Introduction to Stream Restoration - Hydro-Environmental Analysis: Freshwater Environments - page 247

also incorporate sediment transport to maintain channel stability. The relationships between sedi-
ment transport and channel geometry are highly variable due to, for example, differences between
materials in the bed and banks. However, to be successful, the design of the restored channel should
be stable, minimizing aggradation and degradation by reestablishing equilibrium between the sup-
ply and transport capacity. The design is commonly based on sediment transport analyses in order
to evaluate the relationships between the sediment supply and the sediment transport capacity.
Rivers with erodible boundaries not only tend to meander, they also tend to migrate. To be success-
ful, a restoration project must also typically prevent channel migration.
It should also be recognized that all meanders are not the same. That is, for example, meandering
rivers vary in their meander geometry and bank materials, such as in the formation of point bars
and chutes. Recall that point bars consist of relatively coarse-grained materials (silts and sands) laid
down on the inside (convex) bend of a migrating stream channel, while chutes form a new channel
across the base of a meander. Based on the classiication schemes (see Chapter 2) of Schumm (1963,
1977), Rosgen (1994, 1996), and Brice (1975, 1984), Soar and Thorne (2001) described three of the
most common types of meander bends found in stable single-thread channels as:

• Equiwidth Meandering (Schumm Type 3a; Brice Type A/B; Rosgen Type E): Equiwidth

indicates that there is only minor variability in the channel width around meander bends.
These channels are generally characterized by: low width/depth ratios; erosion-resistant
banks; ine-grained bed material (sand or silt); low bed material load; low velocities; and
low stream power. Channel migration rates are relatively low because the banks are natu-
rally stable.

• Meandering with Point Bars (Schumm Type 3b; Brice Type C; Rosgen Type C): Meandering

with point bars refers to channels that are signiicantly wider at bendways than at cross-
ings, with well-developed point bars but few chute channels. Point bars consist of rela-
tively coarse-grained materials (silts and sands) laid down on the inside (convex) bend of
a migrating stream channel. These channels are generally characterized by: intermedi-
ate width/depth ratios; moderately erosion-resistant banks; medium-grained bed material
(sand or gravel); medium bed material load; medium velocities; and medium stream power.
Channel migration rates are likely to be moderate unless the banks are stabilized.

• Meandering with Point Bars and Chute Channels (Schumm Type 4; Brice Type D; Rosgen

Type C/D): Meandering with point bars and chute channels refers to channels that are very
much wider at bendways than at crossings, with well-developed point bars and frequent chute
channels. These channels are generally characterized by: moderate-to-high width/depth ratios;
highly erodible banks; medium- to coarse-grained bed material (sand, gravel, and/or cobbles);
heavy bed material load; moderate-to-high velocities; and moderate-to-high stream power.
Channel migration rates are likely to be moderate to high unless the banks are stabilized.

Soar and Thorne (2001) indicated that other types (such as in the Rosgen classiication system
Aa+, A and B, D, DA, F and H) would, for various reasons, not present realistic or attractive targets
for a restoration scheme.

8.4.18.2 Restoration Techniques
One approach to restoring a river to “a close approximation of its remaining natural potential” is to
base the restoration design on its predisturbed condition, the “carbon copy” approach. Determining
and mimicking the “natural” condition, where possible, is perhaps the best method for restoring
the sinuosity of channels. The analysis may be based on historical records, such as maps, aerial
photographs, or soil surveys. However, in many cases, the natural condition is unknown. Also, dis-
turbances in the river's watershed, resulting in changes in low and sediment loads, may make what
was once a stable condition, unstable under present conditions. Thus, past conditions are often not
good estimators for future impacts.