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04/05/2019 09:17:11

Hi all, 

I am using the generic rainfall/runoff unit for the input into a small network I have created. For transformation, I am using the 'watershed lag method'. Is anybody able to clarify the flow length for me? I assumed it was the average distance from any part of the catchment to the channel? 


Many Thanks


Flood Modeller Support

04/07/2019 11:11:46


The Flood Modeller generic rainfall/runoff unit uses US hydrology methods detailed in Technical Release 55 [https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1044171.pdf]. Chapter 3 is on travel time and you will see at the start of this chapter, the time of concentration is defined as follows:

Tt is a component of time of concentration ( Tc ), which is the time for runoff to travel from the hydraulically most distant point of the watershed to a point of interest within the watershed.

For the watershed lag method used in Flood Modeller, we are in fact calculating this time of concentration. As such, your flow length needs to be the length from the most distance point of the watershed to the outlet.

Also, watch out for your units as American custom units are inches/feet/miles. You can adjust between metric and US customary units if needed in 'general system parameters' in the Flood Modeller interface (double click the first row named 'General' in your Network table to access this).

Hope this helps!


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