Abstract
Artificial reefs are being implemented around the world for their multi-functions including coastal protection and environmental improvement. To better understand the hydrodynamic and morphodynamic roles of an artificial reef (AR) in beach protection, a series of experiments were conducted in a 50 m-long wave flume configured with a 1:10 sloping beach and a model AR (1.8 m long × 0.3 m high) with 0.2 m submergence depth. Five regular and five irregular wave conditions were generated on two types of beach profiles (with/without model AR) to study the cross-shore hydrodynamic and morphological evolution process. The influences of AR on the processes are concluded as follows: (1) AR significantly decreases the incident wave energy, and its dissipation effect differs for higher and lower harmonics under irregular wave climates; (2) AR changes the cross-shore patterns of hydrodynamic factors (significant wave height, wave skewness and asymmetry, and undertow), leading to the movement of shoaling and breaking zones; (3) the beach evolution is characterized by a sandbar and a scarp which respectively sit at a higher and lower location on the profile with AR than natural beach without AR; (4) the cross-shore morphological features indicate that AR can lead to beach state transformation toward reflective state; (5) the scarp retreat process can be described by a model where the scarp location depends linearly on the natural exponential of time with the fitting parameters determined by wave run-up reduced by AR. This study demonstrates cross-shore effects of AR as a beach protection structure that changes wave dynamics in surf and swash zone, reduces offshore sediment transport, and induces different morphological features.
Highlights
Beach erosion is a worldwide problem and intensified by natural effect and human activities in recent years [1], and the available measures are hard structures and beach nourishment [2].Scientific knowledge and practical experience since 1950s have led to principles of working with natural processes, in the long term and broad overall perspective [3]
Short [73] established the relation between 3-dimensional morphology and sediment-wave parameter (Dean’s parameter Hb /ws T) to identify beach state classified as two extremes and four intermediate states
This experimental study investigated the cross-shore hydrodynamic and morphodynamic processes of beach profiles with and without artificial reef (AR) under regular and irregular wave climates, and comparative analysis of the results revealed the influences of AR
Summary
Beach erosion is a worldwide problem and intensified by natural effect and human activities in recent years [1], and the available measures are hard structures and beach nourishment [2].Scientific knowledge and practical experience since 1950s have led to principles of working with natural processes, in the long term and broad overall perspective [3]. In the operational level, engineering should work with the nature to better achieve sustainable protection by proper use of beach nourishment and submerged coastal structures [4,5,6,7,8]. In the view of erosion prevention, an investigation on the natural (e.g., rocky headlands, offshore reefs, cobble, and boulder stream deltas) and artificial structures (e.g., groins, jetties, and breakwaters) in central and southern California [9]. In the view of coastal engineering, wave propagation on natural reef coasts had been studied in terms of sea and swell waves, wave group, infragravity wave [10,11,12,13,14], wave set-up and run-up [15,16,17], Water 2020, 12, 2947; doi:10.3390/w12102947 www.mdpi.com/journal/water. Reefs are still rarely assessed for their morphological effect despite that some field investigations have highlighted the positive contribution of natural reef in shoreline stabilization [22,23]
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