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: R

Recurrence Interval:

A statistical expression of theaverage time between floods equalling or exceedinga given magnitude (see flood frequency).

Reflected wave:

That part of an incident wave that is returned seaward when a wave impinges on a steep beach, barrier or other reflecting surface.

refraction (of water waves):

The process by which the direction of a wave moving in shallow water at an angle to the contours is changed. The part of the wave advancing in shallower water moves more slowly than that part still advancing in deeper water, causing the wave crest to bend toward alignment with the underwater contours.

Regulatory Floodplain:

That portion of the floodplainsubject to floodplain regulations (usually thefloodplain inundated by the one-percent chanceflood).

Regulatory Floodway:

The channel and that portionof the adjacent land area that is required throughregulations to pass flood flows without increasingthe water surface elevation more than a designatedheight.

Reservoir:

A natural or artificially created pond, lakeor other space used for storage, regulation orcontrol of water. May be either permanent ortemporary.

Revetment:

A facing of stone, concrete, etc., built to protect a scarp, embankment or shore structure against erosion by wave action or currents.

Riprap:

A layer, facing or protective mound of stones randomly placed to prevent erosion, scour or sloughing of a structure or embankment; also the stone so used.

Riverine:

Relating to, formed by, or resembling ariver (including tributaries), stream, brook, etc.

Roller Compacted:

Roller Compacted Concrete (RCC)I s "concrete compacted by roller compaction; concrete that, in its unhardened state, will support a roller while being compacted." Properties of hardened RCC can be similar to those of conventionally placed concrete. However, Roller Compacted Concrete can also be made with hardened properties that are outside the range of typical properties of conventionally placed concrete. The term "roller compaction" is defined as "a process for compacting concrete using a roller, often a vibrating roller."

Roller Gates:

Roller gates are large cylinders that move in an angled slot. They are hoisted with a chain and have a cogged design that interfaces with their slot.

Runoff:

That portion of precipitation which is notintercepted by vegetation, absorbed by the landsurface or evaporated, and thus flows overlandinto a depression, stream, lake or ocean (runoffcalled 'immediate subsurface runoff' also takesplace in the upper layers of the soil).