terrigenous sediment dynamics in a small, tropical, fringing-reef embayment
TRANSCRIPT
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
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
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
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
Chapter Two: How is water circulating over the reef?
• Lagrangian = spatial coverage
Lagrangian driftersGPS-tracked drifters, to determine spatial patterns related to wind and wave forcing
Chapter Two: How is water circulating over the reef?
• 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
Conclusions from Chapter 2:
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
High SSY from watershedand/or
Low wave-driven circulation
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
High SSY from watershedand/or
Low wave-driven circulation
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
Spatial patterns of sediment accumulationB
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
Spatial patterns of sediment accumulationB
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
Spatial patterns of sediment accumulationB
ENTH
IC S
EDIM
ENT
*Note: different chart scales
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
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
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
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)
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
Conclusions from Chapter 3:
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