Comprehensive Approach for Flood Hazard Mitigation in Rio Grande de Añasco

Basic Project Information:

Problem and Research Objectives

Mayor floods from the Rio Grande de Añasco spread a total of 8.1 kilometers along the coast of the Mayagüez and Añasco Municipalities. This area has been totally or partially inundated more than seventeen times in the last thirty years. Damages to residential areas, sugar cane crops, pasture lands, roads, bridges, and utility services have been reported continuously. It is urgent to adequately analyze the flood situation in order to implement protection and mitigation measures for the mayor communities inside the flood zone.
The objectives of this project are:

• To study the performance of hydraulic models for the analysis of different alternatives for the protection and mitigation of flood hazards in the mayor communities located in the floodplain of Rio Grande de Añasco.
• To analyze the appropriateness of using one-dimensional models for H&H studies in the Rio Grande de Añasco flood are and to suggest techniques to improve simulations for the studies within the limitations of the models.
• To produce a detailed case-study where the implemented procedures and techniques could be used as raw model for other areas in Puerto Rico and abroad.

Methodology

The project proposes a detailed hydrologic and hydraulic study of the Rio Grande de Añasco. The study is oriented to compare the capacity of hydraulic models as a reliable tool to propose flood control and mitigation measures in highly populated zones such as the city of Añasco, and the warps of La Playa, Algarrobo and Sabanetas. The project will use state-of-the-art GIS and GPS techniques to update field information. A two-dimensional hydraulic model will serve as the basic tool to describe the flood conditions and will be compared with the one-dimensional model to determine the suitability of using current model in H&H studies for the area.

Data Collection and Generation:
Updated data for the Rio Grande de Añasco watershed will include ground elevations, soil types, land coverage, precipitation, and discharges:

• Digital levels combined with GPS will be used to establish a vertical control and differential leveling network. Dual frequency GPS recievers will be used.
• State of the Art GIS techniques will be used to make an exhaustive analysis of land cover within the watershed. Aerial photographs, DLGs, LANDSAT and SPOT imagery will serve as raster data for land cover reclassification.
• Digitized NRCS soil type data layers will serve as base data for soil classification.

The NRCS method is proposed for the hydrologic model. Modeling will be preformed using the USCoE's Geo-Hydraulic Modeling System (GEO-HMS) and the EMSI's Watershed Modeling System (WMS) software package. Results from the two models will be compared in order to provide recommendations on the approprate use of each.

• Precipitation Data: Records from 5 meteorological stations will be used as base data to perform analysis to supplement TP-42 regionalized precipitation values.
• Discharge Data: USGS gage station records will be used to calibrate output from the two models. A discharge frequency analysis would be done to compare the 100-yr flood with that obtained with the models at the USGS's station. Results from the hydrologic investigation will provide the flow data needed by the hydraulic models.

A two-dimensional hydraulic model will be created for the Rio Grande de Añasco using the finite element program FESWMS from the federal Highway Administration. This program is part of the Surface Water Modeling System package.

• The model will be completed with representative eddy viscosities and Manning's coefficients acording to the soil cover and other topographic features.
• Several flood protetion measures will be considered in the model.