Shallow Concentrated Flow to Open Channel 2

[civlgrrl]

I feel really silly asking this but I have a municipality who requires me to designate flood plain on a ditch. I have explained that there is no floodplain on site, but they say that since there is a ditch that a floodplain exists (we're talking 100-yr).

So, looking at their ordinances, they go into the standard sheet flow (which is defined NTE 300 ft), shallow concentrated and open channel.

But, help me out here, when would you consider defining the difference between shallow concentrated flow and open channel? I have always considered small basin ditches to be shallow concentrated and moved on. This municipality wants me to run a HEC-RAS study on the site.

Thanks in advanced.
Shelly Hattan
CivilGrrl Engineering, Inc.

 

[francesca]

It shouldn't be too hard to take your 100-year flow and calculate a flow depth.

 

[RWF7437]

First, if you are going to do a HEC-RAS study and want your results to be acceptable to FEMA you'll need to use flows that are consistent with previous studies on the stream or ditch you are studying. On most FEMA Flood studies these are often flows derived from Regional Regression Equations but you need to find out by looking at previous studies.

If you find this to be true, then you probably don't need to know the shallow concentrated flow distance at all.

If you find that flows used on previous studies are based on some other method then you need to use that method also.

Even if your study will NOT be reviewed by FEMA, using FEMA's methodology should be acceptable to your local agency. If it is not then they should tell you what is acceptable. Their direction must be clear and complete. When a local agency requires some other method they accept some responsibilty for the results; but of course you'll have a heckofatime suing them.

As for your original question, once you know the method for calculating the flow (anything BUT the Rational Method !) you can make your best guess as to what the shallow concentrated flow is. BUT THEN, estimate how sensitive the method is to changes in your assumed distance. To be conservative, use the shortest reasonable time of concentration you have calculated. My guess is that most methods will not be very sensitive to changes in the shallow concentrated part of the total flow path.

Besides, your rainfall data will probably be "accurate" only within a range of 30% to 50% and the "flood plain" for a ditch is not a regulatory flood plain under the National Flood Insurance Program.

And besides, the so called 100 year flow is NOT magic and has nothing to do with 100 years.

good luck

 

[civlgrrl]

It's not going to be a FEMA floodplain. The drainage area is far too small (<15 acres).

The ordinance is not clear AT ALL. When I ask for more clarification, they said it's when sheet flow turns to concentrated flow.

The municipal ordinance says I can't disturb the area within 550 feet of a floodplain. BUT, staff says a floodplain is not limited to JUST FEMA floodplain.

I think I'm gonna punt on this and figure out a Q based on Rational Method (I'm not going to pull out a hydrograph for <15 acres) and use that to determine depth of the ditch all the way up. It's the gotta get this thing out the door method. The depression of the ditch is no greater than 3 feet deep and it's fairly MAYBE 10 ft wide at the widest point.

I just didn't realize there wasn't a real specific point that defines the when a Shallow Concentrated Flow turns into and Open Channel. I even pulled out Chow and couldn't find anything in there.

Thanks for the help.

 

[RWF7437]

Can't disturb an area within 550 feet of a flood plain ? Never heard of this. I wonder if people in New Orleans or New York, or Los Angeles have or....never mind. Did you mean 50 feet ?

Try this link:

http://www.hydrocad.net/tc.htm

Pulling out a hydrograph can't be that tough, can it? Not much tougher than pulling out Chow, I imagine.

good luck with agencies like yours

 

[civlgrrl]

NO. I mean 550 from the floodplain. It's for a gas well pad.

I'm not a big fan of this part of the ordinance and tried to get it repealed when it underwent revision. One Council member was pretty unreasonable when it came to backing off this requirement.

And there is a reason why I'm going to use RM instead of a hydrograph (which I'm very qualified to do, BTW). It's because the kiddo reviewing the submittal won't understand what I'm submitting. I have a higher likelihood of the submittal getting accepted.

You still didn't answer my question, though. When does shallow concentrated turn into open channel?

 

[RWF7437]

"NO. I mean 550 from the floodplain. It's for a gas well pad. "

Thanks for this information. It appears the requirement has more to do with preventing contamination of the creek or ditch than it has to do with hydrology.

"And there is a reason why I'm going to use RM instead of a hydrograph (which I'm very qualified to do, BTW). It's because the kiddo reviewing the submittal won't understand what I'm submitting. I have a higher likelihood of the submittal getting accepted. "

There are several possible answers to this. You could try to educate the "kiddo" and introduce him/her to the wonders of hydrology. But, for a 15 acre drainage basin this may not be the best example. Still, following FEMA methods should be acceptable to any local agency, regardless of how many Kiddos they employ. Doesn't the Kiddo have a Boss ?

"You still didn't answer my question, though. When does shallow concentrated turn into open channel?"

The link provided should answer your question as completely as it is possible to answer it. A short answer is; there are cases where there is no "shallow concentrated flow" and other cases where there may be considerable distances characterized by rivulets on the surface over which the runoff travels.

A very simple, and I admit I'm simpleminded, method I often use is:

1. Trace the basin boundary on a USGS Quadrangle map

2. Measure from the boundary a distance (<300'). This is the overland flow distance and is usually much lees than 300 feet. From that point measure to the first discernable stream. Such streams are usually shown as blue lines or by sharply "v-ed" contours on the map. If in doubt, I'll usually make this distance as short as seems reasonable. This is the shallow concentrated flow distance.

3. Because the shallow concentrated flow segment is usually a very small part of the total flow path, and because he purpose of this whole exercise is to arrive at the time of concentration, errors in this method are minimal compared to the other errors inherent in the Rational Method itself.

4. I get out of the office and go look at what I've just done. That's what Dr. Chow told me to do when I was in College many moons ago.

good luck with Kiddo

 

[TerryScan]

From the TR-55 manual:

"Open channels are assumed to begin where surveyed
cross section information has been obtained, where
channels are visible on aerial photographs, or where
blue lines (indicating streams) appear on United States
Geological Survey (USGS) quadrangle sheets."

This sounds as simple and clear as one is likely to find.

 

[queque]

Your local NRCS office has a Hydrologist on board. If not , he is within the State you are in. He can look at your watershed and share the TR55 methodology he uses.
Also, he can explain the answers to your question. Each NRCS State office has a Hydrologist that is versed on their methodoloy.
Good Luck.

 

[Rye1]

Hey Shelly,
There is not often an answer to your question - "When does sheet flow become shallow concentrated flow?" In our area, we generally use NTE 200' unless you are in an very urban area and then it's NTE 100'.

We only really use this in calculating time of concentration.

If I was required to map and move on, I think I would take your approach also RM for the flows but I may run a quick and dirty HEC-RAS to determine basic WSE.

 

[blueoak]

This is like a water of the state or water of the US question. It is whateveryour regulator can accept. I am a big fan of USGS blue lines. It is pretty easy otherwise I like bed and bank. You get these swales sometimes. From your description I doubtsome technical manual will help since I can find another showing it different. Can they state what they used previously?

 

[harringr]

Sheet flow is usually 0.1' or less and shallow concentrated flow is usually between 0.1' and 0.5'. Muncipalities can usually define when a floodplain study is to be performed in any natural low. This information should be contained in a manual or ordinance. Floodplains are not usually defined in an engineered ditch. These engineered ditches can have 100-year limits and usually you want to keep houses and buildings out of those areas. In my jurisdictional area I have defined a flooplain for any natural low with a contributing drainage area of 50 acres or greater. I am flexible with the method of calculating the floodplain for the unmapped low since it is not a regulatory floodplain I do not allow more than 100' of sheet flow ... 200' - 300' is much too long.

Good Luck
Bob H

 

[Crystalct1]

This is what is required:

Get topography for the area. A ditch will usually not be visible on USGS so survey will be needed unless the municipal office has something on file.

Pick a spot on the ditch as the point of interest. This should be either where the ditch empties into another waterbody or where the ditch enters another municipality. Delineate the drainage area to that spot.

Looking at the contours, determine the longest flow path (not necessarily the longest length, but the longest time when you consider whether the flow path is through a smooth area or grass/forested area).

The path will begin at the edge of the delineated polygon and head perpendicular to the contours for the ditch in question.

Refer to TR-55 (google and download if necessary):

The first segment is sheet flow. Use their standard of sheet flow NTE 300 ft, but to be conservative, use a maximum of 100 feet. Use TR 55 formula for sheet flow to obtain sheet flow travel time.

The next segment is shallow concentrated flow. This segment length is until the path hits the ditch. Use TR-55 formula for shallow concentrated flow travel time.

Use manning equation for the flow in the ditch. TR-55 recommends assuming full bank flow in the computation for velocity. Divide ditch length by velocity to obtain open channel travel time.

Open channel flow must be within a defined channel, meaning definable bed and banks. Sheet flow is very slow and begins to "concentrate itself" into rills and gullies as it flows overland toward an open channel of some sort. Shallow concentrated flow is the flow in these rills and gullies. There is no accurate way to determine where or when it begins, so we pick arbitrary lengths that have been observed in the field. I've seen the recommended max sheet flow anywhere from 100 to 400 feet. I guess it depends on your area. That's why we take the ordinance's direction.

Next you need to take the total travel time (or time of concentration) to the point of interest and convert that into a 100-year flow. TR-55 has a tabular method for peaks, but I recommend HEC-HMS. HEC-HMS uses the NRCS method, but automates it. You need basin lag time, total area, curve number and 24-hour storm precipitation in inches.

Once you get that peak, use it in HEC-RAS to obtain your elevation and limits of 100-year floodplain.

Hope this helps someone.