No-Rise Certification Question

[Builder604]

I have a client who is planning to add on to his dock. The property is located in Northampton County, NC. The addition will include 4 – 4x4 posts to be in constructed in the lake. The county is requiring that our client get a No-Rise Certification before they issue a building permit. Are there any guidelines/methods for correctly modeling the effect of the dock posts in the lake and floodway? Any guidance would be appreciated.

 

[gbam]

Is the lake within a floodway or floodplain?

 

[Builder604]

The reservoir is dam controlled and the dock location is in the floodway.

 

[beej67]

Can you post a FIRMette or tell us where this place is so we can look one up? I'm having a hard time visualizing a lake in a floodway.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[Builder604]

The firmette is attached. Thanks.

 

[Builder604]

If there is any problem opening the firmette let me know.

 

[Ryb01]

Just thinking out loud but trying to calculate the rise in water elevation for instaling 4- 4x4 posts...in a lake? Are they serious? It's like a drop in a bucket.

I don't have much exertise in this area, however just trying to rationalize the request. I look forward to see other posts from the forum.

 

[beej67]

Yeah, I might be going out on a limb here, but to heck with it.

The only way a bureaucrat can lose his job is if he actually makes a judgment call. That's just the nature of the regulatory environment. Following the rules ad-absurdum not only ensures you don't lose your job, it positions you well for promotion. I suspect you have run into one of these people. They're quite common, in truth.

So the natural thing to do is to rant and rave about how stupid their request is, but the better thing to do is try to see the issue from their point of view. They're a bureaucrat looking after their job security, so they want you to provide them some sort of documentation they can point to that shows they followed the "no rise rules" in your design review.

So here's what I would do. First I'd try to get a hold of the hydrology study for the lake, presuming there was one. That study probably had a stage storage table in it somewhere, just subtract the volume of the posts from the lake storage, show the reviewer that the stage storage didn't change due to roundoff error, and submit a 'compliance study' with them that acts as an amendment to the approved hydro study on file. If such a study doesn't exist, then compensate for the volume of the posts by digging a few wheelbarrows full of dirt out of the shore somewhere to compensate.

If the reviewer wants HEC-RAS for this, then make a painstakingly detailed effort to explain to your client how unreasonable the reviewer is being and ask him what approach he'd like to take. If it gets that far, then it might not be an engineering issue at all. The whole thing could be political, and may need to involve his boss.

The two most likely scenarios I see are either A) you ran unto Ultimate Bureaucrat and just need to work it out with him somehow, or B) the owner pissed off local government and the issue isn't the dock at all, it's something else.

This sort of thing is what makes engineering fun, right?



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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[Builder604]

Is the rise in water elevation due to the additional volume or from restricted flow due to the obstruction. If it is merely a volume issue then just removing the volume of the posts from the bank would be the simplest fix. If it is from having a restriction in flow then it would require some analysis to prove that the restriction in flow doesn't cause the water elevation to rise. In our case would be negligible.

 

[beej67]

In theory, rise in elevation could be from either decrease in lake storage or increase in flow obstruction. In practice, there will be no rise because your dock is a drop in the bucket. Your reviewer is being nitpicky, but apparently needs something on file saying there's no rise due to your dock.

Talk to the reviewer, try to handle this with a conformance letter, or an addendum to whatever hydrology study was performed for the lake, based on volume. Avoid HEC-RAS if possible.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[SMIAH]

Additionnal volume from the posts vs estimated actual lake volume.

In %.

That could be a killer.

 

[Builder604]

The contact at FEMA said that the rise in water elevation needed to be evaluated from a voloume and restriction of flow standpoint. FYI

 

[beej67]

Does the 100 year flood top the dam?

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Builder604]

The normal pool is 132' and 100 yr flood is 133' and the top of the dam is 142. They have the right to flood the lake up to 133'

 

[beej67]

Then there is no "restriction" because there is no flow to restrict. Say that explicitly in your conformance letter.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[SMIAH]

How can someone at the FEMA says that it has to be evaluated from a restriction of flow standpoint?!

4 4x4 posts in a reservoir...

Is there something else we should konow?

 

[beej67]

I'm having a hard time believing this is classified as floodway instead of floodplain.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Builder604]

The planner at the County is requiring the no-rise cert. The southern part of Roanoke Rapids lake in in Halifax county and they don't require anything but the northern area is in Northampton county. We have done a no-rise cert for a small dock in creek several years ago where we actually surveyed cross sections of the creek; however, it isn't very practical to do that on the reservoir. We are planning getting data from FEMA and using HEC-Ras, for what its worth on this small project, to prove the elevation rise is negligible. I hate to have to go though using HEC-Ras but the planner wants to see some technical data.
I do have a question concerning HEC-Ras. I have little experience with the software but upon looking at it the information for the dock would be input into the pier tab on the bridge scour window, correct?

 

[beej67]

Is the lake even in the RAS model at all? There's no flow in the lake.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[SMIAH]

Using HEC RAS for this problem is an overkill.

In the Geometric Data/bridge Culvert Data, you add the dock as a bridge deck/roadway between 2 cross sections and then you add piers.