6.2 Overland flow and channel flow processes

Overland Flow and Channel Flow Processes

Overland flow and open channel flow describe how water moves across land surfaces and through natural or artificial channels. Understanding these processes is essential for predicting floods, designing drainage systems, and managing water resources across a watershed.

This section covers how overland flow is generated, how open channel flow behaves, and the key equations used to estimate flow velocity and discharge.

Overland Flow Generation and Characteristics

Mechanisms of overland flow

There are two primary mechanisms that generate overland flow, and they arise from very different conditions.

Infiltration-excess overland flow (Hortonian overland flow) occurs when rainfall intensity exceeds the soil's infiltration capacity. Water that can't soak in has nowhere to go but across the surface. The infiltration capacity depends on:

• Soil properties: texture, structure, and how wet the soil already is
• Surface characteristics: vegetation cover, land use, and whether a surface crust has formed

This mechanism dominates in arid and semi-arid regions, heavily compacted soils, and urban areas where impervious surfaces (pavement, rooftops) prevent infiltration almost entirely.

Saturation-excess overland flow occurs when the soil becomes fully saturated from below as the water table rises to the surface. At that point, even light rain can't infiltrate because there's simply no pore space left. This is common in:

• Areas with shallow water tables
• Low-lying zones near streams and wetlands
• Convergent hillslope hollows where subsurface flow accumulates

Antecedent soil moisture and local topography are the biggest controls here. A landscape that received heavy rain yesterday is far more likely to produce saturation-excess flow today.

Other factors that influence overland flow generation include:

• Rainfall characteristics: intensity, duration, and spatial distribution
• Topography: slope gradient and slope length (longer, steeper slopes accumulate more flow)
• Soil frost, which can seal the surface and block infiltration
• Urbanization, which replaces permeable soils with impervious cover

Open Channel Flow Characteristics

Laminar vs. turbulent channel flow

Flow in open channels falls into two regimes depending on how the water particles move relative to one another.

Laminar flow occurs at low velocities in smooth, small channels. Water moves in parallel layers with minimal mixing between them. The velocity profile is parabolic, with the highest velocity at the center of the channel. Viscous forces dominate.

Turbulent flow occurs at higher velocities in rougher, larger channels. Flow paths become irregular, eddies form, and there is significant mixing across the cross-section. The velocity profile is more uniform and follows a logarithmic shape. Inertial forces dominate over viscosity.

The Reynolds number determines which regime applies:

$Re = \frac{vR}{\nu}$

where $v$ is mean flow velocity, $R$ is the hydraulic radius (cross-sectional area divided by wetted perimeter), and $\nu$ is kinematic viscosity.

• $Re < 500$: laminar flow
• $500 < Re < 2000$: transitional flow
• $Re > 2000$: turbulent flow

Nearly all natural streams and rivers are turbulent. Laminar flow in open channels is mostly a theoretical reference point.

Manning's equation for flow estimation

Manning's equation is the most widely used formula for estimating mean flow velocity in open channels:

$v = \frac{1}{n} R^{2/3} S^{1/2}$

where:

• $v$ = mean flow velocity (m/s)
• $n$ = Manning's roughness coefficient (dimensionless)
• $R$ = hydraulic radius (m), calculated as cross-sectional area divided by wetted perimeter
• $S$ = channel bed slope (m/m), representing the energy gradient

To get discharge, combine Manning's equation with the continuity equation:

$Q = vA$

where $A$ is the cross-sectional area of flow. So the full expression becomes:

$Q = \frac{1}{n} A R^{2/3} S^{1/2}$

Selecting Manning's $n$: The roughness coefficient is chosen from published tables based on channel conditions. Some representative values:

• Smooth concrete-lined channel: $n \approx 0.013$
• Clean, straight natural stream: $n \approx 0.030$
• Winding stream with pools and vegetation: $n \approx 0.050$
• Floodplain with heavy brush: $n \approx 0.100$

Higher $n$ values mean greater flow resistance and slower velocities.

Channel factors in flow resistance

Flow resistance in a channel depends on both roughness and geometry, and understanding their interaction is key to predicting how water moves.

Channel roughness is captured by Manning's $n$. Roughness increases with coarser bed material (boulders vs. sand), denser vegetation, and obstructions like fallen logs or bridge piers.

Channel geometry affects how efficiently the channel conveys water:

• Cross-sectional shape varies from rectangular and trapezoidal (engineered channels) to irregular (natural streams)
• Hydraulic radius $R$ measures conveyance efficiency. A larger $R$ means less of the water is in contact with the channel boundary relative to its volume, so there's less frictional drag. Wide, shallow channels have low $R$ and are less efficient; deep, narrow channels have higher $R$
• Slope $S$ directly controls the gravitational driving force. Steeper slopes produce faster flow and higher energy