hydraulic question (Is it a bridge or a culvert??) 1

[bgrove]

Recently I submitted an H&H report in which I modeled a culvert as a bridge in the HEC-RAS computer program. I recieved a comment back that it should have been modeled using the culvert editor within HEC-RAS. I have always been under the impression that from a hydraulic perspective the difference between a culvert and a bridge is the fact that culvert allow for more submergence in which case the hydraulic calculations become more complicated while a bridge is more simplistic since very little submergence is allowed. In my particular case I have an exisitng bridge and I am replacing it with a culvert which has the exact same hydraulic opening and over all superstrucutre depth. I did not want to change modeling appraoches in HEC-RAS for fear that a "glitch" in the program would show a watersurface increase that common sense tells you does not exist.

Bottom line here are my questions: 1. First of all do the you guys generally agree with my logic. 2. Does anyone know of an offical publication or text book that I could site in my response to this reviewer which will help validate this opinion? Thanks you for your help.

 

[Qshake]

Sounds like your reviewer is a possible State entity.

In many cases culverts and bridges as well as watershed areas are divided by empirical definitions.

Bridges have long be classified as having a span greater than 20'. Thus if your culvert has a span greater than 20' its a bridge according to the FHWA and likely a client.

Drainage areas are often classifed by maginitude. These areas are then assigned different freeboard requirements etc. It also stands to reason that larger areas will require larger openings and thus require bridges.

Having said that, it is your client and your client has asked for a revision. And if your client is knowledgeable about the process, as many State DOTs are, your wasting time looking for reasons not to change it.

 

[unclesyd]

Echoing "Qshake" if it's a state enity give up.
I have a friend who has considerable acreage around his home of many years that has a wooden bridge that has been repalced several times, no problem. It needs replacing and he wanted to go to a culvert system. When he tried to get a construction permit the answer he got was that you can't go from a bridge to culvert, but you can go from a culvert to an approved bridge. He has another problem in that while inspecting his bridge and drainage area they classified part of the adjacent property a wetlands. Now the little branch drains a wetland. So the bridge cannot be replaced in kind, it has to be engineered, both from the mechanical and hydraulics side. His first quote was for $11,000 for the engineering.

 

[KRSServices]

Sometimes, it absolutely bogles the mind as to how some bureaucrats can become so entrenched in structure of habit. I have personally had many designs approved for the replacement of an old bridge with a culvert or multiplate. This has been done up here for years. Once the hydraulics aare resolved, this issue becomes one for the fish cops, because the designs are approved. Some regulators move slower than others, but perhaps they can utiliize the work being done in other jurisdictions in America to view how there are different approaches to the same problems.

KRS Services

http://www.krs-services.com

 

[RWF7437]

Although not mentioned by the others who have posted, another suggestion would be to look at the definitions of "bridge" and "culvert" in the HEC-RAS documentation. My best recollection is that bridge openings have "natural" bottoms while culverts have bottoms made of the culvert material. Thus, a concrete culvert might have an "n" value of 0.013 while a bridge might have a stream bottom with an "n" value mor like 0.020 to 0.040 or rougher. Either a bridge or a culvert may be overtopped.

For the fish police, a bridge is almost always preferable because it retains the "natural" conditions during the flow regimes when fish can actually swim upstream. No fish swim upstream during the regulatory flood in most places I know of.

Distinguishing between a bridge and culvert on the arbitrary basis of span seems, hydraulically useless.

good luck.

 

[bltseattle]

I believe the hydraulic difference is primarily the bottom condition, as noted by RWF7437. I've had a number of projects where the goal is to provide fish passage in either a bottomless 3-sided box culvert (ie a bridge) or a arch/round CMP filled partly with gravel. In either case HEC-RAS can not effectively model a conduit with a rough bottom and smooth walls using the culvert routine.

One key aspect is the goal of the analysis. If you are looking at partially full and low flow scenario with a natural bottom than the bridge routine is the way to go, but if HW/D>1 and the effect of bottom roughness is not as much on the overall capacity then the simpler culvert approach may be accurate enough.

I would present the agency with the case that since it's a bridge now, it is likely that the bottom will be rough (either countersunk by design or by wash-in of streambed gravel) and therefore a bridge best represents the long-term operating condition of the structure.

 

[RWF7437]

bgrove,

You didn't say whether fish passge is a concern or not. If not, the low flow analysis may not be necessary. But if so, you'll need to analyze your bridge or culvert for a range of flows.

Still, from the point of view of hydraulics, the nature of the bottom will dictate which approach to use.

Do you have the latest version of HEC-RAS ? My recollection is that it will allow you to model a culvert which has been "depressed" by filling the bottom to some depth with rock.

good luck

 

[bltseattle]

My recollection is that the HEC-RAS partially filled culvert routine is limited in that the bottom is assumed to be flat and the culvert must be round. I'm not sure how it handles the longitudinal slope, but I recall it was insufficient for a partially filled pipe arch culvert installed flat but with a longitudinal channel slope (so the upstream end was more "filled" than the downstream end), and slight v x-section, which is somewhat typical for fish passage improvements.

I admit to being a stickler for detail when modeling fish passage, however, and every project is different.

 

[unclesyd]

Just to add a tidbit of information about my post. The jurisdictional people were questioned about using the oval type two culverts with the relative flat bottom and the answer was still no. He has another proposal of using a three channels with the ovals that will give him about 80% open area compared to the bridge.

Another twist is that they would also like something other than CCA treated wood in the bridge.