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Technical Posters

The Error Indexfor Beach Planform Models

G. IglesiasUniversidad de La Corua, Spain

The beach planform is a result of the general morphodynamical system, in whichmany physical processes intervene. Hence studying the planform is not simple. Inprinciple, the relevant processes should be analysed in order to establish how thesystem functions. This involves applying a suite of numerical models to study waves andtides, currents, and sediment transport.

A beach section may be eroding or accreting steadily, and the above procedurewill be the only reliable way to analyze its evolution. However, most beaches do notexperience long-term variations, although they do respond to changes in the waveclimate a state often referred to as dynamical equilibrium. In this situation and forcertain shoreline configurations, the beach planform may be predicted by a simpleformula, without detailing how the general morphodynamical system functions.

This paper deals with beaches partially sheltered by a headland or a man-madestructure. Their typical planform, referred to as bayed beach, crenulate-shapedbeach, spiral beach, half-moon bays, zeta bays, etc., is primarily the result of wavediffraction caused by the structure or the headland. The planform formulae or modelsusually consider the position of thepole of diffraction (the breakwater tip or the headlandapex); the direction of the incoming wave fronts (before they pass the pole of diffraction);and the control point, the shoreline point from where the beach is aligned to the abovedirection marking the end of the diffractive effects on the planform.

Two of the most recent models, Hsu & Evans (1989) and Tan & Chiew (1994),have been tested on twelve beaches in Northwestern Spain: Mio, Vilario, Cedeira,Carnota, Ladeira, Pontedeume, Panxn, S. Xurxo, Corrubedo, Laxe and Espasante. Theresults have been compared with the real planforms, obtained from aerial photographs.In general, the Tan & Chiew formulation leads to a better fit of the actual beach shape,and the rest of this paper will deal with it.

Obviously the quality of the adjustment between predicted and real planforms willnot always be the same. In order to assess it in a simple way, a new parameter hasbeen introduced: the error index ( ). It is defined with resort to a least-squarestechnique, and measures the deviation of the predicted planform from the real beachcurve. When the index is positive or negative, the beach area is overestimated or

underestimated, respectively. Beaches can thus be classified into two groups:Group I: > 0

Group II: < 0

Moreover, some peculiarities of the coastal morphodynamics affecting the modelperformance have been analysed. If the beach is in the lee of a breakwater that hasbeen recently extended, it is possible that the system has not attained its equilibrium


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form yet. In this case, the model will overpredict the beach area ( > 0), and the beachwill belong to group I. This is the situation, for instance, in Laxe ( = +0.350).

When a stream or a small river flows into the sea at the beach, the sediment dischargedon the system cannot be taken into account by the model. Therefore, the actualshoreline lies ahead of the predicted curve. This underestimation of the beach area

implies a negative error index ( < 0), and the beach belongs to group II. Such is thecase of Carnota ( = 0.570).

Finally, at the beaches were none of the above peculiarities are present, themodel gives a good prediction of the real planform. The error index may be eithernegative or positive (and the beaches may belong either to group I or II), but its value isnegligible in any case (| |


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Big Sur Coast Highway Management Plan

John D. DuffyandAileen K. LoeCalifornia Department of Transportation

Erosion and sedimentation along the Big Sur coastline comprises a broadspectrum of processes. Landsliding, bluff erosion, and discharge from inlandwatersheds are most prominent. This is an emergent coast with the young CoastRanges rising steadily from a constantly changing Pacific Ocean. This part of theCalifornia coast is unique in that human intervention has been relatively minimal: noneof the watersheds have been dammed and development is limited mainly to scatteredsingle-family homes and roadways. The most prominent man-made feature along thecoast is California State Route 1. Managing the roadway within this setting is thechallenge of the California Department of Transportation (Caltrans). Historically,management efforts have been largely reactive to events impacting the highway. Today,Caltrans has initiated a proactive approach known as the Big Sur Coast HighwayManagement Plan (CHMP). A primary objective of the CHMP is to work with diversestakeholders to achieve shared ownership in the management of this prime coastalaccessway.

Landsliding is one of the most noticeable erosional features along the coast andhas the greatest impact to the highway. In the past, Caltrans has implemented variousprogressive methods of dealing with landslides. Today, we are living with many of thelarge Quaternary landslides instead of attempting to stabilize these slides with grand civilengineering projects. The results are highway repairs with fewer direct environmentalimpacts, but requiring continual maintenance and associated traffic delays.

This change in engineering approach is illustrated over two recent El Nino stormperiods. After the storms of 1983, highway repair from one large landslide resulted inthe removal of 3.1 million cubic meters of earth and a one-year road closure. After thestorms of 1998, highway repairs from four large landslides resulted in the removal ofonly 700,000 cubic meters of earth and a three-month road closure. Handling anddisposing of the residual material remains a challenge.

The current management strategy for handling residual material from any sourceor phenomenon (e.g. whether by slope erosion or watershed discharge) is exporting tolandfills. The landfills vary from commercial operations to sites on privately- or publicly-owned land. These sites are located at some distance from the source and result indirect impacts to coastal upland habitats and indirect impacts from truck-haulingoperations.

Without human intervention, a portion of material generated from large erosional

events would enter the marine environment; but to what extent this would occur isunknown. Likewise, the extent to which current practices upset the sediment balance isalso unknown. As part of the CHMP, several technicalstudies have been performed orare underway by Caltrans in coordination with the USGS, CA Division of Mines &Geology, UC Santa Cruz and the CSU Monterey Bay to provide support better-informeddecisions for corridor management.


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Monitoring of the San Diego Regional Beach Sand Project: Lessons forFuture Beach Replenishment

Teri FennerandShawn ShamlouEDAW, Inc.

The RBSP will have been constructed along San Diego County, California, by theSan Diego Association of Governments (SANDAG), during spring and summer of 2001.The project involved beach nourishment on 12 receiver sites with 2 million cubic yards ofmaterial dredged from six offshore borrow sites, and was the largest of its kind along thewest coast of the US.

As part of the construction effort, SANDAG has been performing a three-phasemonitoring program. Baseline characterization was performed to establish pre-projectconditions in the nearshore environment. Monitoring occurred throughout the six-monthconstruction period to ensure no impacts to resources as directed by the permitting

agencies, and will continue for four subsequent years to verify no long term, significantimpacts to marine resources. The data gleaned from SANDAG monitoring will providemore information on a number of topics, including seasonal sand movement in thedynamic ocean system of the San Diego region, waterbird foraging patterns in turbidwaters, and grunion activity, and will help to determine the accuracy of model predictionsfor sand movement used in the environmental analysis.

The poster will display the monitoring requirements placed on the RBSP as partof the environmental review and permitting process and explain the intent behind therequirements. The baseline monitoring effort will have been completed, and the findingswill be described and illustrated on placards. Most elements of the constructionmonitoring will have been implemented at the time of the conference, and the results todate will be displayed on individual placards showing text and photos of the project.Individual placards will show monitoring results of grunion runs, underwater culturalresources, water quality, and the relationship between turbidity and kelp habitat and birdforaging. As no monitoring results are yet available for nearshore resources, the posterdisplay will present the ongoing monitoring plan for nearshore reefs, including generallocations, habitat types, and the data expected to be generated by the long-termmonitoring program.

The intent of this presentation will be to inform others working on beachreplenishment projects of the type of data this project will provide. It is hoped that thesedata will help inform the design and implementation of similar projects, particularly inCalifornia where scant monitoring has occurred and pressure is mounting to restoreeroding beaches.


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A Dynamic Policy Model for Sustainable Beach Management:The Case of the Sand Budget

David TurbowUniversity of California, Irvine

California's beaches are unique economic resources, yet beach erosion poses aformidable challenge to scientists and policy makers alike. Linking upstream land use tobeach erosion, a dynamic simulation model was constructed in order to examine thesustainability of beach preservation goals under different scenarios regarding beachreplenishment costs and available funding to coastal cities. Adjustable physicalparameters affecting sediment supply to beaches included river inputs from adjacentwatersheds, cliff sand, littoral drift, and dams. Model results indicate that with low costsof sand replenishment and moderate annual sand loss assumed, a hypothetical beach50 meters in width could be sustained for 40 years. Using the model, economictradeoffs of sand replenishment interventions can be readily examined, thus making it asimple yet integrative tool for decision-makers in coastal cities.

INTRODUCTION

In addition to physical factors beyond human control, the amount of sand on agiven beach can be deprived by man-made structures located in inland watersheds,thereby impeding beach preservation efforts. Due in part to loosely defined institutionalstructures for California's regional planning process, sand replenishment projects onbeaches often entail incompletely defined economic tradeoffs for coastal cities.Therefore, to implement a sustainable set of policies to protect beaches in the state ofCalifornia, a future emphasis will likely be placed upon making stronger policyconnections between upstream land use and beach erosion. The aim of the project was

to create a model linking key physical factors and economic factors related to sandsupply in order to establish the feasibility of beach preservation goals in coastal cities.

METHODS

The model was constructed in a system dynamics framework using STELLA5.1 Research software. The model was loaded with data and run. A forty-year timehorizon was established with a time step of 0.1 years using Euler integrationmethodology.

DATA

Sand inputs and outputs to a hypothetical beach were simulated. Sand supplywas dependent upon input from a single river of initial sand volume 200,000 m 3. Erosion

of soil into the river provided 4,000 m3 of river sand per year. The rate of sand supplyfrom the erosion of rocks along the coast, referred to in the model as "Cliff Sand"supplied 500 m3 of sand per year. The rate of river sand reaching the beach wasassumed to be dependent upon both river residence time (40 years) and resistance toflow from a dam. The default multiplying factor used to determine resistance of sandleaving the river from the dam was 0.3. Littoral drift rate was assumed to be 3,000 m3/yr.

Sand loss was computed as a function of the drift rate, and defined as the crosssectional area of sand multiplied by the rate of littoral drift in meters. The default beach


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width goal was 50 m. with an initial beach length of 1,000 m. and a constant depth of 3m. was set.

Replenishment funding each year was computed as a function of the minimum ofeither dollars needed or the replenishment funding rate. Sand cost was set at $15.50per m3. The adjustable maximum amount of funding spent by the state was set at

$200,000 per year. Replenishment funding needs were defined in the model as theamount of money required to replenish the beach to a width desired by the city, andwere assumed to be dependent on both sand price and on volume of sand needed.

RESULTS

The delivery of river sand to the beach increased from 1,500 m3 over the first fiveyears to a maximum of 2,077 m3 during the last five years of the simulation period.Required sand replenishment funding exceeded $2 million dollars per year after the firstfive years, but did not exceed $2.1 million per year during any 5 year-period throughoutthe simulation period. The required sand replenishment stayed at a near constant13,000 m3/yr. Sand losses, estimated at 150,000 m3 in the first five years were reducedto 15,466 m3 for each subsequent five-year period of the simulation period.

DISCUSSIONModel results indicate that when sand costs were set at a low value of $.10 per

cubic meter, a beach of 50 meters in width could be sustained over a forty-year period.When the price of sand was increased to $15.50 per cubic meter, however, the initialbeach width of 50 m was reduced to below 6 meters within 5 years, and did not recoverto a point above 5.2 meters for the duration of the simulation period.

With a low and relatively constant volume of sand loss assumed, beaches couldbe protected both through recovery of sediment delivery to the beach, and throughsteady funding for sand replenishment. Given a true sediment budget for sand gainedthrough river inputs and cliffs an accurate assessment of sediment delivery rates, as wellas knowledge of full losses due to littoral drift, the feasibility of sand replenishment

programs could be determined with increased precision. By tying inland land use tovolume of sand on the beach, a more holistic view of the total costs incurred throughbeach protection programs becomes evident.

CONCLUSIONS

Through the creation of a simulation model, policy scenarios can be examined todetermine whether a beach can be preserved given a limited financial budget, limitedsand supply, and multiple sources of sand loss associated with both uncontrollablephysical processes and man-made influences to watersheds. Many policy argumentscan be reduced to disagreements about assumptions regarding the characteristics ofsuch complex systems, which have been simplified here but made explicit in order todemonstrate the utility of the model. It is hoped that such dynamic models of the social

and ecological interactions between inland and coastal systems may help to identifyhigh-leverage and robust regional planning policies for decision makers.


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The Spatial and Temporal Variability of Sediment Discharge to the SantaBarbara Channel, CA

Jonathan A. WarrickWhittier College

Leal A. K. MertesUniversity of California, Santa Barbara

The coastal watersheds of the Santa Barbara Channel (SBC) drain the westernTransverse Ranges and are known to produce the highest sediment yields in southernCalifornia. Recent (1997-2000) field sampling of suspended sediment in 13 rivers andcreeks during major winter storm events (>2 cm precipitation) shows that the maximumrates of sediment discharge are often dominated by very high, suspended sedimentconcentrations (sometimes >40 g L^-1). When concentrations are greater than 40 g L^-

1, hyperpycnal plumes at the river mouths may transport the majority of the sediment.The buoyant surface plumes observed by both remote sensing and oceanographicmeasurements, although turbid (up to 200 mg L^-1) and extensive (10s of kilometersoffshore), carry

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