Bedform spacing from defect dynamics

Spacing is a time-varying characteristic of bedform fields deriving from th e behavior of defects (ends of crest lines) in the bedform pattern. In a mo del based on this hypothesis, crest line length is lost and spacing increas es because bedform defects, which are smaller in height and faster migratin g than surrounding bedform crest lines, merge with larger bedforms as defec ts migrate through bedform fields. Spacing in large bedform fields asymptot ically increases with the logarithm of time as pairs of oppositely facing d efects meet and annihilate. Spacing in small bedform fields, such as flumes , exponentially approaches a fixed value as defects are eliminated at the b oundaries of the field. Model predictions are compatible with observed spac ing of transverse bedforms, ripples formed under waves and linear dunes, ca lling into question the widespread assumption that bedform spacing approach es a steady-state value characteristic of fluid flow and sediment transport over two-dimensional bedforms.