MEAN HIGH WATER SPRINGS - The average height of the high waters occurring 

 at the time of spring tide. Frequently abbreviated to HIGH WATER 

 SPRINGS. 



MEAN LOWER LOW WATER (MLLW) - The average height of the lower low waters 

 over a 19-year period. For shorter periods of observations, cor- 

 rections are applied to eliminate known variations and reduce the 

 results to the equivalent of a mean 19 -year value. Frequently 

 abbreviated to LOWER LOW WATER. 



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

 year period. For shorter periods of observations, corrections are 

 applied to eliminate known variations and reduce the results to the 

 equivalent of a mean 19-year value. All low water heights are in- 

 cluded in the average where the type of tide is either semidiurnal 

 or mixed. Only lower low water heights are included in the average 

 where the type of tide is diurnal . So determined, mean low water in 

 the latter case is the same as mean lower low water. 



MEAN LOW WATER SPRINGS - The average height of low waters occurring at 

 the time of the spring tides. 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. Frequently abbreviated to LOW WATER SPRINGS. 



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

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

 hourly height readings. Not necessarily equal to MEAN TIDE LEVEL. 



MEAN TIDE LEVEL - A plane midway between MEAN HIGH WATER AND MEAN LOW 

 WATER. Not necessarily equal to MEAN SEA LEVEL. Also called 

 HALF-TIDE LEVEL. 



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

 sample into two equal parts by weight, one part containing all 

 grains larger than that diameter and the other part containing 

 all grains smaller. 



MEGARIPPLE - See SAND WAVE. 



MIDDLE-GROUND SHOAL -A shoal formed by ebb and flood tides in the middle 

 of the channel of the lagoon or estuary end of an inlet. 



MINIMUM DURATION - See DURATION, MINIMUM. 



MINIMUM FETCH - The least distance in which steady state wave conditions 

 will develop for a wind of given speed blowing a given duration of 

 time. 



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