TRENCH DRAIN

Useful for Erosion Processes:

	Toe erosion with upper bank failure
	Scour of middle and upper banks by currents
	Local scour
	Erosion of local lenses or layers of noncohesive sediment
	Erosion by overbank runoff
	General bed degradation
	Headcutting
	Piping
	Erosion by navigation waves
	Erosion by wind waves
	Erosion by ice and debris gouging
	General bank instability or susceptibility to mass slope failure

Spatial Application:

	Instream
	Toe
	Midbank
	Top of Bank

Hydrologic / Geomorphic Setting

	Resistive
	Redirective
	Continuous
	Discontinuous
	Outer Bend
	Inner Bend
	Incision
	Lateral Migration
	Aggradation

Conditions Where Practice Applies:

Should be considered when shallow, water bearing strata conduct groundwater that emerges (daylights) at the streambank. A good example would be relatively permeable surface strata or water bearing sands up to 3 m (10 ft) thick, e.g., outwash sand or coarse alluvium, overlying relatively impermeable silty clay deposits, e.g., clay till or fine alluvium. This is a fairly common stratigraphic sequence in glaciated terrain and alluvial valleys.

Complexity:

Low. Technique is relatively simple and straight forward to implement. Basically requires the use of a backhoe to excavate and backfill a narrow trench parallel to the crest of a streambank.

Design Guidelines / Typical Drawings:

Trench Drains constructed without a pipe at the bottom are commonly known as French Drains (see Figure 1a). An efficient, well-constructed Trench Drain requires the use of perforated, jointed, slotted or porous pipe placed near the bottom of a trench (see Figure 1b) that is surrounded with pea gravel or selected pervious filter aggregate. When a drain is excavated in erodible materials, synthetic filter fabrics (geotextiles) should be used (see Figure 1c) to line the sides and bottom of the trench to prevent soil fines from entering the coarse backfill in the drain (see Special Topic: Role of Geotextiles). The main backfill should be specially selected pervious filter aggregate designed to allow unrestricted flow of water to the pipes. The minimum required permeability, k, of the trench fill material can be determined by Darcy's law, using a downward gradient of 1.0 and a factor of 10 to 20 to allow for seepage flow reduction caused by contamination. Assume, for example, a flow of 5.7 m³/day (200 ft³/day) will enter a longitudinal drain with a width of 0.5 m (1.5 ft). The minimum required k = 20 (5.7)/0.5 = 228 m/day (k = 20 (200)/1.5 = 2670 ft/day). Pea gravel, stone chips, or comparable materials would be needed to ensure this level of permeability and flow throughout.

Figure 1. Cross sections of subsurface drains. a) French drain, b) conventional trench drain with pipe, c) trench drain with filter fabric.	TABLE 1. Discharge capacities of 1 x 0.7 m (3 x 2 ft) cross sections of stone filled, trench drains.
	Figure 2. Nomograph for computing required size of circular drain, flowing full (US Army Corps of Engineers, 1974).

Most drains should be equipped with pipes because gravel or rock-filled trenches have limited discharge capabilities even when clean aggregates are used. The discharge capabilities of drainage trenches backfilled with clean stone or coarse gravel, as estimated by Darcy's law, are given in Table 1. The required diameters of both corrugated metal, concrete, and polymeric (smooth) drain pipes for a wide range of discharge quantities can be determined from the nomograph in Figure 2 (USACE, 1974).

The location of perforations and open joints in pipes should always be placed to allow unobstructed flow to pipes. If a drainage pipe is completely surrounded with specially selected coarse filter aggregate (refer to Figure 1b), perforations can completely surround the pipe. Unjointed sections of pipe should be used to convey water across areas where the discharge of water into the soil from drains must be prevented. The same injunction holds for the final discharge of collected drain water, viz., it must not be allowed to discharge on to a slope and instead must be conducted safely down a slope using a chute or slope drain (see Technique: Slope Drains).

Trench Drain Typical Drawing
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