Environmental Engineering Reference
In-Depth Information
Another approach is the so-called natural approach. This approach involves simply creating a
valley and letting the channel form naturally. Due to the instability of the system, this approach is
often unsatisfactory. The best design procedure is a systems approach, based on an analysis of the
disturbed site by meander analysis and consideration of the undisturbed sites nearby (Hasfurther
1985; Gore et al. 1995).
The initial steps in the restoration project are to (FISRWG 1998):
•
Describe the physical aspects of the watershed and characterize its hydrologic response.
•
Select a preliminary right-of-way for the restored stream channel corridor and compute the
valley length and valley slope.
•
Determine the approximate bed material size distribution for the new channel.
•
Conduct a hydrologic and hydraulic analysis to select a design discharge or a range of
discharges.
•
Predict a stable planform type (straight, meandering, or braided).
The last step, predicting a stable channel type, may be based on a sediment transport analysis or
an analysis of similar (reference) stable systems. Figure 8.31 provides a deinition sketch of cross
section geometries and dimensions through a meander.
Following the preliminary steps, the systems approach to design (Hasfurther 1985) is based on
performing a basinwide analysis of the stream channel to determine the meander characteristics,
such as the meander wavelength (
L
m
), the belt-width or amplitude (
A
m
), the radius of curvature (
R
c
),
and the channel's alignment (see Figure 8.31). The sinuosity is computed from the channel length
and slope (channel length = sinuosity × valley length; channel slope = valley slope/sinuosity), and
a preliminary design meander and sinuosity are determined.
An alternative to the systems approach is to base the meander design on empirical equations that
relate meander properties (e.g., belt-width, wavelength, and radius of curvature) to other properties,
such as low, width, radius of curvature, sediment load, and other factors (Rosgen 1994). Generally,
the most effective are those that are related to the discharge or the bank-full width (USACE 1994;
Copeland et al. 2001). For example, formulations are often of the form:
b
AaW
=
m
d
L W
=
m
f
R W
=
c
where
a
to
f
are coeficients. For example, in Leopold and Wolman (1966)
11
.
A W
=
27
.
m
101
.
L
=
10 9
.
W
m
10
.
R W
=
24
.
c
in units of feet. Leopold et al. (1964) suggested that meander wavelengths are most commonly in
the range of 10-14 times the channel width (
c
= 10-14). Soar and Thorne (2001) indicated that
an unbiased morphological expression for the meander wavelength within 95% conidence lim-
its on the mean response suitable for engineering design is given by meander wavelengths within
11.26-12.47 times the channel width (
L
m
= 11.16-12.47 *
W
, in meters). However, the ranges are
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