Engineered, man-made channels are typically built with an established/documented depth and width.
Let’s call that Original Configuration A. Over time, a portion of that depth and width may be partially impacted with layers of sediment and other material that ends up in drainage channels, such as vegetative debris and trash. Channels can also erode over time. This erosion could be a source of material in a channel. Let’s call that Eroded Configuration B.
While sediment generally tends to shift each time there is a major rain event, causing little impact on flood risk, in extreme cases this accumulated material potentially could reduce “channel conveyance,” which is the channel’s ability to move stormwater. Erosion of the channel side slopes could also potentially impact channel conveyance.
A maintenance project removes the sediment and repairs erosion, restoring channel conveyance and returning the channel to its Original Configuration A. This is potentially better for stormwater conveyance than Eroded Configuration B, but it does not widen or deepen BEYOND the Original (engineered) Configuration A.
In other words: Let’s say a channel is originally designed and built with a 1 percent (100-year) level of service. Accumulated sediment or erosion reduces that level of service to something less than the 1 percent (100-year) level of service. Removing the sediment and repairing the erosion allows the channel to carry more stormwater than it did WITH the sediment or erosion, but it is still no more than the original 1 percent (100-year) level of service.
Maintenance is positive, but it has limits.
Maintenance projects, in general, do not require the same degree of environmental permitting and mitigation as channel widening/deepening/straightening/concrete-lining, because maintenance projects do not alter the channel’s original configuration.
Maintenance can also take place on a non-engineered, natural channel, but that is a much more rigorous, time-consuming process, because of environmental permitting and compensatory mitigation requirements.