From Defra 

About blocking in-field drains in grassland

You can block in-field drains or remove them to restore natural flows of water through grassland.

The natural passage of freshwater through rocks, soil and over land supports many habitats and species.

If you use artificial surface and sub-surface drainage, it will change the natural flow of water. This has led to the loss of many wet habitats and their species.

Benefits of blocking in-field drains

Blocking in-field drains to create small areas of wet ground on grassland will improve:

• natural water flows and drainage
• plant and invertebrate species diversity
• habitat for breeding wading birds like snipe
• flood resilience by slowing runoff into watercourses
• soil carbon storage and water quality
• the quality of natural wetland habitats

Wet grassland provides more cool refuges for species threatened by climate change. It will also help to reduce sediment-rich runoff into watercourses and sensitive habitats.

You can slow the flow of water as part of a natural flood management scheme.

Read more here.