AWRA SYMPOSIUM ON GIS AND WATER RESOURCES
Sept 22-26, 1996
Ft. Lauderdale, FL

USING RASTER AND VECTOR GIS DATA FOR COMPREHENSIVESTORM WATER MANAGEMENT

Stephen P. Phipps⁽¹⁾

ABSTRACT: Many communities are developing comprehensive storm water management programs--funded by storm water utilities--to comply with Environmental Protection Agency (EPA) regulations and the National Pollutant Discharge Elimination System (NPDES) permitting process. The presenter will discuss how to use raster and vector GIS data to establish a storm water utility, manage storm water infrastructure, and conduct basin master planning.

• Utility Setup: Topics include determining impervious areas, applying billing credits for detention structures, and using other best management practices (BMPs).
  
  
• Infrastructure Management: Topics include establishing a spatial inventory of storm water features collected using existing source documents or field methods, and linking inventory features to a third-party work-order processing system to improve routine maintenance and more easily address customer complaints and emergencies such as flooding.
  
  
• Basin Master Planning: Topics include using available digital terrain models (DTMs) for modeling applications, and using DTMs with ARC/INFO data linked to the HEC-1 and HEC-2 hydrologic and hydraulic models for basin master planning.
  
  

This paper will illustrate the many opportunities and benefits of using raster and vector GIS data for comprehensive storm water management--including faster, more cost-effective decision making.

INTRODUCTION

Many communities are developing comprehensive storm water management programs--funded by storm water utilities--to comply with Environmental Protection Agency (EPA) regulations and the National Pollutant Discharge Elimination System (NPDES) permitting process.

Developing a comprehensive storm water management program requires three primary components, among others:

• Establishing a funding source for ongoing storm water management;
• Creating a program for operations and maintenance (O&M) of storm water infrastructure; and
• Conducting modeling and master planning to effectively plan for growth and storm water capital improvements.
  
  
  

Establishing a Utility

A continuing revenue source for storm water management can be established with a storm water utility, based on service fees or user charges. Local governments that have established utilities have found the method to be effective, reliable, and equitable because residents, businesses, and industries pay based on their contribution to runoff and receive benefits from corresponding improvement projects.

While flat rates, based on equivalent residential units (ERUs), are typically assigned for most residential parcels, utility service charges for nonresidential parcels are frequently based on impervious-area calculations, often made using data from aerial photographs.

Creating an O&M Program (Infrastructure Management)

Ongoing O&M is also critical to successful storm water management. If not maintained, even a well-designed and constructed storm water system will not perform as intended. Proper maintenance includes periodic cleaning of inlets and basins and mowing and repair of land covers. However, because of the age of storm structures in many cities and the lack of detailed engineering data and location maps, public works officials often find that the location and condition of many storm water structures is a mystery to them and to private property owners. Thus, the first step in implementing an O&M program is establishing the current locations and conditions of storm water structures in the community.

Conducting Modeling and Master Planning

A master drainage plan, coupled with computer modeling, is an important element in a comprehensive storm water program. The master drainage plan should address short- and long-term drainage system improvements covering basins within the incorporation and other basins that drain into the local watershed. Developing the modeling and the associated plan requires an up-to-date, accurate, and efficient mapping system.

Using Raster and Vector GIS Data

Communities pursuing a comprehensive storm water management program have found that raster and vector GIS data can be combined efficiently and cost-effectively to help establish a storm water utility, manage storm water infrastructure, and conduct modeling and basin master planning.

By creating a storm water GIS with both raster and vector data, communities have discovered that implementing a comprehensive storm water management program and complying with EPA/NPDES regulations is not as painful as they once thought.

USING RASTER AND VECTOR GIS DATA FOR UTILITY SETUP

Once a community has received an NPDES permit or has committed to establishing a comprehensive storm water management program, a utility is often created to begin funding the storm water plans and programs. Raster and vector GIS data can be used not only to calculate impervious areas and establish utility billing rates for nonresidential parcels but also to identify utility billing credits for storm water "best management practices" (BMPs).

Identifying Impervious Areas

1. Using Airborne Digital Imagery and Spectral Analysis to Identify Impervious Areas

Airborne digital imagery can be used economically and effectively to identify impervious areas. Red, green, and near-infrared color bands--which include both true-color and color-infrared data sets--would have to be collected to distinguish impervious areas from pervious areas; impervious areas, for example, would appear in red on a workstation. Imagery can be captured at a resolution of 0.5 feet per pixel. While such files are large, they are still manageable using today's storage technologies such as RAIDS (Redundant Array of Inexpensive Disks).

ERDAS image processing/spectral analysis software is often used to automatically generate polygons (raster data) of impervious areas, which can then be translated to ARC/INFO for further manipulation.

The next step: digitizing parcel lines (vector data) based on maps and other data usually collected from the local auditor or assessor's office. Parcel lines input into ARC/INFO can then be overlaid with the impervious-area polygons; the square footage of impervious areas in each nonresidential parcel can then be calculated and passed on to the storm water utility billing system so that appropriate fees for commercial and industrial parcels can be assessed.

Using raster and vector data to calculate impervious areas in this way is efficient because polygon processing is automatic; heads-up digitizing of impervious-area polygons is not required. This not only saves time but potentially produces fewer input errors.

2. Using Digital Orthophotos to Identify Impervious Areas

Digital orthophotos, raster images of ground features in their true map positions, are another alternative for identifying impervious areas of nonresidential parcels. Once impervious areas are identified, the digital orthophotos can be used as a strong foundation or backdrop for establishing a storm water GIS, which can be used for storm water system O&M and as well as modeling and basin master planning.

Digital orthophotos are created by removing image displacements on aerial photography through computer image processing. The first step is acquiring aerial photography and obtaining sufficient horizontal and vertical ground control to orient the photographs to known coordinates and ground features. Fully analytic triangulation is performed to mathematically densify the ground control.

Aerial photographs are then scanned with a micron-based film scanner to produce continuous-tone, raster images of the photography. (The emergence of digital cameras is slowly eliminating the need for image scanning.)

The aerial photographs are used again to compile digital terrain models (DTMs), a series of X, Y, Z coordinates that accurately depicts ground elevations. The DTMs must be both accurate and dense enough to adequately define the terrain. Two kinds of points are input to create DTMs: breaklines, which indicate an abrupt change in elevation, and spot elevations in a grid-type pattern (mass points).

The final step in digital orthophoto production is image rectification. The raster images of the photography are overlaid with the DTMs and corrected based on ground coordinates so that image displacements can be removed. The result: Accurate, rectified raster images of the aerial photography, which allow users to work with "soft copies"--computerized maps.

The digital orthophotos, with impervious areas clearly visible, are then used to perform heads-up digitizing of impervious areas as polygons. Parcel lines (vector data) from the local auditor or assessor's office can be input into ARC/INFO as an overlay to the impervious-area polygons digitized from the digital orthophotos.

The square footage of impervious areas in each nonresidential parcel can then be calculated and passed on to the storm water utility billing system so that appropriate fees for commercial and industrial parcels can be assessed.

Using Digital Orthophotos to Identify Utility Billing Credits
for Storm Water BMPs

Some communities have chosen to further enhance the equitability of their utilities by rewarding BMPs; these communities issue storm water utility billing credits to parcels that have installed structures or implemented programs that help alleviate drainage problems.

The following are three common practices that warrant storm water utility credits in some communities with utilities:

1. Practices That Reduce Discharge Peak. A credit may be given for structures that decrease or slow down storm water from gaining access to the city's storm water system. Ponds or detention structures are examples of such structures. Credits are awarded based on the surface square footage of the pond or detention structure (as calculated from digital orthophotos, for example) and the amount of water trapped as determined by engineering calculations.

2. Practices That Reduce the Discharged Volume to the Surface Drainage System. A credit may be given for structures such as French drains that use infiltration techniques to allow storm water to reach the ground water system rather than the surface drainage/utility management system. Credits are awarded based on the existence of such structures, which can be identified during field data collection.

3. Practices That Increase Water Quality Using BMPs. A credit may be given for structures that allow a storm's initial runoff, called the wash-off load, to be stored long enough so that heavy metals and other pollutants that affect water quality can be eliminated before they reach the municipality's system. A wet detention structure is one such structure. Typically, the first half-inch of a storm event must be detained for at least two hours for a credit to be allowed. Credits are awarded based on the surface square footage of the wet detention structure (as calculated from digital orthophotos, for example). Credits for BMPs can also be awarded by using raster data such as digital orthophotos to calculate surface square footage of other structures that help alleviate drainage problems. Such structures include constructed wetlands, Austin first-flush filtration basins, and grassed swales.

Raster and vector data used during utility setup also provides a side benefit for a community: an instant customer-service system and a permanent record, based on impervious-area calculations, of how the storm water bill for each property was derived. Customers who contact the municipality with questions about their storm water bills will be able to see--via a map/photo on the screen or a plot of their property--the basis for the impervious-area measurement and the storm water utility charge.

USING RASTER AND VECTOR GIS DATA FOR
INFRASTRUCTURE MANAGEMENT

Once a utility is established and a revenue stream begins, a community can start implementing an ongoing O&M program for better managing its storm water infrastructure. Raster and vector GIS data can be used to establish a spatial inventory of storm water features and link inventory features to a third-party work-order processing system. These activities will help the community improve routine maintenance and more easily address customer complaints and emergencies such as flooding.

A flooding or drainage problem can occur for several reasons:

• Storm water structures are damaged or obstructed because they have not been regularly inspected or maintained.
• Homes are built too close to streams.
• Storm water structures are undersized because poor methodology and incomplete data have been used to determine which size structures to install.
• Urban development upstream has produced more storm water flow downstream; structures downstream often have not been planned to handle this additional flow.
  

Controlling flooding problems and initiating routine O&M means knowing the locations, conditions, and other characteristics of structures within the community's storm water system. The first step: establishing a strong GIS foundation for adding storm water system features, structures, and attributes. (If digital orthophotos were used during utility development, they can also be used as the basis for adding storm water graphic and nongraphic data. Otherwise, a base mapping foundation would have to be established.)

Using Raster and Vector GIS Data to Establish a
Spatial Storm Water Inventory
 

If digital orthophotos have been created, a community can effectively combine both raster and vector storm water data to establish a spatial inventory of storm water features and structures.

1. Creating Raster Data of Storm Water System Details

If fairly accurate and up-to-date storm water maps exist depicting fine details about the storm water system (for example, exact manhole and inlet locations), they can be scanned and incorporated into the GIS system. Scanning instead of digitizing/vectorizing allows more data to be converted in less time; moreover, valuable storm water data that otherwise may not have been included will be available in the storm water GIS for O&M applications.

2. Creating Vector Data of Major Storm Water Structures

Next, a basic GIS map of major storm water structures and features would be digitized as vector data. (Digital orthophotos would be used as the accurate backdrop for adding storm water graphic features.) The resulting map would include structures such as pipes, open channels, and ditches; ID numbers would be assigned to all structures input.

The raster images containing fine details about the location of other storm structures would be available as "visual attributes" to the major storm water structures and features. When detailed locational data was needed on a particular storm structure, the raster image--linked to the structure via an ID number--could be accessed via a point-and-click operation.

Using Field Methods to Establish a Spatial Inventoryof Storm Water Structures

Instead of converting existing storm water maps to raster and vector data, a spatial inventory of storm water structures could also be established using field methods in conjunction with the accurate digital orthophoto base map. This alternative would result in more detailed, current information than would be available if only existing maps were used.

There are two options within the field-methods alternative:

1. The Total GPS Inventory Option

This option is preferred when existing storm water system maps are outdated. Inventory crews would use GPS methods and hand-held data collectors to occupy storm water structures, determine locations, and collect attributes needed for O&M, modeling, and master planning applications. Structures to be located include manholes, pipes (inlets and outfalls), and culverts. Channels, including streams and open channels, would also be located.

The result would be the locations (x,y coordinates) and attributes needed for each structure (such as the elevation). Data collected in the field could then be downloaded, translated into ARC/INFO, and overlaid and linked with the digital orthophoto base map. The ID number for each storm water structure would be its x,y coordinate.

2. The Data Prep/Field Method Inventory Option

This option is preferred when existing storm water system maps are fairly accurate and current and tied to an accurate base map (but not necessarily digital orthophotos). Storm water structures on the existing hard-copy maps would be heads-up digitized into the ARC/INFO GIS; ID numbers would be assigned automatically to storm water structures input. The ARC/INFO system would then be used to plot a map containing the storm water structures and the newly assigned ID numbers.

Inventory crews would use these maps in the field to collect attributes on the identified storm water structures via hand-held data recorders. Attributes collected and tied to the structure ID number on the map would later be downloaded into the ARC/INFO GIS and linked to the graphic structure via the ID number.

Even if storm water maps had been updated regularly, it is possible that inventory crews may find structures not identified on the newly plotted maps. Therefore, such structures would be sketched on the maps and assigned an ID number in the field based on a predetermined formula so that attributes could still be collected and attached.

Using Raster and Vector GIS Data to Link Inventory Features
to a Third-Party Work-Order Processing System

Once a storm water inventory/GIS system is established, raster and vector data can also be used with a work-order processing system to ensure that storm water complaints from customers are addressed and resolved quickly.

A storm water structure's ID number provides the link to the work-order processing system. For example, a dispatcher receiving a customer compliant via telephone would input the customer's address into the GIS system; the address would be linked to its appropriate pipe ID numbers, which would automatically tell work crews where they might begin their investigation of the drainage problem. If desired, crews could also choose to plot a vector map of the affected area as well as storm water detail maps on structures in the immediate area (if available in the GIS as raster images). But raster and vector storm water GIS data can also be used for scheduling routine maintenance. Structure IDs within a certain range could be selected and maps containing those structures plotted. O&M crews could take these maps into the field to locate, clean, and perform any needed maintenance on targeted structures. This method provides a consistent means to ensure all structures are properly and routinely maintained.

In the past, a community may not have attempted a regular maintenance program for all structures because of a lack of funding. This strategy for routine maintenance would allow communities--many for the first time--to begin a proactive storm water management program. Such a program would not only allow problems to be addressed as they occur but also prevent certain new drainage problems from occurring at all. As a result, the number of flooding complaints and emergencies reported by customers during a storm will likely decrease significantly.

USING RASTER AND VECTOR GIS DATA FOR
BASIN MASTER PLANNING

The raster and vector storm water GIS data collected for utility development and infrastructure management can also be used for modeling and basin master planning: a solid foundation has been established; storm water structures have been located and tagged with ID numbers; and attributes have been collected. But before modeling can be performed and a master plan developed, DTMs must be collected and included in the GIS.

Collecting DTMs

DTMs, a series of X, Y, Z coordinates that accurately depicts ground elevations, are required for storm water modeling so that accurate, hydrologically correct surface models can be generated. (A surface model is a digital 3-D representation of the ground surface.)

Having a hydrologically correct surface model lets the ARC/INFO system automatically cut channel cross sections by "feeling" the digital topography. The resulting channel profiles show graphically how much storm water the channels can handle before flooding occurs.

Aerial photography is used as the basis for generating DTMs. If digital orthophotos were produced as the accurate backdrop for the storm water GIS, DTMs were created; however, the process used to create DTMs for digital orthophotos and the process used to create DTMs for modeling applications is not the same. DTMs used for digital orthophoto creation are typically not suitable for modeling applications; however, if the digital orthophoto project was planned with the idea that DTMs would eventually be used for modeling applications, it is possible that existing DTMs could be used.

If no DTMs are available, USGS digital elevation models (DEMs) could be used. However, USGS DEMs have an accuracy of +/-30 meters; therefore, the resulting modeling data would only be reliable for regional planning and not detailed basin master planning.

Linking ARC/INFO GIS Data with Hydrologic and Hydraulic Models Storm water GIS data already collected could be used to perform hydrologic and hydraulic modeling. By linking the ARC/INFO storm water GIS with HEC-1 (hydrologic) and HEC-2 (hydraulic) models, enhanced modeling capabilities are available. Moreover, modeling becomes easier because manual calculations are not required. For example, if the models were not linked to the GIS, manual calculations would be required for the cutting of cross sections, which would take at least five times longer than using the ARC/INFO system linkage.

ARC/INFO Data Required for Linkage with the HEC-1 and HEC-2Models

The following storm water structures should be collected in the field and included in the ARC/INFO GIS for modeling and master planning as well as the O&M application (see Table 1):

The following storm water features, collected or compiled from aerial photography sources and other data, should be included in the ARC/INFO GIS for modeling and master planning as well as the O&M application (see Table 2):

Characteristics of the Interface between ARC/INFO
and the HEC-1 and HEC-2 models

Woolpert has successfully linked the HEC-1 and HEC-2 models with ARC/INFO and implemented the system's capabilities on several modeling projects. Primary characteristics of the interface include the following:

1. The HEC-1 model uses vector data--streams and open channels--and the ARC/INFO networking function to show the locations of and relationships between the different types of flows: sheet flow, shallow channel flow, and concentrated channel flow. The linkage allows users to see graphically, on a map, how fast the three flow types are moving at different locations within the model (volume/time). These unit hydrographs, which make up the HEC output card, are then used to feed the HEC-2 hydraulic model.

2. ARC/INFO can be used to graphically show the output of the HEC-2 model--the extent of flooding for various storm events. A calibrated and verified HEC-2 model can also be used to develop two-, 10-, 25-, 50-, 100-, and 500-year frequency flood profiles for streams and open channels within the watershed. The linkage with ARC/INFO allows users to see these results graphically.

3. The interface also allows for easier model calibration. Because the models are linked to ARC/INFO, model calibration is easier because results are posted graphically and any needed changes to cross sections can be made via a user interface. Calibration ensures that scenarios the model will predict are based on results from actual rain events. To ensure model predictions will be realistic, adjustments are required to correct cross sections, unit hydrographs, and other data.

Modeling Results

Once the modeling is performed, modeling results are typically summarized in a report. This report, which could then be used as a starting point for basin master planning, might include the following:

• Modeling assumptions;
• Watershed-specific hydrologic and hydraulic results; and
• Modeling recommendations.
  

Because the models were linked to ARC/INFO, channel cross-section locations and the two-, 10-, 25-, and 100-year frequency floodplains and flooded structures--delineated in different and corresponding colors--could be plotted on maps along with base mapping data.

Once the modeling is performed and conclusions reached, the basin master plan can be developed. A comprehensive basin master plan might include the following:

• Detailed topographic mapping of stream channels and floodplains;
• A review of existing basin and data reports;
• Delineation of existing and potential flood-prone areas;
• Conceptual designs of alternative drainage schemes with varying levels of protection;
• Basic design criteria for drainage improvements;
• Capital and O&M cost estimates; and
• A prioritized construction schedule.
  

The HEC-1 and HEC-2 models, linked with ARC/INFO, can also be used to do "what if" scenarios for basin master planning applications:

• What if a shopping center is built in X location?
• What if Y parking lot is destroyed and replaced with a detention pond?
  

By modeling these scenarios, users will be able to determine what impact--positive or negative--a proposed development will have on downstream structures.

CONCLUSION

Establishing a storm water utility, managing storm water infrastructure, and conducting basin master planning to comply with EPA/NPDES regulations and meet the community's needs can best be accomplished by using raster and vector GIS storm water data as an information management and planning tool. Once a storm water GIS is established, more data can be added in the future to further enhance the system's usefulness.

However, if a community chooses not to establish a GIS as it goes about implementing a comprehensive storm water management program, all subsequent tasks will be significantly more difficult. But more importantly: without a GIS to help organize, store, analyze, update, and manage such large amounts of data, it's unlikely that communities will be able to successfully implement critical storm water management plans and programs needed to properly manage the quality and quantity of storm water.

ACKNOWLEDGEMENTS

The author wishes to thank Teresa Zumwald of Zumwald & Company in Englewood, Ohio, for her assistance in writing and editing this paper.

REFERENCES

Brinkman, Robert F., C.P., Woolpert, Dayton, Ohio. "Digital orthophotos for utility applications," 1992.

Reese, Andrew J., P.E., Ogden Environmental and Energy Services, Inc., Nashville, Tenn. "Understanding storm water problems," in APWA Reporter, January 1993, pp. 7-8, 22.

Woolpert, Louisville, Ky. "Storm water management in the '90s," 1992.

1. GIS Project Manager, Woolpert, 409 East Monument Avenue, Dayton, Ohio, 45402-1261.

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