Water on the Land

Water arrives on the land as either surface water runoff or as groundwater. Some of this water originates on the coastal property. Other surface water and groundwater is flowing through on its journey to the lake from inland sources.

Surface water runoff may come from rain water, snow melt, groundwater seeps or springs, and lawn or garden sprinkling systems.

There are a number of indictors of surface water problems. Many exposed soil surfaces on bank and bluff slopes have miniature troughs or larger gullies. Some slopes have exposed lengths of drain pipe or exposed foundations of stairways or other structures. Areas of decayed vegetation in low areas indicate possible prolonged periods of standing water that may have infiltrated into the groundwater, rather than evaporated. Exposed soil surfaces on the land indicate possible easy infiltration into the groundwater.

Groundwater infiltrates into the soils of coastal properties and moves to the slope face from surface water sources, off-site groundwater sources, septic systems or dry wells. Active bluff slumping (FIgure 1) is a visual indication that on-going erosion is occuring due to groundwater. The hidden activity of groundwater can be more dangerous than the visible effects of surface water runoff because groundwater can trigger large, deep landslides that sometimes have catastrophic consequences (Figures 2 & 3). The presence of water in soil pores and soil fractures beneath a slope weakens the soil by adding weight and by reducing the frictional resistance among soil particles that are in contact with one another.

All coastal properties have groundwater flow beneath them; the ground adjacent to and lower than the lake surface elevation will generally be saturated. The surface of this zone of saturation (called the water table) is at lake level at the shoreline and rises gradually in the inland direction. For any banks consisting entirely of sand and/or gravel, this will be the only groundwater flow system present. Infiltrating water moves directly into the lake-level groundwater flow system and causes little weakening of the soil.

Many coastal bluffs contain soil layers (clays and tills) that retard water flow into the water table near lake level. Coastal landslide problems develop primarily where there are zones of water saturation above the lower, main water table; these are called perched groundwater. At such sites, groundwater collects in the sand and gravel layers because underlying soil layers that are resistant to flow slow downward movement of the water. The water flow in these sand and gravel layers is usually toward the slope face, where the water emerges in the form of seeps or springs.

Groundwater's influence on slope stability is controlled by several factors, including the quantity and distribution of groundwater beneath coastal property. The amount and rate of water infiltration is also important. The greatest infiltration comes from prolonged, slow application of water at infiltration locations. The soil moisture content and the soil structure's ability to pass water through the soil are also important.