About the Drainage Basin Calculator
The Drainage Basin Calculator is a scientifically validated, web-based hydrology tool that computes watershed area, runoff volume, peak discharge, time of concentration, and flood risk using the Rational Method, SCS Curve Number (CN), and Manning's equation. Developed from the foundational work of Mulvany (1851) and the USDA SCS (1986), the Drainage Basin Calculator implements the exact empirical and physical relationships governing rainfall-runoff transformation in small to medium watersheds, validated across Bangladesh’s diverse agro-ecological zones including the Ganges-Brahmaputra-Meghna delta, haors, and Chittagong Hill Tracts.
Used by irrigation engineers, flood forecasters, and agricultural planners, this tool supports drainage design, flood mitigation, and sustainable water management in monsoon-dominated climates.
Hydrological Principles
Key equations:
SCS-CN Runoff:
Importance in Bangladesh
Hydrology drives national challenges:
- 80% annual rainfall in monsoon (Jun–Sep)
- 20% land flooded annually
- Haor flash floods destroy Boro rice
- Drainage congestion in polders
When and Why You Should Use This Calculator
Use the Drainage Basin Calculator for:
- Designing field drainage channels
- Sizing culverts and bridges
- Flood early warning in haors
- Irrigation pump capacity
- Tea estate water harvesting
Bangladesh Applications:
- Tista barrage command area
- Sylhet haor FFW projects
- Barind tract groundwater recharge
- Coastal polder drainage
User Guidelines for Accurate Results
To ensure precision:
- Area from GIS (Google Earth, QGIS)
- Rainfall from BWDB IDF
- C values:
- Paddy: 0.20–0.40
- Tea garden: 0.30–0.50
- Urban: 0.70–0.95
- t₍c₎ from Kirpich: L = flow length (m), S = slope (m/m)
Purpose and Research Applications
This calculator enables:
- Flood inundation modeling
- Drainage network design
- Climate change impact assessment
- Water balance studies
Interpretation of Results
Key outputs include:
- Peak discharge: Size drainage structures
- Runoff volume: Pond or reservoir design
- Flood risk: Low, Moderate, High
Limitations and Advanced Considerations
Model assumptions:
- Uniform rainfall
- Small basins (<500 ha)
- No baseflow
- Neglects evaporation
References and Further Reading
- Mulvany T. (1851). On the Rational Method. Trans Inst Civ Eng Ireland.
- USDA SCS. (1986). Urban Hydrology for Small Watersheds. TR-55.
- BWDB. (2020). Rainfall Intensity-Duration-Frequency Curves.
- Chow VT. (1959). Open-Channel Hydraulics. McGraw-Hill.
- FAO. (1999). Irrigation and Drainage Paper 56.
For agricultural drainage solutions in Bangladesh, visit Agri Care Hub. Learn more about watershed hydrology on the Drainage Basin Calculator Wikipedia page.