Academic journal article Southeastern Geographer

Bankfull Indicators in Small Blackwater Streams in Peninsular Florida: Reliability and Relations with Hydrology

Academic journal article Southeastern Geographer

Bankfull Indicators in Small Blackwater Streams in Peninsular Florida: Reliability and Relations with Hydrology

Article excerpt

INTRODUCTION

Bankfull stage, or the elevation at which the stream just begins to overflow onto its floodplain, is considered an important threshold in geomorphology. Bankfull discharge, the streamflow associated with that stage is considered "the most effective streamflow for moving sediment, forming or removing bars, forming or changing bends and meanders, and generally doing work that results in the average morphological characteristics of channels" (Dunne and Leopold 1978). It is related to meander geometry characteristics (Ackers and Charlton 1970), and is the breakpoint between processes of channel formation (erosion) and floodplain formation (deposition) (Copeland et al. 2000). The floodplain is defined as the relatively flat, depositional surface adjacent to the stream that is being built and rebuilt by a stream in the present hydrologic regime (Emmett 2004).

Field identification of bankfull stage is the method most often used to estimate the channel-forming flow, though its correct identification in the field can be difficult and subjective (Williams 1978; Johnson and Tei11996; Knighton 1998), in part because there are many different indicators and there are also streams where several benches or geomorphic breaks (i.e. sudden transitions such as change in slope) exist (Woodyer 1968). The U.S. Forest Service has published a field guide for both determining bankfull stage and conducting a stream channel survey (Harrelson et al. 1994).

Bankfull stage has been related to flood frequency in the literature, considered to average a 1.5 year return period on the annual maximum series in the United States, ranging from 1.01 to 32 years (Leopold et al. 1964). Examining a wide range of rivers and streams, Williams (1978) confirmed that the modal recurrence interval was about 1.5 years on the annual maximum series and about 1.0 years on the partial duration series, finding a similar mode but also several stations above and below these values.

The physical environment largely controls species composition and abundance of stream-dependent fauna (Allan 1995; Gordon et al. 2004), thus restoring streams to a more stable and biologically productive state has become a priority for many government agencies and private organizations, and approximately $10 billion has been spent on 30,000 river restoration projects in the United States as of 2004 (Malakoff 2004). More knowledge of channel form including the bankfull levels and the flow duration and flood frequency of these systems has the potential to assist in creating target natural channel designs for stream restoration efforts. Because many Florida streams have been degraded due to land use changes and channel and floodplain alterations, examining the least impacted streams in peninsular Florida will provide the necessary data to implement natural channel designs as a stream restoration technique in Florida. These data will be useful to public agencies, as well as to private industries such as the phosphate mining industry. The latter is required to rebuild topography and streams on the landscape after removing vegetation and several meters of overburden and phosphatic sediments.

Although there have been studies of bankfull indicators elsewhere in the southeastern United States Coastal Plain including northwest Florida (Metcalf 2004), Alabama (Metcalf 2005), Georgia (Buck Engineering 2004), North Carolina (Doll et al. 2003; Sweet and Geratz 2008), and Virginia and Maryland (Krstolic and Chaplin 2007), no prior work has been conducted in peninsular Florida. Metcalf (2004) examined some bankfull indicators and published regional curves for "Florida streams," yet his study sites were confined to extreme north Florida and the Panhandle and even included sites in Georgia and Alabama. Thus these relationships may not be applicable to streams in peninsular Florida, as it is quite different in physiography, geological context, and rainfall patterns (Fernald and Purdum 1992). …

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