A drainage easement is a part of your property where the City has limited rights of access and/or use. Generally, you cannot make any improvements in a drainage easement. That means no fences, sheds, walls, trails or buildings. You should avoid planting trees or much landscaping as well.
A drainage easement has two possible purposes. It may be needed for the flow of storm water. For example, drainage ditches and creeks are typically within a drainage easement. In this case, anything that prevents the flow of water; that might catch debris; that might be washed away; or that might cause a dam-like effect is problematic.
Alternatively, the easement may be needed to access drainage infrastructure. In this case, anything that might make it difficult to drive a truck through or dig up an underground pipe is problematic.