All up and down the East Coast of New Jersey, especially in the Northern half of the state (Ocean and Monmouth Counties) there is continual and serious concern about beach erosion. I won't belabor the "encroaching developement" arguments. Suffice to say the ocean frontage is about as developed as it can get. Every time there is a storm, especially a serious one, it brings out all the bewailers and bemoaners predicting catastrohic damage (which seldom happens) but we do lose our beaches for awhile and occasionally our boardwalks.
The beaches come and go but we do seem to have less than we used to. Some summers there seems to be hardly any, and others there's plenty. Sand bars move in until they meet the shore and become a part of the beach (then we have plenty) and then move off (or north or south) and then we have little. It's never in any predictable pattern but is a source of constant concern.
How Erosion Occurs
At any rate I have devised a simple mechanism to keep our beaches intact. First a quick synopsis of how the erosion occurs. When a storm developes the wind creates huges walls of energy in the ocean, called waves, that can be 20 or 30 feet high or more. These waves of energy come roaring in from deep water, are forced up the relatively steep incline near shore (say within the last 100 meters or so) and carry the ocean with them. When the energy gets to a height where it can no longer support the weight of the water it's dragging with it and/or the energy gets expended climbing the incline, the water crashes down and the energy is dissapated.
Visualize holding a ruler perpendicular in front of you and slowly move it across and up an imaginary gradual incline. As you move it up slowly let it tip forward in the direction of your movement. The tipping is caused by the greater friction on the bottom of the wave as it climbs the rising ocean floor and rises above what would be the level of the ocean if there were no waves. At some point (it's your vision) you can imagine it becoming top heavy and crashing down.
What happens then is the top of the sand becomes momentarily "liquified" and as the water runs back down the incline towards the ocean it carries sand with it. At some point the sand settles out out and that's how the sand bars get formed. The entire process is governed by the power (velocity and size) of the waves.
Later on after the storm passes, smaller waves cross the sand bar and that very same friction action that caused the tipping of your ruler gradually drags the sand back towards shore. You can see this for yourself if you look underwater when a wave moves by you. You'll see the surface of the sand sort of raise up and settle down, each time it moves in toward the beach fractions of an inch, until of course it meets the surf line where it meets the returning water of a broken wave. Usually that's where you'll find a narrow stony section. That's caused by the movement of the sand constantly filling the smaller spaces under the stones forcing them to the surface.
Undertows
Undertows are caused by the volume of water in a wave coming across the sand bar, breaking, then making its way back out. It follows the path of least resistance following the inside of the sand bar until it finds a break in it and flows back out to sea. That flow (or current) is called an "undertow".
Water Power
It's clear there's a lot of power in ocean waves but let me put it in perspective for you. If a given volume of water (say one gallon) moving at one mile per hour (mph) will move a given weight (say one pound); if it moves at two mph it will move 9 lbs and if it moves at three mph it will move 625 lbs. And so on up the speed curve. I'll leave it to the math wizards to work out what it will move at ten mph (probably Mount Etna).
This power is what thwarts beach replenishment projects. We've all seen huge volumes of sand pumped onto the beaches, only to see it disappear in one or two winter storms. Huge dredges pump millions of cubic yards of sand from offshore. The storms wash it back out and the dredges start all over again. The only difference between that and making little rocks out of big ones is that it pays a lot better. As for its usefulness, well the politicians get to say they're doing something and a lot of tax money gets pumped into the economy but other than that ....
Making that Power work for us
I have come up with an idea I'm certain can reverse the near shore erosion process. Essentially it boils down to dissipating that predogious power before it reaches the shore line (say the 100 meters or so we are interested in). In effect what I would do is install an incline for the wave to climb and force it to dissipate its energy before it reaches the shore.
The object is for the wave to run up the ramp and fall from its own weight, expending its destructive energy (from a human point of view) before it reaches our precious beach. It's pretty simple really. (I got the idea from a cartoon a long ago, only in that the shore side was sort of inclined as well.)
Sand will get carried up the incline (or pumped over) and fill in underneath the ramp, stabilizing it in place. The sand will further fill in the beach from its current height on shore out to the top of the ramp, providing a constant wide beach for swimming and onshore protection.
The ramp is solid concrete but the surface facing the beach and sides are open to allow the sand to fill in. (The diagram shows the sides but the 6' high surface would be directly perpendicular to the beach) I have it here with a 15' ramp which yields roughly a 30 degree upward slope. I'd like to make it longer, say 20' or even 30', to give a more gradual incline (less resistance to that terrific power and making it work even better) but you get into serious weight considerations (discussed below).
What I would do is position this ramp in about 12' of water at normal low tide. That leaves the top edge 6' below water, deep enough not to bother swimmers or even small boaters, but shoal enough to be effective in major (even "minor") storms.
Ramp Construction
I would make the ramps out of 6" thick reinforced concrete. The thickness is needed to protectect the reinforcing bars (Rebar) from corrosion. It's my understanding that at least 2" of concrete is required to prevent water penetration so 6" would allow something more than 2" covering of the rebars. There's a little more involved in strength than just rebars (ex. wire mesh, etc.) but I won't go into it here. At any rate, a four foot wide ramp would weigh about 30 tons, which is, I believe, about the practical (reasonable) handling limit of weight in this type of project.
I have read about foam impregnated concrete that is much lighter but I have no personal knowledge of it. Also there are likely corrosion resistant rebar materials (stainless steel for example) that would allow a thinner coat of concrete. To do a mile of beachfront would require over 1300 4' wide ramps and if the ramps could be made lighter (wider at the same or less weight) then proportionately fewer would be needed to be handled. At any rate let's take a look at how I'd place these ramps.
Installing the Ramps
A large barge specifically outfitted for this job would have to be built. I envision one 50' wide and 150' long. We'll be working in an ocean environment and it would have to be that big to provide a reasonably stable working surface. At the same time it has to be small enough and shallow enough draft to get in and out of the local inlets when nasty weather is coming.
(1) The crane (often called a "cherry picker"). It would be rated at least 100 tons and have a 60-75' hydraulic boom. It's primary use would be to lift and position the ramps after they are dragged to the barge. You see it here lifting the ramp after it has been dragged out to the barge by the winch (2) on the deck of the barge.
(2) A heavy winch (or series of winches) used to manuever the barge via anchors, raise and lower the "spuds" (3), and primarily used to pull the the ramps from shore out to the barge. The dotted line you see leading directly down from the front of the barge down to the ramp is a cable connected to the winch and was used to drag the ramp out from the beach.
(3) 50' long "Spuds" used to anchor and stabilize the barge. Spuds are long heavy timbers (usually but these would probably have to be fabricated out of heavy steel) that are raised and lowered. They can even be used to jack the barge up out of the water, necessary in this case to get away from wave action that could move the barge during ramp positioning.
(4) A sand suction pump (inside the barge) has a 100' of 8" suction hose connected to a 10" 20' long pipe. The pipe held upright (by a small tender boat) and lowered to the ocean floor where it sucks a huge hole in the sand bottom. As the pipe is lowered the hole gets deeper and the sides keep caving into the suction pipe. (Remember trying to dig a hole in the sand by the water's edge as a kid?)
(5) The sand discharge hose running up onto the beach filling in as the ramps are placed. The object is not only to fill in the beach but to get the sand under the ramps (between the base and the ramp) to anchor them in place. If it's not done now there's a possibility of a storm moving them before they get filled in as it will leave a huge wall of resistance to "back flushing" waves as they break over the ramp. (Recall the power of moving water above).
Costs
What are the costs of doing a project like this? Obviously huge. My best guess would be something under $10 million dollars a mile, with about 2/3's of that taken up by the cost of building the ramps (@ a fabrication cost of say $200 per cubic yard of concrete, a very high estimate, it probably could be done for half that or less.)
The barge and beach equipment costs, while considerable (somewhere between 1 and 2 million), aren't really significant as they can be amortized over a long period (10 years or more).
The installation costs certainly wouldn't exceed $2 million per mile (and that's allowing for only installing one ramp every 2 hours (12 men @$50 per hour). Once set up, it should go a lot faster but does allow for a lot of down time (weather, delivery delays, etc.)).
Of course, as you can see, there's lots of variables involved here and I haven't detailed all phases (but I have considered them and factored them into my cost estimates) but have picked the highest outside costs to estimate with (I know, the cynics will say you have to double them but they would be wrong)
and that's what I come up with.
Presuming low interest bonds amortized over 40 years it would amount to a bonding cost per household in my town (which has just about 1 mile of beachfront) of somewhere between $200 and $250 per year in increased taxes on an average tax bill of about $3,000. That's presuming no financial help from either the state or the feds and the town would have to carry the entire burden by itself. We're a very small town with a relatively large beachfront area. Most other towns have a far larger tax base to spread the cost over.
It would cost maybe $50 a year per state income tax payer to do the entire coastline of New Jersey if it were done as a state wide project.
Is it worth it?
It would be to me.
End of Beach Erosion
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The ramp is solid concrete but the surface facing the beach and sides are open to allow the sand to fill in. (The diagram shows the sides but the 6' high surface would be directly perpendicular to the beach) I have it here with a 15' ramp which yields roughly a 30 degree upward slope. I'd like to make it longer, say 20' or even 30', to give a more gradual incline (less resistance to that terrific power and making it work even better) but you get into serious weight considerations (discussed below).
(1) The crane (often called a "cherry picker"). It would be rated at least 100 tons and have a 60-75' hydraulic boom. It's primary use would be to lift and position the ramps after they are dragged to the barge. You see it here lifting the ramp after it has been dragged out to the barge by the winch (2) on the deck of the barge.
