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Effects of Streambed Periphyton on Hydraulics and Sediment Deposition in Streams. Nira Salant Department of Geography University of British Columbia. What is periphyton?. What does periphyton do?. Food and habitat. Physical effects?.  I. Hydraulics.  II. Sediment deposition.

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slide1

Effects of Streambed Periphyton on Hydraulics and Sediment Deposition in Streams

Nira Salant

Department of Geography

University of British Columbia

slide3

What does periphyton do?

Food and habitat

Physical effects?

 I. Hydraulics

 II. Sediment deposition

slide4

Sediment deposition

Algae: High profile

Diatoms: ‘Sticky’

Turbulence

Trapping,

Adhesion,Clogging

slide7

6

7

6

5

5

4

4

Depsoitional velocity wd (cm/h)

TKE shear stress (Pa)

3

3

2

2

1

1

0

0

0.0

2.0

4.0

6.0

8.0

10.0

12.0

AFDM(g/m2)

Deposition from water column: Diatoms

Depositional velocity

Biomass increases:

 Near-bed shear stress increases (structural roughening)

Max shear stress

Near-bed shear stress

 Deposition velocity decreases

(high upward stresses and infiltration decreases = ‘clogging’)

Highest deposition velocity

when near-bed and upper flow shear stresses are low

and biomass is moderate (moderate adhesion, low clogging)

slide10

6

7

6

5

5

4

TKE shear stress (Pa)

4

Depositional velocity wd (cm/h)

3

3

2

2

1

1

0

0

0

5

10

15

20

25

30

AFDM(g/m2)

Deposition from water column: Algae

Deposition decrease with biomass? Clogging?

Unclear relation between biomass, shear stress, and depositional velocity

But…

slide11

6

7

25

7

6

6

5

20

5

5

4

15

4

4

Depositional velocity wd (cm/h)

AM(g/m2)

TKE shear stress (Pa)

3

3

3

10

2

2

2

5

1

1

1

0

0

0

0

0

5

10

15

20

25

0

5

10

15

20

25

Growth stage (Weeks)

Growth stage (Weeks)

Deposition from water column: Algae

Max shear stress

Shear stress increases with growth stage

Surface samples AM

Depositional velocity

Surface deposition decreases with growth stage

Near-bed shear stress

Later growth stage 

Increase in shear stress

 Less surface deposition

  • BUT
  • Higher advection and infiltration (subsurface deposition)

High biomass reduces infiltration

Total deposition = balance of surface and subsurface deposition

slide12

Deposition from water column: Algae

Turbulence  Less surface deposition, deeper infiltration (A8  A20)

Biomass  Reduced infiltration despite high advection (A16)

slide13

Implications

Flow conditions, sediment accumulation, interstitial infiltration  Habitat condition

 Organism behavior

Streambed patchiness and complexity

…a function of periphyton structure and distribution

slide14

Decrease in concentration over time

Exponential model

C0 = peak concentration at time t = 0

k = decay (or deposition) rate (T-1)

ws = settling velocity (D/T)

= depositional velocity wdwhen fit to exponential model

h = flow depth (D)

slide15

I. Hydraulics

Filamentous periphyton ‘patches’

‘Closed’

‘Open’

slide17

umax

Ux

u0

0.6

0.0

10.0

20.0

30.0

40.0

50.0

50.0

u (cm/s)

u (cm/s)

Velocity distribution

slide18

Peak shear = top of Roughness layer

Shear stress distribution

Two-layered flow

Closed

Shift in height of roughness layer top

Same thickness

Logarithmic layer

Periphyton

Open

No periphyton

slide19

Periphyton

None

0.3

0.25

0.2

z/H

0.15

0.1

0.05

0

0

0.005

0.01

0.015

0.02

0.025

Re/ρUx2 (

Near-bed turbulence reduction

1) Shift in location of peak shear (Open mats)

2) Hydrodynamic smoothing

(Closed mats)

Reduced turbulent transfer

Higher upper flow stress

slide20

None

Diatoms 4 Weeks

Diatoms 24 Weeks