Catch Basin vs. Dry Well: Sizing and Use-Case Comparison
These two components usually work together rather than as alternatives to each other.
On this page8 sections
- Catch Basin vs. Dry Well: Role Comparison
- Why These Two Components Usually Appear Together
- Dry Well Terms Explained
- Percolation Rate and Dry Well Feasibility
- Alternative: Dry Well vs. Direct Storm Drain Connection
- Sizing Sequence When Using Both Components
- Quick Reference: When Each Component Is Needed
- Factors That Affect This Decision
It’s common to see these two terms used as if choosing one replaces the other, but in most residential systems a catch basin feeds a dry well rather than standing in for it. The Catch Basin Sizing Calculator handles the basin side of this pairing, and this guide covers how dry well sizing works alongside it.
Catch Basin vs. Dry Well: Role Comparison
| Feature | Catch Basin | Dry Well |
|---|---|---|
| Function | Collects surface water, traps sediment | Disperses collected water into surrounding soil |
| Sized by | Flow rate (GPM) | Storage volume (cubic feet) |
| Typical position in system | First component, at the surface inlet | Final component, at the discharge point |
| Requires | Adequate inlet capacity for peak flow | Soil that percolates well enough to absorb stored water |
Why These Two Components Usually Appear Together
Sending water from a catch basin directly into bare soil without a dry well would just create a new wet spot at the pipe’s outlet, while sending water from an open surface area directly into a dry well without a catch basin first would quickly clog the dry well with sediment and debris. Pairing the two lets each handle the part of the job it’s actually designed for.
Dry Well Terms Explained
| Term | Meaning |
|---|---|
| Percolation Rate | How quickly soil absorbs water, usually measured in inches per hour |
| Storage Volume | The usable void space inside a dry well available to hold water before it infiltrates |
| Infiltration Chamber | A plastic or structural unit, often perforated, used to build a modern dry well |
| Overflow | A secondary outlet from a dry well used when storage capacity is exceeded |
Two Separate Formulas for Two Separate Components
Catch Basin: Q (cfs) = C × I (in/hr) × A (acres), sized for flow rate
Dry Well: Required Volume (cu ft) = Total Runoff Volume (cu ft) − (Percolation Rate × Dry Well Surface Area × Storm Duration), sized for storage
How to Determine If You Need a Dry Well
- Check whether a daylight discharge point or storm drain connection is available on your property
- Test or estimate your soil’s percolation rate, since clay soil may not support a dry well effectively
- Confirm local code allows dry well discharge, since some jurisdictions restrict it near property lines or structures
Example 1: Catch Basin With Daylight Discharge, No Dry Well Needed
- Site: yard slopes to a natural lower point at the property edge
- Discharge option: daylight at the slope’s base
- Dry well needed: no, gravity discharge handles the water directly
Properties with a convenient daylight discharge point often don’t need a dry well at all, since the catch basin can simply release collected water above ground at a lower elevation without any additional infiltration component.
Example 2: Flat Yard With No Daylight Option
- Site: flat yard, no lower point to discharge to, well-draining sandy soil
- Catch basin: sized for 25 GPM based on drainage area
- Dry well: sized for approximately 40 cubic feet of storage based on soil percolation rate and expected storm volume
Flat yards with good percolating soil are strong candidates for a catch basin and dry well combination, since there’s no practical daylight option but the soil can absorb the collected water effectively.
Example 3: Clay Soil Limiting Dry Well Effectiveness
- Site: flat yard, slow-draining clay soil
- Catch basin: sized for 20 GPM
- Dry well: oversized storage needed due to poor percolation, or alternative discharge recommended instead
Clay soil’s slow percolation rate can make a dry well impractically large for the storage volume actually needed, which is often the point where connecting to a municipal storm drain or installing a sump pump system becomes the more practical choice.
Size the catch basin side of your system first.
Use the Catch Basin Sizing CalculatorPercolation Rate and Dry Well Feasibility
| Percolation Rate | Dry Well Feasibility |
|---|---|
| Over 2 in/hr (sandy, well-draining) | Good candidate, reasonable dry well size |
| 0.5-2 in/hr (loam, average soil) | Workable, may need a larger dry well |
| Under 0.5 in/hr (clay, compacted soil) | Poor candidate, consider alternative discharge |
Alternative: Dry Well vs. Direct Storm Drain Connection
Where a municipal storm drain connection is available and permitted, routing a catch basin’s discharge directly into it bypasses the need for a dry well entirely, regardless of local soil conditions. This is often simpler in areas with poor percolating soil, though it depends on whether local code allows private stormwater connections to the public system.
Common Mistakes With This Pairing
- Installing a dry well in clay soil without checking percolation rate first
- Connecting a catch basin directly to open soil without a dry well, risking erosion at the outlet
- Undersizing the dry well’s storage volume relative to the catch basin’s peak flow contribution
- Placing a dry well too close to a foundation, risking water migrating back toward the structure
- Skipping an overflow path for the dry well in case storage capacity is exceeded during an unusually heavy storm
Sizing Sequence When Using Both Components
Start by sizing the catch basin for peak flow rate using the standard sizing formula, since this determines the basin’s footprint and inlet capacity. Then calculate the dry well’s required storage volume separately, based on total expected runoff volume and the site’s percolation rate, since storage and flow rate are different engineering questions that don’t scale directly from one another.
Quick Reference: When Each Component Is Needed
| Site Condition | Recommended Setup |
|---|---|
| Daylight discharge available | Catch basin only, no dry well needed |
| No daylight option, good percolating soil | Catch basin plus dry well |
| No daylight option, poor percolating soil | Catch basin plus storm drain connection or sump pump |
Factors That Affect This Decision
- Availability of a daylight discharge point on the property
- Soil percolation rate at the proposed dry well location
- Local code restrictions on dry well placement and storm drain connections
- Total expected runoff volume from the contributing drainage area
- Proximity of the planned dry well to foundations or property lines
Steps to Plan a Catch Basin and Dry Well System
- Size the catch basin for peak flow rate using your drainage area and rainfall intensity
- Check whether a daylight discharge option exists before assuming a dry well is needed
- Test or estimate your soil’s percolation rate if a dry well is being considered
- Calculate the dry well’s required storage volume separately from the basin’s flow rate
- Confirm local code allows the planned dry well placement and discharge method
- Add an overflow path for storms exceeding the dry well’s storage capacity
Frequently Asked Questions
Is a dry well a replacement for a catch basin?
No, they typically work together: the catch basin collects and filters surface water, while the dry well disperses that already-collected water into the soil.
Do I need a dry well if I have a catch basin?
Only if there’s no daylight discharge point available and the water needs somewhere to go after leaving the basin, such as when the property is flat with no lower point.
Can I install a dry well in clay soil?
It’s possible but often impractical, since clay’s slow percolation rate requires a much larger dry well than sandy or loam soil would need for the same runoff volume.
How do I size a dry well?
Dry wells are sized by storage volume, based on total expected runoff and the soil’s percolation rate, which is a different calculation than the flow-rate sizing used for a catch basin.
What happens if my dry well is undersized?
An undersized dry well can overflow during heavy storms, so including an overflow path to a secondary discharge point is a reasonable safeguard.
Can I connect a catch basin to a storm drain instead of a dry well?
Yes, where a municipal storm drain connection is available and permitted, this bypasses the need for a dry well regardless of local soil conditions.
Size the catch basin portion of your system accurately.
Try the Catch Basin Sizing Calculator