Countermeasures
Many streams have a strong propensity to meander. The sinuosity of the main channel is a general characteristic of a stream and can vary with the discharge and the type of soil that the stream passes through. The erosive force of the stream water forms meanders as it undercuts the main channel bank. The bank support is lost and material caves into the water to be deposited downstream. As the erosion on the outer bend of the meander migrates in a downstream direction, material from upstream deposits on the inside of the bend. This progression of stream meandering can have serious effects on highway crossings. This migration often threatens approach roadway embankment and bridge headers such as shown in Figure 7‑8.
Figure 7-8. Meandering Stream Threatening Bridge and Approach Roadway
In order to protect the roadway from the threat of meanders, yet remain synchronous with nature, it is important to devise countermeasures that are environmentally sound, naturally acting, economically viable, and physically effective.
Possible countermeasures include the following:
- Bridge lengthening -- With reference to the example given in Figure 7‑8, lengthening the bridge may not always be cost-effective as a countermeasure to the damage potential from the meander. In this example, the natural meandering course of the river threatens both the bridge and the approach roadway.
- Bridge relocation – In extreme situations, it may be necessary to relocate the bridge. Generally, it is good practice to locate the bridge crossing on a relatively straight reach of stream between bends.
- River training or some type of erosion control – River training or some type of erosion control may be more effective and economical. Designers have used several measures and devices successfully in Texas to counter the effects of serious stream meandering.
- Linear structures -- When it is not practical to locate the bridge on a relatively straight reach of stream, countermeasures such as spur or jetty type control structures may be needed (see Figure 7‑9). These are sometimes referred to as linear structures, permeable or impermeable, projecting into the channel from the bank to alter flow direction, protect the channel bank, induce deposition, and reduce flow velocity along the bank.
Figure 7-9. Permeable Fence Spurs as Meander Migration Countermeasures
Control structures may or may not cause the typical cross section of flow in a meandering stream to become more symmetrical. For many locations, countermeasures may not be required for several years because of the time required for the bend to move to a location where it begins to threaten the highway facility. In other streams, however, bends may migrate at such a rate that the highway is threatened within a few years or after a few flood events. In such cases, the countermeasure should be installed during initial construction.