seawalls and required design storm

[asabender]

I'm a civil engineer working on a project in Massachusetts.

Do seawalls need to be designed to meet the loading/overtopping/scouring conditions of a 100 year storm? Most certainly have not been (by inspection), and I have never before been asked to check the recurrence at which a wall will be overtopped. The wall is not relied upon by any adjacent structures.

IBC seems to say that the locally codified FIRMs govern for "other structures", but when I asked a state code official, I was told that the IBC does not apply. The state official also told me that, if the project doesn't need a permit, it doesn't need to meet code (which was an interesting point of view from a code official). ASCE 7 and 24 seem fairly tight lipped on the matter. None of the codes seem to say "this code does or does not apply to retaining/seawalls." Most seem to be restricting the code to avoid walls (FEMA not only says it won't touch them, but that they can't be relied upon).

My gut reaction is that a seawall is only there to subdue scour, and should be designed to a point determined by the owner (aka, they want 50y: 50y is the design; if they want 500y: sure, why not?). If the wall were supporting a structure directly, that, obviously, would be a different scenario. Most walls I've seen protect structures which don't meet 100y design themselves.

Thanks in advance.

 

[SMIAH]

No clue about Registration.
But there has to be something about waves. I'd treat this as a breakwater design.

 

[cvg]

Your project is likely within Zone V, described as follows:

Zone V: Areas along coasts subject to inundation by the 1-percent-annual-chance flood event with additional hazards associated with storm-induced waves. Because detailed hydraulic analyses have not been performed, no BFEs or flood depths are shown. Mandatory flood insurance purchase requirements apply.

It is unlikely that your wall will be able to control flooding caused by waves. There will be some overtopping. I would say it should be able to withstand the 1% flood but not necessarily high enough to prevent overtopping. You probably need to get a permit from the local floodplain management agency to construct within the floodplain, so they can give you specific requirements. Around here that would be either the county or city level.

 

[chicopee]

Trust me in Massachusetts you are going to need a permit for about any project that affects public safety, air/water/soil quality; Massachusetts is stifling itself with regulations and permits so your project will not be an exception. If one official tells you no "problem " get another official's opinion before embarking on any project in Massachusetts that's the reason the industrial sector is leaving this State. It's a shame because Massachusetts was at the forefront of the American industrial revolution and now it is a state devoid of such activity.

 

[asabender]

Thanks for the responses.

I am in a VE zone. I am familiar with V-zone residential/commercial design. I have checked the city's FIRMs and know that I will be overtopped by a 51 year storm or so (the FIRM places a 50 year storm BFE within a fraction of an inch of the top of wall). I am aware that the wall will fail very quickly once it starts getting regularly overtopped. I have protected the toe from scour per the ACoE design methodology.

CRMC in RI does not require design for VE100, but DEP in MA is unclear about their stance. DEP has reviewed this wall and had questions, but did not indicate that they thought it should be designed to VE100. It was the state code official that gave the comments I provided above. I would not rely on the letter from DEP as some sort of final word on the matter, as it demonstrated a dangerous lack of understanding regarding stillwater depth vs flood elevation in derivation of wave height.

The real question is: Are there any codes which are enforced that actually dictate what my design storm should be on a seawall or retaining wall under 4 feet? The wall is not a hazard to human life in the event of failure. The closest I've seen to anything addressing design recurrence requirements for low hazard structures is that FEMA states in table 8-1 of FEMA-55 that structures which present a low hazard are to be assigned a coefficient based on a .5 (I wonder if they meant .05!) recurrence, while residential structures are to be based on .01 recurrence.

Admittedly, I find this whole situation insultingly asinine, in strict accordance with chicopee's post above; but here I sit. The wall is actually behind an old pile of small stones acting as a seawall, which everyone seems loath to mention. I find the idea of designing this wall for a VE100 about as logical as designing a hedge for a VE100.

 

[cvg]

you are looking for a statute or an ordinance, not a code or a design guide. again, you need to determine what ordinance's apply to you and those will be set and enforced at the local level, not by FEMA. not sure why you are concerned with requirements in rhode island. and it sounds like the state does not have jurisdiction either. I would have started with city hall and then perhaps to the county if not within city limits.

 

[asabender]

Cvg,

The project is in Fall River on Mount Hope Bay, which is mostly CRMC controlled. I know that they have no power in Fall River, but was using them for comparative purposes as a group that takes a definite stance on the matter.

The state has jurisdiction through DEP due to it being on the water.

Thank you for the suggestion regarding Fall River ordinance. I will follow up with the city.

 

[eea]

Any waters of the United States, also falls under the US Corps of Engineers. I imagine your projects meets a 404 Nation Wide Permit category, but you should review there also.

 

[asabender]

@eea:

NWP 13 comes close, but it's too small to apply. We're pretty far inland. Only the 50 year storm and up hits the wall with breakwater.

Great link though, thanks!