Landscape Grading & Drainage Slope Calculator

ANA›Life Services Authority›National Calculator Authority›Landscape Grading & Drainage Slope Calculator

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Landscape Grading & Drainage Slope Calculator

Calculate slope percentage, grade ratio, elevation change, and estimated surface runoff flow for landscape grading and drainage design.

Elevation Change / Rise (ft or m):

Horizontal Distance / Run (ft or m):

Drainage Area (sq ft or sq m):

Rainfall Intensity (in/hr or mm/hr):

Runoff Coefficient (C, 0–1):

Lawn ≈ 0.25–0.35 | Paved ≈ 0.70–0.95 | Bare soil ≈ 0.50–0.60

Unit System:

Imperial (ft, in/hr, sq ft) Metric (m, mm/hr, sq m)

Calculate

function lanCalc() { var rise = parseFloat(document.getElementById('lan-rise').value); var run = parseFloat(document.getElementById('lan-run').value); var area = parseFloat(document.getElementById('lan-area').value); var rainfall = parseFloat(document.getElementById('lan-rainfall').value); var C = parseFloat(document.getElementById('lan-runoff').value); var unit = document.getElementById('lan-unit').value; var resultDiv = document.getElementById('lan-result');

// --- Validation --- var errors = []; if (isNaN(rise) || rise 1)) errors.push("Runoff coefficient (C) must be between 0 and 1.");

if (errors.length > 0) { resultDiv.style.display = 'block'; resultDiv.innerHTML = 'Input Error:' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// --- Core Slope Calculations --- // Slope percentage: (rise / run) * 100 var slopePct = (rise / run) * 100;

// Slope ratio: 1 : (run / rise) — expressed as "1 in X" var slopeRatio = (rise > 0) ? (run / rise) : Infinity;

// Slope angle in degrees: arctan(rise / run) var slopeAngleDeg = Math.atan(rise / run) * (180 / Math.PI);

// Slope in decimal form var slopeDecimal = rise / run;

// --- Minimum recommended slopes --- // General landscape: 2% minimum away from structures // Swales/channels: 1% minimum // Paved surfaces: 0.5% minimum (ADA allows 1%) var minLandscape = 2.0; var minSwale = 1.0; var minPaved = 0.5;

var slopeStatus = ""; if (slopePct ⚠ Below minimum for any drainage ('; } else if (slopePct ⚠ Marginal — acceptable only for paved surfaces (0.5%–1.0%).'; } else if (slopePct ⚠ Acceptable for swales/channels but below 2% landscape minimum.'; } else if (slopePct ✓ Good — within ideal landscape grading range (2%–5%).'; } else if (slopePct ⓘ Steep slope (5%–10%). Erosion control measures recommended.'; } else { slopeStatus = '⚠ Very steep (> 10%). Significant erosion risk. Retaining structures may be needed.'; }

// --- Rational Method: Peak Runoff Flow Q = C * i * A --- // Imperial: Q (cfs) = C * i (in/hr) * A (acres) → A_acres = area_sqft / 43560 // Metric: Q (m³/s) = C * i (m/s) * A (m²) → i_m_s = rainfall_mm_hr / (1000 * 3600) var flowHTML = ""; var hasFlow = !isNaN(area) && area > 0 && !isNaN(rainfall) && rainfall > 0 && !isNaN(C);

if (hasFlow) { var Q, flowUnit; if (unit === 'imperial') { var areaAcres = area / 43560; Q = C * rainfall * areaAcres; // cfs flowUnit = 'cfs (cubic feet per second)'; } else { var i_m_s = rainfall / (1000 * 3600); // mm/hr → m/s Q = C * i_m_s * area; // m³/s flowUnit = 'm³/s (cubic meters per second)'; }

// --- Manning's Equation: Estimated flow velocity in a grass swale --- // V = (1/n) * R^(2/3) * S^(1/2) // Assume: n = 0.035 (short grass), hydraulic radius R = 0.1 m (≈ 0.33 ft) for small swale var n_manning = 0.035; var R_m = 0.1; // hydraulic radius in metres var R_ft = 0.328; var S = slopeDecimal;

var V_m_s = (1 / n_manning) * Math.pow(R_m, 2/3) * Math.pow(S, 0.5); var V_ft_s = (1.486 / n_manning) * Math.pow(R_ft, 2/3) * Math.pow(S, 0.5);

flowHTML = 'Runoff & Flow (Rational Method)' + 'Drainage Area' + area.toLocaleString() + (unit === 'imperial' ? ' sq ft (' + (area/43560).toFixed(4) + ' acres)' : ' m²') + '' + 'Rainfall Intensity' + rainfall + (unit === 'imperial' ? ' in/hr' : ' mm/hr') + '' + 'Runoff Coefficient (C)' + C.toFixed(2) + '' + 'Peak Runoff Flow (Q)' + Q.toFixed(4) + ' ' + flowUnit + '' + 'Manning's Velocity (Grass Swale, n=0.035)' + 'Estimated Flow Velocity' + V_m_s.toFixed(3) + ' m/s  |  ' + V_ft_s.toFixed(3) + ' ft/s' + '' + 'Velocity Status' + (V_m_s Low — sediment deposition risk' : V_m_s Good — non-erosive velocity' : 'High — erosion risk, consider lining or check dams') + ''; }

// --- Build Result Table --- var unitLabel = unit === 'imperial' ? 'ft' : 'm'; resultDiv.style.display = 'block'; resultDiv.innerHTML = '### Results ' + '' + '' + 'ParameterValue' + 'Slope & Grade' + 'Rise (Elevation Change)' + rise.toFixed(3) + ' ' + unitLabel + '' + 'Run (Horizontal Distance)' + run.toFixed(3) + ' ' + unitLabel + '' + 'Slope Percentage' + slopePct.toFixed(2) + '%' + 'Slope Ratio (1 in X)' + (slopeRatio === Infinity ? 'Flat (no rise)' : '1 in ' + slopeRatio.toFixed(1)) + '' + 'Slope Angle' + slopeAngleDeg.toFixed(2) + '°' + 'Slope (Decimal)' + slopeDecimal.toFixed(5) + '' + 'Slope Assessment' + slopeStatus + '' + flowHTML + 'Reference Minimums' + 'Min. slope — landscape (away from structures)2.0%' + 'Min. slope — drainage swales1.0%' + 'Min. slope — paved / hardscape surfaces0.5%' + 'Ideal landscape grading range2% – 5%' + ''; }

#### Formulas Used

Slope Percentage: Slope (%) = (Rise / Run) × 100

Slope Ratio: Ratio = 1 : (Run / Rise) — read as "1 unit rise per X units of run"

Slope Angle: θ = arctan(Rise / Run)

Rational Method — Peak Runoff Flow: Q = C × i × A Where: Q = peak flow (cfs or m³/s), C = runoff coefficient (dimensionless), i = rainfall intensity (in/hr or m/s), A = drainage area (acres or m²)

Manning's Equation — Flow Velocity: V = (1/n) × R2/3 × S1/2 (SI)  |  V = (1.486/n) × R2/3 × S1/2 (Imperial) Where: n = Manning's roughness coefficient, R = hydraulic radius (m or ft), S = slope (decimal)

#### Assumptions & References

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References

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