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6. CONCLUSIONS AND RECOMMENDATIONS
The inventory of temporary runoff controls on TxDOT construction sites
indicated that silt fences and rock berms were the most commonly used runoff controls on
construction sites. Rock berms were used to treat the drainage from 53% of the area of
the six sites in the study area. Silt fences and sedimentation ponds were the next most
common runoff controls treating 23% and 22% of the total area, respectively. Sediment
ponds were the most inexpensive control on a cost per area basis and were used more
frequently in the earlier stages of construction. Erosion control blankets were the most
expensive controls and tended to be used in the later phases of construction.
Field evaluation of the efficiency of silt fences in removing sediment in runoff
from highway construction runoff showed that the median removal due to filtration was
0%. Additional removal occurred due to particle settling, but was not quantified in the
field portion of the study. The median concentration of solids discharged from the silt
fence controls was approximately 500 mg/L. Geotextile silt fences also proved to be
ineffective in reducing turbidity. The median turbidity reductions for the sites monitored
was about 2%. Monitoring of a single rock berm also showed negligible TSS removal.
The poor filtration performance of the geotextile fabrics alone indicates the
disparity between test efficiency and actual field performance. The bulk of the difference
could be credited to an unrealistic particle size distribution in the slurry mixtures of
previous laboratory studies. Silt and clay size particles were the primary constituents of
construction site generated sediment in this study. The observed data indicated that silt
and clay size particles comprised 92% of the total suspended solids.
The field efficiency of silt fences appears to be dependent mainly on the detention
time of the runoff behind the control. The detention time is controlled by the geometry of
the upstream pond, hydraulic properties of the fabric, and maintenance of the control.
Despite comments by project supervisors that little maintenance of controls was required,
numerous installation and maintenance deficiencies were noted during the study. Holes
in the fabric and inadequate “toe-ins” that result in under-runs reduced the detention time
available for particle settling. In addition, the openings released the discharge in a
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