Ocean Energy Dissipation System
Described here is a system to dissipate the energy in ocean waves before they hit the boardwalk - Ocean Energy Dissipation System (OEDS). Effectively what it will do is dissipate wave energy in a channel for destructive waves to fall into. Something akin to a moat protecting castles of old.
(((Background for web surfers - The town of Lavallette, NJ is on a narrow sand barrier island. It has a boardwalk running the length of the town (just over 1 mile long) on the beach. The boardwalk was regularly damaged in winter storms (some years more than others, many years not at all, but seldom severely - ie structural damage, but expensive to repair nonethe less. It could keep a crew of 3 or 4 men busy for weeks at a time).
In the late 80's, a series of snow fences were installed on the then wide beaches to trap blowing sand creating a dune that eventually grew enough to protect against pretty severe storms.
However, due to various political pressures, the dune has grown so much (ever wider and higher) it is starving (narrowing) the beach to the point the beach is often unuseable in many places at high tide in the summer. Further the dune has grown so high the awesome ocean is no longer visible from the much/most of the boardwalk (you might as well be walking in a theme mall) or the houses that line the oceanfront. So much sand blows off the top of the dune during an easterly (fairly frequent in winter), it creates a (continual) maintenance problem for the town to clean it off. And often becomes almost impassible for all but the most hardy walkers.
Contrast these conditions with the generous beaches Lavallette (and all of Ocean County) had for literally 100 years or more before the dune creation. Some years more than others depending on winter storm frequency, etc., sand would get moved just offshore during storms creating a bar (that could be walked on at low tide sometimes), 100 yards or so out, providing a naturally occurring self protection forcing wave to break further offshore. When the storm abated the smaller ocean waves would gradually move it back to the beach. A process that has gone on for centuries, even millenia, all over the world.
The beach level (for the reference points used in OEDS) before the artificial dune creation started out just about 3' (plus or minus every year) below the current boardwalk level and was descended onto via stairs or ramps. )))
On the eastern edge of the boardwalk a vinyl bulkhead 6' long will be driven down to the same height of the boardwalk (but not be part of the boardwalk). Then 15' east (toward the ocean) of that another bulkhead 20' long will be driven to the same height as the boardwalk.
The sand between the two will be dug out to a depth of 3' and re-deposited on the beach (where it originally was until an artificial dune building system was initiated about 20 years ago) creating a channel between them 15' wide and 3' deep.
Now when storm seas are driven on shore, they will hit the outside of the OEDS, spill into the channel and have their energy dissipated before they reach the boardwalk. When a sea higher than the OEDS comes rushing in, the lower part of the wave will drive up against the outside, forcing the rest of the top the wave to crash relatively harmlessly into the channel. ("Relatively" when compared to the harm it would cause if it hit the boardwalk directly or drove up underneath it.)
By far the most damage to the boardwalk is from seas eroding the sand in front and rolling up underneath driving the decking up. The inside of OEDS on the ocean side of the boardwalk prevents seas from washing up underneath and forcing the decking loose from the heavy framework underneath.
OEDS will effectively "cut the legs out from under" large seas driven onshore, causing them to "trip" and expend their energy falling into the channel.
That's the principle behind the OEDS. Pretty simple.
to more than one one vinyl bulkheading company and/or from their websites. Here are two:
http://www.everlastseawalls.com & http://www.cmilc.com/applications/seawalls.htm .)
First of all, material cost to do all 6,000 feet of oceanfront is about $1,300,000 (estimated high). Presuming installation cost of $1,700,000 (estimated extremely high using figures of 12 men @$50 per hour for 4 months = $400,000 & $1,000,000+ for walers, tiebacks, pilings, etc.).
That's a $3,000,000 total cost, which ain't chicken feed, but amortized over 40 years in an a fully amortizing project bond comes out to less than 2 cents on the annual property tax bill, or maybe a 2% increase from current levels. Further OEDS would last a lot longer than 40 years.
Actually the cost would be offset somewhat by fewer boardwalk repairs and no street end repairs, but the real return is the beautiful beaches we used to have before all the sand was artificially transported into the dune.
When a large sea crashes over the outside of OEDS and falls into the channel, the huge amount of water has to be disposed of. OEDS is designed so at regular intervals (say at street ends), a 10' wide opening (Escape Vents - more details below) left in the outside of OEDS. For that 10' section, the outside of the OEDS channel would be 3' lower. Water would rush down the channels and pour back out through the openings. Depending on ocean height, wave frequency and other factors, this huge volume of water rushing back out the EVs could again undercut oncoming seas heading for the opening.
OEDS Channel Bottom - The bottom of the OEDS channel would be laid with several layers of heavy fiberglass porous landscape cloth. That lets water drain through to the sand underneath (rain, etc). On top of the cloth will be 8-12" of large river rap (which are stones 4"-6" each in size, or about the size of a softball), then 12" or more inches of sand on top of the river rock. The cloth will prevent the rock from disappearing into the sand below. The river rap to provide a stable non-eroding base for water to run across in case the sand gets washed out of the EVs. The sand is there for appearance and soften any (rare) fall from the boardwalk. The object here to keep the water rushing to the EVs instead of eroding and undermining the boardwalk or street ends.
(It should be noted here the 3' below boardwalk level is not a magic number pulled out of the air. It's the level of the beaches were at for decades or longer before the establishment of the artificial dune system (by putting up snow fences ever wider and ever higher to trap blowing sand.)
Escape Vents (EV) - As seas break over the outside of OEDS falling and expending/dissipating their energy into the channel, they bring huge volumes of water and that water has to go somewhere. The outside of the OEDS channel (recall is the same height as the boardwalk) would have 10' wide openings at every street end (about every 250') and the top of the ocean side of OEDS would be 3' lower at the openings. When the channel fills with water, it will rush out the EVs as the wave recedes. (Note engineers may determine a wider opening is necessary.)
EVs at each street provide easy beach access, exactly as we always have had. Some mechanism may be seen as needed to deter storm seas from rushing in through the EVs and running down the street. I don't believe that's a serious threat as only two seas came over the street end on Trenton Avenue (for example) during the '92 Easterly (the worst storm in over 40 years driving the highest water levels in anyone's memory in Monmouth and Ocean Counties) . The narrow EV opening (say 10') in the outside of OEDS will curtail the amount of water driving in by as much as 80%. Further, much of the power of the wave will be sheared off on each side as it hits the EV and the lower sides of the wave collapse/expand into the sudden openings in the channel.
EVs will reduce/eliminate the municipal problem of seas washing down the streets, carrying a lot of sand with them in all but the very very worst of storms (so called "100yr storms"), a problem Lavallette has with street end walk-throughs in the dune in severe storms. What happens as a sea drives against an immoveable surface (sand dune or OEDS) is the energy is directed literally up down and sideways. The sideways push increases the height and volume of the sea going through the lowest part of the surface. With OEDS that sideways energy collapses when it reaches the EV and the downward energy is turned back at the sea (see Scour Plate next).
Scour Plate - bulkheads are blamed by (some) experts for actually increasing erosion more than would happen if they weren't there. The theory is that seas breaking against the bulkhead have (some of) the force directed downward driving sand away thereby increasing beach erosion and threatening bulkhead integrity (causing it to wash out from underneath).
OEDS negates/eliminates that problem because bolted to the top of a high "waler" (a longitudinal brace outside the bulkhead tie rods are fastened through and which increases bulkhead integrity) would be an 18" to 24" wide scour plate sticking out horizontally (like a shelf from a wall).
The scour plate waler would be at least 4' down from the top of the outside of OEDS on the ocean side (normally covered up with sand and only exposed during bad easterlies). (Note - Engineers may want it even lower, say as far down as the low tide level to reduce reflective energy driving sand away - details in next paragraph.) As waves drive against the OEDS any energy that is directed downward would hit the scour plate and be directed back toward the ocean, further undercutting the wave. The OEDS scour plate actually turns some of the destructive wave energy against itself.
Further the scour plate will cause suspended sand to settle in front of OEDS rather than getting pushed away as it might if the energy were reflected down. As further assurance of this, the scour plate could be in the form of a half pipe with the missing top half facing up. As wave energy comes down the face of OEDS, it will enter the open half pipe and be directed upwards and outwards. Any suspended sand would settle more quickly, causing a bar to form in front of OEDS. After the storm passes, this bar will will naturally migrate westward, replenishing the beach. Just as it does now when a bar forms offshore during a storm. The only difference is this (additional) bar will be closer to shore and speed natural replenishment.
The 20' depth of the outside of OEDS should be plenty long enough so the bottom doesn't get eroded under in a really nasty NorthEaster. If erosion experts believe even that's not enough, the sheathing could easily extend to 30', or even 40' without significantly increasing cost (say adding 10% to the project). (Note the DPW super will know how deep the street end bulkheads are and that could be a guide as some of them washed out in the '62 NorthEaster.)
The channel may occasionally get (partially) filled with sand washing or blowing over the outside of the OEDS in a strong (North)Easterly. After the storm passes, it's a relatively easy job to scoop it out back on to the beach with a side dumping front end loader. Note the integrity of OEDS relies on keeping the channel clear so destructive wave energy is dissipated by harmlessly falling down into the channel. If the channel is filled then large seas could just roll across the top and smash into property and down streets. Actually the "excess" sand would pour out the EVs, but not probably until at least some (unnecessary if the channel were kept clear) damage was done.
Tie rods to the outside of OEDS would be lower than the channel depth (say 4' - 6' from the top) and cross to tie into the inside of OEDS which will act as an anchor ("dead man" in construction parlance). In addition the outside of OEDS would have pilings driven every so often (10'-20') to stiffen the exposed surface of OEDS (the 3' above the bottom of the channel) against the force of waves driving against it. The inside of OEDS may need stiffening pilings as well as sand under the boardwalk would be returned to the each during OEDS construction.
(Note here, a considerable "reserve" of sand that is under the boardwalk now could be returned to the beach. Just how much (beyond the 3' that would be recovered during OEDS construction) is a function of current street end bulkheads and inside OEDS sheathing depth. In theory one could probably excavate 20' down or more, though unlikely. No reason 6'-8' couldn't practically be returned to the beach though. At maybe a 5% increased OEDS cost.)
Vinyl as an OEDS sheathing is incredibly strong laterally; is virtually indestructible in terms of weathering, rotting, aging, etc., is far easier to install than traditional wood sheathing (1/2" thick to drive as opposed to 2"-4" wood). It comes with a 50 year guarantee for example. OEDS would last as effective beach preservation well into the new century and probably on into the next.
The beach sand on the ocean side of OEDS could be allowed to accumulate as high as desired, even rising above OEDS if desired though there would be no need. Only the OEDS channel depth has to be maintained 3' at between EVs.
(See also http://www.SwedesDock.com/beachero.sht )
End Ocean Energy Dissipation System
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