Question 5.4: Using the annualised wave climate data given in Table 5.2, e......
Using the annualised wave climate data given in Table 5.2, estimate the net longshore transport rate for a natural beach site with a beach slope of 1 in 100 and a D_{50} grain size of 0.4 mm.
| Table 5.2 Annualised wave climate | |||
| H_{sb} (m) | T_{p} (s) | θ_{b} (deg.) | Frequency (%) |
| 0.8 | 4.5 | 25 | 5 |
| 1.2 | 5.5 | 15 | 10 |
| 1.5 | 6 | 5 | 15 |
| 1.3 | 6 | -5 | 12 |
| 1.1 | 5.5 | -15 | 8 |
| 0.5 | 4 | -25 | 5 |
| Note: +ve and –ve wave angles refer to opposite sides of the beach normal. | |||
Apply Equation 5.60
Q_{LS}=6.4\times 10^{4} H_{s b}^2 T_p^{1.5}(\tan \beta)^{0.75} D_{50}{ }^{-0.25}\left(\sin 2 \theta_b\right)^{0.6} . (5.60)
to each wave component, multiplying by the frequency of occurrence, and then sum to find the net annual longshore transport rate. The results are tabulated in Table 5.3 and illustrated in Figure 5.11
It can be seen from Table 5.3 that waves only occur for 55% of the year, representing the percent of time for onshore winds, and from Figure 5.11 that the net longshore transport rate is much less than the gross rates up and down coast.
| Table 5.3 Longshore transport results | |||||
| H_{sb} (m) | T_{p} (s) | θ_{b} (deg.) | Frequency (%) | Q_{ls} (m^{3}/annum) | ΣQ_{ls} (m³/annum) |
| 0.8 | 4.5 | 25 | 5 | 3725.5 | 3725.5 |
| 1.2 | 5.5 | 15 | 10 | 17536.9 | 21262.4 |
| 1.5 | 6 | 5 | 15 | 24829.5 | 46091.9 |
| 1.3 | 6 | -5 | 12 | −14919.8 | 31172.2 |
| 1.1 | 5.5 | -15 | 8 | −11788.7 | 19383.4 |
| 0.5 | 4 | -25 | 5 | −1219.6 | 18163.8 |
| Σ = 55 | Σ = 18163.8 | ||||
