Washington State Department of Ecology
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drain pipe on bluff
Drainage improvements can be the most cost-effective way to reduce slide risks.

Water often triggers slides
Water is commonly the primary factor triggering a landslide. Slides often occur following intense rainfall, when stormwater runoff saturates soils on steep slopes or when infiltration causes a rapid rise in groundwater levels. Human actions can exacerbate sliding when drainage systems fail or when development increases runoff near steep slopes.

Cost effective prevention
Drainage improvements may often be the most cost-effective means of reducing the likelihood of landslides. They may include simple measures such as inspecting and repairing existing drainage systems or directing runoff to a storm sewer or in a tightline to the base of the slope. Drainage measures can also be more complex, requiring the drilling of wells and installation of pumps to lower groundwater levels.
surface water

Keep uncontrolled runoff away
from steep slopes

Stormwater can drain directly to a steep slope or can infiltrate rapidly into soils behind the slope face, resulting in a landslide. Property owners along the top of bluffs and steep slopes should take great care to avoid uncontrolled runoff toward the slope.
slope damage near gutter

Plants, porous surfaces, and pipes
Where possible, site design and landscaping should minimize the amount of runoff that flows toward the edge of the slope. Through grading, vegetation choices, and minimizing impervious surfaces, the volume and rate of surface flow can be reduced. Runoff from driveways and gutters can be collected and directed to a storm sewer or to a tightline (a closed pipe) that carries water to a safe point below the slope.
tightline on bluff

Trench and interceptor drains
Groundwater is a major factor in both shallow and deep landsliding, but is much harder to control than surface water. Shallow groundwater, which typically flows several feet below the ground surface, can often be captured with a trench or interception drain. Such systems need to be designed and constructed carefully and require regular inspection and maintenance. Collected water must be directed to a safe location.
interceptor drain

Vertical wells and pumps
Deeper groundwater is much more difficult and expensive to control. Vertical wells, usually in conjunction with pumps, can assist in lowering water levels, but pumps must be maintained and a reliable power supply provided. Electricity may fail during rain storms, just when pumps are needed most.

Horizontal or directional drilling
Horizontal or directional drilling can be used to effectively tap zones where the accumulation of water might contribute to slope instability. Typically, multiple drains are placed, based on a careful analysis of the hydrology of the site. The boreholes, which may be drilled from the slope face or from an upland location, are lined with perforated pipe that drains toward collection points on the slope.
online resources
Free drainage information

For more information about managing surface and groundwater along coastal slopes, see Managing Drainage on Coastal Bluffs.

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