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How to Review a 2D Hydraulic Model andWhat to Submit to FEMA Workshop
2D Modeling GuidelinesBy
Dragi Stefanovic, PhD, PE, DWRE
“Everything is hard before it is easy.
-Goethe
Anything that can go wrong, will go wrong.
-Murphy
Pre-Modeling Guidelines
Modeling Guidelines
Post-Modeling Guidelines
Presentation Overview
How to Review a 2D Hydraulic Model andWhat to Submit to FEMA Workshop
Pre-Modeling Guidelines
Failing to plan is planning to failHave work plan in placeWho, what, when – resources, budget, milestonesQA/QC, training needs
Plan
Where input data is coming fromAgencyDesktop search Site visitOther
Plan
Folder StructureNaming convention Include major tasks from
work plan
Plan
Check agency standards on 2-D models Is model approved? If no standards, use well
known standards from other agency
Agency Standards
How to Review a 2D Hydraulic Model andWhat to Submit to FEMA Workshop
Modeling Guidelines
A log should be kept of modelsConsist of the following:Model name Inflow/outflow Terrain usedManning’s roughness layer usedNote – describing any significant changes
Modeling Log
Start with a large model extentRun modelAdjust model extentsMinimize areaWater can’t touch except
at outflow boundaryBC don’t impact project
area
Model Extents
One of most important factor that will impact accuracy resultsGet more data than you
think you will needCheck to see if bridges
and channels are in terrainDo you need 1D survey
Topographic Data
Decide on horizontal projection and convert all dataDecide on vertical datum and convert all dataUse a standard projection/datum not local
Projection and Datum
One of most important factor that will impact accuracy resultsVerify all boundary
conditions requiredWhere is boundary
condition coming from?
Model Boundary Conditions
0 6 12 18 24Di
scha
rge
(cfs
)Time (hours)
TIME STEP REQUIREMENTSCourant number – numerical stability
Explicit Methods
Implicit Methods
HEC-RAS 2D diffusion wave
HEC-RAS full dynamic wave
𝐶𝐶 = 𝑉𝑉𝑆𝑆∆𝑡𝑡∆𝑥𝑥
, 𝑉𝑉𝑆𝑆 = 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑠𝑠𝑠𝑠𝑤𝑤𝑤𝑤𝑠𝑠 (𝑤𝑤𝑤𝑤𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 + 𝑤𝑤𝑤𝑤𝑤𝑤𝑤𝑤 𝑣𝑣𝑤𝑤𝑣𝑣𝑤𝑤𝑐𝑐𝑣𝑣𝑣𝑣𝑣𝑣)
𝐶𝐶 ≤ 1
𝐶𝐶 > 1
𝐶𝐶 ≤ 2 (max𝐶𝐶𝑣𝑣𝐶𝐶 5)
𝐶𝐶 ≤ 1 (max𝐶𝐶𝑣𝑣𝐶𝐶 3)
Simulation time should extend to required timeDo one run with a
long simulation time Reduce simulation
time to meet requirements
Simulation Time
Pick size that will adequately pick up terrain featuresCell size must be able to
pick up water surface slope correctly Sensitivity Analysis
performed
Grid Size/Cell Face Placment
Elements of different sizes should not change by more than a 2:1to neighboring elements (twice or half as large)
Maximum Area Change
TYPICAL FRICTION VALUES 1D
TYPICAL FRICTION VALUES 2D
Manning’s n for ShallowFlow Conditions
Cowan MethodCowan published the following relationship for estimatingn values.
𝑛𝑛 = 𝑛𝑛0 + 𝑛𝑛1 + 𝑛𝑛2 + 𝑛𝑛3 + 𝑛𝑛4 𝑚𝑚𝑠𝑠
Where:
n0 = base roughness for a straight, smooth, uniformchannel
n1 = degree of surface irregularities of the channel
n2 = variation of the channel cross-section
n3 = relative impact of obstructions
n4 = impact of vegetation and flow conditions
ms = relates to the degree of meandering
Cross-sections trimmedCorrect placementConnected correctly
1D/2D Connections – Cross-sections
Cross-section connect correctly to lateral structureWeir coefficient is set
correctlyHEC developed
guidance for HEC-RAS
1D/2D Connections - Lateral Structures
Take a lot of effortModel all significant
structures
Hydraulic Structures
All models need to be calibrated if data is available
Calibration/Validation
How to Review a 2D Hydraulic Model andWhat to Submit to FEMA Workshop
Post-Model Guidelines
Volume error <1%Check for surgingCheck structure
hydrographsCheck sticky elements
Numerical Health
Develop QA/QC standards Each model is different Requires a lot of time
QA/QC
