Experiment VII-1

Project to Understand Beach Erosion
untangling the factors which move beach sands and gravels


This section attempts to convey key aspects of engineering.  As in other parts of science, the distinction between physics and engineering is blurred.  The engineering task is to utilize what is understood about the world to design a practical solution.  As in basic research, there is often a mix of what techniques have been previously established combined with creative ways to achieve an understanding not previously achieved.

Unlike much of what you can study elsewhere in ie-Physics, here many of the choices about what to describe have been based on the personal experiences of the site author.  And in a way, this is more typical of engineering where the focus is on practical problems rather than systematically understanding useful knowledge.

mapThis engineering assignment has roots which extend nearly as far back as the written history of the region.  While a student, the author of this web site read some of the tales of Theodore Winthrop, 25 year old Yale graduate and descendent of the first governor of Connecticut.  Winthrop journeying around the Far West.  Canoe and Saddle describes 11 days of his adventures in 1853 as he utilized the regional transportation system then available in the Pacific Northwest.

The region now has a W shaped, roughly half a modern multi-laned, high speed highway system hugging the mountains and the drooping fjord which extends about 200 miles in from the Pacific Ocean.  And a railroad is being upgraded for higher speed travel up the East side of what is called Puget Sound.  But getting across this body of salt water is still primarily by slow 18 Knot auto ferries.

In 1998 Washington State introduced passenger ferries traveling twice as fast as the auto ferries, connecting the cities of Seattle and Bremerton immediately to the West on a route which winds around the southern tip of Bainbridge Island.  But home owners along the narrowest part of Rich's Passage claimed that, unlike earlier ship traffic, these new ferries eroded their beaches.  This problem caught the interest of the author of these web pages, who spent thousands of hours of childhood playing on small water-craft along the beach at the family home along Rich's Passage.  Many of the old auto ferries which routinely plied the Passage had been repeatedly upgraded, and re-engined.  As a result of their less than optimum, compromised redesigns, most of the old ferries generated wakes much larger, more choppy, and with shorter wavelengths than the new, smaller but faster passenger ferries.  So it seemed a bit odd that the new, well designed boats would create this problem not produced earlier.  And not all locations along Rich's Passage experienced the recent erosion.  Even though some locations had obvious and severe beach erosion, the unwanted movement of beach sands and gravel was unlikely to have a simple explanation.  Fortunately an engineering study was commissioned, funded by the federal government to understand why the new fast ferries were causing the beach erosion and to propose a solution.  Eventually for several reasons, use of the fast ferries was suspended, further complicating the ferry route

The expression the plural of antidote is not data has important application for science and engineering.  While individual stories and the practical experiences and observations of peoples such as the home owners and the author may serve to formulate hunches, they do not provide the firm understanding of science.

  1. Science requires that we formulate ways to make measurements,
  2. develop models which can be used to make predictions, and
  3. test those predictions against the measurements.

There are several factors which influence the movement of sand and gravel along beaches: tidal currents, wind generated waves, and a wide variety of ship traffic including the largest vessels of the U.S. Navy which visit the shipyard in Bremerton.  Beach erosion and deposition have occurred for millennium.  Now there was practical need to understand all the factors involved.

Project development

So the challenge has been to sort out the various causes which move sand and gravel along the several miles of shoreline, to develop mathematical models which match the measurements of each factor, and allow the determination of the cause of the beach erosion.  That would then be used to attempt to produce a ship hull design which would minimize the beach erosion damage.

For such a project, it is necessary to determine what information is needed.  This is requires an interplay between good logic and well educated hunches.  Antidotal and historical evidence suggests that the relatively small, prototype fast ferries produced wakes which eroded some of the beaches along Rich's Passage while wakes from much larger vessels and old ferries which produced much larger wakes seem to have done less damage.  This suggested that research is needed to determine which wake characteristics cause beach erosion.  To accomplish that, it seems essential to understand the transport of beach materials, identifying the natural factors which move materials so they can be separated from vessel caused damage.  And finally, it might be valuable to understand any effective mitigating activities which might compensate for the vessel caused erosion.  For example, it is widely believed that in primitive conditions beach erosion is balanced by the headlands providing new materials as a part of the erosion process.  It is thought that bulkheads built by land owners to prevent the loss of property block the natural supply of fresh beach materials.  But it might be possible to provide fresh materials from other locations to compensate for the vessel erosion.

A preliminary plan is needed for each information to be gathered.


As typical with contemporary science experiments, engineering projects require funding.  A detailed proposal with objectives, planned research activities, and all costs is prepared and submitted to agencies which might provide the funding.


Unlike many science research projects, engineering project typically require permits from various governmental agencies.  This can sometimes be a frustrating process since each agency has different objectives and criteria which may conflict and prevent completion of the project.  Generally every project needs to be evaluated as whether it can be safely accomplished and completed providing more benefit than harm.

project research

The Rich Passage Fast Passenger Ferry Study resulted in four separate research activities:

  1. Physical and Biological Field Studies
  2. Integrated Modeling of Wake Impacts
  3. Beach Nourishment Demonstration Project
  4. Wake Trials and Low Wake Vessel Optimization

The field studies provided baseline conditions of the beaches allowing comparison with test results.  Beach response to the existing conditions in the study area is dominated by the wake climate in the summer and the wind-wave climate in the winter.  An extensive wake climate analysis was undertaken to quantify and distinguish wakes produced by the various ships that transit Rich Passage.  The beaches in Rich Passage fluctuate in volume from season to season.  Quarterly beach profiles, ground-based photographs, and observations indicated that many of the beaches have recovered from erosion induced by earlier operation of fast passenger ferries.  The beaches exhibited a minor or insignificant responses to the trial wakes created by a similar fast vessel loaded and operated to create comparable wakes.

Modeling involved accurate tidal flows and water levels provided by a finite element model of the region.  Wind wave predictions for the area were obtained using three wind-wave models of the region, oriented to match the open wind fetches.  Predictions of wake trains were obtained using the LSV model.  The results of these detailed models were used to build a new integrated impact assessment model which can be used to simulate many months at a time and compute processes such as sediment transport and beach profile changes.  The analysis showed significant spatial variability in the results.  Because the wind climate is not extreme and the fetch distances are short, wind-wave conditions are fairly mild over most of the Rich Passage.  However along some sections of beach wind waves dominate because the wake conditions are weak.  Analyses showed that vessel type is an important factor in beach impact.  It was found that optimizing the path of a vessel through the passage could minimize beach effects.  Operating the high speed ferries at higher speeds would also minimize overall impacts!  (These vessels travelling lower in the water at subcritical speeds generated larger wakes.)  The integrated model's prediction of sediment transport was found to match the actual monitored movement of radio signal marked tracer gravel carefully placed along 25 beach sections.  Wind wave-driven transport was found to be greatest in the winter months and at profiles that are at the end of the longest wind fetches in the area.  Wave transport of sediments by the roughly hourly scheduled passage of auto ferries was found to be of the same order of magnitude as the winds along much of the Passage.  Tests with a fast ferry generally generated less wave transport of sediments than the slower but much larger auto ferries generated.

Beach nourishment appears to have been postponed by permit issues.

Computer simulations suggested that wakes of fast ferries could be substantially reduced by design.  As a result development, construction and testing of a research vessel was authorized.


At this point it might be useful to create or select a small engineering project and see it through to completion.  As in previous lab experiments, the purpose is to provide experience to help understand the concepts presented herein and to develop useful skills.


Use the information and hints above to propose a small engineering project.  This might be similar to Boy Scout Eagle projects, or school projects required for graduation.  In any case, provide for some sort of permit project where impartial, outside expertise evaluates the safety and benefit of the project before doing the project.



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created 25 May 2007
revised 26 September 2011
by D Trapp
Mac made