EM 1110-2-1100 (Part V)
31 Jul 2003
EXAMPLE PROBLEM V-3-1
FIND: Functional design of a nearshore breakwater system coupled with beach nourishment for shore
protection, but maintain recreational beach.
GIVEN:
A sandy beach shoreline, 685 m in length with historic erosion rate of 0.6-0.9 m/year that
threatens a 2.4-7.3-m bank, with road and sewer line parallel to the shoreline along the top
of the bank. For an annual probability of exceedance of 2 percent (50-year recurrence
interval design storm) the storm surge level is 1.83 m (mlw) and corresponding wave
characteristics are Hs=2.2 m, Tp=9.7 sec at the -9.0 m (mwl) depth contour. The design, still
water, storm depth is 3.05 m at this location and includes a 0.3 m, astronomical tide. Net
sediment transport (sand, d50 = 0.6 mm) is 3,750 to 7,500 m3/year to the north. The existing
beach berm is at +0.76 m (mlw) elevation.
Solution:
1. To maintain the longshore transport rate, the desired planform is subdued to well-developed
salients. From a table of possible Ls and Y values, the ratio 0.75 is selected giving a breakwater
length, Ls=30m and the offshore distance, Y=40 m. This gives the beach response index, Is=4.1,
hence, subdued salients are expected.
2. For shore protection, waves entering the breakwater gaps and diffracting behind the structures will
reach the shoreline. Sufficient beach width (Ymin) and berm height (Zs) are required to dissipate
this wave energy prior to reaching the toe of the banks. Analysis of nearshore, wave diffraction
diagrams indicate that the 50-year design wave height of 2.2 m will be reduced to about 0.9 m at
a distance of about 14 m from the bank toe when the breakwater gap is about 30 m. This gap
width, Lg=30 m is considered a practical minimum width for this project. For shallow-water wave
breaking, (γb=0.78, II-4-3), this wave will break in about 1.16 m depth of water. For a design
storm surge of 1.83 m (mlw) and existing beach berm elevation of 0.76 m (mlw), these waves will
break directly on the bank toe and cause significant erosion. The existing beach width and height
are not sufficient to dissipate the storm wave energy at the 50-year frequency level. Two options
exist. One is to further decrease the wave energy propagating through the gaps by using smaller
gap widths, and resulting longer lengths of breakwater segments. The second option is to add
beach fill to the shoreline area and this option is selected to provide the desired protection for the
bank area.
3. A beach-fill plan was considered that would increase the beach width to 10 m from the toe of the
bank and raise the berm elevation to + 1.8 m (mlw) at a 1:8 construction slope. Wave heights are
now reduced to less than 30 cm near the bank toe for the 50-year storm event. The beach fill
would be expected to evolve (see Part V-4) to a stable planform with salients behind each
breakwater and embayments opposite each gap. The mhw shoreline would recede about 4.5-6 m
opposite the gaps as estimated from analysis of diffraction patterns. The beach shape would also
evolve to a more natural and milder slope to match those in the area (1:10 to 1:15).
(Continued)
V-3-58
Shore Protection Projects
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