What stormwater managers should know about fluvial geomorphology

Robert Prager, P.E., Principal River Engineer, Intuition and Logic, St. Louis, Missouri; Presenter, 2003 APWA Congress
William Heatherman, P.E., Stormwater Engineer, City of Overland Park, Kansas; Presenter, 2003 APWA Congress

The old world of concrete pipes and ditches is giving way to a new era in stormwater, where riparian buffers, natural streams, and wetlands are the preferred approach to meeting flood, erosion control and water quality goals. Managing these natural systems can be a challenge for both city engineers and designers because the old, tried and true rules used to design the "hard" engineered systems no longer apply.

To understand how natural creeks work, stormwater managers and engineers need to become familiar with the science of fluvial geomorphology, a technical field that has been around for over 40 years, but has only recently found itself on the radar of most stormwater engineers.

The changing world of stormwater management
Until recently, the organizing principle of stormwater management has been to concentrate and convey runoff away from populated areas. This approach made sense when streams were the primary means of waste disposal. When the threat of typhoid or cholera looms, it is understandable to see how protecting the public from disease took precedence over other considerations. However, even as we developed better methods of sewage collection and disease control, we continued with our conveyance-based approach to stormwater management.

The consequences of this practice included widespread elimination of natural streams and their riparian corridors, as well as flooding and bank erosion issues that continue despite (or in some cases, because of) our reliance on hard armor and constructed channels. Driven by water quality and environmental concerns, and a better understanding of natural streams, it's time we look for other options that make engineering, financial and ecological sense.

Fluvial Geomorphology
Fluvial geomorphology is the science that describes how moving water shapes the land. All streams exist in a state of dynamic equilibrium, constantly adjusting their size and shape in response to the forces exerted by the flows of both water and sediment. To successfully manage streams for any purpose, we must understand how the streams will respond when we change the conditions around them.

Common activities such as land development, road or utility crossings, or channel alterations have predictable consequences. Accelerated runoff in urban streams is a consequence of increased impervious surface and the faster flow time through pipes and ditches. Streams typically respond to increased flows by first scouring out their bed material and thus "incising" or "downcutting." As the bed of the channel falls away, the stream banks may become unstable and slump into the river, where the soil is then carried away.

The bank scour is most pronounced along the outside of bends, which can cause the stream alignment to migrate, threatening adjacent homes or streets. As the stream migrates, it may eventually get too wide, causing normal depths of flow to become shallower, making it more difficult for the water to transport sediment. Point bars and small deposits begin to reform, particularly on the inside of bends. Eventually, something close to the original channel shape may reemerge, though at a lower elevation and in a different spot than where it started.

The cycle of incision, widening, and deposition is known as channel evolution. The process continues until the channel achieves an optimum cross section capable of transporting the flows of both water and sediment. Along the way, the migration of the channel may also change the curvature of the main channel enough to alter its total length, which in turn affects channel slope, which affects sediment transport. The factors are all intertwined.

The application of fluvial geomorphology allows us to make reasonable predictions about channel response and to avoid or minimize adverse responses. Advances in stream channel management and fluvial geomorphology have accelerated in the past two decades. In 1998, fifteen federal agencies jointly released Stream Corridor Restoration: Principles, Processes, and Practices, a comprehensive introduction to natural stream geomorphology and ecology (copies may be downloaded from www.usda.gov/stream_restoration).

For many designers and stormwater managers, it has been difficult to translate this technology into practice. Some of the practical recommendations that some geomorphologists might make include:

  • Update stormwater codes to encourage or require the preservation of natural streams and their buffer zones as the preferred method of conveyance.

  • Locate sanitary and utility crossings at "riffles" or natural weir points and protect them with grade control structures so they aren't exposed over time.

  • Use energy dissipaters at culverts and storm sewer outfalls.

  • Design culverts so that the width of opening at low flow more closely matches the original channel width, so that silt doesn't build up from frequent storms. Additional culvert openings may have raised flow lines for high flows only.

  • On flood control projects, focus on improving high-flow bypass in the floodplain instead of widened, deepened, or straightened main channels.

  • On bank stabilization projects, emphasize grade control, in-stream guides, and natural vegetation, sometimes combined with carefully planned "hard armor" in high stress locations.

  • Adjust detention strategies to consider smaller, more frequent storms, and to consider the total duration and volume of high flow, rather than merely the magnitude of the peak.

  • Adopt a cautious attitude towards construction in natural streams.
The authors will present an educational session at Congress entitled "Fluvial Geomorphology and Other Big Words Stormwater Managers Need to Know." The session begins at 4:00 p.m. on Sunday, August 24. Robert Prager can be reached at (314) 647-1161 or by e-mail at robert@ilincworld.com. Bill Heatherman can be reached at (913) 895-6050 or by e-mail at bheather@opkansas.org.