New York's coastal bluffs are made up of various erodible sediments,
such as sand, silt, clay and gravel. Rock, such as shale or sandstone,
may be
exposed at the base of a bluff, or the sand, silt and till may extend
below the water's edge. The causes of erosion on coastal bluffs
(toe
erosion, surface runoff, groundwater seepage), combined with the varied
composition of coastal bluffs, result in average long-term annual recession
rates from less than 1 foot per year to 5 feet per year or even more!
Bluff erosion is a primary source of material for beaches where the
bluffs occur with some regularity. Although the erosion of bluffs can
be mitigated in a variety of ways, reduction of bluff erosion
can deprive the associated beaches of their supply of sand and gravel.
Complete stabilization of the bluffs may, in some cases, reduce the
size of the beaches or eventually cause the beaches to disappear altogether.
Plants are useful to stabilize bluffs in as steep an angle of incline
as possible to maximize usable land area at the top of the bluff. Generally,
the maximum slope to be considered for vegetative stabilization projects
is a 1:1.5 slope, that is, 1-ft. vertical rise for each 1.5-ft horizontal
run.
Top of the Bluff Vegetation
The vegetation along the edge of the
bluff serves as a protective buffer for
the bluff face. It should be maintained
or reestablished as a "buffer," a
strip of undeveloped land. This is particularly
important in areas where bluffs are too
steep and too high for economically feasible
stabilization methods. Therefore, a wide
strip (100m, or 300ft, is desirable; 300
m or 1000 ft is optional) of dense natural
vegetation should be maintained along the
bluff edge. This strip precludes human
activity too close to the bluff edge and
retards surface runoff from upland areas.
Also, the roots of the vegetation strengthen
the bluff's resistance to slumping. Read Recommended
Plants for the Lake Ontario Shoreline Property
Owners [632 KB PDF] for help selecting
plants.
Bluff Face Vegetation
Vegetation should be established on patchy
and barren bluff faces to protect them from
erosion and improve their appearance. Whether
or not this is possible depends greatly on
the character of the bluff, particularly on
the steepness of the slope. A slope ratio of
1:1.5 can be considered the dividing line between
a manageable slope and a slope so steep that
vegetation would be difficult or impossible
to establish.
Where possible, steep slopes
should be graded back to a more gentle configuration
(1:3 or flatter is ideal). In cases where
bluffs cannot be practically regraded to
a 1:3 slope, modifications to the existing
slope that will allow some vegetation to
become established may still be made. This
can be accomplished by terracing - providing
horizontal steps in which to plant vegetation - or by breaking the slope with the addition
of contour wattles.
Wattles are
bundles of fresh willow or dogwood cuttings
that are anchored with willow stakes
in trenches along the bluff face. They
act as a base for vegetation growth and
as a trap to slow surface runoff. These
stakes and cuttings are capable of rooting
in the bluff soil provided there is sufficient
moisture.
Plants recommended for stabilizing bluff faces include:
- chokecherry
(Prunus virginiana)
- gray dogwood (Cornus racemosa)
- red-osier dogwood (Cornus sericea)
- wild grape (Vitis riparia)
- common juniper (Juniperus
communis - not recommended for north-facing
or shady bluffs)
- staghorn sumac (Rhus typhina)
- dune willow (Salix cordata)
- bearberry (Arctostaphylos uva-ursi - not
recommended for north-facing or shady bluffs)
- rugosa rose (Rosa rugosa)
Major trees should be sparingly used with caution on the face of bluffs. Such trees, if they collapse because of undermining of the
root system by erosion, will pull tremendous amounts of earth with
their roots as they topple. The resulting large, bare areas are
open to further, accelerated erosion, endangering adjacent land and
vegetation.
Other guidelines to follow to prevent bluff erosion are:
- Avoid placing heavy materials on the bluff, such as a shed, vehicle
or garage.
- Avoid creating narrow, unvegetated pathways on bluffs. These can
become deep gullies, cut by runoff from above, and enlarge rapidly.
Instead of surface pathways, wooden stairs should be constructed
for access up and down the bluff.
- Avoid adding extra water to the surface of the bluff. This includes
use of lawn sprinkler systems and septic-system leach fields.
Sources:
The above information is
from the New York Sea Grant publications: Vegetation
Use in Coastal Ecosystems [14.4MB
PDF] and A
Guide to Coastal Erosion Processes [7.2MB
PDF].
