Deep-Draft Entrance Channels: Preliminary Comparisons Between Field and Laboratory Measurements

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PURPOSE: This Coastal and Hydraulics Engineering Technical Note (CHETN) summarizes

preliminary comparisons between field and laboratory measurements of wave-induced vertical

motions at Barbers Point Harbor, HI (Figure 1). The importance of these wave motions to ship

underkeel clearance allowances and channel depth requirements is described.

Figure 1. Aerial view of Barbers Point Harbor, HI

NEEDS AND BENEFITS: The next generation of ships will require deeper and wider

entrance channels to provide safe navigation. Channel width and depth depend on vessel size,

traffic flow, and environmental conditions such as tides, water levels, winds, waves, and

currents. Channel depth is determined by ship draft and trim (T), and gross underkeel clearance

allowances (hUKC). The hUKC is composed of several factors including (a) tides (hTI), (b) wave-

induced motions (hWA), (c) squat (hSQ), and (d) a safety factor for seabed type (hSF). Additional

clearances can also be included for dredging tolerance and advance maintenance (hAM). Squat is

a drawdown of the ship due to decreased pressure beneath the hull as it moves ahead at nearly

constant speed in shallow and confined seaways. Wider and deeper channels require project and

maintenance dredging, with costs of the order of 0,000 per vertical foot per acre, depending

on the type of seabed. The U.S. Army Corps of Engineers spends over 0 million per year