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

2. Watch these videos if you haven’t already

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 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)

7. 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

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 “
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 Data Required
National Water Management Center (NWMC)
( )
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
Moderately close agreement between exponents on Q and DA
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
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. 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 ?

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

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

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

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

32. 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

33. 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

34. 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