We have believed in multiple use concepts at UDFCD since the early 1970's. Stream or channel corridors can and have been used for more than just flood control. They can also serve as open space corridors, hiker/biker trail corridors and habitat corridors. That thinking has paid off as the Denver metro area has many miles of drainageways that are available for other uses besides drainage and flood control.
In the early days we thought a grass-lined channel with 4:1 side slopes, a concrete low flow channel, and a trail provided for multiple uses. And it did and still does. The trend today, however, is for preservation of natural stream areas and, where possible, restoration of deteriorated urban streams to more "natural" conditions; or where right-of-way is limited, construction of grass-lined channels without any hard bottom. Preservation of a stream corridor has to take place as the land is developed. Many developers wish to develop as much of their available land as possible, so they are looking for ways to minimize the space taken for drainageways. Pressure is increasing, however, for developers to leave the corridors in as natural a state as possible and some developers are moving in this direction. The reality is that the public or the consumer likes riparian corridors and they can enhance a development and be an asset and not a liability.
There are still many situations where homes and businesses are situated in floodplains and some sort of remedial action needs to be taken. Removal of structures from the floodplain and restoration of the channel are options that are looked at very seriously. It is not always possible, however, and in many cases the most appropriate solution is to construct some sort of "soft" channel with water trickling through the middle of it. Our approach now is to make the channel look as natural as possible within the constraints of the available right-of-way. In some cases, the stream can be made to look quite natural, but in other situations, a more formal grass-lined channel is constructed. However, in most of these cases a hard bottom low flow channel is avoided in lieu of a low flow area that will allow surface waters to interchange with ground waters. So the changing face of flood control is away from formalized drainage channels to more natural channels.
However, it is a simple fact that as undeveloped rural areas are converted into urban uses the hydrology and hydraulics of the streams change dramatically. In spite of detention systems that are almost universally required, low flows tend to increase dramatically both in terms of frequency and magnitude. Even if a stream corridor is left open and natural, erosion will eventually become a problem and erosion control measures will eventually be needed. We have addressed this problem through our maintenance program and much of the work that maintenance does is erosion control. Projects can take the form of bank protection or grade control structures. In some cases, like through formal parks, low flow channels have been lined with large boulders. In other situations, such as natural stream corridors, spot bank protection may be necessary at isolated locations or drop structures may need to be constructed to arrest channel degradation. Our drop structure of choice has been one constructed of large boulders that are grouted in place that allow water to fall over them in a waterfall type fashion. These structures can be located at intervals while leaving the rest of a natural stream alone.
Also, stormwater quality is now a factor that must be considered. In the early 1970s stormwater quality was not an issue. Stormwater quality places an emphasis on the more frequent runoff events; whereas, flood control places the emphasis on the infrequent large flooding events. We are finally recognizing the obvious by paying more attention to what happens to the streams because of the more frequent events. Where we had at one time detention ponds to store water to reduce the downstream quantity of water, we are now incorporating into these detention ponds water quality features as well an attempt to improve the quality of stormwater.
The basic trend now is to preserve stream corridors and improve the water quality as much as possible. We are not always able to accomplish stream preservation, but it is always given consideration. In addition we are trying to restore streams where possible; and, where it is not possible, to provide as much opportunity for interaction between the water and people as possible. It still has to be recognized that urban conditions warrant urban solutions, which require careful engineering as well as development of natural flora and fauna. What we are doing now will become a more and more valuable asset as urban areas expand and open space becomes more and more limited. It will be interesting, say in 25 years, to look back and see how good the decisions are that we are making today. In looking back over the last 25 years we have been doing a pretty good job.