Area Drain Spacing Calculator

Calculate optimal area drain spacing based on surface slope, ponding constraints, and drain capacity. Uses sheet flow analysis from TR-55 and HEC-22 methods. Professional drainage design tool for civil engineers.

Calculate Area Drain Spacing

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Surface & Slope

Surface Type

Type of surface for sheet flow

Surface Slope

Slope in ft/ft (or m/m)

Custom Manning's n

Override default surface roughness (optional)

Custom Runoff Coefficient

Override default C value (optional)

Design Constraints

Max Ponding Depth

Maximum allowable water depth on surface

Design Rainfall Intensity

in/hr

Peak rainfall intensity for design storm

Max Travel Time

min

Maximum sheet flow travel time

Drain Configuration

Drain Type

Type of area drain inlet

Drain/Grate Size

Diameter or width of drain inlet

Drainage Pattern

How water flows to drains

Total Drainage Area

Total area to be served by drains

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