Drainage Systems

Approaches to drainage issues will usually fall into two general types of solutions which incorporate drainage minimization solutions and drainage control system solutions. Each group of solutions helps manage slope stability and slope erosion at your property. The practices introduced in this section are general and cover basic drainage management techniques around coastal slopes. Consequently, any single technique by itself may not sufficiently address the broad range of drainage issues occurring on your property. Nor may this website discuss in sufficient detail the application of each technique to the unique characteristics of your property. Therefore, it is always a good practice to reference other information sources before making the final selection of your drainage system.

General slope shapes for Puget Sound shorelines used in this website are shown in Figure 11 (below). Use Figure 11 to approximate some of the slope shapes that are familiar to you. When you identify your general shape(s), you can match drainage control techniques with your property characteristics identified in your planning efforts. You then can implement a number of drainage control measures presented in this section.

Key to Applicability and Compatibility Graphics

Use the key provided to review the drainage system techniques introduced later in this publication and determine which approaches may be suitable for a general slope configuration.

Figure 11. Indicates general applicability of a drainage system technique for typical slope shapes and compatibility of drainage system approaches with existing shoreline issues. The degree of general applicability or compatibility ranges from Not Recommended to Excellent and is indicated accordingly in the graphics window.

Drainage improvements on a coastal property can be located in any of the following areas: above the slope crest, on the slope face, and near the slope toe. In each of these slope zones, a variety of construction techniques can be applied. Nevertheless, some generalizations can be made about each group of techniques based on their locations. The compatibility of each group in different slope areas is summarized on Table 2.

Above Slope Crest

Managing surface and groundwater before flow reaches a slope is usually the most prudent approach to improving slope stabilization soil and erosion control. Improving drainage at the top of the slope typically requires an integrated approach to drainage control instead of a single, large-scale approach which may be seen on the slope face and along the slope toe. In a system there can be many different drainage control elements including: groundwater interceptor and relief drains, surface water interceptor swales, tightlines, catchbasins, landscaping yard drains, and detention storage. The construction approaches for systems with these components are similar.

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Slope Face

Designing and installing drainage control systems on slope faces can be risky for both the slope and for slope workers. Make sure that you and other people are secure on the slope and that you are not working in unstable areas. Drainage control on slope faces typically addresses issues such as groundwater seepage from the slope and surface water erosion control. Depending on the geometry and characteristics of your slope, you can consider different control techniques. Construction disturbance risks to your slope should always be weighed against the potential gains in slope stability. A number of the construction practices are low impact techniques while others may require heavy equipment.

Below Slope Toe

Slope drainage in Puget Sound has been historically modified by the construction of retaining walls and bulkheads near shorelines. Wall designs should have adequate drainage to keep water pressures from developing behind walls. Presently many bulkheads and wall systems around Puget Sound are in varying conditions of failure and consequently are not performing as designed. Should you suspect that drainage at your slope toe accumulates behind structures, you should seek technical assistance to assess your site conditions before performing improvements in this high risk area.

There are twelve drainage systems detailed on this website. You can use the links below to choose one to read about.

  1. Groundwater Interceptor Drains (Curtain and Trench Drains)
  2. Relief Drains (Tile Drains)
  3. Strip Drains
  4. Interceptor Swales
  5. Stormwater Detention Storage
  6. Slope Drains (Tightlines)
  7. Additional Drainage Control (Horizontal Drains, Well Systems, Drainage Blankets)
  8. Contour Wattling
  9. Revegetation
  10. Slope Toe Drainage Control
  11. Erosion Control Techniques
  12. Source Control

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