Terrigenous sediment dynamics in a small, tropical, fringing-reef embayment

Terrestrial sediment dynamics in a small, tropical, fringing-reef embayment

By Dr. Alex MessinaSDSU/UCSB Joint-Doctoral Program in Geography

photo: Messina

N

Pago Pago Harbor

Pacific Ocean

South ReefNorth

ReefStreamOutlet

Faga’alu, American Samoa

https://msb.unm.edu/divisions/mammals/research/index.html

https://msb.unm.edu/divisions/mammals/research/index.html

Motivation and Research questions

Chapter 1: Where is sediment coming from? and What to do about it?

Chapter 2: How does water circulate over the reef?

Chapter 3: Where is sediment accumulating on the reef?

Sediment accumulation in Faga’alu, Jan 2012

video: Messina

Sediment harming coral in Faga’alu

1. Watershed inputs 2. Hydrodynamics 3. Sediment Accumulation

RIDGE to REEF

Chapter 1: Where is sediment coming from?Sediment from Natural Sources and Human Sources

Human sources:• Quarry• Storm drains• Roads

Natural sediment from forest

QuarryRoad runoff Storm drains

Subwatersheds to isolate sediment sources:Natural, quarry, village

2 PT’s (Pressure Transducers)2 Turbidimeters 1 Autosampler1 Grad student

Sediment yield measured at three locations using:

QUARRY

10km

Measurements:• Water discharge (Q) (L/sec) • Suspended Sediment Concentration (SSC) (mg/L)

Depth with pressure transducer (PT)

Flow measurements relate depth to water discharge(Q, volume/time)

De

pth

SSYEV = Q x SSC

1. Measure SSC in water samples collected by Autosampler and grab

2. Model SSC from Turbidity data

Autosampler

Retrieving samples

Turbidimeter in stream

Grad student

Measuring sediment and discharge during stormsTimelapse videos!

Filtering and weighing sediment in laboratory

Auto-sampler

Measuring Q with flow meter

Detecting changes in sediment

Q-SSC problematic due to scatter

1. Discharge-Concentration relationship

2. Changes in annual yields

3. Event-wise analysis

UPSTREAM DOWNSTREAM

CO

NC

ENTR

ATIO

N

DISCHARGE (Q)

FOREST QUARRY VILLAGE

Detecting changes in sediment

Sequential downstream sources are confused

Q-SSC problematic due to scatter

1. Discharge-Concentration relationship

2. Changes in annual yields

3. Event-wise analysis

UPSTREAM DOWNSTREAM

CO

NC

ENTR

ATIO

N

DISCHARGE (Q)

FOREST QUARRY VILLAGE

FOREST QUARRY VILLAGE FOREST QUARRY VILLAGE

Non-storm

Storm

Continuous Turbidityto…

Continuous SSC

Q(from depth and rating curve)

Integrated over storm to get total

SSY = Q x SSC

KEY METRIC:Total SSY from storm event

KEY METRIC:Total SSY from storm event

TimeStormStart

StormEnd

Storm Event

SSYEV vs. “Storm Metrics” (precipitation and discharge)

How to compare sediment yield from different sources and events? (1)SS

YEV

(to

ns/

km2)

Maximum event discharge (Q) (m3/sec/km2)

Example of a “Storm Event”

Maximum Event Q

Total SSYEV

102

101

100

10-1

10-2

10-3

142 Storm Events measured

• Compare total and % contributions from sources• KEY METRIC: Disturbance Ratio (DR): DR = SSY / SSYFOREST

DR = 1 is no disturbance

How to compare sediment yield from different sources and events? (2)

SSYEV can be used to make a budget of sources

Results from 8 storms Precip SSYEV (tons)

mm Upper Lower_Quarry Lower_Village Total

Min 12 0.06 0.08 0.3 0.7

Max 86 9.6 8.2 5.3 23.1

Total 299 13.4 16.4 16.0 45.7

% 29 36 35 100% Area 50 16 34 100

DR 1.0 4.1 1.8 1.7

From 42 storms (UPPER and LOWER only):•Human-disturbed subwatershed contributed

~87% of SSYEV to the Bay•Human-disturbed areas have increased SSY

~3.9x above natural yields to the Bay

How to compare sediment yield from different sources and events? (2)

SSYEV can be used to make a budget of sources

Results from 8 storms Precip SSYEV (tons)

mm Upper Lower_Quarry Lower_Village Total

Min 12 0.06 0.08 0.3 0.7

Max 86 9.6 8.2 5.3 23.1

Total 299 13.4 16.4 16.0 45.7

% 29 36 35 100% Area 50 16 34 100

DR 1.0 4.1 1.8 1.7

SSY from forested and disturbed areas

Upper Lower_Quarry Lower_Village Total

Area disturbed (%) 0.4 6.5 11.7 5.2

Forested areas (tons) 13.3 3.7 7.8 25.0

Disturbed areas (tons) 0.1 12.7 8.2 20.7

% from disturbed areas 1 77 51 45

DR for disturbed areas 3 49 8 15

•Quarry makes up small area but high SSYEV

•High DR at quarry due to constant disturbance

• Compare total and % contributions from sources• KEY METRIC: Disturbance Ratio (DR): DR = SSY / SSYFOREST

DR = 1 is no disturbance

From 42 storms (UPPER and LOWER only):•Human-disturbed subwatershed contributed

~87% of SSYEV to the Bay•Human-disturbed areas have increased SSY

~3.9x above natural yields to the Bay

Conclusions from Chapter 1:

Where is anthropogenic sediment coming from?Quarry!

• Quarry covered ~1% of watershed, but contributed ~36% of SSYEV

• Mitigate sediment discharge from quarry

Methodological contributions:-Automated storm identification-Quantify change with event-wise SSY-Disturbance Ratio

Messina, A., Biggs, T. (2016) “Contributions of human activities to suspended sediment yield during storm events from a small, steep, tropical watershed.” Journal of Hydrology, in press

Retention ponds installed Oct 2014

Chapter Two: How is water circulating over the reef?

Water circulation controls sediment dynamics

Energetic hydrodynamic forcing compared with other reefs:

-Variable winds-Variable waves-> High spatial variability in

current velocity and direction

How do currents vary spatially over the reef?How do currents vary under calm conditions, high winds, and high waves?

WIND/WAVES


Water circulation controls sediment dynamics

Energetic hydrodynamic forcing compared with other reefs:

-Variable winds-Variable waves-> High spatial variability in

current velocity and direction

How do currents vary spatially over the reef?How do currents vary under calm conditions, high winds, and high waves?

WIND/WAVES

Exposed to big waves!

Wave height recorder

Building drifters

3 acoustic current profilers

5 GPS-recording drifters Deployed via paddleboard

EULE

RIA

NLA

GR

AN

GIA

N

METHODSTwo ways to observe flow:• Eulerian: flow past fixed point• Lagrangian: follow water parcel


• Lagrangian = spatial coverage

Lagrangian driftersGPS-tracked drifters, to determine spatial patterns related to wind and wave forcing


• Eulerian = temporal coverage

Eulerian current metersCurrent meters at fixed points to determine temporal patterns related to wind and wave forcing

Unprecedented spatial coverage:30 deployments of 5 drifters

Wide range of forcing conditions -> “end members”

Gridded drifter observations: 100m x 100m

Divided into three periods, isolating forcing conditions:

-Tide (Calm)-Strong onshore winds-Large waves

100 m

10

0 m

TIDES (CALM) STRONG WINDS LARGE WAVES

Spatial patterns:1. Faster speeds, consistent directions

over southern reef (crest)2. Slower flow, variable direction over

northern reef and channel

Forcing patterns:1. Tides (calm): Slow speeds, variable directions 2. Strong Winds: Slow speeds, toward stream outlet3. Large Waves: Fastest speeds, most uniform directions;

clockwise flushing pattern

DRIFTERS: Mean flow speed and direction

Slow, variable direction Slow, onshore direction Fast, clockwise circulation

TIDES (CALM) STRONG WINDS LARGE WAVES

Spatial/Forcing patterns:• Similar to Drifters, but no spatial

variation over the reef, clockwise pattern•Contextualize drifter measurements, and

show flow decreases with tide stage

Comparing Eulerian/Lagrangian:1. Speeds faster for drifters (50-650%):

• Point – Area• Surface – Water column• Stokes’ drift• Sampling/Analytical error

2. Implications

ADCPs: Mean flow speed and direction

Fastest, esp. on southern reefSlow, less variable directionsSlowest, most variable directions

Water residence time in each grid cell

Spatial patterns• Lowest over southern reef (crest)• Highest over northern reef and near stream outlet

Forcing patterns• Lowest during large waves• Highest during calm and strong onshore winds

Implications:• Stream discharge deflected over northern reef• Potential for sediment impacts highest over

northern reef, under calm or onshore wind


How is water circulating over the reef?

• Wave-breaking on southern reef crest strong control on circulation

• Highly heterogeneous currents over short spatial scales

• Stream discharge likely deflected over northern reef and channel

• Lagrangian velocities were faster than Eulerian; can overestimate flow

Methodological contributions:-Combined Lagrangian/Eulerian approach-Spatial coverage of drifters over reef flat-Spatially distributed residence time-End member forcing

Messina, A., Storlazzi, C., Cheriton, O., Biggs, T. (in review) “Eulerian and Lagrangian measurements of water flow and residence time in a fringing reef flat-lined embayment: Faga’alu Bay, American Samoa.”

Future work: real-time tracking

Chapter 3: WHERE is sediment accumulating? and WHEN?

What processes control sediment accumulation,in space and time?gross and net?

How sediment input and hydrodynamics interact?Monthly? Seasonal?

Are accumulation rates above harmful levels?

High waves > Low water residence time > prevent deposition & remove deposited sediment

High SSY from watershedand/or

Low wave-driven circulation

Hypotheses

High sediment accumulation when:

SamplingGross and Net accumulation

-10 quasi-monthly, for 1 year - gross -> in TRAPS- net -> on PODS

“sediment trap” “sediment pod”

Methods: Sediment Collection & Analysis

AnalysisGrain size and Composition

-fine/coarse fractions separated-rinsed of salts-analyzed for composition:0rganic, Carbonate, Terrigenous

Sieving/Filtering apparatus

Organisms/Gravel removedSediment collection on SCUBA Rinse and Oven-dry

Interaction of Waves and SSY



Hypotheses

Increased sediment accumulation from:

Hypothetical phasing of Waves and SSY

Removal Deposition

SSY (tons): Measured/Modelled-Qmax model (Ch1)Waves (mean height, m): Model-NOAA WaveWatch3

Daily mean wave height, and total SSY over deployment period (dashed lines)

Interaction of Waves and SSY



Hypotheses

Increased sediment accumulation from:

SSY EV

(to

ns/

km2)

Maximum event discharge (Q) (m3/sec/km2)

Qmax – SSYEV model (Ch 1)

**Sediment mitigation decreased SSY, so two models calibrated

Time-Lapse photography

Moultrie GameSpy I-35(8MP, 15 min interval)

Sediment plume following large rain 2/21/14 – Calm conditions

15:45

North Reef

South ReefStream

16:15

Sediment plume deflected

over North reef and Channel

17:00

Spatial patterns of sediment accumulationB

ENTH

IC S

EDIM

ENT

TRA

PS

(GR

OSS

)• Higher accumulation on north reef and near channel• Composition reflected surrounding benthic sediment


ENTH

IC S

EDIM

ENT

*Note: different chart scales

TRA

PS

(GR

OSS

)P

OD

S (N

ET)

• Higher accumulation on north reef and near channel• Composition reflected surrounding benthic sediment• Higher accumulation in traps vs. on pods


ENTH

IC S

EDIM

ENT


TRA

PS

(GR

OSS

)P

OD

S (N

ET)

• Higher accumulation on north reef and near channel• Composition reflected surrounding benthic sediment• Higher accumulation in traps vs. on pods


ENTH

IC S

EDIM

ENT


PODS

TRAPS

Seasonal SSY and Wave patterns:• Highest SSY in July (dry season) due to

one large storm• Waves were larger in dry season (May-

Oct), smaller in wet season (Nov-Mar)N

OR

THER

NSO

UTH

ERN

PODS

MEA

N A

CC

UM

ULA

TIO

N

PODS

TRAPS

Temporal patterns – PODS:• Accumulation on Pods did not correlate with SSY or Waves• Much higher accumulation (esp. terrig) on northern reef• Higher terrigenous accumulation after large SSY event



Oct), smaller in wet season (Nov-Mar)N

OR

THER

NSO

UTH

ERN

PODS

MEA

N A

CC

UM

ULA

TIO

N

Temporal patterns – TRAPS:• Carbonate accumulation in Traps correlated with Waves• Similar to Pods, much higher on northern reef• Similar composition as on Pods• Highest accumulation due to large wave events, esp. southern reef

TRAPS



Oct), smaller in wet season (Nov-Mar)

MEA

N A

CC

UM

ULA

TIO

N

NO

RTH

ERN

SOU

THER

N

Large Waves

Temporal patterns at sites:

TRAPS

Exceeded coral health thresholds in some cases, mostly on northern reef

Carbonate accumulation correlated with Waves on reef crest (1C, 2C, 3C) and reef crest (1B, 3B)

Accumulation low where surrounding availability is low (2B)

Terrigenous accumulation correlated with SSY only near stream (2A)

Controls on sediment accumulation

NO

RTH

ERN

SOU

THER

NC

ENTR

AL

Accumulation in TRAPS vs. SSY, Waves

Sediment accumulation correlated with Waves

Suggests waves resuspend and transport carbonate sediment over the reef

SSY only near stream (2A)

SED

IMEN

T A

CC

UM

ULA

TIO

N


WHERE is sediment accumulating?• Northern reef and near Channel• Due to circulation patterns and SSY from stream

WHEN is sediment accumulating?•High waves transport benthic sediment•SSY is important, but complex and short time scale

Methodological contributions:-Combined sediment traps and pods: gross and net-Related accumulation to measured SSY-Sampled across gradients in distance from stream outlet

and hydrodynamic energy

Messina, A., Storlazzi, C., Biggs, T. (2016) “Watershed and oceanic controls on spatial and temporal patterns of sediment accumulation in a fringing reef flat embayment: Faga’alu, American Samoa.” in preparation

Conclusions

photo: Messina

For Faga’alu

• SSY significantly increased by human disturbance (mostly the quarry)But now it’s fixed!

• Waves cause currents, protect southern reef but stress northern reef

• Sediment accumulation influenced by surrounding sediment

• Terrigenous accumulation correlated with SSY only near stream, impacted northern reef over longer timescales

• Reef recovery is anticipated but uncertain timescale and flushing of deposited sediment

• Daily sediment accumulation patterns and impacts on coral health are still unknown

Conclusions

photo: Messina

For Fringing Reefs

• Rare to have all three R2R components, baseline for management assessment

• This study provides example of relatively simple Ridge-to-Reef study to inform coral management

• SSY in steep, tropical islands is sensitive to human disturbance

• Waves and currents can significantly alter LBSP impacts

• Time scales of sediment transport, deposition, and reworking are uncertain, so watershed restoration may take a long time to observe

• Coral is under threat from global stressors, but we can save coral from terrigenous sediment stress!

Meagan CurtisJameson NewtsonTrent BiggsCurt StorlazziDr. Mike Favazza

QUESTIONS?

Fa’afetai tele lava (big thanks) to all who helped in the field!

Thanks to Mayor Uso and Faga’alu Village

Rocco Tinitali

Mr. JeffreyRoger

“Young” Greg McCormick

Science

Terrigenous sediment dynamics in a small, tropical, fringing-reef embayment