In order to avoid increasing flood risk elsewhere, it is preferable for these to be sloped towards permeable ground or to be made of pervious materials.
Er mwyn sicrhau nad oes mwy o berygl o lifogydd mewn mannau eraill, mae'n well i'r rhain oleddu tuag at dir hydraidd, neu gael eu gwneud o ddeunyddiau hydraidd.