Calculate the Minimum size of drain required. Also you can check if the drain size is sufficient to carry rainwater
The main formula for design is the Rational formula (which gives the amount of discharge with rainfall) and the Manning’s formula (which gives the section of drain).
However, to use the calculator tool for general estimation, you can just use the suggested values in the tool.
Rational Formula
Q = C × i × A
Q = Peak discharge (m³/s)
C = Runoff coefficient
i = Rainfall intensity (mm/hr)
A = Catchment area (hectares)
Manning’s Formula
V = (1 / n) × R2/3 × S1/2
V = Velocity of flow (m/s)
n = Manning’s roughness coefficient
R = Hydraulic radius (m)
S = Slope of energy grade line (m/m)
Q = A × V
Q = Discharge (m³/s)
A = Flow area (m²)
How to Use This Calculator
Follow this step-by-step workflow to determine the required runoff and verify your channel design.
Step 1: Calculate Peak Discharge
- Input Rational Parameters: Enter the Rainfall Intensity (i), Catchment Area (A), and Runoff Coefficient (C).
- Get Result: The calculator determines the Peak Discharge (Q). This is the amount of water your drain must handle.
Step 2: Determine Minimum Section
- Choose Section Shape: Select Rectangular, Trapezoidal, or Circular.
- Input Design Parameters: Enter the Slope (S) and Manning’s Roughness (n).
- Calculate Minimums: Based on the Peak Discharge from Step 1, the calculator computes the Minimum Required Dimensions (e.g., minimum diameter or width/depth) needed to carry that flow.
Step 3: Verify Your Design
- Select Practical Size: Choose a standard size slightly larger than the “Minimum Required” dimensions calculated in Step 2.
- Input Custom Dimensions: Enter your chosen width, depth, or diameter.
- Check Capacity: The calculator computes the capacity of your chosen section.
Rule: If Capacity > Peak Discharge, your design is SAFE.
Worked Examples
Case 1: Small Residential Drain (Detailed Walkthrough)
Scenario: Designing a concrete pipe drain for a 2-hectare residential colony. We need to find the Peak Discharge and the minimum pipe diameter required.
Step 1: Calculate Peak Discharge (Q)Using the Rational Formula: Q = (C × i × A) / 360
- Inputs:
- Runoff Coefficient (C) = 0.50 (Residential mix)
- Rainfall Intensity (i) = 100 mm/hr
- Catchment Area (A) = 2.0 hectares
- Calculation:
Numerator = 0.50 × 100 × 2.0 = 100
Q = 100 / 360 - Required Q = 0.278 m³/s
Using Manning’s Formula for a full circular pipe to find Diameter (D).
- Inputs:
- Roughness (n) = 0.013 (Concrete)
- Slope (S) = 0.005 (1 meter drop in 200m)
- Target Q = 0.278 m³/s
- The Math:
Manning’s Equation rearranged for Diameter (D) flowing full:
Q = (0.3117 / n) × D8/3 × S1/2
Rearranging for D:
D8/3 = (Q × n) / (0.3117 × √S)
Substituting values:
D8/3 = (0.278 × 0.013) / (0.3117 × 0.0707)
D8/3 = 0.00361 / 0.0220 = 0.164
Solving for D:
D = (0.164)3/8 - Calculated Minimum D: 0.518 meters
Since 0.518m is not a standard size, we select the next available standard size.
- Selection: Standard 600mm (0.60m) Pipe.
- Verify Capacity of 0.6m Pipe:
1. Area (A) = π × D² / 4 = 3.1416 × 0.6² / 4 = 0.2827 m²
2. Hydraulic Radius (R) = D / 4 = 0.6 / 4 = 0.15 m
3. Velocity (V) = (1/n) × R2/3 × S1/2
V = (1/0.013) × (0.15)0.667 × (0.005)0.5
V = 76.92 × 0.282 × 0.0707 = 1.53 m/s
4. Capacity (Q_cap) = A × V
Q_cap = 0.2827 × 1.53 = 0.433 m³/s - Result: 0.433 m³/s > 0.278 m³/s (SAFE)
Case 2: Commercial Plaza (Rectangular)
Scenario: A paved parking lot of 1.5 hectares requiring a covered concrete drain.
Step 1: Rational Method (Get Load)- Inputs: C = 0.90, i = 120 mm/hr, A = 1.5 ha
- Q = (0.90 × 120 × 1.5) / 360
- Required Q = 0.450 m³/s
- Shape: Rectangular Box Drain
- Inputs: n = 0.015 (Rough concrete), S = 0.01 (1%)
- Calculated Min Area: ≈ 0.21 m²
- Selection: Try 0.6m Wide × 0.5m Deep.
- Capacity Check: This size carries ≈ 0.65 m³/s.
- 0.65 m³/s > 0.45 m³/s (SAFE)
Case 3: Public Park (Trapezoidal Ditch)
Scenario: An earthen channel for a 10-hectare park.
Step 1: Rational Method (Get Load)- Inputs: C = 0.30, i = 80 mm/hr, A = 10.0 ha
- Q = (0.30 × 80 × 10) / 360
- Required Q = 0.667 m³/s
- Shape: Trapezoidal (Earth channel)
- Inputs: n = 0.025, S = 0.002 (1 in 500)
- Constraint: Max depth available is 0.8m.
- Selection: Base Width = 1.0m, Side Slope = 1:1, Depth = 0.8m.
- Capacity Check: This profile carries ≈ 1.50 m³/s.
- 1.50 m³/s > 0.67 m³/s (SAFE)
Case 4: Industrial Estate (Box Culvert)
Scenario: Heavy runoff area (4 hectares) requiring a large drain.
Step 1: Rational Method (Get Load)- Inputs: C = 0.80, i = 150 mm/hr, A = 4.0 ha
- Q = (0.80 × 150 × 4.0) / 360
- Required Q = 1.33 m³/s
- Shape: Rectangular
- Inputs: n = 0.013, S = 0.005
- Selection A (Failed): 0.8m x 0.8m. Capacity ≈ 1.05 m³/s. (UNSAFE)
- Selection B (Revised): Increase to 1.0m x 1.0m.
- Capacity Check: This carries ≈ 2.50 m³/s.
- 2.50 m³/s > 1.33 m³/s (SAFE)
Frequently Asked Questions
1. What is the Rational Method best used for?
It is best used for estimating peak runoff from small urban catchments (typically less than 50 hectares) where the time of concentration is short.
2. What is Manning’s ‘n’ value?
It is a coefficient representing the roughness or friction of the channel surface. Lower values (0.010) mean smooth surfaces (PVC), while higher values (0.030+) mean rough surfaces (gravel, earth).
3. How do I determine the Runoff Coefficient (C)?
This value is based on land use. Use 0.90-0.95 for pavement/roofs, 0.20-0.35 for lawns, and 0.50-0.70 for industrial areas. It represents the percentage of rain that becomes runoff.
4. What unit inputs are required?
For the Rational Method: Rainfall in mm/hr and Area in hectares. For Manning’s: Lengths in meters. The calculator outputs Discharge (Q) in cubic meters per second (m³/s).
5. What is Hydraulic Radius (R)?
It is the ratio of the cross-sectional Area (A) to the Wetted Perimeter (P). Formula: R = A / P. For a full circular pipe, R = Diameter / 4.
6. How does slope (S) affect the flow?
Slope is the energy gradient. Steeper slopes increase the velocity of the water. Since it is a square root relationship, quadrupling the slope doubles the velocity.
7. Can I use the Rational Method for large rivers?
No. The Rational Method loses accuracy on large catchments because it assumes uniform rainfall intensity over the entire area for the duration of the storm.
8. Why is velocity important in drain design?
If velocity is too low, sediment settles and blocks the drain (silting). If velocity is too high, it may damage the drain lining (scouring). Target velocities are usually between 0.6 m/s and 3.0 m/s.
9. Does this calculator handle partial flow in pipes?
This basic Manning’s calculator assumes you calculate the Hydraulic Radius (R) for the specific water level. If you input the radius of a full pipe, it calculates full-pipe flow.
10. What is 1 Hectare in square meters?
1 Hectare = 10,000 square meters.
11. How do I convert ‘C’ if my area has mixed surfaces?
You should calculate a “Weighted C”. Multiply the C of each surface by its specific area, sum them up, and divide by the total area.
12. What happens if I use the wrong ‘n’ value?
Using an ‘n’ that is too low will overestimate the capacity of your drain, potentially leading to flooding. Always be conservative with roughness estimates.
13. Is the output ‘Q’ the total volume of water?
No, Q is the rate of flow (discharge) at that specific moment, measured in cubic meters per second.
14. What is the standard formula conversion for Rational Method?
The metric formula is often written as Q = (C × i × A) / 360 to handle the unit conversion from mm/hr and hectares to m³/s.
15. Can this be used for sanitary sewer design?
Manning’s equation is used for sanitary sewers, but the Rational Method is specifically for stormwater (rain) runoff, not sewage flow.
The link below shares a drain calculator tool (in detail and accurate) just for the roadside drain in an urban setting.