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

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

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

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

6. SWS 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.

7. North 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.

8. Panoramic to Plan Views – 26 Sept 04
Submerged sandbars
Panoramic to Plan Views – 26 Sept 04 N
A 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.
670m
Submerged sandbars
400m
250m

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

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

14. Ground Truth of Timex Images
Comparison of transect intensity data with measured bathymetry at Duck, NC (Lippmann and Holman, 1989)
5.0
Bathymetry
Peak at Offshore Bar
Peak at Shoreline
Intensity Along Transect
Elevation (m)
SWL
Cross-Shore Profile 16 Oct. 86
SD200
1142m
-5.0 50
100
150
200
250
300
Cross-Shore Distance (m)

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

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

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

18. The summer intertidal terrace incorporates the middle bar

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
Reference 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]).

20. 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
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).
Changes in intertidal sand volumes
Extreme 2005/2006 Winter storms – all sections of the beach experienced sand loss

21. Further indication that the dominant morphodynamics on Benson Beach is sand bar movement Comparison: Planar and Sand bar beach morphodynamics
Planar Beaches:
Sand Bar Beaches:
Sand volume is defined here as that within a fixed intertidal range
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

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 (1st 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. Bar System Review: Feb 2004-Sept 2005 (before the 2005/2006 Extreme Winter Storms)
Shoreline
Inner Sand Bar
Middle Sand Bar
Outer Sand Bar

24. N
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

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

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

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

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

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

32. N
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)

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
Seasonal dynamic equilibrium assumed if
Sand bar morphodynamics stable, and
No significant net annual change in intertidal volume.

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. 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. Intertidal 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

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

38. N
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
The middle (blue) and the inner bars will merge on the north end over the next three months

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

40. Beginning of merging of inner bar with middle bar

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. N
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

43. N
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

44. N
What 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

45. N
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

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

47. N
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

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. Remnant fork of middle bar

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

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

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

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. 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. Nascent Musings
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.
Does Argus have insufficient volume information above MHW to determine this?

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. 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.