The Coastal Compartment Concept and its Importance for Coastal Management (part I)

What is a Coastal Compartment, what is the sediment budget, how it is defined, why it is important for adaptive coastal management strategies and policies?

INTRODUCTION

The interface between land and ocean is one of the most dynamic regions on the Earth’s surface (Harley et al., 2011). Thus, coastal management and planning is crucial to a sustainable and durable land use and to the appraisal of this complex ecosystem. A systematic approach is needed in order to assess and quantify the fluxes of energy and sediment that are involved in such a relentlessly changing area. Part 1 of this article (shorter) will describe the components of the coastal system, examining what it is, how it is defined and the sediment budget. Part 2 (longer) will give a broad overview of the natural agents acting as inputs, outputs, sinks or external forcing (such as beach rotation) to the coastal compartment. In addition, part 2 will highlight the importance of considering the coastal compartment concept and all its components prior undertaking any coastal decision affecting its sedimentary regime.

Good read!

THE CONCEPT

Inspired by the relatively easier stream catchment basin, the coastal compartment concept tries to conceptualise the energy and sediment fluxes interacting with the compartment itself. The pioneering works appeared in the USA (mainly California) around the end of the 1960s (Sanderson and Eliot, 1999); a catalyst for further conceptual developments like those of Davies (1974). In this conceptualisation of the headlands-embayment system, balancing the coastal compartment sediment equation means taking into account its sources and sinks, such as:

- onshore transport

- offshore transport

- longshore drift

- deflation

- sinks (submarine canyons, tidal deltas, inlets, dunes)

- internal biogenic production

- human engineering activities (dredging, nourishment, mining)

In figure 1 we can see the coastal sediment concept applied in a simplified way to Wonoona Beach (NSW) with the major coastal processes influencing the sediment budget of the compartment.

(Figure 1: Simplified sediment budget model for Wonoona-Bellambi beach. Click on the image to get the full beach assessement in my ResearchGate)

Figure 2 schematises figure 1, and Longshore drifting (LST) is considered both source and sink as it passes through the compartment.

(Figure 2: Rosati (2005). General sediment compartment conceptual model.)

This systemic approach is very useful as “it gives us an unit within which we can monitor what is happening, do our sums and calculate sediment budgets” (Davies, 1974).

THE SEDIMENT BUDGET

Also known as beach budget (Bird, 2008), the sediment budget for a defined beach compartment is fundamental in coastal science and engineering for not only understanding behaviour and pathways of sediments and energy but it often forms a common framework for discussions with colleagues and sponsors involved in a study (Rosati, 2005).

The coastal sediment budget deals “ […] with the volumes of sediment supplied to a particular sector by onshore and longshore drifting and yields from the hinterland and the volumes of sediment lost offshore, alongshore or landward over a specific period.” (Bird, 2008). Dolan et al. (1987) recommend elaborating a conceptual sediment budget as a qualitative model prior the actual computation of the fluxes magnitudes. This gives a regional perspective of the major processes in play, because “By definition, examination of sediment budgets at the littoral cell scale involves research of large-scale coastal behaviour.” (List, 1993).

The boundaries of the coastal compartment

Rocky headlands protruding seaward, river mouths (Tamar River, Tasmania), tidal entrances, submarine canyons (Santa Barbara, California) or even groynes can virtually isolate an embayed beach, creating a beach compartment (see figure 3).

( Figure 3_Above: Tamar River, northern Tasmania. Below: Santa Monica (California) submarine canyons acting as sinks (source: www.mbari.org) )

Most of the beaches in New South Wales are bounded by headlands, however, defining boundaries for a coastal compartment is not easy task. A first identification of beach compartment can be drafted by examining charts, maps and vertical photography of the coast but it is necessary to study the patterns of coastal sediment flow to determine actual self-contained compartment boundaries (Bird, 2008). In the past boundaries were placed arbitrarily where it was visually most meaningful (Davies, 1976) but subaerial projection is not a reliable indicator of longshore drift blockage as beach sediment may drift around the rocky headlands (nearshore up to 20m) (Bird, 2008). Davies (1974) states that boundaries have different “filtering effect” (blockage efficacy) creating hierarchies of compartments of varying size and degree of exclusiveness. Tracer studies of sediment movement, geomorphological and sedimentological evidences or analysis of the spatial distribution of wave energy alongshore are common practices when it comes to define whether sediment surpasses headlands or not (Sanderson and Eliot, 1999). Alternatively, statistical methods have been used. Sanderson and Eliot (1999) define the boundaries of multiple compartments by combining the collection of in situ data (sedimentological and environmental variables, such as width of beach, surf zone and nearshore), local knowledge, and a suite of statistical techniques. Their methodology can be summed up as follows:

1) petrological survey of the region

2) identification of geographically clustered similar sediment occurrence (auto-correlation)

3) identification of broad relationship between the sedimentary, geomorphologic and environmental characteristics of the sediment clusters (variance analysis)

4) identification of the likely boundaries

Now that the system has been reviewed, let's focus on the agents that influence the balance of the sediment budget within a coastal compartment, such as marine erosion, rivers contribution, onshore/offshore sediment movement, subaerial erosion, internal accretion, deflation, dunes, inlets, submarine canyons, longshore drift and some external forcing like ENSO and the beach rotation phenomenon (case study: Narrabeen Beach, NSW, Australia).

For every agent a description and an Australian example is provided. I hope you will like it..Stay tuned!

Nicolas Pucino

MSc (distinction) in Coastal Planning and Management, BSc in Physical Geography

References and further readings

• Bird, E.C.F., 2008. Coastal geomorphology: an introduction, 2nd ed. ed. Wiley, Chichester, England ; Hoboken, NJ.
• Carvalho, R. C. & Woodroffe, C. D. (2013). Shoalhaven river mouth: a retrospective analysis of breaching using aerial photography, Landsat imagery and LiDAR. Proceedings of the 34th Asian Conference on Remote Sensing 2013 (pp. SC03-592-1-SC03-592-7). Indonesia: Indonesian Remote Sensing Society.
• Chapman, D.M., 1980. Coastal erosion and the sediment budget, with special reference to the gold coast, Australia. Coastal Engineering 4, 207–227. doi:10.1016/0378-3839(80)90020-4
• Davies, J.L., 1974. The coastal sediment compartment. Australian Geographical Studies 12, 139–151. doi:10.1111/j.1467-8470.1974.tb00270.x
• Dolan, T.J.; Castens, P.G.; Sonu, C.J., and Egense, A.K., 1987. Review of Sediment Budget Methodology: Oceanside Littoral Cell, California. Proceedings, Coastal Sediments ’87 (Reston, VA, ASCE), pp. 1289–1304.
• Eliot, I., Clarke, D., 1982. Temporal and spatial variability of the sediments budget of the subaerial beach at Warilla, New South Wales. Marine and Freshwater Research 33, 945. doi:10.1071/MF9820945
• Fotheringam, D.G., and Goodwins, D.R. (1990) Monitoring the Adelaide beach system. Proceedings 1990 Workshop on Coastal Zone Management, Yeppon, Queensland, pp. 118-132.
• Goodwin, I.D., Freeman, R., Blackmore, K., 2013. An insight into headland sand bypassing and wave climate variability from shoreface bathymetric change at Byron Bay, New South Wales, Australia. Marine Geology 341, 29–45. doi:10.1016/j.margeo.2013.05.005
• Harley, M.D., Turner, I.L., Short, A.D., Ranasinghe, R., 2011. A reevaluation of coastal embayment rotation: The dominance of cross-shore versus alongshore sediment transport processes, Collaroy-Narrabeen Beach, southeast Australia. Journal of Geophysical Research 116. doi:10.1029/2011JF001989
• List, J.H., Terwindt, J.H.J., 1995. Large-scale coastal behaviour. Marine Geology 126, 1–3. doi:10.1016/0025-3227(95)00062-4
• Ranasinghe, R., McLoughlin, R., Short, A., Symonds, G., 2004. The Southern Oscillation Index, wave climate, and beach rotation. Marine Geology 204, 273–287. doi:10.1016/S0025-3227(04)00002-7
• Rosati, J.D., 2005. Concepts in Sediment Budgets. Journal of Coastal Research 212, 307–322. doi:10.2112/02-475A.1
• Sanderson, P.G., Eliot, I., 1999. Compartmentalisation of beachface sediments along the south-western coast of Australia. Marine Geology 162, 145–164. doi:10.1016/S0025-3227(99)00046-8
• Short, A.D., 1993. Beaches of the New South Wales Coast. Australian Beach Safety and Management Program, Sydney, 356 pp.
• Short, A.D., Woodroffe, C.D., 2009. The coast of Australia. Cambridge University Press, Cambridge.