Messina dissertation defense-4_27_16

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

    by Alex MessinaSDSU/UCSB Joint-Doctoral Program in Geography

    photo: Messina

    N

    Pago Pago Harbor

    Pacific Ocean

    South ReefNorth

    ReefStreamOutlet

    Fagaalu, American Samoa

    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 Fagaalu, Jan 2012

    video: Messina

    Sediment harming coral in Fagaalu

    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 isolate sediment sources:Natural, quarry, village

    2 PTs (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)

    Dep

    th

    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 fluvial 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 fluvial 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 / SSYFORESTDR = 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 BayHuman-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 SSYEVHigh DR at quarry due to constant disturbance

    Compare total and % contributions from sources KEY METRIC: Disturbance Ratio (DR): DR = SSY / SSYFORESTDR = 1 is no disturbance

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

    ~87% of SSYEV to the BayHuman-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 patternContextualize 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

    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 wate