Coast Eng . Res Ctr
MR 76-11
Measurement Techniques
for
Coastal Waves and Currents
by
P.G. Teleki
F.R. Musialowski
D.A. Prins
MISCELLANEOUS REPORT NO. 76-11
NOVEMBER 1976
foe re
\ coLiecrion
ee
U.S. ARMY, CORPS OF ENGINEERS
COASTAL ENGINEERING
RESEARCH CENTER
Sa vs 3ap60 :
Reprint or republication of any of this material shall give appropriate
credit to the U.S. Army Coastal Engineering Research Center.
Limited free distribution within the United States of single copies of
this publication has been made by this Center. Additional copies are
available from:
National Technical Information Service
ATTN: Operations Division
5285 Port Royal Road
Springfield, Virginia 22151
Contents of this report are not to be used for advertising,
publication, or promotional purposes. Citation of trade names does not
constitute an official endorsement or approval of the use of such
commercial products.
The findings in this report are not to be construed as an official
Department of the Army position unless so designated by other
authorized documents.
Tiny
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REPORT DOCUMENTATION PAGE BEFORE COMPLETING FORM
1. REPORT NUMBER 2. GOVT ACCESSION NO.| 3. RECIPIENT'S CATALOG NUMBER
MR 76-11
4. TITLE (and Subtitle) 5S. TYPE OF REPORT & PERIOD COVERED
MEASUREMENT TECHNIQUES FOR COASTAL WAVES AND
CURRENTS
Miscellaneous Report
6. PERFORMING ORG. REPORT NUMBER
8. CONTRACT OR GRANT NUMBER(a)
- AUTHOR(s)
PeGeetelc
F.R. Musialowski
D.A. Prins
- PERFORMING ORGANIZATION NAME AND ADDRESS
Department of the Army
Coastal Engineering Research Center (CEREN-GE)
Kingman Building, Fort Belvoir, Va. 22060
10. PROGRAM ELEMENT, PROJECT, TASK
AREA & WORK UNIT NUMBERS
D31191
12. REPORT DATE
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- CONTROLLING OFFICE NAME AND ADDRESS
Department of the Army
Coastal Engineering Research Center
Kingman Building, Fort Belvoir, Va. 22060
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UNCLASSIFIED
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Approved for public release; distribution unlimited.
17. DISTRIBUTION STATEMENT (of the abstract entered in Block 20, if different from Report)
18. SUPPLEMENTARY NOTES
- KEY WORDS (Continue on reverse side if necessary and identify by block number)
Current meters Telemetry
Dye tracers Towed Oceanographic Data Acquisition System
Nearshore currents Wave gages
Sea sled Waves
20. ABSTRACT (Continue an reverse side if necesaary and identify by block number)
A Towed Oceanographic Data Aquisition System (TODAS) consisting of a towed
platform (sea sled) with current meters and a wave gage has been developed for
collection of information on nearshore currents and waves. Data acquired by the
sensors are telemetered to shore and digitally recorded. TODAS is used for real
time evaluation of flow characteristics between shore and a depth of 9.14 meters
(30 feet); this mobile battery-operated system can be used at remote locations.
The system has been used principally in two experimental designs: (a) Monitor-
ing the distribution of longshore currents in _shore-normal profiles, and
DD , FES 1473 ~~ EDITION OF 1 NOV 65 1S OBSOLETE
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(b) a combination of Eulerian-Lagrangian experiments where fixed-point metering
is supported by aerial photography of diffusing dye plumes and concentration
measurements of the dye tracers.
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PREFACE
This report is published to provide an analysis and discussion of the
Towed Oceanographic Data Aquisition System (TODAS), developed for the
collection of information on nearshore currents and waves. The work was
carried out under the coastal processes research program of the U.S. Army
Coastal Engineering Research Center (CERC).
The report was prepared by Dr. P.G. Teleki, F.R. Musialowski, and
D.A. Prins under the supervision of Dr. David B. Duane, former Chief,
Geological Engineering Branch, and Dr. William R. James, his successor.
Several people were instrumental in designing and constructing parts
of the TODAS: E.A. Maiolatesi, N.F. Lang, and B.W. Keebaugh of the
Instrumentation Branch, who designed the circuitry of the wave gage and
current meters; Dr. J.R. Weggel, L.L. Watkins, and C.D. Puglia, who helped
with the design and modification of the sea sled; and G. Sine and other
members of the Hydrographic Survey Section, Survey Branch, U.S. Army
Engineer District, Los Angeles, whose enthusiastic participation in the
field experiments was much appreciated. The support and ideas of Dr.
Duane on the sea sled and experimentation are gratefully acknowledged.
Comments on this publication are invited.
Approved for publication in accordance with Public Law 166, 79th
Congress, approved 31 July 1945, as supplemented by Public Law 172, 88th
Congress, approved 7 November 1963.
WG!
OHN H. COUSINS
Colonel, Corps of Engineers
Commander and Director
CONTENTS
Tig INTRODUCTION.
1. General Concepts.
2. Background and Tjeczines
IEALS TOWED OCEANOGRAPHIC DATA ACQUISITION SYSTEM (TODAS)
1. Sea Sled.
2. Instrumentation .
I POWER REQUIREMENTS.
l ELE Ctroniics .
2. System Gan ahexeion.
IV. EXPERIMENTAL DESIGNS.
1. Continuous Surveys Along chonee Normal POeLLCS
2. Fixed-Point Measurements.
3. Fixed-Point Measurements Goneaned Ten Lacmemciian
Techniques
4. Aerial phe conraphys
We CONCLUSIONS AND RECOMMENDATIONS .
LITERATURE CITED.
APPENDIX.
TABLES
Scan rate selections
Timed start intervals.
Comparison of scans-record times records-file versus time of
recording .
Properties of water-tracing dyes
FIGURES
Sea sled .
Re versus z/d versus Cp.
max
ea d
Distribution of U
Distribution of ———
Tax> au/dt.. case SD.
Case 5-B, fp and f; versus phase angle
du/dt, case 5-B .
CONTENTS-Continued
FIGURES
Case 3-D, fp and f; versus phase angle.
Engineering drawing of sea sled .
Reference axes for sensor alinement .
B-10 current meter with schematic
Diagram reed switches and closure sequence.
B-7 sensor.
Response curve, B-10 sensor .
Photos of wave gage .
Batteries, charger, and cylinder for power package .
Bulkhead connectors .
Generalized schematic of onboard electronics.
Instrument package and cylinder .
Transmitter .
Onshore electronics
Flow diagram, instrument components
Data logger with enlargement of control and monitoring
panel.
Magnetic tape recorder and specifications .
B-7 sensor, Vinnax versus Ana.
B-7 sensor, Vmax Versus ane
Phase lag versus frequency.
True cosine response curve and actual B-10 sensor response.
Field testing of current meters
Experimental grid for sea sled experiments.
Page
7.
20
23
25
26
27
28
29
Sl
32
SS
34
34
55
37
38
40
44
45
27
48
49
51
29
30
31
32
33
34
35
36
37
38
CONTENTS -Continued
FIGURES
Orthogonally mounted current meters
Pairs of horizontally mounted current meters.
Example of longshore current distribution .
Example of energy spectra along shore-normal profile.
Current meters alined for measurement in vertical profile .
Comparison of velocities from dye transport and current
meters
Buoy shore-marker grid.
Typical flight lines for imaging coastal dye dispersion .
Zoom Transfer Scope .
Oblique photo of dispersing dye and map of rectified photo.
Page
53
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54
55
S7/
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66
67
MEASUREMENT TECHNIQUES FOR COASTAL WAVES AND CURRENTS
by
P.G. Telekt, F.R. Mustalowskt, and D.A. Prins
I. INTRODUCTION
1. General Concepts.
The coastal zone is the most frequently observed part of the ocean.
Its shorelines have been explored and described for at least two cen-
turies, but only in the last 20 years have coastal resources been studied
or damages to property systematically documented; however, considerable
knowledge about some of its most important physical processes remains to
be secured.
This report examines a manner in which coastal currents, one of the
least understood processes, may be studied. Currents represent the flow
of water which transport and disseminate heat, salt, and particulate
matter. Currents converge near headlands and marine structures, diverge
near islands and in bays, and influence navigation. Because they are
the main transportive mechanism for the sediments on the ocean floor,
currents in coastal areas often interfere with human intentions by erod-
ing and depositing sand in undesired locations and in undesired quantities.
Probably half of all problems in coastal engineering can directly be
traced to this physical process.
The study of currents requires recognition that the flow of water in
relatively shallow water may originate in one or more of the following:
Wind, waves, tides, density stratification, and even internal waves. The
most difficult part of analyzing current records is perhaps in assigning
the proper weight to each of the components in a field of complex or
interactive flow; also difficult is the assessment of temporal and spa-
tial variabilities (or stabilities) or the modeling of the currents for
predictive purposes.
Coastal currents characteristically exhibit the effect of seasonal
changes and trends which may be peculiar to a geographic area of study,
and incorporate components of the large-scale circulation of the oceans.
Experiments which include consideration of long-term changes in current
velocities, distribution, and structure are not commonly planned for the
coastal zone.
Engineering practice often classifies coastal currents by direction
(onshore versus alongshore) and by area shoreward or seaward of the zone
of breaking waves. Coastal engineers prefer to emphasize the longshore
currents generated by breaking waves, a natural consequence of its rela-
tion to littoral drift; oceanographers tend to view coastal currents in
a slowly varying, wind- and pressure-driven, stratified, and bathymetry-
dependent sense. In either case, currents represent a sediment-moving
force responsible for sediment entrainment, transport, and deposition,
whether occurring in open shelves or in entrances to harbors and tidal
inlets. Recent evidence suggests that sediment transport is not limited
to the narrow zone between the breaking waves and the limit of their up-
rush, but that currents of various origin actively transport sediments
parallel to shore as distantly as the edge of the Continental Shelf.
This sediment transport has implications in offshore engineering, an area
of increasing concern.
In nearshore areas, the main contributing factor to current generation
is the waves shoaling over the Continental Shelf and refracting with de-
creasing depth. Depending on the angle of incidence between the wave
orthogonal and the bottom contours (ultimately the shoreline), coastal
currents might be generated parallel or normal to shore, a matter of
considerable engineering significance. The currents contain an admixture
of periodic and aperiodic water motions. However, in several geographic
areas superimposed wave trains of different amplitudes, frequencies, and
directions will combine to produce random wave fields. The flow generated
by these waves may be stochastic or ergodic (or neither). During the
passage of storms, the wave field and the resulting flow field may also
become nonstationary.
The laminar nature of the flow to this point was implicitly assumed,
and for the potential flow region of relatively deep waters, this assump-
tion may often be correct. However, at the sea floor the boundary layer
may be intermittently to fully turbulent independently of the nature of
flow above it. As the water depth decreases shoreward the scales of
motion become compressed from three into two dimensions and the result is
that coastal, shallow-water currents become increasingly more turbulent
for the full depths of flow. This action induces mixing which is charac-
teristically at maximum near breaking waves and by its rotational nature
drastically modifies the current-induced motion of sediments. Other
effects, such as wave-wave interaction and wave-current interactions,
also influence forces imparted to the flow and the sediments. When ex-
periments are designed for the measurement of currents, essentially two
techniques are available: the Eulerian method and the Lagrangian method.
With the Eulerian method, water motion is observed past one or more fixed
points; with the Lagrangian method, a water particle is followed down-
stream, and changes in its motion (speed and direction) are observed.
Neumann (1968) stated, ''The most complete description of oceanic currents
is obtained from a combination of both Eulerian and Lagrangian methods."
This statement is equally appropriate to shallow-water currents whose
variations in space and time near the coast, are even more complex than
those of major ocean currents.
This study describes Eulerian and Lagrangian techniques used in the
measurement of coastal currents. Through technological improvements for
the survey of the nearshore flow characteristics, new experimental ration-
ale were developed. The report also documents data collection methods and
discusses problems attendant to obtaining accurate measurements.
2. Background and Objectives.
Research in the harsh physical environment of the nearshore zone has
always been difficult to implement. The problem area is the zone of
breaking waves where navigation and the placement and operation of instru-
ments have had many failures. This zone is of interest to those involved
in the study of nearshore sediment transport mechanics, the distribution
of longshore currents, and the dispersal and mixing of water masses, the
physical description of which has suffered for the lack of continuity in
surveys between the beach and the offshore area. Since the wave climate
is seldom constant, these surveys must be rapidly conducted, the sensors
used must be rugged enough to withstand the wave impact forces,. and the
environmental parameters measured should be analyzed in real time to
collect representative data.
The first requirement is for a device which will negotiate the surf
and carry oceanographic instruments at the same time. Several precedents
for such an implement exist, and most result from the interest in military
amphibious operations during World War II, although the type developed by
Isaacs (1945) was intended to carry demolition charges. Isaacs' sled was
perhaps the first of a generation of similar devices designed explicitly
to negotiate the surf. Another who experimented with self-propelled
(or wave-action propelled) sleds was Johnson (1949) in measuring water
depth.
The sled described in this report (Fig. 1) was originally designed by
Robert Sears of the U.S. Army Engineer District, Baltimore. Kolessar and
Reynolds (1966) named it the Sears sled. It was engineered to be a towed
stadia rod (a mast riding on a frame) for sounding the nearshore zone.
The sled could be winched to shore from 9.14 meters (30 feet) of water
or less but had to be deployed again at the beginning of the next survey
line; Kolessar and Reynolds engaged a helicopter for this phase of the
work.
Certain aspects of the Coastal Engineering Research Center (CERC) sea
sled (using Johnson's (1949) terminology), such as the sled's framework
and onshore incremental movement in one mode of operation, reflect its
heritage. However, the use of the sled in the measurement of nearshore
currents and waves is a departure from previous experience. The sled is
a component in TODAS (Towed Oceanographic Data Acquisition System), which
includes the amphibious craft towing the sled, the sensors and the asso-
ciated telemetry, data conversion and storage units, future improvements
(cathode ray tube (CRT) minicomputer), auxiliary dye studies, and aerial
photography. At present the capabilities of TODAS extend to measuring
currents and waves in shallow water (in 0.91 meter (3 feet) < depth <
9.14 meters (30 feet)) in "quasi-real time," with provisions made for
increased capabilities (more and varied-purpose instrumentation and on-
site data analysis).
The development of TODAS was a 3-year effort, during which the system
was continually upgraded and tested under both laboratory and field con-
acai
et
Figure 1. Sea sled.
ditions. Between 1972 and 1974 several experiments were held in
California and one in Michigan. These experiments included navigation
tests, instrument calibration, different data collection modes, evaluation
of experimental designs, and timing’ of support operations (or the support
of other experiments).
The goal of all field experiments was to collect the most comprehensive
environmental data possible with the given capabilities. Thus, many sup-
port operations required collection of wind, temperature, and salinity
data, the areal reconnaissance of bottom roughness, and sediment prop-
erties. TODAS was used on several occasions to support radioactive
sediment tracing (RIST). Most operations were conducted near coastal
engineering structures so that the results could provide insight to their
performance.
TODAS is unique in that it is self-contained, i.e., requiring no
field installations or commercial power sources, and therefore can be used
at remote sites. It is applicable to both coastal and estuarine research.
II. TOWED OCEANOGRAPHIC DATA ACQUISITION SYSTEM (TODAS)
The separable and identifiable components of TODAS are: (a) The sea
sled; (b) the current meters; (c) the wave gage, onboard electronics, and
power supply; (d) the telemetry; (e) the onshore signal conversion; and
(f) the data conversion and recording units. Certain other components
attached by experimental design, such as the multispectral photographic
equipment used in imaging tracer dyes for charting surface currents, may
also be included.
1. The Sea Sled.
The functional parts of the sled are a 9.14-meter-long mast, attached
crossmembers (spars) supporting current meters, a frame, and a pair of
skis on which the mast-spar assembly and two cylinders containing an
electronic package and the power supply ride. The total weight of the
sea-sled frame and the components of the data acquisition system mounted
on the sled is 347 kilograms (765 pounds), exclusive of additional weights
which can be added for more stability in a high-energy surf. Although the
sea sled has a low gravitational center, certain conditions can overturn
the platform, e.g., excessive wave forces, steep slopes, or a combination
of both.
A calculation of the critical overturning moment and a description of
the upper (safe) limit of environmental conditions under which the sled
may be used in the field are given below.
a. Wave-Force Analysis. In the operation of a structure at sea, two
commonly encountered hazards are structural damage (i.e., bending, break-
ing, and twisting of members) and overturning of an unanchored platform.
Both hazards are the result of excess wave forces. Thus, the design of
a structure requires a survey of wave and bottom conditions likely to be
encountered by the towed sea sled and the subsequent calculation of wave
forces on the components of the platform.
The total force exerted by waves on a unit section, dz, of an object
is composed of the drag force and the virtual mass force:
Ge = (Gi) 2 a) ce, (1)
where the drag force is proportional to the square of the orbital velocity
in the horizontal plane:
fy = 1/2 EpoD (lulu) (2)
and the virtual mass force is proportional to the horizontal acceleration
force exerted on a mass of water displaced by the object:
f; = 1/4 CypmD? 2 (3)
In equations (2) and (3), D is the diameter of the cylindrical
object, p is the mass density of seawater, u is the horizontal compo-
nent of the orbital wave velocity, and Cp and Cy are the coefficients
of drag and inertia, respectively.
The horizontal components of velocity and acceleration can be approx-
imated using:
= ah coshvki@at i) eOSunt
aR cosh kd Q (4)
and
oul.) 2neHiicosh kdl (ated) Sie (5)
ot 12 sinh kd
where k = 2n/L and w = 27/T are the wave numbers, y is the vertical
coordinate measured upward from the bottom, d is the water depth, H
is the wave height, T is the wave period, and L is the wavelength.
Expanding equation (4), the force exerted on a section of a pipe, dz,
at any position z _ becomes:
df s maoDHe
dz 2T2
The overturning moment about the bottom according to Morison, et al., (1950)
TSS
fefinsin wt + fpcosquutiia (6)
y DHL? (_np
Maa Go jer = eee
Because the small-amplitude theory underestimates u in shallow water,
substitute the empirical stream function theory of Dean (1965) using cases
5-B and 3-B (Dean, 1975) for illustration (Figs. 2 to 6). Case 5-B repre-
sents the deepest water in which the sled is operated; case 3-B represents
the breaking wave conditions.
Cyk Sinwt ar CpK,e0s%ut). (7)
Upper Section
Middle Section
’
?
Mast, Lower Section
o o
Sj o
= =
z/d
cv 0)
ke
rs)
u
Le)
[=)
3
£
5
oO
104 108 io®
_ Umax 9j
ey
Figure 2. R, versus z/d versus Cp (U.S. Army, Corps of Engineers,
Coastal Engineering Research Center, 1975; Fig. 7-58).
Z/d
O | 2 3 4 5 6 7 8 9 Lop
U=[U /(H/T)]}, du/at
Umax
Figure 3. Distribution of ———,
(H/T)
du/dt, case 5-B.
Ome 2 AS) 4S Ger 7) nO GamtO™ oll
U, du/dt
Figure, 4... Dist rubutron of Ura. du/dt, case S—D.
Case 5-B
for z/d =O.
fo) 1/4 = wW/2 30/4 Vy ues sit /2 eur A 27
Figure 5. Case) 5-B,, fy and f; versus phase angle.
180
160
140
100
60
fo ofj
-60
-100
fo) Wea fe Suffre WAS 57/4 3n/e 71/4 27
Figure 6. Case 3-D, fp and f. versus phase angle.
Members used in the sled construction with components contributing to
fp and fj, have "pipe-equivalent"' diameters in the range 4.87 milli-
meters (0.016 foot) < Dj; < 27.3 centimeters (0.896 foot). The distribution |
of Reynolds numbers dependent on Dj is:
ms Umax (9)i Di
v
, (8)
with a kinematic viscosity of 9.29 X 107% square centimeters per second™!
(1 X 107° square feet per second!) (Fig. 2) indicates the steady flow
Cp = 1.2 is applicable in all but the case of the two cylinders. The
variation of Re in the vertical is the result of the variation of Umax
(9 = 0°) with depth, for case 5-B (Fig. 3) and for case 3-D (Fig. 4).
Figures 3 and 4 also show the depth-dependent distribution of the hori-
zontal acceleration for 6 = 20° which is near the maximum for fj. As
fpmax and fimax are not in phase (Figs. 5 and 6), the maximum total
force on the sled can vary (9 = 22° to 45° when fj is nonnegligible).
The maximum force corresponding to the wave crest is always larger than
the maximum under the trough of the wave.
The representative wave parameters for case 5-B are H = 3.53 meters
(11.6 feet), T = 12.25 seconds, and d = 9.14 meters (the normal limit of
seaward excursion). The total drag force, calculated for case 5-B is
fp = 4,151.2 newton and the inertia force is 168.3 newton; this is balanced
by the submerged weight of the sled (225.89 kilograms or 498 pounds). The
overturning moment, combining both fp and fj, is 18,853 newton meters
(8,741.6 foot pounds) against 2,886.65 newton meters (2,196.8 foot pounds)
resistance along the short axis of the sled, and 3,376.44 newton meters
(2,490 foot pounds) in the direction of towing. Therefore, the sled will
be unstable under these wave conditions.
In the examples of breaking wave conditions (case 3-D), only a part
of the sled is submerged. The overturning moment along the short axis
is 1,115.58 newton meters (822.7 foot pounds) (inertia forces not contrib-
uting), balanced by 2,978.86 newton meters (2,196.8 foot pounds) of resis-
tance. This demonstrates that the sled is operable in 1.52-meter depths
(5 feet) and 1.34-meter-high (4.38 feet) breakers at T = 10 seconds,
commonly encountered under fair-weather conditions in coastal areas.
The above examples demonstrate that the danger of overturning in deep
water is greater than in the breaker zone. Where the combined forces
become excessive the corresponding sea state usually prevents operations
a prtort. However, with the sled parked offshore for continuous measure-
ments, the possibility of loss in changing wave conditions is real.
b. Construction and Materials. The two main functional components of
the sea sled are combinations of a mast and spars used to support oceano-
graphic instrumentation and a pair of runners on which the device slides
along the ocean floor. The runners are constructed of a 1.27-centimeter
(0.5 inch) aluminum plate, a 7.62- by 1.5-centimeter (3 by 1.5 inches)
aluminum channel, and a 8.89-centimeter-diameter (3.5 inches) aluminum
pipe. The center platform is 0.61 meter (2 feet) square, constructed
18
of 0.635-centimeter (0.25 inch) aluminum plate, and welded to two 7.62-
by 3.8l-centimeter (3 by 1.5 inches) cross channels (Fig. 7).
An additional cross channel is attached across the edge of the runners.
The 9.14-meter-high mast consists of three sections. The lower section
is a 6.03-centimeter (2.375 inches) outside diameter 2.77-meter-long
(9.08 feet) aluminum pipe; the middle section is a 4.82-centimeter
(1.9 inches) outside diameter, 3.03-meter-long (9.96 feet) pipe; and the
upper section is 3.34-centimeter (1.315 inches) outside diameter, 3.04-
meter-long (9.98 feet) aluminum pipe. These sections are connected by
coupling joints and the bottom of the mast is attached to a flange on
the center platform. The mast is guyed at 12 points, in groups of four
guys each at elevations of 3.048, 6.096, and 9.14 meters (10, 20, and
30 feet), and supports three or more adjustable spars to which the current
meters are attached.
All members were constructed from extruded, aluminum alloy 6061-T6.
Aluminum is preferable because of weight and corrosion resistance; the
6061-T6 alloy also has higher tensile strength than other alloys. The
corrosion resistance is not affected by welding although there is some
loss of strength in the weld area or heat-affected zone. Alloy 6061-T6
is the least expensive of all the heat-treatable alloys produced.
Where magnetic fields are present, stainless steel is not preferred
to aluminum (see discussion of current meters); also, aluminum better
absorbs impact loads from waves.
Erection of the structure takes about 2 hours and requires no special-
ized tools for assembling. The main structure is similar to equipment
used in nearshore surveying; the difference being the spars, the skid
design, and the instrument powerpacks. The sled's weight is increased by
bolting steel bars to the runners, preferably to the bow part.
Damaged sections of the sled can be repaired at the field site with
an arc welder.
c. Operation Modes and Required Support. Two standard operations
have been practiced with the sled, the choice depending on the type of
flow-measurement experiment. One operation is locating the sled at a
desired point offshore and measuring currents im situ; the other requires
towing of the device by a LARC V or LARC XV (lighter amphibious resupply
cargo) vehicle along a given profile, usually alined normal to shore.
The latter operation is an incremental movement of the platform, usually
commencing in 9.14 meters of water to avoid submerging the antenna, and
currents are measured for a given length of time at stations in sequence
along the profile. During operation in this mode, the sled is decoupled
from the LARC during recording time to prevent any movement of the sled
due to drifting of the LARC. This procedure is repeated for each succes-
sive position of the sled along the profile (positions are usually pre-
calculated). A pair of shore targets normal to the profile gives the
LARC operator his control when moving in on the profile. Actual locations
19
Part
Part Number
RUNNER ASSEMBLY
Bottom Plate 1
Channel 2
Spacer 3
Bearing Plate 4
Brace 5
Guy Cable Anchor 6
Skirt 7
PLATFORM ASSEMBLY
Cross Channel 8
Cross Member Support 9
Cross Member Connector Plate 10
Brace Connector Plate ll
Platform Plate 12
Mast Support Plate 13
Mast Support Tube 14
Mast Support Brace 15
Cradle Anchor 16
Cradle Anchor 17
Platform Brace 18
Cradle Spacer 19
FRONT CROSS MEMBER
Cross Channel 20
Cross Member Connector Plate 21
Brace Connector Plate 22
Tow Cable Connector Plate 23
REAR CROSS MEMBER
Cross Channel 24
Cross Member Connector Plate 25
Brace Connector Plate 26
MAST ASSEMBLY
Lower Section 27
Middle Section 28
Upper Section 29
Upper Couple 30
Lower Couple 31
Couple Guy Anchors 32
Upper Mast Guy Anchor 33
Upper Spar 34
Middle Spar 35
Lower Spar 36
Spar Mount 37
Turnbuckles 38
Guy Cable 39
CYLINDER CRADLE
End Plate 40
Channel 41
Base Plate 42
Brace 43
POWER PAK CYLINDER
Main Cylinder 44
Flange 45
Lid 46
Handle 47
Bulkhead Connector 48
ELECTRONICS CYLINDER
Main Cylinder 49
Flange 50
Lid 51
Handle 52
Bulkhead Connector $3
Figure 7.
fe) | 2
Engineering drawing of sea sled.
20
A we
X VANE
ai
Scale in Feet
()
PEASE] = 3
Figure 7. Engineering drawing of sea sled (continued)
2|
of the sled on the profile line are determined with transits while re-
cording is taking place.
The sled is towed with a 0.794-centimeter-diameter (0.3125 inch)
galvanized steel cable coupled to harnesses on the sled and on the LARC.
Cable lengths vary depending on the mode of operations. Several 30.48-
and 91.44-meter (100 and 300 feet) sections of cable which can be attached
consecutively are carried onboard the LARC. About 152.4 meters (500 feet)
of cable are needed to turn the sled around in depths of 9.14 to 15.24
meters (30 to 50 feet) and to aline the sled on a profile headed toward
the shore. Care must be taken when using shorter lengths of cable since
the sled has a tendency to hydroplane and could possibly capsize if an
obstruction was hit. Extra cable is also added to avoid coupling and
decoupling when passing through the surf zone.
While the sled is under tow, a minimum of two people are needed, a
LARC operator and a deckhand. Experimental designs requiring these oper-
ations are discussed in the next section.
2. Instrumentation.
The sea sled was principally designed for deploying current meters in
depths not over 9.14 meters. The sled's most important assets are the
versatility of measurements provided by the spars which can be raised,
lowered, or rotated, and the current-meter mounts which are also adjust-
able to alinements on any of three axes. Thus, sensors can be placed in
any configuration and at any elevation below 9.14 meters. In practice,
the sensors are seldom placed closer than 0.91 meter (3 feet) of the
bottom because streaming around the cylinders is expected to bias records
produced by the sensors. In addition to the current meters, a pressure
wave gage is commonly installed on the sled. However, the number and
kinds of instruments are not limited by the sled, as long as a method to
collect the data from the instruments is provided.
Realining a sensor or moving it from one elevation to another can be
executed underwater by a diver. This operation requires the loosening of
setscrews of the cross fittings which support the spars.
Upward or downward movement is limited to each 3.048-meter length of
each section of the mast. While the spars can be rotated in horizontal
plane or about their axes, the current meters can also be rotated 360°
about the spar axis; this is easily done even underwater by loosening a
pair of U-bolts holding the mount. If necessary, the sensors can also
be moved laterally. Rotation about the spar, if alined normal to shore,
will change a shore-parallel recording to a vertical or vice versa; to
aline in a shore-normal position will require complete removal of the
mount and the attached sensor (Fig. 8).
a. Current Meters. Of the four types of current sensors commonly
in use (acoustic, inductance (EM), force, and mechanical), the mechanical
current meters are the most reliable in the surf zone. The principle of |
22
nor?
ae S
o
re
@
>
Figure 8. Reference axes for sensor alinement.
23
operation is to channel flow across an impeller mounted in a duct (Fig. 9).
Magnets embedded in the blades of the impeller close reed switches, and in
a bidirectional sensor the switching sequence (Fig. 10) determines the
direction of flow.
Ducted, impeller current meters by Bendix were used in experiments
during the last 3 years. The first set of meters had four-blade impellers
in a 7.62-centimeter (3 inches) duct (Bendix Model B-7) modified for
bidirectional response (Fig. 11); the second set of four meters (Bendix
Model B-10) was designed to be bidirectional sensors with five-bladed im-
pellers in 10.16-centimeter (4 inches) ducts. In the latter model, range
is 0 to 5 knots (0 to 257 centimeters per second) with a threshold velocity
of 2 centimeters per second. In the range 1 to 100 pulses per second,
meter response is linear (Fig. 12). For calibration purposes, 16 pulses
per second equal to 1-knot current is used.
b. Wave Gage. Measurement of the height and period of waves, con-
currently with the sensing of current speed, is obtained from a pressure
wave gage (Fig. 13). This instrument consists of a solid state, strain
gage pressure transducer with a range of 0 to 11.34 kilograms per square
centimeter-absolute (0 to 25 pounds per square inch-absolute) and housed
in a 0.63- by 27.3-centimeter (2.0625- by 0.896-foot) (outside diameter)
aluminum cylinder. The gage senses the total pressure at a given depth
from which the dynamic pressure component is extracted and used as the
measure of wave height given the pressure correction factor at that loca-
tion. Since it is difficult to determine the pressure correction under
various conditions of shoaling waves, the gage is usually mounted adjacent
to one or more current meters which provide an independent estimate of the
correction factor. The wave gage is designed to switch ranges in incre-
ments of 3.048-meter depths with a +0.6l-meter overshoot. This gage was
designed at CERC, and permits conduct of continuous surveys in variable
water depths to a maximum of 9.14 meters.
c. Auxiliary Instrumentation. A comprehensive coastal oceanographic
experiment usually requires the measurement of several additional param-
eters, such as windspeed and direction, barometric pressure, salinity, and
water temperature. The TODAS is capable of accommodating these future
requirements in one of two ways. Sensors can be added to the sea sled
part of TODAS, thereby requiring only the installation of one voltage-
controlled oscillator (VCO) for each new sensor (assuming an output of
0 to 5 volts (direct current)). Signals from these sensors can be mixed
and demodulated in the same fashion as for existing current meters and
the wave gage. One frequency discriminator has to be installed at the re-
ceiving unit for each sensor.
Another capability of TODAS is the direct input of signals from sen-
sors not housed on the sled (e.g., from a shore-based anemometer). The
Signal input, in this case, can be either analog or digital (Binary Coded
Decimal-BCD) format; a total of 32 sensors can be accommodated.
24
Bearing Pin
Reed Switch
Housing
Impeller
Bearing Pin
Magnets
Figure 9. B-10 current meter with schematic.
ZS
Electrooceanics
R | 5IOF4F Receptacle A
220 2 ; :
1 To
Instrument
O T Package
O
Reed Switch *| yi.
Sige rene Cable
Reed Switch #2 a
Electrooceanics
52F4M-1 Connector
witch
aa Closes *| | |
Reed
Switch #|
Positive Direction
+
Reed | | Closes *2 | |
Switch #2 | |
ted
Switch
—<+—,
is Cl # ira
Read oses 7|
Switch |
Negative Direction
Switch
Reed | | Closes #2 | |
Switch #2
Time —————>
Figure 10. Reed switches and closure sequence.
26
Roteguaaey Ils) = 7/ Sensone.
Oil;
“IOSUdS QT-g
‘aaano osuodsay
(s/sasind) jndjno s8,9W
“ZI oansty
(Uy) paads juesing
28
Figure 13. Photos of wave gage.
(as)
III. POWER REQUIREMENTS
TODAS was designed to operate on a direct-current power supply which
would give experimental capability in remote localities. In previous
experiments, either a conventional source or a portable generator (alter-
nating current) was used to avoid the problems of handling and charging
batteries. During field experiments, a small portable gasoline-powered
generator proved adequate for operating the receiver, discriminator,
data logger, analog, and digital recorders. The power consumption of
the onshore system is about 7 amperes per hour.
All components onboard the sea sled are powered by four 12-volt
rechargeable Gel Cell batteries housed in an aluminum cylinder (0.63 by
27.3 centimeters, outside diameter) that rides on the sled (Fig. 14).
Each battery is rated at 20 ampere-hours, giving a total of 80 ampere-
hours (equivalent to 260 hours) continuous underwater operation without
recharging. The battery and instrument package are connected by an eight-
conductor neoprene cable with bulkhead connectors (Fig. 15).
1. Electronics.
In an earlier version of the present system, all data were obtained,
processed, and recorded as analog signals (voltage) onboard the sled.
Since this system precluded real-time observation of the data flow needed
for appraising the performance of sensors and for determining whether
Significant values of ocean parameters are being recorded, modifications
were undertaken which separated data acquisition into onboard (on the
sled) and onshore components.
a. Signal Acquisition, Mixing, and Telemetry. Outputs of the current
meters are dual overlapping pulses which are converted to an analog signal
(+ volt, direct current) by the signal-conditioning circuitry, one for
each sensor (Fig. 16 and App.). The processed signals are amplified to
match the input of the VCO and then mixed before being relayed to the
transmitter (Fig. 16). Pulse to analog conversion, amplification, analog
to frequency conversion, and mixing take place within the instrument pack-
age housed inside an aluminum cylinder (0.61 meter long by 27.3 meters in
diameter) which rides on the sled (Fig. 17).
Output of the pressure wave gage (0 to 5 volts, direct current) and
the direct-current signals from each current meter are converted into
frequencies by the VCO's, mixed through a series of operational amplifiers,
and transmitted to a shore-based receiver on 27.454 megahertz frequency
band. The transmitter, with its antenna mounted at the top of the mast,
has a power output of 2.2 watts (Fig. 18). Effective operational limits
of the telemetry system is about 8.05 kilometers (5 miles). Radio noise
becomes a problem beyond this distance.
b. Receiving and Demodulation. The modulated signals are usually
received onshore (Fig. 19) (although signals can also be received on the
30
SLEPT ATT LEA PT
Amphenol Connector ~
Cat i
Bulkhead Connector
Amphenol Receptacle
“Globe Gel-Cell Batteries &
Battery Charger
Figure 14. Batteries (upper photo), charger, and cylin-
der (lower photo) for power package.
3|
5 wat
1% HEX QO Ring
Part 739-217
$-}
16 5
3%
1%
1-14 THD
%
Sin Del aA VDE ne
Type Voltage Rating
RM 'S' 4S BCL (Brass) 300
RM 'S' 4S BCL (SS)
Ef. Ba
'
le
ot eae Plug 5%
HO (On:
MALE FEMALE
RM 'S' 4S MP RM 'S' 4S FS
Figure 15. Bulkhead connectors.
32
Current
Pulse to
Meters anolgaue
Line ae
Dual
Overlapping
Pulses Calibration Constant
Circuitry Selection
Antenna
-l2vde.
Voltage
Regulators
+l2vd.c.
Oto Svd.c. from
Pressure
Transducer
Figure 16. Generalized schematic of onboard electronics (letters
refer to enlarged schematics in Appendix).
39)
Instrument package and cylinder.
leauieqbbetey ALT) «
Transmitter.
Liz
Figure
34
Figure 19. Onshore electronics.
35
towing vessel), and discriminated according to channel number and infor-
mation content. The discriminated signals are monitored on a six-channel
analog recorder for real-time visual observation and for a permanent
continuous record.
c. Analog-Digital Conversion and Recording Modes. Output from the
discriminators enters a data logger consisting of several major parts
(Fig. 20). The data logger converts the analog signals to digital form,
assigns the time to the data points, identifies the channels (sensors),
and outputs the digitized data for recording on magnetic tape.
The data logger (Fig. 21) is a portable alternating or direct current-
powered analog to digital converter scanning up to 32 analog channels at
the rate of 33.3 channels per second, converting 200 characters per second,
each character having six BCD bits. Analog multiplexing is done with
COSMOS switches. The unit accepts up to 12 parallel bits of external data
recorded in 10 (4-bit) bytes.
The main feature of the data logger is the system's variable scan rate
(Table 1) Of On 3336 Os SOS oOo 5 OLWemandms Som channel sper:
second (BCD formating), limited by the tape recorders' 300-character per
second recording speed. Other recorders are available with: higher scan
rates up to 1,000 channels per second; 2° and 2% to 2!2 scans per record
and records per file; single, continuous and monitor-scanning modes; auto-
matic start at intervals of 109 between 1072 and 10! seconds, 1, 5, 10
minutes, and 1, 2, and 24 hours with an interval multiplier of 1 to 9
(Table 2); choice of end of record (EOR), end of file (EOF) or no stop,
visual monitoring of data and time automatic and manual channel advance,
and inter-record gap (IRG) or file gap control.
Multiplier) CCG)
1 2 1
1
Table 2. Timed start intervals.
01
S)
01
0.
10
15
20
25
30
35
40
02
-03
-04
-05
06
-07
-08
20
30
40
50
60
70
80
Woy foo) SS) ony Cn eS eet)
ey ey te) Ss eo se) OO SS
Vey fo SS) ey Ch eS OLS
Wey (oe) St) On, OS DES)
Data are recorded on a seven track, 556 bits per inch magnetic tape
recorder (Fig. 22) which operates on either 110 volts (alternating current)
or 12-volts battery power. Data are recorded on 1.27-centimeter-wide tape
36
Speed-
Sensing A
Circuitry cex/cett |
Operational model
Amplifiers Se e208
Batteries
SONEX
model
TEX-3085 OCEAN-
BENDIX vcOs APPLIED
model 8-10 FAIRCHILD RESEARCH
pucted made! TFISO CORP
Meters Pressure Transducer OT-217
Transmitter
Ngee
Es IS3it MAG model Receiver
Tape Recorder 9100 ADC
CALCOMP
Plotter
cOC
6600
Computer AIRPAX
Recorder Frequency
Discriminators
Analog Single-Channel
Figure 20. Flow diagram, instrument components.
Bil
2n
YEAR HOUR SCAN/REC REC/FILE
WARM UP
1 10 60
SEC
CHANNEL TIME SCAN MOD
1M 5M
Eeieacel ree : . ioe
15 elH
ADC TRIG STOP CONT 15° Te °10H
015 24H
NO
50,'0° 250 asd
25. ie 2500 3e 6
cS Gebel Feller] 3
jo) °2.5K le °8
yar “5K OFF °9
MULTIPLIER
CHAN/SEC
Figure 21. Data logger with enlargement of control and monitoring
panel.
38
Table 1. Scan rate selections.
Switch Data Character Actual Applicable Recorder
position format rate rate Options, MP
_(chan/s) | characters | (character/s) (chan/s) Gal fixer |[Sos
1 2 1 X X X X xX
| 4 0.5 x X X X X
6 0.333] X xX X xX X
5 2 10 § xX X X x X
4 10 2.5 X x X X X
6 1 VAG Oia | sox x x X X
10 2 20 10 X xX xX x X
4 20 S x X X x X
6 20 SSS SN eX X X X X
25 2 50 25 X X X x
4 50 Zi X X X X X
6 50 SSS Sule aeX X x X X
50 2 100 50 x X X
4 100 25 X Xx x X
6 100 16.7 X x X X X
100 2 200 100 X xX xX
4 200 50 X x X
6 200 33/53 X X xX
250 2 X
4 X
6 x x
S00 2 Xx
4 X
6 x
1,000 2 X
4 xX
6 X
2,500 2 X
4 X
6 xX
5,000 2 X
4 x
6 X
10,000 2 x
4 X
6 X
1. Option presently used.
39
OA
<<
l
|
l
I
l
Recording rate
Packing density.
Number of tracks
Tape width. .
Parityae. cuts:
POWeT -trcnnenious
Figure 22
Magnetic
0 to 300 characters per second
. 556 bits per inch
7
0.5 inch
odd
105 to 125 volts, 50 or 60
hertz, 1.5 amperes, 12 volts-
direct current (switch select-
able)
tape recorder and specifications.
40
on 2.16-meter (8.5 inches) coplanar-mounted reels equivalent to 365.76
meters (1,200 feet) of 1.5-mil tape. Recording rate is 0 to 300 characters
per second. Data formated as follows:
———_—$§$ —_—$ $$ —__—_ RECORD errr
DATA
nCHANNELS
FIRST POINT DATA LAST
(Date and Time)
OOYDDDHHMMSS O IRG RECORD IRG RECORD
OS aaeaeee TO IRCOUNDIS) POEURS TRIE Bh
Symbols are: Y = year, D = day, H = hour, M = minute, S = second,
C = channel number, P = polarity, and X = data. Each record is arranged
according to code (each block signifies a bit):
7 TRACK TIME EXT BCD DATA CHANNEL DATA
| jofolo|
exe
400|_40|_, 4
aoa
ae
peer
Track 1248AB
Plus OC © 1 1
Minus OOOOO!
Typical files are composed of 2* scans per record and 2© records per
file, a total of 1,024 data points per channel. Generally, this is con-
sidered a convenient length of information for wave and current data,
requiring 4 minutes and 55 seconds of recording time. A comparison of
different settings for recording data (Table 3) indicates the time required
to record 2!9 data points per channel in the continuous mode can vary be-
tween 1,224 and 2,216 seconds. Records in excess of 212 words are usually
too long for time-series analysis.
2. System Calibration.
Several calibration procedures must be observed with TODAS. he eas sit
is calibrating the current meters for frequency response, accuracy in re-
cording speed and directivity (cosine response), threshold velocity, line-
arity, and environmental response (ruggedness). Coupled to the frequency
response and threshold velocity is the bidirectivity of the sensor, i.e., the
4
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plodey-sueds
42
ability to record continuously across the line of zero flow from the
negative to the positive domain. The threshold frequency determines how
well the meter responds to the oscillatory flow under waves. However, an
ideal sensor should equally be responsive to quasi-steady motions and the
capillary wave range. The limiting factor for ducted meters is the iner-
tia of the impeller's mass (and indirectly by the fit and wear of the
bearings); the B-10 sensors response time to an impulse force is 0.1
second. The system has three options for the time constant, 0.22, 2.2,
and 22.2 seconds.
Directivity of the meter, commonly referred to as a costne response,
is important for determining the velocity of the flow at the point of
measurement, i.e., both the rate of flow through the sensor and its direc-
tion. Since the B-10 meters have near-cosine response, speeds recorded by
three sensors placed in an orthogonal configuration can be resolved into
a resultant vector representing velocity.
Information available on tests performed with the B-7 sensors (Fig. 11)
discussed below demonstrates the utility of a directed, impeller-type
meter for measuring currents which include wave components.
a. Laboratory Tests. Four B-7 current meters were tested in 1972 in
a 29.26- by 0.457-meter (96 by 1.5 feet) wave tank in 0.61 meter of water.
The sensors were attached to a carriage driven by a bulkhead frame with
the bulkhead removed, and oscillated in still water using a programable
wave generator. The reason for a simulated wave condition rather than
real waves was to eliminate reflected waves from either end of the tank.
Two amplitudes of the sine wave input (a, = 25.4 centimeters (10 inches)
and ay = 50.8 centimeters or 20 inches) were used with frequencies of
OROSPOR062 5; 0001714), “ON08,) On 1) Or, 125: 0-0538540.2,, O25, and 0 4ehextizr
The sensors were tested for linearity of response, phase lag, voltage,
and pulse output as functions of frequency and amplitude of oscillation.
Figure 23 shows that the relationship between the maximum voltage,
Amax, and the corresponding maximum velocity, Vmax, is linear:
Vieng Co = -1.499 + 1.983 Amax (mV)
Since a high-frequency oscillation and a long time-constant preclude the
output voltage to reach zero for zero velocity, the sensors' responses
were evaluated with respect to the offset voltage which follows the
relationship:
cm
Vmax (sso) = 3-95 + 2.095 Amin (nV)
as shown in Figure 24. The correlation coefficients were Re = 0.9955 and
0.9907, respectively. Calibration tests for the flow velocity versus
pulses per second output of the B-7 sensors indicated that towing tank-
generated response curves cannot be used in bidirectional flow conditions.
The phase-frequency relationship is a concern in oscillating flow, i.e.
the real time associated with the function forcing the response recorded
43
Vmax (cm /s)
20in Stroke
@ Meter |
ry Meter 2
a Meter 3
v
Meter 4
Rigune: 25:
15 20 25 30
Amax (millivolts)
B-7 sensor, Vpay versus Naot
44
35
40
Vmax (¢m/s)
Stroke 10in
Meter | °
Meter 2
Meter 3
Meter 4
10 Te) 20 25 30
a (millivolts)
35
Figure 24. B-7 sensor, Vnax Versus Amin:
45
by the sensor must be known. Figure 25 shows that the response of the
B-7 current meter is phase-dependent, and the lag (and scatter) increas-
ing toward lower frequencies.
An extensive calibration test has been completed by National Oceanic
and Atmospheric Administration (NOAA), National Oceanographic Instrumen-
tation Center (NOIC) on the B-10 current meters. A later evaluation of
the meters will be expanded to include prototype waves, environmental
tests, response to vibration, and virtual mass effects. Results of the
calibration test show a nearly true cosine response of the B-10 meters
(Ee 26) i
The wave gage was calibrated statically, applying compressed air to
the pressure transducer. The output voltage was then recorded as a
function of pounds per square inch units of pressure.
bs Firelid tests. Calabration of thevcircuitry cis importants inecoulecr—
ing good data. The following steps are observed before each deployment
of themsea) siiedi:
(1) Check power supply voltage output;
(2) calibrate current meter pulse to analog conversion
circuitry with a 16-hertz source (16 hertz = 1 knot);
(3) check radio frequency output from voltage controlled
oscillators;
(4) check radio frequency input to discriminators;
(5) check wave gage voltage output;
(6) check directional response of current meters; and
(7) record offset.
To verify that the performance of all current meters are identical
under field conditions, the sensors are mounted side by side on one of
the spars (Fig. 27) and towed offshore. Consequently, data are recorded
for 3 to 4 orientations of the sled (with respect to the base line), thus
evaluating if any differences in voltage output or phase are present in
the meters.
IV. EXPERIMENTAL DESIGNS
Three experimental designs for the measurement of coastal currents
and the corresponding operations are discussed in this section.
1. Continuous Surveys Along Shore-Normal Profiles.
a. Rationale and Needs. The purpose of surveying flow properties
along a profile is threefold: (a) To record the variation in current
magnitude near the surface or on the surface with distance from shore;
(b) to record the spatial structure of currents near the boundary and
46
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00
Oro
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"SZ oan3 Ty
UOI}OIINSQ yo Aouanbes4
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(ww
)
Ol
47
ve)
\ 7
IS :
if
2 fi
\\U/
PRS
Ir
we
al
ap
Figure 27. Field testing of current meters.
49
relate these to sediment properties and bed form types; and (c) to deter-
mine what the longshore current distributions are on both sides of the
breaker zone. All of these relate to the need for understanding coastal
sediment transport.
Several models have been proposed in the past for the distribution of
longshore currents, the causes of current generation, and the resulting
littoral drift. However, systematic studies for testing these models in
the field have not been carried out, even for simple topographies, mono-
chromatic waves, and unobstructed coastlines. Verification of the shore-
normal distribution of longshore currents is important, the studies to
begin under the simplest environmental conditions, progressing to include
complex bathymetry, tides, and low-frequency components of general
circulation, to application of the modified models to the design, evalua-
tion, and maintenance of coastal structures.
Since numerical models of coastal currents are currently based on
mean velocities, the vertical distribution of currents combined with their
lateral variability is unknown. First-order importance in sediment trans-
port studies is knowing the structure of the velocity field and being
able to extrapolate profiles to the bottom, especially where entrainment
is concerned as boundary layer development and the boundary where dis-
tribution governs the motion of sediments. Therefore, experiments must
be designed to obtain current data near the bottom.
In surface current measurements it is necessary to keep a constant
distance between the reference water level (mean lower low water (MLLW)
where tides are present) and the point at which currents and waves are
measured (Fig. 28a). If the nearshore slope is uniform, all sensors can
be arranged to move downward only and parallel to the mast at constant
increments. Although this operation is not difficult, it may require
extended periods of underwater work to adjust spars and realine meters
into the three coordinates during which the main features of the nearshore
circulation system (wave, wind, and current directions) can change.
In the study of sediment entrainment, transport, bed form generation
and maintenance, and bottom friction, the flow of water near the bottom
must be measured (Fig. 28b). For this measurement (but not exclusively)
at least three points of flow readings in the vertical are needed to
define the velocity gradient and in turn imply the form of the boundary
layer, and the distribution of boundary shear. This is a relatively
straightforward procedure when operating on gentle, uniform slopes; how-
ever, both steep or barred offshore profiles can complicate the design of
an experimental grid. The problem in data analysis of such a scheme is
the evaluation of the variable pressure-response factor resulting from the
continuously changing depth at the wave gage. Since equidistant or "equi-
depth" spacing of the sled positions usually generates data gaps (Fig. 28c),
it is more advantageous to survey the profiles beforehand and use the fa-
thometer profile to determine positions for the sled; i.e., on bar crests,
bar troughs, or midpoints on bar slopes where data will contain similarities
50
Station
4
et
Station
4
Station
4
Station
3
Figure 28. Experimental grid for sea sled experiments.
S|
(Fig. 28d). For this control, a minimum of a pair of onshore navigation
targets to sight on and a transit to read cutoff angles to the sled are
required.
b. Sensor Configurations. The optimum design for measuring the dis-
tribution of coastal currents along a horizontal profile should consist
of at least nine bidirectional ducted current meters, in groups of three,
and alined orthogonally to one another (Fig. 29) at elevations near the
surface, at middepth,-and near the bottom. Each sensor measures the cur-
rent speed concurrently along the three axes and from these data the mean
vector can be resolved and velocity computed. Alternately, paired electro-
magnetic sensors may be used. Where vertical orbital velocities are
negligible (shallow water or high-frequency waves), pairs of B-10 meters
mounted horizontally at right angles have been used (Fig. 30); however,
in the reconstruction of the vertical velocity profile, less than three
points in the vertical are inadequate. The wave gage is best situated
adjacent to the lowermost group of meters, thus assuring that pressure
records will be collected even with the least number of submerged current
meters.
c. Operations. Common to all these objectives is the incremental
movement of the platform, usually commencing in deep water and ending
where water depth becomes less than 0.91 meter. The reason for not
carrying the measurements up to the dry beach is that the placement of
sensors below the upper rim of the cylinders would introduce a bias into
the records. The actual operation requires towing the sled to 9.14 meters
offshore, where the first record is usually obtained, followed by its
consecutive shoreward displacement to a predetermined location or pre-
determined water depth. In most operations the sled needs to be decoupled
from the towing vessel, especially in heavy swell.
Minimum personnel required for continuous surveys where sensors are
used in fixed position throughout the experiment, include a LARC operator,
a deckhand to release the sled cables, a transit operator, and a data
station operator. However, standby divers must be available for emergency
underwater work; when surface currents are measured the divers are used
to readjust spars on the sled each time the water depth has changed.
d. Data Collection. Several data recording options may be exercised
with each experiment (Table 3). A data file convenient to analyze should
consist of less than 2!2 words per sensor. Usually 5 to 7 minutes of re-
cordings per station along the shore-normal profile are adequate for ana-
lyzing that part of the record which contains the wave-induced currents.
Since the principal aim of such a study is to observe the spatial actions
without a change in the local wave, current, and wind climates with longer
recording times, the uniformity of conditions during the measurement
period may be foresaken. An example of the mean longshore velocities
along a profile is shown in Figure 31. The time-series analysis which
includes Fast Fourier Transform (FFT) programing is also used to calculate
the energy spectra for each sensor (Fig. 32).
32
Figure 29. Orthogonally mounted current meters.
Figure 30. Pairs of horizontally mounted current
meters.
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54
Fraction of Total Energy
Period (s)
IO 8 (238)
Pressure at Z,
0.6%
5.7| cm/s
3.305
5.6lcem/s
3.305
O 0.2 0.4 O 0.2 0.4
Longshore Currentat Zo Onshore Current at Z, Onshore Current at Zo
Frequency (Hz)
Figure 32. Example of energy spectra along shore-normal profile
(Teleki, Schwartz, and Musialowski, 1976).
0)
2. Fixed-Point Measurements.
a. Rationale and Needs. At times it is desirable to obtain a time-
series record of flow at a given point to (a) assess the contribution of
slowly varying ocean parameters, such as connected to tides and seiches,
and (b) record the wave and the directional current spectrum with changing
barometric pressure, wind velocity, and direction at a particular site
of interest. In this case deployment of current meters in the vertical
is preferred to allow measurement of the propagation of long waves through
time and to determine attenuation with depth.
b. Sensor Configurations. The manner in which sensors are deployed
during tm sttu measurements is a variation of designs discussed previously.
The configuration chosen was either an equidistant placement of four sen-
sors in the vertical, alined parallel to shore (Fig. 33), or three de-
ployed in the same manner with the fourth oriented shore-normal adjacent
to the wave gage. The latter design assures that some record of the inci-
dent wave orbital velocities is collected while the velocities in the
vertical profile are also recorded.
c. Data Collection. In 260 hours (10.83 days) of continuous record-
ing by the system onboard the sea sled, long time-series records can be
obtained with TODAS at any site, even if remotely located. This capa-
bility can be extended to 65 days if data were to be collected once every
hour for 10-minute periods. Because the batteries are regenerative, 80
to 90 days of operation may be feasible in this manner.
3. Fixed-Point Measurements Combined With Lagrangian Techniques.
The measurement of flew at a fixed point is regarded as the Eulerian
technique of measurement. The technique is a record of events passing
through the fixed point, and these events can be related to the Lagrangian
velocity by:
WAGs 2 he CeseBe) 5
where uj is the Eulerian velocity measured at the fixed probe, t is
time, Xj(a,0) = aj is the Lagrangian position, and Vj is the Lagrangian
velocity.
The Lagrangian position of a moving fluid particle in space is:
Xq (a,t) = ay o { Vi (ae ide! F
(@) .
and the measurement of Vj requires tagging the particle so that it can
be followed and recorded in the interval t-t'. Because the statistics
of uj are not related to V; in a simple way (Tennekes and Lumley,
1972), it is best to record them simultaneously to evaluate both the time-
dependency and space-dependency of the moving fluid in the experimental
area.
56
Figure! 35.0) Cumrentameters alaned for ‘measurement
in vertical profile.
If
For Lagrangian measurements, several techniques have been developed,
tested, and used. Methods included are the use of dyes, or floats,
drogues, cards, confetti, and other free-floating objects. While drogues
are designed to eliminate measurement of all but the mean fluid motion,
dyes on the opposite end of the scale respond to microfluctuations in the
movement of water masses, and thus are true tracers of water particles
themselves. However, their use requires evaluation of both the advective
motion and the diffusive and dispersive properties of water masses.
a. Rationale and Need. Dyes have been used as tracers of the quan-
tity V;, probably because of the difficulties experienced in designing
drogues which readily responded to wind drag and wave motion. Although
some of the reasoning for this and some of the techniques have been re-
ported in Teleki, White, and Prins (1973) and Teleki and Prins (1973), it
is best to enumerate the conceptual model required to carry out combined
Eulerian-Lagrangian experiments.
Nearshore waters are generally turbulent which means considerable
mixing (momentum exchange) is taking place throughout the water column.
In this environment, simple measurement of the flow with a fixed probe
cannot provide an estimate of the eddy diffusion coefficient, E;, which
is the measure of dispersion of mass resulting from the turbulent veloc-
ity fluctuations and can be written as:
more (GL 25 5 4) 8
Ej is related to the dispersion coefficient in:
sci = =—— dC
ES = ul Ca (ky + cq) 57 ry
where C; = C3 - Ci’ is the deviation of the concentration of a conservative
tracer from the mean, U; = uj - ul! is the deviation of the velocity from
the mean, k; is the molecular diffusion coefficient, and E;j is the
diffusion coefficient. The Eulerian velocity, uj;, is used in the above
expression because its statistics are better known than that of Vj.
Fixed-probe measurements provide an easier solution of a given algo-
rithm than a probe moving with the velocity Vj, because the techniques
developed for the statistical manipulation of time-series data are well
understood, e.g., compared to the Lagrangian integral scale in which such
quantities as the mixing length and dissipation rate are needed but are
difficult to estimate.
Near the coast and especially near engineering works, there may be an
interest in what is referred to as "general circulation." This is a series
of synoptic snapshots of currents past an object designed to train, divert,
or obstruct the same flow, or the sediment it carries. The engineer is
asked to evaluate the performance of the structure for which he needs to
know what the areal variations in waves and currents are. Measurement of
velocities in the Lagrangian domain is an answer. Unfortunately, present
techniques are designed for surface measurements only and not for the flow
58
near the bottom boundary. To overcome these limitations, experiments were
conducted to combine the surface tracing of dyed waters with the velocity
measurements in the vertical profile at a stationary point. The purpose
is to correlate Vj; and uj;, corresponding to the dye velocity and the
velocity measured by metering, respectively, and requires that the sled-
mounted meters be positioned in the field where the dye is dispersing.
Correlations can only be carried out in the area immediate to the fixed-
point sensors, irrespective of the type of dye injection used (Fig. 34).
However, this method can be improved by spacing two sets of current meters
at the upstream and downstream boundaries of the area where dyes are
traced, correlate the Eulerian velocity scales for the Lagrangian domain,
then use the results as a check on the dispersion and diffusion of dye
through the control area.
Dye releases require preselecting the algorithm which will later
govern the analytical procedures. Three injection techniques will satisfy
this: (a) Continuous injection at a point, (b) continuous line injection,
andy (¢)) point, Source (slug), injection. Choice of injection is influenced
by geographical, physical, or operational constraints and whether the
experiment is designed to study convective properties of the flow.
Continuous injection requires flow conditions in the control (measure-
ment) area to be homogeneous. The technique is based on the principle of
"steady dilution,'' i.e., the tracer concentration stabilizes in time as
its properties become representative of the flow properties themselves.
Steady dilution requires a complete mixing over time-length of the system
and a single sample from the well-mixed (equilibrium) condition. The
disadvantage of this algorithm for use in coastal areas where the time
scales associated with molecular diffusion, advection, and turbulent
diffusion vary in space and time, is that the additional requirements of
steady, unidirectional flow and high discharge rates cannot be met; thus,
the mixing length cannot be estimated. Use of a line injection will not
materially alter the analytical problems experienced with point injections.
However, this technique is useful for charting flow lines near coastal
engineering works because the variability in surface flow velocities, the
mixing zone, and the pulsating and meandering nature of coastal currents
can be observed.
If a single-point injection is used, time integration may be applied.
The drawbacks associated with the algorithm are that complete lateral
mixing and high-discharge steady flow are still required, and the dye
field must be sampled continuously at a fixed point and fixed depth for
the estimation of the mixing length. This method is best applied to
tidal inlet flow studies where the physical boundaries present and the
quasi-steady, high-discharge tidal flows staisfy the minimum requirements.
Another algorithm built around single-point injections is the spatial-
integration technique where the center of gravity of a diffusing-advecting
cloud is found and the concentration is measured. If the instantaneous
position of the centroid is denoted by xj(tp), the difference
ico WqGeG), becomes; aimeasure of V4. Therefore, 1t ws critical to
know the depth to which the tracer cloud has diffused at each position and
59
(#1)
16 April 1972
Shore Marker
: A
1 ' ' 0 Buoy
Le OP Faas he SO © Seo Sled
Bose Line
-18 Deer nies ss Fix on Dye Potch
'
' i '
‘ Scole m Feet
' ' Se eS 100 @ 100 800 300 400 S60
i a ert H ' Scole in Meters
SS ——
=24 ' 1 a 100 ° 108
|
‘ q !
‘ ' i e@ ° ize e
' ! t
‘ te 1
' is o7:0 a ‘ '
@ ' H '
' '
'
-30 1 '
V (cm/e)
Dye transport
(cm/s)
oS O- On 0 O1:0n O- 6.56
N Ne © © O OO OO OO
Figure 34. Comparison of velocities from dye transport and
current meters.
60
instance corresponding to each x;(t). After the concentration contours
are derived, the evaluation of the eddy diffusivity coefficients, ce
is simplified.
aD
b. Selection and Use of Dye Tracers. Water-tracing dyes must be
chosen according to whether their colorimetric or fluorometric properties
are to be measured or imaged. Generally, the emissivity of fluorescent
substances (measured in milliwatt per square centimeter steradians) is
much higher than nonfluorescing chemicals and the bandwidth of transmit-
tance is usually narrower, all of which aid the spectral separation of
one tracer from another. In tests at Oceanside and Point Mugu, California,
and at Pentwater, Michigan, three dyestuffs in green, yellow, and red
bands of the visible spectrum were commonly used (Table 4). These sub-
stances have been used in pollution research and rescue operations; none
is toxic to living organisms.
c. Dye Injection Procedures. Dyes are normally injected by placing
100 grams of the material in a water-soluble film or bag, which in turn is
placed in a porous sand-sample bag. The inner container retards solution
of the dye until the film dissolves; the porous sand-sample bag allows the
solution to pass through and keep the dye fixed in position. Six bags are
usually attached to an anchored buoy line and positioned 0.3048 meter
(1 foot) below the water's surface. This method does not satisfy any of
the injection schemes discussed previously since the dye release is
neither instantaneous nor at a constant discharge; however, this type of
injection is useful in aerial photography of nearshore circulation. Where
the spatial integration method is used, the dye packets are tied to a
free-floating buoy which marks the advective path, and the surrounding
developing dye patch will indicate the rate of diffusion outward from the
source. In steady dilution, a pump must be used to inject the tracer at
a known discharge rate.
A typical experiment involves designing a control grid to locate the
centroid or leading edge of the dye; e.g., where a combination of six
buoys and six shore targets were used to provide sufficient control to
(a) determine the position of the cloud, and (b) rectify oblique images
to vertical equivalents (Fig. 35).
Although drogues and floats are used in nearshore experiments for the
charting of currents, their physical size is not representative of a
volume of water small enough to have the scales of molecular diffusion.
Wind stress on the surface also tends to modify their path to the extent
of overriding the advective properties of the fluid. Where offshore ba-
thymetry is undulating (bars, shoals), these objects often ground for
short periods. Therefore, correctly traced dye clouds will better depict
the true course of nearshore currents.
4. Aerial Photography.
a. Survey Flight Lines and Frequency of Coverage. To piece together
the distribution of currents both in the lateral (y-coordinate) and the
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62
4 Shore Targets
Primary Grid, 600 ft/Side (182.88m)
-----+---- Secondary Grid, 424.26 ft/Side (129.42)
0) 2) @ @ © ©
1437+6l 1443+6l 1449+61 1455+61 1461+61 l467+6
—— Base Line —— j=:
Experimental Groin
BUOYS
oO Planned Location
* Actual Location |2 April 1972
e Actual Location |4 April 1972(a.m.)
2° Actual Location I4 April 1972 (p.m.)
®@ Actual Location 16 April 1972
Sccle In Feet
[_— a oon]
0 200 400 600
Figure 35. Buoy shore-marker grid.
63
longitudinal (x-coordinate), the position of the dyed water mass must be
recorded from the time of injection to time t,, where n signifies the
limit of control area delineated by the grid. Photo coverage in time
increments is a first-order necessity for this purpose. The flight lines
for an area of 1,280.16 meters square (4,200 feet) can vary from a single
pass at 2,316.48 meters (7,600 feet) to multiple passes at lower altitudes.
Given a cluster of 70-millimeter cameras with 100-millimeter focal lengths,
the typical field of view for 1,066.8 meters (3,500 feet) (flight alti-
tude) is 591.62 meters square (1,941 feet) (a photo scale of 1:10,672)),
typically with 30-percent sidelap and 50-percent endlap (Fig. 36). Expe-
rience indicates that 2 to 2.5 hours flight time is required for an area
of this size to complete advection of the 600 grams of dye used per in-
jection point, resulting in about 80 to 90 frames exposed per camera.
b. Multispectral Photography. Aerial films should be exposed for
water, rather than land. The product can be considerably enhanced by
selecting the films in combination with filters responding to the band-
width at which either the fluorometric or colorimetric reflectance of the
dyes in the field is at maximum. Several film-filter combinations can be
selected in this manner, preferably one for each tracer and without spec-
tral overlap. The resulting system becomes multispectral (whether black
and white or color films are selected for each bandwidth) and the extended
dynamic range thus allows certain processing techniques to be applied and
analytical methods (color recontruction; density slicing) to be practiced
for the evaluation of the image content.
If vertical photos are difficult to obtain, oblique photos can be used
by rectifying the images with the aid of an instrument such as the Zoom
Transfer Scope (Bausch and Lomb) (Fig. 37). The scope functions as a
camera luctda, i.e., the superposition of two images, two maps or an image
and a map through an optical system becomes possible. Rotation, stretch-
ing and zoom capabilities of the scope allow the user to match control
points, scales, and within limits rectify the obliquity in an image for
the vertical resolution or vice versa. This instrument is useful in the
collation of information from imagery of varying scales onto a common
engineering map. Use of oblique photos in dispersing dye and the reduced
information are shown in Figure 38.
Data to be extracted from photos include the measure of Vj, the
advective property of the dye plume. Vj can be estimated from the propa-
gation of the leading edge of the plume with time; however, it is not
indicative of the true transport of mass. To obtain the correct estimate,
the center of the propagated mass must be determined and its displacement
traced with elapsed time; e.g., methods used include: (a) The sampling
of dye patch from boats or with automatic samplers at various geographic
points and in time (Kilpatrick and Cummings, 1972); (b) the measurement
of the tracer concentration with a aerial Fraunhofer camera or Fraunhofer
line discriminator (Hemphill and Stoertz, 1969); and (c) the analytical
processing of imagery which includes density slicing. ;
Although the last method is strictly qualitative, it can be quantized
with controlled sampling of the water mass during the course of photo-
graphic flights.
64
SITE VIB!
tecrsste-s| =
Avg. Ground Surface
60 Pct
Endtap
t)
o
a ‘
— Airbose
‘
4 ) 5 5 9
Control
Paints
Overlapping Photos
Yu
ao
210
en
o>
Ground Scale (ft)
Figure 36. Typical flight lines for imaging coastal dye dispersion.
65
Eueurceresi/... 200m iugansiter Scope.
66
Oceanside Harbor
Base Line _
MLLW
Scale in Feet
Pee. ee Depth Cont i
400 O 400 800 1200 P onourey mitt
Figure 38. Oblique photo of dispersing dye and map of rectified
photo.
67
V.. CONCLUSIONS AND RECOMMENDATIONS
Techniques related to the measurement of flow in the nearshore zone
have been surveyed, and the advantages and disadvantages associated with
the various approaches discussed. The advantages are: (a) The ability
to negotiate the surf zone with instruments mounted on the sea sled;
(b) a near-continuous mode of operation for measuring waves and currents
contemporaneously and chart their distribution in the spatial sense;
(c) to see the data in real time for maximizing the conditions in collect-
ing useful information; and (d) the ability to combine Eulerian and
Lagrangian techniques for the complete description of flow.
The disadvantages are: (a) The present inability to obtain Lagrangian
correlation scales independent of dye studies by measuring currents si-
multaneously with sets of meters at two locations; this requires another
sea sled and corresponding instrumentation for the two platforms to be
deployed at the upstream and downstream boundaries of the test area where
diffusion studies are conducted; (b) the time needed to adjust the spars
so that data points in space would be dispersed equidistantly from one
another and from the reference water surface, thus providing a means to
record wave attenuation along a shore-normal profile using a constant
pressure correction factor; and (c) the present method of measuring cur-
rents near the bottom boundary. The location of the instrument and power-
packs restricts mounting of sensors to elevations 0.91 meter above the
bottom, when placements closer to the ocean floor may penetrate the
boundary layer of slowly varying shelf currents.
The sea sled is an ideal vehicle for the measurement of currents and
waves (as presently done), or salinity, temperature, and sediment content.
Water temperatures were read during one of the experiments, including wind-
speed and direction necessary to estimate the influence of wind stress on
the time required to set a lake into motion or on the dispersal of a dye
plume.
The dye experiments are an integral part of a nearshore current study,
because mixing (turbulent diffusion) is nonnegligible near the coast and
the records obtained from in sttu sensors contain information on the tur-
bulent part of the spectrum. The development of a dye plume is followed
by aerial cameras which record the zones of intense mixing, allow the
experiments to assess the aerial variations in current speed at the surface
near the fixed-point sensors, and thus gain insight of upstream histories
for the coastal flow. Repetitive photo coverage is useful in estimating
horizontal and lateral diffusion coefficients when the proper injection
algorithm is applied and the dye concentrations are measured in the field.
The ultimate use of such information is for sediment transport calcu-
lations in coastal areas and for the evaluation of the performance of
engineering structures.- Necessarily, these are often interrelated. How-
ever, a sufficiently wide range of environmental conditions may be tested
in time, the evaluation of which would result in producing predictive
models and design curves useful to the coastal engineer. This study has
documented these needs and expectations, provided an insight into the prob-
lems, and recorded some of the accomplishments.
68
LITERATURE CITED
DEAN, R.G., "Stream Function Wave Theory; Validity and Application,"
Coastal Engtneering Spectalty Conference, ASCE, Santa Barbara, Calif.,
1965.
DEAN, R.G., "Evaluation and Development of Water Wave Theories for
Engineering Application," SR-1, Vol. I, Stock No. 008-022-00083,
Vol. II, Stock No. 008-022-00084, U.S. Government Printing Office,
Washington, D.C., 1974.
HEMPHILL, W.R., and STOERTZ, G.E., ''Remote Sensing of Luminescent
Materials," Proceedings of the Stxth International Symposium on Remote
Sensing Envtronment, 1969, pp. 565-585.
ISAACS, J.D., "Report on Beach Survey with Sea Sled and Mast at Pismo,
California," Report HE-116-135, University of California, Berkeley,
Celsliey , | Gals?
JOHNSON, J.W., "Camp Pendleton Sea Sled," Technical Report 1 55-11,
University of California, Berkeley, Calif., 1949.
KILPATRICK, F.A., and CUMMINGS, T.R., ''Tracer Simulation Study of
Potential Solute Movement in Port Royal Sound, South Carolina," U.S.
Geological Survey Water Supply Paper 1586-J, Reston, Va., 1972.
KOLESSAR, M.A., and REYNOLDS, J.L., ''The Sears Sled for Surveying in the
Surf Zone," CERC-Bulletin and Summary Report of Research Progress,
1965-1966, Vol, II, 1966, pp. 47-53.
MORISON, J.R., et al., ''The Force Exerted by Surface Waves on Piles,"
Petroleum Transacttons, AIME, Vol. 189, 1950, pp. 149-154.
NEUMANN, G., Ocean Currents, Elsevier, Amsterdam, 1968.
TELEKI, P.G., WHITE, J.W., and PRINS, D.A., '"'A Study of Oceanic Mixing
with Dyes and Multispectral Photography," Proceedings of the American
Society of Photogrammetry, Symposium on Remote Sensing in Oceanography ,
1973, pp. 772-787 (Also CERC Reprint 2-74, NTIS Number AD 775 561).
TELEKI, P.G., and PRINS, D.A., 'tPhotogrammetric Experiments on Nearshore
Mixing and Diffusion," Proceedings of the Second Internattonal Con-
ference on Port and Ocean Engineering under Arctte Conditions, 1973,
pp. 251-265, (Also CERC Reprint 9-74, NTIS Number AD A014 216).
TELEKI, P.G., SCHWARTZ, R.K., and MUSIALOWSKI, F.R., "Nearshore Waves,
Currents and Sediment Response," Abstract, Proceedings of the 15th
Coastal Engineering Conference, Honolulu, Hawaii, June 1976.
TENNEKES, H., and LUMLEY, J.L., "A First Course in Turbulence,"
Massachusetts Institute of Technology Press, Cambridge, Mass., 1972.
U.S. ARMY, CORPS OF ENGINEERS, COASTAL ENGINEERING RESEARCH CENTER,
Shore Protection Manual, Vols. I, II, and II, 2d ed., Stock No.
088-022-00077, U.S. Government Printing Office, Washington, D.C.,
LO Silt pp):
69
APPENDIX
Schematics of onboard electronics; letters refer to generalized
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‘69 *d : Aydeasortqta
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