Railway Structures Identified: Workflow Assistance

[PeterWilson]

We are currently designing a new Railway line and need to define where structures are required along the alignment. I have come up with the following workflow and would like to ask for assistance in implementing my workflow.

1: Convert the Railway Alignment into a raster grid (same cell size as flow accumulation grid)

What is the best method to convert the railway alignment polyline into a raster grid that has the same cell size and overlays correctly on top of the flow accumulation grid?

2: Use a conditional statement (Map Algebra) to generate a new flow accumulation grid based on the intersection of the flow accumulation grid and railway alignment grid.

How would I structure my conditional statement to generate a new flow accumulation grid based on where the two grids intersect if I don't know the range of values beforehand?

3: Use the new flow accumulation grid to define the type and location of structures are required based on the flow accumulation values. (i.e. higher = bridge and lower = culvert)

Any advice in developing a criteria based on the flow accumulation value along the railway alignment to define where structures will be required as well as type will be appreciated?

Regards

[AlvaroFonseca]

Hello Peter

This is quite interesting. I am also currently working on 51 km of a whole new railway line (with velocities up to 260 km/hour). So I have some comments:

- What do you mean by structures? Is it bridges and/or dams? Underways for rivers, channels and/or roads?
- I suppose that, when you say alignment, you mean a CAD file where the alignment (on 2 dimensions) is shown, right? Or you mean the DTM of the railway? Because converting a 2-D alignment to a raster grid does not make much sense to me. Right?

Assuming you have a new DTM for the railway, and another DTM for the existing terrain area surrounding the railway, then what about doing this?:

- Merge both terrain datasets, but make sure to give highest priority to the new railway DTM, so that this "comes on top" on the merging process.
- With the result, a simple Cut/Fill operation should give you the areas where your new DTM is above/below the existing terrain
- Alternatively, with the merged DTM, run ArcHydro Terrain Pre-processing functions, all the way down to mapping of Depressions and the Flow Direction Grid. These results will indicate where you have low areas requiring culverts, and where water would flow from/to.

Hope this helps! You are welcome to ask/comment on my suggestions.

Kind regards,
Alvaro
Consultant Drainage and GIS Engineer
Grontmij Denmark A/S
www.grontmij.dk

[PeterWilson]

Hi Alvaro


I have come up with a solution, based on my discussions with Dean Djokic from the Arc Hydro Team. I'll firstly try to explain what I was trying to accomplish.

I'm currently working on two railway alignment projects, the one is within Liberia/Guinea which is +/- 760 km in length and the second is within Malawi/Mozambique which is +/- 1000 km in length

The engineers have supplied us locations where structures (Bridges/Culverts) are required based on there preliminary assessment using a strip survey +/- 100m on either side on the alignment.

We were tasked to determine the hydrology characteristics for each structure as well as identify where additional structures would be required. In order to determine the catchment characteristics (Catchment Area; 10/85 Slope; Length to Centroid; Longest Water Course & Average Slope) for each structure, we used the SRTM 90m DEM.

We originally ran the entire terrain preprocessing steps within Arc Hydro and then used a manual approach, using the Trace Flow Path tool which is found within the Arc Hydro toolbar to identify additional flow paths in order to determine where the structures (Bridges/Culverts) should be placed and to calculate the catchment characteristics. This was tedious and very slow.

Our new approach involves the following steps

1: Convert Railway Alignment Polyline into a Raster Grid
2: Convert the Railway Alignment Grid into a Point Feature Class
3: Interpolate to shape the FAC (Flow Accumulation Values) based on the location of the points
4: Filter out the points that have a FAC (Flow Accumulation Value) which is 0 or is below a predetermined threshold
5: Load the FAC Points into the BatchPoint Feature Class
6: Generate Watersheds based on the new BatchPoint's
7: Eliminate unnecessary Watersheds based on Visual Inspection
8: Generate the remainder of the Catchment Characteristics for the Watersheds

Regards