Nearshore placement of a sand dredged mound Ernest R. Smith a , Felice D'Alessandro b, * , Giuseppe R. Tomasicchio b , Joseph Z. Gailani a a U.S. Army Corps of Engineers, Coastal & Hydraulics Laboratory, 3909 Halls Ferry Road, Vicksburg, MS, USA b University of Salento, Engineering Department, Ecotekne, Corpo O, Via Monteroni, 73100 Lecce, Italy ARTICLE INFO Keywords: Nearshore mound Dredged material Longshore transport Sand migration Physical model experiments ABSTRACT As a part of the Dredging Operations and Environmental Research (DOER) Program, movable-bed physical model experiments were performed at the U.S. Army Engineer Research and Development Centre's (ERDC), Large-scale Sediment Transport Facility (LSTF) to investigate the fate and evaluate the benefits of nearshore-placed dredged material. The resulting bathymetry was measured with detailed surveys, the migration of the mound was quantified and comprehensive observations of hydrodynamics were obtained. The potential suitability of dredged material placement in the nearshore/surf zone was demonstrated after 10 h of oblique wave attacks. It has been shown that, as the mound was located at the edge of the surf zone, very likely wave breaking induced horizontal circulation may be dominant. A downdrift accretion of the submerged beach was observed, which is due to the transport of part of the sediment suspended by breakers at the mound and captured by the longshore currents. The experiments provided useful validation data for numerical morphological models. 1. Introduction The environmental impacts on adjacent coastlines resulting from removal of sediment by harbor and inlet maintenance dredging opera- tions are a worldwide problem and the aim of recycling appropriate dredged sediments back in the nearshore littoral system has been a common objective in many previous studies [18]. In this context, it is worth to point out that environmental impacts may result from both dredging activities and the disposal of dredged material. The present work, however, deals only with aspects related to the disposal of dredged material. Typical practice for a century has been to transport dredged sand to an offshore disposal site, in deep water, where the sediment is lost from the littoral cycle. A potentially viable alternative to offshore disposal is the placement of sediments, if clean and with good quality, in the near- shore region. Positioning of dredged material in the nearshore allows natural migration and dispersion of the sand under the wave action and currents. The nearshore mound location must be chosen judiciously and different factors should be taken into account to assure that material remains in the littoral zone and nourishes the beach. An important factor to take into account when placing a feeder mound is the depth of placement. If placed properly, the mound can be considered “active” or “dispersive” helping the nearshore region in two ways. First, wave breaking over the mound promotes the movement/translation of its centre of mass (centroid) and/or significant dispersion of sand, maxi- mizing the chance of sediments reaching the littoral system and indi- rectly nourishing the beaches. Second, such a placement scheme gives added protection to the beach by dissipating wave energy. Although the potential benefits of this concept are commonly recog- nized, guidelines on optimal placement of the dredged mound and on the onshore sand migration processes are still lacking. Nearshore morphology models predicting storm-scale erosion have been in use for the past decade and demonstrate an acceptable level of accuracy as a result of well-defined equations, established numerical solutions and quality laboratory and field data (e.g. Refs. [15,34]). However, modelling of long-term material fate for use in determining whether an existing or proposed disposal site will be dispersive or non- dispersive has not yet approached a similar level of accuracy. Indeed, sediment transport models developed to predict cross-shore storm erosion are dominated by the seaward-directed wave and roller return current predicted in energetic seas. The onshore migration of sand and beach recovery is a gradual process and only prevails during period of low wave steepness (e.g. Ref. [4]), and the erosion models are poorly suited to longer-term simulations [13]. Notable exceptions are [20,21,35]; but these are restricted to longshore uniform behavior. Noteworthy attempts to model the 2DH evolution of an offshore nour- ishment is presented in Refs. [33] and [8]. Generally, in practical * Corresponding author. E-mail addresses: Ernest.R.Smith@usace.army.mil (E.R. Smith), felice.dalessandro@unisalento.it (F. D'Alessandro), roberto.tomasicchio@unisalento.it (G.R. Tomasicchio), Joe.Z. Gailani@usace.army.mil (J.Z. Gailani). Contents lists available at ScienceDirect Coastal Engineering journal homepage: www.elsevier.com/locate/coastaleng http://dx.doi.org/10.1016/j.coastaleng.2017.05.002 Received 29 August 2016; Received in revised form 22 May 2017; Accepted 22 May 2017 0378-3839/© 2017 Elsevier B.V. All rights reserved. Coastal Engineering 126 (2017) 1–10