Flood plane

[Anonymous]

I have a river with a 100-yr design storm profile (flood plane height) that I would somehow like to translate onto the plan view to show what areas would be under water if the water breached the top of bank in a few locations (which it does). I have tried creating a corridor with a horizontal top that daylights to the existing grade at the elevation of the water surface, but this only shows the perpendicular distance that the water goes out to at that cross section.

Manually, I could try to delineate the area for a particular breach that is at the water surface elevation where the water breaches the top of bank, but I don't want to do it manually. And the flood height varies of the river reach so at each breach location the flood plane water surface elevation is different.

So I want AutoCAD to automatically delineate the flood plane for me. Is that possible? Does anyone have any pointers on how to do that?

[KirkNoonan]

How about creating points at your flood stages, offsetting them from the river's axis and then creating a 'flood plane' surface. Then you could make a volume surface with your existing surface and the flood plane surface. Do an elevation analysis on the resulting volume surface to produce shading of the flooded areas. Might not be perfect, but it wouldn't be too hard.

[Anonymous]

thanks, that is not a bad idea. I will give it a try and see how it works out.

[Anonymous]

Just for the record, it's a flood "plain," not "plane."

[KirkNoonan]

If it is a flat surface - it is a plane. If it depicts flooded areas - flood plane. 😉

[Anonymous]

Hi kirknoonan,

The real trouble is that the flood surface level is never a plane.

In the lower reaches of a river where the flow velocities become very
low it may approximate one.

In the upper reaches with higher flow velocities the departure from
predicting the water level at the boundary of the stream by assuming it
is level with the centre of the stream can be seriously different. I
have observed differences of over 2m by comparing the water surface
levels at the ends of long horizontal bridge over a flood plain with the
levels at the stream centreline

Regards


Laurie Comerford

kirknoonan wrote:
> If it is a flat surface - it is a plane. If it depicts flooded areas -
> flood plane. 😉

[Anonymous]

I think the easiest thing to do is to create the floodplain surface
(flat or not it doesn't matter) that accurately represents the
elevations of the floodplain near the limits of the floodplain.

Create a volume surface that compares the EG from the Floodplain.
Display that volume surface as a Zero contour. That Zero contour is
your floodplain.

If you want, create an elevation analysis that shades floodplain depths
only.

Matthew Anderson, PE



shaneelliss wrote:
> I have a river with a 100-yr design storm profile (flood plane height)
> that I would somehow like to translate onto the plan view to show what
> areas would be under water if the water breached the top of bank in a
> few locations (which it does). I have tried creating a corridor with a
> horizontal top that daylights to the existing grade at the elevation
> of the water surface, but this only shows the perpendicular distance
> that the water goes out to at that cross section. Manually, I could
> try to delineate the area for a particular breach that is at the water
> surface elevation where the water breaches the top of bank, but I
> don't want to do it manually. And the flood height varies of the river
> reach so at each breach location the flood plane water surface
> elevation is different. So I want AutoCAD to automatically delineate
> the flood plane for me. Is that possible? Does anyone have any
> pointers on how to do that?

[Anonymous]

Hi Matt,

Hate to disagree, but a flood plain is nothing but an area with a
reasonable probability of being flooded. Usually such areas a fairly
flat such that the extent of shallow flooding will be quite extensive.
In the flood I mentioned earlier an area of approximately 33,000 acres
was flooded with a maximum depth of about 1.5m downstream of the bridge
I mentioned.

The zero contour of a volume surface will give the boundary of the
flooded area and this is usually of concern to designers, but the volume
surface will only be reliable if the predicted water surface is reliable.

Shane's original query has to be resolved before you can deal with
creating a volume surface.

Aerial photography of existing floods will provide a better starting
point for these predictions than surface models alone

Regards


Laurie Comerford

Matt.Anderson wrote:
> I think the easiest thing to do is to create the floodplain surface
> (flat or not it doesn't matter) that accurately represents the
> elevations of the floodplain near the limits of the floodplain.
>
> Create a volume surface that compares the EG from the Floodplain.
> Display that volume surface as a Zero contour. That Zero contour is
> your floodplain.
>
> If you want, create an elevation analysis that shades floodplain depths
> only.
>
> Matthew Anderson, PE
>
>
>
> shaneelliss wrote:
>> I have a river with a 100-yr design storm profile (flood plane height)
>> that I would somehow like to translate onto the plan view to show what
>> areas would be under water if the water breached the top of bank in a
>> few locations (which it does). I have tried creating a corridor with a
>> horizontal top that daylights to the existing grade at the elevation
>> of the water surface, but this only shows the perpendicular distance
>> that the water goes out to at that cross section. Manually, I could
>> try to delineate the area for a particular breach that is at the water
>> surface elevation where the water breaches the top of bank, but I
>> don't want to do it manually. And the flood height varies of the river
>> reach so at each breach location the flood plane water surface
>> elevation is different. So I want AutoCAD to automatically delineate
>> the flood plane for me. Is that possible? Does anyone have any
>> pointers on how to do that?

[Anonymous]

At the risk of beating this horse to death, it was traditionally referred to as a flood "plain." I Googled it after sending my first message, and read that the word "plane" is being used more and more for this term and is now considered good form. So, I stand corrected. I'm old school, I guess.

[Anonymous]

Laurie -

As I typically and routine perform flooding analysis here in the States
- the limits of the floodplain is what is regulated and is part of the
National Flood Insurance Program in the U.S.

For example, using an alignment and cross-sections, we can export that
information out to HEC-RAS to create a surface water profile for the
100-year event. Bringing that SDF information back into Civil 3d to
create the 100-year water surface would make delineating the SFHA simple.

Until a simple tool becomes available to round trip this information
(I've used many different tools on Land etc), I've also done this manually.

What gets a little complicated is the designing and calculating the
regulatory 0-10 and 10-100 year floodplain cut and fill easily. Civil
3d does make this task simple.

Matthew Anderson, PE



Laurie Comerford wrote:
> Hi Matt,
>
> Hate to disagree, but a flood plain is nothing but an area with a
> reasonable probability of being flooded. Usually such areas a fairly
> flat such that the extent of shallow flooding will be quite extensive.
> In the flood I mentioned earlier an area of approximately 33,000 acres
> was flooded with a maximum depth of about 1.5m downstream of the bridge
> I mentioned.
>
> The zero contour of a volume surface will give the boundary of the
> flooded area and this is usually of concern to designers, but the volume
> surface will only be reliable if the predicted water surface is reliable.
>
> Shane's original query has to be resolved before you can deal with
> creating a volume surface.
>
> Aerial photography of existing floods will provide a better starting
> point for these predictions than surface models alone
>
> Regards
>
>
> Laurie Comerford
>
> Matt.Anderson wrote:
>
>> I think the easiest thing to do is to create the floodplain surface
>> (flat or not it doesn't matter) that accurately represents the
>> elevations of the floodplain near the limits of the floodplain.
>>
>> Create a volume surface that compares the EG from the Floodplain.
>> Display that volume surface as a Zero contour. That Zero contour is
>> your floodplain.
>>
>> If you want, create an elevation analysis that shades floodplain depths
>> only.
>>
>> Matthew Anderson, PE
>>
>>
>>
>> shaneelliss wrote:
>>
>>> I have a river with a 100-yr design storm profile (flood plane height)
>>> that I would somehow like to translate onto the plan view to show what
>>> areas would be under water if the water breached the top of bank in a
>>> few locations (which it does). I have tried creating a corridor with a
>>> horizontal top that daylights to the existing grade at the elevation
>>> of the water surface, but this only shows the perpendicular distance
>>> that the water goes out to at that cross section. Manually, I could
>>> try to delineate the area for a particular breach that is at the water
>>> surface elevation where the water breaches the top of bank, but I
>>> don't want to do it manually. And the flood height varies of the river
>>> reach so at each breach location the flood plane water surface
>>> elevation is different. So I want AutoCAD to automatically delineate
>>> the flood plane for me. Is that possible? Does anyone have any
>>> pointers on how to do that?
>>>

[Anonymous]

Thanks for the suggestion Matt. I may not be doing this in the most efficient manner, but here is what I have done. I imported the water surface profile from HEC-RAS for the 100-yr storm, converted that to a profile on my river centerline profile view, created a corridor with horizontal daylights to the existing grade, created a corridor water surface from that corridor, created a feature line from the river centerline alignment and draped it to the new water surface elevations, then offset the feature line a couple thousand feet on either side of the river using a zero elevation offset, created a flood water elevation surface using those feature lines as breaklines, then created a volume surface using the flood water elevation surface as the base and the existing grade surface as the comparison surface (I don't know if this should be the other way around). I have never used a volume surface before so I am not real experienced in that, but it seems to have created something anyway.

My question now though is, how do I display the volume surface as a "Zero contour"? I am not sure what that is. Would you mind explaining that a little?

Thanks.

[Matt.Anderson]

A volume surface is just a comparison surface between the base and comparison surface. That is, for each TIN triangle, or GRID coordinate, the surface will create a triangle corner with the mathematical subtraction. Its a still a surface.



With that being said, if you use your standard Existing Ground Surface Style, you will show contours ranging from the depth of flooding (EG less BFE), including the Zero Contour line. This is the line where the water meets the ground. (EG less BFE = 0)



I assume you are using a surface style that displayed your 1' and 5' contours. Create a new surface style and call it Base Flood Elevation. Turn off High and Low. Turn on User-Defined Contour. See yesterday's post at civil3d.com about User-Defined Contours.



Use the Surface Analysis to create a user-defined contour at elevation 0.



There are probable a few alternative ways to skins this cat.



There are a tons of different possibilietes to shade the floodplain, or do earthwork calcualtions...lots of possibilities...maybe I will write a 1/2 day or day long webinar if interest is there...



Matthew Anderson, PE

anderson@jaseng.com

[Anonymous]

Awesome. That shows it alright. It would be nice to know how to hatch those flooded areas. I will have to play around with it. Thanks for the help.

[mjfarrell]

you might experiment with creating boundaries, and using the boundary styles to do the hatching.

or you might even get away with one of the surface anlysis functions and the right hatch, line, color settings....

[Anonymous]

So I created a volume surface comparing my flood water surface elevation and my existing ground elevation and created some user-contours at an elevation of 0. It created the contours but I thought it looked like there were some contour lines missing. So I did an elevation analysis with everything above zero feet being brown, and everything below zero feet being blue. I have attached the screen shot of what I got. As you can see, the zero elevation contours (the white lines) are missing in some spots. Is there something I am doing wrong? If I change the user-contour to an elavation of say 0.001' it shows the zero elevation contour where it is missing now, but doesn't show it where it shows now.

[Anonymous]

Contours are calculated only when they fall along a line of
interpolation between points. In your case, when you had a surface with
0 elevation, some of the lines were lost - I'm surprised you got any
. Change your user contours (for water elevation) to -.0000001 and
give it a crank. You will still be 'off' a tad but not much.

Karl

shaneelliss wrote:
> So I created a volume surface comparing my flood water surface elevation
> and my existing ground elevation and created some user-contours at an
> elevation of 0. It created the contours but I thought it looked like
> there were some contour lines missing. So I did an elevation analysis
> with everything above zero feet being brown, and everything below zero
> feet being blue. I have attached the screen shot of what I got. As you
> can see, the zero elevation contours (the white lines) are missing in
> some spots. Is there something I am doing wrong? If I change the
> user-contour to an elavation of say 0.001' it shows the zero elevation
> contour where it is missing now, but doesn't show it where it shows now.
> ------------------------------------------------------------------------
>

[Anonymous]

This is actually very easy to do. Create a proposed profile showing the floodplain in relation to a centerline alignment down the middle of your stream/river/etc. Use a generic link subassembly with a simple corridor to target the existing surface at a very low slope (.05%....). Extract the feature line from the corridor where you link assembly hits the surface. Voila, you have your floodplain.

Alan
http://civil3d.streamrestoration.info

[Anonymous]

Alan,

My problem isn't in getting the line where the water first hits the bank, as the bank is well above the water level in most places. The problem is that in my case the bank is a levee and the natural elevation of the land beyond the levee is lower than the flood water elevation for some ways. In in a few places there is a low spot in the levee where water can "escape" the river. I want to delineate where the water will go when it gets out in those few locations.

I have done the things suggested above, and have a volume surface that shows me what area is below or above the flood water height, but I still have not been able to figure out how to cross hatch those areas. I can only get it to do a solid fill using an elevation analysis.

Any recommendations on how to get a cross hatch in the low lying areas?

Shane

[Anonymous]

Shane,

Sorry for not reading your question more thoroughly. I would do the same thing I suggested for your problem by creating an alignment on the opposite side of the berms as the stream/river. Then target the floodplain profile on both the left and right sides of the alignment with a generic subassembly link that targets the floodplain profile on existing ground. You will see the contour that your water surface elevation is escaping by doing this as long as your tinn is large enough to account for the areas that water is "escaping". Extract the feature lines, convert them to 2d polylines, then hatch the area. Hope I understood you this time and was able to help.

[O.Maille]

You could create a seperate surface for levee and compare to you flood area, it would give you solid fill for the low lying areas along the bank independent of the overall flood surface style.
If you are analysing the volume of flooding (this can be done by adding volume column to legend table) be careful if you edit the ranges as there is a bug whereby if ranges overlap volume calculations give incorrect answers. You can edit ranges to eliminate certain elevations from you surface plotting. For example if you remove all the negative ranges the 2d solid fill will not hatch over land above your flood level.