Real-world drainage problems with step-by-step calculator walkthroughs. Each scenario explains what to enter, why, and what the results mean for your project. Click “Try This Scenario” to open the calculator with the values pre-filled.
Homeowner Scenarios
Sizing Drainage for a Suburban Backyard
Your backyard floods every time it rains. Learn how to calculate the runoff and determine what size drainage system you need using the Rational Method Calculator.
Drying Out a Soggy Side Yard
The narrow strip between your house and fence is always muddy. Walk through sizing a French drain to fix the problem permanently.
Building a Rain Garden for a Downspout
Redirect a roof downspout into a rain garden instead of letting it erode the yard. Size the garden for your specific conditions.
Grading a Swale Along a Property Line
Your neighbor’s runoff flows into your yard. Design a swale to intercept and redirect the water along the property line.
Contractor Scenarios
Will This 6-Inch Pipe Handle My Runoff?
A client asks if their existing 6-inch pipe is large enough. Use Manning’s equation to check the capacity and recommend an upgrade if needed.
Detention Pond for a Small Development
Size a detention facility for a 5-acre residential subdivision to meet post-development runoff requirements.
Selecting a Catch Basin for a Parking Lot
Choose the right catch basin size and spacing for a commercial parking lot with known drainage areas.
Runoff Estimate for a Mixed-Use Site
Calculate the curve number and runoff volume for a site with multiple land cover types using the SCS method.
How Long Does Runoff Take to Reach the Outlet?
Determine the time of concentration for a small watershed to use as input to the Rational Method.
Sizing Drains for a Flat Commercial Roof
Calculate the number and size of roof drains needed for a flat-roof commercial building.