This glossary defines every technical term used across DrainageCalculators.com in plain English. Each definition includes links to the calculators that use it, so you can go straight from understanding a concept to applying it.
58 of 58 terms
A
Acre-Foot
A volume of water that would cover one acre of land one foot deep, equal to about 325,851 gallons. Commonly used to measure the capacity of detention ponds and reservoirs.
B
Backwater
Water that is backed up or slowed because of a downstream obstruction or high water level. Backwater effects can cause flooding upstream of bridges, culverts, or channel constrictions.
Bioretention
A stormwater management practice that uses a specially designed planting area with engineered soil mix to filter and absorb runoff. Similar to a rain garden but typically includes an underdrain for poor soils.
BMP (Best Management Practice)
A technique, method, or structural control used to manage stormwater runoff. BMPs include rain gardens, detention ponds, permeable pavers, and vegetated swales. The goal is to reduce runoff volume, slow peak flows, and improve water quality.
C
Catch Basin
A box-shaped drain structure with a grated top that collects surface water and directs it into an underground pipe system. Typically found along streets, parking lots, and at low points. Also called a storm drain inlet.
cfs (Cubic Feet per Second)
A unit of flow rate used in the United States for drainage and river flow. One cfs equals about 449 gallons per minute. A typical residential storm drain might carry 2-5 cfs, while a large river carries thousands.
cms (Cubic Meters per Second)
The metric unit of flow rate, equal to about 35.3 cfs. Used internationally for drainage and river flow calculations.
Cistern
A tank that collects and stores rainwater for later use (irrigation, toilet flushing, etc.). Cisterns reduce stormwater runoff while providing a free water supply.
Critical Depth
The water depth in an open channel at which flow transitions between subcritical (slow, deep) and supercritical (fast, shallow) regimes. At critical depth, the Froude number equals 1.0. Important for designing channel transitions and drops.
Culvert
A pipe or boxed opening that carries water under a road, railroad, or embankment. Culverts range from small corrugated pipes under driveways to massive concrete box culverts under highways.
Curve Number (CN)
A number between 0 and 100 that describes how much rainfall becomes runoff based on soil type, land use, and soil moisture. Higher numbers mean more runoff. A parking lot might be CN 98, while a forest on sandy soil might be CN 30. Developed by the NRCS (formerly SCS).
D
Design Storm
A hypothetical rainfall event of a specific intensity and duration used to design drainage systems. For example, a "10-year, 24-hour storm" has only a 10% chance of occurring in any given year and lasts 24 hours. Larger storms (25-year, 100-year) require bigger drainage systems.
Detention
Temporarily storing stormwater runoff and releasing it slowly to reduce peak flow rates downstream. A detention pond fills during a storm and drains out over hours or days through a controlled outlet. All water eventually leaves.
Drainage Area
The land area that contributes runoff to a specific point, also called a watershed or catchment. All rain falling within this boundary eventually flows to the same outlet. Drainage area is a key input for most runoff calculations.
Dry Well
An underground pit filled with gravel or a perforated chamber that collects stormwater and lets it slowly soak into the ground. Often used to handle roof downspout water.
E
Energy Dissipator
A structure placed at a pipe outlet or channel drop to slow water velocity and prevent erosion. Common types include riprap aprons, stilling basins, and impact basins.
F
Flow Rate (Discharge)
The volume of water passing a point per unit of time, typically measured in cfs (cubic feet per second) or cms (cubic meters per second). Also called discharge. The Rational Method and other formulas calculate the peak flow rate for design purposes.
Freeboard
The vertical distance between the water surface and the top of a channel, levee, dam, or structure. Freeboard provides a safety margin to prevent overtopping. Typical freeboard requirements are 6 inches to 2 feet depending on the structure.
French Drain
A trench filled with gravel containing a perforated pipe that collects and redirects groundwater and surface water. One of the most common residential drainage solutions for wet yards and foundation waterproofing.
Friction Slope
The rate of energy loss due to friction as water flows through a pipe or channel. In uniform flow, friction slope equals the channel bed slope. Measured as a dimensionless ratio (feet per foot or meters per meter).
Froude Number
A dimensionless number that indicates whether flow is subcritical (slow, tranquil, Fr < 1) or supercritical (fast, shooting, Fr > 1). At Fr = 1, flow is at critical depth. Important for understanding hydraulic jumps and channel transitions.
G
gpm (Gallons per Minute)
A flow rate unit commonly used for pumps, downspouts, and small drainage systems. One cfs equals about 449 gpm.
Green Infrastructure
Stormwater management approaches that use natural processes like infiltration, evaporation, and plant uptake to manage runoff. Includes rain gardens, bioretention, permeable pavers, green roofs, and vegetated swales.
Green Roof
A roof covered with vegetation and a growing medium, designed to absorb rainfall and reduce runoff. Green roofs can retain 50-90% of small storms and reduce peak runoff significantly.
H
Head Loss
The reduction in water pressure or elevation as water flows through a pipe, fitting, or structure due to friction and turbulence. Measured in feet or meters of water.
Headwall
A concrete or masonry wall at the inlet or outlet of a culvert or pipe. Headwalls prevent erosion, provide structural support, and direct water into or out of the pipe.
Hydraulic Grade Line
A line showing the water pressure level along a pipe or channel system. In an open channel, it matches the water surface. In a pressurized pipe, it represents the level water would rise to in vertical tubes inserted along the pipe.
Hydraulic Radius
The cross-sectional area of flow divided by the wetted perimeter, used in Manning's equation and other flow formulas. A larger hydraulic radius means more efficient flow. A full circular pipe has a hydraulic radius of diameter/4.
Hydrograph
A graph showing flow rate over time at a specific point. The peak of the hydrograph is the maximum flow rate. Hydrographs are essential for designing detention ponds because they show the total volume of runoff, not just the peak.
Hydrologic Soil Group
A classification system (A, B, C, D) that groups soils by how quickly water infiltrates. Group A (sand/gravel) drains fastest. Group D (clay) drains slowest and produces the most runoff. Used as an input for the SCS Curve Number method.
Hyetograph
A graph showing rainfall intensity over time during a storm. Different storm distributions (SCS Type I, IA, II, III) have different hyetograph shapes. Used as input for detailed hydrograph calculations.
I
IDF Curve
An Intensity-Duration-Frequency curve showing how rainfall intensity varies with storm duration for different return periods. For example, a 10-year, 15-minute storm might have an intensity of 5 inches/hour, while a 10-year, 60-minute storm has 2 inches/hour. Essential input for the Rational Method.
Impervious Surface
Any surface that prevents water from soaking into the ground: roofs, paved roads, concrete, asphalt, compacted gravel. Impervious surfaces generate the most runoff. The percentage of impervious area is a key factor in runoff calculations.
Infiltration
The process of water soaking into the ground through the soil surface. Infiltration rate depends on soil type, moisture content, and surface condition. Sandy soils infiltrate fast (1-8 in/hr); clay soils infiltrate slowly (0-0.2 in/hr).
Infiltration Trench
A long, narrow trench filled with stone that captures stormwater and allows it to infiltrate into the surrounding soil. Similar to a French drain but designed primarily for volume reduction rather than conveyance.
Inlet
An opening in a drainage system where surface water enters. Types include grate inlets, curb inlets, combination inlets, and slotted drains. Inlet capacity depends on the grate type, street slope, and gutter spread.
M
Manning's Equation
The most commonly used formula for calculating flow in open channels and partially full pipes: V = (1.486/n) * R^(2/3) * S^(1/2) (US units). It relates velocity to the channel roughness (n), hydraulic radius (R), and slope (S).
Manning's n (Roughness Coefficient)
A number that describes how rough a pipe or channel surface is, affecting flow resistance. Smooth PVC pipe has a low n (0.010). A natural rocky channel has a high n (0.040-0.070). Concrete pipe is typically 0.012-0.013. Selecting the right n value is critical for accurate flow calculations.
N
Normal Depth
The depth of flow in a channel when the gravitational force driving the flow exactly balances the friction resisting it, resulting in uniform flow at constant depth. Determined by solving Manning's equation for depth.
O
Orifice
A controlled opening (circular or rectangular) through which water flows, typically in a detention pond outlet structure. The flow rate through an orifice depends on the size of the opening and the water depth above it.
Outlet
The downstream end of a pipe, culvert, or drainage system where water is discharged. Outlet design must address erosion protection and energy dissipation to prevent scouring of the receiving channel.
P
Peak Discharge
The maximum flow rate during a storm event, measured in cfs or cms. This is the critical value used to size pipes, channels, and inlets. The Rational Method calculates peak discharge using Q = CiA.
Permeable Paver
Interlocking concrete or brick pavers with gaps that allow water to infiltrate through the surface into a gravel base below. Reduces runoff from driveways, parking lots, and walkways while still providing a hard surface.
Pervious Surface
Any surface that allows water to soak through, such as natural grass, forest floor, or permeable pavers. Pervious surfaces generate less runoff than impervious surfaces.
R
Rain Garden
A shallow planted depression designed to capture and absorb stormwater runoff from roofs, driveways, or yards. The soil mix and plants filter pollutants while the water soaks into the ground over 24-48 hours.
Rainfall Intensity
The rate of rainfall, typically measured in inches per hour (in/hr) or millimeters per hour (mm/hr). Higher intensity means rain is falling harder and faster. Intensity varies with storm duration and return period, described by IDF curves.
Retention
Permanently capturing and holding stormwater, typically through infiltration into the ground. Unlike detention (temporary storage), retained water does not leave the site as surface runoff. Rain gardens and dry wells are retention practices.
Return Period
The average number of years between storms of a given magnitude. A "10-year storm" has a 10% chance of occurring in any given year, not that it happens exactly every 10 years. Also called recurrence interval. Common design return periods are 2, 5, 10, 25, 50, and 100 years.
Riprap
Large stones or rock placed on slopes, channels, or at pipe outlets to prevent erosion from flowing water. Riprap size (D50) is calculated based on flow velocity and channel slope.
Runoff Coefficient (C)
A number between 0 and 1 that describes how much rainfall becomes runoff versus soaking into the ground. A paved parking lot has a high coefficient (0.85-0.95) because almost all rain runs off. A grassy field has a low coefficient (0.25-0.35) because most rain soaks in. Used in the Rational Method (Q = CiA).
S
Subcritical Flow
Slow, deep, tranquil flow in an open channel where the Froude number is less than 1. Most drainage channels are designed for subcritical flow because it is more stable and easier to control.
Supercritical Flow
Fast, shallow, shooting flow in an open channel where the Froude number is greater than 1. Supercritical flow can cause standing waves, erosion, and difficulty controlling direction. It occurs on steep slopes or at drops.
Swale
A broad, shallow channel with sloping sides, usually lined with grass or other vegetation, that conveys and filters stormwater. Swales slow water down, allowing sediment to settle and some water to infiltrate.
T
Tailwater
The water depth at the downstream end (outlet) of a pipe, culvert, or channel. High tailwater can reduce the capacity of the drainage system by causing water to back up.
Time of Concentration (Tc)
The time it takes for water to travel from the most distant point in a drainage area to the outlet. Tc determines the rainfall intensity used in the Rational Method: a longer Tc means lower intensity (and lower peak flow). Calculated using methods like Kirpich, FAA, and NRCS.
U
Unit Hydrograph
A hydrograph showing the runoff from one unit (typically 1 inch) of rainfall applied uniformly over the drainage area during a specific duration. Used as a building block to construct hydrographs for any storm by scaling and superposition.
W
Watershed
The entire land area that drains to a common point. Same as drainage area or catchment. Watersheds are defined by topography: ridges and hills form watershed boundaries, while valleys and streams form the drainage network.
Weir
A barrier across a channel or at a pond outlet over which water flows. Weirs are used to control water levels and measure flow rates. Common shapes include rectangular, V-notch, and broad-crested weirs.