Many thanks for your recent enquiry concerning the proposed siting of your raised beds on a grass surface.
To maximise the drainage potential of the raised bed, I suggest you leave the areas to be occupied by the raised beds free of membrane and bark chips and just carry out some basic groundworks, beginning with the removal of grass and about an inch of soil before taking up your fork or spade and very lightly digging over the area where the beds are to be placed. This will allow soil added to the raised bed to mix with the ground and also help retain water – raised beds can dry out very quickly. Not gently cultivating this area can result in even faster water loss, especially if the undersoil is very compacted.
It's also important to incorporate plenty of organic matter to your raised bed soil as this will both help with drainage (water can pass through the air spaces it creates) and water retention (the bulky matter and sticky black humus hold onto and store water). Home-made garden compost or well rotted manure are generally the best forms of organic matter to add to your bed mix, but if you can’t lay your hands on these immediately, then most good garden centres actually sell organic matter/farmyard manure by the bag.
Following these guidelines should help you get the most from your raised beds and hopefully solve any drainage problems before they occur - always handy when you've plenty to do in the garden!
I do hope my information proves to be of some help; many thanks once again for your enquiry and the best of luck with your raised bed project.