Advanced Catch Basin Sizing Calculator

The implemented equations are screening-level approximations. Inlet hydraulics can depend on grate pattern, splash-over velocity, local depression geometry, gutter section, roadway cross slope, curb geometry, clogging mechanism, approach flow distribution, and manufacturer test data.

1. Define the design basis

Start with the governing return period, local rainfall source, allowable roadway spread, maximum ponding depth, minimum pipe diameter, freeboard target, clogging policy, and maintenance standard. These criteria control the meaning of the final result.

2. Enter drainage areas

Use either a total drainage area, an impervious and pervious split, or the roof, pavement, and landscape fields. When detailed surface areas are present, the calculator creates a weighted runoff coefficient. Confirm that component areas agree with the total area.

3. Select hydrology

The Rational Method is suitable for many small drainage areas where local guidance allows it. The Modified Rational option applies a user-entered factor. The NRCS option estimates runoff depth before approximating a peak flow. Direct-flow mode accepts a known hydrograph peak from another model.

4. Define the inlet

Select the inlet style and placement. Enter grate clear opening information, curb opening dimensions, discharge coefficients, ponding depth, clogging allowance, and any redundancy divisor. Sag inlets are checked with weir and orifice concepts. On-grade inlets receive an additional interception-efficiency adjustment.

5. Check gutter or ponding

For roadway installations, enter longitudinal slope, cross slope, allowable spread, roughness, and local depression geometry. Compare the estimated gutter capacity with design flow. For sag locations, confirm the design ponding depth remains below the emergency overflow level.

6. Size the chamber and sump

Enter internal dimensions and sump depth. Sediment storage is based on annual loading, a bulk factor, and the cleanout interval. Add any local water-quality volume. The calculator also checks whether plan dimensions leave room for the outlet pipe and a working-space allowance.

7. Verify outlet capacity

Enter outlet diameter, count, slope, length, roughness, downstream flow, minor-loss coefficients, and tailwater. The calculator estimates full-flow capacity, velocity, losses, hydraulic grade elevation, and freeboard. A complete design may require dynamic network routing.

8. Review warnings and reports

Do not rely on the green status alone. Read every warning, examine assumptions, compare the calculated standard basin with approved products, and inspect bypass behavior across the sequential inlet table. Export the report for review and independent checking.

Inlet capacity is not chamber capacity

The grate or curb opening controls how quickly runoff enters the structure. The chamber controls temporary internal storage and physical pipe accommodation. The outlet controls how quickly captured water leaves. A design can pass one check and fail another.

Sag and on-grade conditions differ

A sag inlet may experience increasing head and transition toward orifice behavior. An on-grade inlet must intercept moving gutter flow. Splash-over, frontal flow, side flow, grate orientation, and bypass to downstream structures can become important.

Clogging should be scenario-based

Leaves, trash, sediment, snow, ice, and construction debris do not block every inlet in the same way. Run at least three cases: clean, expected blockage, and severe blockage. Critical sag locations often need redundant relief paths.

Tailwater may control the system

A large outlet pipe can still perform poorly when downstream water levels are high. Tailwater, junction losses, downstream surcharge, and reverse flow require a connected network model when they are significant.

Maintenance is part of sizing

Sump volume depends on sediment load and cleanout frequency. A larger sump does not eliminate maintenance. Access, vacuum-truck reach, confined-space requirements, traffic control, and disposal procedures should be considered.

Standard structure selection

The built-in size list is generic. Replace it with local precast catalogs, approved standard drawings, traffic-load classes, knock-out restrictions, pipe connection rules, frame and grate options, wall reinforcement, and buoyancy criteria.

Hydraulic grade line

The displayed hydraulic grade elevation is a simplified static estimate. Real systems may require junction loss methods, gradually varied flow, dynamic wave routing, pressure-flow transitions, storage nodes, pumps, control structures, and time-varying tailwater.

Structural design

This calculator does not design reinforcement, wall thickness, base slabs, top slabs, frames, grates, lifting inserts, bedding, buoyancy resistance, seismic forces, traffic loads, or construction joints. Obtain structural design where required.