Hydrodynamic Force on Inundated Bridge
Hydrodynamic Force on Inundated Bridge
(OP)
I don't usually design bridges so I'm asking for as much imput as possible on this one. I've been asked to design the abutments for a 90' span to go on private land over a creek. The owner wants to use a flat rail way car as the superstructure. The traffic on the bridge will be very light: atv's and pick-up trucks.
Because it's a private bridge on private land, I'm not sure what flood interval to use for the water height. If it were a public structure, I'm sure I'd be under AASHTO code. Determining the elevation of the bridge is something I need to do.
ASCE 7 flood hydrodynamic loads are based on the DFE, design flood elevation. This is based on the "community's flood hazard map." This is a very rural area and I can't find this information. What percentile and year interval are flood maps usually based on?
I recreationally kayak so I'm pretty familiar with the USGS water gages. This particular creek doesn't have a gage, but a similar one only a few miles away does, so I'm using it's numbers. The gage for the similar creek has velocity and height data dating back to 1980. I'm thinking about using the 95% percentile numbers over this time period (setting the bridge elevation to be NON inundated 95% of the time over a 29-year span).
I'm going to design the bridge for the inundated flood loads, but I'd like to have some justification for the expected interval.
Because it's a private bridge on private land, I'm not sure what flood interval to use for the water height. If it were a public structure, I'm sure I'd be under AASHTO code. Determining the elevation of the bridge is something I need to do.
ASCE 7 flood hydrodynamic loads are based on the DFE, design flood elevation. This is based on the "community's flood hazard map." This is a very rural area and I can't find this information. What percentile and year interval are flood maps usually based on?
I recreationally kayak so I'm pretty familiar with the USGS water gages. This particular creek doesn't have a gage, but a similar one only a few miles away does, so I'm using it's numbers. The gage for the similar creek has velocity and height data dating back to 1980. I'm thinking about using the 95% percentile numbers over this time period (setting the bridge elevation to be NON inundated 95% of the time over a 29-year span).
I'm going to design the bridge for the inundated flood loads, but I'd like to have some justification for the expected interval.






RE: Hydrodynamic Force on Inundated Bridge
Remember that an area can experience any kind of flood (if there is such potential) at anytime that is outside of the return period suggested by statistics.
RE: Hydrodynamic Force on Inundated Bridge
RE: Hydrodynamic Force on Inundated Bridge
RE: Hydrodynamic Force on Inundated Bridge
I don't think losing access to the bridge is a concern if it were to be inundated. It won't be used for any critical purpose. Plus, flood level waters don't stay at that elevation for long.
RE: Hydrodynamic Force on Inundated Bridge
You might want to check that out..
Just remember - Mother Nature is a bitch and she will always win - we just try to put off the inevitable.
RE: Hydrodynamic Force on Inundated Bridge
RE: Hydrodynamic Force on Inundated Bridge
If I am you, I would set at least a few feet clearance in between the anticipated maximum flood level and the deck.
RE: Hydrodynamic Force on Inundated Bridge
RE: Hydrodynamic Force on Inundated Bridge
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Hydrodynamic Force on Inundated Bridge
By the way, if you are in the US, you may want to check into FEMA criteria.
RE: Hydrodynamic Force on Inundated Bridge
The length of the bridge is already set. The owner is buying a 90' long flat rail car. I have no control over that. Therefore I can't set the length of the bridge, just the elevation. To set the rail car high enough to clear the 100-year FEMA flood mark would be very unreasonable in this case. I've determined what I believe to be the 10-year flood mark and have set it just above and am designing it for the hydrodynamic loads from inundation. Locks of concrete and dywidag anchors into the bedrock should keep it in place.
I appreciate all the help and comments, but some of the suggestions have gone over and above the nature of this project. It's a small creek on private land (100's of acres) that the owner just wants to drive his atv's over and go hunting on. Hydrologic studies and AASHTO HS ratings are beyond the scope of this bridge.
RE: Hydrodynamic Force on Inundated Bridge
We all know about these types of projects very well and most of us won't touch them which is how they wind up in your hands..."I don't usually design bridges" whereas we're concerned with codes, legal matters and ethics.
You asked for our help "asking for as much 'imput' as possible" and then you have the audacity to criticize it.
Regards,
![[pipe] pipe](https://www.tipmaster.com/images/pipe.gif)
Qshake
Eng-Tips Forums:Real Solutions for Real Problems Really Quick.
RE: Hydrodynamic Force on Inundated Bridge
Contact the local DOT and ask them where to obtain a copy of the local roads manual.
Also, you cannot look a 1 stream and expect another steam to act the same way. The have completely different watersheds. That is like saying all W12's have the same capacity just because they are both the same shape and are 12" deep.
Here is a suggestion, follow the stream downstream and take a look at the geometry of the nearest downstream structure. Then contact the landowner of the ground on the upstream side of this structure and ask him if he has ever seen any major backup from the structure. If your bridge opening is as large or larger then you have something to base your design on.
RE: Hydrodynamic Force on Inundated Bridge
Qshake, you should probably give your high horse a rest. He's tired, hungry, and needs some water from carrying you so long. Just as I assume you are, I am a professional licensed in multiple states with many years experience. I'm not throwing codes and legal matters to the wind and designing something that's unsafe to the owner (the performance of the bridge) or the rest of the public (it's affects on the environment downstream). However, I was just reminding this forum that this scale of this work is small and private. We has engineers tend to go overboard sometimes with "hire a geotech", "hire a hydrologist", etc. I'm trying to provide the ownwer with an economical bridge that will not wash away during a flood event, will not adversely affect upstream or downstream environment, but will also not be 30' above grade so that he has to drive his atv up a spiral ramp to get to it.
RE: Hydrodynamic Force on Inundated Bridge
A draw bridge could be raised after it was used and would help to reduce the effects of flooding.
RE: Hydrodynamic Force on Inundated Bridge
RE: Hydrodynamic Force on Inundated Bridge
RE: Hydrodynamic Force on Inundated Bridge
Down the rushy glen,
We daren't go a-hunting
For fear of little men
Hmmm, may be a X year flood where X the number of years the owner is expected to still go hunting will fare better. Also, from your scheme, a bit of encasing on the hard ground will enhance higly the survability of the bridge on its enhanced capacity to sustain moment from hydraulic forces and less heavy scour. Sometimes a small difference gives noticeable advantage.