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UW ESS 230 - Beaches and Coastal Environments

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Beaches and Coastal Environments of WashingtonWind patterns affecting WA stateSeasonal changes in water level and wave heightSeasonal changes in wave period and directionSediment source for southern WA coast beachesSouthwest Washington Coastal Erosion StudyObservations of submarine beach profileShoreline changes over many decadesChanges in Wave ClimateElwha RiverHolocene sea-level riseRecent rise in Puget Sound sea levelLocal variability in longshore transportCommon profile for Puget Sound beachesImpact of waves and tidesDifferences in beach profilesImpacts of biogenic materialsImpacts of seawallsOther stupid human tricksBeaches and Coastal Environments of WashingtonSouthern WA Coast – sandy beaches, spits, lagoons, sediment supply from Columbia River, northward longshore transportNorthern WA Coast – headland beaches, cliff erosion, gravel beaches, stacks and island remnants offshoreStraits of Juan de Fuca – headland beaches, cliff erosion, river supply, eastward swell from ocean, large and complex spitsPuget Sound– headland beaches, cliff erosion, rivers with deltas, much local variability due to morphologyWind patterns affecting WA stateWinter ConditionsLow-pressure systems generate winds from S and SW, which push water against coast – causing storm surgeSummer ConditionsHigh-pressure systems generate winds from N and NE, which pull water away from coast – causing deep water to upwell with nutrientsWinter conditions dominate longshore transportSeasonal changes in water level and wave heightMay August December MarchSea level ~0.5 m rise due to winds during winterWave height increases from ~1 m in summer to ~4 m in winterSeasonal changes in wave period and directionMay August December MarchWave periods (and wavelengths) reach a maximum in winterWave direction from SW during winter, from NW during summer290deg250Pathways for Columbia River sandPredominant direction of transport is northwardSand going to beach is small portion of total supplySediment source for southern WA coast beachesDams placed on Columbia River during 20thcentury likely trapped much of sand supplied to beachesColumbia discharge ~10 million tons/y[Mississippi~200 million tons/yAmazon~1000 million tons/y]Southwest Washington Coastal Erosion StudyBeach profiles collected seasonal from a number of beach-perpendicular transectsObservations of submarine beach profileVery difficult to obtain accuratelyData now collected using jet skis with depth recorder and GPSChanges in beach profilesWinter 98 profile (yellow) is lower than Summer 97 profile (red) due to seasonal erosionSummer 98 profile (green) is higher and shows berm and bars moving onto beachS 00S99S 98S 97W 99W98W972- and 3-m contours (above MLLW) show erosion during winter (landward migration) and deposition during summer, with a net movement landward between S 97 and S 00Shoreline changes over many decadesSand accumulation occurred before 1950Rate of accumulation slowed since thenSevere erosion on north side of Willapa Bay Inlet has continued through periodSouth side of Grays Harbor Inlet accumulated much sediment before 1950, but has eroded since building jettyChanges in Wave ClimateElwha RiverDrainage basin within Olympic Mountains, supplies sediment to Straits JdFTwo dams have trapped sediment since 1912 and 1926, nearly eliminating supplyScheduled for removalPuget SoundPredominant north-south orientationComplex morphologyGlacial historyCliff erosion and river supplyEnergetic, but variable wave activityStrong tidal currentsHolocene sea-level riseGlacial rebound had strong impact on local sea level during early Holocene, especially in northern Puget sound – sea level fellRecent rise in Puget Sound sea levelFor past century, sea-level rise has been ~2 mm/y, similar to global sea-level riseLocal variability in longshore transportDue to orientation of land masses and direction of windsResults in dramatic heterogeneity of transport processes: convergent and divergent transport causes deposition and erosiondifferences over small scalesTypical Puget Sound BeachGravel foreshore and sandy low-tide terraceCommon profile for Puget Sound beachesImpact of waves and tidesWater level rises, exposing higher areas (including cliffs) to erosionWave energy increases, moving sediment seaward and alongshoreEquilibrium profile develops, depending on importance of across-beach and alongshore transportDifferences in beach profilesWhere across-beach transport dominates, sediment profile changes depending on wave conditionsWhere alongshore transport dominates, profile remains similar but moves landward or seawardImpacts of biogenic materialsOyster beds Drift WoodStabilize intertidal and supratidal regions of beach profileImpacts of seawallsThey help stabilize shoreline, but increase wave energyWaves reflect back, rather than breakSuperimposed wave height causes stronger wave motions – coarsens seabed and removes subtidal eelgrass (which is important for stabilizing seabed, and providing biohabitats)Other stupid human tricksOther Stupid Human TricksSand mining and slope failures destabilize beach


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UW ESS 230 - Beaches and Coastal Environments

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