Glossary

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A B C D E F G H I L M N O P Q R S T U V W Z

Glossary terms for letter: D

Datum, plane:

The horizontal plane to which soundings, ground elevations or water surface elevations are referred. Also REFERENCE PLANE.

Deepwater wave:

Waves which develop in water of sufficient depth that they are not influenced by the friction of the lake bottom.

Design Flood:

Commonly used to mean the magnitude of flood used for design and operation of flood risk management structures or other protective measures. It is sometimes used to denote the magnitude of flood used in floodplain regulations.

Designated Floodway:

The channel of a stream andthat portion of the adjoining floodplain designatedby a regulatory agency to be kept free of furtherdevelopment to provide for unobstructed passageof flood flows.

Diffraction (of water waves):

The phenomenon by which energy is transmitted laterally along a wave crest when a part of a train of waves is interrupted by a barrier, such as a breakwater, the effect of diffraction is manifested by propagation of waves into the sheltered region within the barrier's geometric shadow.

Downdrift:

The direction of predominant movement of littoral materials.

Drum Gate:

Drum gates are constructed as a segment of a circle and hinged on its downstream end. A watertight sill chamber is provided for the gate. To raise the gate, upper pool pressures are introduced to the chamber. This force may be supplemented by flotation chambers or hydraulic cylinders. The major difficulty encountered with this gate is the necessity to exclude silt and sand from and maintain seals on the chamber.