What is the correct way?

I'm kind of guessing here, but it looks the elevations changes as you go up and down the creek. It seems like the thing to do would be to prorate those listed elevations across your property. At least thats what I would do.

How much difference in elev.?
And, what datum is the rest of things on?
(could be NAD 29 vs NAD 83)

N

My first thought was to prorate as well.
I'm on the city's control but it is NAD 83.
The elevations change from 542 to 538.

I assume you are talking about the changes at each cross section on the FIRM map. i have always prorated between the cross sections and referenced the FIRM panel as my basis. I can't imagine that they would expect anything different from a Surveyor.

Have you looked at the FIS map (study) for this area, sometimes there is additional information - or clarifies elevations.

There is no such thing as NAD29 and NAD83 is a horizontal datum with an associated ellipsoid height. The current national vertical datum is NAVD88 and its predecessor was NGVD29, known as the Sea Level Datum of 1929 until 1973.

I would look at the FIS and base it on that.

arctan(x), post: 382661, member: 6795 wrote: I have been instructed to dedicate a utility/floodway easement to the City of Garland on a tract of land that I am platting. I've been instructed to give a 1-foot vertical freeboard from the flood elevation and then offset that contour to 10 horizontal feet.
My problem is that my subject tract has 2 different elevations along the creek according to the FIRM panel.
How would be best to proceed? I've done a topo of the creek and subject tract.
I've attached an image to assist with the visual.

I think that the correct answer for Texas is that the determination of floodwater surfaces is not within the practice of land surveying, but is engineering hydrology. If the elevation of the water surface of the 100-year flood is to control the easement location, then you will need to generate cross sections of the creek and have a qualified engineer model the flow sufficiently well to generate floodwater elevations at each cross section which you can then offset as required to define the easement.

The alternative would be to use the upstream floodwater surface as the basis of the easement location, which would be quite a conservative approach (but may not be feasible, depending upon terrain).

The flood elevation has been determined by FEMA, use the FIS as mentioned. You'll have to determine where the nearest cross section is to your property then scale, you can show the flood zone to the nearest 10th, the zone line will cross contour elevations and be in NAVD88

I prorate between the two different elevations shown on the firm.....that's what I have to do for elevation certs anyway....

Follow the FIS profile along the creek.

arctan(x), post: 382661, member: 6795 wrote: What is the correct way?

Surely you know that "THE correct way" is an oxymoron. Opinions will vary on whether there is any correct way or multiple correct ways, but rarely exactly one way.

Glad someone straightened out the NAD/NGVD/NAVD thing. Even when we know what was intended, that's a more serious error than just riling up the spelling police.

A flood study would require a detailed topo (1ft contour) of the entire watershed area that water would come from to get there.

That info would be plugged into hydro program that would analysis by rate, time, and yearly averages to determine 100, 500 and probable flood elevation levels.

A quick analysis can be done with a USGS Topo, planimeter to determine area and vertical averages plugged into an engineering equation to determine rise at distance downstream from origin.

It is all an assumption and none of it is absolute.

I've done it before while planning lake subdivisions and I am no longer in possession of that particular engineering text.

A few years ago I did a relative topo of an 1800 ft dam in Nad27 that the State determined a flood elevation of a 250 acre lake.

I am still waiting for my copy of the results, which will probably never arrive.

The HOA of the lake subdivision ask for it and nobody is talking...........lol

As I understand it, The FIS rules - MOST of the time.

First, does the 538 elevation have freeboard under the bridge? If not the stream profile should indicate pooling.

Second you have more than 2 elevations to deal with between 538 and 542. At a minimum 1 footers and possibly smaller intervals.

How many cross sections have you done along your parcel and how far off the site did you go?

I note that the cross sections need to go to an elevation above your freeboard requirement.

If the scaled floodway width is less than your measured width at the required elevation you are OK. If the scaled floodway width is greater than your surveyed width to elevation then you need to raise your elevation to attain that width. If you want to argue that point hire a hydrographic engineer and do a full analysis. If your inside cross section lines cross, analyze rationally. If there is a great angle between the outside curve cross sections you will need to add rational breakpoints. Possibly 0.5' intervals on the outside curve only.

Consult with your client what his intentions are with the Southwest corner. He may want to consider placing the entirety of it within the easement. You should prepare to respond with pros and cons.

Paul in PA, PE, PLS

arctan(x), post: 382661, member: 6795 wrote: I have been instructed to dedicate a utility/floodway easement to the City of Garland on a tract of land that I am platting. I've been instructed to give a 1-foot vertical freeboard from the flood elevation and then offset that contour to 10 horizontal feet.

As I read between the lines of the easement configuration requirements, it will probably be used in connection with platting with a statement to the effect that the 100-year floodplain is contained within the limits of the easement that is to be created. If this is so, fast forward to the next major flood when structures are in place that get flooded under any of a number of possible scenarios. The object of any Texas surveyor who will have his or her name on the plat would rationally be to be as much of a bystander as possible. That means (a) don't practice outside the limits of one's license and (b) don't make any assumptions whatsoever that cannot be absolutely, positively defended in the future when things go wrong.

Life is too short not to have a civil engineer with a substantial E&O policy right at the front of the line. Life is too short to be a land surveyor having to explain why so-and-so said that it would be okay to just whip something in that turned out to result in significant losses to innocent homeowners who, not unreasonably, want checks written to cover them.

So, unless a surveyor figures out how to make sure that none of the future property owners will ever be damaged by flooding (and I want to know how much that crystal ball cost), it is a guaranteed losing proposition to be standing anywhere near that one when it all goes South unless you have a simple and solid story to account for your role in things.

I would not be surprised to find that for the city to accept the plat that the flood determination has to be signed off on by an engineer. My local wants an engineer to sign almost anything that has to do with possible changing of flow of floodwaters, or adding things to the watershed that affects how much water goes into said watershed.

Update...
There was already a flood insurance study completed for the stream. I used the published stream profile plus 1 foot vertical freeboard to create a surface and then used my topo as a second surface and then used Carlson to plot where the two surfaces intersect.
I called a couple of engineers who didn't think that they needed to get involved with the project. My client really didn't want to spend the money on an engineer either. So it seems to have all worked out. Thank you all for your input!

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I am with the crowd pointing you to the FIS. One more tip.
The BASE Flood Elevation from the flood study may or may not relate to flood risk. It is a regulatory line. Anyone can read the FEMA bulletins and generate the line requested. Regardless of opinion it is written in a federal rule how to do it. The key part in this is to clearly state what the line is and is not.
Good luck, Tom