Gravel Filter Design. A filter should be placed under the riprap unless the material forming the
core of the structure is coarse gravel or of such a mixture that it forms a natural filter. Two
types of filters are commonly used: gravel and geotextile filter.
The sizes of gravel in the filter layer are calculated as explained in Chapter 6 and the results
are given in Table 10.2. Two layers of filter material are required because the riprap sizes are
Thickness of the filters may vary depending on the riprap thickness, but should not be less than
6 to 9 in. (15 - 23 cm). Filters that are one-half the thickness of the riprap are satisfactory and
provide a great degree of safety.
Height and Depth of Riprap. The design height of riprap protection above the existing bed
level (Figure 10.10) must provide for freeboard, water depth, superelevation and wave height.
The normal depth from the thalweg level and the superelevation were determined earlier.
Because the forms of antidunes are a series of inphase symmetrical sand and water waves,
the wave height at the water surface can be assumed equal to the antidune height. The
minimum height required for riprap protection above the existing bed level can be determined
by considering normal depth, superelevation, and antidune height. In order to provide
additional protection against the breaking waves, an extra foot of freeboard is added to the
required minimum height of riprap above bed level.
The riprap (Figure 10.10) must extend some distance below the thalweg level to provide safety
against possible local scour, general scour, and troughs of passing sand waves. The general
scour is assumed to be negligible because the proposed structures do not significantly
constrict the flow. Some local scour is expected at the leading portion of the riprap revetment.
Hence, a larger depth of riprap is required at this location. The minimum depth of riprap below
thalweg level can be determined by considering the potential scour and the antidune height.
In general, the riprap should extend a minimum of 5 ft (1.5 m) below thalweg level in order to
protect against possible long-term degradation of the river reach. If the computed depth of
riprap protection below thalweg level is less than 5 ft (1.5 m), the design depth is set at 5 ft (1.5
10.1.4 Design for Alternatives 4 and 5
Alternative 4. The fourth alternative is simply to provide sufficient buffer strip distance between
the railroad and the existing river bank so that bank protection may not be required. The
distances the north bank will migrate under different design flood conditions can be evaluated
by utilizing sediment transport rates and the migration history of the 1965 flood. The estimated
bank migration distances for the design floods are given in Table 10.4. In order to provide
extra protection against slope failures and long-term bank migration due to smaller sizes of
floods, the design buffer strip distances should be at least twice the computed migration
distances (factor of safety - 2.0) because the accumulated bank migration distance due to
smaller sizes of floods can be very significant.