Perhaps you've noticed that we generally use the term "rolling drain dip" when describing those mounds of soil on a trail that create a high point that directs water to a nearby drain system rather than running down the length of a section of trail, which create serious erosional issues. Here's an example of a rolling drain dip that needs rebuilding.
A damaged or blown out rolling drain dipWhy don't we call these things "water bars"? Because they're not. A water bar is a point in the trail supported by artificial support buried perpendicular to the trail (e.g., the tires and logs on East Boundary Trail). Frequent trail riders recognize the drawbacks to water bars: eventually, the soil erodes around the artificial support and essentially creates holes in the trail, which require maintenance time to fill.
The two images below illustrate an intact and functional rolling drain dip. The rolling dip makes it fun for cyclists while the drain helps maintain trail integrity and optimizes the trail's surface.
A functional rolling drain dip, viewed from one direction The same section, viewed from the opposite direction; note the drain
If a trail is designed and built sustainably, it should not require as much maintenance to maintain at optimum conditions. FYI, IMBA refers to the trail feature that we call a "rolling drain dip" as a "rolling grade dip." (photos by Tim Sawchuck)