FEMA Benchmark

I wouldn't think they would use that bolt, every time it's opened the elevation would change.

The first PLS that I worked for in the 80's used a bonnet bolt "between city and state", I use the top bolt.

With all the horror stories we hear about fire hydrant stability/replacements, it might turn out that neither checks well.

If I were to ever use a fire hydrant bolt I would use one of the flange bolts at the base...

:good:

Old school = flange bolt between Mueller and Chattanooga.

It's always been a bonnet bolt, either with a tag ("tag bolt") or between letters like tom said. Not the top nut that controls the flow, it goes up an down, and I've never known the bottom flange bolts used, the hydrant gets in the way and I'm not sure how it would be identified.

You can usually tell if the hydrant has been rehabilitated since the description was written, but you always check another bench anyway, right?

As an old guy I agree with Tom. I called them cap bolts, maybe NE cap bolt between the R in Mueller and the C in Chattanooga.

I hate Hydrants!
car hits the hydrant, the vertical extension off the water main breaks. city crew unbolts and replaces the extension and the same hydrant is back... but the extension was not the same length.
everything looks the same as the day before.

Always use more than one benchmark.

I'm pretty sure Deral wouldn't mind; I copied and pasted this from the old RPLS site. I added the paragraph breaks and the bold reference. Deral had some good information concerningthe stability of hydrants.

Ken

Fire Hydrants as BM's.
Posted by Deral at Home on Mar 13, 2010 10:55 am

I, like many others, probably have used a hydrant as a BM on many projects over the years. I failed to realize some very basic engineering functions of waterlines under pressure. I hope this post helps others to understand some of the dynamics of these BM's.

There are two basic designs of hydrants. Dry barrels and wet barrels. A wet barrel is active all the time as water is inside the barrel all the time and pressured up. The dry barrel is not active until the operating nut is turned and the hydrant is flooded. Wet barrel are sometimes used in the warmer climates while wet barrels are used in climates that do have winter freezes.This action in itself, of operating the hydrant creates something called a water hammer. If you turn the nut to fast, either in opening or closing, you can bust lines for blocks because of the water hammer. Even slowly, you still have a minor impact which can, and will, shift the height of the hydrant.

The next time you see them install a waterline then take a look in the ditch. Each line is bedded in a mixture of soil and rock that will allow it to move slightly. If it does not move at all, and is too rigid, then the line will break when even a slight water hammer hits the line. And examine a hydrant being installed. It is also bedded and sometimes blocked by concrete to help in reducing the amount of buckle (movement) that it can experience if it is near an angle point.But they all can have some movement, and it tends to be vertically. And even without the dynamics of a water hammer from slamming a nut open or closed then you have the constant battle within the system of maintaining an equal balance. Water lines are systems and in general, a line can be feed from multiple directions. Otherwise, when you shut down a line for a break then you would kill the entire line downstream. Add to this the availability of high and low zones, and other parts of the operating system then it is constantly undergoing micro and sometimes macro changes which impact the lines and the hydrants.

Contrary to what someone said, the five sided 'nut' on top is called the operating nut. It does not move up nor down but is a odd sided (to prevent the casual user from operating it) nut on top of the stem that operates the valve below. It merely is the turning mechanism.

And from experience on many road project topo's then I've seen a hydrant move over a tenth over night. But we always use multiple marks when setting up each day to have some checks. And these are checks on our own personal points from the day before. Not something on a set of plans done six months ago.

Additional information-Most towns are required to operate their hydrants once per year and this in Lawton is done by the Fire Department during their annual flow test procedures.Sometimes a city will replace a line but reuse the hydrants. There are never set back in the same place but the dates and brand could still match an old set of plans. Just a word to the wise.The fire department uses a long wrench to remove the access covers for the steamer cover and the hose nozzle covers. Often they list in their reports that the hydrant has sunk so that the wrench cannot be spun freely before hitting the dirt. The water department will come and raise the hydrant by adding a section below the hydrant.Hydrants are changed and replaced regularly around Lawton. At least with our database we can tell if the date and manufacture are different than what was shown. Many stakeout plans on show a symbol with the label FH. Is is the same one as when the survey was done?

All in all, we quit using them as BM's for any work in Lawton. I even worried when using them as a temporary turning point. What if it was undergoing a change at that time? Maybe paranoid but after seeing the actual changes in a short period of time then it made me very skeptical about using them for any vertical control.

Your mileage may vary but this has been my experience and documented in field books.Deral

and I always thought the operating nut went up and down..

I know of a field crew around this area that uses hydrants as traverse points. They will actually occupy the top spindle with the instrument and call out the hydrant as a trav point/benchmark on their plans. I guess to kill two birds with one stone. I've never seen or heard of anyone else doing this. Seems crazy to me.

If I use a hydrant as a temporary benchmark, I'll use the text, instead of a bolt. The o in "open" for example. Then there is no question as to which bolt.

-V

be careful with fema benchmarks.

i tied into 2 different fema benchmarks and they did't even check within themselves.

tied into a state D.O.T. benchmark and neither fema benchmark checked with it.

> be careful with fema benchmarks.
>
> i tied into 2 different fema benchmarks and they did't even check within themselves.
>
> tied into a state D.O.T. benchmark and neither fema benchmark checked with it.

This has been my experience too. I do not trust any of them, unless I've tied to another independent BM.

Whether they change or shift or whatever, FEMA recommends using their BMs; Preferably the closest to your project area. Who am I to worry about FEMA's BMs? List the BM you used and let FEMA worry about the accuracy of the BMs. You as a surveyor are building their network of BFEs on the backs or your client's wallets. When you play with FEMA, you play by FEMA's rules.

> Whether they change or shift or whatever, FEMA recommends using their BMs; Preferably the closest to your project area. Who am I to worry about FEMA's BMs? List the BM you used and let FEMA worry about the accuracy of the BMs. You as a surveyor are building their network of BFEs on the backs or your client's wallets. When you play with FEMA, you play by FEMA's rules.

You had better worry about them. Are you aware that some of them have been published incorrectly on the older panels, either fat-fingered by the original surveyor or by a typo when printed? I've seen 1' and 2' flat typos. Some of them are completely out in left field. So you're going to blindly accept those and sign/seal a FEMA Elev. Cert. knowing that?

I always check between two or more if I don't know the pedigree of one that I want to use, including a check into an NGS monument. That being said, the current panels reference NGS CS (Control Stations) monuments, which is due to the RMs being so inconsistent. Those NGS CS monuments are mostly old USGS monuments (some newer NGS stations too) which served as the basis for the RMs.

I did a survey last month that I had to reference the FIRM map. The BM's listed on the FIRM were 40+ year old NGS monuments that were long gone... and the FIRM map was revised 2-3 years ago....

Def use more than 1 BM... More than 2 if possible....

Few years ago, I was running a loop and one of my FIRM BM's was a "brass disk in SE bridge abutment". Well, we found the brass disk, and shot it. When we tied to another BM, we missed by 6+ feet. After some investigating, I realized the bridge had been replaced after the FIRM was issued. The newer bridge was 6' higher. However, someone had replaced the brass disk in the called for location......

FEMA Benchmark> Well Now

After I reduced my gps static data from the day before I was able to determine that is actually the top of the bolt on the main discharge valve.

After reducing the level notes, and having run through both points, then reducing my OPUS selections, and then converting the FEMA BM to '88' (Yes, we are still on the '29' NGVD datum for FEMA) I found only 0.11' difference. And that's about the error we are finding here on the Cape.

Thadd has done some comparisons on GPS derived elevations (long term occupation) and CorpsCon conversion and has found about a tenth error in them.

Since the FEMA BM's are just being updated by conversion and not by field run levels I'm holding my OPUS reductions. It's much easier for me to provide back up material using the OPUS elevations rather than the converted FEMA BM's....

But yes, the message here is VERIFY VERIFY VERIFY Or as one president said "Trust but verify!"

I use the city monumentation which FEMA also uses for their BMs in my area. I live and work in Flood City. Pretty much everything is in some sort of flood hazard zone. These MBs have been around for a while. When you fill out the certificate, you specify what panel and date of panel you used. Now that FEMA is online with DFIRMs, I can double check the BM elevation against the published elevation of the city monument. If that turns out to be different, I use the most recent elevation published by FEMA. The real problem with all this, at least in the big J, is that contractors hired to install handicap ramps in the sidewalk disregard the note on the monument: DO NOT DISTURB. They dig it out and it's never to be seen again. We are losing our monuments at an alarming rate. The city has been informed and has done nothing to stop it.

We could what-if ourselves into a frenzy. What if, after I submit an elevation certificate, they change the elevation of the bench?

And don't think for a minute that I don't worry. Even when the job is good, I still worry. Unfortunately, I don't get paid enough to worry as much as I do.