The role of the CNA
Mexico’s National Commission of Water (CNA) is heir to a great tradition of hydraulic engineering, and during the course of its existence has been fortunate in having outstanding professionals and specialists from various disciplines within its ranks. The organization has achieved international acclaim for its dedication and technical capabilities.
Its remit is to administer and preserve Mexico’s waters, with public participation, in order to achieve sustainable use of resources. To help it fulfil its obligations, CNA is divided operationally into central offices and regional and state management departments. CNA Chiapas is the regional department for the southern border state of Chiapas, dealing with technical hydrological analysis.
The headquarters of the CNA is in the capital, Mexico City. Its main aims are:
- to support the regional and state managements in realizing the actions necessary to achieve sustainable use of water in each region;
- to establish the politics and national hydraulic strategies;
- to integrate the institution’s budgets and to watch over their use;
- to work together with national and international financial bodies on the credits they require for the hydraulic sector;
- to establish programs to support the municipalities in supplying potable water and maintenance work in cities and rural communities;
- to promote the efficient use of water for irrigation and industry.
Central offices also establish collection policies and inspection regimes for abstraction and discharge permits, coordinate any modifications necessary to the National Water Law and support their application. They set out standard procedures for water-related issues, operate the national meteorological service, and maintain a close and fruitful relationship with the Congress of the Union, which provides assistance to the national media and is linked to the federal organizations (dependencies) that coordinate actions that benefit the water sector.
The Gerencia Regional Frontera Sur project
The Gerencia Regional Frontera Sur project is being undertaken with the CFE (Mexico’s Energy Commission, responsible for hydro-electric dams), and aims to identify areas of potential flooding in local Federal Zones – sections of rivers and nearby territories in public ownership and managed by the state that are regulated by the CNA.
CNA’s regional offices have been given the task of estimating the federal zones of the rivers in their areas, as a vital part of planning and characterization of urban developments in cities close to streams and lakes. The project also examines flood risks in these areas.
Rivers’ federal zones are defined in many different ways: in Spain, the zone is a 5m service zone along river banks with a 100m wide management area; in Bolivia, rivers have to have a 3m service zone along their banks for public use; and in Chile the federal zone is obtained from the river’s maximum height over a ten-year period, with an 80m extension beyond that line.
Mexico’s Water Law defines a 10m contiguous strip next to a river channel that is national property, measured horizontally from the water level, averaged over a period of five years. The boundaries are from 100m of the river’s source to its point of discharge into the sea.
The Grijalva river is the main river in the Chiapas region, and was one of the key rivers in the CNA Chiapas study. The main elements within the study were AutoCAD files and digital elevation models, utilized by a topographic working group, working on desktop and notebook computers.
Setting out the boundaries of the federal zones of channels is mainly undertaken using topographic characterization by traditional methods.
The current project has focused on determining federal zones using remote monitoring data, emphasizing the economic advantages and reductions in run times. The objective has been to provide a method guide for working out federal zone boundaries in future, mainly based on integrating photogrammetric data to help work out channel topography.
Modeling the river channel
The section of the river between la cortina de la Presa La Angostura and the beginning of the Vaso de la Presa Chicoasén was modeled using InfoWorks RS. The objective was to determine possible flood zones along the river channel, created by discharges from outfall pipes from La Angostura y Chicoasén hydro-electric dam, which is sited between La Presa Angostura and El Vaso de la Presa Chicoasén.
The Presa La Angostura outfall was blocked in 1995 after a series of flood events in the area, but in 1999 a combination of prolonged, major abstractions by the dam’s turbines and unusually high flows contributed by the rio Santo Domingo in the Vaso de Chicoasén river basin caused flooding in some parts of Chiapa de Corzo city.
It was therefore decided that it was necessary to discover the flood potential of the downstream river with the obstruction removed from the outfalls. Some 150 cross-sections were taken at 500m intervals, and records of levels in the La Angostura y Chicoasén dam were utilized. Relevant rainfall data was also added to the CNA and CFE networks.
For the simulation, 77km of topography was mapped, at 1m contours. The first stage involved generating digital elevation models (*.TIN and *.GRD) followed by integration of the numeric model and calibration of basic parameters, which was followed by a 1000m3/sec flow simulation.
To obtain the channel topography, photogrammetric digital restitution was used, and in the case of the Santo Domingo river, a comparison was made between photogrammetric data and topographic surveys.
The working group began by using regular grids, which take up less hard disk space but don’t define topographic details. The federal zone was estimated by calculating flow profiles for the course of the river generated for a return period of five years.
It was discovered that a photogrammetric scale of 1:7500 is able to provide an acceptable resolution for channels over 60m wide.
The project found that defining the boundaries of Federal Zones using photogrammetry is fast and economic, the precision of the process being dependent on the scale of the photogrammetric restitution and the width of the river channel. The advantages are the speed, economy and precision of the method.
The second stage involved simulating flows of 2100m3/sec and 3000m3/sec. The contributions of the Santo Domingo and Suchiapa rivers without considering simultaneous 100-year returns were estimated at 3100m3/sec.
It was also necessary to focus on vital urban zones that might be prone to flooding, and determine zones affected by ground use. The run time for the simulations was 15 days.
The third stage, which remains to be done, will involve estimating potential damage for construction regulator Secretaría de Obras Publicas y Vivienda