Inlet Capacity Gutter Calculator

• Triangular gutter flow (screening): Q = (K/n) · √S · Sx^(5/3) · T^(8/3). Typical constants: K≈0.56 (US) and K≈0.375 (SI).
• Curb opening weir: Qw = Cw · L · h^(3/2), where L is opening length and h is ponding depth.
• Grate orifice: Qo = Cd · A · √(2 g h), where A is open area and g is gravity.
• Total theoretical capacity: Qt = Qw + Qo (combination), or the applicable term only.
• Effective captured capacity: Qcap = Qt · (1 − clog) · E.
• Design demand: Qd = Q · safety, and Inlets = ceil(Qd / Qcap).

These relationships are common for preliminary checks. Detailed inlet capture can depend on approach velocity, gutter depression, splash-over, inlet geometry, and agency criteria.

1. Select a unit system and choose a flow method.
2. If using spread-based flow, enter n, slopes, and spread T.
3. Select inlet configuration and enter ponding depth h.
4. Provide curb opening length L and/or grate area A.
5. Set efficiency, clogging allowance, and safety factor to match your practice.
6. Press Calculate. Review inlet count and optional spacing.
7. Use the export buttons to download CSV or PDF for documentation.

Drainage intent and performance targets

Inlet capacity is used to confirm that gutter flow is intercepted before spread reaches critical lanes, sidewalks, or building lines. This calculator supports preliminary checks by pairing approach flow with inlet hydraulics, then applying practical allowances for clogging and capture efficiency. Use it early to screen layouts and later to document consistent assumptions.

Inputs that control inlet capacity

Ponding depth is a primary driver because both weir and orifice relationships increase rapidly with depth. Curb-opening length affects the weir component, while grate open area affects the orifice component. Coefficients represent inlet geometry and approach effects. Keep units consistent and confirm that depth is measured at the inlet throat location.

Effective captured capacity and reliability

Theoretical capacity is reduced to an effective captured capacity by applying clogging allowance and capture efficiency. Clogging accounts for leaves, sediment, and debris, while efficiency reflects bypass and approach flow patterns. Increasing these conservatism factors reduces capacity, which can increase the recommended number of inlets and improve robustness during intense storms.

Spacing, maintenance, and constructability

When a total run length is provided, spacing is estimated by distributing the recommended inlet count along the run. Final spacing should also consider low points, sag vertical curves, driveway crossings, utility conflicts, and maintenance access. If frequent clogging is expected, reduce spacing or select inlet types that are easier to clean.

Example data for documentation

The following example set can be copied into design notes for review. It demonstrates a combination inlet with modest ponding depth and typical adjustment factors. Your governing criteria may specify different coefficients or safety factors.

• Unit system: SI, Flow Q: 0.030 m³/s (direct input)
• Ponding depth h: 0.10 m, Curb length L: 1.20 m, Grate area A: 0.040 m²
• Coefficients: Cw 1.84, Cd 0.62, Efficiency 0.80, Clogging 0.15, Safety 1.10
• Output snapshot: Qt ≈ 0.086 m³/s, Effective Qcap ≈ 0.058 m³/s, Inlets = 1

1) When should I use direct flow instead of spread-based flow?
Use direct flow when hydrology or a network model provides inlet approach discharge. Use spread-based flow for quick screening when you only have slopes, roughness, and target spread for the gutter section.

2) What does capture efficiency represent?
Capture efficiency approximates the portion of approach flow intercepted by the inlet. It implicitly includes bypass, splash-over, and approach alignment effects. Local standards may recommend typical values by inlet type and roadway class.

3) Why include a clogging allowance?
Debris and sediment reduce open area and create partial blockages. A clogging allowance introduces conservatism so inlet count remains adequate during adverse conditions. Use higher values in leafy corridors, industrial areas, and low-maintenance routes.

4) How do I select weir and orifice coefficients?
Coefficients depend on geometry and approach conditions. Start with typical practice values and then update using agency guidance or manufacturer data. If detailed inlet capture curves are available, calibrate coefficients to match those references.

5) Can the calculator replace agency inlet capture methods?
No. It is best for preliminary checks and documentation. Final design should follow the governing standard for inlet types, spread limits, bypass evaluation, and sag/grade inlet selection using the required capture methodology.

6) What does the safety factor do?
The safety factor increases the design demand above the approach flow to provide a margin for uncertainty. Raising it increases the recommended inlet count. Choose a factor consistent with risk tolerance and project criticality.

7) How should I interpret the recommended spacing?
Spacing is a simple run-length distribution of the recommended inlet count. Adjust it to place inlets at low points and where bypass would be unacceptable. Confirm field constraints and maintenance access before finalizing locations.