Technical Posters

The Error Index for Beach Planform Models G. Iglesias

The beach planform is a result of the general morphodynamical system, in which many physical processes intervene. Hence studying the planform is not simple. In principle, the relevant processes should be analysed in order to establish how the system functions. This involves applying a suite of numerical models to study waves and tides, currents, and sediment transport. A beach section may be eroding or accreting steadily, and the above procedure will be the only reliable way to analyze its evolution. However, most beaches do not experience long-term variations, although they do respond to changes in the wave climate a state often referred to as dynamical equilibrium. In this situation and for certain shoreline configurations, the beach planform may be predicted by a simple formula, without detailing how the general morphodynamical system functions. This paper deals with beaches partially sheltered by a headland or a man-made structure. Their typical planform, referred to as bayed beach, crenulate-shaped beach, spiral beach, half-moon bays, zeta bays, etc., is primarily the result of wave diffraction caused by the structure or the headland. The planform formulae or models usually consider the position of the pole of diffraction (the breakwater tip or the headland apex); 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 above direction 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. The results 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 will not always be the same. In order to assess it in a simple way, a new parameter has been introduced: the error index ( ). It is defined with resort to a least-squares technique, and measures the deviation of the predicted planform from the real beach curve. 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 model performance have been analysed. If the beach is in the lee of a breakwater that has been recently extended, it is possible that the system has not attained its equilibrium

form yet. In this case, the model will overpredict the beach area ( > 0), and the beach will 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 discharged on the system cannot be taken into account by the model. Therefore, the actual shoreline 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 the case of Carnota ( = 0.570). Finally, at the beaches were none of the above peculiarities are present, the model gives a good prediction of the real planform. The error index may be either negative or positive (and the beaches may belong either to group I or II), but its value is negligible in any case (| |<<1). A good example is the Panxn beach ( = 0.050). The relation between the error index and the beach morphodynamics has thus been shown. REFERENCES Hsu, J. R., and Evans, C. (1989). Parabolic bay shapes and applications. Proc. Institutions of Civil Engineers, London, England, Vol. 87 (part 2), 557570. Tan, S.K., and Chiew, Y.M. (1994). Analysis of bayed beaches in static equilibrium. J. of Wtrwy., Port, Coast. and Oc. Eng., Vol. 120, No 2, 145153.

Big Sur Coast Highway Management Plan John D. Duffy and Aileen K. Loe
California Department of Transportation

Erosion and sedimentation along the Big Sur coastline comprises a broad spectrum of processes. Landsliding, bluff erosion, and discharge from inland watersheds are most prominent. This is an emergent coast with the young Coast Ranges rising steadily from a constantly changing Pacific Ocean. This part of the California coast is unique in that human intervention has been relatively minimal: none of the watersheds have been dammed and development is limited mainly to scattered single-family homes and roadways. The most prominent man-made feature along the coast is California State Route 1. Managing the roadway within this setting is the challenge 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 Highway Management Plan (CHMP). A primary objective of the CHMP is to work with diverse stakeholders to achieve shared ownership in the management of this prime coastal accessway. Landsliding is one of the most noticeable erosional features along the coast and has the greatest impact to the highway. In the past, Caltrans has implemented various progressive methods of dealing with landslides. Today, we are living with many of the large Quaternary landslides instead of attempting to stabilize these slides with grand civil engineering projects. The results are highway repairs with fewer direct environmental impacts, but requiring continual maintenance and associated traffic delays. This change in engineering approach is illustrated over two recent El Nino storm periods. After the storms of 1983, highway repair from one large landslide resulted in the removal of 3.1 million cubic meters of earth and a one-year road closure. After the storms of 1998, highway repairs from four large landslides resulted in the removal of only 700,000 cubic meters of earth and a three-month road closure. Handling and disposing of the residual material remains a challenge. The current management strategy for handling residual material from any source or phenomenon (e.g. whether by slope erosion or watershed discharge) is exporting to landfills. The landfills vary from commercial operations to sites on privately- or publiclyowned land. These sites are located at some distance from the source and result in direct impacts to coastal upland habitats and indirect impacts from truck-hauling operations. Without human intervention, a portion of material generated from large erosional events would enter the marine environment; but to what extent this would occur is unknown. Likewise, the extent to which current practices upset the sediment balance is also unknown. As part of the CHMP, several technical studies have been performed or are 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-informed decisions for corridor management.

Monitoring of the San Diego Regional Beach Sand Project: Lessons for Future Beach Replenishment Teri Fenner and Shawn Shamlou
EDAW, Inc.

The RBSP will have been constructed along San Diego County, California, by the San 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 of material dredged from six offshore borrow sites, and was the largest of its kind along the west coast of the US. As part of the construction effort, SANDAG has been performing a three-phase monitoring program. Baseline characterization was performed to establish pre-project conditions in the nearshore environment. Monitoring occurred throughout the six-month construction 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, significant impacts to marine resources. The data gleaned from SANDAG monitoring will provide more information on a number of topics, including seasonal sand movement in the dynamic ocean system of the San Diego region, waterbird foraging patterns in turbid waters, and grunion activity, and will help to determine the accuracy of model predictions for sand movement used in the environmental analysis. The poster will display the monitoring requirements placed on the RBSP as part of the environmental review and permitting process and explain the intent behind the requirements. The baseline monitoring effort will have been completed, and the findings will be described and illustrated on placards. Most elements of the construction monitoring will have been implemented at the time of the conference, and the results to date will be displayed on individual placards showing text and photos of the project. Individual placards will show monitoring results of grunion runs, underwater cultural resources, water quality, and the relationship between turbidity and kelp habitat and bird foraging. As no monitoring results are yet available for nearshore resources, the poster display will present the ongoing monitoring plan for nearshore reefs, including general locations, habitat types, and the data expected to be generated by the long-term monitoring program. The intent of this presentation will be to inform others working on beach replenishment projects of the type of data this project will provide. It is hoped that these data will help inform the design and implementation of similar projects, particularly in California where scant monitoring has occurred and pressure is mounting to restore eroding beaches.

A Dynamic Policy Model for Sustainable Beach Management: The Case of the Sand Budget David Turbow
University of California, Irvine

California's beaches are unique economic resources, yet beach erosion poses a formidable challenge to scientists and policy makers alike. Linking upstream land use to beach erosion, a dynamic simulation model was constructed in order to examine the sustainability of beach preservation goals under different scenarios regarding beach replenishment costs and available funding to coastal cities. Adjustable physical parameters affecting sediment supply to beaches included river inputs from adjacent watersheds, cliff sand, littoral drift, and dams. Model results indicate that with low costs of sand replenishment and moderate annual sand loss assumed, a hypothetical beach 50 meters in width could be sustained for 40 years. Using the model, economic tradeoffs of sand replenishment interventions can be readily examined, thus making it a simple yet integrative tool for decision-makers in coastal cities. INTRODUCTION In addition to physical factors beyond human control, the amount of sand on a given beach can be deprived by man-made structures located in inland watersheds, thereby impeding beach preservation efforts. Due in part to loosely defined institutional structures for California's regional planning process, sand replenishment projects on beaches often entail incompletely defined economic tradeoffs for coastal cities. Therefore, to implement a sustainable set of policies to protect beaches in the state of California, a future emphasis will likely be placed upon making stronger policy connections 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 sand supply in order to establish the feasibility of beach preservation goals in coastal cities. METHODS The model was constructed in a system dynamics framework using STELLA 5.1 Research software. The model was loaded with data and run. A forty-year time horizon was established with a time step of 0.1 years using Euler integration methodology. DATA Sand inputs and outputs to a hypothetical beach were simulated. Sand supply was 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 supply from 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 was assumed to be dependent upon both river residence time (40 years) and resistance to flow from a dam. The default multiplying factor used to determine resistance of sand leaving 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 cross sectional area of sand multiplied by the rate of littoral drift in meters. The default beach

width goal was 50 m. with an initial beach length of 1,000 m. and a constant depth of 3 m. was set. Replenishment funding each year was computed as a function of the minimum of either dollars needed or the replenishment funding rate. Sand cost was set at $15.50 per 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 the amount of money required to replenish the beach to a width desired by the city, and were 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 five years 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 first five years, but did not exceed $2.1 million per year during any 5 year-period throughout the simulation period. The required sand replenishment stayed at a near constant 13,000 m3/yr. Sand losses, estimated at 150,000 m3 in the first five years were reduced to 15,466 m3 for each subsequent five-year period of the simulation period. DISCUSSION Model 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 initial beach width of 50 m was reduced to below 6 meters within 5 years, and did not recover to 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 could be protected both through recovery of sediment delivery to the beach, and through steady funding for sand replenishment. Given a true sediment budget for sand gained through river inputs and cliffs an accurate assessment of sediment delivery rates, as well as 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 to volume of sand on the beach, a more holistic view of the total costs incurred through beach protection programs becomes evident. CONCLUSIONS Through the creation of a simulation model, policy scenarios can be examined to determine whether a beach can be preserved given a limited financial budget, limited sand supply, and multiple sources of sand loss associated with both uncontrollable physical processes and man-made influences to watersheds. Many policy arguments can be reduced to disagreements about assumptions regarding the characteristics of such complex systems, which have been simplified here but made explicit in order to demonstrate 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 identify high-leverage and robust regional planning policies for decision makers.

The Spatial and Temporal Variability of Sediment Discharge to the Santa Barbara Channel, CA Jonathan A. Warrick
Whittier College

Leal A. K. Mertes
University of California, Santa Barbara

The coastal watersheds of the Santa Barbara Channel (SBC) drain the western Transverse Ranges and are known to produce the highest sediment yields in southern California. Recent (1997-2000) field sampling of suspended sediment in 13 rivers and creeks during major winter storm events (>2 cm precipitation) shows that the maximum rates of sediment discharge are often dominated by very high, suspended sediment concentrations (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 oceanographic measurements, although turbid (up to 200 mg L^-1) and extensive (10s of kilometers offshore), carry <2% of the total sediment load of the rivers. Based on order-ofmagnitude rating curves derived from the field measurements and USGS records, an average annual sediment budget for the Santa Barbara Channel was computed for the available record of water discharge (1927-2000). Our results confirm that the Santa Clara River is the dominant sediment source (51%) for the SBC, yet it covers 70% of the total watershed area. Using regionalization calculations to compensate for lack of historical data on water discharge, the smaller coastal watersheds are shown to produce sediment at relatively higher rates. Analysis of digital elevation maps and ancillary data on land use and geology shows that, the highest sediment yields are correlated with the presence of young (Plio-Pleistocene) siltstones. Additionally, a disturbed land use category that includes agriculture, barren, and non-native grassland shows an additional significant correlation to high sediment yields.