Bottom Boundary Shear Stresses on a Model Beach

Abstract

The maximum amplitude of shear stress in the bottom boundary layer of water waves was evaluated with a Preston probe inclined on a 1 12 5 slope beach Near bottom velocity profiles were obtained in laminar and developing turbulent flow conditions from which the experimental boundary layer thicknesses were evaluated Agreement between experimental bottom velocities and those calculated from Airy theory deteriorate with decreasing depth on the beach resulting m lower shear stress values than predicted by linear theory The measured boundary layer thickness on the slope exceeds the predicted for horizontal bottom, increasing shoreward to some critical depth outside the breaker zone from where it decreases shoreward The influence of roughness on the shear stress distribution is considerable m the "offshore" region, but becomes negligible near the breaker zone On a smooth bottom the coefficient of friction agrees with Kajiura's expression.

A Shear Plate for Use in Oscillatory Flow

“…They define the quantities damping ratio, ( and undamped natural frequency, ƒ" as: c = BENDAT et PIERSOL [16] ont décrit la réponse dynamique du système représenté par l'équa tion (3). Ces auteurs définissent comme suit Ie rapport d'amortissement ( et la frequence propre sans amortissement ƒ":…”

“…Wall similarity techniques (one indirect approach) rely on a knowledge of the bound ary layer velocity profile above the bed. JONSSON [1] and KAJIURA [2] used velocity profile methods while TELEKI and ANDERSON [3] used the Preston tube. Since the study of the velocity distribution in oscillatory bound ary layers is only just beginning, because suitable instrumentation was not previously available, the determination of shear by this method is based on assumed velocity profiles which have yet to be confirmed experimental ly.…”

“…Les methodes de similitude pariétale, qui constituent un exemple des procédés indirects, sont fondées sur la connaissance du profil des vitesses dans la couche limite au-dessus du lit. JONSSON [1] et KAJIURA [2] ont utilise des methodes de profils de vitesses; TELEKI et ANDERSON [3] se sont servi du tube de Preston. L'étude de la distribution des vitesses en couche limite oscillante est encore a ses debuts car un materiel de mesure adéquat a longtemps fait défaut.…”