Lecture6 2 runoff and stream flow dung

Streamflow measurement • Stage height of water is measured readily at some point on a stream reach with staff gauge or water level logger pressure transducer... Streamflow measurement •

VNUF - 2016

Principles of

Watershed management

Lecture #6

Runoff and stream flow

Dr Bui Xuan Dung- Department of Environment Management

Stream or

river flow

measurement

Q

Streamflow

• Streamflow, discharge, runoff all mean the same

• Discharge is the most important info for the water resources

manager

• Peak flow data are needed for flood control planning

• Low-flow data are needed for estimating the dependability of water supply

• Total runoff needed for design purposes (e.g reservoir storage)

Discharge

• Q = Discharge (volume/time)

= flow, streamflow, stream runoff

• 1 m3/s = 35.32 cfs

Amount of runoff measured as

flow, streamflow, discharge (all the same thing)

• Large river discharge – 180 m3/s (6358 cfs)

• Small stream flow = 0.01 m3/s = 10 L/s (0.35 cfs)

Streamflow measurement

• Stage (height of water) is measured readily at some point on a stream reach with staff gauge or water level logger (pressure transducer)

Staff Gauge

Streamflow measurement

• Stage data are used for calculating discharge

• Common methods that use stage data for discharge calcs are:

• Stream gauging

• Flumes and weirs

• Empirical equations

Parshall flume

Water stage data logger monitor water level at Parshall flume

Where: H is water level; k, n

are constants

Discharge measurement

V-weir

V-notch weir

• Precalibrated structures for measuring flow

• Commonly used on small (< 800 ha) or experimental watersheds

• Can be constructed or concrete, metal, wood …

• Flume is an open channel built to contain flow in a cross section and length

• Weir is a small over flow dam used to alter stream flow so it can be measured

Flumes and Weirs

Flume

• HS, H, and HL flumes

• Developed by USDA NRCS

• Measure intermittent runoff

Flumes

• Parshall flume

• Developed by Dr Ralph L Parshall in 1922

• It measures water in open conduits and is also used for measuring irrigation water

• It has three sections: a converging section, a throat section and a diverging section

Flumes

http://www.brighthub.com/engineering/civil/articles/61462.aspx http://www.tracomfrp.com/parshall.htm

• Weirs include a notch control

• Stilling basin is formed upstream

• Stilling well and stage recorder

• Edge over which water flows is called

the crest

Weirs

How do you measure flow (“discharge”)?

• Q at each section = velocity * area

• Q at each section = velocity * distance * height

Stream gauging

River flow

Area

Velocity

Discharge (streamflow) is:

Q = Area (L2) times velocity (L T-1);

Units: L3 T-1 (e g., m3 s-1)

Q = ΣAiVi [ Q = Σ (A*V) in lab handout ]

= depth measurement locations

= velocity measurement locations

Stream discharge from velocity and depth measurements

Stream bank

Measuring discharge

Stream gauging

Boat equipped with Acoustic Doppler Current Profiler for calculating streamflow

Measuring river runoff

Steps:

1.Measure stream stage (height of water surface)

2.Measure discharge

3.Define relationship between stage and discharge

Image credit: Vermont EPSCoR CWDD ,

http://www.uvm.edu/~streams/index_old.php?Content=pages/2010_2011_Undergraduates

Measuring river runoff

Measuring river runoff

Measuring river runoff

RATINGS CURVE

Collect stage data continuously, transform it to discharge data

To get a bit of experience with stream gaging and analysis of stream data, visit

http://vcourseware4.calstatela.edu/VirtualRiver/FloodingDemo/index.html and play with it!!!

Manual Measurement of discharge

HYDROGRAPHS

AND RUNOFF RESPONSE

Some definitions:

Definitions

Uses of hydrographs

• Determine total volume of runoff

– Water use planning

– Land use planning

– Water storage planning

• Determine hydrologic response

– Rs = (Annual stormflow/annual ppt)100

• Determine runoff delivery ratio

– (Storm RO/ Storm PPt)100

Uses of hydrographs

• Compare seasonal differences in flow

– Annual hydrograph

• Compare annual differences in flow

– Many years record

• Know water level (for irrigation, rating…)

• Determine baseflow and stormflow

– Estimate groundwater contributions

– Determine runoff processes

• Saturation overland flow

– Saturated soil reaches surface

– Areas with high infiltration capacity

Rainfall and runoff Watershed 5

15 min ppt (mm) Runoff (L/s)

Rangelands runoff – river interactions

• River stage increased up to 0.3 m after a flashflood event that yielded a peak discharge of 17.9 m 3 s -1 at the tributary stage-measuring station

• Variable source area

– Area of saturation overland flow grows during storm

– Increasing area contributing runoff during

storm

Various baseflow

separation procedures

Hydrograph separation analysis 26

Different runoff processes vary

in terms of:

reactions, and hence water quality;

Predicting runoff pathways

which runoff processes will occur where:

Predicting runoff pathways:

Qualitative signs

Can measure erosion pedestals

Predicting runoff pathways:

Qualitative signs

at the base of slopes;

 Gleying (dark or gray color due to extended saturation);

 Mottling (iron oxides, manganese);

 Hydrogen sulfide smell;

Predicting runoff pathways:

Qualitative signs

 Field observations during storms:

by roads or other locations?

 Steep or flat?

 Convergent, planar or divergent?

Predicting runoff pathways:

Quantitative indicators

intensity and peak runoff:

Predicting runoff pathways:

Quantitative indicators

intensity and peak runoff:

Predicting runoff pathways:

Predicting runoff pathways:

Quantitative indicators

Predicting runoff pathways

is complex

than one mechanism, and the

importance of the different flow

pathways will change:

Factors controlling runoff pathways

Tell me the dominant runoff mechanism:

Runoff from

convergent versus planar hillslopes

Runoff: tussock (bunchgrass) vs pine (GH2)

Watershed condition and discharge

• As watershed area increases, discharge increases and lag time increases

• Bigger watersheds have larger water

storage capacity

Importance of runoff pathways

decreases with increasing basin size

and less time getting to the channel;

runoff patterns?

Watershed morphometry and

20 m

5 km

Max elevation:

30 m Min elevation: 10m

Watershed morphometry and