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Streamflow analysis and modelling

Streamflow analysis

•  Analysis of streamflow data is important aspect of hydrology –  describes flow regime –  enables comparison between rivers –  predicts possible future river flow

•  The most important methods of streamflow analysis are: –  hydrograph analysis –  flow duration curves –  frequency analysis

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Stream Hydrograph •  Hydrograph – a plot of stream discharge at a point over time

–  Records fluctuations in discharge or stream height over time –  Useful tool to monitor stream behavior remotely

•  Creating the Hydrograph - plot discharge or stage on the vertical axis; plot time on the horizontal axis

Stream Hydrograph

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Stream Hydrograph

Stream Hydrograph

Fraser River at Steveston

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Stream Hydrograph

Stream Hydrograph

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Stream Hydrograph

Hydrograph

Q = volume of water passing a plane in a unit of time

Q

watershed

Q = stream discharge

•  a plot of stream discharge at a point over time

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Hydrograph •  Stream discharge is influenced by five important components

–  Channel width –  Channel depth –  Velocity –  Gradient –  Bed roughness

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Stream Discharge (Q)• Five important

conditions:

– Channel width

– Channel depth

– Velocity

– Gradient

– Bed roughness

Q = v x A (depth x width)

A

v

Discharge (m3/s) = width (m) × depth (m) × velocity (m/s)

water cross section x velocity

30 m3/s 180 m3/s

Stream Flow Continuity

Q = v A

• Given constant discharge

–Reduction in area ==> faster flow

–Changes in slope and roughness influence velocity and mustbe compensated by change in area

• A balance of driving

and resisting forces

determines the nature

of river flow :

Driving force = water weight x sin(bed slope)

Resisting force = river-bed area x river-bed shear stress

Q = v x A (depth x width) Discharge (m3/s) = width (m) x depth (m) x velocity (m/s)

Hydrograph •  Base flow occurs during times with no significant precip

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Hydrograph •  Base flow occurs during times with no significant precip

Q = stream discharge Time

Hydrograph

QQ (cfs)

Base Flow is sustained streamflow in between rain events

base flow

Hydrograph

Austin Creek

In lowland streams, baseflow is primarily sustained by groundwater

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Hydrograph

In mountain streams, baseflow is primarily sustained by snowmelt, glacier melt, and groundwater

Deming Glacier and the Middle fork of the Nooksack River

Photo by John Scurlock

Hydrograph •  Storm flow occurs during and immediately after significant rainfall events

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Hydrograph •  Rising limb represents increase in discharge due to gradual build-up of

storage in catchment

Hydrograph •  Rising limb represents increase in discharge due to gradual build-up of

storage in catchment

Q = stream discharge

Q

Time

Hydrograph

Q (cfs) rising flow

rain

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Hydrograph •  Receding limb represents withdrawal of water from storage build-up

during early phase

Hydrograph •  Receding limb represents withdrawal of water from storage build-up

during early phase

Q = stream discharge

Q

Time

Hydrograph

Q (cfs)

receding flow

rain stops

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Hydrograph •  Base flow occurs during times with no significant precip

Q = stream discharge Time

Hydrograph

QQ (cfs)

Base Flow is sustained streamflow in between rain events

base flow

Hydrograph •  Most often the recessional limb is broader than rising limb thus most

flood hydrographs are skewed

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Hydrograph •  Lag period between peak rainfall and peak discharge

Hydrograph •  Peak flood discharge is highest point on hydrograph when greatest

amount of water is in river

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Hydrograph •  Bankfull discharge occurs when river is just contained within its banks

Hydrograph •  Basin lag refers to the time needed for water to discharge through the basin •  The larger the basin the slower the discharge will be, the lower the peak will

be •  Floods are more frequent in small basins, but also dissipate faster; in large

basins floods are less frequent, but when they occur it can take a long time to discharge all the water

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Hydrograph

Flood Hydrograph •  Flood hydrographs show reaction of streams to excess inflow often due

to access rainfall

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Stream Hydrograph •  Hydrographs allow easy comparison between rivers, but also between

years or extended periods for the same river

Stream Hydrograph •  Hydrographs allow easy comparison of river behaviour due to

changes in its catchment

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Stream Hydrograph

Stream Hydrograph

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