Many different methods are available for shoreline protection ranging in price, material and classification. The most broad definitions include structural, bio-technical and non-structural.
In order to avoid paying hefty price and liability the property owner shall clearly understand two important points:
1. He/she is not a professional, i.e. not qualified by education and experience to determine which method may be the right application for his/her specific condition.
2. Different contractors may be motivated for different reasons to recommend different methods.
It does not really matter, because the origin of the problem has to be identified first through proper research and investigation by learned professional. Depending on complicity such task would involve civil engineer, structural engineer, environmental engineer, geotechnical engineer, surveyor, attorney and other disciplines or specialties.
Selection of viable alternative may only be done based on scientific and practical design approach.
Sea wall or retaining wall
At the price of more than $2000 per linear foot many solid concrete sea walls observed by our company were slowly deteriorating, usually within 20 - 30 years, due to corrosion of steel reinforcement and cracking under hydrostatic pressure behind the wall. Sea wall application destroys the beach of water front property and becomes a safety issue, if railing and fencing are not properly installed to prevent person from falling, since that person will not be able to climb up a vertical wall and get out of water.
It is always better to perform preventative replacement before failure occurs, otherwise dewatering and sheet piling becomes very complicated by necessity to remove collapsed structure and backfill, leaving unprotected property behind it, with risk of further damages which may be triggered by rain, vibration or construction equipment operation due to temporary equilibrium condition after collapse.
Sheet Piling or shoring
At the price of approximately $1000 per linear foot sheet piling is typically used as temporary solution. Sheet pile design requires geotechnical engineer to determine soil properties necessary for structural calculations and at least two types of calculation: stability and deflection. Without proper backfill and drainage sheet piles frequently fail under hydrostatic pressure. Another concern during installation in the close vicinity of existing home on the property – is seismic, which may negatively affect the building and frequently leads to cracking.
Gabions, sand-cement bags, concrete or rock blocks
People frequently prefer using cement bags as alternative to costly sea walls, mostly because it may be installed directly by themselves of contractor without usage of special equipment. Unfortunately they do not realize that mechanical properties of soil may not support the weight, which leads to settlement, inward rotation, outward rotation and failure.
Without geo-synthetic liner rocks are only creating turbulence around them and accelerated erosion while sinking deeper into the shoreline every time when a wave hits them. In addition to unsightly appearance – it is a major safety hazard, since they quickly become extremely slippery due to micro-organisms growth.
Geo-tubes, geo-wraps, geo-socks.
Polyamide sandbags were first used by Dutch government in 1957, quickly followed by Enkalon sand mattresses as part of Delta program aimed to protect dykes and dams of Eastern Schelde lock system from raging storms.
At the price as low as $30 per linear foot geo-synthetic containers became very popular in USA over past few years, due to overall economic situation and individual financial conditions of property owners. Originating in Europe and thoroughly tested by Army Corp. of Engineers this method got abused by some contractors, who are not careful enough about stability or safety, disappointing many property owners as a result making them believe that method does not work all together. Additional weight added to the top of the slope without proper geotechnical investigation and bathymetric survey frequently causes unstable block to tear off and fall into the water body.
The main reason for geo-tube failure - is improper installation.
Sometimes people working in the company for a few years begin considering themseves an "expert". Installation appears easy for them, because it was properly designed by our engineering department. Motivated by greed but without having a degree from recognised university or alternative prior experiance - they steal a company's client list, in order to open their own comnpany and undercut industry prices.
Unfortunately, they are not aware of tedeous work, that had to be performed in the field and in the office by other employees doing research, surveying and design, involving accurate identification of soil criteria, slope stabitity calculation, study of site hydrology, wave action, etc. They also trying to assure their clients that engineer's supervison or government oversight are not necessary for this type of work - in order to make it even cheaper.
Exposure to the sunlight causes deterioration of material, filling with aggregate prevents the grass from growing. While it may initially appear cheaper to the client – steep slope, over-pumping and staggering is causing excessive tensile forces and unsafe condition which violates typical guidelines and regulations of many government agencies.
Coir logs may be used along shoreline where erosion does not exceed one foot vertical height and velocity of water is not sufficient to wash it out. This method is not effective when drop-off is 1 ft. or higher or when shear stress caused by movement of water exceeds 3 psf.
Burlap may be used in combination with other stabilization methods to protect shelf from erosion during low water events. This method is not effective when drop-off is 6 inches or higher or when shear stress caused by movement of water exceeds 3 psf.
Aquatic planting only works really well in combination with other stabilization methods, where shelf is already established or as preventative measure. It may somewhat protect the lake bank form erosion when wave action is very mild and velocity of water is very low. However, all by itself - it is unable to solve stability or seepage problems.
Enkamat™ was originally developed in Netherlands on 1973 and over a period of 40 years proved to be excellent liner for establishing and promoting ground vegetation cover on steep slopes in many countries.
One example of improper application observed by our company include mat being stapled to the underwater slope by dredging contractor who would then pump material beneath the mat attempting to fill eroded portion on the slope and bring it into conformance with typical section.
Such applications are totally wrong, because it makes impossible for material particles suspended in the water to consolidate, unless water body is been pumped down significantly below problem area, suitable material is imported and properly compacted, which becomes extremely problematic and costly on any slope exceeding 4H:1V.
Enkamat™ and equal alternatives are wonderful aid in erosion protection of steep slopes mainly for two reasons: 1. extensive root system improves physical parameters of top and underlying soils, 2. dense vegetative cover prevents infiltration. Both play major role in slope stability analysis, which shall be performed by structural or geotechnical engineer.
Concrete blocks and fabric formed concrete revetments
Cabled concrete blocks such as Armorflex™ with geotextile filter provide flexible stable protection in high flow, high-scour application. Concrete blankets provide excellent cover for geotextile in such places where vegetation cover has low survival rate due to restricted access, shadow or absence of irrigation.
Due diligence has to be exercised by the engineer of record in slope stability calculations and hydrogeological research in order to prevent possible failure.
Grouting and tie-back.
Our firm is using variety of methods to stabilize existing retaining walls, which show signs of backfill erosion, unstable foundation, buckling or rotation. Those methods include tie-backs with highest quality American made Magnum Helix Anchors and cement or resin injection pressure grouting.
Unfortunately, it may not always be possible to predict size of voids washed out in the porous material and provide equal spreading of grout during pumping while avoiding bleeding of suspension. There exists no model that describes the penetrability of the grout and it is also difficult to evaluate the grouting result. Therefore only experienced licensed contractor trained and authorized by the manufacturer shall be allowed to perform such stabilization - based on report and recommendations from geotechnical or structural engineer.