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Sediment Transport Analysis for Riverbed Mining According to New Puerto Rico Regulations

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Soil Erosion Research Under a Changing Climate, January 8-13, 2023, Aguadilla, Puerto Rico, USA  .(doi:10.13031/soil.23502)
Authors:   Walter F Silva-Araya
Keywords:   River dredging, River sustainability, Sediment transport.

Abstract

The Puerto Rico Department of Natural Resources adopted new guidelines for preparation of sediment transport studies for the extraction of riverbed material; these are the “Guías para la Elaboración de Estudios de Transporte de Sedimentos para la Extracción de Material en los Ríos de Puerto Rico” or “Guidelines for the Preparation of Sediment Transport Studies for the Extraction of Material in the Rivers of Puerto Rico” (Departamento de Recursos Naturales y Ambientales, 2016). This document aims at minimizing the environmental impact and increasing the sustainability of river resources while allowing the use of alluvial materials for construction. The general objective is to manage river extraction such that the affected reach approaches its natural condition with materials from the river and its watershed in a reasonable time after the dredging operations have ended. Dredging must be done at a rate not exceeding the threshold beyond which the replacement of materials will not be possible. Based on the information available, the methodology in the Puerto Rico guidelines allows for: 1.) A geomorphological and vertical stability analysis of the bed to determine if the formation of an armor layer is possible and to compare the present riverbed slope with the equilibrium slope; 2.) A monthly sediment volume analysis under the quasi-equilibrium assumption to examine if the river supplies recoverable material; and 3.) A sediment transport model with the simulation of dredging scenarios.

The case study was made for a reach of Río Majada in the municipality of Salinas, Puerto Rico. Daily flows from USGS gaging stations were available, Yang‘s sediment transport function was selected based on grain size and river velocities, and different scenarios of mass removal based on weekly dredging operations were simulated. The sediment transport capacity was studied using HEC-RAS 5.0.7. A qualitative analysis of other possible sources of sediments not considered by this model was performed to supplement the findings. The climatology of the project site indicates that the average daily and monthly flows are low, however, larger flows of shorter duration do occur providing a significant source of sediment. The soil composition in the banks, composed mainly of a mixture of alluvium with easily erodible stones and steep slopes, is another source of lateral sediment into the river.

The study computed the amount of dredged material, riverbed changes in elevation during several years, including two years of dredging, and recommendations about where and how to proceed with the extraction operation to obtain a significant recovery of the river characteristics after the cessation of operations. Material from a point bar and a sidebar at the project site provided a source that could be naturally refilled as seen in Figure 1.

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