# Jetties (or groins) on a beach..how exactly do they prevent erosion? **URL:** \ **Category:** Factual Questions\ **Created:** [July 20, 2003, 4:08pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756 "2003-07-20T16:08:34Z")\ **Posts on this page:** 11\ **Page:** 1
**Author:** ![ChryslerBldg](https://avatars.discourse-cdn.com/v4/letter/c/e480ec/32.png) [@ChryslerBldg](https://boards.straightdope.com/u/ChryslerBldg)\ **Post date:** [July 20, 2003, 4:08pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/1 "2003-07-20T16:08:34Z")
My beach has a groin (more commonly called a jetty) consiting of 2 rows of about 4 telephone pole size piling driven into the sand and connected by a 2 foot wide wood beam. The pilings and beam are coated with the same type of “tar” you’d see on a telephone pole. At the end of this string of pilings are a line of boulders 15ft wide and going out about 200 ft. in to ocean. I know the jetty is there to prevent beach erosion, but how exactly doe it accomplish this? ---
**Author:** ![Little\_Bird](https://avatars.discourse-cdn.com/v4/letter/l/3ec8ea/32.png) [@Little\_Bird](https://boards.straightdope.com/u/Little_Bird)\ **Post date:** [July 20, 2003, 4:23pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/2 "2003-07-20T16:23:52Z")
I would assume that since waves tend to travel one way down the beach that the waves would hit the jetty and then stop there. This would leave the area in lee of the jetty free of the waves’ erosive action. Whaddya think? ---
**Author:** ![Qadgop\_the\_Mercotan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/qadgop_the_mercotan/32/83_2.png) [@Qadgop\_the\_Mercotan](https://boards.straightdope.com/u/Qadgop_the_Mercotan)\ **Post date:** [July 20, 2003, 4:24pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/3 "2003-07-20T16:24:23Z")
currents running parallel to the beach will sweep sand away and deposit it elsewhere. Jetties interrupt the current, causing the sand to deposit at the base of the jetty. That’s the theory anyway. In practice it gets more complicated. QtM, part-time seawall architect (from when Lake Michigan tried to eat my house) ---
**Author:** ![MaryEFoo](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@MaryEFoo](https://boards.straightdope.com/u/MaryEFoo)\ **Post date:** [July 20, 2003, 5:02pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/4 "2003-07-20T17:02:53Z")
Yes, if the current is running from right to left, and your jetty protrudes straight out, the sand deposits to the right of the jetty, but it erodes away from the left hand side. —\_\_\_|— Dashed line is the original beach, underscore the new cut-away shoreline, vertical is the groin, backslash is piled up sand, last dashed line the original beach up-current from your groin. Typically the backslash area gets protected, and the uproar comes from the underscore area. If per your description the pilings allow water and sand to filter through, the cut-away area will be much less cut away. I wonder about the 200’ of rocks. Maybe it is to prevent the sand from washing out away from the beach, to the bottom of the bay. This might work like a funnel, sorta, actually increasing the supply of sand to the down-current side of the jetty, as well as the up-current side. ---
**Author:** ![Ringo](https://avatars.discourse-cdn.com/v4/letter/r/779978/32.png) [@Ringo](https://boards.straightdope.com/u/Ringo)\ **Post date:** [July 20, 2003, 5:19pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/5 "2003-07-20T17:19:22Z")
Sand in transport _will_ be deposited where the jetty faces the current, and the beach sand in the lee of the jetty will be somewhat protected from erosion by the current. On the whole, the idea is to minimize the erosional effect of the longshore currents by setting out impediments. ---
**Author:** ![Trigonal\_Planar](https://avatars.discourse-cdn.com/v4/letter/t/0ea827/32.png) [@Trigonal\_Planar](https://boards.straightdope.com/u/Trigonal_Planar)\ **Post date:** [July 20, 2003, 5:22pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/6 "2003-07-20T17:22:03Z")
Yes, Qadgop is correct. The idea is to force the water currents to drop their loads of sand instead of carrying them out to sea. ---
**Author:** ![NinetyWt](https://avatars.discourse-cdn.com/v4/letter/n/cc9497/32.png) [@NinetyWt](https://boards.straightdope.com/u/NinetyWt)\ **Post date:** [July 20, 2003, 5:40pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/7 "2003-07-20T17:40:18Z")
This works for rivers, too. In instances where the river channel begins to move to one side, “training walls” can be used to “train” the river back to its original position. It’s accomplished, again, by slowing the water down at the “wall” so that it drops its sediment there. ---
**Author:** ![MC\_Master\_of\_Ceremonies](https://avatars.discourse-cdn.com/v4/letter/m/bb73d2/32.png) [@MC\_Master\_of\_Ceremonies](https://boards.straightdope.com/u/MC_Master_of_Ceremonies)\ **Post date:** [July 20, 2003, 5:52pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/8 "2003-07-20T17:52:13Z")
QtM is in indeed correct, sand depoists build up on the side of the jetties facing into the current. btw though just checking at [dictionary.com](http://dictionary.com) “groin” is an acceptable spelling usually “groyne” is prefered (or maybe it is just a UK thing). ---
**Author:** ![barbitu8](https://avatars.discourse-cdn.com/v4/letter/b/839c29/32.png) [@barbitu8](https://boards.straightdope.com/u/barbitu8)\ **Post date:** [July 20, 2003, 8:03pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/9 "2003-07-20T20:03:11Z")
> [@](#): > > \*Originally posted by MC Master of Ceremonies \* > \*\*btw though just checking at [dictionary.com](http://dictionary.com) “groin” is an acceptable spelling usually “groyne” is prefered (or maybe it is just a UK thing). \*\* That’s just a UK thing. In fact, I’ve never seen it spelled “groyne” before. ---
**Author:** ![Ficer67](https://avatars.discourse-cdn.com/v4/letter/f/c57346/32.png) [@Ficer67](https://boards.straightdope.com/u/Ficer67)\ **Post date:** [July 21, 2003, 1:37pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/10 "2003-07-21T13:37:47Z")
Here in Louisiana, we have similar constructions all around. Every two years or so, the structures disappear, and get replaced. This leads me to the conclusion that there is no way to stop erosion, and that eventually the jetties on your beach will have to be replaced too. ---
**Author:** ![Phlosphr](https://avatars.discourse-cdn.com/v4/letter/p/c68b51/32.png) [@Phlosphr](https://boards.straightdope.com/u/Phlosphr)\ **Post date:** [July 21, 2003, 2:38pm UTC](https://boards.straightdope.com/t/jetties-or-groins-on-a-beach-how-exactly-do-they-prevent-erosion/189756/11 "2003-07-21T14:38:17Z")
Ahh, up here in Connecticut we have WAAAAY too many rocks all around thus the plethora of Stone walls, and Granite Jetty’s. By the way. The beach where we usually launch our boat fills in in the summer and receeds in the winter. The Sand height I mean. And we have two quarter mile granite Jetty’s one behind the other making the ocean spilling into long island sound much less harmfull to our million dollar properties.