Selection and Design of Channel Rehabilitation Methods
188.8.131.52 Flexible Mattress
The basic concept of a flexible mattress is that material or objects which cannot resist erosive forces
separately can be fastened together or placed in a flexible container to provide adequate resistance to
erosive forces, while partially retaining the desirable characteristics of adjustable armor, especially that of
The most common flexible mattress materials are: concrete blocks; fabric; and gabions. Materials
which have a more limited use are: grids (for confining earth or other fill material); used tires; and wood.
This compromise between adjustable armor and rigid armor is most attractive when economical
materials can be used for the mattress. In fact, the origin of some variations can be traced directly to
creative use of local materials. Where no protective material of local origin is adequate to withstand the
erosive forces in a given application, the most suitable method may be the one which requires the least
amount of costly imported material, a requirement which is often met by a flexible mattress.
Advantages: Flexibility to adjust to scour or settlement and still remain in contact with the bed
and bank is the most obvious trait. Most mattress materials which are sold under trade names share
another advantage - they are available in various configurations, thus can be applied to a variety of
Flexible mattresses can be placed underwater with a relatively high degree of confidence. If
properly anchored to a geotechnically stable bank, they can be placed on steep slopes. They can be
walked upon easily, thus are suitable for slopes used by pedestrians.
Mattress components are
subject to deterioration from the elements and
vandalism. However, the damage is often within acceptable limits and since the various types are affected
differently, identification of the hazards enables the designer to select an appropriate mattress for a given
application. The construction of some types of mattresses are labor intensive, and may require skills not
commonly available. However, the labor intensive aspect may not be a disadvantage in all cases, and may
be an advantage in some cases.
INDIRECT TECHNIQUES FOR EROSION PROTECTION
Indirect protection structures extend into the stream channel, and redirect the flow so that hydraulic
forces at the channel boundary are reduced to a non-erosive level. Indirect protection techniques can be
classified as follows: dikes and retards; and other flow deflectors such as bendway weirs and Iowa vanes.
184.108.40.206 Dikes and Retards