August 2, 1999
Prepared for:
V.I. Department of Planning and Natural Resources, Division of Environmental
Protection
by Julie Wright, University of the Virgin Islands Cooperative Extension
Service
Growth, soil cover and survival rates were evaluated during a six-month period. With occasional watering, talinum, inch plant, spider lily, ground orchid and vetiver were all easily propagated. Sunrose did not propagate well and died during the dry period. Talinum and inch plant provided the greatest percentage of ground cover within six months. Ground orchid did not provide sufficient ground cover and was taken over by weeds. Native plants out-performed exotics (with the exception of vetiver) in this trial.
A second trial was initiated in May 1999. Exotic plants lantana (Lantana
sp.), mini rhoeo (Rhoeo sp.), purple queen (Tradescantia pallida), wandering
jew (Zebrina pendula), blue daze, purslane and "jump-up-and-kiss-me" (all
Portulaca sp.) were planted on cleared slopes (20 - 30%) above the CES
St. Thomas office driveway and parking area. Growth, soil cover and survival
rates are being evaluated for these plots also.
on
and establishment rates were quicker than those of bermuda grass (Cynodon
dactylon), which was previously used. It was found that bahia grass germination
and propagation rates were in fact slower than those of bermuda grass.
However, once established, bahia grass tended to be hardier than bermuda
grass.
A
variety of ESC practices were installed during the field day demonstration
sessions of the Practical Approaches for Effective Erosion &
Sediment Control (ESC) training workshops held May 30, 1998 on
St. Thomas and June 2 on St. Croix. Workshop participants were given installation
instructions during in-class training the day before the field sessions.
During
the St. Thomas workshop, steep, south-facing slopes at the Cost-U-Less
development site were hydroseeded using bahia, rye (Lolium perenne) and
bermuda grass seeds, cellulose (paper) fiber hydromulch, and a soil binder.
Bermuda grass seed was also hand-broadcast in a sediment retention pond
overflow swale at the site. On St. Croix, bahia grass was hydroseeded,
along with cellulose fiber hydromulch and a soil binder, and hand-broadcast
at a home/farm site in Estate St. George's Hill.
Bermuda
grass hand-broadcast at the St. Thomas site germinated well beneath the
straw/coconut fiber mat, but did not become well established, possibly
because it was inhibited by an adjacent tamarind tree. The bahia and bermuda
grass that was hydroseeded germinated and is becoming well established
on the shady slopes. The grass hydroseeded on unshaded slopes died because
the seedlings did not get enough water to grow after germination and were
burnt by the hot sun.
Bahia grass hydroseeded at the St. Croix sites was quickly over grown
by guinea grass. However, bahia grass seed that was hand broadcast and
then covered with the excelsior erosion control mats germinated and propagated
well.
CES
and the VI Housing Authority cooperated to demonstrate the use and effectiveness
of erosion and sediment control blankets/mats for slope stabilization.
Erosion control mats were installed in November 1997 on disturbed slopes
along the road at the Donoe Housing Community (St. Thomas).
CES purchased 100% straw fiber (S2) and 70% straw/30% coconut fiber
mix (CS2) erosion control mats and staples and provided VIHA grounds personnel
with training on proper installation procedures.
VIHA
grounds personnel prepared the area (cleared brush, removed debris, stones
and stumps, and raked to smooth the slope surface) and installed the matting
along steep (> 60%), stony, droughty slopes. CS2 mats were used for the
steepest sections (specified for 1:1 slopes up to 75' in length), and S2
mats (specified for 2:1 slopes up to 75' in length) were used for less
steep sections. Mats were installed around desirable vegetation (ginger
thomas - Tecoma stans, pink cedar - Tabeburia heterophyllo and bougainvillea
- Bougainvillea glabra) that was kept on the slope during the clearing
process. Ground orchid (an exotic ground cover adapted to steep, stony,
droughty slopes) and ginger thomas (a native bush that was already present
at the site) were planted within the matting to help further stabilize
the soils. CES provided technical assistance, training, and materials to
the Virgin Islands Housing Authority (VIHA), and continues to work with
VIHA and Donoe residents on ESC projects.
A CS2 erosion control mat was installed during the St. Thomas ESC training
workshop to demonstrate the use of ESC mats for channel stabilization,
proper installation techniques for the practice, and to evaluate its effectiveness
in promoting grass seed germination, as well as its durability. The mat
was installed in the overflow swale from a sediment retention pond.
At the St. Croix ESC training workshop, excelsior (wood fiber) mat and
hay mulch were installed on a steep slope (>45%) at a home/farm site in
Estate St. George's Hill (see photo, page 2). Workshop participants first
seeded the cleared slope with bahia grass seed and then installed excelsior
matting on one segment and hay mulch on an adjoining segment in order to
compare the two practices for erosion control effectiveness and longevity.
The mats installed at all sites have biodegraded significantly. The
straw/coconut fiber (CS2) mat has been the most durable, taking approximately
6 months to disintegrate, whereas the excelsior mat installed on St. Croix
completely disintegrated after only 2 ½ months; this may be partially
due to the high rainfall that occurred during June and July 1998.
Silt fencing was also was installed at the CES - St. Thomas Home Demonstration
Garden in March 1999 to prevent soil on the slope above the driveway from
eroding and silting in the driveway. Signs describing the project and comparing
good and bad silt fence installation practices were produced and the fence
is being maintained as a demonstration of proper installation. The fence
is functioning properly in keeping sediment from flowing off the slope
onto the driveway.
A
triangular dike was also installed during the St. Croix ESC training workshop
to demonstrate how it can be used as a check dam in a swale leading to
a drainage culvert. Participants cleared the area for the dike, anchored
it in, and backfilled soil and rock on top of dike flaps to hold it in
place. During a follow-up site visit on June 29, 1998, the triangular dike
was found to be functioning properly, with approximately one inch of sediment
observed accumulated behind the dike. Triangular dikes were also used on
the St. Thomas CES parking lot during excavation and grass germination
to prevent soil from washing off the driveway and into the road (see below).
CES
staff prepared the CES Home Demonstration Garden (removed weeds, cut back
invasive bush, dug out and removed diseased trees) prior to the demonstration.
Staff worked with a UVI architect to map out the St. Thomas CES office
driveway and parking lot demonstration sites. The parking lot and driveway
were then excavated and graded for the demonstration; the lot was also
rolled with a vibrating roller to smooth it.
Installation occurred in early February 1999 by CES staff with assistance
from crew and the product's distributor in Puerto Rico. The web installation
in the driveway was conducted as part of a hands-on workshop for thirty-seven
(37) attendees. Participants were shown a video describing the use and
proper installation of the material, and then participated in installing
the webbing. Each 20' long section of the GeoWeb® was stretched and
anchored (using j-bars along the sides and end pieces) down the driveway,
and the sections were joined with staples. The webs were then filled (by
backhoe) with crushed stone and soil, and the stone and soil was rolled
with a vibrating roller to enhance compaction.
CES
staff installed the GeoBlocks® in the parking lot the following week.
The blocks were laid flat and interlocked, with the edges anchored using
j-bars. Uneven edges were cut using a saw to provide more level coverage.
A backhoe was used to filled the blocks with topsoil, the soil was raked
level by hand, and stones were removed. The parking lot was then seeded
with a mix of bermuda, rye and bahia grasses and covered with straw erosion
control mats to hold in moisture and prevent birds from eating the grass
seed.
The parking lot was watered and fertilized to promote faster grass germination
and establishment. Parking was prohibited on the lot for three (3) months
so that the grass could become well established. Heavy use of the drive
portion of the lot has damaged some of the grass, but the grass in the
parking areas along the side is holding up well.
lot
demonstration.Inch plant is very successful at covering steep, droughty, rocky slopes. Talinum is very successful in covering bare soils ranging from moist to dry conditions. These species will work well in reducing erosion from small storm events, but neither of these species has a thick root system (such as bermuda, bahia, hurricane or carpet grass) or deep root system that can prevent mass wasting (landslides) that may occur during large storms or hurricanes.
Vetiver is easily propagated from cuttings. The first method of propagation involved removing sections from a parent plant (root and leaf material), trimming excess roots (leaving only 3") and (leaving 4" of leaf). "Plugs" were then planted directly in the demonstration plot and watered daily. This first planting took 4-5 weeks to develop new shoots. However, we discovered a faster, more reliable method of propagation: materials were prepared as before but then rooted in water for about 4 days (until new root material was observed). The plugs were then transplanted into the demonstration plots. This second planting took 2 weeks to develop new shoots. Using this method of propagation, no mortality occurred, whereas the previous method resulted in approximately 20% plant mortality.
Hydroseeding can be a very effective
erosion control practice, but only if sufficient moisture is available
for grass germination and propagation. This means that bare soils should
be seeded either during the rainy season or else supplemental water must
be provided to ensure success. Success rates increase in areas where shade
is present, particularly on north-facing slopes, and if a mulch is provided.
Straw mats (100%) are specified for use on 2:1 (25%) slopes up to 75' in length, but were observed to prevent erosion on shorter slopes up to 1:1 (50%) steepness. Straw (70%)/coconut (30%) fiber mats are specified for use on 1:1 slopes up to 75' in length, and were observed to perform well on even steeper slopes (~60%) that were shorter in length (no greater than 50'). Excelsior fiber mats were not sufficiently durable under demonstration conditions.
Grassed block/grid cellular confinement systems can work well
in controlling erosion and stormwater on fairly level, low-use traffic
areas, such as emergency access areas, driveways or parking lots. For higher
or heavier used parking areas, gravel fill would be more appropriate, as
continual vehicular traffic damages the grass base. Some disadvantages
for use in the V.I. have been observed on the grassed demonstration system.
First, the subsurface must be completely level for the blocks to lay properly.
Secondly, for a grassed parking area that requires soil fill, the blocks
should be filled by hand in order to avoid over-filling with soil. Over-filling
reduces the blocks ability to protect grasses planted within the system
so that continual traffic will damage the sod.
CES will continue to maintain and install erosion and sediment control practices to promote pollution prevention in the territory. For more information on erosion and sediment control, contact the UVI Cooperative Extension Service at (340) 692-4080 on St. Croix or (340) 693-1080 on St. Thomas or visit our website at: http://rps.uvi.edu/CES/wqhome.htm
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For further information on UVI-CES's Erosion Control Demonstration Project, or CES Water Quality Programs, or to provide us with feedback on this website, please contact Julie Wright at (340) 693-1082.
