MEAN LOW WATER (MLW) - The average height of the low waters over a 19- 

 year periodo For shorter periods of observations, corrections are 

 applied to eliminate known variations and reduce the result to the 

 equivalent of a mean 19-year value » 



All low water heights are included in the average where the tj^se 

 of tide is either semidiurnal or mixed. Only the lower low water 

 heights are included in the average where the type of tide is diurnal. 

 So determined, mean low Vater in the latter case is the same as mean 

 lower low water » 



EEAN LOW WATEP SPRINGS - Frequently abbreviated low water springs. The 

 average height of low waters occurring at the time of the spring 

 tides o It is usually derived by taking a plane depressed below 

 the half-tide level by an amount equal to one-half the spring range 

 of tide, necessary corrections being applied to reduce the result 

 to a mean value. This plane is used to a considerable extent for 

 hydrographic work outside of the United States and is the plane 

 of reference for the Pacific approaches to the Panama Canal. 



MEAN SEA LEVEL - The average height of the surface of the s ea for all 

 stages of the tide over a 19-year period, usually determined from 

 hourly height readings. See also SEA LEVEL DATUM, 



MEAN TIDE LEVEL - Also called half-tide level. A plane midway between 

 mean high water and mean low water. 



MEDIAN DIAMETER - The diameter which marks the division of a given sample 

 into two equal parts by weight, one part containing all grains larger 

 than that diameter and the other part containing all grains smaller. 



MINIMUM DURATION - The time necessary for steady state wave conditions 

 to develop for a given wind velocity over a given fetch length, 



MIXED TIDE - A type of tide in which the presence of a diurnal wave 

 is conspicuous by a large inequality in either the high or low 

 water heights with two high waters and two low waters usually 

 occurring each tidal day. In strictness all tides are mixed but 

 the name is usually applied without definite limits to the tides 

 intermediate to those predominantly semidiurnal and those pre- 

 dominantly diurnal, (See Figure A-IO) , 



MOLE - In coastal terminology, a massive solid-fill structure of earth, 

 (generally revetted), masonry, or large stone. It may serve as a 

 breakwater or pier, 



MONOLITHIC - Like a single stone or block. Therefore in (say) break- 

 waters, the type of construction in which the structure's component 

 parts are bound together to act as one, 



MUD - A fluid- to -pi as tic mixture of finely divided particles of solid 

 material and water. 



A-20 



