River Ecology and Restoration - River Dynamics and Integrated River Management - page 691

4) Design a habitat improvement plan. Quantify the desired results in terms of hydraulic changes,
habitat improvement, and population increases. Integrate selection and sizing of habilitation works with
instream flow requirements. Select potential schemes and structures that will be reinforced by the
existing stream dynamics and geometry. Test designs for minimum and maximum flows and set target
flows for critical periods derived from the historical mass curve.
5) Implement planned measures. Arrange for on-site location and elevation surveys and provide advice
for finishing details in the stream.
6) Monitor and evaluate results. Arrange for periodic surveys of the rehabilitated reach and reference
reaches, to improve the design, as the channel ages.
Evidence suggests that traditional design criteria for widespread bank and bed stabilization measures
(e. g., concrete grade control structures, homogeneous riprap) can be modified, with no functional loss, to
better meet environmental objectives and improve habitat diversity. Weirs are generally more failure-prone
than deflectors. Deflectors and random rocks are minimally effective in environments where higher
flows do not produce sufficient local velocities to produce scour holes near structures. Random rocks
(boulders) are especially susceptible to undermining and burial when placed in sand-bed channels,
although all types of stone structures experience similar problems. Additional guidance for evaluating the
general suitability of various fish habitat structures for a wide range of morphological stream types is
provided by Rosgen (1996). Seehorn (1985) provides guidance for small streams in the eastern U.S.
Nowadays numerous design web sites are available (White and Brynildson, 1967; Seehorn, 1985;
Wesche, 1985; Orsborn et al., 1992; Orth and White, 1993; Flosi and Reynolds, 1994). The use of any of
these guides should also consider the relative stability of the stream, including aggradation and incision
trends, for final design.
Hydraulic conditions at the design flow should provide the desired habitat; however, performance
should also be evaluated at higher and lower flows. Barriers to movement, such as extremely shallow
reaches or vertical drops not submerged at higher flows, should be avoided. If the conveyance of the
channel is an issue, the effect of the proposed structures on stages at high flow should be investigated.
Structures may be included in a standard backwater calculation model as contractions, low weirs, or
increased flow resistance (Manning) coefficients, but the amount of increase is a matter of judgment or
limited by National Flood Insurance Program ordinances. Scour holes should be included in the channel
geometry downstream of weirs and dikes since a major portion of the head loss occurs in the scour hole.
Hydraulic analysis should include estimation or computation of velocities or shear stresses to be experienced
by the structure.
If the hydraulic analysis indicates a shift in the stage-discharge relation, the sediment-rating curve of
the restored reach also may change, leading to deposition or erosion. Although modeling analyses are
usually not cost-effective for a habitat structure design effort, informal analyses based on assumed
relations between velocity and sediment discharge at the bankfull discharge may be helpful in detecting
potential problems. An effort should be made to predict the locations and magnitude of local scour and
deposition. Areas projected to experience significant scour and deposition should be sites for visual
monitoring after construction.
Materials used for aquatic habitat structures include stone, fencing wire, posts, and felled trees.
Priority should be given to materials that occur on site under natural conditions. In some cases, it may be
possible to salvage rocks or logs generated from construction of channels or other project features. Logs
give long service if continuously submerged. Even logs not continuously wet can give several decades of
service if chosen from decay-resistant species. Logs and timbers must be firmly fastened together with
bolts or rebar and must be well anchored to banks and bed. Stone size should be selected based on design
velocities or shear stress.