reinaldo garcia, phd a proposal for testing two-dimensional models to use in the national flood...
TRANSCRIPT
Reinaldo Garcia, PhD
A proposal for testing two-dimensional models to use in the National Flood Insurance Program
Presentation Outline
MotivationStatus quo
Why we need improvementsWhat are others doing
Suggested components for new model review guidelines
Model performance and predictive capability
Depends on many factors, including:• Model mathematical formulation• Numerical method• Appropriate use of boundary conditions• Adequate configuration of grid or mesh• Choice of roughness coefficients and time steps• Data quality• Expertise of the modeler to perform the simulation.
To ensure adequately testing the predictive capabilities of a 2D flood model, testing guidelines should be based on standardized
benchmarks and specifications
Status Quo
FEMA requires that any hydrologic or hydraulic computer model used in support of flood insurance studies must be reviewed, tested and accepted by a government agency responsible for the implementation of programs for flood control and/or regulation of floodplains National Flood Insurance Program (NFIP) regulations. Subparagraph 65.6. FEMA relies on other agencies to review and test a model and determine whether it is scientifically correct and technically sound.FEMA List: Numerical Models Meeting the Minimum Requirement of NFIP Current Nationally Accepted Hydraulic Models
Why we need improvements
FEMA does not establish a standard testing procedure to determine if a model is scientifically or computationally correctNo specifications or metrics to verify that the model can provide adequate or accurate information to support NFIP studies. Inconsistent testing methods depending on the agency or modeler doing the evaluation. Federal agencies often have no budget or time test a model that is not on the FEMA list. Some government agencies also develop their own models…
Status Quo
Is not openIs not fully reliable
Hampers innovationDoes not serve well FEMA’s mission
What are others doing?
Components of sound model review guidelines
Model verificationModel validation
Standardized benchmarksMetricsOpen
Model vs. Real World
CalibrationAdjusting numerical or physical parameters in the program to improve comparison with data
VerificationDetermining that the program accurately represent the conceptual model (program solves the equations right)
QualificationFidelity of the equations to reality
ValidationDetermining the degree to which the model is an accurate representation of the real world.
Verification
Example of model verificationComparison with Lynch & Gray 2-D analytical solutions
Example of model verification
Validation
“No one believes model results except the one who performed the calculation,
...everyone believes observed or measured data except the one who did the measurements”
P. J. Roache
Validation
UK Benchmarks
Provide a dataset against which 2D hydraulic modeling packages can be evaluated
Provide evidence to ensure that such models used for flood and coastal risk management are capable of adequately predicting the variables upon which flood risk management decisions are based.
UK BenchmarksFlooding of a disconnected water body Assess basic capability to simulate flooding of disconnected water bodies on
floodplains or coastal areas and wetting and drying of floodplains
Filling of flood plain depressions Tests capability to predict inundation extent and final flood depth flow low momentum flow over complex topographies. Also tests flooding of disconnected water bodies and wetting and drying of floodplains. Predicts inundation extent, with an emphasis on final distribution of water rather than peak levels.
Momentum conservation over a small (0.25m high) obstruction
Tests capability to simulate flow at relatively low depths over an obstruction with an adverse slope.
Speed of flood propagation over an extended flood plain
Tests simulation of speed of propagation of flood wave and the prediction of velocities at the leading edge of the advancing flood.
Valley flooding Tests simulation of major flood inundation at the valley scale. Dam break Tests simulation of shocks and wake zones close to a failing dam. This could also
be applied to other examples like rapid opening of sluice gates, failures of embankments, etc.
River to floodplain linking Evaluates capability to simulate flood volume transfer between rivers and floodplains using 1D and 2D model linking.
Rainfall and sewer surcharge Tests capability to simulate shallow flows in urban areas with inputs from rainfall and sewer surcharge.
Test relevance for specific applications
Application Test Large scale flood risk mapping 1 & 2 Catchment Flood Management Plan 1, 2 & 7 Flood Risk Assessment and detailed flood mapping
1, 2, 3 & 7
Strategic Flood Risk Assessment 1, 2, 3, 4, 7 & 8 Flood hazard mapping 1, 2, 3, 4, 7 & 8 Contingency planning for real time flood management
1, 2, 3, 4, 5, 7 & 8
Reservoir flooding mapping 1, 2, 3, 4, 5 & 6
Benchmark Examples
Flooding a disconnected water body
UK Benchmark Examples
Assess model’s ability to conserve momentum over an obstruction in the topography.Important when simulating flooding in urbanized floodplains.
UK Benchmarks
Should be complemented with verification
Americas Society of Civil Engineers
ASCE: Verification and Validation of 3D Free-Surface Flow Models, 2008Three main components:– Analytical solutions– Lab/flume experiments– Field scale real-world applications
Provides a data set for 3D models only
Suggestions for Model Review Guidelines
Verification should precede validationValidation based on set of benchmarks Acceptable error tolerances (metrics)Benchmarks based on intended use: not all models are necessarily adequate for all projectsStandardized test and benchmark data sets should be public and freely available
Implementing these model review guidelines would better serve FEMA’s mission.
Thank you / [email protected]