Assessment Of Degradation. Field sites having degradation problems are more numerous
than sites having aggradation problems. Annual rates of degradation averaged from past
records such as the closure of a dam give poor estimates of future rates of degradation.
Typical situations exhibit an exponential decay function of the rate of channel degradation.
Recent evidence of degradation can be detected from field surveys or by stereo viewing of
aerial photographs. Indicators of degradation are: (1) channel scarps, headcuts and
nickpoints; (2) gullying of minor side tributaries; (3) high and steep unvegetated banks; (4)
measurements of streambed elevation from a bridge deck; (5) changes in stream discharge
relationships; and (6) measurements of longitudinal profiles.
Assessment Of Scour And Fill. Natural scour and fill refer to fluctuations of streambed
elevation about an equilibrium condition. These fluctuations are associated mainly with floods
and occur by three different mechanisms operating jointly or independently: (1) bed form
migration; (2) convergence and divergence of flow; and (3) lateral shift of thalweg or braids.
The maximum scour induced by the migration of a dune is almost one-half the dune height,
and dune heights are roughly estimated as one-third of the mean flow depth. In gravel bed
streams, most migrating bed forms can be regarded as bars, the height of which is related to
flow depth. The migration of a bar through a bridge waterway is mainly of concern because of
the deflection and concentration of flow. Bar migration tends to be a random process and its
motion can best be tracked from time-sequential aerial photographs.
Gravel bars tend to migrate on braided streams and to remain fixed at riffles on unbraided pool
and riffle streams.
Flow convergence in natural streams is associated with scour, whereas divergent currents are
associated with deposition. Persistent pools have the strongest convergence of flow and the
greatest potential for scour. Such pools are best identified by a continuous bed profile along
the thalweg. In braided streams, scour holes are found at the confluence of braids. Field
measurement of cross-sectional area and flow velocity at an incised reach near bankfull stage
provides a good basis for calculation of scour by extrapolation to the design flood.
Instability of the streambed that results from shift of thalweg is related to stream type and can
be assessed from study of aerial photographs. On sinuous canal form streams, shift of the
thalweg during flood is minimal. A greater shift of the thalweg can be expected on sinuous
point-bar streams. In straight reaches, alternate bars visible on aerial photographs taken at low
stage are commonly present. These alternate bars indicate the potential for thalweg shifting
and also for bank erosion when the current is deflected against the bankline. Shift of the
thalweg with increase in stage must be considered when determining the location of the point
of maximum bed scour, bank erosion, and the alignment of piers with flood flow.
substantially greater at specific places along the channel. Bends and narrow sections may
scour at high stages regardless of the effect of bridge structures. Straight or gently curved
reaches with stable banks are preferred.
Considerations for the selection of a crossing site on a non-sinuous reach include: (1) is the
site at a pool, riffle or transition section; (2) are alternate bars visible at low stage; and (3) what
is the effect of migration of mid-channel bars, if any? With respect to meandering reaches,
questions requiring solution include: (1) what has been the rate and mode of migration of the
meander; (2) what is the probable future behavior, as based on the past; (3) is the site at a
pool, riffle or transition section; and (4) is meander cutoff probable?