Rainfall Accumulation and Flood Risk Estimator

ANA›Life Services Authority›National Calculator Authority›Rainfall Accumulation and Flood Risk Estimator

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Rainfall Accumulation and Flood Risk Estimator

Estimates total rainfall volume, runoff volume, and flood risk level based on rainfall intensity, duration, catchment area, and surface runoff coefficient.

Rainfall Intensity (mm/hr)

Typical: light <2.5, moderate 2.5–7.5, heavy 7.5–50, extreme >50 mm/hr

Storm Duration (hours)

Duration of the rainfall event (0.1 – 720 hours)

Catchment Area (km²)

Total land area draining to the point of interest

Surface Runoff Coefficient (C)

-- Select surface type -- Impervious (concrete/asphalt) — C = 0.95 Urban residential (dense) — C = 0.80 Urban residential (sparse) — C = 0.65 Mixed urban/suburban — C = 0.50 Grassland / pasture — C = 0.35 Forest / woodland — C = 0.25 Sandy / well-drained soil — C = 0.15 Clay / poorly drained soil — C = 0.70

Rational Method coefficient (0 = all absorbed, 1 = all runs off)

Antecedent Soil Moisture

Dry (no recent rain) — factor 1.0 Moist (rain within 5 days) — factor 1.2 Saturated (heavy recent rain) — factor 1.5

Prior soil moisture increases effective runoff

Calculate Flood Risk

function raiCalc() { // --- Inputs --- var intensity = parseFloat(document.getElementById('rai-intensity').value); var duration = parseFloat(document.getElementById('rai-duration').value); var area = parseFloat(document.getElementById('rai-area').value); var C = parseFloat(document.getElementById('rai-runoff').value); var amf = parseFloat(document.getElementById('rai-antecedent').value);

var resultDiv = document.getElementById('rai-result'); resultDiv.style.display = 'block';

// --- Validation --- var errors = []; if (isNaN(intensity) || intensity 500) errors.push("Rainfall intensity seems unrealistically high (max 500 mm/hr)."); if (!isNaN(duration) && duration > 720) errors.push("Duration cannot exceed 720 hours (30 days)."); if (!isNaN(area) && area > 100000) errors.push("Catchment area cannot exceed 100,000 km².");

if (errors.length > 0) { resultDiv.innerHTML = 'Please fix the following:' + errors.map(function(e){ return ''; }).join('') + ''; return; }

// --- Core Calculations ---

// 1. Total rainfall depth (mm) var rainfallDepth = intensity * duration;

// 2. Total rainfall volume over catchment (m³) // area km² → m²: × 1,000,000; depth mm → m: ÷ 1000 var rainfallVolume = rainfallDepth * area * 1000; // m³

// 3. Effective runoff coefficient (capped at 1.0) var C_eff = Math.min(C * amf, 1.0);

// 4. Runoff volume (m³) — Rational Method extended var runoffVolume = C_eff * rainfallVolume;

// 5. Peak discharge (m³/s) — Rational Method: Q = C·i·A // i in m/s = intensity(mm/hr) / 3,600,000 // A in m² = area(km²) × 1,000,000 var peakDischarge = C_eff * (intensity / 3600000) * (area * 1e6); // m³/s

// 6. Flood Risk Score (0–100) // Weighted composite of normalised sub-scores var scoreIntensity = Math.min(intensity / 100, 1.0); // normalise to 100 mm/hr extreme var scoreDuration = Math.min(duration / 48, 1.0); // normalise to 48-hr event var scoreRunoff = C_eff; // already 0–1 var scoreDepth = Math.min(rainfallDepth / 200, 1.0); // normalise to 200 mm depth var scoreDischarge = Math.min(peakDischarge / 500, 1.0); // normalise to 500 m³/s

var floodScore = ( scoreIntensity * 0.25 + scoreDuration * 0.15 + scoreRunoff * 0.25 + scoreDepth * 0.20 + scoreDischarge * 0.15 ) * 100;

// 7. Risk category var riskLabel, riskColor, riskAdvice; if (floodScore ' + 'ParameterValue' + 'Total Rainfall Depth' + fmt(rainfallDepth, 1) + ' mm' + 'Total Rainfall Volume' + fmt(rainfallVolume, 0) + ' m³ (' + fmt(rainfallVolume/1e6, 3) + ' GL)' + 'Effective Runoff Coefficient (Ceff)' + fmt(C_eff, 3) + '' + 'Estimated Runoff Volume' + fmt(runoffVolume, 0) + ' m³ (' + fmt(runoffVolume/1e6, 3) + ' GL)' + 'Peak Discharge (Rational Method)' + fmt(peakDischarge, 2) + ' m³/s' + '' +

'' + 'Flood Risk Score: ' + fmt(floodScore, 1) + ' / 100' + '' + '' + '' + '' +

'' + 'Risk Level: ' + riskLabel + '' + '' + riskAdvice + '' + ''; }

#### Formulas Used

1. Total Rainfall Depth (mm): D = I × t where I = intensity (mm/hr), t = duration (hr)

2. Rainfall Volume (m³): Vrain = D × A × 1000 where A = catchment area (km²)

3. Effective Runoff Coefficient: Ceff = min(C × AMF, 1.0) where C = Rational Method surface coefficient, AMF = antecedent moisture factor

4. Runoff Volume (m³): Vrunoff = Ceff × Vrain

5. Peak Discharge — Rational Method (m³/s): Q = Ceff × i × A where i = intensity (m/s), A = area (m²)

6. Flood Risk Score (0–100): Score = 25·Sintensity + 15·Sduration + 25·Srunoff + 20·Sdepth + 15·Sdischarge Each sub-score is normalised to [0, 1] against reference extreme values.

#### Assumptions & References

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References

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