Flood Modelling with HEC-RAS

This course provides a practical, step‑by‑step workflow to perform flood (inundation) analysis using HEC‑RAS (steady and unsteady flow, 1D/2D coupled), plus methods to assess and manage hydraulic risk. It covers data needs, model setup, calibration/validation, uncertainty analysis, hydraulic hazard mapping, exposure & vulnerability assessment, risk quantification, and recommended risk reduction and management actions.

Objectives

• Build an accurate HEC‑RAS model (1D/2D as appropriate) to simulate design storms and observed events.

• Produce inundation maps and hydraulic outputs for selected return periods (e.g., 2, 5, 10, 25, 50, 100, 200 years).

• Quantify hydraulic risk by combining hazard, exposure and vulnerability.

• Provide actionable recommendations for flood risk reduction, monitoring and emergency planning.

Scope

The workflow is applicable to river reaches, floodplains, urban channels and mixed rural/urban catchments. It assumes available topographic and hydraulic data; it includes optional steps for data‑poor contexts (empirical methods and sensitivity analysis).

Required Data

1. Topography / Bathymetry

   • LiDAR DEM (preferred) or contour survey

   • Cross‑section surveys along the river

   • Channel bathymetry for deeper rivers

2. Hydrologic data

   • Streamflow/gauge records (continuous or event-based)

   • Rainfall records and IDF curves

   • Design hydrographs or frequency analysis (e.g., statistical IDF, flood frequency)

3. Hydraulic data

   • Roughness/Manning’s n estimates (channel and floodplain)

   • Structures: bridges, culverts, weirs (geometry and invert elevations)

4. Land use / Exposure

   • Building footprints, critical infrastructure locations

   • Population or economic value layers

5. Observed flood extents (for calibration/validation)

6. Metadata (datums, coordinate systems)

Software & Tools

• HEC‑RAS (latest stable release) — for 1D, 1D unsteady, 2D flow modeling and 1D/2D coupled setups

• HEC‑GeoRAS (ArcGIS) or RAS Mapper for GIS integration and pre/post‑processing

• QGIS/ArcGIS — for preparing terrain, land‑use and producing maps

• Hydrologic tools — HEC‑HMS, rainfall‑runoff models or frequency analysis scripts

• Optional: Python/R for batch processing, Monte Carlo uncertainty, economic analysis

Workflow (Step‑by‑Step)

1. Project Setup

• Define study reach, objectives and design scenarios (return periods, climate scenarios).

• Establish coordinate system and vertical datum (consistent across datasets).

2. Data Preparation

• Process DEM (fill sinks where appropriate, remove artifacts), create terrain for RAS Mapper.

• Extract cross‑sections: ensure sufficient spacing to capture geometry changes and structures.

• Digitize structures and lateral boundaries; derive conveyance lines for 2D mesh.

3. Hydrology

• Select design hydrographs: peak flows and hydrograph shapes for each return period.

• If gauge data available: perform frequency analysis (e.g., Log‑Pearson III) to obtain design discharges.

• For ungauged basins: use regional regression, HEC‑HMS, or scaling from nearby stations.

4. HEC‑RAS Model Building

• Create geometry (river stations and cross‑sections) in RAS Mapper or geometry editor.

• Input structures (bridges, culverts) with accurate geometry and loss coefficients.

• Choose modeling approach:

  • Steady flow (1D) for cross‑sectional water surface profiles (quick assessments).

  • Unsteady (1D) for hydrograph routing, backwater effects and dam break.

  • 2D or 1D/2D coupled for detailed floodplain flows and urban overland routing.

• Set boundary conditions (flow hydrographs or stage) and initial conditions for unsteady runs.

5. Manning’s n and Roughness

• Assign Manning’s n values based on land cover; define spatial variation for floodplains.

• Document assumptions and ranges for sensitivity testing.

6. Calibration & Validation

• Calibrate using observed water levels, high‑water marks or historic flood extents.

• Adjust roughness, lateral flow parameters or structure losses within reasonable ranges.

• Validate model on an independent event if available; document performance metrics (NSE, RMSE, bias).

7. Uncertainty & Sensitivity Analysis

• Perform sensitivity runs for Manning’s n, boundary hydrographs, and structure parameters.

• Consider Monte Carlo sampling or scenario analysis to propagate input uncertainties to flood extents and depths.

• Present confidence intervals for water depth and inundation area.

8. Inundation Mapping & Post‑Processing

• Export HEC‑RAS water surface rasters or depth grids for each scenario.

• Combine with DEM to generate depth and velocity maps (depth = W.S. − DEM).

• Produce maps for specific return periods and probabilistic hazard (e.g., 5th–95th percentile inundation).

9. Exposure & Vulnerability Assessment

• Overlay inundation maps with exposure layers (buildings, infrastructure, population).

• Assign vulnerability curves (depth–damage functions) for asset types.

• Calculate expected annual damage and scenario damage for each return period.

10. Risk Quantification

• Compute risk as Hazard × Exposure × Vulnerability.

• Aggregate results spatially (per parcel, administrative unit) and by asset type.

• Optionally compute economic metrics: Expected Annual Damage (EAD), Benefit‑Cost Ratios for mitigation.

11. Decision Support & Risk Management

• Prioritize interventions using risk rankings: structural (levees, bypasses), non‑structural (zoning, early warning), nature‑based solutions (riparian restoration).

• Define monitoring plans (gauges, telemetry), maintenance of structures, and emergency response triggers.

• Consider adaptation pathways and climate change scenarios in long‑term planning.

12. Reporting & Communication

• Produce technical report: methods, data, calibration results, uncertainty, maps, recommended measures.

• Create clear non‑technical summaries and maps for stakeholders and the public.

• Provide GIS layers and HEC‑RAS project files as deliverables.

Quality Assurance / Quality Control (QA/QC)

• Maintain a data log and metadata for all inputs.

• Version control for HEC‑RAS projects and scripts.

• Peer review of model setup, calibration and assumptions.

• Sensitivity checks for critical parameters and structure geometry.

Deliverables (Suggested)

• HEC‑RAS project folder (geometry, plan files, flow files, 2D mesh if used).

• GIS layers: DEM, inundation rasters, depth maps, velocity maps, exposure overlays.

• Technical report and non‑technical summary with maps and recommended measures.

• Spreadsheet of damage estimates, EAD, and mitigation cost‐benefit analysis.