New GPS/GIS technologies help Miami solve water and sanitary sewer issues
Water may well be Miami’s blessing and curse. Miami’s water – the ocean, the Everglades – that draws millions of tourists to the region each year. It’s water – the kind whipped up by tropical storms and hurricanes – that creates a variety of problems for Miami during the wet season.
One of those problems is sanitary sewer overflows (SSOs). In the late 1980s and early 1990s, a series of SSOs flooded downtown intersections and spilled raw sewage into the Miami River.
Miami is particularly hard hit by SSOs because of its terrain. The highest elevation is only 40 feet above sea level, and the groundwater table is 3 to 6 feet below the earth’s surface. When it rains, the ground sucks water through the sandy earth; some makes its way into the cracks of the sanitary sewer pipes. When a lot of unplanned-for water gets into these pipes, the system is overloaded. Clogs occur. Pump stations fail. And sanitary sewers overflow.
And today the Miami-Dade Water and Sewage Department (MDWASD) is in the midst of a $1.1 billion project to upgrade its sanitary sewer system, a sprawling utility created in 1973 from 30 smaller ones. The project (due for completion by 2002) includes major force main and pump station capacity improvements; upgrades and expansions of three wastewater treatment plants; and studies of how the department manages and operates the water and wastewater utility.
Then there is the creation of a geographic information system (GIS) for MDWASD, which will depict the entire 414-square-mile service area under its jurisdiction. The system is being created using global positioning system (GPS) satellite surveying and GIS methodologies. It will be one of the largest, most accurate utility infrastructure management systems ever constructed.
The project involves converting MDWASD’s 1 106 paper water and sewer atlas maps to digital format; combining planimetric data created in ARC/INFO with the digital water and sewer atlas maps; using GPS surveying to locate more than 180 000 aboveground water and sanitary sewer structures at near-centimeter-level accuracy; and developing an accurate and complete utility network for use in ARC/INFO and ArcView.
WHY GPS AND GIS WERE USED
MDWASD knew it needed accurate information to help meet consent decrees and upgrade both the sanitary sewer and potable water systems. Although the utility had thousands of source documents, even greater accuracy would be required for modelling purposes. And structures were being added to or altered daily, demanding a better method than producing paper maps twice a year to keep the data current.
Real-Time Kinematic (RTK) pen-based PC GPS – an industry first – was selected as the in-the-field data-collection method, because it could identify aboveground structures at near-centimeter-level accuracy. Originally 170 000 structures (valves, hydrants, treatment plants, manholes, pump stations and so on) were to be identified, but that number increased to 182 000 as additional structures not featured on the source documents were found in the field. Now, with the GPS primary sweep almost finished, MDWASD has a near-complete inventory of its sanitary sewer and water features.
Real-Time means raw data is collected and processed immediately via radio link between a base station and GPS surveyors using pen-based PCs. Surveyors see results – position and accuracy – in real time, and can confirm that sufficient data has been collected while still in the field. Kinematic means three-dimensional centimeter-level accuracy, obtained in as fast as five seconds as opposed to minutes, allowing data collection to be completed with fewer crews and within a one-year time frame.
RTK pen-based PC GPS was a new marriage of technologies created in a collaborative effort between Woolpert, the Dayton, Ohio-based prime contractor, and Trimble, a Sunnyvale, Calif.-based GPS technology firm. RTK GPS and pen-based PC data-capture solutions were available as separate technologies, but merging the RTK technology with software and a rugged pen-based PC was an industry first – and one that met MDWASD’s needs for quick, accurate data collection.
GPS-to-GIS integration proved easy with the new GPS method. Prior to the GPS phase, MDWASD’s 1 106 1"00′ scale water and sewer atlas maps were scanned. The resulting images were georeferenced to the department’s existing photogrammetrically created planimetric base map containing edges of pavement and property data; and these images were loaded into the pen-based PCs and used as a backdrop GIS by surveyors in the field. Attribute data collected in the field was automatically formatted for bulk-loading into the correct GIS data layers. The process ensured immediate quality control and eliminated the need for cumbersome paper maps.
The primary sweep has gone five times faster and cost up to 50% less than post-processed kinematic GPS and conventional utility-location surveying techniques such as photogrammetry.
THE GIS – VIRTUAL REPRESENTATION
Creating a GIS from the highly accurate GPS data was the next step. And like the GPS, the GIS proved groundbreaking. All geographic information systems are spatial: structures are identified in relation to other structures. The MDWASD GIS is not only spatial – it’s horizontally and vertically accurate down to the near-centimeter-level collected in the field-inventory phase. In other words, the GIS is a virtual representation of what’s on top of and in the ground.
This kind of precision posed cartographic challenges. No GIS had achieved legible near-centimeter-level accuracy at a usable map scale. Miami commonly used 1"00′ scale, but needed the GIS to retain the high accuracies of structures captured during the GPS utility inventory phase for engineering purposes. As a result, a new programming methodology had to be developed to `squeeze’ symbols for various structures onto tiny spaces in the GIS layers, still accurately represent each structure’s location and network connectivity, and be cartographically legible.
These needs required cartographic adjustment of the highly accurate network to allow for GIS analysis and plot generation at various scales. While `cartographic adjustment of a highly accurate network’ may seem an oxymoron, it produced a solution to the problem of representing highly accurate information at a 1"00′ scale. Network connectivity (x,y,z co-ordinates) and the precise spatial locations of GPSed utility structures are maintained as attribution (along with structure ID number, feature type, and as-built page number for pipes) and are not shown on the cartographic output; instead they are stored in the database. Features and the network are represented cartographically to make the GIS easy to use and to reflect MDWASD’s standard, preferred map scale.
Solving problems with the GIS
Approximately 40% of the total sewage flow to treatment plants during rainy weather is linked to infiltration/inflow (I/I), which causes SSOs. By repairing or replacing problem pipes, MDWASD has slashed peak flow to the plants by 50 million gallons per day. There is thus no need for capacity upgrades to pump stations, saving MDWASD $10 million in construction. The I/I condition is very difficult to quantify, but the GIS will allow MDWASD to develop a model of I/I problems, and get a more accurate assessment of pipe repair and replacement needs.
The GIS will allow MDWASD to preserve pipes by using modelling and data analysis to predict problem areas – the cost of maintaining a pipe is usually less than the cost of replacing it. The near-centimeter-level accuracy obtained in the field-inventory phase allows maintenance crews to locate ailing pipes or other problematic components more precisely, thus reducing the size of construction zones or even allowing for trenchless technology. This reduces disruption to economic activity, and minimises the risk of damage to nearby buildings and electrical, phone and gas lines.
The GIS accuracy will also help MDWASD deal with a crisis situation, such as a water-main break requiring immediate valve identification and closure to isolate the failing pipe segment. It will help staff alert customers to service interruptions. And finally, the accurate GIS will help locate utility structures covered by debris in the aftermath of major storms.
COMPUTERISED MAINTENANCE USING GIS
Utility maintenance efforts can be based on a computer maintenance management system (CMMS) that uses the GIS to organise information about each system component, allowing for a variety of maintenance and prioritisation programmmes. A work-order processing system can be related to the CMMS, resulting in better customer service, despatching, and follow-through for complaints and repairs. The system will also unify MDWASD’s 30 office/plant locations, allowing uniform querying and plotting capabilities, and eliminating duplicated work.
It will also be used to create a capital improvement plan to keep pace with Miami-Dade’s booming residential and commercial development. A model using GIS data will predict the impact of new construction on transmission capacity and pressure levels at connection points.The model will foresee possible overflow points within the system – it will even recommend new infrastructure needs and help MDWASD budget for them.
Finally, the GIS will help MDWASD deal with the impact of inclement weather. Combining pump station and force main data from the GIS, a model of the sanitary sewer system will be created to determine how much wastewater the system can store and transport – in other words, identify those pipes capable of handling increased sewage loads. If the model predicts what officials hope it will, sewage will be stored in certain pipes during heavy rains before being released through the system. This will reduce sanitary sewer overflows and eliminate the need to construct expensive force mains and expand treatment plants.