The potential environmental impacts and the possible need for stream impact mitigation measures should be primary considerations. (See in Section 2 for more information.) Mitigation practices are not generally warranted but may be mandated by the cognizant regulatory agency. As such, you may need to coordinate with Texas fish and wildlife agencies before determining mitigation. Consult the Environmental Affairs Division and the Design Division, Hydraulic Branch, to determine the need for mitigation when you deem stream modifications necessary.

Channel modifications may be necessary and also can provide environmental enhancement (see the previous subsection). Also, channel modifications that are compatible with the existing aquatic environment can sometimes be constructed at little or no extra cost.

There will be less aquatic habitat where a channel is shortened to accommodate highway construction. This not only decreases the aquatic biomass, but also reduces the amount of surface water available for recreation and sport fishing. Estimate the significance of this effect by comparing the amount of surface water area, riparian and upland wetland area, and stream length that will be lost with the existing amount in the geographic area. If there will be a loss, particularly of wetlands, resource and regulatory agencies may raise objections in light of the national “no net loss” policy currently prevailing. In some instances, such habitat loss may be acceptable when combined with mitigation measures, but such measures should prevent habitat damage beyond the channel change limits.

Enhancement of the channel may be accomplished during stream reconstruction at little additional cost, and perhaps at less cost where reconstruction is essential to the needs of the highway project. It may even be possible to reconstruct the surface water resource in one of the following manners that eliminates an existing environmental problem:

The most common practices are using a drop-type grade control structure (check dam), maintaining the existing channel slope, and increasing the channel change length by constructing an artificial meander.

Culverts can provide another alternative similar to using drop structures. You can increase the culvert flowline slope to accommodate the elevation difference caused by shortening a channel. The increased erosion associated with steep culverts is localized at the outlet that can be protected.

Simulate the existing channel cross section if it is relatively stable, has low flow depths and velocities, or has adequate minimum flow requirements.

Determine the cross-sectional shape by hydraulically analyzing simple and easy to construct shapes that approximate the preferred natural channel geometry. The analysis generally compares the stage-discharge, stage-velocity, and stage-sediment relationships of the natural channel with the modified channel.

Stream relocations may temporarily impair water quality. The problem is primarily sediment-related, except for those rare instances where adverse minerals or chemicals are exposed, diverted, or intercepted. With a channel relocation, the new channel should be constructed in dry conditions wherever possible. Following completion, the downstream end should be opened first to allow a portion of the new channel to fill as much as possible. Next, the upstream end should be opened slowly to minimize erosion and damage to habitat mitigation.

Where the channel relocation interferes with the existing channel, it may be desirable to construct rock and gravel dikes or to use other filtering devices or commercially available dikes to isolate the construction site, thereby limiting the amount of sediment entering the water.