Instream flow assessment in New Zealand

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Flow assessment framework

Morphology

Evaluate for changes in flow

Water qualityMethods andparameters

DOTemperature

Biologicalevaluation

Water qualitysuitability

Flow regime requirementsMethods Flushing flow analysis (shear stress)

Fluctuating flow habitat analysis Sediment deposition

Habitatsuitability

Velocity/depthHydraulic geometry

2d modelsStage/Discharge

Water surface profile

Flow assessment based on flow responsecurves of biological indicators

Instream habitat

• Habitat suitability is a widely used concept e.g., angling and hunting

• No suitable habitat - no fishSuitable habitat - maybe..

Three basic life requirements for fish and benthic invertebrates that

live in flowing water

• Velocity

• Depth

• Cover

New Zealand adult brown trout (Hayes & Jowett 1994)

Depth (m) Velocity(m/s)

0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.0

0.2 0.2

0.4 0.4

0.6 0.6

0.8 0.8

1.0 1.0

Adult brown trout (Raleigh et al. 1984)

Depth (m) Velocity(m/s)

0.0 0.3 0.6 0.9 1.2 1.5 0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.0

0.2 0.2

0.4 0.4

0.6 0.6

0.8 0.8

1.0 1.0

Offset (m)

0 4 8 12 16 20 0.0

2.0 -2.00

5.3 m3/s

11 m3/s

Prediction of depth and velocity

Predicted water level and depth

Predicted velocity

Level at 5.3 m3/s

Level at 11 m3/s

Habitat quality across river

1 m3/s

6 m3/s

11 m3/s

Discharge (m3/s)0 5 10 15 20 25

Maximum

Point where habitatbegins to reduce sharply

Usable habitat summed over reach for each flow

NZ case studies

• River flows are affected by :– Hydroelectric diversion,– Hydroelectric flow control, or– Irrigation diversion

• Studies between 1979 and 2001

• Trout, benthic invertebrate, and native fish goals

1. Description of river and flows

2. Habitat analysis and flow recommendation

3. Before and after photographs

4. Biological response

For each project:

• Lake fed river - natural mean flow 450 m3/s

• River diverted for hydropower generation in 1976

• Minimum flow of 0.3 m3/s since dam construction

Waiau RiverHydroelectric diversion

Discharge (m3/s)0 5 10 15 20 25 30

Adult brown trout

Food production

Maximum habitat at >15 m3/s Sharp drop as flow goes below 10 m3/s

Goal: Trout and food production

Summer flowWinter flow

Before: flow about1 m3/s

After: flow about16 m3/s

Minimum flow of 12-16 m3/s since

August 1997

Before After minimum flow

1 year

Waiau River

Numbers of trout (>20 cm)

1996 1997 1998 1999 2000 2001Bro

Brown and rainbow trout densities ranked nationally

River (300+ reaches in descending order)

Waiau River

Before

Monowai RiverControlled flow to hydroelectric plant

• Lake fed river - mean flow 13.8 m3/s

• Dam at lake controls flow to downstream power station

• Minimum flow of near zero, usually each night

Discharge (m3/s)0 2 4 6 8 10

Food production

Maximum habitat at 5-7 m3/s Sharp drop as flow go below 3 m3/s

MaximumSharp drop

Goal: Invertebrate abundance and diversity

1-JAN-95 1-JUL-950

1-JAN-92 1-JUL-92

Minimum flow since July 1994

1991 1992 1993 1994 1995 1996 1997 1998

After minimum flow increased to 6 m3 s-1

Invertebrate abundance doubled Number of species doubled

Density

Taxon richness

Invertebrate abundance and diversity

Before

Ohau RiverHydroelectric diversion

• Lake fed river - mean flow 80 m3/s

• Flow diverted for hydropower in 1979

• Minimum flow of near zero

Discharge (m3/s)0 5 10 15

Adult brown trout

Food production

Maximum

Habitat analysis indicated excellent adult trout and food producing habitat at 10 m3/s

Goal: High quality brown trout fishery

Before:Flow < 1 m3/s

After:Flow 10 m3/s

Minimum flow of 10-14 m3/s since 1994

Failed to meet expectations

Trout present, but not many

Reasons unknown

Food ?

Recruitment?

Flow too high?

Tekapo RiverHydroelectric diversion

• Lake fed river - mean flow 90 m3/s

• Lake flow diverted in 1978 leaving 10 m3/s from tributaries

• Excellent adult trout spawning and food producing habitat at >10 m3/s

0 5 10 15 20

Discharge (m3/s)

Trout spawning

Food production

Habitat begins to drop sharply

Goal: High quality trout fishery

Just below damZero flow

45 km downstreamFlow 10 m3/s

Tekapo River

• Before diversion (flow 90 m3/s)– Not mentioned in angling surveys – Some trout spawning

• After diversion (flow c. 10 m3/s)– Up to 240 brown and rainbow trout (> 20 cm) per

km– More than 200 juvenile brown and rainbow trout

per km

River (300+ reaches)

Brown and rainbow trout densities ranked nationally

Before?

Moawhango RiverHydroelectric diversion

• Natural mean flow 9.3 m3/s

• In 1991, all water was diverted to another river system

Discharge (m3/s)

Goal: benthic invertebrate communitycomposition

0 1 2 3 4 50

15DeleatidiumZelandoperla decorataAoteapsycheHydrobiosis parumbripennisMaoridiamesaOrthocladinaeTanytarsus vespertinus

MaximumSharp drop

• Sharp reduction at flows < 0.8 m3/s

After 0.52 m3/s

Before 0.06 m3/s

Minimum flow of 0.6 m3/s since

June 2000

Before (1997)

After (2002)

Desirable species Undesirable species

Waipara RiverDiversion for irrigation

• Small gravel-bed river

• Mean flow 2.5 m3/s

Discharge (m3/s)

0.0 0.1 0.2 0.3 0.4 0.5 0.60

Common river galaxiasLongfin eel (<300 mm)Upland bullyTorrentfishBluegill bullyCommon bully

Sharp drop

Goal: maintain native fish population

Habitat for a species with “intermediate” velocity preference begins to decline sharply at 0.12 m3/s

Dec 1998-May 1998

Dry summer• Less than recommended

minimum (0.12 m3/s) for 36% of time

Dec 1999-May 2000

Wet summer• Less than recommended

minimum ( 0.12 m3/s) for 11% of time

Effect on Fish Population

Beginning of summer

Upland

y gala

Bluegil

l bull

End of summer

Dry 1998/99 summer

Upland

y gala

Bluegil

l bull

Wet 1999/00 summer

Conclusion

• This is all the NZ data on biological response to flow changes

• Flow assessments were based on habitat requirements

• 5 out of 6 were successful• The outcomes don’t support commonly held

views that more flow is better and that all aspects of a natural flow regime are important

Thank you

Instream flow assessment in New Zealand

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