HEC-RAS Overview

Last updated: 2026 · US Army Corps of Engineers

HEC-RAS (Hydrologic Engineering Center — River Analysis System) is a computer program that models the hydraulics of water flow through natural rivers and other channels. Developed by the US Army Corps of Engineers, it is the industry standard for 1D and 2D hydraulic modeling used in floodplain management, bridge design, dam breach analysis, and environmental flow studies.

HEC-RAS 6.x introduced a fully integrated 2D flow solver based on the depth-averaged shallow-water equations (Saint-Venant), enabling high-resolution flood simulations over complex terrain.

2D Flow Modeling

The 2D solver in HEC-RAS uses an unstructured computational mesh that conforms to terrain features such as roads, levees, and channel banks. Each computational cell stores depth-averaged velocity vectors and water surface elevation at every time step.

Two governing equation sets are available:

Shallow Water Equations (SWE) — Full Saint-Venant equations with inertial terms; suitable for rapidly-varying, supercritical flow.
Diffusion Wave (DW) — Simplified momentum, faster computation; best for slow, subcritical floodplain flow.

The Saint-Venant Equations

The 2D depth-averaged Saint-Venant equations govern the motion of shallow water bodies. In conservative form they describe conservation of mass and momentum across a thin fluid layer.

∂h/∂t + ∂(hu)/∂x + ∂(hv)/∂y = 0                  (continuity)

∂(hu)/∂t + ∂(hu² + ½gh²)/∂x + ∂(huv)/∂y = ghS₀ₓ − ghSfₓ  (x-momentum)

∂(hv)/∂t + ∂(huv)/∂x + ∂(hv² + ½gh²)/∂y = ghS₀ᵧ − ghSfᵧ  (y-momentum)

Where h is water depth, u and v are depth-averaged velocity components, g is gravitational acceleration, S₀ is bed slope, and Sf is friction slope (computed via Manning's equation).

Computational Mesh

HEC-RAS 2D uses an unstructured hybrid mesh of quadrilaterals and triangles. Cell faces align with terrain breaklines (channel banks, roads, levee crests) to capture hydraulic controls accurately. Typical cell sizes range from 1 m (channel) to 50 m (floodplain).

Manning's Roughness

Bottom friction is parameterized through Manning's n values assigned to land-cover polygons. Common values: open water 0.03, short grass 0.035, dense vegetation 0.08–0.15.

Flood Inundation Mapping

Model output (water surface elevation at each cell centroid) is differenced against terrain elevation to produce depth grids. These are visualized as animated color maps showing flood wave propagation, velocity vectors, and Froude number distributions.

Key outputs used in this animation platform:

Water depth (m) — time series at every cell
Velocity magnitude and direction (m/s, degrees)
Froude number — identifies sub/supercritical transitions
Inundation extent — binary wet/dry mask per time step

Applications

HEC-RAS 2D is used across a broad range of civil and environmental engineering applications:

FEMA Flood Insurance Rate Map (FIRM) production
Dam breach consequence assessment
Urban stormwater and combined sewer overflow analysis
Coastal storm surge and sea-level rise inundation
River restoration and levee setback design
Emergency action planning for critical infrastructure