There are a number of reasons why an existing field drainage system may be inadequate.
- Scheme may have been designed to work with mole drains that have since collapsed
- Drainage system may have reached the end of its useful life
- Land use may have changed
- Drains may have been installed without permeable backfill
To find out more have a look at the ADDB Field Drainage Guide which looks at the principles, installation and maintenance issues.
http://cereals.ahdb.org.uk/media/725158/g68-ahdb-field-drainage-guide.pdf
