Bankfull / Effective / Dominant Discharge Brian Bledsoe Department of

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

2. Watch these videosif you haven’t already • http://www.stream.fs.fed.us/publications/videos.html

5. 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

6. And… • Consistency of reference • 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)

7. Definitions: Physical • 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 Theoretical Physical – but takes LOTS of data

8. 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

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

11. 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

12. 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)

13. Regional Curves • Regional Hydraulic Geometry Curves • 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

14. Physiographic Provinces of Regional Curves CIVE 521 – Fall 2009

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

16. Regional Curve Example Example results from Castro & Jackson (2001) Pacific NW CIVE 521 – Fall 2009

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

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

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

20. 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)

21. 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)

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

23. 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)

24. Effective Discharge Product of Magnitude and Frequency - C Sediment Discharge Rating Curve - B Flow Frequency - A Effective DischargeCalculation Discharge

25. Logarithmic Bins

27. Arithmetic Bins

31. 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

32. 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

33. 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