Coastal Services Center

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Sediment Budgets in Coastal Systems

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Introduction

In the planning phase of a beach nourishment project or an inlet improvement project, it is important to develop an accurate understanding of the sediment budget. The sediment budget provides a framework for understanding the complex processes that can take place in the vicinity of inlets and beach nourishment projects. This budget is critical to understanding the complex sediment pathways and the magnitudes of those sediment movements, at inlets and along beaches. The accuracy with which the estimates of different gains and losses are calculated is also critical because the sediment budget will be significantly influenced by these estimates.

A sediment budget is used to analyze existing site conditions, hind-cast historical conditions, and predict future performance of constructed projects. For this reason comprehensive physical surveying programs to monitor the beaches are essential in developing an understanding of the sediment quantities representing key components of the sediment budget. The premise behind a coastal sediment budget is relatively simple. If more sediment is transported into an area than transported out of an area, shoreline accretion results. Conversely, if more sediment is transported out of an area than is transported in, shoreline erosion results. Balancing the inflow and outflow of sand resources for a given region is important to maintaining stable beaches, and is used by coastal engineers and geologists to analyze and understand shoreline changes as well as to design beach nourishment projects and predict their future behavior. The amount of sand in a given area is a balance between the inflow, outflow, and amount stored in the littoral cell (or nearshore and onshore area).

Sediment contributions to the beach can come from many sources such as rivers, offshore bars migrating onshore, beach nourishment, and, most commonly, longshore transport. In warm latitudes, living organisms produce sand biogenically. As a result, these littoral systems are predominantly carbonate sand beaches comprised of shells and shell-derived sand.

Erosion, or sediment lost from the beach, arises from such processes as the longshore movement of sediment out of a given area and the movement of sediment to offshore areas as a result of human actions such as dredging or mining. These are the major sources and losses but there are minor components that are also important. Wind-blown sand is a typical minor loss, where sand is moved from the beach to the dune system and is an important component of a sediment budget. Another is the development of offshore, ebb-tidal and flood-tidal shoals (associated with inlets). Washover fans on the landward side of the beach or barrier island represent another type of minor sediment loss.

A primary goal in the design development of a beach nourishment project requires that the design professional accurately quantify the components of the sediment budget for the project site. This is critical when estimating the design level of the project and the renourishment or maintenance interval for the project. Thus, familiarity with the physical setting is essential in developing a working sediment budget that can be used to determine how and where sand should be placed for the most stable sand placement configuration (for example, more rapidly eroding beach areas would receive a greater amount of sand).

Sediment Sources and Sinks in a Sediment Budget

It is more difficult to accurately quantify the inflow and outflow (that is, pathways of sand movement within the beach and nearshore system) to the system than it is to determine the sources and their associated pathways. There are models that allow prediction of the major contributors. Using these models requires detailed knowledge of various conditions that occur at the site such as wind, tide, storms, and waves. These elements are the basis of the climatic conditions that affect the movement of sand and, thereby, the stability of the beach sediments over the short and the long term. The results of these models are then compared with historical conditions or measurements to see how well they perform, that is, match the past observations of the given beach system. Then the models are used to predict the evolution of a beach nourishment project in the years following its construction. If the information needed for the model is not available, then the only recourse is to depend upon the historical information, or, at the least, information from similar sites, to predict the performance of the project. Models can be used to estimate the losses from the sediment budget that cannot be measured, such as sand lost to the offshore area that occurs during major storms, that is, does not move back into the "active" littoral system, or only partially recovered to the system. There are limitations to these models, but they still represent the best tools to quantifying primary components of a sediment budget and predicting the performance of alterations to the natural system (that is, beach nourishment).

Common transport components of a littoral system, which should be identified and quantified when developing a sediment budget, are shown in Table 1.

TABLE 1. Gains and Losses of Sediment to the Littoral System
Gains	Losses	Net
Longshore transport into area River transport Sea cliff erosion Onshore transport Biogenic deposition Wind transport onto the beach Beach nourishment	Longshore transport out of area Offshore (or cross-shore) transport Wind transport inland Deposition in maintained navigation channels Transport through submarine canyons Solution and abrasion Mining Other anthropogenic causes	Beach accretion or erosion (sand gains and losses)

Figure 1 illustrates many of the typical components and sediment flow pathways found at tidal inlets. The main problems encountered when developing a sediment budget are associated with quantifying the primary components of the sediment budget, as listed in Table 1. The summation of the gains and losses at a beach will provide an understanding of the erosion rates following construction of a project, and thus the long-term maintenance costs. Historical accretion or erosion rates for the given shoreline are also used to quantify the gains or losses that are otherwise difficult to ascertain.

Erosion

Beach erosion can be determined in a general way by measuring shoreline or dune scarp retreat from aerial photographs or in detail using ground (beach profile) surveys. The volume of material eroded from cliffs and/or dunes can be quantified by determining the long-term recession of the dune scarp using aerial or ground surveys spaced over a designated period of time. These long-term erosion losses are reduced to an annual rate and then further analyzed to determine the net amount of sediment that is predicted to stay on the beach once it erodes from the dunes. The volume of sand eroded or accreted from the beach is derived from the overall sediment budget.

Offshore or Cross-Shore Sediment Transport

Sand can move offshore due to the erosion caused by severe storms that carry the material into deep water which prevents typical waves from moving the sand back onto the beach. In other locations, sand is carried shoreward from offshore shoals or bars from shallow depths during mild wave conditions. The design of some beach nourishment projects, like the one constructed at South Padre Island, Texas, reflects the expectation of cross-shore sediment transport from offshore shoals or bars. Sediments dredged from maintenance of the Brownsville ship channel were placed offshore of South Padre Island with the expectation that the material would be moved onto the beaches or nearshore littoral system.

Inlet Shoals

Flood tidal and ebb tidal shoals, often called tidal deltas (such as the Mississippi River Delta) are typically large repositories of sand within the littoral system and are important components of a sediment budget near an inlet. Thus, tidal shoals represent significant sinks and sources of sediment. As they reach equilibrium state, the shoals maintain a relatively constant total sediment volume. A mature flood tidal delta affects the flow of sand across an inlet allowing sediments to move across the inlet between adjacent barrier islands. An ebb tidal delta will grow until it reaches a stable configuration. In its stable form, an ebb tidal delta will bypass sediment around the tidal delta to the downdrift shoreline. The point at which the material enters the downdrift shoreline may be well beyond the inlet. An example of this type of downdrift bypassing is shown in Figure 2, at Moriches Inlet on Long Island where the ebb tidal delta is stable and thus provides for effective sand bypassing across the inlet.

River Supply

Sediment contributed to the littoral system by a river can be an important source to beaches and may be approximated using rating curves for the sediment transport quantities as a function of the daily or annual river flows (discharge values). The US Geological Service maintains these types of flow records for most rivers. However, estimating sediment contributions for large complex estuaries can become much more difficult than open coast beaches. In these areas the sediment typically does not reach the coast as a direct function of river discharge. Within complex estuaries, it may be necessary to develop an annual sediment quantity based on the annual flow, size of the drainage basin, sediment "yield" of the basin, and the amount of material that will remain in the littoral system once it reaches the coast.

Wind Transport

Quantifying sediment contributions to the sediment budget from wind transport can be accomplished using formulas developed from wind speeds. Using daily wind speeds and directions, the gains or losses to the littoral system can be approximated. Alternatively, wind transport of sand can be quantified by the measured beach profile changes or by the migration of dunes either landward or seaward. In many locations the net wind transport of sand is lost from the littoral system because the backbeach area is not well vegetated, whereas when the dune is well vegetated these plants tend to catch and hold the sand in place.

Anthropogenic Activities

Damming rivers has significantly reduced sediment delivery to beaches and deltas on the coasts. Mining sand and gravel for purposes of construction has long been a common practice in the coastal zone in some local areas. However, this sediment loss typically occurs further inland and not in the littoral system directly surrounding the beaches. Nevertheless, the sand that would have been transported to the coast does not reach the littoral system and must be subtracted from the potential river contribution.

Effects of Coastal Structures

Additionally, the construction of coastal structures such as breakwaters, groins, and jetties can interrupt the longshore sediment movement. This can reduce or possibly eliminate the natural delivery of sand to the downdrift beaches. Also important is the dredging that takes place on the ebb tidal or flood tidal shoals for the purposes of navigation. Historically this material has not been placed on the downdrift shorelines, causing it to be lost from the active littoral system. More recently, the concept of regional sediment management (RSM) has been adopted by state and federal agencies. This concept has demonstrated the importance of placing material into the system when it is removed by various means. This includes creative management of sediments on a much larger scale than previously considered. This clearly forces a new regulatory paradigm.

Beach nourishment may add sediment to the littoral system (that is, sediment trucked to the beach, dredged from back bay areas, or introduced from other sources outside of the littoral system). Beach nourishment activities may also redirect sediment within the littoral system. One method of sediment redirection is the bypassing of sediment at inlets. It is important to realize that this type of human activity modifies the natural processes and pathways of sediment transport within the littoral system.

Longshore Transport

Longshore transport is the most significant component of the sediment budget in terms of total movement of sand within the littoral system, especially along the open coasts of the Atlantic, Pacific and Gulf of Mexico. Quantifying this component (that is, the amount of material transported/moved) in this pathway requires knowledge of the wave climate and the availability of sediment within the littoral system. Typically, material may move in both directions along the coast depending on the direction of the waves and longshore current. Defining the net transport that results from the sediment budget for a given area is very important to estimate the net erosion of the shoreline. Similarly, the gross transport, which is the sum of the transport in both directions, is very important to the maintenance period or dredging cycles at a navigation channel that crosses the littoral system.

Biogenic

Biogenic production of carbonate sediments is only important at the most southerly shorelines on the Atlantic and Gulf Coasts of the US mainland. It is extremely important in Hawaii, Puerto Rico, and other areas located in the tropics. Production rates of biogenic material vary considerably but are typically quite small. The same processes that affect the more typical, quartz-rich sediments found along most Atlantic Coast beaches also affect carbonate sediments but the effects of waves and currents are not identical for these two different types of sediment. Because the production rates are slow for carbonate sediments, it is most important that there are no disturbances to the natural rate of production or the transport processes.

Estimation of Quantities

Komar (1996) provides a summary of the methods available to estimate the quantities for each component in the sediment budget shown in Table 1. In addition to the empirical tools available for estimating the quantities for the littoral sediment gains and losses, historical shoreline monitoring and beach nourishment performance plays a significant role in developing useful estimates for developing a sediment budget. When neither adequate historical evidence nor data for the empirical relationships is available, estimates can be made from cross-shore and longshore transport models. Rosati and Kraus (1999) demonstrate a framework to assemble the estimated quantities into a working sediment budget to provide future predictions based on varying assumptions. At inlets, sediment budgets are difficult to formulate because the pathways are quite complex.

Often the budget requires information for several unknown quantities with varying degrees of uncertainty. As a part of the Coastal Inlets Research Program a methodology has been suggested to quantify the uncertainty in commonly occurring processes. Kraus and Rosati (1998) provide a mathematical framework for a systematic means of evaluating the uncertainty for each of the variables included in the sediment budget. An example of a sediment budget for a portion of coastline is given in Figure 3. The arrows in the figure show the pathways of sediment travel. Quantities for each of these pathways need to be established to properly determine the sediment budget. A more complete case is given in Figure 4 that illustrates the net sediment transport components surrounding South Lake Worth Inlet. Numbers in the figure refer to the percent of the net inflow of sediment to the system from Palm Beach. For example, 37 percent of the material arriving from Palm Beach naturally bypasses the inlet while 26 percent is mechanically bypassed via the sand transfer plant that is shown in Figure 5.

Conclusions

A sediment budget is used to design beach nourishment or an inlet improvement project and thus it is essential to develop an understanding of the primary components and quantities of sediment movement within the littoral system. This budget is particularly important to understanding the complex sediment pathways around and through an inlet. The accuracy with which the estimates of different sediment gains and losses are calculated is also critical because the sediment budget will be significantly influenced by these estimates.

References

Coastal Inlets Research Program (CIRP) web site: http://www.cirp.wes.army.mil/cirp/cirp.html.

Komar, Paul D. 1996. "The Budget of Littoral Sediments – Concepts and Applications." Shore and Beach. Volume 64. Pages 18 to 26.

Kraus, Nicholas C., and Julie D. Rosati. 1998. "Estimation of uncertainty in coastal-sediment budgets at inlets." U.S. Army Corps of Engineers, Coastal Engineering Technical Note IV-16.

Rosati, Julie D., and Nicholas C. Kraus. 1999. "Sediment Budget Analysis System (SBAS)." U.S. Army Corps of Engineers, Coastal Engineering Technical Note IV-20.

U.S. Army Corps of Engineers. 1984. Shore Protection Manual (SPM). 4th ed., 2 Vols., U.S. Government Printing Office. Washington, DC.