Comparison of a Physical and Numerical Mobile-Bed Model of Beach and T-Head Groin

Introduction

Numerical modeling technology for simulating beach morphology in the presence of complex structures is advancing rapidly, though limitations remain in the ability to simulate detailed beach contour movement near and above the water line. Mobile-bed physical models are also limited, primarily due to issues of model scale constraints. The primary aim of this paper is to compare the performance of a numerical coastal morphological model with a series of physical model tests, for an open-coast sandy beach modified with T-head groins. It is important to understand the relative benefits available through applications of the combination of numerical and physical models to projects. The present study compares planform and profile changes in the physical model with simulations using the Coastal Modeling System (CMS) numerical model (http://cirp.usace.army.mil/wiki/CMS).

Physical Model Description

The physical model is based on a proposed shoreline development project to include, among other features, nourishment and maintenance of an approximately 7km long stretch of the barrier island and construction of several emergent T-head groins (Figure 1). The three-dimensional physical model was undertaken at a geometric scale of 1:25 at the Canadian Hydraulic Centre’s Large Area Basin (LAB), utilizing a set of moveable wave generators capable of providing long-crested waves to match a variety of spectral conditions. Tests from three dominant design wave directions were conducted to investigate the performance of the proposed design under storm wave and water level conditions. Wave heights were measured at several wave probes, some of which are indicated by the labels in Figure 2. A fine silica sand with a median diameter of approximately 0.13mm was employed in the model to represent the beach fill. This sand was the finest non-cohesive material readily available. According to the expression for fall velocity developed by Soulsby (1997), the 0.13mm sand has a fall velocity of 1.1cm/s. Applying the Froude scaling law for velocity, this material represents prototype median grain diameter of 0.39mm with a fall velocity of 5.7cm/s. The model beach fill was constructed to crenulate shaped planforms in the bays between T-groins based on GENESIS (Hanson, 1989) simulations of typical annual transport at the site. The initial model profiles were constructed to a 1:10 slope above the Mean Sea Level (MSL = +1.32mMLLW) and 1:25 slope below MSL. The mobile bed portion of the model ranged from the -2mMLLW contour offshore to the +4mMLLW contour on land (Figure 2). This study focuses on the first series of wave tests to limit uncertainty due to rebuilds of the model bathymetry that were carried out between wave test directions. The incident waves have the direction of most severe storm waves at the project site (285ON), equivalent to a 20O counterclockwise angle from shore-normal. The waves in the physical model were selected to match the intensity, profile, and duration of a realistic design storm as in Table 1 and Figure 3. After the model beach was constructed, a small wave segment was run in the model for a short duration (8-hr prototype scale) to smooth out any small aberrations remaining from the construction. The beach was then in its initial condition, t=0 hr.

Beach profile morphology was measured after each wave segment on a transect between Groin 3 and Groin 4 for each test case. Measurement of the beach profile was conducted manually from a bridge as shown in Figure 4. The location of the transect (Profile 2) is shown in Figure 2. Planform morphology was measured within the mobile bed area of the model at the end of the full set of tests from a given wave direction.