What This Solves
Calculates optimal spacing between area drains in flat or gently-sloped surfaces based on allowable ponding depth, surface slope, and drain capacity.
Best Used When
- You are laying out area drains in a parking lot, plaza, or athletic field
- You need to determine the maximum spacing that keeps ponding below an allowable depth
- You want to verify that existing drain spacing will prevent nuisance flooding on a flat surface
Do NOT Use When
- You need a linear trench drain rather than individual point drains — Use Trench Drain Calculator
- You are designing a roadway inlet system with curb and gutter — Use Inlet Interception Calculator
Key Assumptions
- Surface slope is uniform between drains
- Sheet flow analysis applies (shallow overland flow to each drain)
- Each drain captures flow from its tributary area without bypass
- Drains are not clogged and operate at full rated capacity
Input Quality Notes
Surface slope is critical — verify with field survey. Even small low spots between drains can cause ponding. Use conservative (lower) drain capacities for areas prone to debris.
Calculate Area Drain Spacing
For educational purposes only. Not a substitute for professional engineering judgment.
Area Drain Spacing Overview
Area drain spacing determines the maximum distance between drain inlets to prevent excessive ponding and ensure timely drainage. Spacing is governed by three constraints:
- Ponding Depth - Maximum allowable water depth on the surface
- Travel Time - Time for water to flow to the nearest drain
- Drain Capacity - Inlet capacity must exceed contributing runoff
Surface Manning's n Values
| Surface Type | Min | Typical | Max |
|---|---|---|---|
| Smooth Concrete | 0.01 | 0.011 | 0.013 |
| Broom-Finished Concrete | 0.012 | 0.014 | 0.016 |
| Asphalt | 0.01 | 0.012 | 0.015 |
| Smooth Pavers | 0.012 | 0.014 | 0.016 |
| Textured Pavers | 0.015 | 0.018 | 0.022 |
| Exposed Aggregate Concrete | 0.015 | 0.018 | 0.022 |
| Turf/Grass | 0.15 | 0.24 | 0.41 |
| Gravel | 0.02 | 0.025 | 0.03 |
| Bare Soil | 0.01 | 0.02 | 0.03 |
Source: TR-55 (1986), Table 3-1
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Last verified: February 2026