sub-bed material:
Material underlying that portion of the streambed which is
subject to direct action of the flow. Also, substrate.
subcritical,
Open channel flow conditions with Froude Number less
supercritical flow:
than and greater than unity, respectively.
tetrahedron:
Component of river-training works made of six steel or
concrete struts fabricated in the shape of a pyramid.
tetrapod:
Bank protection component of precast concrete consisting of
four legs joined at a central joint, with each leg making an
angle of 109.5- with the other three.
thalweg:
The line extending down a channel that follows the lowest
elevation of the bed.
tieback:
Structure placed between revetment and bank to prevent
flanking.
timber or brush mattress:
A revetment made of brush, poles, logs, or lumber interwoven
or otherwise lashed together. The completed mattress is then
placed on the bank of a stream and weighted with ballast.
toe of bank:
That portion of a stream cross section where the lower bank
terminates and the channel bottom or the opposite lower bank
begins.
toe protection:
Loose stones laid or dumped at the toe of an embankment,
groin, etc., or masonry or concrete wall built at the junction of
the bank and the bed in channels or at extremities of hydraulic
structures to counteract erosion.
total scour:
The sum of long-term degradation, general (contraction) scour,
and local scour.
total sediment load:
The sum of suspended load and bed load or the sum of bed
material load and wash load of a stream (total load).
tractive force:
The drag or shear on a streambed or bank caused by passing
water which tends to move soil particles along with the
streamflow.
trench-fill revetment:
Stone, concrete, or masonry material placed in a trench dug
behind and parallel to an eroding streambank. When the
erosive action of the stream reaches the trench, the material
placed in the trench armors the bank and thus retards further
erosion.
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