Calculator Inputs
Formula Used
Weighted runoff coefficient: Cw = Impervious fraction × 0.90 + Pervious fraction × 0.25.
Raw water quality volume: WQV = Drainage area × Water quality depth × Runoff coefficient.
Design water quality volume: Design WQV = WQV × Safety factor.
Drawdown flow: Flow = Design WQV ÷ Drawdown hours.
Storage credit depth: Credit = Ponding depth + Media depth × Void ratio + Infiltration rate × Drawdown time.
Required bottom area: Area = Design WQV ÷ Storage credit depth.
Pollutant load: Load kg = Concentration mg/L × Design volume m³ ÷ 1000.
Orifice diameter: Diameter comes from Q = Cd × A × √(2gh).
How to Use This Calculator
Enter the drainage area and choose the matching unit. Add the water quality rainfall depth from the project standard. Enter a manual runoff coefficient or use the weighted impervious option. Then add storage, soil, drawdown, forebay, pollutant, and cost values. Press calculate to show results below the header and above the input form. Use CSV for spreadsheets. Use PDF for a quick report.
Example Data Table
| Input | Example Value | Purpose |
|---|---|---|
| Drainage area | 10,000 m² | Defines contributing site area. |
| Water quality depth | 25 mm | Defines treatment rainfall depth. |
| Runoff coefficient | 0.85 | Converts rainfall into runoff. |
| Safety factor | 1.15 | Adds design allowance. |
| Drawdown time | 24 hours | Controls outlet release rate. |
| Media depth | 0.45 m | Adds void storage volume. |
| Infiltration rate | 10 mm/hr | Adds soil disposal capacity. |
Construction Water Quality Sizing Guide
Purpose of Water Quality Sizing
Water quality sizing helps construction teams estimate treatment volume before runoff leaves a site. It supports detention basins, bioretention cells, sediment forebays, infiltration beds, and underground storage. The goal is simple. Capture the frequent runoff event. Hold it long enough for treatment. Release it at a controlled rate.
Core Design Inputs
The drainage area is the first driver. A larger area creates more runoff. The water quality depth is the rainfall depth selected by the local manual or permit. The runoff coefficient reflects surface cover. Pavement, roofs, and compacted construction areas create higher values. Landscaped areas produce lower values. The calculator also estimates a weighted coefficient from impervious percentage.
Storage and Drawdown
Design volume is increased by the safety factor. This allows for survey tolerance, sediment buildup, and uncertain surface conditions. Drawdown time converts volume into a required release rate. A short drawdown period needs a larger outlet. A long drawdown period needs less flow, but may keep the facility wet for longer.
Footprint Planning
The basin footprint depends on available storage depth. Surface ponding stores water above the bed. Media or stone stores water in voids. Infiltration adds disposal capacity during the drawdown period. These credits create the storage depth used to size the bottom area. The side slope and freeboard then estimate the top area.
Pollutant and Cost Review
The pollutant calculation estimates load from concentration and volume. The removal value shows expected capture based on selected efficiency. This helps compare alternatives during early design. The cost output multiplies design volume by a unit cost. It is not a bid. It is a planning value for screening options.
Practical Use
Use this tool during concept design, submittal checks, and value comparisons. Confirm final criteria with local codes, rainfall data, geotechnical testing, and engineer review. Field conditions can change quickly on construction sites. Update inputs when grading, soil exposure, or impervious cover changes.
FAQs
What is water quality volume?
It is the runoff volume selected for treatment. It normally represents a frequent rainfall event that carries sediment, nutrients, oils, and construction pollutants.
Can this calculator size a detention basin?
It can estimate water quality storage and drawdown flow. Flood detention and peak routing need separate hydrologic modeling.
What runoff coefficient should I use?
Use a value from the local design manual. For quick planning, use the weighted impervious method or enter a project-specific coefficient.
Why is a safety factor included?
It adds allowance for sediment, construction variation, survey tolerance, and uncertain infiltration performance during early design.
What does drawdown time mean?
Drawdown time is the period allowed to empty the water quality volume through infiltration, outlet control, or both.
Is infiltration always allowed?
No. Infiltration depends on soil tests, groundwater separation, contamination risk, and local rules. Confirm suitability before final design.
What is forebay volume?
Forebay volume is a portion of storage reserved near the inlet. It helps trap coarse sediment before flow reaches the main cell.
Are the exported files final engineering documents?
No. They are calculation summaries for review. A qualified designer should verify assumptions, codes, survey data, and site constraints.