Good point! Yes the culvert should be grated. It is about a 4'-0" diameter pipe ... so scampering inside is quite doable. This is part of a Storm Runoff Drainage System which is a completely separate system from the sewer. The culvert empties into a concrete channel along side a walking and horse trail.
View attachment 21848
The open culvert is on the other side of this bridge (the bridge supports a four lane roadway). The left side is for water and the right side is for horses and people.