What is a Trench Drain?

Trench Drain

Definition

A trench drain is a long, narrow drain that sits flush with a pavement and collects sheet flow through a grate or a slot.

FAA AC 150/5300-13B calls trench drains and slotted drains linear drains. It recommends linear drains for the collection of sheet flow, and grated inlet catch basins for the collection of channelized flow.

The drain page covers all airport drains, from the pavement slope to the pipe outlet. This page covers the trench drain only.

Trench drain and slotted drain

The two names describe two ways to let the water in.

• Grate inlet. FAA AC 150/5320-5D describes it as an opening in the gutter or ditch that a grate covers. A trench drain with a grate works this way.
• Slotted drain. FAA AC 150/5320-5D calls it a slotted inlet. It is a pipe cut along its length, with bars across the opening to keep the slot open.

AC 150/5320-5D puts the slotted inlet in the class of continuous inlets. A continuous inlet can intercept the flow over a wide section. It suits the places where the designer must stop sheet flow before the water crosses a pavement.

Where airports use them

• Taxilane. AC 150/5300-13B 4.14.3 asks for a flush trench drain or slotted drain inlet across a taxilane. It deals with surface sheet flow.
• Apron. The same circular limits an apron linear trench drain or slot drain to 38 m (125 ft). The interval between trench lines is at least 1.8 m (6 ft). The limit follows NFPA 415 and acts as a fire stop (5.10.1).
• Hangar apron. For a hangar apron, AC 150/5300-13B Appendix E asks designers to locate drainage inlets outside the wheel paths of aircraft. It also says to consider intermittent trench style inlets where the slope between hangars is flat.
• Ponding limit. An apron must not hold surface water deeper than 6 mm (1/4 in), measured with a 3.7 m (12 ft) straightedge (5.10.1).

Grates and aircraft loads

A grate on an airfield carries aircraft wheels. FAA AC 150/5320-5D gives these rules in paragraphs 7-6.1.4 and 7-6.1.5:

• Inlet grating and frames must withstand the wheel loads of the largest aircraft that uses the facility.
• Cast iron grates may not stand where wheel loads can reach them. A grate may be steel or ductile iron.
• Manufacturers make grates for airport loadings from 345 to 1,724 kPa (50 to 250 lb/in²). Hold-down devices stop aircraft from displacing a grate.
• If a grate is bought from a manufacturer, the vendor must certify its design load.
• The vendor ships grates without paint or coating, so that a crack or flaw shows before installation.

AC 150/5300-13B asks that the design accounts for the load transition from the drain to the pavement next to it. It also recommends that surface inlets stand apart from the wheel paths of aircraft. This avoids direct aircraft loading on the drain and limits early damage to the pavement around it.

Clogging

Slotted inlets are very susceptible to clogging from sediment and debris. FAA AC 150/5320-5D does not recommend them where the water carries much sediment or debris. On an apron, the design must also control and collect sediment, fuel, oil and deicing fluid (AC 150/5300-13B, 5.10.1).

Trench drains, runways and strips

ICAO Annex 14 and EASA say little about the trench drain itself. They set two rules that touch it.

• Smoothness. Annex 14 Attachment A and EASA GM1 ADR-DSN.B.090 give a runway surface tolerance of 3 mm under a 3 m straight-edge. The tolerance does not apply across the crown of a camber or across drainage channels. Both ask for caution when drainage grilles go into a runway surface, so that the surface stays smooth.
• Strips. Drains on a runway strip need careful location and design, so that an aeroplane that runs off the runway is not damaged. Annex 14 3.4.6, Note 1, and EASA GM1 ADR-DSN.B.165 say that suitably designed drain covers may be required. The Annex 14 note on a taxiway strip (3.11.3) says the same. Guidance on the design of covers is in ICAO Doc 9157.

Inspecting a trench drain

The FAA ties the operator’s maintenance duty in 14 CFR 139.311(d) to drainage grates. FAA CertAlert 22-05 lists them with manhole covers and other bolted-down equipment in the pavement. It asks operators to inspect them and to keep them secured.

FAA AC 150/5380-6C, paragraph 2.3.2, advises periodic inspection of drainage systems and of edge drains and catch basins. It suggests each season, and after unusually heavy rainfall. The signs of a problem that apply to a trench drain are:

• ponding of water
• broken or displaced inlet grates
• clogged or silted inlet grates
• erosion around inlets
• discoloration of pavement at joints or cracks

EASA AMC1 ADR.OPS.C.010 says drainage systems should be checked periodically, and cleaned or maintained if necessary.

What a drone survey shows

A drone survey photographs the pavement surface. Its images can show ponding, a missing or displaced grate and a drain that is full of debris. A drone does not see the pipe below the grate or measure the flow. TarmacView’s drone airport inspection plans the survey flights and captures the photos. It does not check the hydraulics of a drain.