I did some reading about drainage easements in Monmouth County. Seems to be quite commonplace.
That 20 foot drainage easement means you have a storm sewer pipe running through the edge of your property.
Unless that pipe collapses the local township/borough will never want to access it on your property.
There is an Englishtown Borough Council meeting from 2004 in which they allow a family to build a shed on the drainage easement. The homeowners had to pay $1.00.
»www.englishtownnj.com/minutes/04 ··· 1404.htm