Pressure Gradients over a Barred Beach

1
Pressure Gradients over a Barred Beach

• Sungwon Shin
• Coastal and Ocean Engineering
• Oregon State University

2
Offshore sandbar movement
(Hoefel and Elgar, 2003)
3
Onshore sandbar movement
(Hoefel and Elgar, 2003)
4
Elgar et al (2001)
Sandbar location Maxima of S(u), A(u),
S(du/dt)
(Elgar et al, 2001)
5
Drake and Calantoni (2001)

• Large acceleration generates pressure gradient

6
Motivation

• Onshore bar migration has still difficulty in
  simulation
• Most of studies are focused on the quantities of
  entire time series (all waves)
• How about individual wave?

7
Objectives

• Lab experiment over a barred beach
• Synoptic data set for free surface, pressure, and
  velocity
• The relationship between pressure gradient and
  acceleration in terms of skewness for both all
  waves and individual wave

8
Measurement Location
Breaking
9
Experimental Setup Pressure Gages

• Pressure gradient

3 cm
1 cm
gt 0 , Onshore
PG 1 2 3 4
onshore
lt 0 , Offshore
10
Experimental Setup
11
Analysis

• Filtering
• High and low frequency components were removed
• Pressure gradient using 4 sensors
• Despike ADV signal (Goring and Nikora, 2002)

12
Time series of analyzed data over the bar
u (cm/s)
du/dt (cm/s2)
13
? ? , o p , ? u500 , ? u507
? S(dp/dx) , ? S(du/dt) , ? A(u500)
? S(dp/dx) , ? S(du/dt) , ? A(u507)

• Cross-shore variation of A(u), S(du/dt),
  S(dp/dx) have good correlation for entire waves

14

• How about individual wave?

15

• Lets see skewness and asymmetry of free surface,
  pressure, and velocity first

.
Good or not?
16
S(p) vs S(?)
? H1/3 o others
17
S(p) vs S(u)
? H1/3 o others
18
A(p) vs A(?)
? H1/3 o others
19
A(p) vs A(u)
? H1/3 o others
20
Joint distribution of ?, P and u

• Good news
• High correlation for individual waves in both
  skewness and asymmetry
• Water surface elevation and near bottom velocity
  has some difference in skewness
• Better correlation with 1/3 highest waves

21
dp/dx vs acceleration and velocity
22
A(u) vs S(du/dt)

• Not good
• Low correlation

? H1/3 o others
23
Cross-shore variation of r2 value
? H1/3 o All
24
S(dp/dx) vs S(du/dt)
Really bad
? H1/3 o others
25
Cross-shore variation of r2 value
? H1/3 o All
26
Time domain sediment transport model

• Rakha et al (1997), Karambas and Koutitas (2002)
  phase-resolving Boussinesq model, time-averaged
• Long and Kirby (2003) Boussinesq model with
  instantaneous wave condition

27
Conclusion 1
Wait !!!!

• Acceleration skewness is used for modeling
  sediment transport rate
• However, acceleration skewness doesnt match with
  pressure gradient skewness for individual wave

28
Exceedance Probability
29
Madsen (1974)

• Large horizontal pressure gradient can make bed
  material lose its stability, i.e. momentary
  failure.
• ?Pcrit 0.5

? density of fluid ?t the density of the
saturated bed material F the angle of internal
friction
30
Cox et al (1991)
Onshore
Offshore
31
Exceedance probability
Rayleigh Distribution
Critical of ?P for sand stability is
0.5. (Madsen, 1974)
32
Exceedance probabilityOnshore
L 3
L 1
L 2
L 4
L 5
L 6
L 7
L 8
L 9
33
Exceedance probabilityOffshore
L 3
L 1
L 2
L 4
L 5
L 6
L 7
L 8
L 9
34
(No Transcript)
35
(No Transcript)
36
Discussion

• S(du/dt), S(dp/dx), A(u)
• Highly correlated for whole time series
• Low correlation for individual wave
• Should be considered to model with instantaneous
  acceleration
• Exceedance probability
• Offshore direction is dominant in this exp.
• Some extreme cases were found

37
(No Transcript)
38
(No Transcript)
39
Madsen (1974)

• Large inertia force combined with the loss of
  stability of the bed result in significant
  onshore sediment transport.
• ?Pcrit 0.5