1 ) You may like to read my ...
Published by Yogesh Agrawal, Director (retired), Minor irrigation schemes, Water Resources Department at Govt of Rajasthan
1 ) You may like to read my article "Disadvantages of Excess irrigation " in this regard, which will give you a clear picture.
2) I think you are not specific. slow means what, with respect to sand or what? Movement of water in unconsolidated clay duly tilled , is very much higher than the clay watered and consolidated to make embankment. The water logging is due to use of excessive use of water for irrgating the crops. You can use tensiometers to give required quantity of water to the crop. There is no fun in using excessive water for irrigation and than draining it out back to river.