Bankfull / Effective / Dominant

Discharge

Brian Bledsoe

Department of

Civil and Environmental Engineering

Colorado State University

Watch these videosif you haven’t already

• http://www.stream.fs.fed.us/publications/videos.html

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

Why do we care?

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

Physical – but takes LOTS of data

Looking for clues…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

CIVE 521 – Fall 2009

Physiographic Provinces of Regional Curves

• Regional curves showing bankfull dimensions by drainage area

• From NRCS - Stream Restoration Design National Engineering Handbook (2007)

CIVE 521 – Fall 2009

Regional Curve Example

Example results from Castro & Jackson (2001) Pacific NW

CIVE 521 – Fall 2009

Regional Curve Example

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

• 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

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 DischargeCalculation

Flow

Fre

quen

cy -

A

Pro

duct

of M

agni

tude

and

Freq

uenc

y - C

Effective Discharge

Sediment D

ischarge R

ating C

urve - B

Discharge

CLASS Bin Freq PROB of AVG Q PROB of SED RATESED RATEPRODUCTOCCUR EXCEED(PDF) (CFS) (CDF) (CFS) (Tons/Day)(Tons)

A B C = A*B1 0 0 0.00 0.00 100.00 0.0000 0.00 0.002 24.00 14 0.04 12.00 99.96 0.0002 0.66 0.003 27.97 339 0.97 25.99 98.99 0.0005 2.11 0.024 32.60 2364 6.75 30.29 92.24 0.0006 2.66 0.185 38.00 4637 13.25 35.30 78.99 0.0008 3.35 0.446 44.29 4204 12.01 41.14 66.98 0.0010 4.22 0.51

19 324.31 229 0.65 301.28 3.45 0.0196 84.63 0.5520 377.98 153 0.44 351.14 3.01 0.0247 106.58 0.4721 440.54 136 0.39 409.26 2.62 0.0311 134.23 0.5222 513.45 133 0.38 477.00 2.24 0.0391 169.06 0.6423 598.43 108 0.31 555.94 1.93 0.0493 212.92 0.6624 697.48 64 0.18 647.95 1.75 0.0621 268.15 0.4925 812.91 60 0.17 755.19 1.58 0.0782 337.71 0.5826 947.45 68 0.19 880.18 1.39 0.0985 425.32 0.8327 1104.26 115 0.33 1025.86 1.06 0.1240 535.66 1.7628 1287.02 101 0.29 1195.64 0.77 0.1562 674.62 1.9529 1500.03 63 0.18 1393.53 0.59 0.1967 849.63 1.5330 1748.30 32 0.09 1624.17 0.50 0.2477 1070.04 0.9831 2037.65 29 0.08 1892.98 0.41 0.3120 1347.64 1.1232 2374.90 33 0.09 2206.27 0.32 0.3929 1697.24 1.6033 2767.95 37 0.11 2571.43 0.21 0.4948 2137.54 2.2634 3226.07 58 0.17 2997.01 0.05 0.6232 2692.06 4.4635 3760.00 17 0.05 3493.03 0.00 0.7848 3390.44 1.65

35006 100 0.69854 31.70

max. flow 11043min. flow 10Q2 5724Interval 0.0922

Log

arit

hmic

Bin

s

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

0 500 1000 1500 2000 2500 3000 3500 4000

Discharge (cfs)

Se

dim

en

t Y

ield

(T

on

s)

Sediment Transport Rate (tons/day) Sediment (Tons)

Bin Freq PROB of AVG Q PROB of SED RATESED RATEPRODUCTOCCUR EXCEED(PDF) (CFS) (CDF) (CFS) (Tons/Day)(Tons)

A B C = A*B1 24 14 0.0000 12.00 100 0.0000 0.00 0.002 210.80 32807 93.7182 117.40 6.28 0.0047 20.47 19.183 397.60 1175 3.3566 304.20 2.93 0.0199 85.87 2.884 584.40 319 0.9113 491.00 2.01 0.0409 176.59 1.615 771.20 114 0.3257 677.80 1.69 0.0664 286.97 0.936 958.00 98 0.2800 864.60 1.41 0.0958 414.03 1.167 1144.80 128 0.3657 1051.40 1.04 0.1287 555.87 2.038 1331.60 98 0.2800 1238.20 0.76 0.1646 711.11 1.999 1518.40 47 0.1343 1425.00 0.63 0.2034 878.69 1.18

10 1705.20 29 0.0828 1611.80 0.55 0.2449 1057.80 0.8811 1892.00 20 0.0571 1798.60 0.49 0.2888 1247.74 0.7112 2078.80 16 0.0457 1985.40 0.44 0.3352 1447.94 0.6613 2265.60 18 0.0514 2172.20 0.39 0.3838 1657.92 0.8514 2452.40 15 0.0428 2359.00 0.35 0.4345 1877.24 0.8015 2639.20 16 0.0457 2545.80 0.30 0.4874 2105.54 0.9616 2826.00 36 0.1028 2732.60 0.20 0.5422 2342.48 2.4117 3012.80 24 0.0686 2919.40 0.13 0.5990 2587.76 1.7718 3199.60 14 0.0400 3106.20 0.09 0.6577 2841.12 1.1419 3386.40 7 0.0200 3293.00 0.07 0.7181 3102.31 0.6220 3573.20 5 0.0143 3479.80 0.06 0.7803 3371.11 0.48

max. flow 3760min. flow 24Q2 5724Interval 186.8

Arithmetic 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

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

Application

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