Drainage for a public park - vegetated swale (existing muddy mess) vs french drain system

[DrainMe]

I am preparing drainage plan for a park located on a hilly part of town. There is an existing path/walkway with a 6" on the lower side and a hilly lawn/grass area on the upper side. There is an existing grassy swale on the hillside next to the path. Unfortunately, the swale has multiple muddy depressed areas. This park has high pedestrian traffic and people do not stay on the paths. I believe that pedestrian traffic caused these depressions, and they have gotten worse overtime. The landscape guy asked me whether we can remove the swale and replace with a french drain system. Please see attachment.

I would think that a swale with a subdrains would help with the ponding problem/muddy areas. Any design recommendations would be greatly appreciated.

 

[cvg]

grade the swale to drain. muddy areas get that way because of standing water. if you cant get the swale to drain without excessive grading than a pipe drain is better than a french drain for surface water drainage. if high groundwater is the problem, then a french drain might be useful.

 

[civilman72]

Re-grade the swale to drain to specific low points with pedestrian-friendly grates to a buried pipe system. A french drain is difficult to maintain (it will eventually clog), possibly more expensive, and is probably not your best solution to drain the hillside.

 

[SMIAH]

As cvg points out, If no groundwater, a trench drain could help.
You could use an infiltration trench.
You could use underground pipe system.
To me, this really depends of local soil properties.

 

[DrainMe]

Thank you all for your replies. The current grassy swale is graded to drain but depressions developed and we are not allowed to change the plant species to provide more durable swale surface. The soil is horrible, HSG group D. Would installing sub drain for the swale and providing better topsoil(sand) be the best solution?

Thanks

 

[civilman72]

With HGS Group D, you obviously have very little infiltration. This can be improved by replacing this material with a more porous material and french drain. But how do you plan to construct this without disturbing/changing the plant species?

When you say plant species, do you mean grass or actaul plants? Can you add plants to the low areas? They will soak up water and keep pedestrians away.

If you're talking grass, why can't you install a drainage fabric that will reinforce the swale and still allow the current plant species to grow back?

 

[cvg]

filling the depressions would be first course of action since as it stands the swale does not fully drain. I see no reason for a subdrain, if your swale drains like you say it does than a subdrain will not improve things.

Since you seem set on installing a french drain, make sure you install inlets at the low areas to drain the surface water. Connect the inlets to a pipe, perforate it and surround with gravel if you want. Make sure your drain daylights or you will still have standing water. Sand will not make a more durable surface for walking, gravel might but I would not call either one topsoil.