Drainage Channel Maintenance

A maintenance plan for a drainage channel also requires consideration of short-term and long-term responses of the drainage channel to maintenance, particularly when the investment of limited financial resources is considered. Additionally the possibility of local channel maintenance having system-wide effects must be assessed.

The most critical factor related to these issues is the influence of natural processes of erosion and deposition within modified channels. In most cases, the channel created through maintenance is much different than the channel natural processes would produce if allowed to operate without inference. Over time, these processes work to reconfigure the bottom of the channel to generate a self-sustaining form.

A good set of channel cross sections will provide a starting point for this analysis, but it can be supplemented with field data on the response of existing drainage channels to past maintenance. The primary reason to collect this information is economic. If maintenance produces a situation that will result in a return to reduced drainage channel capacity in a short time then the real, or annual, cost to taxpayers is increased. It is simply prudent to consider alternatives that are more sustainable by natural processes to avoid the need for costly frequent maintenance or inefficient expenditures of funds.

If maintenance produces an expected instability in tributaries, upstream or downstream, then this is important information both in maintenance planning and a common sense good neighbor policy for any district. This information is particularly important to address issues of possible sediment pollution produced by the planned maintenance.

The development of silt bars, or benches, within the channel bottom reflects natural adjustments to maintenance. The development of bars or benches creates an inset channel that is sustainable by natural erosional and depositional processes. Recent scientific studies of bars and benches in the Spoon-River channel in Champaign County, a tributary to the Salt Fork, indicate that these features: 1) are stable, and 2) generally lie below the levels of tile outlets. Studies also revealed that these bars and benches provide needed habitat for fish. Studies also indicate that removal of benches during channel maintenance often results in rapid redevelopment of these features after maintenance. Thus the short-term drainage benefit achieved from removing the benches may be rapidly offset by bench redevelopment. Over the long term it may be more economically efficient to retain stable benches in channels, rather than expend funds frequently for maintenance efforts that are fundamentally at odds with natural erosional and depositional processes within the channel. Benches also help to confine low-flow in the large drainage channel within a well-defined small channel in the center of the large channel, thereby protecting banks of the large channel from erosion.

Any action locally within a river system can produce responses upstream and downstream of the affected area. An evaluation should be made of possible effects of maintenance on downstream flooding and sedimentation. Deepening of a main channel can also trigger erosional responses in tributaries draining into the main channel, thereby causing channel stability problems upstream and deposition within the main channel that could offset sediment removal during maintenance and contribute to sedimentation problems downstream.

Leave a Comment

Previous post:

Next post: