Arved J. Raudkivi

FLOW CHARACTERISTICS IN LOCAL SCOUR AT BRIDGE PIERS

The paper summarises the major results of an investigation of How patterns, turbulence intensity distributions and boundary shear stress distribution in the scour zone of a circular pier under clear water scour conditions.

Bed forms in alluvial channels

A solution is offered for the relative phase relationship of bed, depth and surface waves observed in alluvial channels, and is found to be in good agreement with observation in laboratory flumes. The solution does not depend on an assumption of a phase lag between the velocity and sediment movement. The emphasis of the paper is on the mechanics of bed-wave formation.

The roughness height under waves

The effect of bed roughness and sediment load on energy dissipation under wave action has been analysed with the aid of unidirectional flow relationships and laboratory data. An expression is proposed for estimation of the equivalent roughness height. The paper also raises questions which need further study.

Reduction of sand demand for shore protection

Beach nourishment is an environmentally preferred method of shore protection, but the annual sand requirement may lead to substantial maintenance costs. The shoreline processes, involving the surf zone, beach and dune, are reviewed with the aim of reducing the annual sand requirement of eroding shorelines. It is shown that surf zones with equilibrium profiles, on which the wave energy conversion is evenly distributed across the surf zone, from experience for given conditions indicate least loss of sand. On steep, eroding shorelines it may be difficult to establish an equilibrium profile. For such cases, the use of perched surf zones is recommended, which are supported at the seaward limit by an underwater sill. For reduction of littoral transport, the use of pervious pile groynes is recommended. These are arguably more efficient than impervious groynes. The sand loss from a usually dry beach by raised water levels is shown to be a function of the beach slope and is least when the storm waves at raised water levels do not cut an erosion escarpment. The loss of sand from a dune by infrequent severe storm tides can be prevented with the aid of a built-in membrane. These sand losses are usually large and constitute an uneconomic use of this sand resource. The proposed concepts and measures are linked to existing knowledge, augmented by data from the large wave flume (LWF) in Germany and field data from the North and Baltic Sea coasts.

BEACH NOURISHMENT AND DUNE PROTECTION

A special reflecting wall 12 m long and 2.1 m high was built off the beach at Reggio Calabria, and 30 wave gauges were assembled before the wall and were connected to an electronic station on land. It was possible to observe the reflection of wind waves generated by a very stable wind over a fetch of 10 Km. The experiment aimed to verify the general closed solution for the wave group mechanics (Boccotti, 1988, 1989), for the special case of the wave reflection.Significant features on Wadden Sea wave climate are evaluated in respect of the state of the art. Main emphasis was laid on an analysis of the governing boundary conditions of local wave climate in island sheltered Wadden Sea areas with extensions being sufficient for local wind wave growth. Explanatory for significant wave heights a reliable parametrization of local wave climate has been evaluated by using generally available data of water level and wind measurements.

THOUGHTS ON RIPPLES AND DUNES

The above discussion of the ripple-dune problem in unidirectional flow is aimed at raising questions and challenging researchers not to accept the partial answers, known so far, as the truth. As a contribution, a hypothesis for the development of bed features is put forward which offers a chance to describe the bed features in concurrent deterministic and random forms.

Role of Turbulence and Particle Exposure on Entrainment of Large Spherical Particles in Flows with Low Relative Submergence

AbstractLaboratory experiments of sediment entrainment were carried out in a flume for different flow configurations with particle image velocimetry (PIV) used to measure the flow velocity. The experiments were done in a low submergence condition. Measurements revealed that the vertical distribution of streamwise flow velocity at the instant of entrainment is significantly different from the temporal mean flow profile. The role of turbulence for a given flow depth is found to decrease with an increase in exposure of the particle. An expression for threshold velocity is proposed that accurately represents the enhanced near-bed turbulence during entrainment. A Shields-type parameter, expressed in terms of critical near-bed flow velocity rather than shear velocity, is used to define the sediment transport threshold.

On transport of bed load by waves

The Bagnolds energy concept for sediment transport is used to estimate the volume of sediment mobilized by strong wave action. The results are compared with wave tunnel data for sheet flow conditions and with bed load formulae by Sleath and by Madsen and Grant. The predictions differ excessively and agreement with wave tunnel data is poor. The results indicate that only a small percentage ofthe energy dissipation at the bed is due to the sediment in motion.

BEACH AND STORM-TIDE PROTECTION ON THE COAST OF THE BALTIC SEA

The coastline of the southern Baltic Sea has been retreating due to the locally rising sea level. The rate of rise is estimated between 70 to 150 mm per century with diminishing trend. This has led to erosion, narrow beaches and wave-cut dunes and cliffs. In addition the coast is subject to storm-tides of the order of up to 2.5 m. Since much of the hinterland is low and flat these can have a devastating effect if the coastal defences are breached.

Beginn der Sedimentbewegung

Wenn die Geschwindigkeit eines Stromes, sei es Gas oder Flussigkeit, uber einem ebenen Bett von Kornern langsam gesteigert wird, wird ein Zustand erreicht, in dem sich einzelne Korner in Bewegung setzen. Uber mehr als zweihundert Jahre haben Forscher versucht, diesen Beginn der Sedimentbewegung analytisch zu erfassen, aber immer noch gibt es keine vollstandige Obereinstimmung.