This invention pertains to the field of erosion prevention, and more particularly to offshore beach and shoreline erosion prevention devices.
Shorelines and beaches are subject to erosion from the action of waves impinging thereon. Wave action erodes beaches by several different mechanisms. Waves mobilize shoreline materials and then redistribute them, leading to erosion. Rising and falling water levels may erode beaches over a long period of time. Shoreline structures, including seawalls, pilings and levees, often increase beach erosion adjacent to those structures, by causing wave reflection, turbulence, eddies and currents. These currents mobilize the beach materials which may be transported along shore or offshore. Offshore currents, transversing the beach, can carry the beach materials many miles away until the current slows and the beach materials sink due to the influence of gravity. Further, heavy storms can impinge high waves on beaches and shorelines, imparting heavy forces which carry away the beach or crumble the shoreline leading to heavy erosion. Portions of Lake Michigan are particularly susceptible to shoreline erosion, and it is not uncommon for bluffs 20 to 40 feet high to erode 5 feet inland during the course of a year. The seawall area of Galveston, Tex. has for many years experienced severe beach erosion problems, where heavy seas carry the sand off the beach face and, because of ocean currents, deposit the sand some 5 to 10 miles away.
In a natural beach/water ecosystem, the shallow water extending up to the beach, and the beach face itself, act to dissipate the energy of the waves, thereby preventing erosion of the land area behind the beach. Typical water front profiles include a surf zone of relatively shallow water where the waves break into surf, a beach zone where a wave expends its last landward energy, and the land area behind the beach. The land areas typically include dunes, low barrier islands, alluvial fans and river deltas, or bluffs. During severe storm conditions when the waves are commonly two to three, and occasionally ten times their normal height, the typical beach response is the loss of material from the beach zone to an offshore sand bar. The sand bar then creates a shallow area offshore with a deeper trough between it and the beach face. The shallow area causes the waves to break on the sand bar, thereby initiating dissipation of wave energy further offshore and providing a wider surf zone. Both of these effects decrease beach erosion. It is not uncommon for a series of multiple sand bars to develop offshore. The sand bars tend to create a series of long ridges parallel to the beach, causing the waves to dissipate much of their energy offshore before they reach the beach. Over time, the beach-sand bar system stabilizes, and includes a relatively constant volume of sand. Heavy wave action tends to carry sand off the beach and deposit it on the sandbars. Long term low energy wave action moves the offshore sand bar back onto the beach face. This is known as the summer/winter beach profile response.
The establishment of an equilibrium between the beach face and the sand bars can take many years to develop, and minor changes in the shoreline, offshore water circulation patterns, and a myriad of other environmental factors can upset the environmental balance and thereby cause substantial beach or coastline erosion. Further, a particular ecosystem may be adverse to the continued existence of a substantial beach or a stationary coastline. The coastline of northern California is continuously eroding, and small beaches exist often only in sheltered coves or cuts in the bluffs which abut the sea. Further, unusual storms having wave heights experienced only every few decades will often severely depreciate the amount of beach material. Finally, municipalities and individuals, in a quest to increase the leisure value of shoreline, place sand on the shoreline to establish a beach. Such artificial beaches commonly wash away, because the shoreline, sea bed or lake bed structure is not compatible with the existence of a beach at the location of the artificial beach.
Many methods have been employed in an attempt to reduce shoreline erosion. These attempts have included both protruding and submerged breakwaters located offshore. The protruding breakwater reflects and/or dissipates the waves. A submerged breakwater also reflects and/or dissipates waves, or causes the wave to break further offshore. These breakwaters are typically constructed of concrete or stone, and are solid structures. Commonly, rubble or rocks are piled in a submerged line off the shoreline to form a breakwater.
Breakwaters have several deficiencies. Foremost, they are expensive to build and maintain. Rubble breakwaters erode by losing rock to the action of waves, and unstable subsoils commonly cause the rocks or concrete segments to sink into the sea or lake bed. The use of larger rocks to prevent wave displacement is expensive, because larger rocks cost more to quarry and transport.
Revetments and seawalls are also used to reduce shoreline erosion. However, these structures actually inhibit beach and sandbar growth. Therefore, although they may protect the shore behind the beach, they tend to erode the beach by requiring materials for offshore sandbar development to be provided by the adjacent unprotected beach and by creating intensified water currents which may permanently transport the beach materials out to sea.
On shore, devices other than revetments and seawalls are used to protect the beach. Beach grass may be planted to create a subsurface root structure which is more resistant to wave action than loose sand. However, this provides a low level of protection, and commonly requires constant nourishment and replanting.
A device known as a sand perch may also be used on the beach to prevent beach erosion. These devices are concrete structures having small channels, commonly less than four feet high, with a generally triangular cross section. Vertical or horizontal slots are found on their upward projecting triangular faces. The horizontal slots allow the seawater to pass through the channel, but the sand is trapped in the device thereby keeping the sand on the beach. Although this device reduces beach erosion, it is an unsightly addition to the beach.