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| Titel |
Efficiency of erosion mitigation strategies in reducing sediment-loading rates from unpaved road networks into coral reef-bearing waters of the Eastern Caribbean |
| VerfasserIn |
Carlos Ramos-Scharron, Sarah Gray, Whitney Sears |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250088210
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| Publikation (Nr.) |
 EGU/EGU2014-3188.pdf |
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| Zusammenfassung |
| Erosion from unpaved road networks represents a critical source of stress affecting the coral
reef systems of the U.S. Virgin Islands in the Northeastern Caribbean. Combined
community- and government-driven efforts to reduce sediment contributions from unpaved
roads in the island of St. John have consisted in improving road drainage design,
paving selected road segments, and constructing sediment retention structures.
Here we describe empirical evidence attesting to the efficacy of these mitigation
efforts.
Road drainage improvements reduced sediment production rates to about a third of
pre-treatment levels. Road-segment scale erosion rates following paving ranged
from 5-30% of pre-treatment levels, depending on road slope and road grading
frequency. A 616-m3 sediment retention pond proved to contain 86 Mg of sediment
annually and about 94% of the runoff generated from a 12 ha sub-catchment with
an unpaved road density of 19 km km-2. Watershed-scale modeling evaluations
suggested that the combination of these three treatments within the 13-km2 Coral Bay
watershed resulted in the reduction of annual sediment delivery rates from 445
Mg yr-1 to 327 Mg yr-1. Cost-effectiveness analyses suggest that road drainage
improvements and construction of the detention pond provided the greatest reductions in
sediment delivery per total amount of funds spent. Even though paving is a proven
erosion control method, the high costs involved made it a relatively cost-inefficient
method.
Marine sedimentation of terrigenous sediment (land-derived) was regularly monitored
(every 26 days) at 15 near-shore and reef sites from 2008 to 2013 below the treated and
undeveloped watersheds. Sediment composition (% terrigenous) determined by loss on
ignition was multiplied by the total sediment accumulation rate in tube sediment traps to
obtain terrigenous sediment accumulation rates (in mg cm-2 d-1). Mean terrigenous
sediment accumulation rates were over 24 (near-shore) and 6 (reef) times greater below
developed compared to undeveloped watersheds. Comparisons of marine terrigenous
accumulation rates between undeveloped and developed areas were consistent with
those based on modeled sediment yields. At all near-shore sites below mitigated
watersheds, mean post-mitigation marine terrigenous sediment accumulation rates
were reduced by up to 70% of mean pre-mitigation values. Nevertheless, further
analysis is required to evaluate whether the observed post-mitigation reduction is
strictly related to watershed restoration. At the developed coral reef sites, acute
terrigenous sediment accumulation rates during major storms equaled or exceeded
coral-stressing levels (>50 mg cm-2d-1). Results from this research are being
employed in the design of mitigation strategies within our study areas as well as in other
parts of the Caribbean where roads are considered a major threat to coral reefs. |
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