Longitudinal Patterns in Stream Channel Geomorphology and Aquitic Habitat in the Luquillo Mountains of Puerto Rico

The hydrologic, geomorphic, and ecological dynamics of tropical montane streams are poorly understood in comparison to many temperate and/or alluvial rivers. Yet as the threat to tropical freshwater environments increases, information on the dynamics of relatively pristine streams is important for understanding landscape evolution, managing and conserving natural resources, and implementing stream restoration. This dissertation characterizes the geomorphology and hydrology of five adjacent watersheds draining the Luquillo Experimental Forest (LEF) in northeastern Puerto Rico, and discusses implications on aquatic habitat. I performed several interrelated studies, including: 1) formulating a geographic information systems (GIS) framework to estimate hydrologic parameters from topographic information and hydrologic records, 2) developing a method to determine active stream channel boundaries (“bankfull” stage) that allows for comparison of channel geometry on the basis of flow-frequency, 3) decoupling the relative influences of lithologic and hydraulic controls on channel morphology using an extensive field-based stream survey and analysis of stream profiles, channel geometry, and sediment dynamics, 4) linking network- and pool-scale geofluvial dynamics to the abundance of migratory fish and shrimp through a collaborative analysis combining geomorphic surveys and aquatic faunal sampling. This research indicates that these streams have some properties resembling both temperate montane and alluvial rivers. Similar to low-gradient rivers where floodplains mark channel boundaries, the active channel stage in these streams is defined by the incipient presence of woody vegetation and soil development. Systematic basin-scale geomorphic patterns are well-developed despite apparent non-fluvial and lithologic control on local channel morphology. This implies that strong fluvial forces are sufficient to override channel boundary resistance; a feature common in self-forming “threshold” alluvial channels. Migratory aquatic fauna abundances are influenced by a variety of geomorphic factors such as barrier waterfalls and suitable headwater habitat, and are consequently highly variable and patchy. These results stand in contrast to the notion that aquatic communities mirror systematic geomorphic gradients, but rather acknowledges the influences of multiscale geomorphic processes. Ultimately, this research provides baseline information on physical and biological processes in relatively unaltered tropical streams and can be used to inform further studies that document human interactions with stream networks.

Map showing susceptibility to earthquake-induced landsliding, San Juan Metropolitan Area, Puerto Rico

Santiago Marilyn, Map showing susceptibility to earthquake-induced landsliding, San Juan Metropolitan Area, Puerto Rico, 2 plate.em>

Analysis of slope angle and rock type using a geographic information system indicates that about 66 percent of the San Juan metropolitan area (SJMA) has low to no susceptibility to earthquake-induced landslides. This is at least partly due to the fact that 45 percent of the SJMA is constructed on slopes of 3 degrees or less, which are too gentle for landslides to occur. The areas with the highest susceptibility to earthquake-induced landslides account for 6 percent of the surface area. Almost one-quarter (23 percent) of the SJMA is moderately susceptible to earthquake-induced landslides. These areas are mainly in the southern portions of the SJMA where housing development pressures are currently high because of land availability and the esthetics of greenery and hillside views. The combination of new development and moderate earthquakeinduced landslide susceptibility may indicate that the southern portions of the SJMA may be at greatest risk.

The Frequency and Distribution of Recent Landslides in three MontaneTropical Regions of Puerto Rico

Larsen, Matthew C.; Torres-Sanchez, Angel J. 1998. The frequency and distribution of recent landslides in three montane tropical regions of Puerto Rico. Geomorphology 24 :309-331.

Landslides are common in steep mountainous areas of Puerto Rico where mean annual rainfall and the frequency of intense storms are high. Each year, landslides cause extensive damage to property and occasionally result in loss of life. Average population density is high, 422 peoplerkm2, and is increasing. This increase in population density is accompanied by growing stress on the natural environment and physical infrastructure. As a result, human populations are more vulnerable to landslide hazards. The Blanco, Cibuco, and Coamo study areas range in surface area from 276 to 350 km2 and represent the climatologic, geographic, and geologic conditions that typify Puerto Rico. Maps of recent landslides developed from 1:20,000-scale aerial photographs, in combination with a computerized geographic information system, were used to evaluate the frequency and distribution of shallow landslides in these areas. Several types of landslides were documented— rainfall-triggered debris flows, shallow soil slips, and slumps were most abundant. Hillslopes in the study area that have been anthropogenically modified, exceed 128 in gradient, are greater than 300 m in elevation, and face the east-northeast, are most prone to landsliding. A set of simplified matrices representing geographic conditions in the three study areas was developed and provides a basis for the estimation of the spatial controls on the frequency of landslides in Puerto Rico. This approach is an example of an analysis of the frequency of landslides that is computationally simple, and therefore, may be easily transferable to other settings.

Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides

Larsen, M.C., 2001, Evaluation of temporal and spatial factors that control the susceptibility to rainfall-triggered landslides, in Gruntfest, E., and Handmer, J., eds., Coping with Flash floods: Kluwer Academic Publishers, p. 277-288.

How wide is a road? the association of roads and mass wsting in a forested montane environment

Larsen, M.C., 1995, How wide is a road? The association of roads and mass wasting in a forested montane environment, Puerto Rico [abs] American Geophysical Union, EOS Supplement, v. 76, no. 17, p. S309.

Mass wasting has confounded road builders for as long as humans have constructed transportation routes through mountainous terrain. The high-intensity rainfall that is typical of mountainous humid-tropical settings results in mass wasting that is exacerbated by construction and maintenance of roads. However, the actual extent of the zone of mass-wasting disturbance associated with roads is not well known. Determination of this zone is important for land use managers, highway engineers, and foresters who must deal with costly and sometimes life-threatening hazards attributed to road-related landsliding. A spatial data base of 1,859 landslides, representing approximately 50 years of landslide activity, was analyzed using a geographic information system to determine landslide frequency in relation to roads. A 268-km-long transportation network in a 276-km2 area of humid-tropical, mountainous, mostly forested terrain in Puerto Rico was used in the analysis by developing a series of 17 buffer zones varying from 5 to 350 m in length, measured perpendicular to road axes. Most of the study area lies within the boundaries of the U.S. Forest Service-administered Luquillo Experimental Forest. Average landslide frequency in the study area was about 7 landslides per km2. In all buffer zones within 100 m of roads, landslide frequency was higher than the average, and in buffer zones beyond 100 m, landslide frequency was about the study area average. The 100-m buffer zone landslide frequency was 2.4 times higher than the average rate, indicating that within 100 m of a road, associated landslide disturbance is significant. The 100-m buffer length is equivalent to a 200-m wide swath along the 268- km road corridor, representing a total of 49 km2 (or 18 percent) of the total surface of the study area. Thi s means that during the 50 years represented by the landslide sample, for every km of road length, 20 hectares of the study area was affected by this high landslide disturbance rate. In addition, the mass of regolith eroded by mass wasting in the 100-m buffer zone is more than 6 times the mass eroded in areas outside the buffer zone. A total of about 94 tonnes per km2 per year is eroded in the buffer zone by mass wasting, compared to about 15 tonnes per km2 per year in areas outside of the 100-m buffer zone. This represents a considerable portion of the approximately 200 to 400 tonnes per km2 per year of fluvial sediment that is exported from Luquillo Experimental Forest watersheds.

The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico

LÓPEZ, TANIA DEL MAR; AIDE, T. MITCHELL; SCATENA F. N. 1998. The Effect of Land Use on Soil Erosion in the Guadiana Watershed in Puerto Rico. Caribbean Journal of Science, Vol. 34, No. 3-4, 298-307, 1998.

The Revised Universal Soil Loss Equation (RUSLE) was used in conjunction with a Geographic Information System to determine the influence of land use and other environmental factors on soil erosion in the Guadiana watershed in Puerto Rico. Mean annual erosion, suspended sediment discharge, and the rainfall-erosion factor of the RUSLE increased with annual rainfall. Median soil erosion rates varied among the seven land uses: bare soil (534 Mg ha-1 yr-1), open canopy forest (26 Mg ha-1 yr-1), agriculture (22 Mgha-1 yr -1), pasture (17 Mg ha -1 yr -1), less dense urban (15 Mg ha-1 yr -1), closed canopy forest (7 Mg ha -1 yr -1), and dense urban (1 Mg ha-1 yr -1). The differences between open canopy forest, agriculture, pasture, and less dense urban were not significantly different but median values for open canopy forests were slightly greater because they occurred on steep slopes. The five-year average sediment delivery ratio for the basin was 0.17, which is comparable to delivery ratios estimated for watersheds of similar size. Simulations of different land use configurations indicate that reforestation of 5% of the watershed with the highest erosion rates would decrease basin wide erosion by 20%. If the entire watershed was reforested, soil erosion would be reduced by 37%.

Water Withdrawn From the Luquillo Experimental Forest, 2004

Crook, Kelly E.; Scatena, Fred N.; Pringle, Catherine M. 2007. Water Withdrawn From the Luquillo Experimental Forest, 2004. U.S. Department of Agriculture, Forest Service, International Institute of Tropical Forestry. Gen. Tech. Rep. IITF-GTR-34.

This study quantifies the amount of water withdrawn from the Luqillo Experimental Forest (LEF) in 2004. Spatially averaged mean monthly water budgets were generated for watersheds draining the LEF by combining long-term data from various government agencies with estimated extraction data. Results suggest that, on a typical day, 70 percent of water generated within the forest is diverted before reaching the ocean. This is up from an estimated 54 percent in 1994. Analysis showed that up to 63 percent of average monthly stream runoff is diverted from individual watersheds during drier months. Watersheds with large water intakes have the most dramatic decrease in streamflow, particularly the Río Espiritu Santo watershed, where 82 percent of median flow is diverted.

Rainfall, Runoff and Elevation Relationships in the Luquillo Mountains of Puerto Rico

Garcia, A.R. Warner, G.S. Scatena, F. and Civco, D.L. 2002. Bisley Rainfall and
Throughfall Rainfall, Runoff and Elevation Relationships in the Luquillo
mountains of Puerto Rico. Caribbean Journal of Science. 2002 (In press).
Published as Scientific Contribution No. 1642 of the Storrs Agricultural
Experiment Station.

Long-terrn rainfall and discharge data from the Luquillo Experimental Forest (LEF) were analysed to develop relationships between rainfall, stream-runoff and elevation. These relationships were then used with a Geographic Information System (GIS) to determine spatially-averaged, mean annual hydrologic budgets for watersheds and forest types within the study area. A significant relationship exists between 1) elevation and mean annual rainfall; 2) elevation and the average number of days per year without rainfall; 3) annual stream runoff and the weighted mean elevation of a watershed; and 4) annual stream runoff and the elevation of the gaging station. A comparison of rainfall patterns between a high and a low elevation station indicated that annual and seasonal variations in rainfall are similiar along the elevational gradient. However, the upper elevation station had greater annual mean rainfall (4436 mm/yr compared to 3524 mn/yr) while the lower station had a greater variation in daily, monthly, and annual totals. Model estimates indicate that a total of 3864 mm/yr (444 hm3) of rainfall falls on the forest in an average year. The Tabonuco, Colorado, Palm, and Dwarf forest types receive an estimated annual rainfall of 3537, 4191, 4167, and 4849 mm/yr, respectively. Of the average annual rainfall input, 65% (2526 mm/yr) is converted to runoff and the remainding 35% (1338 mm/yr) is lost from the system by evapotranspiration and other abstractions. In comparsion to other tropical forests, the LEF as a whole has more evapotranspiration than many tropical montane forests but less than many lowland tropical forests.

Rainfall-induced landslide susceptibility zonation of Puerto Rico.

Lepore, C. Kamal, S. A., Shanahan, P. Bras, R. L., Rainfall-induced landslide susceptibility zonation of Puerto Rico. Environmental Earth Science, 2011. DOI 10.1007/s12665-011-0976-1

Landslides are a major geologic hazard with estimated tens of deaths and $1–2 billion in economic losses per year in the US alone. The island of Puerto Rico experiences one or two large events per year, often triggered in steeply sloped areas by prolonged and heavy rainfall. Identifying areas susceptible to landslides thus has great potential value for Puerto Rico and would allow better management of its territory. Landslide susceptibility zonation (LSZ) procedures identify areas prone to failure based on the characteristics of past events. LSZs are here developed based on two widely applied methodologies: bivariate frequency ratio (FR method) and logistic regression (LR method). With these methodologies, the correlations among eight possible landslide-inducing factors over the island have been investigated in detail. Both methodologies indicate aspect, slope, elevation, geological discontinuities, and geology as highly significant landslide-inducing factors, together with land-cover for the FR method and distance from road for the LR method. The LR method is grounded in rigorous statistical testing and model building but did not improve results over the simpler FR method. Accordingly, the FR method has been selected to generate a landslide susceptibility map for Puerto Rico. The landslide susceptibility predictions were tested against previous landslide analyses and other landslide inventories. This independent evaluation demonstrated that the two methods are consistent with landslide susceptibility zonation from those earlier studies and showed this analysis to have resulted in a robust and verifiable landslide susceptibility zonation map for the whole island of Puerto Rico.
Syndicate content