drainage ditches in coastal areas

[Gumhead]

location: East Coast, within 50-150 yards of beach front, extremely flat terrain, water table very close to ground

background: A neighborhood association paid a firm to design and build some drainage improvements. These 'improvements' included ditches to carry water out of the neighborhood and into a canal located inland.

The problem is that these ditches get standing water in them since the water table is regularly above the bottom of the ditches. Also, after it rains, water never appears to move through the ditches...water always appears stagnant. Some neighbors are thinking the improvements are a waste of money, and are also concerned about aesthetics, and mosquito breeding.

Is there anybody out there who has experience and advice on designing ditches for coastal areas? Would it be wise to just accept puddling water on properties and streets during and after storms, or has anyone had success with coastal ditches? Also, it has been suggested that some ditches be replaced with French drains in order to avoid the standing water problem. Could this work? Are you aware of any resources that have information on coastal area ditches? Thanks!

 

[johnhan76]

The only way to avoid water in the ditch is to establish a ditch outlet that is not submerged. Otherwise groundwater will not have a way out of the ditch and will accumulate. This is probably hard to do in your coastal environment. I would consider building the site up by bringing fill in and then piping the water. If the outlet of the pipe is submerged, then you will still have water backing up into the storm boxes but at least it will not be visible.

 

[Gumhead]

Thanks for getting back w/ me. The neighborhood is already fully developed and building the area up would not be financially realistic.

As it is now, the ditches go to a storm sewer pipe, which then goes into the canal. The outlet of the storm sewer pipe is submerged.

 

[cvg]

ditches simply are not feasible in coastal areas. The engineer who designed them blew it! The engineer(s) who designed the subdivision or the grading plans for the houses also blew it. The only way to keep water out would be to seal the ditch lining against inflow. Concrete lining likely won't work, as any cracks will allow seepage in. In addition, if you do manage to prevent the cracks, the hydrostatic force would likely crack the lining or float it out. Seriously, the only method in this situation is to floodproof (elevate) the structures.

 

[johnhan76]

If the outlet of the pipe is submerged but not the inlet, then the ditches should at least flow to the inlet even with the groundwater. It sounds like the ditches are not properly graded if there is stagnant water in them. if regrading doesnt work then Like CVG said, floodproof the structures.

 

[cvg]

the likely reason for water standing in the ditch is the high ground water. The invert of the ditch is below the groundwater elevation. The flow seeps through the ground and exits into the ditch. In effect, you are attempting to dewater the area - which in this case would be difficult (impossible?) given your close proximity to the ocean. During storm events, these ditches are also likely to backup due to storm surge. This will reduce the effectiveness when it is needed the most.

 

[johnhan76]

Yes, I would agree that the water in the ditch is groundwater, but a properly graded ditch with a free outfall should move the groundwater and not produce stagnant water that gumhead referred to. But I doubt that there is a free outfall - gumhead will have to answer that.

 

[Gumhead]

In an effort to reduce the standing water problem, the ditches were raised slightly (filled in). In theory this should slightly reduce the frequency at which the water table is above the bottom of the ditches, but there is still standing water on a regular basis. It didn't appear to make much of a difference.

If there is a sudden deluge, I suppose the ditches could carry some water, but with the ground being 100% sand I would think overland flow to the ditches would be minimal. Also, I believe the water table would rise very quickly after the storm, and that the ditches would quickly be filled by the rising water table. I really don't see a situation where these ditches could ever carry a significant amount of water. Am I off base here?

A suggestion was made to put fabric in the ditches, fill them with gravel, pull fabric over the top, and then restore the ground to original level. I'm thinking that if the ditches don't convey water, then the gravel/fabric idea won't work either.

 

[cvg]

the gravel / fabric idea will not work to convey storm runoff (which I assume was the original purpose of the ditches. This flooding would occur quickly and before significant water could soak into the gravel/fabric - french drain system. I would guess that you could show that the percolation rate is much lower than the amount of rainfall runoff. Plus, the fabric/gravel layer can lose effectiveness after time if a layer of silt begins to form on the surface. It sounds like a lot of work and money for a mostly ineffective system. I stand by my original post that raising the houses or lots is the only practical method in a coastal area.

 

[dicksewerrat]

Could you line the ditches with an impermeable membrane, weight that down with enough concrete to hold out the highest groundwater elevation and slope the dithces a little better?

 

[Gumhead]

There is not a free outfall.

As far as dicksewerrat's suggestion, I think that lining could work to a certain degree. However, this area is a beach resort, and aesthetically I don't think it would fly.

And for an important clarification...I should have caught this before... The purpose of the drainage project was to remove water from the streets and front yards. Rain water getting into people's houses is not really a problem (storm surge is a problem, but rain is not).

I'm thinking that the street puddling could be solved by raising the roads slightly and putting in a decent pavement crown, but this would be very expensive. It would also just force the water that would have been in the street into people's yards.

Ultimately, I think that this is a case where the "do nothing" alternative would have been best. When you live at 3-5 feet above sea level, I think it should be expected that there will be puddled water everywhere when and after it rains.

 

[TerryScan]

Folks seem quick to fault the engineer. I believe he probably did the best possible improvements. I live 3 miles from the beach. The beach town near me is quite similar to that which Gumhead has described.

This community was well established 30 years ago, so one cannot raise the whole town. One cannot alter the tailwater either since it is waterways connected to inland bays and the ocean.

The only possible SOLUTIONs to the problems are economically impractical (raise the town...or dike it off and drain by massive pumping). One is left to try to mitigate the problems.

Some of the worst flooding occurs from tidal surges backing into the drainage systems. Check valves (flaps, or tideflex valves) at storm outlets may help. This could allow for some storage in the ditches until tailwater lowers.

I believe part of the reality is that there were always problems in these communities. I believe the changes have been in the expectations of the residents. There are more year-round vs. seasonal residents all the time. They are not accustomed to the flooding that has always occured (due to the upland community they moved from and they only came before in dryer summer months). Perhaps community education is what is needed.

 

[cvg]

If developed as you say without floodproofing, there will always be problems in these communities. As I stated originally the engineer (read "developer&quot constructed these homes in the cheapest possible manner to maximize profit. However, very little effort was put into protection of these homes from flooding. The unsuspecting homeowners bought them, not realizing that they will experience frequent flooding. Since the roads and lots were not graded properly (or maybe they are just located too close to the ocean and shouldn't have been built there at all?) there is very little that can be done after the fact. I completely agree that at this point, there is no economically practical solution to the problem.

I really can't agree with the statement by Terryscan

"I believe he (the engineer) probably did the best possible improvements".

I believe the engineer did what the developer told him to do. It's unfortunate however, that the county or city in charge of issuing the building permits would not require due dilegence in providing necessary flood protection for these areas.

 

[TerryScan]

CVG-

In my reference town, there was no engineer or developer. There was a beach. A group would camp there in the late 1800s. (a church camp to be exact). A church auditorium was built around 1900. At some point in time someone built a cottage... sometime later he got a neighbor...etc.
By 1920, there were some paved roads and a couple dozen homes. I doubt there were building permits required at that time.

But, I suppose you are correct. In 1901, the developer or his engineer did not properly safeguard the community with regards to flooding as well as many other aspects of infrastructure.

 

[Gumhead]

In this particular case, a neighborhood association went to a construction services/engineering firm and asked them to come up with drainage improvements. They did what they were asked, and I believe they designed the improvements to the best of their abilities. And in theory, on paper, it looked ok. However, I believe a critical part of the design process was omitted: benefit vs cost. Is it really worth it? Just because a system is the "best possible" doesn't mean that it is worth the costs (in money, and in intangibles such as aesthetics).

To me this is a perfect example of forgetting the big picture...zooming in close to examine how to fix the problem, and never stepping back to say "does this make sense?"

 

[cvg]

terryscan
you have hit on the precise reason professional engineers and architects are registered in this country. It is to provide the public with a measure of assurance that public (and private) works will be designed and built in a safe manner. It is entirely likely that there was no grading plan at all for the church camp or for the cottages. It's also likely that most of the structures would be considered substandard when compared to todays building codes.

 

[lsp01]

Gumhead,

When I was designing some stormwater improvements in an older neighborhood in south Florida we had a very similar situation. The problem was not really flooding into homes etc, just nuisance water in the yards and roads.

In this instance the area had already installed roadway piping and a tide-flex flange at the outfall. This helped the situation mostly, however if the tide was high enough, then the water would still back up into the streets since there wasn't enough head to open the valve.

The ultimate solution was the installation of a storm water pumping station which pumped stomwater from a wet well into the canal. This will worked well for nuisance water, but I wouldn't recommend a pump station for true flood protection.

I guess it still comes down to is it cost effective? These people thought so!

And just for the record, the original developer and engineer for the neighborhood were VERY reputable and designed according to the standards of the time. But 50 years ago the area around them was undeveloped, so when they constructed the canal the peak storm stage was about 1.5 feet lower then it is today, thanks to the neighboring developers also discharging into the canal.

Lsp01

 

[dicksewerrat]

You could use decorative riprap to hold down the membrane. Then it would be landscaping not a channel for storm water.