STREAM CHANNEL RESPONSE TO URBANIZATION IN THE HUMID TROPICAL REGION OF NORTHEAST PUERTO RICO

This study investigates the effects of urbanization on channel geometry and morphology in the humid sub-tropical region of North East Puerto Rico, an area that is characterized by frequent high-magnitude storms, and rivers with steep coarse-grained channels. This study compares grain size, low flow channel bed roughness, and the hydraulic geometry of streams across a land use gradient that ranges from pristine forest draining the Luquillo Mountains to high density urbanized catchments in the San Juan Metropolitan area. The measurable effect of urbanization on these streams is less than that reported for comparable temperate streams. Continued development in the region has resulted in increased fine sediment in urbanized catchments and the introduction of anthropogenic debris. Materials of anthropogenic origin comprise an average of 6% of the bed material in streams with catchments with 15% or greater impervious cover. No measurable difference was discernible in stream bed roughness between forested and urban channels. 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. On average bank-full cross sectional area of unchannelized 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 did not change with urbanization while the world average increase is 1.5 times. Direct channelization in urban areas resulted in reaches with average cross sectional areas 3.3 times larger than comparable forested reaches.