A number of different numerical codes were required to build a complete, integrated model of the environment.  Accurate tidal flows and water levels were 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 dominant wind directions.  Predictions of the temporal and spatial wake train patterns produced by POFF and non-POFF vessels were obtained using the LSV model, which was developed in Phase 1 of the study.  The results of these detailed models were used to build a new integrated impact assessment model, ProfileAnalysis, which was the primary tool for investigating the impacts of tides, waves and wakes on the shores of the study area.  Unlike the detailed models, ProfileAnalysis can be used to simulate many months at a time, which is required in order to assess the processes that affect beach evolution.  As well as hydrodynamics calculations (flows, waves and wakes), the model computes processes such as sediment transport and profile evolution.  

Wake height predicted by LSV for Bremerton to Seattle route at 37 knots: Snohomish (top); Spirit (bottom)

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 – Sinclair Inlet region.  In certain areas however, such as the western side of Point White and along the shores of Port Orchard, wind waves dominate because wake conditions are weak. 

Wake climate analyses showed that vessel type is an important factor in beach impact.  Comparisons of time series of wake conditions in the nearshore indicated that, although the wake heights produced by the Chinook-class and Spirit POFFs were similar, the wake produced by the Chinook-class vessels were twice as energetic due to the longer periods.  Breaking waves from Snohomish-class vessels were concentrated in the 4 to 4.5 s range, with heights up to 0.45 m, whereas most of the breaking wave activity for Spirit is concentrated in the 3 to 3.5 s periods, with heights less than 0.35 m.  Sensitivity tests showed that more wake energy reaches the Point White shore than the Point Glover shore and that, as expected, more wake energy reaches the shore if the vessel is closer to that shore.  Overall, wake energy reaching shore does not seem to be sensitive to vessel direction and a trend was found in the Spirit results in which wake impacts reduced with increased vessel speed.  These findings suggests that moving the sailing line through Rich Passage closer to the Point Glover shore and, for vessels similar in behavior to Spirit, operating the POFF at higher speeds would minimize overall impacts and provide a more balanced distribution of impact between the two shores.

Frequency of wave breaking by height and period for a Bremerton-bound Snohomish

Frequency of wave breaking by height and period for a Bremerton-bound Spirit

Frequency of wave breaking by height and period for a Bremerton-bound WSF car ferry

The sediment transport estimates of the ProfileAnalysis model were found to mimic the movement of the centroids of the gravel tracer particles at Point White.  Simulations were performed at 25 shore-normal profiles distributed across the study area for typical summer and winter periods.  Each test was performed using a climate comprised of various combinations of wind-waves, vessel wakes and tidal flows. Wind wave-driven transport was found to be greatest in the winter months and at profiles that are at the end of the longest fetches in the area.  WSF-driven transport was found to be of the same order of magnitude as the winds in Point White - Pleasant Beach - Point Glover area, but much less in the Port Orchard area.  Simulations with POFF vessels showed that, overall, Spirit has a smaller contribution to the net alongshore sediment transport in the study area than a Snohomish-class vessel.  Because POFF wakes tend to reach shore in a fairly shore-normal orientation, the greatest potential impact of POFF wakes on beach evolution is in cross-shore direction.

Alongshore movements of centroid of Tracer Group 2 (red circles), and net sediment transport rate predicted by the model at location PW_A of Tracer Group 2 (blue line)

Alongshore movements of centroid of Tracer Group 3 (red circles), and net sediment transport rate predicted by the model at location PW_B of Tracer Group 3 (blue line)

Comparison of the overall summer and winter sediment transport factors for five scenarios (Snohomish climate=100)

An assessment of morphological evolution and an impact assessment attempts to quantify the hydrodynamic forcing that drives beach response.  Four indicators were investigated: wake intensity, potential sediment transport, wake climate, and toe erosion at bulkheads.  The results, grouped together to yield an overall assessment, were found to be fairly insensitive to the weighting of the individual impact factors.  With an impact value of 100 allotted to a system using Snohomish-class vessels operating at the same frequency as the former WSF POFF schedule, the impact value of the present, non-POFF environment was computed as approximately 55 and the impact value of a POFF operation of Spirit as approximately 70.  If the operating schedule is reduced to one-half the number of runs that Snohomish had during its operations in the year 2000, the impact for Spirit reduces to approximately 60.  If the operating schedule is increased to double the former WSF POFF schedule, this increases to approximately 90.

Report 2 also includes a self-contained section, Assessment of Wake Impacts on Structures, which provides baseline information on bulkhead conditions and an examination of those bulkheads that might be at risk to overtopping at high water levels.  Although overtopping does not necessarily lead to property damage, since houses and other structures (excluding boat sheds) are generally well set back and in many locations are significantly elevated above the crest of the bulkhead, it can cause damage to gardens and vegetation.  An analytical method was utilized to predict the probability of overtopping of structures under POFF and wind waves.  Almost all of the bulkheads along the project area are of sufficient height to withstand wave attack from POFF vessels during normally occurring water levels.  At higher water levels, some structures in Rich Passage were found to exhibit vulnerability to POFF wakes, whereas structures near Bremerton were shown to have only slight exposure.  In the case of wind waves, the risk of overtopping is dependant upon the direction of exposure of the bulkhead.  Overall, wind waves are predicted to generate more overtopping than is expected from Spirit-class POFF wakes for a given water level.  Thus, the analysis indicates there is a greater risk of damage to property from wind waves during storm events combined with elevated water levels, in which repeated and sustained overtopping conditions may occur for several hours, than from POFF wakes from Spirit-class vessels.

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