Drainage Glossary

35 terms used in residential drainage work, defined in plain English with their sources. Jump to a category or scroll through.

Drainage Systems

French Drain: A trench filled with gravel and a perforated pipe that collects subsurface water and carries it by gravity to a discharge point. The gravel acts as a collection zone; the pipe provides the conveyance.
Catch Basin: A below-grade box with a grated top that collects surface water at a low point and routes it into a buried pipe. A sediment sump at the bottom traps debris before it enters the pipe.
Area Drain: A flush-mounted grate set at the lowest point of a flat surface — a patio, planter bed, or lawn — to collect standing water and feed it into a drain pipe.
Channel Drain: A long, narrow trench drain with a continuous grate, installed across a driveway, patio edge, or doorway threshold to intercept sheet flow before it enters a structure or low area.
Trench Drain: A linear drain set in a narrow channel, typically precast concrete or polymer, that collects water along its full length. Functionally similar to a channel drain; the term is often used interchangeably in residential work.
Sump Pump: An electric pump installed in a below-grade pit (sump) that activates when water rises to a set level, discharging it through a pipe to a point away from the structure. Used where gravity drainage alone cannot move water out.
Pop-Up Emitter: A spring-loaded cap at the end of a buried drain line that opens under water pressure to release flow at grade, then closes to keep debris out. Commonly used as the discharge point for buried downspout extensions.
Dry Well: A buried chamber — gravel-filled pit, perforated barrel, or modular cube — that receives stormwater and allows it to infiltrate into the surrounding soil. Effective only where soil permeability is high enough to empty the chamber between storms.
Rain Garden: A shallow, planted depression designed to capture roof or yard runoff and allow it to infiltrate through an engineered soil mix. Functions as both a landscape feature and a stormwater best management practice (BMP).
Swale: A broad, shallow channel — vegetated or lined — graded to convey surface water overland at low velocity. Swales slow runoff and can promote infiltration, depending on soil type and lining.
Curtain Drain: A shallow French drain installed along the uphill side of a structure or yard to intercept groundwater flowing laterally through the soil before it reaches the protected area. The term distinguishes it from a foundation drain, which sits at footing level.
Downspout Extension: A pipe — above-grade or buried — that carries roof gutter discharge away from the foundation wall to a remote outlet such as a pop-up emitter, dry well, or curb. Moves the roof's concentrated water load off the building pad.

Grading & Slope

Positive Drainage: A grading condition in which the finished ground surface slopes away from a structure in every direction, so that surface water moves away from the foundation under gravity alone. Also called positive slope or positive fall.
Negative Grading: A condition where the ground surface slopes toward a structure rather than away from it, directing surface water against the foundation. The opposite of positive drainage and a common cause of water intrusion at the slab or footing.
Grading Correction: Reshaping the ground surface to restore or establish positive drainage away from a structure. On tract lots, this typically means re-establishing the original pad swales that settlement, landscaping, or hardscape additions have flattened.
Settled Pad: A building pad — the flat, graded area a house sits on — where fill soil has consolidated over time, lowering the surface and reducing or reversing the original slope away from the foundation.
Compaction: The mechanical densification of soil to a specified percentage of its maximum dry density, measured against a laboratory standard (typically a Proctor test). Proper compaction of fill reduces future settlement that would flatten drainage grades.
Finish Grade: The final elevation and contour of the ground surface after all grading, fill placement, and compaction are complete. Drainage design references finish grade, not the rough-graded surface beneath it.
Rough Grade: The preliminary shaping of a site's contours before structures, utilities, or finish surfaces are placed. Rough grade establishes the general drainage pattern; finish grade refines it.

Materials & Fittings

Perforated Pipe: Drain pipe with holes or slots along its length that allow groundwater to enter from the surrounding gravel bed. Installed with perforations facing down in most residential French drain applications so the pipe collects water that has risen to the invert.
Filter Fabric: A permeable geotextile wrapped around the gravel bed of a French drain or placed between soil layers to let water pass while blocking fine soil particles that would clog the drain. Also called geotextile fabric.
Drain Rock: Clean, washed gravel — typically 3/4-inch to 1-1/2-inch crushed stone — placed around a perforated drain pipe to create a high-permeability zone that channels water toward the pipe. Also called drain gravel.
NDS Fittings: A line of residential drainage connectors, adapters, catch basins, and emitters manufactured by NDS, Inc. Widely used in residential yard drainage because the components are designed to connect without solvent welding.
Corrugated Pipe: Flexible, ribbed drainage pipe (typically corrugated polyethylene) that is lightweight and easy to curve around obstacles. Its ribbed interior creates more friction than smooth-wall pipe, which reduces flow capacity at a given slope.
Smooth-Wall Pipe: Rigid or semi-rigid drain pipe with a smooth interior bore (typically SDR-35 PVC or dual-wall HDPE). Its lower internal friction moves more water per inch of slope than corrugated pipe of the same diameter.

Codes & Permits

Grading Permit: A local government authorization required before moving earth above a specified volume threshold. In Riverside County and its cities, the threshold and conditions vary by jurisdiction; the permit ensures grading does not obstruct drainage courses, undermine slopes, or direct water onto neighboring property.
Stormwater Management: The regulatory and engineering framework for controlling how rain and runoff are collected, conveyed, treated, and discharged. In California, stormwater management is governed at the state level by the State Water Resources Control Board and locally by Municipal Separate Storm Sewer System (MS4) permits.
Easement: A legal right granting access across or use of another party's land for a specified purpose. Drainage easements reserve a corridor for water conveyance; structures, fences, and plantings within the easement may be prohibited or restricted.

Soil & Water

Expansive Clay Soil: Clay soil with a high proportion of smectite minerals that absorbs water and swells significantly, then shrinks and cracks as it dries. The volume change exerts lateral and vertical pressure on foundations, slabs, and retaining walls. Common in the Temecula Valley (Auld and Murrieta soil series).
Percolation Rate: The speed at which water moves downward through soil, typically measured in inches per hour. A low percolation rate — common in clay soils — means water accumulates near the surface rather than draining through the ground, which determines whether infiltration-based drainage (dry wells, rain gardens) is viable.
Water Table: The underground boundary below which soil and rock are fully saturated with groundwater. A seasonally high water table can push water up into basements, crawl spaces, and subsurface drain systems from below.
Hydrostatic Pressure: The pressure exerted by standing or slow-moving water against a buried surface — a foundation wall, retaining wall, or slab. Increases with depth and is the primary force that drives water through cracks and joints in below-grade construction.
Surface Runoff: Rainwater or irrigation water that flows across the ground surface rather than infiltrating into the soil. On compacted or clay-rich ground, runoff begins almost immediately; on sandy or well-structured soil, it starts only after the infiltration capacity is exceeded.
Subsurface Water: Water present below the ground surface, whether perched above an impermeable layer, moving laterally through a permeable stratum, or part of the permanent water table. French drains and curtain drains are designed to intercept and redirect subsurface water before it reaches a structure.
Saturation Point: The condition in which all void spaces in a soil are filled with water and no additional water can be absorbed. Beyond saturation, any added water becomes surface runoff or builds hydrostatic pressure against adjacent structures.