Armor Unit Stability A key element in the design of any rubble mound structure is to determine the required armor unit size. This in turn dictates much of the remaining geometry of the structure (i.e., layer dimensions, sublayer unit sizes, structure slopes, crest width of the structure). The factors a V ecting the armor unit size, usually speci W ed as the weight of the unit, are listed below. Design wave characteristics . The design wave height at the structure is of critical importance. Some designers use the signi W cant height although more conservative heights such as H 10 have been used. The importance of the struc- ture, how well the design wave height is known, and the desired safety factor will a V ect this choice. The expected duration of attack by high waves during a design storm is also an important factor. If the water depth at the seaward side of the structure is su Y ciently shallow the design wave will break before it reaches the structure. This will limit the wave height attacking the structure and will a V ect the type of attack that occurs. Runup deflector Gravel Filter cloth Embankment Riprap 3 Cutoff wall MLW 1 Figure 7.5. Typical revetment cross-section. 216 / Basic Coastal Engineering
Armor unit characteristics . The shape, speci W c gravity, and range of unit sizes in the armor layer are important. More angular rock will be more stable than rock that is rounded. Arti W cial concrete armor units are designed to have irregular interlocking shapes while maintaining su Y cient layer porosity when in place. Narrower stone size ranges are preferred for armor layers as very W ne sizes are easily removed from the structure and W ner sizes decrease the layer porosity which decreases its ability to relieve underlayer pressure exerted by wave action. Lower layer porosities will also increase the elevation of wave runup on the structure. However, specifying too narrow of a size range to a quarry operator increases the e V ort and thus cost of obtaining the rock. Armor layer slope . The X atter the slope the smaller the size of the armor unit required for stability to wave attack. Smaller sizes yield thinner armor layers but X atter slopes require longer layers. Trunk versus head . The head or end of a structure is typically exposed to more concentrated wave attack than the trunk or side of the structure. For more complex structures it is common to have X atter slopes and/or larger armor unit sizes at the head to protect against the greater wave attack. Overtopping . The structure may be designed for moderate or signi W cant wave overtopping depending on the structure purpose and design criteria. For sign- i W cant overtopping there is less return X ow on the seaward face of the structure but greater action on the leeward face. This might allow smaller seaward armor unit sizes and steeper slopes but more attention will have to be placed on the design of the structure crest and lee side.
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