Nearshore Sediment Transport

Models for predicting the transport of sediment along straight coastlines in general use in the mid-1970’s were derived empirically from sparse measurement of both the forcing function (waves and currents) and the response function (sediment motions). In addition to the unsatisfactory nature of the basic measurements upon which they were based, the models were deficient because they failed to employ such potentially significant factors as wind stress, sediment size, bottom slope and spatial variations in waves and currents, including the effects of rip currents.

Torrey Pines Beach is part of the Torrey Pines State Preserve located along the coast between the communities of La Jolla and Del Mar, San Diego County, in the southern part of California. The coast is relatively straight with nearly north-south trending beaches backed by 90 m high wave-cut sea cliffs. The 2 km long section of beach selected for the experimental site is 6 km north of Point La Jolla headland and 3 km north of the head of Scripps branch of La Jolla Submarine Canyon (Figure 1A-1).

Santa Barbara is located on a sandy lowland on the coast of southern California, 153 km northwest of Los Angeles and 563 km southwest of San Francisco. It borders the Santa Barbara Channel which is bounded on the north and east by the mainland shoreline of Santa Barbara and Ventura Counties, on the south by the Channel Islands (San Miguel, Santa Rosa, Santa Cruz, and Anacapa) and on the west by the open waters of the Pacific Ocean (Figure 1B-1).

Rudee Inlet and the beaches to the north and south were selected as the site of the East Coast NSTS Total Trap experiment due to:

Wind generated surface gravity waves are the dominant driving force for surf zone dynamics on open coasts. A major thrust of the NSTS study was an investigation of the relationship between the incident waves and the processes that they drive (e. g., surf zone currents and sediment motion). A useful description of a linear wave field is obtained by the measurement of the frequency-directional spectrum. Particular attention is paid to certain moments of this spectrum. For example, S _xy , the onshore flux of longshore directed wave momentum, is an important wave parameter involved with longshore currents and resulting sediment transport. These wave field statistics were sampled with a linear array of pressure sensors in a mean depth of 9.6 m and by a pressure sensor and orthogonal-axis current meter in 5.7 m depth.

Accurate, repetitive surveying of the subaerial beach and shallow nearshore out to depths of about −1 m relative to Mean Sea Level (MSL) was performed as part of Task 4E of the Nearshore Sediment Transport Study (NSTS). This surveying was used to measure changes in beach shape and volume caused by cross-shore and longshore sediment transport during the inter-survey periods. The surveys also provided a daily measurement of the height of surf zone instrumentation above the sand bottom. Because accuracies of better than 5 cm were required for this task, the survey technique selected was similar to that described by Nordstrom and Inman (1975). A self-leveling engineers level and fiberglass, extendable survey rod were used for measuring vertical changes in the beach profile. The self-leveling level had a standard deviation of approximately ±2 mm over 1.6 km of double-run leveling. It was water resistant, with a magnification of about 32x and minimum focus distance of about 2 m. The leveling rods were graduated in 0.01 foot increments, with a linear accuracy of better than 1 in 4000. The fiberglass construction insured that the rod did not swell when wet and affect instrument accuracy. The survey line was a thin, plastic-sheathed steel line graduated at 3.0 meter increments. The construction of the line minimized stretching/contraction which would detract from survey accuracy. The maximum separation between level and rod was maintained at less than 60 m where possible; the primary exception to this occurred in the seaward portions of the profile when the level could not be relocated in the swash zone.

The primary purpose of the offshore soundings conducted in conjunction with the NSTS studies at Santa Barbara, California and Rudee Inlet, Virginia was to determine sediment volumes in water depths too great for standard rod and level surveying procedures employed on the dry beach or in wading depths. Soundings are subject to particular types of errors which can lead to bias in the results and substantially misleading volume errors. These errors at Santa Barbara and Rudee Inlet included: data contamination by dredging, the horizontal position of the survey, the correct temporal and spatial tidal level, the fathometer calibration, the noise introduced into the data by waves and the requirement to survey a sufficient distance offshore to ensure that substantially all of the volumetric changes occurring are being measured. These individual error components and the means taken to minimize them are the subjects of later paragraphs in this section, followed by an overall error assessment viewed in light of the intersurvey volume changes measured. The errors will be discussed in the context of the Santa Barbara measurement program; however, they are similar for the Rudee Inlet study, except dredging occurred more frequently at the latter site. The survey plan for Santa Barbara, California is presented in Figure 3B-1 and comprised a total of 64 beach profiles and 62 sounding lines. Most (59) of the beach profiles and offshore sounding lines were colinear.

The objective of the surf zone dynamics task was to characterize the horizontal velocity and sea surface elevation fields within the surf zone, and to relate them to the observed incident wave climate. Properties of the waves during shoaling, breaking, and run-up were measured with current meters, pressure sensors, wave staffs, and a run-up meter. This chapter is a brief description of the various sensors, and estimates of their accuracy. General considerations in the design of the experiments, and maps of typical sensor locations, are also given. More detailed information about the sensors (including manufacturers’ specifications and close-up photographs) are given in the Torrey Pines and Santa Barbara experiment reports (Gable, 1979, 1981).

The objectives of this task were to measure the characteristics of the nearshore circulation system at both field experimental sites and to develop a predictive model to:

The Nearshore Sediment Transport Study (NSTS) field experiments relied on Marsh-McBirney electromagnetic current meters to provide estimates of currents in the surf zone, for modeling of both hydrodynamics and sediment transport. These meters were chosen because of their history of successful use in previous surf zone studies, relying on their compactness and durability in this rough environment. The present work was motivated by examination of the NSTS experimental data from Santa Barbara, California, in which the higher-order velocity moments calculated from current meter time series showed a time-variability not obviously related to time scales of change of forcing (e. g., wave groupiness, infragravity waves), and by field studies which show persistent offshore near-bottom flow which is not yet satisfactorily explained by nearshore circulation theories. Because of the importance of these quasi-steady flows and higher order velocity moments to sediment transport in the nearshore, the present study examined under carefully controlled laboratory conditions the dynamic response of electromagnetic current meters (EMCM) typically used for field experimentation.

This aspect of the program included the development of fluorescent sand tracer and suspended sediment sampling devices and procedures for their use in measuring the transport rate of sand. These devices and procedures were employed at Torrey Pines Beach and at Leadbetter Beach.

An acoustic bedload sensor was designed and developed as a part of the program. Its operation is based on the fact that sand at rest must dilate before moving and remain dilated in motion. Thus, it is possible to obtain reflections from the surface of the moving sand as well as from the layer at rest due to the different acoustic properties of the stationary and dilated sand.

An important aspect of environmental engineering is the prediction of the behavior of a foreign substance introduced into the environment. In environmental fluid mechanics a knowledge of the environmental conditions (the transporting system) is generally assumed and the problem is that of predicting the behavior of an ensemble of contaminated (marked) fluid particles released into the transporting system. The fundamental idea behind the use of tracers for the determination of transport is essentially the inverse problem, i. e., one deliberately marks a finite number of particles within the transporting system with the expectation of gaining information about the characteristics of the transporting system itself by monitoring the behavior of the marked particles (the tracers).

The successful use of tracer sand to determine sediment transport rates relies on the two basic premises that the tracer behaves exactly in the same way as the native sand and that the motion of the tracer centroid can be determined. The centroid velocity provides an estimate of the transport velocity, which when multiplied by the thickness of the moving sand, yields the transport rate.

The sediment trapping experiments, with durations of six months to more than a year, required a completely different data recording system than the one employed for the one month intensive experiments. It must be capable of reliably logging data samples several times each day without requiring any resident personnel. In 1976, the Nearshore Research Group at SIO had completed the development of a system for monitoring wave climatology at distant locations (Figure 7B-1). This wave network, which is described in its initial form in Seymour and Sessions (1976), is funded jointly by the U. S. Army Corps of Engineers and the California Department of Boating and Waterways.

Wave-induced velocities are the primary driving force for littoral sand transport. For this reason, a major component of the NSTS program was to measure wave associated velocity and elevation fluctuations. A description of the shoaling wave transformation is a necessary ingredient in the development of any sediment transport model.

Measurements of velocity and elevation in the inner surf zone usually show that a substantial fraction of the total variance is at surf beat periods, roughly 30–200 sec. (Inman, 1968; Suhayda, 1971, 1974; Goda, 1975; Huntley, 1976; Wright et al., 1979; Holman, 1981; Wright et al., 1982; and others). A typical current record from an inner surf zone sensor at Torrey Pines Beach, and a low passed version of the same record are shown in Figure 9-1. Although wind wave motions are usually the most obvious component of the unfiltered cross-shore velocity field (upper panel), surf beat motions are certainly significant (lower panel). Below, we first briefly review some theoretical ideas about the nature and origin of surf beat. Then experimental evidence supporting the various theories is discussed, placing special emphasis on the NSTS Torrey Pines results.

The description of nearshore currents is of primary importance to the development of littoral transport models. The bases are given here for the current models to be described in Chapter 15. A convenient starting point for the study of unsteady flow phenomenon is a statement of the general conservation equations of mass, momentum and energy, following, for example, Mei (1983). In the first part of this chapter, the conservation equations are examined and discussed. A basic element of the NSTS dynamics task was to develop longshore current formulae for the case of statistically stationary waves (statistics do not change with time) propagating over straight and parallel contours. Making these assumptions, the momentum balances are simplified and compared with some of the NSTS field data in the second part of this chapter.

The field data collected at Torrey Pines Beach in November 1978, were used to study the offshore wave groups, the wave-induced nearshore circulations and their generation mechanisms on a beach with relatively simple bathymetry. Unlike the theoretical models of nearshore circulation based on deterministic mechanisms, the mean currents at natural beaches of dissipative type (where the ratio of deep water wave steepness to beach slope is high and waves break cleanly) are superimposed on very energetic infragravity waves. Migrating and pulsating rip currents with long alongshore wave length scales were particularly difficult to detect, using the current meter array, despite the visual observations of rip currents. A special instrument, consisting of tripod-mounted current meter and a pressure sensor, linked to shore by telemetry, was designed to make measurements in a stationary rip current (see Chapter 4); however, it proved of little value, due primarily to the lack of stationarity of the nearshore circulation system.

Field measurements of suspended sediment in the nearshore zone have been carried out with a variety of instruments, including pumps operated from ocean piers (Watts, 1953; Fairchild, 1972, 1977) and from sleds pulled through the surf zone (Coakley, 1980); self-siphoning samplers (Wright et al., 1982a); light-scattering devices (Thornton and Morris, 1978; Downing, 1983); light transmission devices (Brenninkmeyer, 1974, 1976a, b); and diver-operated samplers [Kana, 1976, 1978, 1979; Kana and Ward, 1980 (modified for use from a pier); Inman et al., 1980]. While each of the above studies have limitations, the combined information presented gives a general view of suspended sediment movement in the nearshore zone. Those studies that provide background data directly applicable to the present investigation are summarized briefly.

Theories and empirical studies of longshore sediment transport rates within the surf zone have generally assumed or have estimated in some ad hoc manner, the contribution of suspended-load sediment to total-load transport. However, there has been broad disagreement upon the importance of suspended-load transport; ranging from dominant (Watts, 1953, Fairchild, 1972; Dean, 1973; Galvin, 1972; Brenninkmeyer, 1976; and Kana, 1977) to less than one-half of the bedload transport (Komar and Inman, 1970; Komar, 1976, 1978; and Inman et al., 1980). Fairchild (1977), Kana (1977, 1978, 1979), Kana and Ward (1980) and Inman et al. (1980) suggest that the ratio of suspended load to total load is not constant, but that the importance of suspended load varies for different surf zone conditions (e. g., breaker type) and wave intensities.

Chapters 13 and 14 describe two relatively straightforward techniques for measuring longshore transport, one involving short-term tracer experiments and the second longer-term trap experiments. Unfortunately, cross-shore transport is not readily amenable to either of these techniques. Cross-shore transport is driven by oscillatory velocities, near the bottom, which are nearly perpendicular to the shoreline. Net cross-shore transport results from a local or general assymmetry in these oscillations. In many cases, this bias is very small compared to the magnitude of the oscillatory component.

This chapter describes the results of experiments using sand tracers to measure the bedload response to forcing by waves and currents. Previous field studies of the longshore transport of sand have used wave arrays to measure the wave field and fluorescent sand tracer to measure the longshore transport of sand (e. g., Inman, Komar and Bowen, 1968; Komar and Inman, 1970). The former field procedures have been modified and improved to include arrays of pressure sensors and electromagnetic current meters and injections of dye in the water to measure the wave and current field. Fluorescently dyed sand is employed as a tracer to measure the motion of the sediment. The methods and assumptions involved in our use of sand tracer are detailed in Chapter 6B of this volume.

The Nearshore Sediment Transport Study (NSTS) included two total trap experiments conducted to evaluate the longshore sediment transport relationship I ₍ = KP _(s in which I ₍ is the total longshore immersed weight sediment transport rate, P _(s is the longshore component of wave energy flux and K is a proportionality factor which could depend on a number of wave and sediment parameters. The locations of the two experiments were at Santa Barbara, California and Rudee Inlet, Virginia. The trap characteristics for each of these two sites are quite different. The Santa Barbara trap consists of the spit formed in the lee of the Santa Barbara breakwater. At Rudee Inlet, the trap is inside of a weir-type jetty with a crest elevation at about mean sea level.

Longshore current models have been tested using NSTS measurements at Leadbetter Beach, Santa Barbara. The NSTS provided data on nearshore currents for a variety of wave conditions including narrow banded (in frequency and direction) swell waves of small and moderate height and wide banded waves during local storms. The theoretical models include a two-dimensional, finite element, monochromatic wave driven model (Wu et al., 1985) and one-dimensional (i. e., no longshore variations) random wave driven model (Thornton and Guza, 1986). These models assume the waves are narrow banded in frequency and direction and use a single frequency and direction to describe the input wave field. In keeping with the narrow band wave assumption, data from four days with narrow band swell (3–6 February) were used for comparisons. Empirical orthogonal eigenfunctions were used to find longshore current patterns during all directional wave conditions (Guza et al., 1986). This chapter summarizes these papers and presents additional comparisons to a simple one-dimensional monochromatic wave model (Longuet-Higgins, 1970).

The chapters following will present the models for longshore and cross-shore surf zone transport that have been developed from the analyses of NSTS data. These are global models in which net transport is integrated in space and time across the surf zone. This chapter is intended to form an introduction to them by reviewing existing models for gross sediment transport, i. e., transport at a point over a half cycle of oscillation. None of these point models were developed within the NSTS program, but all of the integrated NSTS net transport models have been developed from them.

Seymour and King (1982) evaluated a number of existing models for cross-shore transport by testing their skill in predicting the observed beach excursions at Torrey Pines. The results were discouraging, in that none of the models showed a useful skill level. As discussed in Chapter 12, the Torrey Pines data set may not have provided a realistic test for these models. Therefore, the other three data sets described in Chapter 12 (Scripps Beach, Santa Barbara and Virginia Beach) were employed to evaluate models for cross-shore transport.