reference Manual HEC-RAS (English) Literature on Uncertainty in Inundation Mapping     Horritt (2006): A methodology for the validation of uncertain flood inundation models Hunter et al. (2005): Utility of different data types for calibrating flood inundation models within a GLUE framework … www.sciencedirect.com for paper Lecture Outline  What is Flood Routing?  Definition  Motivation  Techniques & Analysis    Hydraulic Flood Routing  Approximations to the St. Venant equations Concept of probabilistic inundation mapping  Motivation  Uncertainty in inundation mapping  GLUE methodology Introduction into the exercise Flood Routing – Definition  What is Flood Routing? Flood /Flow routing is… a mathematical procedure for predicting the changing in magnitude, speed and shape of a flood wave as a function of time at one or more positions along a river. (Fread (1993): Handbook of Hydrology) Flood Routing – Definition  How does the runoff change from point A to B? Flood Routing – Motivation   Floods  Predict flood propagation  Protection  Warning Design     Water conveyance systems Protective measures Hydrosystem operation Water dynamics    Ungauged rivers Peak flow estimation River-aquifer interaction Flood Routing – Techniques  Hydrologic Routing    Continuity equation and analytical /empirical relationship between storage and discharge (e.g. Muskingum method) For watershed simulation studies  from upstream to downstream No significant backwater effects or discontinuities!  Hydraulic Routing   Solution of partial differential equation of unsteady open channel flow ( St. Venant equation) Accounts for backwater effects and internal boundaries (culverts, bridges, weirs) Flood Routing Analysis  Channel Inputs  Reach properties  length LAB  slope SAB  roughness nAB  width wAB  Water Inputs  Basin response  the rainfall-runofftransformation gives us QS(t), ql(t)  Output  ??? Flood Routing Analysis Steady Uniform Flow Gradually Varied Flow dv/dt=0 dv/dx=0 Unsteady Flow (Hydraulic Routing) St. Venant Equation St. Venant Equation St. Venant Equation St. Venant Equation Limitations in the real world Probabilistic Inundation mapping Motivation  Past and Now: deterministically created inundation maps  Futur: Mapping of flood hazard and risk for low, medium and high probabilities  high uncertainty in results of 1 D- or 2D hydraulic simulations Sources of uncertainty  Model structure   Implementation of numerical scheme    Small errors can have significant impact on results Model input/output   Choice of type of numerical solution (implicit or explicit) Optimal time step Topography   Model approximates nature  simplifications Flood hydrograph Parameters  Surface roughness GLUE methodology = Generalized Likelihood Uncertainty Estimation Each member of Monte Carlo ensemble is assigned a weight according to how well it fits observed calibration data  Cj Lw   L i i i   ij i Cj - weighted average flood state for jth cell Li - weight for each simulation i Wij - simulation result for jth element (wij =1 for wet, wij =0 for dry) Reject poor simulations Calibration/validation with remote sensing data and/or high water marks GLUE methodology  Compare observed and predicted inundated ares A B F A B C F…measure of fit A…wet area correctly predicted by the model B …area predicted as wet but observed dry (over-prediction) C …area predicted as dry but obeserved wet (under-prediction)  Weigths for uncertain flood inundation map Fi  min i ( Fi ) Li  max i ( Fi )  min i ( Fi )  Mini(Fi)…minimum measure of fit throughout the ensamble Maxi(Fi)… maximum measure of fit throughout the ensamble GLUE can be used to evaluate different sources of uncertainty (initial conditions, boundary conditions, topography…) GLUE methodology  Result: uncertain inundation map Exercise: Hydraulic Modelling of Open Channel Flow - Objectives  Get to know…      a numerical hydraulic model for flood routing (HEC-RAS) the general methodology, possibilities and limitations of numerical flood routing the role of different parameters and their influence on the simulation the concept of probability in spatial inundation extends by creating probabilistic inundation maps And not…  to reproduce a flood event as accurate as possible Exercise: Hydraulic Modelling of Open Channel Flow - Task Description 1 Steady State Simulations (d/dt = 0)   1.1 Rating curve (Q-H function) 2 Unsteady Flow Simulations      2.1 Influence of initial conditions (=> “warm up” time) 2.2 Investigation of the temporal discretization 2.3 Rating curve for unsteady flow 2.4 Numerical damping (influence of weighting factor ) 2.5 Influence of downstream boundary condition  Uncertainty analysis of roughness parameters using probabilistic innundation mapping  Paper assignment Exercise: Hydraulic Modelling of Open Channel Flow- River Reach & Flood Events   River Reach  Thur length of about 3 km  Geometric data available on the course webpage Flood events      Each group chooses and signs in for a flood event Flood hydrograph: hourly mean flow for the Thur in m3/s with a duration of 30 to 50 hours Too flashy to be realistic - but suitable for this exercise To be used for unsteady flow simulations Flood hydrographs available on the course webpage Exercise: Hydraulic Modelling of Open Channel Flow - Flood Routing Model HEC-RAS  HEC-RAS -> Hydrologic Engineering Centers River Analysis System  developed by the US Department of Defense, Army Corps of Engineers in order to manage the rivers, harbors, and other public works  1D - hydraulic flow model    Steady & Unsteady Flow Sediment Transport Water Temperature & Water Quality Exercise: Hydraulic Modelling of Open Channel Flow - Flood Routing Model HEC-RAS  Steady Flow  based on the solution of the one-dimensional energy equation  Energy losses evaluated by friction and contraction / expansion   momentum equation may be used in situations where water surface profile is rapidly varied (hydraulics of bridges ) Unsteady Flow  full, dynamic Saint-Venant equations  using an implicit, finite difference method Exercise: Hydraulic Modelling of Open Channel Flow  And Now...   Form groups and choose a flood event Get to know HEC-RAS and start working on the exercise (Very useful: hecrasmodel.blogspot.ch)  Start paper assignments at home  Next week: Working on the exercise  Deadline 17th December 2014