Response Possibilities and Solutions
Increased Slope
. The following are possible responses to increased slope:- The stream response may be in the form of a regime change from a meandering to a braided channel, or sediment transport through the steepened reach may be increased enough to cause degradation upstream of the realignment and aggradation downstream.
- Banks may become unstable and require structural stabilization measures to prevent erosion.
- Tributary channels entering the steepened main channel may be subject to headcutting, with deposition occurring at or downstream of the junction.
The following are possible solutions to increased slope:
- You may use grade control structures (such as check dams, weirs, or chutes) to minimize increases in gradient, provided there is some assurance that the normal meandering tendency of the channel will not bypass these structures in time.
- If topography permits, use meanders to reduce the stream gradient to existing or threshold levels. These meanders may require stabilization to assure continued effectiveness and stability.
Encroachment.
Highway locations or modifications in certain terrain conditions may result in an encroachment such as that illustrated in Figure 7‑7.
Figure 7-7. Highway Encroachment on Natural Streams and Stream Relocation
This type of channel realignment may require providing a channel of sufficient section to convey both normal and flood flow within the banks formed by the roadway and the floodplain. The low flow channel may require realignment, in which case a pilot channel could approximate the existing channel characteristics of width, depth, gradient, and bottom roughness. Where no pilot channel is provided, the average daily flow is likely to spread over a much wider section, and flow depth will be reduced in such a way that water temperature, pool formation, and sediment transport are adversely affected. These modifications may result in a braided channel condition and hamper the re-establishment of the natural aquatic environment.
Clearing of vegetation along stream banks may remove root systems that have contributed to bank stability. Clearing and grubbing reduces the bank and floodplain roughness and contributes to higher velocities and increased erosion potential for those areas. However, the limited clearing of adjacent right-of-way involved with transverse encroachments or crossings does not normally affect the overall conveyance capacity of a channel to any significance.
A water surface profile analysis is necessary to establish the
for channels with varying roughness characteristics across the channel. The
of estimating stage-discharge relationships can be subject to significant error if the typical section used does not represent the actual conditions upstream and downstream of the crossing site. Therefore, the
is recommended. (See
for more details on these methods.)
Channel enlargement or cleanout through a limited channel reach is sometimes proposed in an effort to provide additional stream capacity. If the stage of the stream at the proposed highway site is controlled by downstream conditions, there can be limited or possibly no benefits derived from localized clearing.