Bankfull / Effective / Dominant Discharge Brian Bledsoe Department of Civil and Environmental Engineering Colorado State University

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Why do we care? To simplify the world (and the design process) by selecting a single, surrogate discharge that best represents the integrated effects of a complex series of flow events

And… Consistency of reference Hydrological significance Among sites and over time Hydrological significance Bankfull stage can tend to occur within a range of recurrence intervals Morphological significance Bed load/flow measurements suggest that bankfull flow may transport the greatest amount of material over many years Point where the active channel stops and the floodplain begins (or the breakpoint between the processes of channel formation and floodplain formation)

Definitions: Bankfull Discharge: fills a stable alluvial channel to the elevation of the active floodplain Dominant Discharge: would produce the same channel geometry that is produced by the long-term hydrograph if constantly maintained in an alluvial stream over a long period of time Effective discharge: transports the largest percentage of the sediment load over a period of many years. Effective discharge is the peak of a curve obtained by multiplying the flood frequency curve and the sediment discharge rating curve Physical Theoretical Physical – but takes LOTS of data

Looking for clues… Stream/river engineering is an in-exact science at best Determination of bankfull stage/flow is one of the least exact tasks, but its one of the highest in importance

Rosgen (1996) considers bankfull discharge “ Rosgen (1996) considers bankfull discharge “...the single most important parameter in Level II classifications.”

Data used to determine bankfull stage Gauge data Regional curves Area history Field indicators Top of point/lateral bars Change in bank slope Vegetation clues Erosional features

Gauge Data Only place you may have flow record AND physical clues Observe “best” local indicators near gauges Relate to local, valley, and basin scale factors for potential extrapolation (in conjunction with other methods)

Regional Curves Regional Hydraulic Geometry Curves Data Required National Water Management Center (NWMC) ( http://wmc.ar.nrcs.usda.gov/technical/HHSWR/Geomorphic/ ) Water in Environmental Planning - Dunne and Leopold Data Required Drainage area (proportional to Qbf flow) Other dominant controls Land use Precipitation amounts History

Physiographic Provinces of Regional Curves CIVE 521 – Fall 2009

Regional curves showing bankfull dimensions by drainage area From NRCS - Stream Restoration Design National Engineering Handbook (2007)

Regional Curve Example Example results from Castro & Jackson (2001) Pacific NW Moderately close agreement between exponents on Q and DA CIVE 521 – Fall 2009

Regional Curve Example From Lawlor (2004) Western Montana data CIVE 521 – Fall 2009

Area History Recent flood and/or drought history Area geologic history Glaciation Native vs. transported material Deposition / Erosion history Piedmont example Settlement  Deforestation  Urbanization

Field indicators of Bankfull Stage Top of point/lateral bars Change in bank slope Vegetation clues Erosional features

Field Determination of Bankfull Discharge (Rosgen, 1996) The presence of a floodplain at the elevation of incipient flooding Elevation of the highest depositional features A break in slope of the banks Evidence of inundation (rock staining, exposed roots, vegetation change)

Williams (1978) Compared 16 different ways of determining the bankfull discharge: depositional features cross-section morphology vegetation, and others Bankfull discharge, as defined by the active floodplain elevation (36 sites), does not have a common recurrence interval (but does have a mode of 1.5 years)

Ease of measure Alluvial material Bedrock channel Connected to floodplain Snowmelt hydrograph Consistent history Sand & gravel dominated system Bedrock channel Incised system Flashy hydrograph Boulder or clay dominated system Complex history Easier ?

Effective Discharge The effective discharge is the discharge that transports the largest portion of the annual sediment yield over a period of years (Andrews, 1980)

Effective Discharge Calculation Product of Magnitude and Frequency - C Sediment Discharge Rating Curve - B Flow Frequency - A Discharge

max. flow 11043 min. flow 10 Q2 5724 Interval 0.0922148 Arithmetic Bins

Logarithmic Bins max. flow 11043 min. flow 10 Q2 5724 Interval 0.0922 Sand-bed river in Mississippi - Hotopha River

Arithmetic Bins max. flow 3760 min. flow 24 Q2 5724 Interval 186.8 Logrithmic bins

Effective Discharge Effective discharge is extremely sensitive to the methods employed: Arithmetic (gravel) vs. logarithmic (sand) approaches Number of classes for flows (# of bins) Selection of sediment transport relationship Modes of sediment transport across flows Overbank flows Availability and temporal density of flow data

Application Computation of effective Q provides another piece of the puzzle when morphologic indicators are sparse in unstable systems in circumstances of rapidly changing land use where significant changes in hydrology are expected

Take home message Bankfull Discharge = Field Measurement Dominant Discharge = Theoretical Effective Discharge = Computational Flow that moves the most sediment Product of Flow PDF and Sediment Transport