Public Comment

Commentary: Natural Creeks Need 30-Foot Buffer to Thrive By LAUREL COLLINS

Tuesday February 28, 2006

Maintaining a setback of 30 feet makes good sense for Berkeley creeks and anything less is short-sighted for long-term restoration, ecological health, and city maintenance requirements. 

Some particular points are as follows: 

1. All natural creeks have have a relatively predictable pattern, dimension, and profile relative to their “bankfull” discharge. An essential component to natural functioning streams is the dimensions of their bankfull width, and flood-prone width. Bankfull width is the width of the active channel at flows that occur about 1.3 to 1.7 years on average. The flood-prone width is measured at a height that is two times the maximum bankfull depth. This represents the width of the channel that would flood during the more rare, infrequent large floods. The flood-prone width for stable channels can often be greater than 2.2 times the bankfull width. For example, this means that a creek that has a bankfull width of 25 feet and a maximum bankfull depth of three feet, must have a flood-prone width of 55 feet or more at a height of six feet in order to maintain a stable configuration, i.e., not aggrade or degrade its bed. However, it may laterally migrate. 

2. To prevent excessive lateral migration, natural channels typically need a healthy riparian corridor (along with the appropriate flood-prone width). Large roots add to the cohesion of banks, which help the bank resist erosion during large floods. An approximately 30-foot-wide buffer might make better sense for the rooting requirements and canopy sizes of healthy, mature riparian vegetation. Furthermore, if a healthy riparian canopy exists, structures will need to have a setback of 30 feet for clearance purposes to meet the code for fire hazard reduction. 

3. Large mature riparian vegetation will benefit streams not only for stability purposes, but for also adding shade and thereby maintaining cool water temperature for fish. A mature riparian corridor is important for long-term recruitment of large woody debris, essential for increasing diversity and structure of pools for fish habitat and other aquatic organisms. 

4. Many streams in Berkeley are highly entrenched and are eroding their nearly vertical banks. This is often an indication that they are trying to increase their flood-prone width. In our urban setting this is often the picture: a narrow, very deep channel, lined with various concrete walls, sacrete bags, rock riprap, or other types of revetments that are in various stages of disrepair, often collapsing into the channel. One of the restoration techniques available to deal with this kind of instability is to lay back the banks at a more stable angle and create an appropriately wide flood-prone width that permits the growth of riparian vegetation. Inner floodplain benches are often created. In order to apply this kind of restoration strategy that provides for the most natural kinds of stream functions, we need to have an appropriate amount of space available. Reducing the setback width will prevent future restoration efforts from being as successful as possible. Instead they will require expensive artificial retaining structures that will require costly maintenance in perpetuity. 

5. A 30-foot setback width allows access for large equipment along a creek for conducting repairs, maintenance, or restoration. A 10-foot setback limits this capability. 

6. Even in urban settings vegetation exceeds the benefits of concrete or other artificial structures that cause increases in water velocity and thereby increase erosive forces on the channel bed and induce more downcutting. As a result of increased velocity, flood peaks and thus flood risks are increased to the downstream urban setting. Having the future option to widen the flood-prone width of entrenched channels allows us or future generations to reduce the risks of urban flooding and property loss from bank erosion. This future option should not be eliminated by short-sighted planning. If structures are allowed to be built to within 10 feet of the banks, the channels will ultimately require expensive structural solutions that eventually fail and/or require expensive maintenance. Many of Berkeley’s streams are already underground, in storm drains or culverts. We are already dealing (or not dealing) with mistakes of the past. Creating a 10-foot buffer will likely lead to more “undergrounding” of streams that are no longer visible or viable. Natural processes that maintain stream functions are not limited to 10 ft buffers. 

7. Ideally, buffer size should be based upon appropriate flood-prone width and ecological requirements. These are founded in good science not politics. 


Laurel Collins is principal geomorphologist at Watershed Sciences in Berkeley.›