Why dig a French drain instead of just an uncovered, unfilled trench ?

Question

I have a 200x80 ft riding arena that is unfortunately lower than the surrounding land. I'm thinking of digging trenches around this arena to intercept the water flow into the arena, and draining the water to another location. All the research I've done suggest gravel-filled covered trenches. Is there a reason why I should not leave the trenches unfilled and open? They would hold more water when not filled with gravel.

Answer 1

The reasons for the suggestions are mainly twofold:
1) allows for the slow dissipation of water and eliminates erosion
2) to prevent a breeding ground for mosquitoes

French drains often include corrugated drainfield plastic tubing
[4-6"] that is surrounded on 4 sides with the gravel. This lets the water traverse a longer/larger area and helps to prevent a saturated or soggy section of land near the point of water entry.

Also consider covering the gravel with a mesh material so that dirt doesn't compact around the rock.

the tubing and mesh are found in the same section at Home D or any building supply.

Might also consider using "drainfield" rock, which is larger than gravel.
Look for septic tank or stone/rock suppliers in your area
they will know about DR

good luck

Answer 2

There are a number of reasons. First of all an uncovered trench is a safety hazard for human and equine alike. Second there is a matter of erosion. The gravel will allow reasonably free flow of water without allowing a great deal of washout. If you are going to leave them open and uncovered, I would recommend lining the trench with concrete or a similar material to prevent erosion.

Answer 3

The idea of the gravel is allow the water to drain away through it slowly. Which is fine if you don't get flood type rain falls. Gravel drains are mostly used when the ground is constantly damp and you require a constant seepage removal.

But if its heavy downpour type rain you'd need the open drain instead so it can take the water away quicker.
It depends on the situation.

Answer 4

With no offense, and without reading deep into other answers very likely valid. The rocks allow penetration without MUD for one thing. The other issue however might be some levels/possibilities in yours or any case; regarding danger/ liability/ possible injuries

Answer 5

The water will move faster over the gravel, and the soil won't get saturated and/or eroded.