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Global Solutions
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Use of GIS for the analysis and design of potable water and sanitary systems
Boris A. Gomez
Civil Engineer/GIS Specialist Louis Berger Group
Via Argentina Edifico del Prado 2A Panama 75
Tel: 011-507-265-3133, Fax: 011-507-213-0544
E-Mail: bgomez@louisberger.com
Abstract
Using case studies in Panamá, Guatemala, México and the Virgin Islands, the
advantages of using GIT to design and develop water and waste water systems are
explored. From site choice to system layout and demand analysis, GIT is key,
especially because it possible to import commercial packages of hydraulic analysis
and use SCADA data, for example, to help achieve optimal, cost-effective solutions.
The benefits of combining environmental and engineering data via a GIS are
described.
Introduction
The process of creating a successful GIS for potable water and sewage system
analysis and design is complex, but not so complex that it cannot be described and
illustrated.
This process should follow the steps outlined in Figure 1 below. The cost of data
acquisition and entry constitute fully 80 percent of the total cost of the system of this
nature, and this stage, are the basis on which the success of the project is measured.
- Data Gathering
This step should start with the compilation of the information related to the project. A
copy of the data obtained will be registered, catalogued and kept in the Project file.
The information consists of electronic information, memories, planes, schemes,
archives, etc.
Along with the compilation of the relevant information, initial visits to the project
area will be made, which will allow to know topographic, city-planning, population
characteristics, the areas of drainage and service area, the water sources and the
locations of the future structures to design.
Figure 1 GIS Database Preparation Flow Chart
A review and analysis of the compiled information is then done. The intention of this
analysis is to acquire a general knowledge of its characteristics and detailed
knowledge of the parameters and bases of the design of the elements and systems.
The Figure 2 shows the relationship between data cost and data quality. This should
be taken in account depending on the definition of the final product level (master plan
or design)
- Preparing Geographic Information.
Based on existing secondary information and field visits (geographically referencing
the key elements using GPS), a database is created that includes aerial photographs,
satellite images, and existing themes files. Layers within the existing systems in the
study area should be obtained from the Client.
Using this database, a hydrologic analysis at the level of sub-river basins should be
prepared, as should a land use map, slope maps, river maps (using commercial models
from Digital Elevation Models), maps of flood plains, maps of existing potable water
and sewage systems, and other maps required for decision-making as the best
alternative is chosen.
This database must also contain the populations to be served, the general location of
infrastructures and all possible water sources and effluent discharge points. All these
maps are laid over aerial photographs that can be acquired, georeferenced and
rectified (depending on where the study is done, these photographs can be recent or a
bit out of date). When possible, three-dimensional images are created. This provides
the specialists with a better idea of the morphology of the zone.
- Modeling
For the modeling development is necessary knows the design criteria as follow:
1. Potable Water
- Pumps in Pumping Station
- Storage
- Minimum Pressures
- Maximum Pressures
- Residual Pressure
- Minimum Pressure Zone
- Additional Minimum Flow for Fire Flow
- Flow for Fire Flow
- Power Supply
- Per Capita Flow
- Pipe Material
- Peaking Factors
2. Sewage System
- Per Capita Flow
- Infiltration
- Pipe Material
- Gravity Pipes Slope
- Velocity in Pressure Pipes
- Peaking Factors
- Land and Existing Network Topology
- In and Out Flows Location
Existing users will first be located within the digitized network based on the
digitalization and incorporation of field information. All the data required by the
modeling are described in GIS Process: Mathematical Model.
Figure 3 Three-dimensional View
GIS Process - Mathematical Modeling
Figure #4 Screen shot of Commercial Sewer System Modeling Software
Potable Water Modeling
The different elements to be included in the Potable Water Model database are:
- Pressure Nodes - As points in the database and having related demand and
elevation alphanumeric fields. The demand is derived from geographic
characterization. The elevation comes from the digital elevation model (DEM)
generated.
- Pipes to Pressure - As lines in the database and having related alphanumeric
diameter, length, initial node and final node fields. The diameters and lengths are
obtained from the base map. The roughness is included as a global value in the
calibration process.
- Pressure Reducer Valves (PRV) - As points in the database and having related
elevation (from the DEM) and setting pressure fields.
- Pumps - As points in the database. The required fields are the elevation and the
assigned power. The elevation is obtained from the DEM
- Reservoirs - As points in the database. These represent the water source
(superficial or groundwater)
- Tanks – As points in the database. Elevation fields are required for the tanks, as
are tank dimensions and initial elevation. The elevation is obtained from the
DEM. The dimensions of the tank and initial elevation are obtained from the
Client.
Sewage Modeling
The elements to be included in the sewage model database are:
- Manholes - As points in the database and having related alphanumeric ground
elevation, incoming flows and losses produced by manholes fields. The elevations
come from the digital elevation model (DEM) that is generated from the
secondary information and from the field for the feasibility stage and after the
final design detailed topography.
- Pipes to Gravity – As lines in the database and having related alphanumeric forms,
material, size, elevations of beginning and end of the pipe fields.
- Wet Wells – As points in the database that represent the chamber where black
water will arrive to be pumped; form, dimensions and elevations are important.
- Pumps - As points in the database that represent black water pumping stations.
The fields that are required for the model are the elevation of the pump, the head
and the flow design.
- Pressure Nodes - As points in the database and related to the elevation field. The
elevations will come from the digital elevation model (DEM) that is generated
from the secondary information and from fields in the feasibility study and after
the detailed topography for the final design is ready.
- Pressure Pipes - As lines in the database and has related the alphanumeric fields of
diameter, length, initial node and final node.
- Outlet - As points in the database that represent the final points of the collection
system. The elevations come from the digital elevation model (DEM).
- Analysis of the Physical, Chemical and Bacteriological Characteristics of
Bodies of Water
The hydrological analysis will be based on the information collected on elevations,
soil types, land use and precipitation rates.
Sample collection and laboratory analysis is needed to know the chemical and
bacteriological characteristics the water. The quality of the water will depend
basically on the geological formations of the area and on human activity (for example,
agricultural, industrial and municipal effluents in waters above the river basin). The
most important parameters to take into account for the chemical and bacteriological
characterization are: total pH, solids, temperature, conductivity, salinity, turbidity,
NO2, NO3, N-NH3, DBO5, fecal coliforms, total coliforms and phosphorus.
- Selection of the Better Alternative
A technical-economic analysis must be undertaken to select and recommend the best
alternative, using a Selection Matrix. This matrix is used to identify the optimal
alternative.
The total points assigned to each alternative is the sum of the assigned points in the
following categories:
- Cost
- Environmental Impact
- Reliability
- Facility of Operation and Maintenance
- Specific Projects
The following table includes the key elements needed for GIS database preparation
and its respective evaluation according to the difficulties found in each country.
| Issue |
Guatemala |
Panama |
British Virgin Islands |
Nogales,Arizona |
| Related Digital Information |
No |
No |
No |
Yes |
| Existing Network Topology |
Regular |
Bad |
Bad |
Regular |
| Costumer Information |
Bad |
Bad |
Bad |
Regular |
| Hydrologic Information |
Regular |
Regular |
Bad |
Good |
| Aerial Photography |
No |
Yes (old) |
Yes (old) |
Yes (recent) |
| Maps at appropriate scales |
Yes |
Yes |
Yes |
Yes |
| Digital Elevation Model |
No |
No |
No |
Yes |
| Existing Models |
No |
No |
No |
No |
| Satellite Images |
No |
Yes |
No |
No |
| Existing Digital Information |
No |
Yes (related to environmental studies) |
Yes (related to environmental studies) |
Yes |
| Land Use Maps |
No |
No |
No |
Yes |
| Groundwater Maps |
No |
Yes |
No |
Yes |
| Related Studies |
No |
No |
No |
Yes |
| Water Quality |
No |
No |
No |
Yes |
Conclusions
Today, in addition to being on time, below cost and above quality, the success of a
GIS project is measured by the creation of a database that makes it possible to carry
out a wide range of analysis. And the quality of a database depends on excellent data
acquisition and definition, which in itself accounts for 80% of the cost of a GIS.
Data definition depends on the country where the project will take place, budget and
the detail required, along with the final drawings and the modeling.
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