Final Technical Summary – Guza and O’Reilly 

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FINAL TECHNICAL SUMMARY 

STUDY TITLE: Wave Prediction in the Santa Barbara Channel

REPORT TITLE: Wave Prediction in the Santa Barbara Channel

CONTRACT NUMBER: 14-35-0001-30758

SPONSORING OCS REGION: Pacific

APPLICABLE PLANNING AREA: Southern California

FISCAL YEAR(S) OF PROJECT FUNDING: FY97, FY98

COMPLETION DATE OF THE REPORT: January 2001

COST(S): FY 97 - $38,975, FY 98 - $49,166, FY 1999, 2000 – no cost

CUMULATIVE PROJECT COST: $88,141

PRINCIPAL INVESTIGATORS: Robert Guza, William O’Reilly

AFFILIATION: University of California, San Diego

ADDRESS: Center for Coastal Studies 0209, UCSD, La Jolla, CA 92093-0209 

BACKGROUND: 

Knowledge of past, present and future wave conditions in the Santa Barbara Channel 

is needed for numerous government, commercial and recreational activities in this heavily 

utilized coastal zone. Accurate historical wave estimates are important for coastal and ocean 

engineering design, planning, and environmental impact assessment. Real time wave 

"nowcasts" and forecasts are needed for efficient and safe marine operations, disaster 

preparedness, and beach safety. 

Wave prediction in the Santa Barbara Channel is extremely complicated owing to the 

coastline orientation, the complex bathymetry, and the spatially variable wind field. The 

Channel Islands block remotely generated waves approaching the Channel from the south, and 

Point Conception blocks waves arriving from the northwest. Deep ocean waves that do 

propagate into the Channel are severely refracted and diffracted by the irregular shallow water 

bathymetry, producing the large spatial variations in swell wave height and direction typically 

observed throughout the region. Wave measurements in the Channel have shown that wave 

refraction-diffraction models underpredict wave heights at the east end of the Channel during 

WNW swell events, and that the source of this underprediction is the (unmodeled) reflection 

of wave energy from the rugged northern (i.e. facing the Channel) coastlines of the Channel 

Islands. 

OBJECTIVES: 

The primary objective is to investigate the reflection of ocean surface waves from the 

Channel Islands, and to use the results to improve existing swell predictions throughout the 
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Channel. A secondary objective is to lay the groundwork for future inclusion of locally 

generated wind seas, and their reflection from the Islands, in Santa Barbara Channel wave 

forecasts. 

DESCRIPTION: 

Waves propagating into the Santa Barbara Channel from the W-WNW impinge on the 

northern coasts of San Miguel, Santa Rosa, Santa Cruz, and Anacapa Islands, and a significant 

amount of the incident wave energy reflects from these rugged coasts back into Channel. This 

adds a significant southerly component to the waves along the Santa Barbara County 

coastline, in the lee of Pt. Conception, during these wave events. 

A detailed analysis was performed on three sets of directional buoy measurements 

made in 50m water depth, ~2km offshore of the north coasts of Santa Rosa and Santa Cruz 

Island, to quantify the frequency and directional characteristics of the reflected wave field as a 

function of the incident wave conditions. In addition, directional wave measurements in the 

center of the Channel and along the Santa Barbara County coastline were analyzed to quantify 

the reflected waves in the far-field of the Islands. 

Results of this analysis were used to model the Islands as a diffuse source of reflected 

wave energy with the amount of energy reflected from the islands being determined by the 

height, direction, and frequency of the incident waves reaching each island. 

SIGNIFICANT CONCLUSIONS: 

The amount of wave energy reflected from all the Channel Islands is frequency 

dependent, with the lowest frequencies (longer wave periods) having larger reflection 

coefficients (The reflection coefficient is defined as the ratio of wave energy reflected by the 

island, to the wave energy incident to the island). Wave reflection coefficients for shorelines 

with rocky shallows (e.g. Santa Rosa Island) are also strongly energy dependent, with much 

smaller wave reflection coefficients during larger wave events when more wave breaking and 

dissipation occurs at the coast. In contrast, wave reflection coefficients from cliff-lined coast 

(e.g. Santa Cruz and Anacapa Islands) only decrease slightly with larger incident waves. In 

both high and low wave energy conditions, reflection from the Channel Islands can contribute 

significantly to the wave energy at the east end of the Channel and along the Santa Barbara 

County coastline 

STUDY RESULTS: 

In general, the relative amount of reflected wave energy depends greatly on the wave 

frequency, with observed reflection coefficients at low frequencies ( f < 0.07Hz) two or more 

times larger than at higher frequencies. The highest reflection coefficients (0.10-0.70, 

depending on the wave frequency) were observed at the western end of the north side of Santa 

Cruz Island, where long, sheer coastal cliffs extend directly into 10-20m water depth. 

Slightly lower reflection coefficients were observed at the north-east end of Santa Cruz Island, 
Final Technical Summary – Guza and O’Reilly 

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where the cliffs are more irregular, and off Santa Rosa Island, where the coast is better 

characterized as rocky shallows than as cliffs. 

While the overall amount of observed reflection was similar for these three different 

types of rugged coasts, the estimated reflection coefficients at Santa Rosa were strongly 

energy dependent. Low energy waves were much more “reflective” (coefficients = 0.20-0.50) 

than higher energy waves (coefficients = 0.05-0.20), presumably owing to a more active 

dissipation region in the rocky shallows with higher waves. A similar, but somewhat weaker 

dependence was observed at the East Santa Cruz Island site, and only a slight energy 

dependence is observed at the West Santa Cruz site with its wall-like coastal cliffs. 

These observations suggest the following wave reflection scenarios: When low energy 

swell propagates into the channel from the WNW, all the islands reflect a significant amount 

of this swell energy back into the Channel. The lower frequency, longer period swell (T > 14 

s.) experience the highest amount of reflection (30%+). However, when larger swell is 

present in the Channel, active dissipation zones develop along most of the island coastlines, 

reducing the relative size of their reflected wave fields (i.e. low reflection coefficient) 

compared to low wave height events. The western half of the north coast of Santa Cruz, and 

perhaps the north side of Anacapa Island, continue to be highly reflective under energetic 

conditions. 

STUDY PRODUCTS: 

Study products include a web page on the Coastal Data Information Program web 

server that is dedicated to providing real-time wave and wind information for the Santa 

Barbara Channel: http://cdip.ucsd.edu/mms. This wave and wind prediction effort continues 

to receive support from the State of California and the products on the Channel web page will 

continue to expand over the next year, including the implementation of a wind-wave 

prediction model for local seas. Additional products include a paper in the proceedings of the 

5th California Islands Symposium and a paper in preparation describing the wave 

measurements and island wave reflection model.