Elsevier

Ocean Engineering

Volume 30, Issue 3, February 2003, Pages 297-307
Ocean Engineering

The formation of offshore ripples in the zone under irregular waves

https://doi.org/10.1016/S0029-8018(02)00040-9Get rights and content

Abstract

In this article, results obtained from an experimental investigation conducted to determine the wave-induced geometric characteristics of offshore ripples and bars are presented. The experiments were performed using irregular waves. Natural beach sand was used in the study, where the mean diameter was 0.35 mm and the specific gravity was 2.63. The initial slope of the beach was 1:5. Different wave groups were generated over the initially flat beach, and a number of characteristics were determined. These include the ripple number, individual and average ripple heights, individual and average ripple lengths and the length of the offshore bar. The results of the experimental study were evaluated and empirical expressions based on the results were formulated.

Introduction

Natural beaches exposed to waves that have a high energy are highly dynamic. Offshore ripples and bars are formed by the impact of these waves. Therefore, knowledge of the geometric characteristics of offshore ripples and bars is of great importance regarding some coastal engineering problems. Various factors play vital roles in the formation of offshore ripples and bars. These factors include wave characteristics, near-shore current, beach slope, sediment property, and bottom friction. The characteristics of offshore ripples can provide valuable information regarding the formation of offshore bars.

Sand transport processes and associated bar behaviour processes were discussed by O’Hare and Huntley (1994). Osborne and Vincent (1996) studied sand concentrations and velocity data at several positions along a rippled bed in laboratory conditions. These investigators determined that the phase relationships between the velocity and the concentration are such that it can give completely opposite depth integrated transport values at different positions along the bed form. Van Rijn and Havinga (1995) analysed the transport processes of fine sand and bed ripple geometric characteristics under combined current and irregular waves in a laboratory basin. Tanaka and Dang (1996) determined the geometry of sand ripples due to combined wave-current flow. The authors proposed a new formulation for the spacing of wave-current induced sand ripples. Mogridge et al. (1994) derived empirical formulas to determine the bed ripple geometric characteristics under pure waves based on collected laboratory and field data of many investigators. Empirical formulas for bed ripples generated by waves were suggested by Nielsen (1981). Tanaka and Shuto (1984) proposed the ripple steepness concept and derived empirical formulas under waves and currents. Khelifa and Ouellet (2000) performed an experimental investigation of bed forms generated by the combined action of waves and currents. The investigators compared test results with earlier studies and suggested some empirical formulas. Grasmeijer and Van Rijn (1999) carried out a laboratory study as well, and presented an analysis of a series of laboratory measurements of the horizontal velocity and the suspended sediment concentration at a number of locations across a simulated shallow water bar built with fine sand and subjected to irregular waves of varying significant wave heights. Kobayashi et al. (1996) conducted a laboratory study to determine wave reflection, overtopping and over-wash of dunes. Hughes (1983) analysed the similitude relationships for the physical modelling of coastal dune erosion in movable bed models, which were developed based on the concentration of inertial forces. A stable profile equation was discussed by Larson and Kraus (1992). The ratio of the depth of the low point and the top point of a bar was obtained as 1.69. Sunamura and Maruyama (1987) studied the secondary bar, which occurs near the shore by conducting laboratory tests. Hsu (1998) performed experimental and theoretical studies in order to define offshore bar geometry under inclined waves. Larson et al. (1999) analysed the equilibrium beach profiles under breaking and non-breaking waves, and developed three different models to derive the profile shape under non-breaking waves. All three models produced equilibrium profile shapes of power types with the power typically in the range of 0.15–0.3.

In this study, the experimental results concerning offshore ripples and bars geometry under irregular waves are presented. As shown in Fig. 1, the geometric characteristics of offshore ripples and bars are defined using parameters such as the total ripple number, rnt; individual ripple number, i(1,2,3,…,rnt);; individual ripple length, rl(i); average ripple length, rla (rla=∑i=1rntrl(i)/rnt); individual ripple height, rh(i); average ripple height, rha (rha=∑i=1rntrh(i)/rnt); individual ripple steepness, rs(i) (rs(i)=rh(i)/rl(i)); average ripple steepness, rsa (rsa=rha/rla); bar length, bl; bar height, bh; and bar steepness, bs. Based on experimental results, these geometric characteristics were formulated. The relationship between the average ripple steepness and bar steepness was evaluated, and an empirical expression was suggested.

Section snippets

Experimental tests

Experimental tests were carried out at the Hydraulic Laboratory of Civil Engineering Faculty of Istanbul Technical University, Istanbul, Turkey. Fig. 2 shows the experimental set-up of the offshore ripple and bar experiment conducted in a wave flume that was 24 m long, 1 m wide, and 1 m deep. The wave channel was made of glass, and water depth in the channel was 60 cm during testing. Irregular waves were generated by using a computer-controlled wave maker joined to the bottom of the flume and

Dimensionless parameters

The following parameters initially play a role in defining geometric characteristics of wave-induced offshore ripples and bars. This study proposes that the geometric characteristics of offshore ripples and bars can be defined by the parameters of rnt, i, rl(i), rla, rh(i), rha, rs(i), rsa, bl, bh, and bs. Significant wave heights (Hs) and significant wave periods (Ts) basically characterise these waves. On the other hand, sand density (γs) and sand diameter (D) are used to define beach

Conclusions

The geometric characteristics of wave-induced offshore ripples and bars are investigated in this experimental study. The experiments were performed using irregular waves. The terms used for the geometric characteristics of wave-induced offshore ripples and bars include rnt, i, rl(i), rla, rh(i), rha, rs(i), rsa, bl, bh, and bs. The most important parameters that affect the given geometric characteristics are defined. All these parameters are written as a dimensionless form by using the π

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