Dewatering Rate Calculator
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Dewatering Rate Calculator
Calculated Pump Requirements
Total Water Volume to Remove:
0 m³
Required Flow Rate (Metric):
0 m³/hr
Required Flow Rate (Liters):
0 L/min
Required Flow Rate (US):
0 GPM
Recommendation:
How to Calculate Dewatering Rates
Dewatering is a critical process in construction, mining, and wastewater management involving the removal of groundwater or surface water from a job site. Calculating the correct dewatering rate is essential for sizing pumps effectively, ensuring project timelines are met, and maintaining structural stability of excavations.
The Dewatering Formula
The fundamental calculation for dewatering involves determining the total volume of water to be moved and dividing it by the available time window. However, unlike simple tank emptying, excavation dewatering must account for the nature of the soil (porosity) and groundwater recharge.
The basic formula used in this calculator is:
Q = (V × P × SF) / T
Where:
- Q: Required Flow Rate (m³/hr)
- V: Volume of the excavation (Length × Width × Depth)
- P: Porosity or Specific Yield (percentage of volume occupied by water)
- SF: Safety Factor (multiplier to account for continuous groundwater inflow/recharge)
- T: Time allotted for dewatering (hours)
Understanding Inputs
- Soil Porosity (Specific Yield): If you are pumping water out of an open pit (like a swimming pool or a flooded basement), the porosity is 100%. If you are lowering the water table within soil, you only pump the water contained in the voids.
- Gravel: ~25%
- Sand: ~20-30%
- Clay: ~3-5%
- Safety Factor / Recharge: In most excavation scenarios, water doesn't just sit there; new groundwater flows in as you pump. A safety factor of 1.5 (50% extra capacity) is standard to account for this recharge and pump inefficiencies.
- Target Duration: The speed at which you dewater affects soil stability. Rapid drawdown can cause bank collapse in certain soils. Always consult a geotechnical engineer for maximum safe drawdown rates.
Pump Sizing and Selection
Once you have the required flow rate (GPM or m³/hr), you must select a pump that operates efficiently at that point on its curve. Note that this calculator determines the flow rate required. You must also calculate the Total Dynamic Head (TDH)—the vertical distance the water must be lifted plus friction losses in the pipe—to finalize your pump selection.
Common Dewatering Methods
Depending on the depth and soil permeability calculated above, different methods may be used:
- Sump Pumping: For shallow excavations in coarse soil.
- Wellpoints: For lowering groundwater levels in sandy soil (up to 5-6m depth).
- Deep Wells: For deep excavations with high volume requirements.
function calculateDewatering() {
// 1. Get Input Values
var length = parseFloat(document.getElementById('length').value);
var width = parseFloat(document.getElementById('width').value);
var depth = parseFloat(document.getElementById('depth').value);
var porosityInput = parseFloat(document.getElementById('porosity').value);
var time = parseFloat(document.getElementById('time').value);
var safetyFactor = parseFloat(document.getElementById('safetyFactor').value);
// 2. Validation
if (isNaN(length) || length <= 0 ||
isNaN(width) || width <= 0 ||
isNaN(depth) || depth < 0 ||
isNaN(time) || time <= 0 ||
isNaN(porosityInput) || porosityInput < 0) {
alert("Please enter valid positive numbers for all dimensions and time.");
return;
}
// 3. Logic and Calculation
// Calculate raw geometric volume in cubic meters
var geoVolume = length * width * depth;
// Apply porosity (convert percentage to decimal)
// If 100%, we take full volume. If 20%, we take 0.2 of volume.
var porosityFactor = porosityInput / 100;
var waterVolumeStatic = geoVolume * porosityFactor;
// Apply Safety Factor (Recharge allowance)
// Total Volume to pump = Static Volume * Safety Factor
var totalVolumeM3 = waterVolumeStatic * safetyFactor;
// Calculate Flow Rates
// m3 per hour
var flowRateM3H = totalVolumeM3 / time;
// Liters per minute: (m3 * 1000) / 60
var flowRateLMin = (flowRateM3H * 1000) / 60;
// US Gallons per minute: L/min * 0.264172
var flowRateGPM = flowRateLMin * 0.264172;
// 4. Update UI
document.getElementById('resVolumeM3').innerHTML = totalVolumeM3.toLocaleString(undefined, {maximumFractionDigits: 1}) + " m³";
document.getElementById('resFlowM3H').innerHTML = flowRateM3H.toLocaleString(undefined, {maximumFractionDigits: 1}) + " m³/hr";
document.getElementById('resFlowLMin').innerHTML = flowRateLMin.toLocaleString(undefined, {maximumFractionDigits: 0}) + " L/min";
document.getElementById('resFlowGPM').innerHTML = flowRateGPM.toLocaleString(undefined, {maximumFractionDigits: 0}) + " GPM";
// Generate Recommendation text
var recText = "";
if(flowRateGPM < 50) {
recText = "Small portable submersible pumps or trash pumps likely sufficient.";
} else if (flowRateGPM < 500) {
recText = "Medium duty 3-4 inch trash pump or wellpoint system required.";
} else {
recText = "High capacity 6+ inch dewatering pumps or deep well system required.";
}
document.getElementById('resRec').innerText = recText;
// Show result section
document.getElementById('resultOutput').style.display = 'block';
}