Quantcast Design Considerations

 
  
 
Surface Armor for Erosion Protection
In most climates wood will deteriorate quickly if exposed to alternate wetting and
drying. Therefore, it is not a suitable material for use above low water unless treated lumber
is used (which may affect water quality), or unless frequent maintenance or the establishment
of vegetation is planned.
The durability of metallic components may be poor underwater. This is a significant
shortcoming, since the mat must remain intact to function properly.
Construction is difficult if currents are swift, depths are great, or the flow carries large
amounts of floating debris.
The designs that use lumber or long poles woven into a mat are stiff, which limits their
capacity to conform to bank and bed irregularities. Severe erosive forces require thick mats,
which reduces flexibility in proportion to thickness, and loss of permeability greatly increases
the difficulty in sinking in swift currents. In fact, the stiffness of sturdy woven pole and
lumber mats led to them being replaced on the lower Mississippi River about 1900 by willow
fascines, or bundles, cabled together into mats. The fascine mat was more flexible. However,
the high labor cost and diminishing willow supply, as well as sometimes ineffectual
performance, led to the fascine mat being replaced in turn about 60 years ago by the much
more successful articulated concrete mat.
7.4.6.4 Design Considerations
The major causes of failure of wooden mattresses on the lower Mississippi River, as
discussed by Elliott (1932). The disadvantages of this technique listed above provide a basis
for defining the most critical elements of design. The most serious shortcomings were found
to be:
Rotting of the mattress where it was alternately wet and dry;
Inability of the mattress to adjust to scour at its toe (riverward edge); and
Failure of fasteners and connecting components from corrosion, abrasion, or
fatigue.
Design of a wooden mattress should address these points of vulnerability by utilizing
the following measures:
A secure, durable interface between the wooden mattress and whatever more
durable material is to be used to armor the upper bank should be specified.
Since this interface will likely be underwater at the time of construction,
unless the work is done at extremely low river stages, a material which is
suitable for reliable placement underwater is dictated. Stone is an excellent
choice, although many other adjustable armors or flexible mattress materials
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