26 Studies of Depositional Systems and Sedimentary Rocks: In Honor of Edward Scudder Belt

Tuesday, 26 March 2002
1:00 PM - 5:00 PM

SEDIMENTATION AND RUNOFF ONTO THE REEF SURROUNDING ROATAN, BAY ISLANDS, HONDURAS

Paper number: 26-2
Program: SYMPOSIA
Time: 3:05 PM – 3:20 PM

Authors

Abstract

The island of Roatan , the largest of the Bay Islands, a chain of Caribbean islands roughly 60 km off the north coast of Honduras, is surrounded by a barrier and fringing reef. It has been estimated that this small island of approximately 160km2 has as many as 82 watersheds. The reef surrounding Roatan, one of the world's most beautiful, is experiencing sedimentation rates that seasonally are 10X those reported from other reefs in the Caribbean. As part of a larger study on the Roatan reef, we are examining the rate of sedimentation, total suspended solids in the reef water, and sediment composition. Some of the sediment accumulating in traps on the fore reef are terrigenous in composition, which raised the question, "what is the contribution of terrignenous material to the reef from the numerous small watersheds?" a question made more relevant by the rapid land use change the island is currently experiencing. Initially we chose 5 watersheds for study however difficulty of access led to only 2 watersheds being studied in detail, seasonally over a 2 year time period. We did not measure stream bedload, so terrigenous loading values presented here represent suspended load only. Because our previous studies indicate that the bedload is trapped on the shoreface and does not mix into the adjacent lagoon we feel that measuring the suspended load is an adequate estimate for what material might ultimately move across, and impact, the reef.

permanent stream

ephemeral stream

1999 fairweather

2001 fairweather

1999 fairweather

2001 fairweather

Q=13,105cm3/sec

Q=56,800cm3/sec

Q=0

Q=0

tss=4mg/l

tss=234mg/l

loading=52.5 g/sec

loading=13,291g/sec

1999 storm

2001 storm

1999 storm

2001 storm

Q=2,289cm3/sec

Q=537,882cm3/sec

Q=639cm3/sec

Q=1,185cm3/sec

tss=74mgl/l

tss=2,280mg/l

tss=14,170mg/l

tss=15,000mg/l

loading=170g/sec

loading=1,226,391g/sec

loading=9,054,630g/sec

loading=17,775000g/sec

We are currently using a fingerprinting technique on selected trace elements in the suspended sediment to see if variations in concentrations reflect bedrock composition on the island. This will enable us to determine how "closed" individual watersheds on the island may be, an important factor on an island characterized by highly variable land use.