Inman, 1952; Folk and Ward, 1957, McCammon, 1962.) These formulas are
averages of 2, 3, 5, or more symmetrically selected percentiles of the
phi frequency distribution, such as the formula of Folk and Ward.
*14 + *S0 + *84
M* =------------------5----------------(4-S)
where
is the particle size in phi units from the distribution curve
at the percentiles équivalent to the subscripts 16, 50 and 84 (Fig. 4-8);
is the size in phi units that is exceeded by x percent (by dry weight)
of the total sample. These définitions of percentile (after Griffiths, 1967,
p. 105) are known as graphie measures. A more complex method - the method
of moments - can yield more précisé results when properly used.
To a good approximation, the médian,
is interchangeable with the
mean, (M), for most beach sédiment. Since the médian is easier to détermine it is widely used in engineering studies. For example, in one CERC
study of 465 sand samples from three New Jersey beaches, the mean computed
by the method of moments averaged only 0.01 millimeter smaller than the
médian for sands whose average médian was 0.30 millimeter (1.74 phi).
(Ramsey and Gaivin, 1971.)
The médian and the mean describe the approximate center of the sédiment size distribution. In the past, most Coastal engineering projects
hâve used only this size information. However, for more detailed design
calculations of fill quantifies required for beach restoration projects
(Sections 5.3 and 6.3), it is necessary to know more about the size distribution.
Since the actual size distributions are such that the log of the size
is approximately normally distributed, the approximate distribution can be
escri e (in p i units) by the two parameters that describe a normal dis«n
7 the?ean and the standard déviation. In addition to these
er^ ^raean and standard déviation), skewness and kurtosis
this theoreticîi* i ° actual size distribution of the sample départs from
this theoretical lognormal distribution.
spreads out
ds^u^aroun^t)1011 ÏS
m®asure of the degree to which the sample
out around the mean and may be approximated using Inman’s (1952)
définition,
=
^84
^16
2
(4-4)
where 4>8k is the sédiment si
cent by weight, of the sample
has a lognormal distribution
ze, in phi units, that is finer than 84 per• If the sédiment size in the sample actually
then 0$ is the standard déviation of the
4-16
averages of 2, 3, 5, or more symmetrically selected percentiles of the
phi frequency distribution, such as the formula of Folk and Ward.
*14 + *S0 + *84
M* =------------------5----------------(4-S)
where
is the particle size in phi units from the distribution curve
at the percentiles équivalent to the subscripts 16, 50 and 84 (Fig. 4-8);
is the size in phi units that is exceeded by x percent (by dry weight)
of the total sample. These définitions of percentile (after Griffiths, 1967,
p. 105) are known as graphie measures. A more complex method - the method
of moments - can yield more précisé results when properly used.
To a good approximation, the médian,
is interchangeable with the
mean, (M), for most beach sédiment. Since the médian is easier to détermine it is widely used in engineering studies. For example, in one CERC
study of 465 sand samples from three New Jersey beaches, the mean computed
by the method of moments averaged only 0.01 millimeter smaller than the
médian for sands whose average médian was 0.30 millimeter (1.74 phi).
(Ramsey and Gaivin, 1971.)
The médian and the mean describe the approximate center of the sédiment size distribution. In the past, most Coastal engineering projects
hâve used only this size information. However, for more detailed design
calculations of fill quantifies required for beach restoration projects
(Sections 5.3 and 6.3), it is necessary to know more about the size distribution.
Since the actual size distributions are such that the log of the size
is approximately normally distributed, the approximate distribution can be
escri e (in p i units) by the two parameters that describe a normal dis«n
7 the?ean and the standard déviation. In addition to these
er^ ^raean and standard déviation), skewness and kurtosis
this theoreticîi* i ° actual size distribution of the sample départs from
this theoretical lognormal distribution.
spreads out
ds^u^aroun^t)1011 ÏS
m®asure of the degree to which the sample
out around the mean and may be approximated using Inman’s (1952)
définition,
=
^84
^16
2
(4-4)
where 4>8k is the sédiment si
cent by weight, of the sample
has a lognormal distribution
ze, in phi units, that is finer than 84 per• If the sédiment size in the sample actually
then 0$ is the standard déviation of the
4-16