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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.
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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.
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.
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 documents Slope
Stabilization and Erosion Control Using Vegetation and
Vegetation Management: A guide for Puget Sound Bluff
Property Owners should be referenced for further information
on slope vegetation management practices and erosion
control. |
