urban development

Reduced channel morphological response to urbanization in a flood-dominated humid tropical environment

Phillips C.B., Scatena F.N. Reduced channel morphological response to urbanization in a flood-dominated humid tropical environment. Earth Surface Processes and Landforms. 2012. DOI: 10.1002/esp.3345

Urbanization through the addition of impervious cover can alter catchment hydrology, often resulting in increased peak flows during floods. This phenomenon and the resulting impact on stream channel morphology is well documented in temperate climatic regions, but not well documented in the humid tropics where urbanization is rapidly occurring. This study investigates the long-term effects of urbanization on channel morphology in the humid sub-tropical region of Puerto Rico, an area characterized by frequent high-magnitude flows, and steep coarse-grained rivers. Grain size, low-flow channel roughness, and the hydraulic geometry of streams across a land-use gradient that ranges from pristine forest to high density urbanized catchments are compared. In areas that have been urbanized for several decades changes in channel features were measurable, but were smaller than those reported for comparable temperate streams. Decades of development has resulted in increased fine sediment and anthropogenic debris in urbanized catchments. Materials of anthropogenic origin comprise an average of 6% of the bed material in streams with catchments with 15% or greater impervious cover. At-a-station hydraulic geometry shows that velocity makes up a larger component of discharge for rural channels, while depth contributes a larger component of discharge in urban catchments. The average bank-full cross-sectional area of urbanized reaches was 1.5 times larger than comparable forested reaches, and less than the world average increase of 2.5. On average, stream width at bank-full height did not change with urbanization while the world average increase is 1.5 times. Overall, this study indicates that the morphologic changes that occur in response to urban runoff are less in channels that are already subject to frequent large magnitude storms. Furthermore, this study suggests that developing regions in the humid tropics shouldn’t rely on temperate analogues to determine the magnitude of impact of urbanization on stream morphology. Copyright © 2012 John Wiley & Sons, Ltd.


Worldwide, suspended sediment flux entering the oceans each year is increasing due to human activities. Land development in Puerto Rico is causing the island’s mean annual suspended sediment flux to increase. These high suspended sediment yields are unfavorable for human recreation, water treatment, as well as habitat protection. This study attempts to determine if land development in eastern Puerto Rico is significantly impacting suspended sediment (SS) levels. In eastern Puerto Rico lies the Luquillo Experimental Forest (LEF), which has been protected from land development since the 1940s. With 75 percent of the Island’s virgin forests, seven streams (Quebrada 1, Quebrada 2, Quebrada 3, Rio Mameyes Puente Roto, Quebrada Sonadora, Quebrada Guaba, and Rio Icacos) were chosen to represent “natural” suspended sediment conditions. Examining the relationships between drainage area, discharge, suspended sediment concentrations, slope and suspended sediment loads, these seven streams were compared to one stream outside of the national forest, Rio Mameyes Rt. 3, whose watershed is partially used for agriculture and urban purposes. Results indicate that humans are not significantly impacting suspended sediment levels in the Rio Mameyes Rt. 3. Because land development in the Rio Mameyes Rt. 3 watershed is relatively small, less than 20 percent, it is feasible that the agriculture and urban development are not drastically affecting SS levels or the sampling period did not capture the full range of variability. A principal component analysis of drainage area, suspended sediment load, discharge, slope and temperature of the eight streams shows that only one factor, which contains all components, can be extracted.
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