EM 1110-2-1100 (Change 1)
31 Jul 03
BOTTOM BOUNDARY LAYER
The lower portion of the water flow that experiences frictional retardation based on its proximity to the bed.
BOTTOMSET
One of the horizontal or gently inclined sediment layers deposited in front of the advancing forest beds of a delta.
BOULDER
A rounded rock more than 256 mm (10 inch) in diameter; larger than a cobblestone. See SOIL CLASSIFICATION.
BOUNDARY CONDITIONS
Environmental conditions, e.g. waves, currents, drifts, etc. used as boundary input to physical or numerical models
BOX GAGE
A tide GAUGE that is operated by a float in a long vertical box to which the tide is admitted through an opening in
the bottom. In the original type of box gauge the float supported a graduated rod which rose and fell with the tide.
BRAIDED RIVER
A river type with multiple channels separated by shoals, bars and islands
BREACHING
(1) Formation of a channel through a barrier spit or island by storm waves, tidal action, or river flow. Usually
occurs after a greater than normal flow, such as during a hurricane. (2) Failure of a dike allowing flooding.
BREAKER
A wave breaking on a shore, over a REEF, etc. Breakers may be classified into four types (See Figure II-4-1):
COLLAPSING--breaking occurs over lower half of wave, with minimal air pocket and usually no splash-up.
Bubbles and foam present.
PLUNGING--crest curls over air pocket; breaking is usually with a crash. Smooth splash-up usually follows.
SPILLING--bubbles and turbulent water spill down front face of wave. The upper 25 percent of the front face may
become vertical before breaking. Breaking generally occurs over quite a distance.
SURGING--wave peaks up, but bottom rushes forward from under wave, and wave slides up beach face with little
or no bubble production. Water surface remains almost plane except where ripples may be produced on the beachface during
runback.
BREAKER DEPTH
The still-water depth at the point where a wave breaks. Also called BREAKING DEPTH.
BREAKER INDEX
Ratio of breaking wave height to DEEPWATER wave height
BREAKER ZONE
The zone within which waves approaching the coastline commence breaking, typically in water depths of between 5
and 10 meters for ocean coasts, but sometimes in shallower water.
BREAKING
Reduction in wave energy and height in the surf zone due to limited water depth
BREAKWATER
A man-made structure protecting a shore area, harbor, anchorage, or basin from waves. A harbor work.
Appendix A Glossary of Coastal Terminology
A-11
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