Evaluating Slope Drainage

Slope surface water and groundwater can have a major impact on coastal slope erosion and stability. In landslide prone areas, the presence of groundwater in your slope or surface water on your slope is usually the primary factor leading to landsliding and erosion.

Severe erosion and landsliding typically occur during or shortly after periods of heavy rainfall. It is important to understand the magnitude of water flows that can be involved with coastal properties. The quantity of water affecting coastal slope areas can be quite significant. A large storm can drop over 4 inches of water in a relatively short (24-hour) period. If this water falls on the roof and driveway surfaces of a typical waterfront residence (say 5,000 to 10,000 square feet of area), it can result in over 25,000 gallons of water. Additionally, if this pattern is repeated on many hundreds of properties which are upslope of your property, it can have serious erosion and slope stability implications for your property.

Although rain falls on property regardless of site features (i.e. trees versus houses), site development does tend to cause more rapid and concentrated runoff. Also, on undeveloped land much of the rainfall never reaches the ground. It is captured by foliage and evaporates back into the atmosphere. If rapid concentrated runoff is permitted to flow onto or into the slopes it can have a serious impact on slope stability and erosion.

Because of the impact to coastal slopes, it is important that you understand your specific slope drainage conditions and identify the sources contributing water into and onto your slope. If you can identify the sources of slope surface and groundwater, you can then take steps to control some of these sources. The Coastal Property Owner Slope Drainage Checklist can help you organize the observation of surface water and groundwater conditions on your property. You may also share your checklist observations with other professionals who assist you in your drainage control efforts.

Evaluating Surface water

The best time to observe surface water flows is during periods of rainfall. Put on your raingear and walk around on your property. Make notes on the locations where you see surface water flows including where they originate, along what path they flow, and where they go. Pay particular attention to roof, driveway, and parking drainage. Also, note the behavior of flows from drainage ditches, culverts, and pipes that are located on your property or that originate on adjacent properties and discharge onto your property. Figure 7 (below) shows some common surface water problems on slopes.

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Evaluating Groundwater

You usually will not know groundwater is present unless you observe it "daylighting" as seepage on your site. Daylighting groundwater can be present on level or gently sloping upland portions of your site or on the slope crest, face, or toe. You might also encounter shallow groundwater while digging into the soil on your site.

Figure 7. Common surface water problems

Groundwater indicators are generally similar for gentle or steep site slopes. However, on level or gently sloping uplands you may have trouble identifying where groundwater daylights due to standing surface water. Distinguishing between surface water and groundwater on level or gently sloping ground may require you to observe the site during drier weather periods when surface water is minimal.

Because seepage is related to the geology of your site, groundwater will often daylight on the slope at the contact between different soil types and often where there is a change in the slope angle. It is important to try and identify the locations of groundwater discharges that appear on your slope (slope crest, face, and toe). The locations may help you and others assess whether there is a practical or cost effective approach to manage slope drainage. The best time to observe your slope for groundwater is late in the wet season (mid to late winter into spring) and during a dry period between storms. During a dry period daylighting groundwater should not be misidentified as strictly surface water flow. Figure 8 shows typical ground-water evidence on a slope.

When looking for indications of groundwater seepage during a dry period look for:

  • A wet sheen or surface water appearing along a zone on the slope.
  • A wet soil zone on the slope.
  • Distinct changes in slope vegetation. Look for vegetation adapted for wet soils located along a zone on the slope. A few common indicator plants are horsetail, willows, salmonberry, and skunk cabbage. These plants may also be located below active seepage zones.
  • Daylighting groundwater seepage at soil layer boundaries.
  • Daylighting groundwater seepage at changes in the slope angle.
Figure 8. Typical groundwater evidence on a slope.

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Changes to Slope Drainage Characteristics

Previous site development grading and landscaping on your property may have significantly changed natural drainage patterns. If your property is undeveloped, site development will significantly affect existing slope surface water and groundwater flow. Modifications to slope drainage that can impact your property include the following actions:

  • Routing new or existing drainage sources to an existing natural drainage course, catch basin, pipe, or culvert or routing drainage directly onto a neighbor's property. Routing drainages to areas that are not able to accommodate the flows can cause erosion and/or slope stability problems.
  • Site grading that changes the slope and contour of the site. Site grading can cause surface water to channelize or sheet flow to flow to a different location which can lead to erosion and/or slope stability problems.
  • Converting permeable surfaces to impermeable surfaces such as roof, driveway, parking, or compacted earth areas. These development features cause an increase in stormwater runoff (compared to rain that falls on native vegetation). This increases the potential sheet and concentrated flows that can cause erosion and slope stability problems.
  • Slope cutting or filling and vegetation removal for views and beach access. Cutting and filling on a slope can impact slope stability and can also alter existing drainage paths. Removing vegetation can significantly reduce slope stability. The companion websites Slope Stabilization and Erosion Control and Managing Vegetation on Coastal Slopes should be referenced for further information on slope vegetation management practices and erosion control.

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Special Needs Areas

Should you recognize or suspect that your property or adjacent properties contain any of the following, you should contact the suggested resources for assistance or information.

  1. Historical or recent landslides based on your observations or from Coastal Zone Atlas Mapping [A,F,J]
  2. Groundwater seepage located more than 10 feet below the crest of slope with landsliding or significant erosion of the slope [F]
  3. Your site drainage will affect adjacent properties [A,F,I]
  4. Your septic system discharges or seeps onto the slope [A,E]
  5. Surface water flows from off-site sources affect your property [A,I]
  6. You need to understand and implement erosion control techniques for property development [A,B]
  7. Pipe outfalls located below ordinary high water elevation of Puget Sound [A,B]
  8. Site ponds, streams, or wetlands [A,B,C,H]
  9. Coastal marine erosion [A,D,F,G,K]
  10. Habitat for endangered, threatened, rare animal species (e.g. bald eagle or osprey nests/alternate nests/perch trees) [D]


A - Local Planning Office and Engineering Department
B - Washington Department of Ecology - Water Quality Program
C - Army Corps of Engineers, Seattle District
D - Washington Department of Fish and Wildlife - Habitat Division
E - Local Health Department
F - Geotechnical Engineer
G - Coastal Engineer
H - Wetland Professional
I - Civil Engineer specializing in drainage
J - Washington Department of Natural Resources - Geology and Earth Resources Division
K - Washington Department of Ecology - Shorelands Program

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