Abstract: Session B  8:30 am (BACK)

Are Our Floodplains Connected?  Our Understanding of Return Intervals and Implications on Pollutant Load Removal

Robert Siegfried
Whitman, Requardt & Associates, LLP
Richmond, Virginia

Authors:  Robert Siegfried and Michael Hearn, P.E.

Our understanding of stream – floodplain interactions is critical to the restoration of healthy streams and floodplains. A natural flow regime can be described by a variety of characteristics including: magnitude, frequency, duration, season, and rate of change (i.e. flashiness). Many coastal plain streams in the Mid-Atlantic access their floodplains 5-20 times per year, sometimes for days or weeks at a time. Their floodplains are extensive forested wetlands which help to trap nutrients and sediment, provide wildlife habitat and reduce flood peaks. But frequent floodplain access is not limited to the coastal plain. Research in headwater streams in the mountains of North Carolina also shows out-of-bank flows occurring 3-5 times per year. Frequent floodplain access appears to be the natural flow regime for stable streams throughout much of the Mid-Atlantic.

Most traditional stream design has focused on only one of these natural flow regime variables – magnitude (i.e. discharge). During the design process, Qbkf maybe assumed to be equivalent to a return interval of Q 1.2 to Q 1.5. This perception may lead designers to assume that a healthy stream should access its floodplain on average no more once per year. This misperception is often derived from 1) the statistic methods used to calculate return intervals, and 2) a lack of understanding of natural floodplain-stream interactions in the Mid-Atlantic.

Return interval discharges can be calculated using the Annual Maximum series which is suitable for large flows, or the Partial Duration series, which is suitable for flows with a return interval of < 2 years. Many regional curve studies along the East Coast report the return interval of Qbkf based on the Annual Max Series even though this statistic is not valid for bankfull discharges. This procedural issue may lead to stream designs with floodplain flows occurring significantly less frequently than found in natural flow regimes.

The Chesapeake Bay Program recently approved the concept of stream restoration pollutant removal rates, in part, being based on frequency of floodplain access. Streams designed to mimic the frequent floodplain access that research shows is common in Mid-Atlantic streams will maximize this pollutant removal mechanism. Localities that design streams well connected to their floodplains will find that stream restoration is one of the most cost effective BMPs available to them to meet the Bay TMDL.