Understanding Your Property

Puget Sound Geology

Most of the Puget Sound geology that influences coastal slope drainage and stability is a result of past glaciations. Figure 2 shows the general geology of the Puget Sound area. The glaciers deposited the soil layers that make up your slope. A common soil layer sequence found along coastal slopes is shown on Figure 3 (below). The layers consist of glacial till, sand and gravel, and silt/clay. Your shoreline bank or bluff could be composed of one or more of these soil layers.

Some isolated areas of Puget Sound shorelines consist predominantly of rock instead of soil.
In the construction industry, glacial till is frequently referred to as hardpan because it is very dense, making it difficult to dig. Hardpan is a mixture of clay, silt, sand, and gravel. Water infiltrates through this soil very slowly and consequently water will often accumulate or "perch" above the glacial till during and after wet weather periods.

Figure 3. Common soil layer sequence found
along coastal slopes.


Sand and gravel soils usually contribute to the stability of a slope in the absence of water. However, water readily flows through sand and gravel. Water can reduce the stability of the slope when it accumulates above a soil layer that is not as permeable as the sand and gravel. Water that accumulates above the impermeable soil layer and may flow laterally until it "daylights" as seepage on the slope face.

In the Puget Sound region, the impermeable soil layer can be a silt layer within the sand and gravel unit or a silt/clay layer located under the sand and gravel deposits. The impermeable silt/clay layer was compacted under the weight of the overlying soil units and more importantly, the weight of the glaciers.

Today's shoreline is a result of continuing geologic evolution since the time of the glaciers. At the top of the slope is a weathered zone (including topsoil) that is usually a few feet thick. Landslide debris and deposits from erosion and weathering can be found on the slope face and at the toe (bottom) of the slope. The debris and deposits usually consist of a jumbled mixture of all the geologic soil layers that make up the slope as well as vegetation that moved down the slope.
Information on the geology and the relative stability of coastal slopes can be found in the Coastal Zone Atlas of Washington (see links section for more information). This atlas can often be found in the public library reference or atlas section or at local county planning and engineering departments.

Return to top

Natural Slope Processes

Under natural forces of gravity, wind, and water, coastal slopes continue to change. The slope will generally change from both erosion and soil mass movement (landsliding). The majority of erosion in the Puget Sound area is due to the wearing away of soil or rock by the movement or flow of water. Erosion can also include wearing away of soil by wind and ice. Landslides occur when a mass of soil moves down the slope under the force of gravity. Figure 4 shows the general form of various erosion and mass movement (landslide) features.
Puget Sound Coastal Slope Processes

Figure 4. Typical Puget Sound Coastal Slope Processes


As erosion and landsliding occur, they change the shape of slopes. In general the overall shape of the slope will become flatter and less steep. This tends to increase the overall stability of a slope. However, erosion and landsliding can leave localized areas of a slope steep and unstable. Other factors can affect slope stability. For example, if slope topography is changed by adding soil to the top of slope or the toe of the slope is removed, the slope angle will eventually adjust under the forces of gravity to achieve a more stable slope configuration. Also, if significant water is added to the slope such as what might occur during and after intense rainfall, the sudden increases in surface water on the slope and ground-water seepage may reduce the stability of the slope to the point where a landslide occurs. Figure 5 schematically illustrates a landslide movement on a bluff face.

Figure 5. Change of a bluff face.


As a property owner, you will want to reduce the hazards associated with these natural processes to acceptable levels of risk. Your efforts should be directed to reducing factors which adversely affect slope stability. In some cases it is not economically or physically possible to eliminate or reduce all the adverse factors associated with slope stability and erosion.
With this website you can generally evaluate your slope drainage and identify measures that can be taken to improve slope stability and erosion.

Return to top

Surface Water and Groundwater

Surface water and groundwater are the two sources of water that contribute to your coastal slope drainage and can accelerate erosion and slope stability problems. Figure 6 shows general drainage movement down a slope. Surface water (as the name indicates) is water that flows across or is ponded on the ground surface. This can include surface water features, sheet flows and concentrated flows.

Surface water features can include puddles that form during rainfall. This water usually infiltrates into the soil and becomes groundwater. Other more permanent features include ponds and wetlands that also contribute to groundwater by infiltration. When surface water features become full and overflow they can contribute to concentrated flows or sheet flows.

Figure 6. General surface water and groundwater
movement down coastal slopes.


Sheet flow is most easily recognized as a thin layer of water flowing over smooth paved areas. Sheet flow is common across parking areas, driveways, and large sloping expanses of lawn. Sheet flow can concentrate into rills and small channels. Sheet flow can also migrate to other areas and seep into more permeable surfaces such as sandy soils. This process converts surface water sheet flow into groundwater.

Concentrated surface water flow can originate as natural or constructed channels, streams, drainage ditches or as discharges from ponds and wetlands. Concentrated surface water flows can also include discharges from rooftops, roads, and parking areas. These flows are often collected and discharged through downspouts, drainage swales, culverts, and other drain pipes. Surface water flow volumes and velocities can be large and will generally occur during and after heavy rainfall periods. Less frequently they occur as a result of discharges from other sources such as fire hydrants, failing stormwater detention facilities, and drainage systems that are poorly maintained. Water velocity increases on steeper ground where the water flows faster due to the force of gravity.

Groundwater location and movement is generally related to the geology and soils comprising your slope. Groundwater can be located within several feet of the ground surface or deeper in the slope at a sand/clay boundary or in cracks and seams in hard soils or bedrock as shown on Figure 6. Groundwater often accumulates or seeps at more than one level in a slope.

Return to top

Slope Stability and Drainage

The characteristics that influence the stability of your slope include geology, slope drainage, slope topography (shape and steepness), changes to slope topography by placing soil or removing soil from the slope, and undercutting from coastal erosion processes. This section focuses on slope drainage and how it affects slope stability. Later sections will discuss what you can or cannot do to change your drainage characteristics to improve slope stability.

Under the influence of gravity, water appearing as surface water or groundwater travels in the direction of least resistance. Water can move in many directions on the surface or in the soil but one fact remains constant - if it moves, it will ultimately move to a position of lower elevation. Often rain or surface water infiltrates into a weathered soil layer, perches on top of relatively impermeable glacial till, and exits on the slope face as seepage where it becomes surface water flow.

Deeper groundwater can also affect slope stability as it travels down through a sand and gravel layer and perches on a silt or clay layer. Perched groundwater can occur at shallow depths (less than 10 feet) or at greater depths below the crest of the slope. Often a slope will have several groundwater zones.

Soil erosion caused by broken pipe.


Groundwater often causes slope stability problems when it is present in a sand and gravel layer and is perched on top of a silt/ clay layer. The water can act as a soil lubricant which reduces the friction between the sand and silt/clay layers causing the sand to slide on top of the clay. In addition, as the groundwater daylights and exits the slope as seepage, it may erode the sand causing the soil above the sand to become unsupported and fail. Sometimes both the erosional and lubricating effects of groundwater are responsible for large landslides.

Surface water flow can infiltrate into the soil and saturate a loose or weathered layer of soil on the slope crest or face. The saturated soil is heavier and may also be lubricated by the water. This can also cause the soil to move downslope as a debris flow or mudslide.

Return to top

Slope Erosion and Drainage

Surface water flow can cause erosion of soils on the crest and face of your waterfront slope. Usually, the most severe erosion occurs where surface water flows are concentrated. The steeper the slope angle and higher the slope, the greater the potential is for erosion. Some soil types are more erodible than others. For example, soils such as sand and gravel are more prone to erosion than are silt and clay. However, once silt and clay are disturbed, they are highly erodible. Erosion can also cause soil on adjacent slope areas to become steeper to the point that they are susceptible to landsliding.

Surface water often causes severe erosion to coastal slopes where pipes are allowed to discharge onto the slope. Over time the water discharging from the pipe erodes away vegetation and soil (or rock). If this process continues, the soil erodes until the pipe no longer directly discharges onto the slope. Instead, the water discharges like a waterfall cascading down onto the slope. The farther water falls, the faster it drops and the more energy the water has when it impacts the soil. With more energy the erosion progresses at a faster and faster rate. Ultimately, the erosion undermines the pipe (which then can break) and potentially results in a rapid retreat or erosion of the top of the slope.

Rill and gully erosion on constructed slope.


This same scenario can be repeated for concentrated surface drainage that flow over the top of a slope. Over time, concentrated flows can create a substantial gully. If water volumes are large and/or velocities are high, a deep gully can form very quickly, even within hours.

Sheet flow can also lead to substantial erosion. Sheet flow naturally tends to concentrate into small rills and channels of water. Flow concentration occurs more rapidly on bare ground, sloping ground, and where long distances are involved. The small rills and channels can concentrate into larger and larger features if left unattended.

It is critical that surface water flows be controlled since they can lead to rapid, severe erosion of your property. Control is particularly important for newly developed or modified property. Something as simple as regrading your driveway can change surface water flow patterns and cause erosion. Often the first heavy rain locates potential problem areas. Immediate adjustments to new site development and existing site properties should be performed to avoid erosion problems. The use of erosion control methods during construction (short term) and for long term erosion prevention are important for reducing the effects of erosion on slopes (see links section for more information).

Previous page: Understanding Your Property > Next page: Evaluating Slope Drainage

Return to top