Rate of Fen-Peat Soil Subsidence Near Drainage Ditches (Central Poland)

This study analyzed design depths ( t o ), post-subsidence depths ( t ), shallowing magnitudes ( d = t o − t ) and ratio values ( d / t ) of 12 drainage ditches in a fragment of the drained Solec fen-peat (central Poland) over a period of 47 years between 1967 and 2014. A significant decrease of the designed depth of the ditches t o was shown, from the average designed value of 0.97 m to their average depth after subsidence, t = 0.71 m. The ratio ( d / t ) of 0.41, which is associated with the degree of organic matter decomposition, indicated medium degree of peat decomposition. The average values of bank and bottom subsidence of the ditches during the analyzed period, 1967–2014, were 0.43 m and 0.17 m, respectively. The values of the average annual rate of land surface subsidence in the vicinity of the ditches were varied and within the range of 0.09 cm year −1 to 1.70 cm year −1 , with an average of 0.92 cm year −1 . Two linear empirical equations were proposed to calculate the amount of subsidence and the average annual rate of subsidence of peat soil surface near the drainage ditch route, based on the knowledge of the initial thickness of the peat deposit. The results of calculations using the equations proposed by the authors were compared with calculations of the same parameters using 10 equations published in the literature. The results obtained using the proposed equations were mostly larger than those calculated with literature-published equations.