Reference / Terminology

Concepts

Concise explanations of the descriptors used in the Atlas. Surface measurements, hydraulic scales and spatial descriptions are kept distinct.

R_aArithmetic mean roughness

The arithmetic mean of the absolute height deviations in an assessed roughness profile. Report the profile preparation and measurement conditions alongside the value.

NIST · Height parameters

R_qRoot mean square roughness

The square root of the mean squared height deviations in an assessed roughness profile. It describes height variation; it is not itself a hydraulic roughness scale.

NIST · Height parameters

kₛEquivalent sandgrain roughness

A hydraulic roughness scale referenced to sandgrain roughness behaviour. Its relationship to a measured surface descriptor depends on the roughness and flow regime.

Flack & Schultz · 2010

Related papers: Flack & Schultz (2010) · Chung et al. (2021)

PSDRoughness spectra

A power spectral density represents surface-height variation across spatial frequencies. It distinguishes spatial scales that a single height statistic does not describe.

NIST · Power spectral density

CoverageCoverage

Here, coverage means the fraction of an assessed area occupied by a specified surface condition. State the assessed area and classification criterion; coverage alone does not describe the arrangement of that condition.

Atlas working description

x, yPatchiness

Here, patchiness describes how different surface conditions are distributed in space. Patch size, arrangement and direction matter to the description, alongside the fraction of area occupied.