Dry Well Calculator

Calculate dry well sizing for stormwater infiltration. Determines storage volume, infiltration capacity, and drawdown time using ASCE MOP 77 methodology.

Calculate Dry Well Design

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

Input Parameters

Design Requirements

Design Runoff Volume

Total volume of runoff to be stored

Storm Duration

hours

Duration of design storm event

Maximum Drawdown Time

hours

Maximum time allowed to drain the dry well (24-72 hours typical)

Dry Well Dimensions

Shape

Geometric shape of the dry well

Diameter

Inside diameter of cylindrical well

Depth

Total depth of the dry well

Soil & Backfill Properties

Surrounding Soil Type

Native soil for infiltration rate determination

Backfill Type

Material used to fill the dry well

Safety Factor

Factor of safety for infiltration rate (1.5-3.0 typical)

Include bottom infiltration Include sidewall infiltration

Dry Well Design Overview

Dry wells (infiltration pits) provide temporary storage and ground infiltration for stormwater runoff. They are commonly used for roof drainage, parking lots, and residential stormwater management.

Storage Volume - Total volume available in backfill voids
Infiltration Area - Bottom and sidewall area for water to infiltrate
Drawdown Time - Time to drain stored volume (24-72 hours typical)
Safety Factor - Applied to soil infiltration rate for conservative design

Soil Infiltration Rates

Soil Type	Min (in/hr)	Typical (in/hr)	Max (in/hr)
Gravel	4	8	20
Sand	2	4	8
Loamy Sand	1	2.4	4
Sandy Loam	0.5	1	2
Loam	0.25	0.5	1
Silt Loam	0.15	0.3	0.6
Sandy Clay Loam	0.1	0.2	0.4
Clay Loam	0.05	0.1	0.2
Silty Clay Loam	0.04	0.08	0.15
Sandy Clay	0.02	0.05	0.1
Silty Clay	0.01	0.03	0.06
Clay	0.005	0.02	0.05

Source: ASCE MOP 77 (2006), adapted from NRCS data

Backfill Void Ratios

Backfill Type	Min	Typical	Max
Gravel (3/4" - 1.5")	0.30	0.35	0.40
Crushed Stone (#57)	0.35	0.40	0.45
Open-Graded Aggregate	0.38	0.42	0.48
Perforated Chambers/Crates	0.90	0.95	0.97

Source: ASCE MOP 77 (2006), Design and Construction of Urban Stormwater Management Systems