French Drain Yard Drainage Calculator

Purpose and model boundary

Routes Rational or direct inflow through eight full-pipe Manning candidates and exposes capacity, velocity, trench, gravel, and cost trade-offs.

Do not treat this result as a hydraulic or plumbing design, flood-protection guarantee, code-compliance determination, permit approval, equipment selection, safety certification, cost quote, or endorsement by a cited source. Site geometry, rainfall, runoff, soil, pipe, pump, tank, irrigation, manufacturer, locally adopted code, and approval inputs remain the user's responsibility.

Inputs and units

• inflow basis: a listed route or text value.
• selection objective: a listed route or text value.
• minimum velocity check: a listed route or text value.
• catchment area acres: entered in acres.
• rainfall intensity in hr: entered in inches per hour.
• runoff coefficient: entered in fraction.
• direct design flow cfs: entered in ft^3/s.
• design flow factor: entered in factor.
• drain length ft: entered in ft.
• maximum capacity utilization: entered in fraction.
• minimum design velocity ft s: entered in ft/s.
• gravel waste factor: entered in fraction.

The candidate grid contains 8 fixed data rows plus one header row with these columns: Candidate label, Include, Pipe ID (in), Pipe OD (in), Slope (fraction), Manning n (Manning n), Trench width (in), Gravel depth (in), Gravel cost (user currency/yd^3), Pipe/install cost (user currency/ft), Fixed cost (user currency). Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

Design inflow is either entered directly or calculated by the US-customary Rational relationship:

Q_d = 1.008 C i A f_d

Q_d is in ft^3/s. C is runoff coefficient (fraction), i is rainfall intensity (in/h), A is catchment area (acres), and f_d is the entered design-flow factor.

For pipe candidate j, full-pipe area, hydraulic radius, and Manning capacity are:

A_p,j = pi (D_i,j/12)^2/4

R_h,j = D_i,j/48

Q_cap,j = (1.486/n_j) A_p,j R_h,j^(2/3) sqrt(S_j)

A_p,j is in ft^2, R_h,j is in ft, and Q_cap,j is in ft^3/s. D_i,j is inside diameter (in), n_j is Manning roughness, and S_j is slope (ft/ft). The velocity the workbook publishes and gates on is the design velocity, which carries the design inflow over the full pipe area rather than the Manning capacity. Design velocity and capacity utilization are:

v_d,j = Q_d/A_p,j

U_j = Q_d/Q_cap,j

v_d,j is in ft/s and U_j is a fraction. Full-flow velocity Q_cap,j/A_p,j is a different quantity, larger than v_d,j by the factor 1/U_j, and the workbook neither publishes nor gates on it. A self-cleaning velocity check must therefore be read against v_d,j, reported as the design velocity output for the selected candidate.

Gravel volume is trench cross-section less pipe outside area, times drain length and (1+w_g), converted to yd^3; w_g is gravel waste fraction. Total entered cost is gravel volume times gravel unit cost plus drain length times pipe/install unit cost plus fixed cost. Candidate j passes only when it is included, Q_cap,j is above zero, U_j is at or below the entered maximum capacity utilization, and, where the minimum velocity check is set to Apply, v_d,j is at or above the entered minimum design velocity. Passing rows alone are ranked by the selected objective.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• design flow cfs (ft^3/s): Peak inflow after the planning factor.
• selected candidate: Passing candidate selected by the active objective.
• selected id in (in): Inside diameter of the selected candidate.
• full flow capacity cfs (ft^3/s): Calculated full-flow capacity of the selected candidate.
• capacity utilization (fraction): Design flow divided by selected full-pipe capacity.
• design velocity ft s (ft/s): Design flow divided by the selected candidate's full pipe area. This is the value the minimum-velocity gate compares, not the Manning capacity divided by that area.
• gravel volume yd3 (yd^3): Gross trench volume less pipe displacement, including entered waste.
• estimated installed cost (user currency): Editable gravel, length, and fixed costs for the selected candidate.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• All shipped default pipe, trench, roughness, and cost values are synthetic illustrative demonstrations.

• Pipe candidates, slopes, roughness, trench geometry, and costs are synthetic editable examples.

• The pipe is screened as a full circular gravity conduit and the entered utilization factor supplies planning reserve.

• Gravel volume subtracts only pipe displacement and does not model void ratio or compaction.

• This does not predict soil infiltration, groundwater drawdown, inlet collection, sediment, freezing, outlet tailwater, erosion, or permitting.

• A qualified drainage professional must verify field elevations, soils, discharge rights, utilities, filter compatibility, maintenance, and local requirements.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 93 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• FHWA Urban Drainage Design Manual HEC-22 - Primary public source for Rational and Manning equations. Official U.S. government publication; equations are independently implemented.
• FHWA Highway Drainage Publications - Official engineering context for gravity drainage and Manning conveyance. Official U.S. government publication gateway; no table or prose is copied.
• NIST volume units - Unit-conversion provenance for cubic feet and cubic yards. Official public U.S. government reference; conversions are independently implemented and no source material is copied.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, forecast, or external dataset is embedded. The calculator independently implements general hydraulic, water-balance, and home-project engineering arithmetic from cited public and official sources; all defaults, labels, conductor/load rows, equipment assumptions, diagrams, and test values are synthetic illustrations.