Coastal Erosion of Atibainha Reservoir As an effect of wind fetch and maritime traffic Gerard L. Block Columbia University

Introduction Soil erosion from land areas is widespread and adversely affects all natural and human-managed ecosystems, including agriculture and forestry. (Pimentel 1998) Soil errosion is not confined to inland areas, however, and can be at times most extreme upon the coasts of landmasses. Waves produced by winds and maritime traffic each effect the composition of the shores upon which they break. An study was planned to determine if the coast of Atibainha reservoir of Sao Paulo state, Brazil had been eroded by waves produced by winds and watercraft. Taking into account the fact that the reservoir was completed in 1972, the experiment was designed to measure soil loss to coastal areas over the controlled period of time from the completion of the reservoir to the present, a time span of 29 years.

Hypotheses for the experiment consisted of the null and an alternate. They were: Null Hypothesis: Boat wakes and wind blown waves do not cause the erosion of the coastal soil of Atibainha reservoir. Alternate Hypothesis: Areas of coast exposed to waves produced by high wind fetch and maritime traffic are subject to coastal erosion.

Methods: Soil Coring and Depth Measurement Soil Coring was conducted with the use of a soil corer. Cores were taken 18 times at each site; 6 at each distance: Shore, 1.5 meters from shore, and 3 meters from shore. The depth measurement process was conducted three times per site, with measurements taken every 0.5 meters to a distance of 4 meters from shore. Study Sites Two sections of the coast of Atibainha reservoir were selected for the study. The first was a peninsula exposed to large amounts of wind and vessle created waves, the second, a bay well protected from the same factors.

Results Depth Measurement returned positive results, showing a marked difference between the high and low errosion sites in terms of underwater drop off, following the prediction that a site of high erosion would have a shallower drop off, due to the build up of silt from the eroded shore. (Figure 1) Soil Coring returned significant data, although it was opposite of that predicted in terms of high and low erosion sites. (Table 2)

Figure 1

Soil Coring Results Table 1

Conclusions Depth measurements followed predictions and suggest that significant erosion was occurring at the high erosion site. Soil Coring was apparently corrupted be rain-based run off erosion, leading to unpredicted findings. However, data was on trend to conincide with predicted findings at greater distances from shore.