How Deep Should a Dry Well Be?

A dry well is an underground structure that helps to divert unwanted water from your home, including stormwater and sometimes gray water. A dry well serves as an excellent defense for your home and landscaping, helping to capture runoff, reduce erosion, and prevent standing water on your property.

A dry well is a simple way to redirect water runoff to dissipate into the ground. However, it is important that it meets the proper specifications. Finding the proper depth of a dry well is essential for ensuring its functionality and protecting your property. Knowing how deep a dry well should be will help ensure your property has the proper support against water runoff.

Factors Influencing Dry Well Depth

Choosing the proper depth for a dry well will ensure that it can retain an adequate amount of water and be able to divert the water away from other areas of your property. There is no one-size-fits-all answer to how deep a dry well should be, as a variety of factors can influence the ideal depth.

Some of these key factors include:

• Soil composition
• Limitations, like tree roots or large rocks
• Soil percolation
• How much water volume you expect on your property

Fortunately, you can gain a complete picture of all these factors by performing some simple tests and making a few calculations.

Understanding Soil Characteristics

Soil infiltration and soil percolation are the two most important soil characteristics to consider when installing a dry well, as the effectiveness of your well relies on it. Soil infiltration is the entry of water down into the soil. 

Meanwhile, soil percolation is the rate at which water moves through soil. 

Typically, soft and sandy soils have better soil percolation. Understanding these characteristics of your soil will help you determine the depth of your dry well.

The best test for understanding your soil’s infiltration and percolation is to follow these steps:

1. Dig a hole about 12 inches deep.
2. Fill the hole with water.
3. Wait 24 hours.
4. If the water completely drains within 24 hours, perform the test a second time.
5. If the water drains completely this second time, then you have good soil percolation for a dry well.
6. If the water does not drain completely, choose a new location for your dry well.

This quick test is a great first step for determining how deep you should dig your dry well. The better the percolation rate, the better the location for your dry well.

Calculating the Ideal Dry Well Depth

Given the number of variables, the ideal dry well depth will differ depending on your property. Primarily, the structure depends on your home and the amount of runoff you can expect from a storm. If you have a lot of stormwater runoff, you will need a larger dry well to account for the extra water and give it time to permeate through the soil.

1. Determine the Water Runoff Volume

To determine the ideal size of your dry well, start by calculating the amount of water runoff that affects your property during a storm. In Portland, rainstorms are usually one-inch or less, so this is a good reference to use for calculating storm runoff. 

You can determine the stormwater runoff for your property using these steps:

1. Calculate the square footage of the drainage area on your property following this equation: 

(Drainage Area Length [ft] x Drainage Area Width [ft] = Drainage Area Square Footage [ft²] )

2. For properties with multiple drainage areas, calculate the square footage of each one and add them together.

3. Calculate the volume of your ideal dry well for a one-inch storm using this equation:

(Total Drainage Area [ft²] ÷ 12 = Stormwater Volume [ft³] )

2. Calculate the Depth of Your Dry Well

After you know the ideal volume for your dry well, you can use it to determine the proper depth and dimensions. Typically, a dry well is about 3 feet deep. However, this depth can change depending on your property. After all, the priority is achieving the proper volume. 

To determine the area your dry well will cover, follow this equation:

(Stormwater Volume [ft³] ÷ 3 ft Depth = Dry Well Area [ft²] )

If there are other limitations, like tree roots or landscaping, that will limit the area of your dry well, then you can complete this equation backward to determine the depth:

(Stormwater Volume [ft³] ÷ Dry Well Area [ft²] = Dry Well Depth [ft] )

For example, if your stormwater volume turns out to be about 12 ft³ and you only have space for a 3 ft area dry well, then you will need a depth of 4 ft.

Consulting Professionals and Permits

Digging a dry well is a challenging but possible DIY project. However, it is important to have a full picture of the product and what you need to begin. Before digging a dry well, you will have to research the necessary permits and regulations for your area.

For example, every dry well in Oregon must be registered with the Oregon Department of Environmental Quality. In some instances, like dry well decommissioning, working on dry wells requires supervision from a licensed geologist.

Even if you plan on doing the project on your own, it’s a good idea to consult professionals with experience in dry wells to ensure you capture any nuance before you begin. Professionals who have gone through the process multiple times will be able to provide you with valuable insight into the regulations you will need to follow.

Although it is possible to do the project on your own, there are several benefits to working with a professional:

• They will have the right equipment to dig a hole and direct the water runoff to the area.
• They have materials like landscape fabric or stone to build a dry well.
• Since they have experience working with various soil types and property layouts, they will be able to adapt the dry well to the needs of your property.
• They have the proper knowledge surrounding local regulations, have the necessary permitting, and have the right certifications for the job.

Ultimately, when you consult a professional, you can enjoy peace of mind throughout the process and leave the work in their experienced hands.

A Dry Well in Action

Many people do not know the extra steps involved with maintaining a dry well and often think the work ends with installation. From regular sampling of the soil to decommissioning, there are a lot of additional factors to consider.

It’s important to be prepared for the additional responsibilities that come with having a dry well on your property, including:

• Dry well sampling for annual compliance with environmental regulations, especially for properties that handle chemicals or petroleum products
• Dry well registration with the Department of Environmental Quality
• Routine cleaning and maintenance to avoid flooding and groundwater contamination
• Dry well decommissioning when the well is no longer needed

Alpha Environmental can support you in any of the above tasks, from dry well sampling to dry well maintenance. By helping you stay on top of these tasks, we help you stay in compliance with local regulations and keep your property safe from flooding or contamination.

Find Support for Dry Well Management from Alpha Environmental

A dry well is a great structure for preserving the health of your property, but it does take some time and experience to ensure that it will be most effective and compliant with local regulations. A dry well of the correct depth protects your landscaping and home against runoff.

At Alpha Environmental, we have experience providing maintenance for dry wells and supporting homeowners in following local regulations and procedures. We will help you with everything from dry well sampling to dry well decommissioning.

Here in Portland, storms are common, and a properly functioning dry well can make a significant difference in preserving the health of your property. At Alpha Environmental, we can provide the assistance you need to keep your property in its best possible condition. Contact us today to learn more about how deep a dry well should be or for support with managing your dry well.