modeling stormwater flow through a stone trench

[DenverJ]

I was hoping for some guidance on how to model stormwater flow through a stone trench in HydroCAD. What I have is a gravel access road which was constructed across the end of a field without a roadside ditch. The property owner is complaining about flows from the road causing erosion in his field. There isn't enough space within the r/w to construct a full ditch but I was thinking I could construct a stone trench along the road which would discharge to the culverts. I'm just not sure how to model that. Any thoughts?

 

[psmart]

Can you please clarify your proposed design? Do you mean a trench, filled flush with stone, that discharges to several culverts? How do you intend to keep the stone out of the culverts?

The answer depends on the design details and how you expect the system to behave. If it behaves like a level pool, you could use a pond with a void setting for the stone, plus appropriate outlets.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[DenverJ]

In this case, this trench is intended to be a conveyance feature not a pond. It is intended to replace a roadside ditch in an area where there isn't sufficient room to grade a ditch. The idea is to have trenches 2' wide and 1.5' deep running along the side of the road to daylight just short of the existing culverts. Each trench would be filled with R-3 riprap so material transport into the culvert should not be a problem. The purpose of the trench is to capture the runoff from the road to prevent it from reaching the edge of the field. The volume and rate of runoff is relatively small since each trench only receives runoff from 300 to 500 feet of roadway; the distance between culverts. The slope of the road is between 7 and 9 percent so a pond node would not work.

What I have done so far is to use PA DEP's E&S Channel Worksheet to determine the cross-sectional area needed carry the flows for the 100-year/24-hour event assuming the trench was empty, then check the cross-sectional area of the trench filled with stone. This is the same method used for riprap channels. I just have no way to show these trenches in the model.

 

[psmart]

If you are expecting Manning's flow in the trench then you might be able to model it as a reach, using the equivalent cross-sectional area and a suitably high Manning's value.

Another option is to estimate the travel time through each trench section and include it in the travel time for the adjacent subcatchment.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[btrueblood]

How are you going to keep the riprap from filling with silt?

 

[cvg]

this sounds like a giant french drain. french drains are meant to drain groundwater, not surface runoff. They do not have the capacity for peaked stormwater runoff flows and yes they clog. furthermore, it sounds very expensive. this does not sound like a viable solution. you might be better off with a storm drain pipe with inlets. Or construct culverts where you need water to cross your road.

 

[chicopee]

Btrueblood has an excellent point. A ski area in my neighborhood had stone filled trenches to capture snow melt within their parking lot. The following ski season, you could no longer tell where the trenches were located as skiers were driving over them to park their cars including "moi" truly.

 

[DenverJ]

Thanks for the comments guys.

psmart, your recommendation for an equivalent cross-section seems to be my best option for modeling.

cvg, I'm not overly concerned with capacity as even my longest stretch of road generates less than 1 cfs and there is no upslope runoff which would reach the trench. There are existing culverts under the road that I am running these trenches to.

As for cost, frankly a pipe and inlet system would double if not triple the cost. Your still importing stone for bedding material and your adding other material. Plus with inlets, you still need to concentrate and convey flows to those inlets which leads back to my original problem. No room.

Siltation may be an issue. Although this is a gas well access road with infrequent use. My concern is more with vegetation working into the stone. I am indicating they should do a biannual inspection and either clean or replace the stone as needed.

 

[cvg]

clean the stone. thats a laugh. they would need to haul it off and replace with new. if you only have 1 cfs, which sounds quite low, than why do you need a drain? 1 cfs would cause negligible erosion