The methods for identifying non-flood years, and adjusting the L-moments to account for these, are described in Flood Estimation Handbook Volume 3, Chapter 19. This adjustment was previously known as the ‘Permeable Adjustment’. Within the report ‘Review of flood frequency estimation in groundwater dominated catchments’ (JBA, 2022) it was noted that catchments with a high proportion of non-flood years tend to be those with low rainfall (SAAR) and are not restricted to permeable catchments hence this is now more widely applied.
Information on non-flood years is presented within WINFAP 5.1 https://winfapdocs.hydrosolutions.co.uk/Pooled-%26-QMED-Analysis/Non%252DFlood-Years-and-Trends/.
WHS have, however, produced the ‘WHS Permeable Adjustment Worksheet’ which can be used to implement this process. This can be downloaded from https://www.hydrosolutions.co.uk/software/free-downloads/.