Distances between dams - worked example
This page walks you through the following scenario; you have a set of points representing barriers to fish migration (e.g. dams) and you want to know the available distance upstream. You can imagine a fish wanting to go upstream, it can go as far as the source of any tributary or up to a barrier point. Anything upstream of a barrier point is not available.
In the image below you want to return the available network distance upstream of barrier 59 up to barriers 42, 61, 62, 63 and any source. RivEX can help you compute this distance. The equation would be upstream lengths of barriers 59 - (62 + 63 + 61 + 42). In this example the available upstream length from barrier 59 is 294.23Km.
1.Encode the following information into your river network: Catchment ID, Distance to mouth and upstream length, these are all attributes that RivEX can generate.
2.Ensure your barrier points are snapped to the network, RivEX has a snapping tool.
3.Run the transfer metrics tool to transfer upstream length to your barrier dataset.
4.Run the find nearest upstream sites tool setting the barrier dataset to be both layers. In this example the dataset is called dams and the ID field is SampleID.
5.Run the Join Field tool as shown below, you are joining the upstream length in your barrier table to your find nearest upstream sites table.
6.Run the summary statistics tool to sum upstream lengths for each barrier.
7.Run the Join Field tool again as shown below, you are joining the SUM upstream lengths from the statistics table to your barrier dataset.
8.Add a new field (type Double) to your barrier dataset and call it AvailUSNet.
9.Run a calculate on AvailUSNet and compute the distance as USLength - SUM_USLength.
Note: any barrier that has no upstream barriers will return the length of available network. Be aware of the issue of barriers on loops as discussed in the find nearest upstream sites tool.