Flow structure in a bouldery mountain stream with complex bed topography

Bouldery, mountain streams often possess highly irregular banks and beds composed of bedrock outcrops and immobile clasts mingled with alluvial bed forms. This complex morphology can induce locally strong flow accelerations and distortions of the water surface. Despite the complexity of flow at a scale of one or two channel widths and smaller, it is possible to identify a filament of high stream wise velocity that exhibits a near‐oscillatory structure, albeit noisy, as it threads back and forth across the channel over tens of channel widths; and transverse water surface slopes locally mimic transverse bed slopes. These features are responses to shoaling of flow over an irregular, nearly random, bed topography. To clarify the mechanisms leading to this structure, linearized forms of the depth‐averaged equations of momentum and continuity are solved in the wavenumber domain, for the case of a straight channel with uniform width, using a doubly periodic description of bed topography as a forcing term. Systematic changes in the strength and phase of velocity and water surface responses with varying wavenumber of bed undulations reflect mutual interaction of streamwise and transverse flow accelerations and transverse water surface slopes. These results are cast in terms of spectral responses to a bed composed of many superimposed waveforms. Then the shapes of spectra describing transverse water surface slopes and the transverse coordinate of the high‐velocity filament, as measured from 100 equally spaced sections along North Boulder Creek, Colorado, are predicted by the analysis. The levels of the spectra are underestimated, however, due to factors not taken into account by the linear analysis, notably variations in width, and form drag associated with coarse roughness.