| A novel large wave flume experiment was conducted on a fixed, barred beach with a sediment pit on the sandbar, allowing for the isolation of small-scale bed response to large-scale wave forcing. Concurrent measurements of instantaneous sheet flow layer and suspended sediment concentration profiles, pore-pressure gradients, near-bed velocity profiles, and velocity profiles spanning the whole water column were obtained on a sandbar. Two sediment distributions were used with median grain diameters, d50, of 0.17 and 0.27 mm. Sheet flow occurred primarily under wave crests, where sheet thickness increased with increasing wave height. A proportionality constant, Lambda, was used to relate maximum Shields parameter to maximum sheet thickness (normalized by d50), with bed shear stress computed using the quadratic drag law. An enhanced sheet layer thickness was apparent for the smaller sediment experiments (Lambda = 18.7), when directly compared to closed-conduit oscillatory flow tunnel data (Lambda = 10.6). However, Lambda varied significantly (5 < Lambda < 31) depending on the procedure used to estimate grain roughness, ks, and wave friction factor, ƒw. Three models for ks were compared (keeping the model for ƒw fixed): constant ks = 2.5d50, and two expressions dependent on flow intensity, derived from steady and oscillatory sheet flow experiments. Values of ks/d50 varied by two orders of magnitude and exhibited an inverse relationship with Lambda, where Lambda ~ 30 for ks/d50 of O(1) while Lambda ~ 5 for ks/d 50 of O(100). Two expressions for ƒw were also tested (with the steady-flow-based model for ks), yielding a difference of 69% (Lambda ~ 13 versus Lambda ~ 22). Intra-wave and wave-averaged observations of sediment flux profiles and transport rates in the lower half of the water column on the sandbar crest are also presented for 19 different wave and sediment cases. The total sediment transport rate was partitioned into suspended sediment (SS) and sheet flow (SF) components to quantify the relative contributions of SS and SF to the total sediment transport rate. Net suspended sediment transport rates were greater than net sheet flow transport rates for the positive half-cycle in 14 of 19 cases, compared to 100% (19 of 19) for the negative half-cycle. Total net sheet flow transport was greater than net suspended sediment transport for 13 of the 19 wave cases. The dominant mode of transport was determined from the ratio of net SF to net SS transport rate. In general, net total transport rate was negative (offshore) when SS dominated and positive (onshore) when SF dominated. Net SF transport rate correlated well with increasing near-bed third velocity moments (r2 = 0.71), and no trend was observed related to the influence of sediment size. |
