The Situation
You are designing drainage for a 15,000-square-foot asphalt parking lot for a small retail building. The lot slopes toward one corner where you need to install a catch basin. The basin will collect all surface runoff and connect to an existing 12-inch storm drain pipe in the adjacent street. You need to select the right grate size and verify it can handle the design flow.
Which Calculator to Use
The Catch Basin Calculator helps you select the grate size, estimate the interception capacity, and verify that the outlet pipe is adequate.
Walking Through the Inputs
Basin Type (Grate inlet)
For a parking lot surface, a flat grate inlet is the most common choice. It sits flush with the pavement and collects water from all directions. Curb inlets are used along roadways with curb and gutter.
Design Inflow (Q = 1.38 cfs)
We can calculate this using the Rational Method:
- Drainage area: 15,000 sq ft = 0.34 acres
- Runoff coefficient: C = 0.95 (asphalt parking lot — nearly all water runs off)
- Rainfall intensity: i = 4.25 in/hr (10-year, 15-minute intensity for this location)
Q = 0.95 x 4.25 x 0.34 = 1.38 cfs (about 620 gpm)
Alternatively, you can enter the drainage area, rainfall intensity, and runoff coefficient directly and let the calculator compute the flow.
Grate Size (24” x 24”)
For a flow of 1.38 cfs, a 24-inch by 24-inch flat grate is appropriate. This grate has a theoretical capacity well above 1.38 cfs at typical ponding depths. A smaller 18x18 grate could work but would have less safety margin, particularly if partially clogged by debris.
Clogging Factor (50%)
Always design with a clogging safety factor. A 50% clogging factor is standard for parking lots — this assumes half the grate openings are blocked by debris (leaves, trash, gravel). Some jurisdictions require 50% clogging by code.
With 50% clogging, the effective grate capacity is reduced by half, so we need the unclogged capacity to be at least 2 x 1.38 = 2.76 cfs.
Outlet Slope (S = 1.0%)
The outlet pipe connects to the 12-inch storm drain in the street. The connecting pipe will be installed at 1.0% slope from the basin to the main line.
Ponding Depth (d = 3 inches)
We allow up to 3 inches of water depth over the grate during the peak of the design storm. This is a balance between interception capacity (more depth = more flow through the grate) and surface flooding concerns (too much depth = vehicles driving through standing water).
The Results
The calculator confirms:
- Grate interception capacity (unclogged): Approximately 3.5-4.0 cfs at 3-inch ponding depth
- Grate interception capacity (50% clogged): Approximately 1.75-2.0 cfs — adequate for the 1.38 cfs design flow
- Safety factor: 1.27-1.45 (after clogging) — acceptable
The outlet pipe (assuming 8-inch PVC at 1% slope) can carry about 0.65 cfs at full flow. Since this is less than the design flow, the basin provides temporary storage while the water drains through the outlet pipe. A 12-inch outlet pipe would be recommended to avoid surcharging.
What This Means for Your Project
Basin Sizing
- Grate: 24” x 24” flat grate (cast iron or ductile iron, rated for vehicular traffic H-20 loading)
- Basin: Precast concrete catch basin, minimum 24” x 24” internal dimensions, 36” deep
- Sump: Include a 12-inch sediment sump below the outlet pipe invert for debris collection
- Outlet: 12-inch PVC SDR 35 pipe at 1% minimum slope to the main line
Key Installation Details
- Set the grate flush with the finished pavement surface
- Grade the pavement to slope toward the basin at a minimum of 1%
- Install a frame and grate rated for the expected traffic loads (H-20 for parking lots)
- Include a silt/trash hood on the outlet pipe inside the basin if required by code
Estimated Cost
- Precast catch basin (24” x 24” x 36”): $400-800
- Cast iron grate and frame (H-20): $200-400
- Outlet pipe (12” PVC, 20 ft to main): $100-200
- Installation labor: $800-1,500
- Total: $1,500-2,900
Try This Scenario
Try this scenario in the Catch Basin CalculatorNext Steps
- Verify the downstream pipe capacity with the Manning’s Pipe Calculator to make sure the existing 12-inch main can accept the additional flow
- Check local codes for required clogging factors and grate ratings
- Consider adding a second catch basin if the parking lot has multiple low points