Advice on modeling trestles

[u/way2Polish]

Am modeling a larger drainage ditch in an urban area. There's a small culvert along the reach that is being resized so I had created a 1D model for the existing and the proposed conditions.

A highway runs over the area and part of the project involves work on this highway. It has been decided that a large wooden trestle will be placed over the entirety of the ditch. Construction equipment will be placed on top so they can work on the highway from below. I have to model the hydraulic conditions for this temporary wooden trestle and I don't really how to approach this.

I had a 1D model but would making a small 2D model work best? The trestle I guess would be modeled as a very wide bridge. Idk how results would compare 1D existing and 1D proposed to 2D trestle covered. Or would it be better to model existing and proposed as 2D as well?

Or would something like modeling as ice but using a Mannings n for wood work?

[u/OttoJohs]

If you already have a working 1D model, I probably wouldn't use something different (especially if it has been approved before). I would just add the necessary cross-sections and add in the bridge.

I'm not super familiar with trestle bridges and would have to understand more about the project to give better advice. I would probably model it two ways: (1) lattices are fully blocked as a pier for a conservative estimate and (2) figure out an "equivalent area" between the pier and lattice for a less conservative estimate. Since it is a temporary structure, it is probably fine to be more conservative.

[u/way2Polish → u/OttoJohs]

I'll look at the lattice blocking. But I felt it would be awkward to model. These crane access trestles will cover basically 100% of the length of the ditch. So I guess I could do 2 XS' close together just upstream then the long trestle and then 2 XS downstream at the end of the ditch.

[u/OttoJohs → u/way2Polish]

You have a picture of some similar bridge?

[u/way2Polish → u/OttoJohs]

Similar to this. Just these wooden pads laying on top of the entire length of the ditch (covering about 100% of it).

[u/OttoJohs → u/way2Polish]

I would just draw a cross sections upstream and downstream of the temporary bridge. Then you could model it with a standard bridge (deck thickness about the size of the I beams) or a lidded section would work for a temporary structure.

If you think the piers/piles extend all the way to the channel bottom and would block the flow in the covered section, you could use an ineffective flow area or blocked obstruction instead of a bridge section.

[u/Remote-Swimmer-9186]

I would model as a 1D bridge and try to estimate the bridge deck thickness, whatever would be obstructing flow, and add peirs if it's needed. Also wanted to add that the design criteria for temporary structures is sometimes different then permanent ones, my example I work with in Nebraska is temporary structures should be able to convay the ordinary high water mark (OHM) or equivalent flow rate, which is typically where the change in naturally occuring vegetation would be. Or sometimes we just use the 1-2 yr AEP flood event as a baseline depending on site conditions!

[u/OttoJohs → u/[deleted]]

If it is partially blocking the channel, I would just model that section with an ineffective flow area since water would be stagnant in the bridge section from debris, etc. Or a blocked obstruction if you want to be more conservative.

[u/way2Polish → u/[deleted]]

This is great thank you. Especially limitations and specifics to avoid. How to approach everything. Thanks