Buhne Point, Humboldt Bay, California: Design for the Prevention of Shoreline Erosion: Hydraulic and Numerical Model Investigations

Source: https://erdc-library.erdc.dren.mil/jspui/ Two numerical models and two physical models were used to investigate the effects of proposed improvement plans with respect to shoreline erosion at Buhne Point, Humboldt Bay, California. Initially, a numerical tidal circulation model was used to determine the tidal current field adjacent to Buhne Point. Maximum flood and ebb tidal currents were identified and used as test conditions for the physical models. A 1:100-scale physical model of central Humboldt Bay included the jettied entrance to the bay, approximately 18,000 lin. ft of shoreline inside the bay (including Buhne Point), and underwater contours throughout the central portion of the bay and the area between the jetties . This model was used to determine the wave climate (angle of wave fronts and wave heights along these fronts) in the vicinity of Buhne Point for a series of incident wave conditions and directions (waves propagated through the Humboldt Bay entrance) and for various water levels and tidal flow conditions. A 30-ft-long wave generator, an Automated Data Acquisition and Control System (ADACS), and a model circulation system were utilized in model operation. The output conditions obtained from the 1:100-scale physical model were input lnto a 1:50-scale physical model of Buhne Point where the effectiveness of various structures proposed for shore protection was evaluated. This model reproduced approximately 9,200 lin. ft of shoreline in the Buhne Point area and the immediate underwater contours in Humboldt Bay and utilized an 85-ft-long curved wave generator, a model circulation system, and crushed coal tracer material in model operation. Through the use of a numerical sediment transport model of Humboldt Bay, the effects of the optimum improvement plan developed from the 1:50-scale physical model on conditions (sediment movement, tidal flushing, etc.) in other areas in the bay (areas not included in the physical models) were determined. The numerical tidal circulation model provided the tidal current field adjacent to Buhne Point for existing (1983) conditions and for the optimum improvement plan (Plan 30) . Based on the results of this model investigation, it was concluded that changes in tidal current velocities and flow patterns will be minimal due to the proposed improvements. The 1:100-scale physical model of central Humboldt Bay provided the wave front and wave heights along the front in the vicinity of Buhne Point for test waves for five water levels and from three directions. Based on the results of this model investigation, it was concluded that: (A.) Regardless of the direction of incident wave approach from the Pacific Ocean, the angle of the wave front in the vicinity of Buhne Point remains essentially the same. (B.) Test waves from northwest (approaching through the Humboldt Bay jettied entrance almost directly up the axis of the channel) result in significantly larger wave heights in the vicinity of Buhne Point, as opposed to test waves from north and/or west. The 1:50- scale physical model of Buhne Point was used to determine the cliuses of erosion at the point and the effectiveness of various structures proposed for shore protection. Based on the results of thls model investigation it was concluded that : (A.) Wave energy approaching Buhne Point from the jettied entrance to Humboldt Bay resulted tn erosion of the original spit. Sediment eroded from the eastern portion of the shoal and migrated westerly where it entered the navigation channel. (B.) For the originally proposed improvement plan (Plan 1), erosion occurred at the eastern portion of the fill with accretion against the proposed groin. Eventually, material migrated around the groin head and toward the nlivigation channel. (C ). For the proposed groin plan (Plan 2), the shoreline did not remain stable. Sediment eroded at the eastern portion of the fill and accreted against the originally proposed westernmost groin. Material eventually migrated around the groin head and toward the navigation channel. (D.) Sediment eroded in the lee of the shore-connected breakwater with the +7 ft elevatlon (Plan 3) for normal high-tide conditions (water el +6.7 ft). (E.) Sediment remained stable in the lee of the shore-connected breakwater with the +10 ft elevatlon (Plan 3A) for normal tide conditions (water el +6.7 ft), but erosion occurred for extreme hightide conditions (water el +9.5 ft). (F.) Sediment remained stable in the lee of the shore-connected breakwater with the +13 ft elevation (Plans 3C and 3D) for all tide conditions (inclurling the extreme +9.5 ft conditions). (G.) A reverse curve in the shore-connected breakwater where it originates from the existing Buhne Drive revetment (Plan 3D) minimized wave convergence and runup in this area. (H.) A 25-ft-wide fill (el +12 ft) in the lee of the shore-connected breakwater lind adjacent to the existing Buhne Drive revetment (Plan 30) prevented transmitted wave energy from running up on Buhne Drive for all test conditions. (I.) Erosion occurred in the lee of the offshore breakwater plans with the +13 ft elevation (Plans 4 and 4A) for extreme high-water conditions (water el +9.5 ft). When the fill was depleted, wave energy transmitted through the revetment adjacent to Buhne Drive and onto the roadway. (J.) A 1,000-ft-long offshore breakwater with the eastern 425-ft portion installed at a crest elevation of +16 ft (Plan 4B) was required to stabilize the fill in the lee of the structure. Slight erosion of the fill at its eastern limit occurred prior to stabilization, but wave runup onto Buhne Drive did not occur. (K.) A 1,200-ft-long offshore breakwater with the eastern 625-ft portion installed at a crest elevation of +16 ft (Plan 4F) resulted in a stable shoreline in the lee of the structure with no erosion. (L.) Sediment stabilized and remained in the area between the structures with the 425- ft-long extension of the original groin for all test waves, tidal currents, and water levels. (M.) Small amounts of sediment penetrated through the voids of the rubble groin head at the downcoast (western) end of the fill for extreme high-tide conditions (water level +9.5 ft). (N.) Of the improvement plans tested, Plan 3D was regarded as the optimum, considering shore protection and construction costs. The shoreline remained stable for all test waves, tidal flow conditions and water levels. The numerical sediment transport model (CELC3D) provided estimates of sediment movements due to residual tidal currents and wave interactions, for both the existing (1983) conditions and for improvement Plan 3D. From this investigation, it was concluded that no new sediment transport patterns are induced by the optimum improvement plan (Plan 3D). NOTE: This file is large. Allow your browser several minutes to download the file.