FLOW STRUCTURE IN A BOULDERY MOUNTAIN STREAM WITH COMPLEX BED TOPOGRAPHY

Bouldery, mountain streams often possess highly irregular banks and be ds composed of bedrock outcrops and immobile clasts mingled with alluv ial bed forms. This complex morphology can induce locally strong flow accelerations and distortions of the water surface. Despite the comple xity of flow at a scale of one or two channel widths and smaller, it i s possible to identify a filament of high streamwise velocity that exh ibits a near-oscillatory structure, albeit noisy, as it threads back a nd forth across the channel over tens of channel widths; and transvers e water surface slopes locally mimic transverse bed slopes. These feat ures are responses to shoaling of flow over an irregular, nearly rando m, bed topography. To clarify the mechanisms leading to this structure , linearized forms of the depth-averaged equations of momentum and con tinuity are solved in the wavenumber domain, for the case of a straigh t channel with uniform width, using a doubly periodic description of b ed topography as a forcing term. Systematic changes in the strength an d phase of velocity and water surface responses with varying wavenumbe r of bed undulations reflect mutual interaction of streamwise and tran sverse flow accelerations and transverse water surface slopes. These r esults are cast in terms of spectral responses to a bed composed of ma ny superimposed waveforms. Then the shapes of spectra describing trans verse water surface slopes and the transverse coordinate of the high-v elocity filament, as measured from 100 equally spaced sections along N orth Boulder Creek, Colorado, are predicted by the analysis. The level s 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.