Directional groin

Apparatus and method are provided for regulating shoreline change in cases where the magnitude of the longshore transport rate varies along a shoreline and is subject to periodic reversals. The method operates by selectively restricting the longshore sediment transport in one of the two possible shore-parallel directions. Some versions of the directional groin operate by changing the effective height of a low profile structure, while in others segments of the structure are selectively opened or restricted to sediment transport. The transport-obstructing portions of the groin may be positioned by flowing water, or may be moved by an electromechanical actuator acting under control of a microprocessor-based control system that also measures the direction of water flow.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
This invention relates to erosion control method and apparatus offering 
asymmetrical resistance to the longshore movement of water and sediment. 
2. Description of Prior Art 
This invention provides means of regulating shoreline changes caused by 
spatial variations in the rate of longshore sediment transport. 
The sediment comprising a shoreline is seldom stationary. Waves striking 
the shoreline at an oblique angle produce longshore, or shore-parallel, 
movement of sediment. The wave forces can be considered as consisting of 
two perpendicular components which are referred to as the normal and 
longshore components. The normal component is that portion of the wave's 
force perpendicular to the shoreline, while the longshore component is 
parallel to the shoreline. Most of the energy of the normal component is 
expended through turbulent dissipation during the wave breaking process. A 
much smaller fraction of the longshore component, however, is dissipated 
and the remaining energy produces a longshore current parallel to the 
shoreline. Sediment, put in motion by the wave breaking process, is 
transported by this longshore current and this movement is referred to as 
longshore transport. 
The longshore transport rate, as used herein, refers to the volume of 
sediment carried in a shore parallel direction for a specified period of 
time and has the dimensions of volume per unit time. In most locations 
wave activity is seasonal, so the most common period of time used for 
transport calculation is a year. Studies of longshore transport indicate 
that the magnitude of the transport rate is a function primarily of the 
wave height at breaking and of the angle between the wave crests and the 
shore line at breaking. Increases in either the wave height at breaking or 
of the wave angle result in higher transport rates. 
Sediment motion can generally be characterized by three modes of transport: 
bed load; saltation; and suspended load. Bed load refers to the rolling or 
sliding motion of sediment in contact with the underwater soil surface. 
Saltation refers to the bounding or hopping motion of sediment that is 
temporarily suspended in the water columns but does not stay in 
suspension. Suspended load refers to sediment maintained in the water 
column through turbulence. For most littoral environments, the sediment 
load is not uniformly distributed through the vertical water column and 
most sediment transport occurs in the lower portion of the water column. 
Sediment motion produced by wave activity can also be resolved into normal 
(i.e., onshore / offshore) and longshore components. Although significant 
shoreline recession can be produced by offshore movement of sand during 
storm conditions, little of this sand is removed from the active beach 
system and, during a subsequent recovery process, most of that sand is 
returned to the beach face. As a result, permanent erosion losses 
resulting from the offshore movement of sediment during storm conditions 
are usually quite small. Permanent erosion losses due to variations in the 
longshore movement of sediment, however, can be dramatic. It is this type 
of erosion loss that the invention addresses. 
Along most sections of exposed shoreline, longshore transport of sediment 
can be considered to be one-dimensional. Continuity considerations for 
one-dimensional transport dictate that if the transport rate increases in 
the direction of transport, the adjacent shoreline erodes. Conversely, if 
the transport rate decreases in the direction of transport, the adjacent 
shoreline accretes. Shoreline stability is preserved only if the longshore 
transport rate is uniform when averaged over a period of interest. 
Due to changes in shore alignment, irregular nearshore bathymetry and 
littoral barriers, longshore transport rates are seldom uniform over large 
sections of shoreline. Areas of change in the shoreline, which comprises 
both erosion and accretion, result from such non-uniformities. 
Groins represent one of the oldest types of structures used to prevent 
shoreline erosion caused by the longshore transport of sediments. Such 
structures consist of partial or complete littoral barriers constructed 
along a line perpendicular to, or at an oblique angle to the shoreline and 
commonly extending both landward and seaward of the surf zone. Although 
usually constructed of stone or concrete, groins have been built of 
timber, steel and various combinations of materials. Aside from the choice 
of construction material, groin design parameters include length, height, 
alignment and permeability. In addition, groins have been designed to be 
adjustable so that their heights and lengths can be periodically changed. 
Descriptions and examples of various groin types can be found in the Shore 
Protection Manual published by the Coastal Engineering Research Center, 
Department of the Army. 
Conventional groins function by creating a physical barrier to longshore 
sediment transport. Since the annual transport rates in the two possible 
longshore directions are seldom balanced, groins initially cause accretion 
on their updrift side (i.e., the direction from which predominant 
transport occurs) and erosion on their downdrift side(i.e., the direction 
toward which predominant transport occurs). This erosion and accretion 
pattern continues until the storage capacity of the groin is reached, at 
which point sediment begins to bypass the groin. Sediment bypassing 
usually occurs around the offshore end of the groin in relatively deep 
water. 
The obstruction to a longshore transport rate afforded by a specific groin 
design depends not only on the groin dimensions, but also on wave climate 
and bathymetry at the site of installation. If wave conditions at a site 
produce a transport rate in one direction which strongly dominates the 
transport rate in the opposite direction, accretion and erosion changes 
will be greater than those occurring at a location where the transport 
rates are more nearly balanced. In a limiting case in which the transport 
rates in both directions are perfectly balanced, the long-term erosion and 
accretion rates will be negligible and shoreline changes will be 
restricted to the near vicinity of the structure. While conventional 
groins can be used to effectively stabilize the shoreline on the updrift 
side of the structure, they provide only a crude means of regulating the 
longshore transport rate and frequently result in accelerated erosion on 
their downdrift side. 
For a given location, updrift accretion and downdrift erosion capacities of 
a groin generally increase as the length of the groin increases. The 
height of the structure also effects accretion and erosion. The crown 
elevations of most conventional groins are above the water line. These 
structures, therefore, provide a longshore transport-obstructing means 
over the entire height of the water column and, when bypassing occurs, it 
is directed around the offshore end of the structure. Two adverse 
consequences are generally associated with such tall structures. The 
longshore current may be redirected offshore along the updrift side of the 
structure so that bypassing, when it occurs, is restricted to relatively 
deep water. This latter feature generally results in increased shoreline 
recession immediately downdrift of the structure. Moreover, sand bypassing 
occurs only when the sand build-up on one side approaches the capacity of 
the structure. Because such build-up usually occurs only on the side from 
which the dominant transport occurs, the sediment moving in the opposite 
direction during periods of transport reversal is essentially prohibited 
from bypassing the groin. 
Low profile groins constructed to have a more or less constant height above 
the littoral surface - i.e., that follow the beach profile --provide less 
of an obstruction to longshore currents. Once these groins have filled to 
capacity, they allow sand bypassing over the structure in the nearshore 
area. For a given length of groin, however, a low profile groin is a less 
effective littoral barrier because it restricts the sediment transport 
over only the lower portion of the water column. 
Although numerous beach erosion control devices have been patented, almost 
all of the devices function by either reducing the wave energy reaching 
the shoreline or by regulating the movement of sediment in an 
onshore-offshore direction, rather than in a shore-parallel direction. 
Examples of nearshore submerged breakwaters can be found in U.S. Pat. Nos. 
4,954,013; 4,896,996; 4,804,294 and 4,711,598. 
Examples of devices which modify the onshore-offshore movement of sediment 
can be found in U.S. Pat. Nos. 4,647,249; 4,954,013. Examples of 
artificial seaweed, intended to perform the same function, can be found in 
U.S. Pat. Nos. 4,490,071; 4,478,533; 4,437,786; 4,374,629 and 4,221,500. 
In U.S. Pat. No. 3,894,397 Fair teaches asymmetrical flow-impeding 
apparatus for onshore-offshore erosion control. Fair's apparatus, of the 
type now known as a "sand grabber", comprises flap-like flow impeding 
elements disposed parallel to the shoreline, the flaps obstructing 
offshore flow through apertures in tubular members oriented with their 
axes in an onshore-offshore direction. 
In U.S. Pat. No. 4,439,058, LeMehaute discloses an erosion barrier 
comprising an array of asymmetrical vertical vanes which promote the 
onshore movement of sediment. LeMehaute's vanes, which have a vee shape in 
horizontal section, flex in response to changes in the direction of water 
flow. 
In U.S. Pat. No. 5,405,217 Dias discloses an erosion control device 
comprising a plurality of tubular units which may be utilized as a groin 
type structure. The device, however, affords the same obstruction to 
longshore transport in both possible shore parallel directions. 
Examples of check values consisting of flexible structures can be found in 
U.S. Pat. Nos. 4,585,031, 4,607,663 and 5,336,018. 
Kumagai, in U.S. Pat. No. 5,388,928, teaches an inflatable weir providing a 
variable height barrier to water flow in a river, the barrier constructed 
so as to minimize riverbed erosion on its downstream side. Another example 
of a flexible or inflatable barriers can be found in U.S. Pat. No. 
4,662,783. 
Waddell, in U.S. Pat. No. 604,810, teaches a jetty projecting outward from 
one bank of a tidal channel to approximately the middle of the channel, 
where the arrangements to used to deepen the channel. Waddell's jetty 
comprises hinged gated arranged so as to allow an incoming tide to pass 
with little obstruction. When the tide begins to fall, the outgoing waters 
close the gates automatically and thus greatly obstruct the flow on one 
side of the channel. Waddell teaches that this will produce a rapid 
outgoing current on the opposite side of the channel, which will carry 
with it much of the sand and silt, and thus rapidly deepen the channel. 
Waddell does not teach the construction of a jetty of groin projecting 
outward from an exposed shoreline. Waddell does not teach a method of 
regulating shoreline changes by retaining sediment along the side of the 
channel from which his jetty projects, but rather teaches a method of 
forcing currents away from that side of the channel. 
SUMMARY OF THE INVENTION 
The present invention provides a method of regulating shoreline change in 
cases where the magnitude of the longshore transport rate varies along a 
shoreline and is subject to periodic reversals, the method operating by 
selectively restricting the longshore sediment transport in one of two 
possible shore-parallel directions. 
It is an object of the invention to provide a directional structure having 
a relatively high resistance to transport in one shore-parallel direction, 
and a lower resistance to transport in the opposite direction. 
A preferred embodiment of the invention controls the degree of restriction 
of longshore sediment transport by adjusting the effective height of a low 
profile groin or by using program control to either lower the height of 
the structure or to open segments of the structure to through flow. The 
invention also provides a means of redistributing erosion losses along a 
shoreline and extending the useful life of artificial beach restoration 
projects by reducing end losses. 
It is an object of the invention to provide directional groin means to 
reduce spatial non-uniformities in a net longshore transport rate by 
judiciously reducing one of the two constituent oppositely directed 
longshore transport rates. Because the net transport rate is the sum of 
two transport rates in opposite shore-parallel directions, variation in 
either of the constituent transport rates will alter the net transport 
rate. It should be noted that complete obstruction of the transport rate 
in one direction is neither necessary nor particularly desirable and that 
any significant reduction of transport in a restricted direction will 
reduce spatial non-uniformities in the longshore transport rate and 
thereby reduce shoreline changes. 
A directional groin of the invention is applicable to shorelines having 
significant seasonal transport in both directions. In addition, because 
each directional groin has a finite area of influence, it is envisioned 
that a plurality of groins will be used in most cases. 
It is a particular object of the invention to provide method and apparatus 
affording significant regulated restriction to longshore sediment 
transport in one direction and minimal restriction to transport in the 
opposite direction. 
It is an object of the invention to provide method and apparatus for a 
passive and unattended control of longshore transport of sediment. In 
particular, it is an object of the invention to provide a groin extending 
offshore from the shoreline, the groin effecting significant resistance to 
longshore transport in one direction and little resistance to longshore 
transport in the opposite direction. 
It is an additional object of the invention to provide method and apparatus 
actively controlling longshore transport of sediment by locally permitting 
or impeding sediment transport responsive to a measured direction of water 
flow. 
It is a particular object of the invention to provide apparatus comprising 
a plurality of movable panels for the active control of longshore 
transport of sediment, the apparatus comprising actuator means moving a 
plurality of panels simultaneously, the actuator means functioning 
reliably even when some of the panels have been rendered immobile because 
of the accretion of sediment about them. 
It is another object of the invention to control the degree of restriction 
of longshore transport with a directional groin that is alterable as 
needed by adding or removing flow control panels or by fixing selected 
panels in either an open or a closed position. 
It is a further object of the invention to provide a variable height 
barrier, which may be rigid or inflatable, for regulating the longshore 
transport of sediment. 
It is a still further object of the invention to provide a directional 
groin having a controllably alterable maximum storage capacity. Such 
changes in storage capacity can be provided by adding or removing flow 
control panels, by fixing panels in their open or closed positions, or by 
changing the heights of the barriers in a raised position.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Turning initially to FIG. 1 of the drawing, one finds a partial sectional 
view of a groin 3 having a plurality of selectively movable members or 
panels 4, each movable member hingedly attached intermediate two adjacent 
support pilings 8. Aside from the movable panels 4 and associated 
hardware, the groin 3 is a conventional structure comprising a headrail 6 
and a plurality of support pilings 8, each having one end embedded in the 
sand 10 or other material making up the beach 12 and littoral subsurface 
14 adjacent the beach 12, the pilings 8 spaced apart along a line or 
curve, which commonly extends landward and seaward of the shoreline 16. 
As described supra, the apparatus of the invention is applicable to cases 
where water currents in the unrestricted direction (indicated with a heavy 
solid arrow 32 in FIGS. 2, 3, 4 and 6) and in the restricted direction 
(which is indicated with an outlined arrow 30 in FIGS. 2, 3, 4 and 6) are 
oppositely directed. In these cases, the invention provides movable panels 
4 closed to flow in the restricted direction 30 and open to flow in the 
unrestricted direction 32. 
Turning now to FIGS. 2 and 3 of the drawing, one finds a movable panel 
structure comprising a generally rigid swinging check panel 22 pivotally 
connected by suitable gusset support means 26 to a support rod 28 running 
between pilings 8 along the groin 3. The density and buoyancy of the panel 
22 and gusset 26 are chosen so that, in the absence of a longshore 
current, the panel 22 will assume a vertical or closed position. Flow in 
the restricted direction 30 forces the panel against a seat surface 25 of 
the mounting collar 24 and thereby blocks transport of water and sediment 
in one longshore direction generally perpendicular to the groin. Flow in 
the opposite longshore direction 32 causes the panels to rotate into an 
open position, thereby allowing transport of water and sediment through 
the structure. It is understood that although the check panel 22 can have 
a range of lengths perpendicular to the support rod, the check panel 22 is 
expected to have a length less than the width between pilings 8 so that a 
deflected panel does not extend very far from the groin. Moreover, 
although the support rod 28 could be oriented in any arbitrarily chosen 
direction parallel to the groin, it preferably has a horizontal or near 
horizontal orientation so that the bottom edge of each panel 22 will be 
more or less parallel to the sand of the littoral surface 14. This 
configuration will reduce the likelihood of the panels being jammed in a 
partially open position by sand build-up. It may be additionally noted 
that the panels 22 may be nearly abutting (as shown in FIG. 2) or may have 
a non-planar shape to allow adjacent panels to overlap horizontally or 
vertically. 
In a second embodiment, shown in FIGS. 4 and 5 of the drawing, a flexible 
panel or baffle 36 flexes about a support rod 28. When the longshore 
current is in the unrestricted direction 32, the baffle 36 deforms away 
from its cooperating seat member 34 (as shown by phantom lines in FIG. 4) 
so as to permit through flow. When the longshore current is in the 
restricted direction 30, the panel 36 seals against the seat member 34 and 
thereby impedes or prohibits flow. The vertically flexible baffles 36 may 
be fabricated using sheets of natural or synthetic elastomers. The baffles 
may be horizontally reinforced with internal stiffening members or 
external battens 38 to add horizontal stiffness to and allow the baffles 
36 to cover larger inter-piling apertures (e.g., as indicated with 
reference character 20 in FIG. 1 of the drawing) than could be achieved 
with non-reinforced baffles. 
The embodiments of the invention described supra represent applications of 
the art of check valves to provide a means of regulating longshore 
sediment transport. It will be recognized to those skilled in the art that 
other mechanisms drawn from the check valve art could provide the same 
function. 
In the embodiments described supra the effective height and shape of the 
groin 3 are determined by the number of panels 4 installed between the 
pilings 8. Hence, panels 4 must be added or removed to adjust the 
effective vertical dimensions of the structure 3. In other embodiments of 
the invention, discussed hereinafter, the need to periodically add or 
remove panels 4 from a groin 3 is overcome by providing an active control 
system 40. 
FIG. 6 shows a plan view of an active control system 40 for a groin 3 
extending offshore from a shoreline 16. The active control system 40 may 
comprise a sensing means 42 to measure the direction and/or magnitude of 
the longshore currents 30,32. The sensor 42 may be located at the groin 3 
or may be remotely disposed, in which case it communicates with the 
controller 46 by known remote communication means (not shown). Signals 
from the current meter 42 are transmitted to the controller 46, which may 
preferably be a microprocessor operating under the control of a stored 
program or may be any of a number of other controllers known to the art. 
Commands from the controller 46 are transmitted to electro-mechanical 
actuators 48 which either open the actuable panels or gates 50 to permit 
flow in the unrestricted direction 32, or close the gates 50 to impede 
flow in the restricted direction 30. A control system 40 may also be 
configured without using a flow sensor 42. In such a case a timekeeping 
means 43, such as the digital clock commonly used in a microprocessor 
controller 46, can be used to open or close the gates 50 in a time 
sequential fashion. 
Although many mechanisms are known in the art for selectively opening and 
closing apertures, the control system 40 of the invention preferably uses 
an actuator mechanism 48 capable of driving a plurality of gates 50 
independently of whether all of those gates are free to move or some of 
them are jammed in one position by an external force. That is, the 
directional groin 3 should function when some of its nominally movable 
gates 50 are actually immobile because of being partially or entirely 
buried in the sand or other sediment 10. As noted supra, in order to 
effectively regulate longshore transport of sediment the asymmetrical 
groin 3 need comprise means offering greater or lesser degrees of flow 
restriction, and need not provide means that either totally eliminate flow 
or that provide no impediment to flow. 
A schematic example of one version of an actuator 48 compatible with a 
panel or gate 50 jammed in a vertical closed portion 66 is shown in FIG. 
7. The actuator 48, which in this embodiment comprises a lifting and 
lowering mechanism 54, is linked by means of a vertical drive rod 56 and 
springs or other flexible coupling means 58 to a support rod 28 having a 
crank 60 configuration at one end. When the rod 56 is driven downward, as 
illustrated in FIG. 7, the flexible couplings 58 bear on the crankshaft 60 
and oscillate the crankshaft 60 (which is mounted in suitable bearings or 
bushings 62) so as to turn the panel of the actuable gate 50 into its 
open, horizontal, setting as indicated by the numeral 64. Enough play is 
provided in the flexible connecting means 58 so that a jammed gate 50, 
nominally driven by the same drive rod 56, can remain in a closed setting 
(as indicated with reference numeral 66 in FIG. 7), an open setting 64, or 
an intermediate setting without preventing the actuator 48 from rotating 
the other controllable panels or gates 50. Moreover, if the jammed gate 50 
subsequently becomes free as a result of flowing water removing the 
obstructing sand or other sediment 10, the flexible connection means 58 
would bias the once-jammed gate 52 into the desired open horizontal 
setting 64. Alternately, when the longshore current is in the restricted 
direction, a signal from the sensing element would, through the action of 
the controller 46, lifting and lowering means 54, and vertical drive rods 
56, produce torque on the horizontal vanes 50 to position them in the 
vertical or closed position 66. 
Yet another embodiment of a groin 3 is shown in FIGS. 8 and 9. Here, a 
plurality of longitudinal inflatable barrier members 72 are utilized to 
provide an obstruction of variable height to longshore transport of 
sediment. Each barrier 72 may be cradled in and secured to a rigid frame 
70 fabricated of steel, concrete or other suitable material. Each frame 70 
is embedded in and partially covered by sand 10 or other material making 
up the littoral subsurface 14. In FIG. 8 the sand line 68 shows the 
boundary between the buried and exposed portions of the barrier 72. 
Similarly, the water line 78 shows the boundary between the submerged and 
emergent portions of the barrier 72. 
Inflation and deflation of the barrier 72 is regulated by an active control 
system 40 having optional sensing elements 42 as previously described. For 
the case of an inflatable barrier 72 the actuator 48 portion of the system 
40 may comprise a submersible pump 74 discharging water through venting 
and filling pipes 76 into the barrier. Deflation of the barrier 72 is 
accomplished by utilizing a vent valve 79 operated by the controller 46 to 
partially relieve the pressure in the barrier 72. When the controller 46 
receives a signal from the sensing means 42 that the longshore current is 
in the unrestricted direction 32, the barrier 72 is vented to relieve 
internal pressure (the collapsed barrier 72 is indicated in phantom with 
the reference character 82 in FIG. 9). When the longshore current reverses 
to the restricted direction 30, the barrier 72 is pressurized and inflated 
to a predetermined elevation as indicated with the reference numeral 80. 
The inflatable barrier elements 72 may be fabricated from natural or 
synthetic elastomers. To add rigidity and strength to the barriers 72, 
reinforcing plies, similar to those used in automobile tires, may be 
employed. Moreover, portions of the inflatable members 72 may comprise 
pleats 84 or other expansile configurations to allow vertical expansion. 
It will be appreciated that similar regulation of longshore sediment 
transport can be accomplished utilizing rigid adjustable barriers or 
barriers which have both rigid and flexible components. 
Although the present invention has been described with respect to several 
preferred embodiments, many modifications and alterations can be made 
without departing from the invention. Accordingly, it is intended that all 
such modifications and alterations be considered as within the spirit and 
scope of the invention as defined in the attached claims.