Management of sedimentation in tropical watersheds

Background

The sedimentation of reservoirs is a serious problem throughout the tropics, yet most attempts to control sedimentation in large river basins have not been very successful. Reliable information on erosion rates and sources of sediments has been lacking. This paper discusses some of the limitations in estimating erosion and sedimentation and encourages development of specific sediment budgets in order to extrapolate equations and generate management procedures.

Research goals & methods

Estimates of natural erosion in the tropics is often hampered by the episodic nature of high-erosion events caused by storms in montane and geologically unstable region. The predominance of biomass degradation in some watersheds can result in an unexpectedly high ratio of bedload to suspended load, shifting sedimentation to ‘‘live’’ rather than ‘‘dead’’ storage within reservoirs. Another limiting factor is an assumption that a primary source of sediments is agricultural with inappropriate use of the Universal Soil Loss Equation. Efforts aimed at reducing reservoir sedimentation by controlling agricultural sources of erosion may have limited benefits if the principal sources are of natural origin or are associated with construction of the dams and reservoirs and with rural roads and trails. Finally, the most appropriate locations for watershed rehabilitation must depend on the magnitude of temporary storage of colluvium and alluvium within the river basin.

Conclusions & takeaways

Systematic development and analysis of sediment budgets for representative watersheds is needed to address these limitations and thereby improve both the planning of river basin development schemes and the allocation of resources towards reducing sedimentation. When sedimentation of reservoirs is the key issue, sediment budgets must focus especially on channel transport rates and sediment delivery from hillsides. Sediment budgets are especially critical for tropical areas where project funds and technical help are limited. Once sediment budgets are available, watershed managers will be able to direct erosion control programs towards locations where they will be most effective.