Development of Closed-conduit Roughness

Non-intrusive measurement of closed-conduit erodible-bed development from plane-bed conditions was undertaken for 12 experiments of ranges of flow strengths and sand sizes. Analogous to open-channel flows, sediment wavelets (which subsequently develop into ripples or dunes) are instigated by discontinuities in plane sections of closed-conduit beds. A random pile-up of sediment, growing by trapping sediment in motion, can thereby eventually be sufficiently large to generate wavelets downstream. For laminar and turbulent open-channel and closed-conduit flows, wavelets generated on a sediment bed will be of a preferred wavelength λ, = 175d ^0.75, where λ and sediment size d are expressed in millimetres. For closed-conduit flows in comparison to open-channel flows, limitation of water-surface deformation generally results in increased rates and magnitudes of bed deformation (in the form of ripple and dune heights and corresponding lengths) with associated increased flow resistance.