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In this paper the effects of boulder concentration on hydrodynamics and local and reach‐averaged sediment transport properties with a flow over submerged boulder arrays are investigated. Four numerical simulations are performed in which the boulders' streamwise spacings are varied. Statistics of near‐bed velocity, Reynolds shear stresses, and turbulent events are collected and used to predict bed load transport rates. The results demonstrate that the presence of boulders at various interboulder spacings altered the flow field in their vicinity causing (1) flow deceleration, wake formation, and vortex shedding; (2) enhanced outward and inward interaction turbulence events downstream of the boulders; and (3) a redistribution of the local bed shear stress around the boulder consisting of pockets of high and low bed shear stresses. The spatial variety of the predicted bed load transport rate qs based on local bed shear stress is visualized and is shown to depend greatly on the boulder concentration. Quantitative bed load transport calculations demonstrate that the reach‐averaged bed load transport rate may be overestimated by up to 25 times when including the form‐drag‐generated shear stress of the immobile boulders in the chosen bed load formula. Further, the reach‐averaged bed load transport rate may be underestimated by 11% if the local variability of the bed shear stress is not accounted for. Finally, it is shown that for the small‐spaced boulder array, the bed load transport rates should no longer be predicted using a normal distribution with standard deviation of the shear stress distribution σ.

Ambient noise tomography has provided essential constraints on crustal and uppermost mantle (isotropic and anisotropic) shear velocity structure in global seismology. Recent studies demonstrate that high frequency (e.g., ~ 1 Hz) surface waves between receivers at short distances can be successfully retrieved from ambient noise cross-correlation and then be used for tomography of near surface shear velocity structures. This approach provides important information for strong ground motion prediction in urban area and near surface structure characterization in oil and gas fields. Here we first give a brief overview about the methodology of ambient noise tomography in global seismology. Then we focus on some recent developments on recovering near surface shear velocity structure using ambient noise tomography. We propose a new one-step iterative surface wave tomography approach that directly inverts all pathdependent dispersion data for 3-D shear wave speeds, in which we perform surface-wave ray tracing at each period using the fast marching method and update ray paths for the next step tomographic inversion. The proposed approach is more efficient than the traditional two-step surface wave tomography and provides a consistent framework for future joint surface wave and body wave travel time tomography.

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