Ima Geek writes, ‘I was fascinated by your thoughts on drainage design. Can you tell me more?’

By: Jeffrey D. Brauer


Ima, for you and the two other readers interested in this subject, I will be glad to. Putting the general golf-drainage plan into effect does require some engineering skills. However, take heart, because if I can design a drainage system, it’s a sure bet that almost anyone can.

There are many sophisticated drainage engineering formulas that I steer well clear of! After laying out a piping scheme, I use the Rational Method to size drainage pipes. Engineers use it on watersheds of less than 200 acres, and most golf course drainage areas fall in that category. On larger watersheds, it tends to underestimate actual runoff, and engineers use other formulas, sometimes as required by law. The Rational formula can fail when actual rains aren’t “average.” A 2-inch rain may occur the day after a 1-inch rain, when soil is saturated, greatly increasing the runoff. Also, a design may anticipate only existing conditions, which might be farmland, and in a decade, it is high-density suburbia.

Short version: For best results, don’t skimp on the percent of runoff estimates, no matter how amazed you are at how large pipes become. The formula is Q=CIA where,

Q = Run-off (in cubic feet per second);

C = Co-efficient (percent of run-off expressed as a decimal);

I = Intensity rate of rainfall (in inches per hour); and

A = Acres of watershed draining to a particular inlet.

The mathematically astute reader will recognize that this formula really estimates acre-inches of flow per hour. However, by coincidence, that is exactly the same value as cubic feet per second, which is the unit used to size pipe.

The Rational Method estimates the percentage of runoff based on site use. “C” is an estimated input. Typical co-efficient values are Urban/Industrial – 70-90 percent, Residential – 50-70 percent, Golf Course – 30-50 percent, and Rural – 10-30 percent.

Generally, we use higher percentages where there are clay soils, steep slopes, and light turf cover, average values for loam soils, moderate slopes and vegetative cover, and minimum values, if you have sandy soils, flat slopes and heavy tree cover. Where a watershed will be partly urbanized, we blend co-efficient values. A watershed that is equally split between golf and housing might rate a C value of 50-60 percent.

“I” is the storm intensity we want to drain, which is usually a two-year storm in 90 minutes. In Houston, that’s 2.0 inches/hour. In Kansas City, its 1.5 inches /hour, and in St. Paul, it’s about 1.0 inch/hour.

These areas are in the “I-35” Corridor, which tends to have intense storms. Thus, you could use these values if you are at similar latitude and have a safety factor. Assuming 50-percent runoff, the typical Houston drainage acre produces 1.0 CFS, Kansas City acres produce 0.75 CFS, and St. Paul acres produce 0.5 CFS of runoff.

To size catch basins, we measure the acreage draining into each inlet, and consult the chart below, which is based on typical light-duty golf course basins, with a safety factor. (Grass clippings reduce capacity.) So, while smaller inlets may have “better aesthetics,” oversize your basins!

Design Intensity in
CFS per acre
CFS Actual
Acres Drained by
 
 
0.25
CFS
ACRE
0.375
CFS
ACRE
0.5
CFS
ACRE
0.75
CFS
ACRE
1.0
CFS ACRE
8 in. round grate
0.3
1.2
0.8
0.6
0.4
0.3
10 inch round grate
0.6
2.4
1.6
1.2
0.8
0.6
12 inch round grate
1.2
4.8
3.2
2.4
1.6
1.2
15 inch round grate
1.7
6.8
4.5
3.4
2.25
1.7
18 inch round grate
1.8
7.2
4.8
3.6
2.4
1.8
24 inch round grate
2.8
11.2
7.4
5.6
3.7
2.8

We size pipes starting at the top of a line, where the pipe needs the same capacity as the first basin. At subsequent basins, the outlet section of pipe must accommodate both that inlet and the water already flowing through the pipe.

Pipe Size
Min. Slope
CFS
Mid Slope
CFS
Max
CFS Slope
4 inch pipe
1.25%
0.2
4.5%
0.4
10%
0.8
6 inch pipe
0.7%
0.6
3.0%
1.33
6%
1.8
8 inch pipe
0.5%
1.0
2.0%
2
4%
3.2
10 inch pipe
0.33%
1.75
1.5%
3.3
3%
5.0
12 inch pipe
0.25%
2.5
1.0%
5
2.5%
7.0
15 inch pipe
0.20%
3.7
0.8%
6
1.75%
11.0
18 inch pipe
0.15%
5.25
0.6%
10
1.5%
16.0
24 inch pipe
0.12%
9.5
0.4%
20
1.0%
30.0

Pipe capacity increases with grade. For example, for 1.0 CFS flow, we can use a 10-inch basin and either a 6-inch pipe at about 3.0 percent, or an 8-inch pipe at 0.5 percent. Many situations require larger pipe at flatter grade.

The minimum slopes shown are those required for “self-cleansing velocity.” (Sanitary sewer engineers use a minimum “self-cleansing” velocity of 2 feet per second, while drain-pipe manufacturers recommend 3 feet per second. I have trouble envisioning storm water flowing less freely than sewage but, nonetheless, use 3 FPS.) The maximum slope is that which limits scoured pipes and exit-area erosion problems from high-flow velocity. The mid range allows you to do some mental gymnastics to arrive at correct pipe size for “in-between” situations.

Ima, I hope this helps you design better surface drainage for your golf course!

Jeffrey D. Brauer and his firm, GolfScapes, have designed 40 golf courses and remodeled 80. Canterberry Golf Course in Parker, Colo., and Giants Ridge are rated among the best affordable public courses in the United States, while his Avocet Course at Wild Wing Plantation in Myrtle Beach, S.C., was a Golf Digest best new course winner, Champions Country Club is rated 5th in Nebraska and TangleRidge Golf Club is 12th in Texas. President of the American Society of Golf Course Architects during its 50th anniversary year in 1995-96, Brauer also designed Colbert Hills Golf Club at Kansas State, which opened in June 2000 as the cornerstone golf course for The First Tee program as well as the first collaboration between the PGA of America and Golf Course Superintendents Association of America. To contact Jeff, call him at 817-640-7275 or send him an email at jeff@jeffreydbrauer.com.

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