Perform Stormwater Retention Pond Recovery Analysis on Exfiltration Trench
Design Example #3
The following design example demonstrates how to use Studio Express Groundwater feature for estimating recovery time for an exfiltration trench. The infiltration design methodologies for retention systems are also applicable to exfiltration trenches.
An exfiltration trench is an excavated trench consisting of a conduit such as a perforated pipe surrounded by stone aggregate used to capture and allow infiltration of storm water runoff into the surrounding soils from the bottom and sides of the trench.
Runoff from each rain event is captured and treated primarily through settling and exfiltration. The perforated pipe is used to increase the storage available in the trench and to help exfiltration by evenly distributing the runoff over the length of the system.
In this example we will estimate the recovery time given the following inputs.
Trench bottom elevation = 20.0 ft
Trench length, L = 480 ft
Trench width, W = 5
Trench depth = 5.0 ft
Side slopes = 0:1
Voids = 44% (details below)
Seasonal high water table = 17.0 ft
Impervious layer elevation = 14.0 ft
f = 0.30 (fillable porosity of surrounding soil)
Kvs = 3.75 ft/day (Vertical)
FS = 2.0
Kh = 16 ft/day (Horizontal)
Treatment Volume = 4,800 cft (based on local drainage authority guidelines)
Compute the Equivalent Voids
One of the key factors when analyzing a trench with a perforated pipe is determining the void ratio. Studio Express Groundwater does not specifically address trenches with embedded pipes. For these situations, accurate results can be obtained by back-calculating a void ratio that provides the equivalent volume.
Compute the actual area of the designed section
Area of the pipe, Ap, = PI x Dia2 / 4 = PI(1.5)2 / 4 = 1.77 sqft
Area of trench = V x 5 x 5 = .40(25) = 10 sqft (40% stone voids)
Total Area = 10 – 1.77 x .40 + 1.77 x 1 = 11 sqft
An appropriate void ratio to specify in Studio Express is the total area of the designed trench divided by the open area of the trench (assuming no voids) or 11 / (5 x 5) x 100 = 44%.
Enter this data into Studio Express Input Grid as shown below, and click [Compute].
Below are the results.
The Infiltration Analysis
Once the Treatment Volume was introduced to this trench, the water elevation rose to 24.55 ft (dotted line). Vertical, unsaturated flow began and later stopped at full saturation. It took 17 hours and used up 2,160 cft of the Treatment Volume. This left 2,640 cft in the trench at a water level of 22.50 ft (solid line). It then took 4 hours for that volume to infiltrate horizontally, until the trench was empty. Total time to recover is 17 + 4 = 21 hours.
Note that L/W = 480/5 = 96. Since 96 is within 10% of 100, the L/W value will be rounded up to 100. Any L/W’s that are not within 10% of 1, 2, 4, 10 or 100 will be not be rounded and Fx will be interpolated between the adjacent charts.
Getting a Printed Report
A final report can now be viewed and printed. Included in this report are detailed calculations to properly document your work. Click the [Reports] button on the Ribbon tool bar and you should see the following:
You can also include the other charts provided in the Groundwater feature, i.e., Stage-Storage and Fx-Fy, by right-clicking on them and selecting “Export this chart…”. It will save to a variety of bitmap formats. In addition, if viewing any of those charts while generating a report, it will be that chart shown on the report.