Jeffrey J. Clark

Effects of land-use change on channel morphology in northeastern Puerto Rico

Between 1830 and 1950 much of northeastern Puerto Rico was cleared for agriculture. Runoff increased by 50% and sediment supply to the river channels increased by more than an order of magnitude. Much of the land clearance extended to steep valley slopes, resulting in widespread gullying and landslides and a large load of coarse sediments delivered to the stream channels. A shift from agriculture to industrial and residential land uses over the past 50 yr has maintained the elevated runoff while sediment supply has decreased, allowing the rivers to begin removing coarse sediment stored within their channels. The size, abundance, and stratigraphic elevation of in-channel gravel bar deposits increases, channel depth decreases, and the frequency of overbank flooding increases downstream along these channels. This is presumed to be a transient state and continued transport will lead to degradation of the bed in downstream sections as the channel adjusts to the modern supply of water and sediment. A downstream decrease in channel size is contrary to the expected geometry of self-adjusted channels, but is consistent with the presence of partially evacuated sediment remaining from the earlier agricultural period. Reverse (downstream decreasing) channel morphology is not often cited in the literature, although consistent observations are available from areas with similar land-use history. Identification of reverse channel morphology along individual watercourses may be obscured in multiwatershed compilations in which other factors produce a consistent, but scattered downstream trend. Identification of reverse channel morphology along individual streams in areas with similar land-use history would be useful for identifying channel disequilibrium and anticipating future channel adjustments.

Experimental Study of the Solute Transport in the Interfacial Exchange Zone (IEZ) of a Gravel Stream Bed

Solute exchange between overlying water and the sediment bed of a stream have important effects on the chemical mass balance and biological activities in both the water column and the sediment. Previous studies did not consider the coupled and cumulative contributions of turbulence in the overlying water, water surface waves, and underflow to the solute transport in the Interfacial Exchange Zone (IEZ), although these processes have been studied individually. To investigate the interaction, eight carefully designed experiments of solute exchange in the pore system of a stream gravel bed were conducted. The data analysis showed that the turbulent flow over the bed, the underflow and the wave induced advection enhanced the solute exchange and penetration of a conservative solute into a stream gravel bed. The maximum penetration depth under the turbulent current was 15cm in 3000s; however, the penetration depth reached 20cm in less than 1000s when surface waves were present. A coupled hydrodynamic and solute transport model was developed to estimate vertical dispersion coefficients for solutes in a porous and permeable streambed. The enhanced dispersion coefficient that combines all three processes, is a function of the near-bed coherent motion due to the turbulent current, relative dispersivity (longitudinal dispersivity/wave length), wave steepness, sediment hydraulic conductivity and sediment porosity, and decreases exponentially with depth; however, the turbulence-enhanced dispersion without the wave effects is smaller and diminishes faster with depth compared to the wave-enhanced dispersion.