Modeling a Synthetic Sports Field

[ptr]

What is the proper way to model a synthetic turf sports field using HydroCAD?

A typical field design consists of a 2" sand/rubber infill turf layer, then 8" stone base layer with flat panel drains (spaced every 20-ft) located on the bottom of the base layer. The flat panel drains then connect into a 10" perf header pipe (in stone trench) before connecting to existing drain line. The field is usually sloped at 0.5 percent.

Doing some web surfing, I found varying CN values (35 to 98) and Tc values (30 min to about 5 hrs). Does anyone have experience in what CN and Tc values to use?

Do I treat the field as a pond or just use the field watershed area with varying CN and Tc values, or both? The goal is to have my post runoff flow be at or below the pre.

 

[psmart]

If the field is intended to intercept all the rainfall, you'll want to use a high CN value such as 98. The Tc in this case will vary depending on the travel time through the turf and underlying material.

The stone base is often modeled as a pond, with the perforated pipe as an outlet control.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ptr]

Thanks psmart.

I agree that CN should be 98, because these fields drain so fast vertically that I have never seen them sheet flow (even in some real downpours).

The problem I have is picking (and justifying) a Tc value. My assumption is that I need a way to calculate the the Tc for the water to drain 8" vertically and then travel 20 feet horizontally to get picked up by the nearest flat panel drain. Does this sound right?

Also, I understand modeling the 8" stone base as a pond adjusted for voids. Would the perf 10" pipe in stone trench running the length of the field be modeled as part of the pond storage and the connection the the existing drain line as the outlet control?

 

[psmart]

Unless there is some type of medium that will delay the flow from the turf to the pond, the Tc would seem to be quite short - perhaps just a few minutes. Remember that the Tc should include just the time required for the water to reach the level pool (pond) underneath the turf, and not the time to the outlet, which is handled by the pond routing.

Selection of pond outlet(s) will depend of the control point(s) for your system. This could be the perforations (modeled as an orifice array), the barrell of the pipe (modeled as a culvert), or some other downstream restriction. Once you determine the control point, you can include any storage upstream of that point as part of your pond.






Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ptr]

Thanks for your help. I believe I'm getting closer to understanding and modeling this field correctly.

The field is surrounded by concrete curbing in order to anchor the turf, so it should act like a stand alone pond and I understand your direction for selecting pond outlets and control points.

I have to revisit the Tc value. It seems like flow is starting from top down, filling the voids in the 8" base layer before reaching the outlet. This takes time. I have witnessed synthetic fields during storm events. During the storm, there was hardly any flow out of the outlets. In fact, the storm event had already passed when flow was observed out of the outlets. So...how to I represent this time lag in the model?

 

[psmart]

A certain amount of time lag will be introduced by the pond routing calculation. I would evaluate this effect before considering a longer Tc.


Peter Smart
HydroCAD Software

http://www.hydrocad.net