A drywell is an underground structure designed to manage and dispose of unwanted water, primarily stormwater runoff. It allows the water to slowly seep into the ground, preventing overflow and reducing the risk of flooding. The size of the drywell is crucial to ensure it can handle the volume of water efficiently. This article explains how to calculate the appropriate size for a drywell using a drywell size calculator.
Purpose and Functionality
The drywell size calculator helps determine the volume of the drywell needed to manage stormwater runoff effectively. By using specific measurements, the calculator ensures the drywell can handle the water without overflowing and that the surrounding soil does not become overly saturated.
Inputs Required for Drywell Size Calculator
To use the drywell size calculator, you’ll need the following inputs:
- Roof Area or Impervious Surface Area: This is the total surface area from which water runoff will be collected. It includes roofs, driveways, and other non-absorbent surfaces.
- Rainfall Intensity: The rate at which rain is expected to fall, typically based on local weather data. This is often based on a specific return period (e.g., a 10-year or 100-year storm).
- Runoff Coefficient: This factor indicates how much rainwater is likely to run off the surface. It varies by surface type. For example, asphalt has a high runoff coefficient (close to 1), while more absorbent surfaces have lower values.
Calculations to Determine Drywell Size
To calculate the volume of a drywell needed to manage runoff effectively, follow these steps:
Step 1: Calculate the Runoff Volume
Runoff Volume (cubic feet)=Area (square feet)×Rainfall Intensity (inches per hour)×Runoff Coefficient×112\text{Runoff Volume (cubic feet)} = \text{Area (square feet)} \times \text{Rainfall Intensity (inches per hour)} \times \text{Runoff Coefficient} \times \frac{1}{12} Runoff Volume (cubic feet)=Area (square feet)×Rainfall Intensity (inches per hour)×Runoff Coefficient×121
The conversion factor (1/12) is used to convert inches of rainfall to feet.
Step 2: Adjust for the Infiltration Rate
Adjusted Volume=Runoff VolumeInfiltration Rate\text{Adjusted Volume} = \frac{\text{Runoff Volume}}{\text{Infiltration Rate}} Adjusted Volume=Infiltration RateRunoff Volume
The infiltration rate measures how quickly the local soil can absorb water, affecting how fast the drywell can dispose of water.
Step 3: Size the Drywell
Drywell Volume (cubic feet)=Adjusted Volume×Safety Factor\text{Drywell Volume (cubic feet)} = \text{Adjusted Volume} \times \text{Safety Factor} Drywell Volume (cubic feet)=Adjusted Volume×Safety Factor
The safety factor accounts for uncertainties in calculations and variations in rainfall and soil absorption. A typical safety factor ranges from 1.2 to 1.5.
Example Calculation
Let’s say we have the following measurements:
- Roof Area: 1000 square feet
- Rainfall Intensity: 2 inches per hour
- Runoff Coefficient: 0.85
- Infiltration Rate: 0.5 inches per hour
- Safety Factor: 1.3
Step-by-Step Calculation
- Runoff Volume:
Runoff Volume=1000 sq ft×2 in/hr×0.85×112=141.67 cubic feet\text{Runoff Volume} = 1000 \text{ sq ft} \times 2 \text{ in/hr} \times 0.85 \times \frac{1}{12} = 141.67 \text{ cubic feet}Runoff Volume=1000 sq ft×2 in/hr×0.85×121=141.67 cubic feet
- Adjusted Volume:
Adjusted Volume=141.67 cubic feet0.5 in/hr=283.34 cubic feet\text{Adjusted Volume} = \frac{141.67 \text{ cubic feet}}{0.5 \text{ in/hr}} = 283.34 \text{ cubic feet}Adjusted Volume=0.5 in/hr141.67 cubic feet=283.34 cubic feet
- Drywell Volume:
Drywell Volume=283.34 cubic feet×1.3=368.34 cubic feet\text{Drywell Volume} = 283.34 \text{ cubic feet} \times 1.3 = 368.34 \text{ cubic feet}Drywell Volume=283.34 cubic feet×1.3=368.34 cubic feet
In this example, the drywell should be at least 368.34 cubic feet to handle the runoff effectively.
Relevant Information Table
| Measurement | Value |
|---|---|
| Roof Area | 1000 sq ft |
| Rainfall Intensity | 2 inches per hour |
| Runoff Coefficient | 0.85 |
| Infiltration Rate | 0.5 inches per hour |
| Safety Factor | 1.3 |
| Runoff Volume | 141.67 cubic feet |
| Adjusted Volume | 283.34 cubic feet |
| Drywell Volume | 368.34 cubic feet |
Conclusion
Using a drywell size calculator is essential for effective stormwater management. By inputting the right data for your specific location and conditions, you can ensure your drywell will function efficiently, managing runoff and preventing flooding. This helps protect your property and the environment by controlling water flow and reducing the risk of erosion and water damage.