Presentation on theme: "Argus Surveying of Benson Beach: Sand bar generation and migration from storm to storm, season to season, and year to year - implications for sediment."— Presentation transcript:
1Argus Surveying of Benson Beach: Sand bar generation and migration from storm to storm, season to season, and year to year - implications for sediment transportJoan Oltman-Shay,Matt Pruis, Dave Berliner, and Dana ShaySponsored by: USACE Portland District
2Argus at the North Head Lighthouse Mouth of the Columbia River Washington State USA
3Eight cameras looking south 50mm lenses Jetties, MCR, and the great state of Oregon in the distanceEight ABMS video cameras are located at the top of the window in the lighthouse lamp room. Care was taken to not obstruct the lamp light and to visually minimize the presence of the hardware and cable (note use of white cables).
4Basic Image Data Types Image & image-derived products snap time exposurevariance
5Visual Signatures in the Nearshore Sand BarShorelineTroughShadowWetted BeachLongshore CurrentsWavelengthWave DirectionWave Celerity
6SWS MCR 2 miles; 3.22 km 4km 1km North Head Lighthouse Aerial view of the site showing the location of the lighthouse and the region spanned by the eight cameras (defined within the black arrows).The Shallow Water Site (SWS) is the location for the placement of some dredge material.
7North Head Panorama – 26 Feb 2005 With knowledge about how each of the cameras and their views register within real-world coordinates (i.e., WA State Plane) the individual pictures can be merged to create a panorama.On this blustery day we show a panorama derived from seven snaps.
8Panoramic to Plan Views – 26 Sept 04 Submerged sandbarsPanoramic to Plan Views – 26 Sept 04NA panorama (top panel) provides a pleasant picture of the beach; however, it isn’t a map.The images can be rectified to present “aerial” or plan view maps of the beach (bottom panel). Plan views allow us to identify and track morphological changes of the beach.White lines define the average location of breaking of waves -- the shoreline and the offshore submerged sand bar locations.Accuracy in mapping shorelines and submerged sand bars comes from using sophisticated image processing methods that have been verified on many beaches around the world.670mSubmerged sandbars400m250m
9Presentation Overview Presentation of North Head Argus Beach Monitoring Station location, view orientation, and image productsPresentation of three “Findings to Date”1 : Benson Beach morphodynamics are dominated by sand bar dynamics (previously presented July 07 in Ilwaco, WA meeting)2 : Sand bar dynamics at the north and south ends of Benson Beach are different:The north end, 3-bar sediment system was likely in a “Seasonal Dynamic Equilibrium” before Winter 2005/2006The south end, 2-bar sediment system was likely not in a “Seasonal Dynamic Equilibrium”3 : The extreme Winter 2005/2006 storms “damaged” the offshore bar systemRebuilding of those sand bars appears to be, in part, derived from intertidal sand along the full length of Benson BeachIt took two years for the offshore bar system to return to a 2004 patternDiscussion of the implications to sediment transport and dredge material disposal
10Definition: Seasonal Dynamic Equilibrium “Dynamic Equilibrium” allows for sand grains moving through a sediment system but with no change in the net system sediment flux (“Dynamic”) and thereforeno net erosion or accretion (“Equilibrium”)“Seasonal Dynamic Equilibrium” explains the winter/summer (“Seasonal”) changes in shoreface morphology but, again, indicates no change in the net seasonal system sand flux (“Dynamic”) and thereforeno year to year net shoreface erosion or accretion (“Equilibrium”)A “Sediment System” is a contiguous coastal region in which sediment is both suspended and advected by waves and currents
11Findings to DateFinding #1: Sand bar movement is a dominant source of change in the intertidal zone of Benson Beach (previously presented at the Ilwaco meeting in July 2007)The dominant seasonal signal of loss and recovery of dry-beach acreage and intertidal sediment volumes on Benson Beach is associated with the summer onshore migration and attachment of sand bars onto the shoreface and the winter detachment of the sand bars and migration offshore.This observation recommends a closer study of offshore sand bar positionsArgus imagery is well suited for this study because of the high temporal resolution (daily observations) of sand bar position data that is critical to the identification and tracking of sand bars through storms and seasons
12Next 4 Images provide an example of bar migration to shoreface - Spring 2005 MHW shorelineRed and blue solid lines mark the crest locations of the outer and middle bars, respectively.The dashed lines mark the crest locations of the same bars observed several weeks earlier.The green line marks the 2.0m NAVD88 shoreline elevationmiddle sand barouter sand bar
13Time Exposure Ground Truth Tests Duck, North Carolina (vintage 1980s)Testtransect
14Ground Truth of Timex Images Comparison of transect intensity data with measured bathymetry at Duck, NC (Lippmann and Holman, 1989)5.0BathymetryPeak at Offshore BarPeak at ShorelineIntensity Along TransectElevation (m)SWLCross-Shore Profile 16 Oct. 86SD2001142m-5.050100150200250300Cross-Shore Distance (m)
15Next 4 Images provide an example of bar migration to shoreface - Spring 2005 MHW shorelineRed and blue solid lines mark the crest locations of the outer and middle bars, respectively.The dashed lines mark the crest locations of the same bars observed several weeks earlier.The green line marks the 2.0m NAVD88 shoreline elevationmiddle sand barouter sand bar
16NThe middle bar is moving onshore at the south end of the beach
17NThe middle bar is attaching to the shoreface at the south end of the beach, moving the 2.0m shoreline seaward
18The summer intertidal terrace incorporates the middle bar
19The effect of sand bar migration on and off the shoreface can be best seen in changes to the Dry-Beach Acreage and Intertidal Sand Volume for the North, Middle, and South sections of Benson BeachReference map for converting local Argus coordinates (m) to WA South State Plane coordinates (m, NAD83). Also shown on this map are the North, Middle, and South section partitions used in the analysis of shorelines, dry-beach acreage, and intertidal volumes for this report. ( X (local Argus) = - [NAD83 Eastings (meters) – meters]; Y (local Argus) = - [NAD83 Northings (meters) – meters]).
20Intertidal sand volume changes due, in large, to on/offshore migration of sand bars – e.g., the sand bar attached primarily onto the south section of the beach in Spring 2005sand bar attachment in the S(outh) section of the beach but not in the N(orth)Intertidal sand volume changes from February 2004 through September 2006 in the North, Middle, and South sections of the beach. Sand volumes are between 0.69 and 2.26m NAVD88 and y = 800 and 3050m. Shown are the change (DT) and cumulative change (TCum).Changes in intertidal sand volumesExtreme 2005/2006 Winter storms – all sections of the beach experienced sand loss
21Further indication that the dominant morphodynamics on Benson Beach is sand bar movement Comparison: Planar and Sand bar beach morphodynamicsPlanar Beaches:Sand Bar Beaches:Sand volume is defined here as that within a fixed intertidal rangeNOTE: Seasonal changes in beach morphology (slope, volume) of planar and sand bar beaches are opposite The observed seasonal intertidal beach slope and sand volume at Benson Beach indicates that its dominant morphodynamics is sand bar movement
22Finding #2:The north portion of the offshore sand bar of Benson Beach was likely part of a sediment system that was a “Seasonal Dynamic Equilibrium” from Feb 2004 (1st Argus observations) until the Winter 2005/2006 extreme storm eventsThe south portion of the offshore sand bar was likely not part of a system in a “Seasonal Dynamic Equilibrium” during the full 4 years of observation
23Bar System Review: Feb 2004-Sept 2005 (before the 2005/2006 Extreme Winter Storms) ShorelineInner Sand BarMiddle Sand BarOuter Sand Bar
24NNote the cross-shore positions of northern end of outer and middle bars – they will not change significantly whereas the southern end positions will change
25beginning of merge of outer and middle bars on southern end
26Outer and middle bar merge remains through Summer 04 and into Winter 04/05
27N Jump from Spring 04 to Winter 05 with little change except - Inner barMerge of outer and middle bars extends 500m further north from position at the end May 2004A 3-bar system remains well-defined on northern end
28NTwo weeks later - the middle bar has merged with the south section of the inner bar
32NMiddle bar position in south is more like an inner bar - compare bar systems on north (3 distinct bars) and south sides (2 distinct bars)
33Review of Finding #2The north portion of the Benson Beach sediment system appears to be in a “Seasonal Dynamic Equilibrium” from Feb 2004 through Sept 2005Seasonal Dynamical Equilibrium is revealed by the seasonal onshore (offshore) migration of a distinct three bar system during summer (winter)The winter three bar system at times moves to a two bar system in the summer when the inner bar attaches to seaward side of the low tide terraceThere little change in the relative positions of the outer and middle bars to each otherThe inner bar location has the greatest seasonal changeThere is no significant net annual change in the intertidal sand volume or dry beach acreage of the northern sectionThere is no evidence that this three-bar system is sediment starved and therefore acquiring sediment from sources such as the beach above MHHWSeasonal dynamic equilibrium assumed ifSand bar morphodynamics stable, andNo significant net annual change in intertidal volume.
34Review of Finding #2The behavior of the south portion of the offshore sand bar system suggests that it has not been in a “Seasonal Dynamic Equilibrium” during the 4 years of observationThere is only a two-bar system suggesting that there is not enough sediment to sustain the three bar system observed to the northThe outer and middle bars merge during winter months, suggesting again, there is not enough sediment at times to sustain the two-bar systemA sediment starved bar system must acquire sediment from other sources outside of its’ nearshore system to rebuild the two-bar system or to detach the the bar from the shorefacePossible sources are from dredge material placed at the SWS and/or erosion of the beach above MHHW
35Finding #3The Winter 2005/2006 extreme storm event “damaged” the offshore bar systemThe sand bars were “blown out”This is particularly evident on the north side where previously there had been a stable system of three sand barsRebuilding of those sand bars appears to be, in part, at the expense of intertidal sand supply along the full length of Benson BeachIt took two years for the offshore bar system to return to the 2004 pattern
36Intertidal sand volume changes due, in large, to on/offshore migration of sand bars sand bar attachment in the S(outh) section of the beach but not in the N(orth)Intertidal sand volume changes from February 2004 through September 2006 in the North, Middle, and South sections of the beach. Sand volumes are between 0.69 and 2.26m NAVD88 and y = 800 and 3050m. Shown are the change (DT) and cumulative change (TCum).Loss of intertidal sand after the extreme 2005/2006 Winter storms – all sections (N,M,S) of the beach experienced sand loss
37Bar System Review: March 2006-Dec 2007 (Post 2005/2006 Extreme Winter Storms) The comparison of Sept 05 (dashed) and March 06 (solid) bar locations indicate large movement offshore (~200m) with the Winter 05/06 extreme storms. The north end of the outer bar (red) has been pushed farther offshore than previously and has broken in two parts
38NThe outer bar will take over the present location of the middle bar within one week; there will remain an remnant outer bar at ~1000mThe middle (blue) and the inner bars will merge on the north end over the next three months
39The outer bar is now 200m closer to shore than has been observed since Argus observations began in Feb 2004Remnant outer bar
40Beginning of merging of inner bar with middle bar
41This piece of bar will eventually be integrated into either, or both the middle and outer bars Both the outer and middle bars are now 200m closer to shore than has been observed since Argus observations began in Feb 2004
42NWinter 06/07 restores the middle and outer bar locations on the north side back to their typical locations (nominally 600 and 800m, respectively); dashed lines are the locations in June 06
43NThe outer and middle bars on south end begin to merge – similar to May 2004 bar movement suggesting that the south end of the bar system is still sediment starved
44NWhat does the merging of the inner bar with middle bar say about sediment supply/starvation?The south end of this inner bar that is forming in April, will feed the middle bar by merging with it in June
45NThe “middle” bar has merged with the southern end of the inner bar and has become forked; this will last until the storms of Winter 07/08
46Nnorth end locations of outer and middle bar remain steady
47NSignificant onshore movement of bar system through summer – dashed lines are the June locationsNote that the southern end of the outer bar is either gone or is no longer revealed by breaking waves
48The base of the forked “middle” bar has moved north 500m since early Sept The outer fork of the middle bar has moved offshore and is now defining the outer bar
50The Dec 2007 bar systemlooks similar to theFeb 2004 bar system
51Mother Nature is not finished – once again the south end of the bar system reveals an absence of a dynamic equilibriumNWinter 07/08 waves are re-defining the south end of the middle bar by merging its’ northern end with the south end of the outer bar
52NA large rip channel?The middle bar is re-defined on the south end farther offshore and the outer bar begins to reform farther offshore
54Looking into the crystal ball: In Summer 2008, the middle bar will likely move toward shore on the southern end, contributing to intertidal terrace rebuildingSediment deposited at seaward end of rip channel?
55Implications for Dredge Material Disposal The previous behavior of the sand bar system suggests that the middle bar on the south end will continue to play a significant role in the movement of sediment between the shore and offshoreThe size of the rip channel along the north side of the jetty (south side of Benson Beach) is a new featurethis channel may serve to move material placed on the southern end of Benson Beach farther and more rapidly offshore than previouslyIt could be argued that if enough dredge material is placed BOTH on the shoreface of the southern section of Benson Beach and at the SWS, the behavior of the southern ends of the outer and middle bars will be modifiedWith the additional sediment, there could be a reduction in the merging of the outer and middle bars on the south end –It is not known how much sediment would be needed to establish a 3-bar system similar to the north end
56Nascent MusingsQuestion: How do sand volumes in the North and South sections, from MSL to 10m depth compare? > Can the USGS/WADOE surveys help answer this questions? Hypotheses: 1) Perhaps the difference in North and South sand volumes can provide a ballpark estimate of the South section sediment deficit and the amount therefore required to establish a Seasonal Dynamic Equilibrium through typical winter/summer seasons 2) Extreme storm seasons, such as Winter 05/06 will continue to erode the beach, setting a new baseline for a new Seasonal Dynamic Equilbrium. Only excess sediment reserves, more than is needed to maintain a typical winter/summer exchange, can continue to maintain the beach through extreme winter seasons.Does Argus have insufficient volume information above MHW to determine this?
57Argus at North Head Lighthouse would not have been possible without funding from USACE and the joint cooperation of Washington State Parks, Coast Guard, and USACE
58NorthWest Research Associates (NWRA) Redmond, WA NWRA is a scientific research group, owned and operated by its Principal Investigators, with expertise in the geophysical and related sciences.