Overfill factor

EM 1110-2-1100 (Part V)

31 Jul 2003

and 0.2 mm, borrow material can be considered compatible if its composite median diameter is within

plus or minus 0.01 mm of the native diameter. For native beach material with a composite median

diameter less than 0.15 mm, use of material at least as coarse as the native beach is recommended.

Even though material is deemed compatible based on these rules, the designer should factor grain-

size differences into estimates of required fill volume through use of equilibrium beach profile

methods, or the overfill factor, or both. Methods for computing beach- fill volumes are discussed

in Part V-4-1-f. These guidelines are based on composite median diameters established for the entire

project and borrow site. Typically, composites for individual profiles, or subsections of the borrow

site, will have variations in median diameter which may exceed the compatibility ranges previously

discussed.

Materials that are not compatible according to these guidelines may still be suitable. Borrow material

that is coarser than the native material will produce a beach which is at least as stable as a fill

comprised of native material. Fills with coarser material provide improved resistance to storm-

induced erosion. A lesser volume of coarser fill will be required to create a beach of a given width,

compared to the volume of native beach sand that would be needed. If the median diameter of the

borrow material exceeds the median diameter of the native material by more than 0.02 mm, a

noticeably steeper beach may form. A steeper beach may become a design issue, along with the

different texture of the coarser fill. Use of material finer than the native material should be avoided,

if possible, but such material still may be suitable. A much greater volume of material will be

required to form a beach of a given width, compared to the volume of native sand that would be

required. Use of finer sand will produce a beach with flatter slopes, which could be a design issue

too. For example, it may be problematic to construct a more gentle beach adjacent to an existing

groin or jetty that is intended to block the longshore movement of sand. Sand transport around the

structure and into a navigation channel may increase.

(i) Overfill factor. The overfill factor, RA, is determined by comparing mean sediment diameter and

sorting values of the native beach and borrow sediments (in phi,φ, units). The phi, φ, scale of sediment

diameter is defined and discussed in Part III-1, and Equations III-1-1a and III-1-1b enable conversion between

sediment grain size diameter in millimeters and the φ scale and vice versa. The overfill factor is computed

using the following relationships between the borrow and native beach material:

(φ84&φ16)

(φ95&φ5)

%

4

6

σφb

b

(V-4-3)

'

σφn

(φ84&φ16)

(φ95&φ5)

%

4

6

n

and

(φ16%φ50%φ84)

&

Mφb & Mφn

3

b

n

(V-4-4)

'

σφn

(φ84&φ16)

(φ95&φ5)

%

4

6

n

Beach Fill Design

V-4-25