GIS and Stormwater Management

GIS and Stormwater Management

Jonathan W. Heald
Assistant Utilities Engineer
City of Bloomington Utilities

Stormwater Management is often referred to as the redheaded stepchild of municipal government. Lacking the political clout and daily exposure given to other departments such as parks, police and fire, and other services such as street repairs, sidewalk extensions, and trash removal, the absence of a comprehensive program is seen as a problem only when it rains. Officials responsible for stormwater management dread going to work on rainy days expecting the deluge of phone calls from citizens complaining of rising floodwaters. Constituents also contact their council representatives demanding action to solve their drainage problems. But as quickly as the waters rise and fall so does the demand for solutions and by the time a plan for stormwater management is completed, memories of the problems have evaporated and the implementation is viewed as a waste of the taxpayers’ money.

Beyond the lack of funding, in most communities, the problems with stormwater management can be attributed to several factors: lack of a central person or department in charge of stormwater, inadequately designed infrastructure unable to handle the demands placed upon it, lack of maintenance of the stormwater system, and decaying infrastructure at the end of its useful life.

Municipalities often diffuse the responsibilities of stormwater management over several departments. Planning Departments are in charge of enforcing floodplain management and erosion control ordinances as new developments petition for required permits. Engineering departments are in charge of completing studies and designing projects to address flooding problems. Street Departments are in charge of maintenance and repairs on the collection system. This separation of responsibility makes Master Planning difficult. Additionally, in all cases, these responsibilities are secondary to the primary duties of each department and are preformed as a reaction to an existing problem. For example, during a street repair, a field crew discovers the pothole is caused by a collapsed section of pipe under the road. In some cases, the solution to the pothole problem would be to fill the pipe with concrete, in others replace the section of pipe. Because the focus of the Street Department is road repair very little effort is made to maintain the existing pipe or verify the condition of the additional sections of pipe.

Even if during the repair the faulty section of pipe is replaced, it is unlikely that the capacity of the pipe is checked to ensure it can adequately handle the runoff reaching it. With stormwater a divided and secondary responsibility, often under-sized infrastructure is installed during repairs and with new development. Cities and towns that lack a central stormwater department also tend to lack clear and precise design standards for sizing stormwater infrastructure. This lack of criteria results in municipalities not checking the capacity of pipes and culverts during repairs and replacement projects, noting any changes in the upstream watershed and private engineering firms submitting varying standards for private developments.

Many communities also tend to be reactive in their maintenance of the stormwater system. Instead of developing an ongoing, daily maintenance program, they rely on citizen complaints of an immediate drainage problem. In a 1992 survey of North Carolina cities, twenty percent stated local flooding problems were due to a lack of maintenance. Surveys of detention ponds in Maryland from 1986 to 1992 indicated a major reason for high rates of failure was poor or nonexistent maintenance.

Another problem is much of the stormwater infrastructure in the U.S. is at the end of its useful life. Most communities have either celebrated or are close to celebrating their centennial birthday. This means the original bridges and culvert, built to carry the roads over creeks and streams, is also close to one hundred years old. If it is not original, much of the infrastructure in the core parts of a community is of the WPA era and is between sixty to seventy years old. With a service life of seventy-five to one-hundred years, coupled with a lack of maintenance, many parts of the stormwater conveyance system are an underground time-bomb waiting to collapse. Back to back culvert failures in Bloomington, Indiana in 1995 and 1996, under roads and homes, prompted an inspection of the major storm tunnels in the city. The 1997 report inspected twenty percent of the culverts over three feet in diameter and identified over twelve million dollars of needed repairs to prevent future collapses.

Whatever the reason - lack of a Master Plan, under-sized pipes half full of debris or culverts collapsing under the street - the flooding problems in our communities can be traced back to a lack of a financially stable, permanent program to address all stormwater issues for a community.

In September 1998, facing many of the problems mentioned above, the City of Bloomington, Indiana established a Stormwater Utility as a permanent program addressing the problems associated with runoff in the community. Title 36-9-15 of the Indiana Code defines stormwater as sewerage and the Stormwater Utility was incorporated as part of the exiting Wastewater Utility. This parent-child relationship has many benefits including an existing billing and collection system, established business practices, and shared resources in administration, engineering, construction and maintenance. Using existing resources allowed for a lower bill to the customer ($28.20 per year for a residential customer) while generating approximately $1.0 M per year in operating revenue. With the service area set at the municipal boundary, the stormwater utility responsibilities currently include: maintaining a fair and equitable user-fee for the customers, reviewing new development to ensure consistent standards are met, performing daily maintenance, and small additions to the existing system, investing in large capital projects to repair the backbone of the conveyance system and preparing to address EPA’s Phase II NPDES Permit requirements for stormwater. In the future, the utility may expand its scope to include maintenance of stormwater management ponds, floodplain management and erosion control enforcement.

Stormwater Runoff
The process by which rainfall becomes runoff is a complicated process with many variables. Theoretical models pour rainfall amount and distributions over the losses associated with land characteristics into a computation and produce runoff amounts. Two methods for determining runoff in the United States are generally used - the Rational Method and the Soil Conservation Service (SCS) Hydrologic Method. Both methods have their advantages, disadvantages, and limitations and will produce good results if applied correctly. The Rational Method uses a simple equation to produce peak discharges for drainage basins up to 200 acres, while the SCS Method requires several equations but can produce hydrographs for drainage basins as large as 2000 acres. Both methods use a variable to relate the land characteristics such as soil types and coverage to the losses such as interception and infiltration that occur prior to runoff. The Rational Method refers to this variable as a runoff coefficient “C” and the SCS Method calls it a Curve Number, CN. To determine a “C” value for a drainage basin, a weighted average is calculated for the entire basin using the runoff coefficients in Table 1.

Table 1. Runoff Coefficients for Various Land Uses

Description of Area		Runoff Coefficients
Downtown Areas	Commercial	0.70 – 0.95
	Neighborhood Areas	0.50 – 0.70
Residential	Single-Family	0.30 – 0.50
	Multi-Family Attached	0.60 – 0.75
	Multi-Family Detached	0.40 – 0.60
	Suburban	0.25 – 0.40
	½ acre lots or greater	0.30 – 0.45
	Apartments	0.50 – 0.70
Industrial:	Light Areas	0.50 – 0.80
	Heavy Areas	0.60 – 0.90
Parks, Cemeteries		0.10 – 0.25
Playgrounds		0.20 – 0.40
Unimproved Areas		0.10 – 0.30
Railroad Yard Areas		0.20 – 0.40
Streets:	Asphalt	0.70 – 0.95
	Concrete	0.80 – 0.95
Drives and Sidewalks		0.75 – 0.85
Roofs		0.75 – 0.95

Source: Hydrology, Federal Highway Administration, HEC No. 19, 1984

Similarly, a curve number for a drainage basin requires the calculation of a weighted average; however, unlike the range of Runoff Coefficients, the curve number has specific values for land uses on different soil types. The Soil Conservation Service defines the four Hydrological Soil types as follows:

Group A	Soils having a low runoff potential due to high infiltration rates. These soils consist primarily of deep, well-drained sands and gravels.
Group B	Soils having a moderately low runoff potential due to moderate infiltration rates. These soils consist primarily of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately course textures.
Group C	Soils having a moderately high runoff potential due to slow infiltration rates. These soils consist primarily of soils in which a layer exists near the surface that impedes infiltration or soils with moderately fine to fine textures.
Group D	Soils having a high runoff potential due to very slow infiltration rates. These soils consist primarily of clays with high swelling potential, soils with high water tables, soils with claypan or a clay layer near the surface, and soils with bedrock at or near the surface.

Using the different soil types and the appropriate land use, a Curve Number can be determined for a drainage basin based on the values in Table 2.

Table 2. Runoff Curve Numbers for Urban Areas

Curve Number for Hydrologic Soil Group
Description of Area		% Imp	A	B	C	D
Lawns, open spaces, parks, golf courses cemeteries, etc.
	Good condition 75% or more grass coverage		39	61	74	80
	Fair condition 50% to 75% grass coverage		49	69	79	84
	Poor condition less than 50% grass coverage		68	79	86	89
Parking Lots, Roofs and Driveways			98	98	98	98
Streets and Roads:
	Paved w/curbs & storm sewers excl. ROW		98	98	98	98
	Gravel including ROW		76	85	89	91
	Dirt including ROW		72	82	87	89
	Paved w/open ditches including ROW		83	89	92	93
Commercial and Business Areas		85%	89	92	94	95
Industrial and Manufacturing Districts		72%	81	88	91	93
Residential
	1/8 acre lot or less, Apartments and Townhouses	65%	77	85	90	92
	¼ acre lot	38%	61	75	83	87
	1/3 acre lot	30%	57	72	81	86
	½ acre lot	25%	54	70	80	85
	1 acre lot	20 %	51	68	79	84
	Agricultural Land Uses
	Pasture or Range Land
	Good condition, protected from grazing		39	61	74	80
	Fair condition, some grazing, mowed for hay		49	69	79	84
	Poor condition, used for grazing		68	79	86	89
Cultivated land with conservation treatments			62	71	78	81
Cultivated land without conservation treatments			72	81	88	91
Wood or forested land
	Mature stand with good cover		25	55	70	77
	Thin stand, with poor cover		45	66	77	83

Source: National Engineering Handbook, Section 4, “Hydrology” Chapter 9, August 1972.

However, hydrology is as much an art as it is a science, and small changes in any of the variables in either method can result in a wide variety of answers. In a recent paper, David Knipe P.E. of INDNR’s Division of Water pointed out inaccuracies with the SCS Curve Number Method. In a study of gauged streams in Indiana, the SCS Method did not reproduce the observed hydrographs from known storm events. Mr. Knipe’s discussion pointed out the inability of the Curve Number to accurately reflect the infiltration of rain water. Furthermore, the paper discusses how Holtan’s Method for determining infiltration through the ground produced more accurate hydrographs and peak discharges.

Whether it is a Rational Method runoff coefficient, an SCS Curve Number or one of the Holtan’s infiltration variables, careful area measurements are necessary to reduce the input errors and improve the final results. Ordinarily, this means initially defining a drainage area, then delineating the permeable and impermeable regions within the drainage area, and finally breaking up the permeable areas into different soil types. Determining all these variables by hand can take roughly an hour per basin. To reduce this time, engineers reduce the number of basins, averaging the land characteristics over the entire basin, and assuming certain types of coverings have certain characteristics. Unfortunately, this leads to generalized results with limited applications. Determining the land characteristics for a complex, multiple basin hydrologic model in an urban area can take months, while compiling and running the model only takes hours. Often models are run several times using different methods and different variables to verify results. While there exist many off-the-shelf programs to perform many of these tasks, they are expensive, limited in their applications and sometimes not compatible with existing data. A simple internal Geographical Information System program, that performs the overlay, cut and paste functions necessary to determine the land characteristics of a given basin, is a useful tool that could increase the number of basins analyzed, reduce calculation time, eliminate errors, and improve results.

GIS Applications
The City of Bloomington, Indiana implemented a citywide GIS in 1996 using aerial photography to generate base map data and manual digitizing to create data layers for the City owned water and wastewater utility. Base map layers included buildings, streets, parking lots, elevation lines/points, drainage basins and water features. Cultural features such as political boundaries, planning zones and properties (maintained and shared by the County in the same GIS software) are also a pert of the system. The GIS data has been kept up to date through periodic import of AutoCAD drawing files showing details of new building, road and subdivision projects. Bloomington uses Genasys GIS software and has distributed a general user interface, called GENIUS, on the desktop of most of the city’s office-based employees.

In the spring of 1998, the first work began using GIS for Stormwater analysis. This initial work was focused on modifying the extents of a drainage basin layer in the City’s GIS. Using two-foot contour intervals and spot elevations, derived from 1996-air photos, the assistant city engineer re-digitized the boundaries of these basins to more accurately reflect their extents. Smaller subsets of original basins were also digitized to generate additional analysis points and isolate areas such as sinkholes. These refined drainage basins were then used to calculate their total area so that more specific calculations for runoff could be generated. These calculations were all done by hand initially, which were not only tedious but also time consuming and inaccurate. With the city exploring using these basins for long-term stormwater inventory planning and the use of runoff-based stormwater user-fees, a more automated process was needed.

The City’s GIS contained other relevant base map data for calculating runoff such as the permeable soil types and impermeable streets, buildings and parking lots. Since the features found within these basin areas were not all closed polygons with stored area values, they were being measured manually and placed into the runoff calculations. The first step on the way to automating this process was to take the existing soil type layer, with hundreds of soil type polygons and generalize it down to the four SCS Hydrological Soil types. Once these soil polygons were reclassified, the like polygons were dissolved into one another to produce a master layer of hydrologic soil types.

The impermeable streets and buildings existed as closed polygons within the City’s GIS and thus could be queried for area measurements. Parking lots, however, were not closed area features and modifications were made to this layer to close off the parking lots using coincident line features shared with roads and buildings (Figure 1). The nature of a traditional map layer based GIS requires true polygon (closed area) features to be entirely contained within one file. Area features that might “share” a boundary with an area on another map layer, required the same edge to be maintained in two files. Ensuring that these edges were indeed coincident, allowed for precise measurement and accurate representation. Bloomington’s parking lots were never digitized or maintained as polygons. Copying these coincident line features from building and street layers proved to be the largest single effort in the use of GIS for stormwater water analysis.

Figure 1

Generating the topology of closed, impermeable surfaces would not have been possible without the aid of aerial photography and ground truthing. Careful analysis of base map data compared with photographs and field surveys was necessary to eliminate ambiguity found in the GIS. Further refinement of these layers required that holes existing in the impermeable layers, such as courtyards in buildings and landscaping within parking lots, were considered permeable. This classification allowed for a conservative estimate of impermeable surfaces, erring on the side of the stormwater water customer. These holes in impermeable surfaces were tagged as “islands”, allowing for reclassification in the future. Sidewalks were not consistently maintained in the GIS and were therefore not considered in this analysis.

With all the necessary base map layers cleaned up and forming closed polygons, the first application, a series of basic overlay commands, were scripted for calculating runoff at the drainage basin level. The basin polygon was used to clip the soil type layer, and the building, parking lot and street layers. With these subsets of base map data, the building, parking lot and street layers were merged together to form one homogeneous, impermeable feature. Areas outside of these features as well as within the “island” holes were not part of the impermeable data. This impermeable layer was then overlaid on top of the soil types. The resulting basin polygon could be queried for total impervious surface and total open permeable soil types given in both percentage and area measurements. These values were used for general basin planning and could be regenerated as new development filled in the basin.

When the City Stormwater Utility was formed and the decision to use a runoff-based user-fee was made, these layers were considered on the smaller scale of permeable and impermeable surface within a given property.

Applications
In order for the Stormwater Utility to function, it must generate the necessary funds to perform it duties for its customers. This means determining a fair and equitable user-fee for all its customers. Initially the Utility based the user-fee on the size of a service water meter and fire lines and correlated it to a residential meter to estimated property size and the amount of runoff generated. A two-tail P-test had a 95% confidence level that the estimate was fair and equitable (n = 0.487). However, there are outliers, predominantly with those customers with fire lines. In an effort to maintain equity, these customers were moved to a runoff user-fee, based on the amount of impermeable surface contained within the parcel correlated to the amount of impermeable surfaces within an average residential lot. Currently, the Utility is working towards moving all customers to a runoff user-fee that accounts for both permeable and impermeable areas within a parcel. The main obstacle to achieving this is the large amount of computations required to implement and maintain accurate bills. The number of variables and amount of calculations required for each parcel could be cumbersome and costly for the Utility as it performs a runoff rate analysis. With the Runoff GIS application, the Utility can determine the land characteristics on a parcel-by-parcel basis. By defining the parcel boundary as a drainage basin and using the address of the parcel as a unique identifier, the GIS program calculates the impermeable and permeable areas for each parcel and creates a file that can be imported into the Utility’s billing system. Once in the billing system, prescribed coefficients for impermeable and permeable areas are attached and a calculation to determine the amount of runoff generated compared to an average single family residence is completed. Since the single family residence is the base unit and receives a base user-fee, currently $2.35 per month, this process results in a runoff bill for the customer responsible for the parcel.

Another problem is that the service area is continually changing as development converts permeable areas to impermeable areas, requiring continual updates to the runoff user-fee. Also, since the service area coincides with the City Limits, additional customers, each with an additional calculation, are acquired annually with each new annexation. As with most systems, the City’s GIS is constantly updated through periodic aerial surveys of the City or daily manual inputs based on either proposed or as-built plans. Because the runoff calculation process can be automated, annual, quarterly, monthly or weekly reports on the entire Service Area can be generated to ensure the accuracy of all the bills. The application can be set to run overnight and checked against current records. With all the parcels in the Service Area calculated, any parcel with the same impermeable and permeable areas could be considered accurate, while discrepancies would be investigated further. As new parcels develop or are added to the service area the application can be used to determine the land characteristics needed for the runoff rate. By using the GIS, human errors in measuring different areas are eliminated and man-hours are saved. Also if a customer questions the amount billed, the application allows for a quick and accurate determination of the parcel’s areas. If the bill is still disputed, surveyed plans can be supplied for comparison and adjustments to the GIS and to the bill can be made when errors are found.

Finally, the Utility bases all bills on the revenue requirements needed to maintain a certain level of service. Currently, the Utility requires approximately $1.0 M per year to perform daily operation and maintenance on the system, design and review extensions to the system and plan and build large capital replacement projects. Depending on how fast the system grows and how many new customers are being served by the Utility, a user-fee adjustment, either up or down, may be necessary. Other factors such as credits given, inflation, costs of living increases, and material and equipment costs also affect the annual revenue requirements of the Utility. With the GIS application, the total amount of permeable and impermeable areas in the Service Area can easily be determined. A rate analysis can then estimate the total revenue generated by the Service Area based on changes to either the runoff coefficients or the base charge. When compared to the funds needed by the Utility to operate, a user-fee adjustment can easily be determined.

Another responsibility is review of new development to ensure they meet the standards of the Utility. This often means reviewing existing and proposed runoff calculations, analyzing pond design and checking inlet location and spacing to prevent local flooding during prescribed storm events. This review sometimes requires independent calculations to verify or challenge submitted results. The same design standards that are applied to private development are also applied to the Utility when making repairs on the system or additions and expansions. In either case, calculations to determine the amount of runoff are necessary. Using the GIS application, an engineer would first define the drainage basins for analysis. These basins are now stored in the system and can used to delineate other basins or referred to at a later date. With the basins defined, the application will determine the total area, impermeable area and the permeable area by soil type. With this information, an engineer can select the appropriated equations or model for the given variables and determine the peak discharges or unit hydrographs for the defined drainage basins. If the GIS stores proposed development on separate layers, the script can be altered to use proposed development layers and determine a pre and post development runoff condition during the review process. The process of determining the land characteristics often takes hours or days depending on the number of basins and the level of detail required. With an average engineer billed around $100 per hour, the application not only can save time and money but also can provide a high level of accuracy on a detailed level. This accuracy can also be correlated to a cost savings to the utility, by eliminating call outs because of flooding and future projects to replace undersized pipes, because the right infrastructure is put in place. It is also can turn out to be a cost savings to private developers as oversized pipes are not used and extra land is not set aside for stormwater management ponds.

The GIS application used to analyze the two to three drainage basins involved on development review or small neighborhood projects can be taken a step further to determine runoff information for an entire systems. Having a hydraulic model of a community with sixty to eighty individual basins averaging two hundred acres in size is a powerful tool in developing a Stormwater Master Plan for a municipality. As the number basins in the model increase the accuracy of the results tend to also increase as the averaging of land characteristics tend to decrease when the basins become smaller. Additionally the number of basins corresponds to the number of analysis points. Having more point allows for a better understanding of the whole system and eliminates interpolation within a basin. Part of the Master Plan should include a Capital Replacement Schedule. While non-hydraulic factors such as structural stability are usually overriding factors, hydraulic inefficiencies cannot be overlooked when replacing large culverts and flood control channels, costing $2,000 to $4,000 per linear foot. Finally, EPA is tightening their regulations regarding non-point source pollution in a continuing effort to achieve the goals of the Clean Water Act, zero pollutant discharge. A community wide system model that analysis sediment transportation and pollutant loading could also be a part of a communities Master Plan to reduce erosion, and pollutants from its streets and prove useful in addressing the new regulations from EPA.

Conclusions
With the formation of the Stormwater Utility in 1998, the City of Bloomington, Indiana has established a permanent, on-going program with a dedicated funding source, independent of political whims. In the past two years, the Utility has spend roughly $4.0 M in Stormwater Management projects, including: large capital replacements, daily cleaning and repairs, development and maintenance of design and construction standards, and upgrading the Utility’s user fee structure. This investment resulted in relief from many of the chronic flooding problems, which plagued Bloomington for many decades and provided insurance against catastrophic failures under streets and buildings. Every aspect of the Stormwater Utility revolves around quickly and accurately calculating the amount of runoff produced during a given storm event. Sizing pipes, spacing inlets, billing customers, reviewing development, developing a Master Plan all require a runoff discharge rate. Since runoff is based on land characteristics, having a GIS application to perform the multiple overlay, cut and paste functions required to determining these variables is a powerful application, that is not only quick and accurate, but has also proven to save time and money.

Since the application is internal to the City’s GIS, there are no data conflicts, which have been experienced with other stand-alone software. While these other software packages are more powerful and contain many more features, the City’s GIS application is easily adaptable to fit a wider variety of situations and doesn’t carry a heavy price tag. One example is calculating the user-fees based on runoff amounts and performing the rate analysis necessary to sustain the Stormwater Utility. While the internal application can set to identify parcel lines as drainage basins and perform citywide calculations for individual parcels, many external software packages cannot handle twenty-thousand quarter-acre basins (parcels). Another example is design of small extensions to the system in neighborhoods to eliminate localized flooding. Often the drainage area for these project are less than one acre and using an off the shelf program to calculate a five cubic foot per second discharge may be viewed as overkill.

As the City’s GIS is periodically updated, from either aerial photographs or ground surveys, and additional features are included in the Base Map Layers, such as sidewalks and landscape plots, the application can revise the runoff characteristics allowing easy updates to, user-fees, stormwater runoff models, and planning documents used by the Stormwater Utility.