Bankfull /
Effective /
Dominant
Discharge
Brian Bledsoe
Department of
Civil and Environmental
Engineering
Colorado State University
Watch these videos
if you haven’t already
• http://www.stream.fs.fed.us/publications/videos.html
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
– 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
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
“...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
– 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
CIVE 521 – Fall 2009
Regional Curve Example
CIVE 521 – Fall 2009
From Lawlor (2004)
Western Montana data
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
• Field indicators
–
–
–
–
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
Easier ?
•
•
•
•
•
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
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
Discharge
Effective
Discharge
Logarithmic Bins
max. flow
min. flow
Q2
Interval
11043
10
5724
0.0922
CLASS
1
2
3
4
5
6
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
Bin
0
24.00
27.97
32.60
38.00
44.29
324.31
377.98
440.54
513.45
598.43
697.48
812.91
947.45
1104.26
1287.02
1500.03
1748.30
2037.65
2374.90
2767.95
3226.07
3760.00
Freq
0
14
339
2364
4637
4204
229
153
136
133
108
64
60
68
115
101
63
32
29
33
37
58
17
35006
PROB of
OCCUR
(PDF)
A
0.00
0.04
0.97
6.75
13.25
12.01
0.65
0.44
0.39
0.38
0.31
0.18
0.17
0.19
0.33
0.29
0.18
0.09
0.08
0.09
0.11
0.17
0.05
AVG Q
(CFS)
0.00
12.00
25.99
30.29
35.30
41.14
301.28
351.14
409.26
477.00
555.94
647.95
755.19
880.18
1025.86
1195.64
1393.53
1624.17
1892.98
2206.27
2571.43
2997.01
3493.03
100 0.69854
PROB of SED RATESED RATEPRODUCT
EXCEED
(CDF)
(CFS)
(Tons/Day)
(Tons)
B
C = A*B
100.00
0.0000
0.00
0.00
99.96
0.0002
0.66
0.00
98.99
0.0005
2.11
0.02
92.24
0.0006
2.66
0.18
78.99
0.0008
3.35
0.44
66.98
0.0010
4.22
0.51
3.45
0.0196
84.63
0.55
3.01
0.0247
106.58
0.47
2.62
0.0311
134.23
0.52
2.24
0.0391
169.06
0.64
1.93
0.0493
212.92
0.66
1.75
0.0621
268.15
0.49
1.58
0.0782
337.71
0.58
1.39
0.0985
425.32
0.83
1.06
0.1240
535.66
1.76
0.77
0.1562
674.62
1.95
0.59
0.1967
849.63
1.53
0.50
0.2477 1070.04
0.98
0.41
0.3120 1347.64
1.12
0.32
0.3929 1697.24
1.60
0.21
0.4948 2137.54
2.26
0.05
0.6232 2692.06
4.46
0.00
0.7848 3390.44
1.65
31.70
5000
4500
Sediment Transport Rate (tons/day)
Sediment (Tons)
Sediment Yield (Tons)
4000
3500
3000
2500
2000
1500
1000
500
0
0
500
1000
1500
2000
Discharge (cfs)
2500
3000
3500
4000
max. flow
min. flow
Q2
Interval
Arithmetic Bins
3760
24
5724
186.8
Bin
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
24
210.80
397.60
584.40
771.20
958.00
1144.80
1331.60
1518.40
1705.20
1892.00
2078.80
2265.60
2452.40
2639.20
2826.00
3012.80
3199.60
3386.40
3573.20
Freq
14
32807
1175
319
114
98
128
98
47
29
20
16
18
15
16
36
24
14
7
5
PROB of
OCCUR
(PDF)
A
0.0000
93.7182
3.3566
0.9113
0.3257
0.2800
0.3657
0.2800
0.1343
0.0828
0.0571
0.0457
0.0514
0.0428
0.0457
0.1028
0.0686
0.0400
0.0200
0.0143
AVG Q
(CFS)
12.00
117.40
304.20
491.00
677.80
864.60
1051.40
1238.20
1425.00
1611.80
1798.60
1985.40
2172.20
2359.00
2545.80
2732.60
2919.40
3106.20
3293.00
3479.80
PROB of SED RATESED RATEPRODUCT
EXCEED
(CDF)
(CFS)
(Tons/Day)
(Tons)
B
C = A*B
100
0.0000
0.00
0.00
6.28
0.0047
20.47
19.18
2.93
0.0199
85.87
2.88
2.01
0.0409
176.59
1.61
1.69
0.0664
286.97
0.93
1.41
0.0958
414.03
1.16
1.04
0.1287
555.87
2.03
0.76
0.1646
711.11
1.99
0.63
0.2034
878.69
1.18
0.55
0.2449 1057.80
0.88
0.49
0.2888 1247.74
0.71
0.44
0.3352 1447.94
0.66
0.39
0.3838 1657.92
0.85
0.35
0.4345 1877.24
0.80
0.30
0.4874 2105.54
0.96
0.20
0.5422 2342.48
2.41
0.13
0.5990 2587.76
1.77
0.09
0.6577 2841.12
1.14
0.07
0.7181 3102.31
0.62
0.06
0.7803 3371.11
0.48
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