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1 Argus Surveying of Benson Beach: Sand bar generation and migration from storm to storm, season to season, and year to year - implications for sediment.

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Presentation on theme: "1 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:

1 1 Argus Surveying of Benson Beach: Sand bar generation and migration from storm to storm, season to season, and year to year - implications for sediment transport Joan Oltman-Shay, Matt Pruis, Dave Berliner, and Dana Shay Sponsored by: USACE Portland District

2 2 Argus at the North Head Lighthouse Mouth of the Columbia River Washington State USA

3 3 Eight cameras looking south 50mm lenses Jetties, MCR, and the great state of Oregon in the distance

4 4 Basic Image Data Types Image & image-derived products snap time exposure variance

5 Shoreline Sand Bar Trough Wavelength Wave Direction Wave Celerity Wetted Beach Shadow Visual Signatures in the Nearshore Longshore Currents

6 6 SWS 2 miles; 3.22 km 1km 4km MCR North Head Lighthouse

7 7 North Head Panorama – 26 Feb 2005

8 8 Panoramic to Plan Views – 26 Sept m 400m 670m Submerged sandbars N

9 9 Presentation Overview Presentation of North Head Argus Beach Monitoring Station location, view orientation, and image products Presentation 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 2005/2006 The 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 system Rebuilding of those sand bars appears to be, in part, derived from intertidal sand along the full length of Benson Beach It took two years for the offshore bar system to return to a 2004 pattern Discussion of the implications to sediment transport and dredge material disposal

10 10 Definition: 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 therefore – no 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 therefore –no 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

11 11 Findings to Date Finding #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 positions Argus 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

12 12 outer sand bar middle sand bar MHW shoreline Next 4 Images provide an example of bar migration to shoreface - Spring 2005

13 13 Time Exposure Ground Truth Tests Duck, North Carolina (vintage 1980s) Test transect

14 14 Ground Truth of Timex Images Bathymetry Intensity Along Transect SWL Cross-Shore Profile 16 Oct. 86 SD m Elevation (m) Cross-Shore Distance (m) Peak at Offshore Bar Peak at Shoreline Comparison of transect intensity data with measured bathymetry at Duck, NC (Lippmann and Holman, 1989)

15 15 outer sand bar middle sand bar MHW shoreline Next 4 Images provide an example of bar migration to shoreface - Spring 2005

16 16 The middle bar is moving onshore at the south end of the beach N

17 17 The middle bar is attaching to the shoreface at the south end of the beach, moving the 2.0m shoreline seaward N

18 18 The summer intertidal terrace incorporates the middle bar

19 19 The 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 Beach

20 20 sand bar attachment in the S(outh) section of the beach but not in the N(orth) Extreme 2005/2006 Winter storms – all sections of the beach experienced sand loss Intertidal 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 2005

21 21 Further indication that the dominant morphodynamics on Benson Beach is sand bar movement Comparison: Planar and Sand bar beach morphodynamics NOTE: 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 Planar Beaches: Sand Bar Beaches:

22 22 Finding #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 (1 st Argus observations) until the Winter 2005/2006 extreme storm events The 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

23 23 Bar System Review: Feb 2004-Sept 2005 (before the 2005/2006 Extreme Winter Storms) Outer Sand Bar Middle Sand Bar Inner Sand Bar Shoreline

24 24 Note the cross-shore positions of northern end of outer and middle bars – they will not change significantly whereas the southern end positions will change N

25 25 beginning of merge of outer and middle bars on southern end

26 26 Outer and middle bar merge remains through Summer 04 and into Winter 04/05

27 27 Inner bar Merge of outer and middle bars extends 500m further north from position at the end May 2004 Jump from Spring 04 to Winter 05 with little change except - A 3-bar system remains well-defined on northern end N

28 28 Two weeks later - the middle bar has merged with the south section of the inner bar N

29 29

30 30 Middle bar moves onshore to feed intertidal zone 3-bar system remains well- defined on northern end (yellow line indicates inner bar) N

31 31

32 32 Middle bar position in south is more like an inner bar - compare bar systems on north (3 distinct bars) and south sides (2 distinct bars) N

33 33 Review of Finding #2 The north portion of the Benson Beach sediment system appears to be in a “Seasonal Dynamic Equilibrium” from Feb 2004 through Sept 2005 –Seasonal 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 terrace There little change in the relative positions of the outer and middle bars to each other –The inner bar location has the greatest seasonal change There is no significant net annual change in the intertidal sand volume or dry beach acreage of the northern section –There is no evidence that this three-bar system is sediment starved and therefore acquiring sediment from sources such as the beach above MHHW

34 34 Review of Finding #2 The 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 observation –There is only a two-bar system suggesting that there is not enough sediment to sustain the three bar system observed to the north –The outer and middle bars merge during winter months, suggesting again, there is not enough sediment at times to sustain the two-bar system –A 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 shoreface Possible sources are from dredge material placed at the SWS and/or erosion of the beach above MHHW

35 35 Finding #3 The Winter 2005/2006 extreme storm event “damaged” the offshore bar system –The sand bars were “blown out” This is particularly evident on the north side where previously there had been a stable system of three sand bars –Rebuilding of those sand bars appears to be, in part, at the expense of intertidal sand supply along the full length of Benson Beach –It took two years for the offshore bar system to return to the 2004 pattern

36 36 sand bar attachment in the S(outh) section of the beach but not in the N(orth) Loss of intertidal sand after the extreme 2005/2006 Winter storms – all sections (N,M,S) of the beach experienced sand loss Intertidal sand volume changes due, in large, to on/offshore migration of sand bars

37 37 Bar 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 N

38 38 The middle (blue) and the inner bars will merge on the north end over the next three months The outer bar will take over the present location of the middle bar within one week; there will remain an remnant outer bar at ~1000m N

39 39 Remnant outer bar The outer bar is now 200m closer to shore than has been observed since Argus observations began in Feb 2004

40 40 Beginning of merging of inner bar with middle bar

41 41 This 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

42 42 Winter 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 N

43 43 The 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 N

44 44 The south end of this inner bar that is forming in April, will feed the middle bar by merging with it in June N

45 45 The “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 N

46 46 north end locations of outer and middle bar remain steady N

47 47 Significant onshore movement of bar system through summer – dashed lines are the June locations Note that the southern end of the outer bar is either gone or is no longer revealed by breaking waves N

48 48 The 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

49 49 Remnant fork of middle bar

50 50 The Dec 2007 bar system looks similar to the Feb 2004 bar system

51 51 Winter 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 Mother Nature is not finished – once again the south end of the bar system reveals an absence of a dynamic equilibrium N

52 52 The middle bar is re-defined on the south end farther offshore and the outer bar begins to reform farther offshore A large rip channel? N

53 53

54 54 Looking into the crystal ball: In Summer 2008, the middle bar will likely move toward shore on the southern end, contributing to intertidal terrace rebuilding Sediment deposited at seaward end of rip channel?

55 55 Implications 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 offshore The size of the rip channel along the north side of the jetty (south side of Benson Beach) is a new feature –this channel may serve to move material placed on the southern end of Benson Beach farther and more rapidly offshore than previously It 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 modified –With 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

56 56 Question: 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. Nascent Musings

57 57 Argus 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

58 58 NorthWest 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.


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