Drainage channel errosion
Flow in drainage channels is accompanied by natural processes of erosion and deposition. Deepening the channel and reducing resistance to flow during maintenance can trigger subsequent channel erosion. This erosion may occur quickly or slowly, depending on specific characteristics of the material properties of the channel, the condition of bank vegetation, and the erosive energy of flow within the channel. Over time, however, erosion can become severe enough to compromise the structural integrity of the drainage channel.
For a full assessment the reviewer should consider a detailed inventory of channel bed and bank conditions, including the location, type and mechanism of bank failures within the channel, and an evaluation of the probable factors producing this erosion.
Numerous rapid assessment procedures exist for evaluating bank stability. This information, in conjunction with engineering considerations about appropriate bank treatments, including recently developed bioengineering approaches that provide for long-term natural maintenance approaches to channel stability, are possible alternatives to traditional bank stabilization methods. What is needed is the identification of effective erosion-control practices for specific sites of concern within the channel system.
An assessment of erosion generated by conditions external to the channel must also be considered.
Spoil from past maintenance has been piled along drainage channels, creating pronounced artificial levees. The tops of these levees often stand above the surrounding farm fields. Although such levees have the advantage of increasing the capacity of the channel during high flows, they also tend to pond water on the field side of the levee during periods of storm runoff from adjacent fields. Without adequate provision for drainage of areas on the field side of elevated levees, ponding can be substantial and can threaten to inundate crops in adjacent fields or compromise the bank/levee, leading to failure during a flood. To manage this issue, information is needed both on the elevations of levees along the channel and on the number of surface drains on the field side of the levees. Moreover, in some cases, drainage of ponded water locally through the levees can produce substantial erosion of the levee and channel bank. In extreme cases, such erosion can produce local failure of the channel bank, promoting the development of a tributary gully that erodes headward toward the adjacent field. Locations where the integrity of the levees has been compromised by erosion from ponded water on the field side of the levees should be systematically mapped and evaluated to determine appropriate erosion-control techniques.