Massive scale, recent Coastal Bank erosion has created some massive scale responses, in an attempt to restore the Coastal Bank itself. This significant, economic response is driven mainly by social-economic concerns, when homes have been built on the edge of eroding Coastal Banks. We need to carefully consider the consequences of these social economic responses, when they are imposed on a geological system driven by the environmental energy of the Coastal Process. Sustainable solutions need to balance Social, Financial and Ecological interests, as part of an alternatives analysis. Coastal Banks are actually “Resource Systems”, with significant, internal linkages within the system and significant external linkages to other systems. The Coastal Process represents a significant player and needs to be part of the the decision making process and not be excluded.
If you would like learn more about the causes of Coastal Bank erosion, click on the link below to download your own copy. The article appears below.
CONTRIBUTING FACTORS TO COASTAL BANK EROSION
Contributing Factors to Coastal Bank Erosion, Gordon Peabody, 2012
What factors would cause these three variations in coastal
bank erosion rates along such a limited area of shoreline?
The historical perspective of viewing resource areas as individual segments is gradually shifting towards the broader concept of viewing resource areas as “Systems”. The “Systems Approach” acknowledges that segments within a resource system may exhibit significant internal linkages with each other. “Resource Systems” in turn, may exhibit significant external links to other systems. The Performance of “Coastal Bank Systems” in the “Public Interest” is defined through significant interactions between: the landform profile and composition; the diurnal tidal prism; and the wind direction and intensity. The challenge with the “Systems Approach” is in our reticence to exchange the oversimplification of segmentation, for the interactive complexity of internal and external linkage. Because Coastal Bank Systems are defined by these complex interactions between wind, tide and landform; Performance cannot be expressed in a linear, cause and effect manner.
In Coastal Bank “Systems”, overall landform profiles link off shore and intertidal bathymetry, with elevations on the lower and upper beach, toe, face (or scarp) and bluff of the coastal bank. Face profiles exhibit layers of deposition from varied rates of glacial melt water flow, over thousands of years. Composition (particle size and mix) varies from clay to fine sand and from coarse sand to stones. These materials have been compacted by thousands of years of ice burden. Coastal Bank Systems may therefore demonstrate significant differences in composition, with profiles inconsistent with the “angle of repose”.
Tidal flow represents the sleeping giant in Coastal Resource Systems. We can quantify the potential of tidal energy by using a basic model: Cape Cod Bay can be viewed as a 20-mile diameter bowl. 1. We obtain the area of the Bay surface by multiplying 3.14 x 10 mile radius, squared = about 300 square miles of surface water; 2. We then convert the 300 square miles to square feet = 1,584,000 sq ft; 3. By multiplying the square feet of the Bay surface by a representative tidal depth of ten feet, we obtain a given tidal volume = 15,840,000 cubic feet of water entering Cape Cod Bay on this tide. This translates to 118,800,000 gallons of water, weighing approximately 986,040,000 pounds or nearly half a million tons of water moving around the arm of the Cape and into Cape Cod Bay within the six hour time period we are measuring. To complete our model for a 24 hour time period, that same approximate volume drains out of the Bay and returns and then drains again. The total volume of tidal water passing around Cape Cod on any given day is approximately 475 million gallons, or nearly two million tons. The weight of this water is combines with tidal current to reshape landforms. Coastal Bank Resource Systems interrupt and redirect tidal energy. When this energy is interrupted by subsurface bathymetry or a landform, the rules of fluid dynamics result in accelerated velocity. It is possible to have significant differences in tidal current velocities near Coastal Bank Resource Systems.
Coastal Bank Resource Systems interrupt and redirect wind energy. Wind energy has characteristic, seasonal patterns: summer patterns are predominantly low velocity south winds; winter patterns are predominantly high velocity, north winds. Onshore winds encountering Coastal Bank Systems experience significant changes in velocity. After crossing open water, winds are interrupted by the Coastal Bank. The rules of fluid dynamics result in interrupted flow generating increases in velocity, along the beach, face, and bluff of the bank. It is possible to have significant differences in wind velocity, within the same, Coastal Bank Resource System.
Wind energy, variable in direction, intensity and duration, generates direct material transport through erosion and deposition. Materials are transported from higher velocity areas to low velocity areas. Material deposition zones can be the upper beach, dune or toe of the bank. Transport also occurs laterally, sideways along the beach and face of the bank. Under intense, onshore wind conditions, accelerated wind blows beach materials against the bank, removing additional material from the face of the bank. This material mix is driven up and over the top of the bank where reduced velocity deposits sand. Safe Harbor has documented the collection of a half million pounds of sand on a 10,000 sq ft area at the top of an ocean front Bluff. Material transport by wind can be horizontal, lateral or vertical.
Wind blows against the fluid surface of the sea, creating uneven ripples known as capillaries or “cat’s paws”. Continued wind pressure against capillaries generates increasingly larger waves, which move with the wind direction. Wave size is determined by three primary variables: wind speed; wind duration; and wind direction. Speed describes the potential energy available for creating waves. Duration describes the amount of time available for energy exchange. Direction is defined as open water distance (known as “fetch”), which translates to greater potential wave size.
Coastal Bank Resource Systems interrupt and redirect wave energy. As waves impact the coastal profile, energy is directly transferred through impact, by material transport and deposition and transferred indirectly through turbidity, altering velocity and direction of near shore currents. As a model, onshore wave energy is first interrupted by sub tidal profiles (bathymetry). Bathymetric friction reshapes waves and begins moving materials shoreward. Waves are interrupted again by inter tidal sand bars. Sand bars Perform when bathymetric friction absorbs wave energy. Linkages between inter tidal sand bars and beaches and also between sand bars, exchange significant volumes of material. This exchange contributes to Performance. When sand bar materials increase or decrease, bathymetric friction decreases or increases. Sand bars exhibit significant, time and tide sensitive differences in their ability to Perform by protecting beaches and banks.
Wave intensity (frequency) contributes to deposition or removal of beach material. Low frequency waves contribute to deposition of material, increasing beach elevation and Performance. High frequency waves contribute to removal of material, decreasing beach elevation and Performance. Removed material is transported through turbulent backwash, away from the beach. Away from the surf zone,reduced turbulence reduces transport and materials are deposited as sand bars, (increasing the Performance of the bars). Beaches with angled (perched) profiles, perform by absorbing wave energy. When wave energy exceeds the ability of a beach to Perform, materials loss transitions the profile from angled (perched) to horizontal and performance fails. Horizontal profile beaches will allow waves to access the toe of the bank and the Coastal Bank itself. The toe Performs by absorbing wave energy, moving materials seaward, to replace some upper beach materials. If the toe is missing from the profile, the bank will Perform by default, absorbing wave energy and collapsing, creating a new toe to restore the profile.
When the toe is removed from the profile, the face of the bank absorbs wave energy, collapsing the bank and restoring profile.
Tidal energy synchronized with wind energy, create significant interactive variables. A synchronous low tide limits storm damage. A synchronous high tide amplifies storm damage. Low tide exposes miles of open area for wind to remove or deposit materials. High tide provides a platform for wave propagation to reach the upper beach. Tidal currents, moving parallel to landforms, create significant transport potential. These lateral currents are referred to as “alongshore” and depending on the tide, move in opposite directions. High frequency, onshore wind and wave energy transports materials “downdrift”, with lateral tidal currents. If sideshore wind and wave direction align with lateral tidal current, there is an exponential potential for materials transport. If sideshore wind counters lateral tidal currents, materials may be transported horizontally to sand bars but there will be minimal net lateral transport, during that particular state of the tide.
When sideshore wave energy aligns with lateral tide currents,
the potential exists for exponential movement of materials.
In studying interactive energy system of Coastal Banks, intriguing clues emerge. Significant variation in rates of Coastal Bank erosion may be linked to the Performance and location of sand bars, with subsequent links to waves and tide. During onshore winds, waves push incoming tidewater inshore and over sandbars. Once the tide reverses, this trapped water drains by creating “rip channels” in the bar. In some cases, channels remain in place for periods of time. New storm waves coming through this open channel will not be reduced by bathymetric friction.
In these areas, full sized waves will directly impact the beach or bank, while waves on either side are reduced by sand bar friction.
Satellite imagery illustrates possible linkage, showing the recently eroded area of Ballston Beach was not protected by a bar.
Notes on restoration efforts:
Restoration efforts on Coastal Banks should be sparing. Alternative responses should consider consequences of creating shoreline anomalies that amplify erosion activity. Renourishment of the eroded toe and part of the eroded face, do not consider the fact that beach elevations have also been significantly lowered by erosion. Lower beach elevations will allow wave propagation right to the toe of the bank. Beaches absorb wave energy but only if they have been restored.
When eroded beach elevations allow wave propagation, wave wash contacts driftline fencing. The narrow slat spacing prohibits water flow. Since water is not compressible, the full wave weight, plus momentum, gets directed at the fence, which essentially, is now behaving like a structure but is not engineered to respond as a structure. Wave wash gaining entry behind driftline fencing cannot escape, resulting in accelerated outwash of recently renourished sand.
The sketch below depicts a Coastal Bank’s net loss to erosion, as part of Cape Cod’s coastal process. Storm winds and surge drive high amplitude, high frequency waves onto beaches. The beaches absorb the high amplitude wave energy but the intense wave frequency doesn’t allow sand to settle out and it is drawn off shore. As the elevation of the sand starved beach drops, sand at the toe of the Coastal Bank gets drawn down onto the beach. Once the toe of the Bank is gone, the face of the bank loses some degree of stability and will collapse to form a new toe. Coastal Banks perform by renourishing eroded coastal beaches.
Driftline Fencing is an engineered, semi-structural erosion response system. Though it has a record of consistent failure, the identical fencing, consistently gets re-installed, always in a continuous line.
Contributing to consistent failure, may be the narrow spaced slats of this semi-structure, which prevent passage of wave wash. That doesn’t mean all the slats should be changed but perhaps the narrow slat fencing itself, could be reconfigured, as shown below.
What effect do thousands of people have on coastal banks? At times of low tide, perhaps not that much.
At high tide, people are packed against the Coastal Bank, with each person displacing their own weight in sand, each time they climb up or down ten feet.
As Coastal Banks erode, shorelines moved landward. The Coastal Bank cannot be moved seaward again. Attempts to fully restore the daunting geo mass of a coastal bank, create shoreline anomalies, which target the restored area for amplified erosion.
When the toe of a coastal bank system is removed by wave action, the bank will collapse, creating a new toe and restoring the system profile.
The use of stairways reduce direct erosion but the steps themselves redirect and concentrate storm wind into the face of the bank. One solution is to utilize “Flo Through” technology, which uses open grate steps, as shown below, to reduce erosion from wind resistance.
Cape Cod isn’t the only place with Coastal Bank erosion issues. Here is a letter from California.
Gordon,
I was so grateful to find the Safe Harbor Website and resources.
The site and the work you are doing is exactly what is needed. I work for California State Parks. We have a historic dump site eroding onto the beach from the coastal hillside in one of our Campground park units. The County Environmental Health Agency is not too happy with us. The site has been tested and PCB and other such stuff is NOT and issue for us – the concern focuses on public safety from broken glass and metal that is washing out on to several shelves above the beach.
We have been looking for “cover” that will help hold the historic material in place and be strong enough to stay intact through winter storms. Maybe even something that hardy native plants might be attracted to root in…
Date September 27, 2010 To: California State Parks, Channel Coast District
Attn:Barbara Fosbrink, District Services Manager, CC: Sarah Lowing
From: Safe Harbor, Contact: Gordon Peabody, Safe Harbor Director
95 Commercial St, Wellfleet, P.O. Bx 880, Wellfleet, MA, 02667
gordonsafeharbor@yahoo.com Phone 508-237-3724
Re: Erosion Response Alternatives Analysis for Unique Coastal Strata
Synopsis: Site specific coastal bank exhibits unique, dissimilar, geological strata: Paleontological tar layers with possible dinosaur bones; Paleo Indian shell middens with possible artifacts, considered “Sacred Ground”; Most recent, subsequent, anthropogenic landfill from Central American migrant camps.
The coastal dynamic processes, interacting with this multi-layered strata, have raised issues regarding: regulations; provenance; public safety and a course of action. Multiple, subsequent subset issues exist and will not be addressed here. The goal of this report is to develop and explore alternative responses and consequences, to determine a course of action. Alternatives are developed using multidisciplinary collaboration and evaluated through ecological, coastal geological and financial consequences and relevance to linkage of scale (how will the solution impact existing, adjacent coastline). While ideal alternatives should empirically balance ecological, social and financial values, coastal erosion tends to create compelling imperatives for response. Permitting protocols are not addressed here.
Note: Tracking erosion rate with 20-year assessment of satellite imagery, provides inconclusive erosion rate/year. For the purposes of this document, a linear value of “X” shall therefore be assigned to the annual rate of coastal erosion, on this site only.
Alternative One: No Action
This classical alternative needs no direct outlay of funding but requires a defacto “sleeping budget” for constant mitigation/removal of debris. This debris may have social (quality of experienced resource) and ecological (elevated presence of iron oxide and mix of non indigenous materials potentially altering interstitial habitat) impacts.
Alternative Two: On Shore, Structural, (Hard Solution) Response
Coastal, engineered structures require significant outlay of funds, have short solution responses and long-term consequences, impacting adjacent shoreline. This may contribute to reduce social quality of the experienced resource. This response fails to meet criteria for linkage to larger scale.
Alternative Three: On Shore, Non Structural, (Soft Solution) Response, Coir Fiber Rolls
Coir Fiber Rolls require moderate financial outlay. Their wave energy absorption function and duration is dependent on wave heights and being properly secured with properly installed, engineered helical anchoring system. This system can reduce erosion. Wave heights for this site could be a factor. This response will not change the rate of coastal erosion, has a limited life span and may eventually impact adjacent shoreline erosion rates by creating an anomalous shoreline feature. This factor reduces its evaluation on criteria of linkage to larger scale.
Alternative Four: On Shore, Non Structural, (Soft Solution) Response, Beach Nourishment
This is an acceptable alternative of moderate financial outlay. This response works with the costal process and creates a successful linkage in scale. The duration of this response is short and will require frequent repetitions, due to lack of subsequent renourishment on adjacent beaches. Challenges arise from source impacts, sand particle size and mix (altering angle of repose). Infrastructure required for enacting this alternative may have on site impacts.
Alternative Five: On Top of Bank, Pro-Active, Surface Layer Removal
This is a preferred alternative, directly utilizing intermediate financial outlay to defer the consequences of the landfill layer coming into contact with a public beach. The amount of surface layer/landfill to be removed would be expressed in linear feet of erosion per year “X”, times fifteen years.
A thin compost/mulch layer would be placed over the open Earth and planted with indigenous seeds to create a sustainable habitat cover layer. The coastal process would not be impacted. This would successfully create a linkage to larger scale.
Alternative Six: Off Shore, Geo Tubes Reduce Wave Heights
This innovative alternative is currently in use for shoreline and off shore erosion control projects internationally but less so in the States. This financially demanding alternative utilizes the “Double Loop Learning” process, where the response is not directed at the problem but at the cause of the problem. By reducing wave heights, erosion is reduced. This would be most successful when implemented on a larger scale to avoid creating anomalous coastal features from differential erosion rates. This would create a moderately successful linkage to larger scale on that basis but otherwise not successfully work within the existing coastal process.
Respectfully submitted for your review, Gordon Peabody, September 27, 2010