Rainwater Runoff Calculator

ANA›Life Services Authority›National Calculator Authority›Rainwater Runoff Calculator

.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }

Rainwater Runoff Calculator

Estimate stormwater runoff volume and peak flow rate using the Rational Method (Q = CiA) and the SCS Curve Number method.

Drainage Area (acres)

Rainfall Intensity (inches/hour)

Storm Duration (hours)

Surface Type

Paved / Impervious (C = 0.95) Rooftops (C = 0.85) Gravel / Crushed Stone (C = 0.70) Lawns / Grass (C = 0.55) Pasture / Meadow (C = 0.40) Wooded / Forest (C = 0.25) Custom Runoff Coefficient

Custom Runoff Coefficient C (0.01 – 1.00)

SCS Curve Number (CN, 1–100)

Common values: Impervious=98, Lawns good condition=61, Woods=55, Pasture=68

Calculate

document.getElementById('rai-surface').addEventListener('change', function() { var customGroup = document.getElementById('rai-custom-c-group'); customGroup.style.display = this.value === 'custom' ? 'block' : 'none'; });

function raiCalc() { var resultDiv = document.getElementById('rai-result'); resultDiv.style.display = 'none';

var area = parseFloat(document.getElementById('rai-area').value); var intensity = parseFloat(document.getElementById('rai-rainfall').value); var duration = parseFloat(document.getElementById('rai-duration').value); var surfaceVal = document.getElementById('rai-surface').value; var cnVal = parseFloat(document.getElementById('rai-cn').value);

// Validation if (isNaN(area) || area ⚠ Please enter a valid drainage area greater than 0.'; resultDiv.style.display = 'block'; return; } if (isNaN(intensity) || intensity ⚠ Please enter a valid rainfall intensity greater than 0.'; resultDiv.style.display = 'block'; return; } if (isNaN(duration) || duration ⚠ Please enter a valid storm duration greater than 0.'; resultDiv.style.display = 'block'; return; } if (isNaN(cnVal) || cnVal 100) { resultDiv.innerHTML = '⚠ Please enter a valid SCS Curve Number between 1 and 100.'; resultDiv.style.display = 'block'; return; }

var C; if (surfaceVal === 'custom') { C = parseFloat(document.getElementById('rai-custom-c').value); if (isNaN(C) || C 1.00) { resultDiv.innerHTML = '⚠ Custom runoff coefficient must be between 0.01 and 1.00.'; resultDiv.style.display = 'block'; return; } } else { C = parseFloat(surfaceVal); }

// ── Rational Method ────────────────────────────────────────────────────────── // Q (cfs) = C × i (in/hr) × A (acres) var Q_cfs = C * intensity * area; // Volume = Q × duration converted to cubic feet // Q in cfs × duration in hours × 3600 s/hr = cubic feet var volume_cf = Q_cfs * duration * 3600; // Convert to gallons (1 cf = 7.48052 gal) var volume_gal = volume_cf * 7.48052; // Convert to acre-feet (1 acre-ft = 43560 cf) var volume_acft = volume_cf / 43560;

// ── SCS Curve Number Method ────────────────────────────────────────────────── // Total rainfall depth P = intensity × duration (inches) var P = intensity * duration; // Potential maximum retention S (inches) var S = (1000 / cnVal) - 10; // Initial abstraction Ia = 0.2 × S var Ia = 0.2 * S; // Runoff depth Q_scs (inches) — only if P > Ia var Q_scs_in = 0; if (P > Ia) { Q_scs_in = Math.pow(P - Ia, 2) / (P - Ia + S); } // Runoff volume over area (acre-inches → acre-feet: divide by 12) var Q_scs_acft = (Q_scs_in * area) / 12; var Q_scs_cf = Q_scs_acft * 43560; var Q_scs_gal = Q_scs_cf * 7.48052;

// ── Format output ──────────────────────────────────────────────────────────── function fmt(n, d) { return n.toLocaleString('en-US', {minimumFractionDigits: d, maximumFractionDigits: d}); }

var html = '### Results ';

html += '#### Rational Method (Q = C·i·A) '; html += ''; html += 'Runoff Coefficient (C)' + fmt(C, 2) + ''; html += 'Peak Flow Rate (Q)' + fmt(Q_cfs, 3) + ' cfs'; html += 'Runoff Volume' + fmt(volume_cf, 1) + ' ft³'; html += 'Runoff Volume' + fmt(volume_gal, 0) + ' gallons'; html += 'Runoff Volume' + fmt(volume_acft, 4) + ' acre-feet'; html += '';

html += '#### SCS Curve Number Method '; html += ''; html += 'Total Rainfall (P)' + fmt(P, 3) + ' in'; html += 'Max Retention (S)' + fmt(S, 3) + ' in'; html += 'Initial Abstraction (Ia = 0.2S)' + fmt(Ia, 3) + ' in'; html += 'Runoff Depth (Q)' + fmt(Q_scs_in, 4) + ' in'; html += 'Runoff Volume' + fmt(Q_scs_cf, 1) + ' ft³'; html += 'Runoff Volume' + fmt(Q_scs_gal, 0) + ' gallons'; html += 'Runoff Volume' + fmt(Q_scs_acft, 4) + ' acre-feet'; html += '';

if (P

#### Formulas Used

Rational Method

Q = C × i × A

SCS Curve Number Method (NRCS TR-55)

S = (1000 / CN) − 10

Ia = 0.2 × S

Q = (P − Ia)² / (P − Ia + S) [when P > Ia, else Q = 0]

Unit Conversions

#### Assumptions & References

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References

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