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2.5 Undergraduate degrees and one master. (Polska wersja)










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Capstone from a year ago

This capstone was the final for my Master in Public Administration

Executive Summary

Problem Statement - Lexington-Fayette Urban County Government (LFUCG) must reduce stormwater and sewer system overflow, increase stormwater capacity and overall, improve its stormwater infrastructure in order to comply with a consent decree between LFUCG and the EPA. Per decree, a stormwater fee must also be established. This paper examines the cost effectiveness of repaving public urban roads and private paved surfaces with porous asphalt. Porous asphalt allows the water to sink through the asphalt and reduces stormwater runoff.

Research Strategy - Through interviews with knowledgeable individuals, public stormwater task force meetings and research, we will examine the feasibility of widespread use of porous asphalt as a viable alternative to storm water tax soon to be imposed on businesses in Lexington, as required by the consent decree. Porous pavement would allow businesses to decrease the stormwater tax based on the area of impervious surface (driveways, parking, sidewalks, streets).

Findings – Repaving of even 1/3 of public roads in urbanized areas in Lexington is impractical and prohibitively expensive, as well as inefficient. Incentives for businesses to avoid the stormwater fee tax through tax credits are problematic. With a 100% credit, the return on investment is more than 100 years for existing pavements and 70 years for new development. The table below shows cost per quantity unit for total volume of stormwater, dissolved solids and total phosphorous between current situation of no porous asphalt and hypothetical situation of repaving 1/3 of urban roads with porous asphalt. Following, the comparison is made for private impervious surfaces changed to porous asphalt surfaces. Lastly, the table shows return on investment for new and existing porous asphalt in relation to $4.16 tax per 1ERU (2,585 sq feet).

Recommendations - Per dollars spent for total stormwater runoff volumes, dissolved solids and phosphorous in stormwater it is the recommendation of the author that porous asphalt should be strongly considered, encouraged and subsidized for new construction. Existing development may rely on conventional approaches to stormwater management such as improving open channels, and increasing capacity of and building new detention basins. Existing development may also investigate other ‘green infrastructures’ (ex. green roofs, green parking) to decrease stormwater tax. This recommendation is influenced by the desire to decrease stormwater runoff, which leads to better water quality, not purely a cost-efficiency aspect.

1. What problems in general are caused by runoff from paved surfaces?

There are numerous problems associated with stormwater runoff from paved surfaces that relate to the chemicals, organic and non-organic matter. In non urban areas nature over time distributes the rainwater in the best way. The problem with stormwater in urban areas is the peak discharges from impervious areas. The water pours more quickly off of city and suburban landscapes, which have high levels of impervious cover. (Cahil)

Stormwater runoff is a major source of water quality impairment. Approximately 40% of all stream impairments are due to storm water. In the early stages of storm runoff from urban watersheds, the land surfaces, particularly the impervious areas such as roofs and paved roadways, are cleansed by the stormwater. The initial flush of contaminants from the catchment, know as the “first” flush” has been confirmed by experimental studies for both pollutant concentrations and pollutant quantities. (Urban Water Management Conference) Nature is the best manager of stormwater. When humans put impervious surfaces into the community, the problems presented by stormwater are significant. (Urban Water Management Conference: Integrated Water Quality Technology & Programs)

The initial flush of water resulting from intensive rain discharges significantly more runoff quantities in post development (purple color on the graph below) than in predevelopment (black color). Somewhat surprisingly, the flush is smaller with porous pavement (green color) even than predevelopment. This would suggest porous pavement is better than natural predevelopment state.

Table 1 - Flush (Peak float) discharges over time

One of the most common pollutants in residential areas is phosphorous, a common ingredient of agricultural fertilizers. If there is too much of it in water, it can speed up reduction in dissolved oxygen in water bodies. Phosphorus gets into water in both urban and agricultural settings and since it tends to attach to soil particles, it moves into surface-water bodies from runoff. (The effects of urbanization on water quality: Phosphorus) Additional education of people in suburban areas is considered by Lexington in its comprehensive plan – application of lawn fertilizers and disposal of animal wastes may be useful to maintain or improve the water quality in streams and lakes near growing urban areas. Stormwater has substantial amounts of phosphorus as it is being washed off from lawns into the streets. There it does not sink into the ground as it would if there were no paved streets, but rather it is washed, untreated, into nearby streams and ultimately to the Kentucky River.

Other pollutants found in stormwater are residues of gasoline and oil associated with the use of automobiles, lawn mowers and industrial equipment found in urban areas. The demand for urban potable water increases as the population grows. There is considerable discussion among scientists warning that the next crisis will be global water shortages. U.S southern states have already experienced severe water shortages over the past decade. (Williams)

2. How big is the problem of runoff from impervious paved surfaces in Fayette Co?

In the words of Lexington’s current mayor Jim Newberry, “Lexington has been ignoring this issue for too long.” The EPA filed a lawsuit against Lexington in 2006 and imposed an immediate fine of $425,000. Currently, the city is debating and planning the best approach to reduce the problem of stormwater runoff including, as required by the EPA decree, establishment of a stormwater fee.

Fayette County uses two sewer systems. One carries residential waste and the other carries stormwater. Wastewater is treated, stormwater is not. Stormwater simply enters ground water streams and eventually ends up in Kentucky River.

Lexington has some seriously inadequate capacity problems with its sewer system and the settlement between LFUCG and EPA clearly identifies very tangible obligations of the city to fix this problem. One of the major items on the list is to eliminate nine interconnections between sanitary and stormwater sewer systems. Interconnections bring at least two major stormwater pipes very close together, in the same way as roads crossing. Therefore, the interconnections have a much bigger potential to bring too much stormwater in one place and cause flooding in this area. This is particularly true if two pipes coming into a location at more than a 90-degree angle cause a “collision” at the intersection. According to the decree, Lexington will also be required to establish a “capacity assurance program to ensure adequate capacity exists before new connections are made to the system.” (Decree at glance) Lexington must ensure that under all possibilities it can deal with all year around stormwater runoffs incidents and ensure it has the capacity with its soon to be improved stormwater infrastructure to be capable of handling the stormwater runoff. Only then, new developments will be able to connect to existing stormwater infrastructures to transfer stormwater from those new areas. (Decree at glance) See Appendix F for map of numerous September 2006 house flooding and sanitary sewer overflows.

Hydrologic and hydraulic issues related to ‘dealing with water’ in any city are very complex. Partly because of the requirements of the EPA, but also recognizing the difficulties in the task, LFUCG is initiating the development of a computer model of the wastewater collection system. Since stormwater runoff also contributes to wastewater problems through flooding the wastewater pipes, the computer model should help illustrate the most cost effective techniques to manage stormwater.

Overall, bad watershed practices cause stormwater overflows of sanitary infrastructure. Too much stormwater in a particular place can cause overflows and mix stormwater with sanitary sewer. Stormwater enters the sanitary sewers. There is then too much water in sanitary sewers which causes overflows. Beyond the unpleasantness to the eye of the observer combined with odor issues, this fact is a major environmental hazard as the sewer is full of parasites and other dangerous bacteria. This aspect of Lexington’s water problems was the main trigger of the lawsuit by the EPA. Sanitary waste is treated either naturally or chemically in two of Lexington’s water treatments. If it overflows, it floods the nearby area and is never treated. Rather, it enters the ground causing potentially dangerous diseases to spread and overall fauna and flora devastation.

Furthermore, the consent decree requires Lexington to establish a fee to pay for the stormwater management practices outlined in the Storm Water Quality Management Program (SWQMP). This fee will, in large measure, be based on the amount of impervious or solid surfaces owned by each property owner in Fayette County. Thus, as a general proposition, the more impervious surfaces that you own, the greater your stormwater management fee will be; although there will be a standard fee imposed for typical homeowners. The fee for residents will be standardized and it will be most likely around $4.16 per 1 ERU (Equivalent Residential Unit) which is an area of (on average) 2,585 square feet. Businesses, however, will have their fee assessed based on paved area and other impervious areas. In other words, the larger the impervious area, the larger the parking space for example, the larger the fee for stormwater runoff from this area.

The “Procedures Manual for Infrastructure Development” describes how LFUCG will manage the design and construction of roads, sanitary sewers and pump stations, and stormwater facilities in developing areas. In the past, the lack of written guidance and procedures caused uncertain responsibilities and expectations for LFUCG, engineers and developers. The new rules shift responsibility from the city to the private owner or developer. Private entities must take responsibility for the public stormwater system that is located on or adjacent to their properties. Private entities include private roads, parking lots and access easements, portions of the public drainage system, including pipes, culverts (conduit enclosing flowing water), bridges, retaining walls (holding back soil that might otherwise slide down a hill), headwalls (retaining soil to prevent erosion), overland flow channels (water channels), swales (areas designated to capture runoff by spreading it horizontally across the landscape), and stormwater practices that carry water to the natural portion of the drainage system. The best management practices and maintenance requirements are contained in the Stormwater Manual.

3. Stormwater best practices in Lexington

Recognizing the detrimental effect that stormwater causes to our streams, costly maintenance, continuing construction of more and more stormwater facilities over the past few years, cities across the nation have put new emphasis on “green infrastructure”. Green infrastructure attempts to hold the stormwater where it falls from the sky rather than relying on carrying it in a controlled fashion to the nearby stream.

There are numerous ways to contain stormwater in a ‘green’ fashion. Some seem small and insignificant, but the opposite is true when all are combined and numerous entities are engaged in those practices. (Green Infrastructure Types, Applications, and Design Approaches to Manage Wet Weather)

• Green Roofs – Nutritious soil is put on the roof to grow vegetation. The vegetation absorbs significant amounts of rain water.
• Rain Harvesting & Downspout Disconnection – residential barrels can be spotted in Lexington where residents collect stormwater for use in their gardens.
• Planter Boxes – Wooden, concrete or other boxes with soil put on impervious surfaces such as sidewalks to grow vegetations which absorb stormwater.
• Rain Gardens – Planting of gardens in residential front (or back) lawns with special rain-liking vegetation to collect water in the garden.
• Green Parking – Changing parts of parking by planting as many trees as possible, putting grassy areas leaving only two strips for wheels when the car is parked.
• Permeable (Porous) Pavements – Putting asphalt, concrete or some other pavement that lets the water through it.
• Green Streets and Highways – Concept similar to porous pavements and green gardens attempting to decrease the impervious surfaces as much as possible
• Vegetated Swales – Areas left for stormwater to run into and left underdeveloped with natural vegetation
• Pocket Wetlands – Similar to above, presumably water would stay there longer.
• Trees and Urban Forestry – Lexington is rather famous for its efforts to put many trees in urban areas. Trees use lots of water and are excellent storm water ‘customers’.
• Riparian Buffers – Vegetated areas next to rivers that protect them from stormwater pollution

All these green infrastructure types, applications and designs are mentioned and encouraged in the Lexington Stormwater Management Plan

This paper focuses only on porous asphalt. But to emphasize how much importance cities assign to all methods, an example is that green roofs lower stormwater runoff from 65-94% in comparison to the same type of roof without soil with vegetation placed. New York pays $4.50 per square foot to an owner who uses green roofs, while Toronto pays $5. Green roofs also have the additional benefits such as air pollution reduction (fighting city smog that contributes to health problems), noise reduction, aesthetic benefits and creation of green jobs (trimming, etc.), energy savings, human use and enjoyment. (New York City Green Roof Buildings Tax Credit.)

The focus of this paper is pollution prevention in residential and commercial areas, as that connects to the requirement of the stormwater fee and the areas of impervious surfaces. The point #2 of measurable goals in Lexington SWQMP states: “By January 31, 2010, […] facilitating the use of, “green infrastructure” alternatives to managing post construction stormwater, such as infiltration, reuse, and evapotranspiration.” (Evapotranspiration is sum of evaporation and plant transpiration from the earth's land surface to atmosphere [Wikipedia].)

One of the “green infrastructure” alternatives is porous pavement. If applicable and appropriate in a particular situation (presumably, subject to permit and approval by city engineers), porous pavements have the potential to infiltrate stormwater, revitalize ground water sources as nature intended and decrease the stormwater fee for businesses. Infiltration in porous asphalt occurs through water going through rocks underneath and then through slow-sinking of the water into the soil. Ground water sources get more water which is the way it would happen in nature if no paved impervious structures were raised.

4. Stormwater – Negative Externality

Clean water for fishing and recreation is a public good. Clean water in our faucets is available for everyone who is willing to pay for it. Clean water versus water that would be of lower quality at the source – water company treatment center - is non-excludable at present but this is not the case in all parts of the world. Also, under the current pre stormwater tax situation, no matter how much one pollutes in reference to another who pollutes less, the cost of clean water in our faucet is the same for all, it is based on price per quantity of the clean water.

Current laws allw an entity to ‘shift’ the full costs of stormwater runoff related problems such as flooding of sewer pipes, environmental hazard, etc. from one party to another. The responsibility of dealing with problems associated with runoff is transferred to the government without any payment to offset costs. The pollution and handling of the stormwater as well as increasing maintenance cost of stormwater infrastructure are by-products of production or consumption of private residents and businesses. We live in our residence with no flooded basements, water dripping on our heads or getting dirty in our dirt-compacted driveways. Businesses in urbanized areas engage in practices that bring them income, loading and unloading of trucks, buildings and other structures storing products and offering them to consumers who parked their cars at provided parking places. The rain coming from the sky washes off the residues of our activities.

The graph above illustrates how stormwater runoff creates negative externality. The supply schedule, S*, reflects only the private marginal costs, for example, a shopping mall supplying their sale items in the mall. The second supply schedule, S#, incorporates the costs that the negative externality imposes on third parties – the stormwater runoff from the mall and its parking structure, as well as the private marginal costs incurred by suppliers.

Stormwater runoff creates a negative externality for some businesses and individuals. Stormwater runoff transfers costs from all taxpayers who may enjoy the benefits of the infrastructures (parking lots, houses, malls, roads, etc.) to people who suffer negative impacts such as flooding because of the misfortune of living in low areas or otherwise areas affected by the stormwater runoff volumes and/or pollutants they carry.

The length of this distance between private supply S* and social supply S# depends upon conditions such as the amount of rain, the area of impervious surfaces, conditions of the parking structures (contaminated or not), number of big storms creating peak flooding, and runoff volumes large enough to create problems down the stream. Peak flooding is primarily due to big storms, common in Lexington during the summer. Big intensive storms can produce large volumes of fast-moving water in the first minutes of the storm. Less intensive rain over a longer time may produce similar quantities of stormwater but with less of peak flooding. The second is preferable as the pipes and other structures have better chances of handling that volume for an extended time.

The standard technique for reducing deadweight loss – triangle area C on the above graph – resulting from negative externalities, is to impose taxes. This would be an additional tax, not an existing income and other tax. This would be a tax in proportion to the amount of runoff from a particular property. Consequently, the price paid by consumers would increase from P* to P#, the net price received by producers would fall from P* to P# - t, and output produced and sold would fall from Q* to Q#.

If we were considering pollution from chimneys instead of stormwater, hypothetically, the fee would increase the cost of a fire for a private consumer from say $5 to $10. This price increase from P* to P# would cause the producers (people enjoying fires in their chimneys) to change their behavior and stop having as many fires per season - Q* to Q#. Consequently, the receivers of the income based on the fee would collect only P# - t and not the entire proposed new price of P#.

However, when it comes to stormwater, no behavior can be influenced as far as quantity through a tax if the impervious surfaces stay as they are or grow. If the quantity of stormwater depends on impervious surface, with a moderate tax, the tax may not induce the owners to change the impervious surfaces and therefore, the quantity of stormwater runoff will not change from Q* to Q#.

The thing that can change the ‘bad behavior’ of residents and industries in the form of stormwater runoff production would be to change their behavior. If the behavior changes, there is no need for tax. If no green technologies are used, the output - stormwater runoff - will not decrease. The increased price (tax) and therefore, ‘income’ will be spent on transferring the output to streams and ultimately the Kentucky River in an orderly fashion (through stormwater infrastructure).

The best scenario would be to not have to pay the tax at all because there would be no reason for it – no impervious surfaces. This would be more efficient than collecting the tax and investing in enlarging ponds, house demolition, etc. Of course, we cannot eliminate impervious surfaces completely. However, we can try to decrease the areas of impervious surfaces. Porous pavements could help in this task. Porous pavements would most likely not eliminate runoffs completely unless very careful analysis of rain quantities was collected and a large enough area of rocks was put underneath the parking or other porous pavement structures. Porous pavement would however, definitively decrease runoffs, substantially filter bad chemicals found in stormwater and, most importantly, decrease drastically the initial ‘flush’ of stormwater during big storms.

5. What is porous asphalt?

Porous asphalt pavements have been used for more than 30 years around the United States to minimize the environmental impact of pavements. Porous asphalt serves two main functions – 1) It is a hard surface, so parking, driving or walking is possible 2) It filtrates stormwater and stores it underneath itself. For most storms, it has the capability of gathering the stormwater and then slowly dissipating it into the ground to recharge groundwater pools.

“A porous asphalt pavement is constructed over a stone filled reservoir to collect and store stormwater and to allow it to infiltrate into the soil between rainfalls.” (Sustainable Construction Using Asphalt Pavement) On the highways, mostly in European countries, a porous asphalt mixture has been popular for reasons other than its infiltration properties. Traffic noise reduction capabilities of porous asphalt resulted in its extensive use and development. In more densely populated areas in Europe, noise is a concern.

There are several different factors that impact whether porous asphalt is appropriate in any particular setting. These are weather, durability, raveling, clogging, and the filtration and water retention characteristics. Each of these is discussed below.

Source: Porous Asphalt Pavements for Stormwater Management

• Weather

The University of New Hampshire Stormwater Center (UNHSC) found that porous asphalt also works well in cold weather regions. “By design, an open-graded, well-drained porous pavement system incorporating significant depth will have a longer life cycle from reduced freeze-thaw susceptibility and greater load-bearing capacity than conventional parking lot pavements.” (Gunderson) One should also note that because friction of porous surface is greater than conventional asphalt surface, the application of salt is either completely unnecessary or much less of it is needed. This translates into significant savings for a city’s winter road salt treatment. The environment also benefits greatly from less salt in ground water. The savings resulting from limited or no salt application were not part of this cost efficiency analysis.

• Durability

Conventional asphalt has a layer that can be recycled every 10-15 years depending on the wear on the pavement. Porous asphalt will last at least as long. In Philadelphia, PA, porous asphalt has been working well for 20 years for a parking structure constructed there. The parking gets daily use with an average parking intensity. (Porous Asphalt Pavement with Recharge Beds 20 Years and Still Working)

The example below “reduces erosion, pollution and the road salt” as well as preserving clean water for this and future generations. It has been in place for over 30 years.

Source: Porous Asphalt Pavements for Stormwater Management

• Raveling

Loss of aggregate - loosening of stones from the surface of a pavement - starts the disintegration of pavement. Raveling is usually caused by water. Snowplows can induce raveling as well. Raveling may be due to low-density compaction when the pavement was placed or an inappropriate adhesive mix. Leaked oil from cars has caused raveling in some parking lots. The Hamilton-Hinkle Paving Company in Georgetown, KY, constructed two small parking lots with porous asphalt. The post-office in Georgetown also has grant-funded small, porous asphalt and porous concrete areas for demonstration purposes. There is no consistent data suggesting that raveling is worse in porous asphalt than in conventional asphalt. Proper construction of the site is crucial as well as the proper adhesive mix used.

• Clogging

Clogging of porous asphalt can occur. This problem does not happen in conventional asphalt as conventional asphalt is impervious and often it is sealed and/or resealed. In Europe large vacuum equipment is used to clean porous highways with pressure water. Water under pressure is directed onto porous pavement while vacuum sucks any debris. In parking structures, clogging would be rather rare as there is not enough traffic to cause substantial clogging problems. In the U.S., there is limited information about restoring permeability to porous pavements. One should remember that even if 99% clogging were to occur, the infiltration rate would still be greater than 10 inches per hour, which is more efficient than most sand and soil mediums. (Gunderson).

• Filtration and Stormwater characteristics

The University of New Hampshire Stormwater Center conducted porous pavement studies. In its 2007 Annual Report they noted: “During the three-year period of monitoring, there has been no surface runoff from the parking lot.” The Northeast region experienced during this time two, 100-year storms, which produced very large quantities of rain. (University of Hampshire Storm Water Center)

To provide sufficient storage to handle the peak rate of stormwater runoff, the volume of the ‘bed’ under the porous asphalt (rocks) should be equal to the volume of the local two-year storm event. If that were the case, there would be virtually no runoff from the parking lot. (Cahil)

According to National Asphalt Pavement Association (NAPA) as well as the University of Hampshire Stormwater study, porous asphalt reduces stormwater peak (otherwise known as ‘flush’) by 88%, increases lag time (time elapsed between initial stormwater peak and end of stormwater runoff) by 460%, and reduces the volume of stormwater runoff by 25%.

Those findings are based on relatively poor infiltrating soils – mostly hydrologic soil group C that are sandy clay loam. Soils of group C have “low infiltration rates when thoroughly wetted and consist chiefly of soils with a layer that impedes downward movement of water and soils with moderately fine to fine structure.” (Hydrologic Soil Groups) This is also the common group for Kentucky soil.

Source: Porous Asphalt Pavements for Stormwater Management

Typical porous construction is very similar to conventional asphalt construction. As the graph above illustrates, the water is let through and allows disseminating over time. However, outlets and conventional pipes are ‘ready’ in case of a big rain and the basin under the asphalt and rocks reaching its capacity.

Porous asphalt, according to NAPA, can reduce toxic chemicals present in stormwater as well as suspended solids and phosphorus. The rates are rather impressive. The rocks and porous asphalt acts as a filter or combination of filters. Reduction in particular filtrates using porous asphalt has been demonstrated at the following rates:

• Total Suspended Solids (TSS) – 99% reduction
  Particles trapped by a filter, typically of a specified pore size often referred to as “non-filterable”.
• Total Zinc (TZ) – 97% reduction
  Metallic chemical element. There are many zinc compounds used industrially.
• Total Petroleum Hydrocarbons (TPH) – 99% reduction
  TPH consists of several hundred chemical compounds with hexane, jet fuels, mineral oils, benzene, toluene, xylenes being some of them.
• Total Phosphorus (TP) – 42% reduction
  Essential to life, but it can be very toxic in large quantities. It has a tendency to bioaccumulation.

Hamilton-Hinkle Paving Company in Georgetown, KY, constructed one small parking lot with porous asphalt for a location that was about 5-7 feet higher than the adjacent property. The adjacent property experienced frequent stormwater flooding primarily because of the parking lot adjacent to it and its higher elevation. After the porous asphalt was constructed, about 2 years ago, there is virtually no runoff from the property and there is virtually no more danger of flooding the adjacent property. (See Appendix H for photo)

• Where not to use

Because porous asphalt has less internal strength capability, due to its porous nature with tiny spaces in-between asphalt components and rocks used for asphalt construction, it is not recommended for such situations as airport taxiways or slopes greater than 6%. There is very limited use of porous asphalt on roadways in the U.S., although it has been applied more extensively in Europe. 20% of Japanese public roads use porous asphalt. (Noise reducing pavement in Japan)

There are also locations where spills and groundwater contamination risk is too great to use porous asphalt. In those situations (such as truck stops and heavy industrial areas) the ability to contain spills must also be considered and built into the system. (Porous Asphalt Pavement with Recharge Beds 20 Years and Still Working)

It is also important to understand that use of porous asphalt cannot be taken as “one size fits all” alternative. There are many places where this alternative may make the flooding and other stormwater related problems worse. An example of one of the worse stormwater problems in Lexington is the Southland Drive, build in natural creek basin. Porous asphalt would not help there and indeed it may let the water flood the basements of businesses and residences even faster. If something is built in the wrong place – meaning it should have never been built there due to hydrology – porous pavement will not help. Consultation with city engineers is vital before installing this pavement. Primarily, also due to possibility of unintentional letting of water into houses foundations and basements, as well as cost concerns (described later), the city of Lexington does not offer any credits or other incentives for residential areas. Residents are subject to a flat tax rate.

6. Alternative methods of preventing stormwater runoffs compared to porous asphalt

In order to compare methods, this paper focuses on volume of stormwater runoff, dissolved solids and amount of phosphorus in storm water. We want to know the cost of each pound of water runoff or dissolved solids. Intuitively, if money was not an issue, we would suggest using it widely and exclusively. However, since budgets are not unlimited, we must find out what is the amount of money spent of each pound of stormwater volume. The study by Driver, N.E, Tasker, G.D. “Techniques for Estimation of Storm-Runoff Loads, Volumes, and Selected Constituent Concentrations in Urban Watersheds in the United States, United States Geological Survey Water-Supply Paper 236” provides a nice mechanism to calculate those numbers. The study was extensive and at the end it produced a regression that can be used to calculate various quantities of different entities associated with stormwater.

• Regression

The regression estimates also allows calculations of other quantities of different entities found in stormwater, but this paper does not calculate those. The paper’s three quantities calculated are highlighted horizontally. The vertical column highlighted on the table below is porous surface, a characteristic of great interest of this paper. I, II, and III refer to different areas of the United States with different amounts of rain per year. Kentucky falls under II area where mean annual rainfall is between 20 to 40 inches. This paper focuses on three quantities with the below regression:

• RUN II – Total volume of stormwater runoff.
• DS II – Total suspended solids (pollutant) in stormwater runoff.
• TP II – Total phosphorous (pollutant) of stormwater runoff.

Table 2 – Summary of regression models for storm-runoff loads and volumes
Source: Techniques for Estimation of Storm-Runoff Loads, Volumes, and Selected Constituent Concentrations in Urban Watersheds in the United States. U.S. Geological Survey Water-Supply Paper 2363. 1990

Other quantities were not calculated due to the difficulty in obtaining appropriate numbers for some variables. The value “--“ in regression shows variables not included in the model.

Another limitation of the regressions’ calculations may be oversimplification of the actual situation. The true volume of the runoff and amount of dissolved solids, as well as phosphorous runoffs, may be dependent on the particular hydrology of a place. Stormwater issues are complex. “In region I models, values of R2 generally were larger and standard errors of estimate were smaller than those in the Region II and Region III models. As mean annual rainfall increased, the ability to estimate storm-runoff loads decreased.” (Driver)

From left to right, each regression has different coefficient values that influence the final quantity calculated by taking each value to the coefficient power.

They are, from the left, the Bo is the below. Underlined quantities measured, values and coefficients represent those used in this paper.

• B0 is regression intercept
• TRN is total storm rainfall
• DA is total contributing drainage area
• IA is impervious area
• LUI is industrial land use
• LUC is commercial land use
• LUR is residential land use
• LU is nonurban land use
• PD is population density
• DRN is duration of each storm
• INT is maximum 24-hour precipitation intensity that has a 2-year recurrence interval
• MAR is mean annual rainfall
• MNL is mean annual nitrogen load in precipitation
• MJT is mean minimum January temperature
• BCF is bias correction factor
• COD is chemical oxygen demand in storm-runoff load, in pounds
• SS is suspended solids in storm-runoff load, in pounds
• DS is dissolved solids in storm-runoff load, in pounds
• TN is total nitrogen in storm-runoff load, in pounds
• TKN is total ammonia plus organic nitrogen as nitrogen in storm-runoff load, in pounds
• TP is total phosphorus in storm-runoff load, in pounds
• DP is dissolved phosphorus in storm-runoff load, in pounds
• CD is total recoverable cadmium in storm-runoff load, in pounds
• CU is total recoverable copper in storm-runoff load, in pounds
• ZN is total recoverable zinc in storm-runoff load, in pounds
• RUN is storm-runoff volume, in cubic feet
• Dashes indicate that the variable is not included in the model
  

• Status Quo – no porous pavements on urban roads

i. What is it?

Without appropriate incentives to change impervious surfaces into porous surfaces, the stormwater runoff quantities must be better handled by current conventional engineering techniques, such as improving open channels and increased capacity of detention basins.

This conventional approach of relying on stormwater pipes, culverts, detention basins and other open water channels that transfer stormwater from urban areas to other places is not inexpensive. The Stormwater Priority Projects Master list has 100 projects of different sizes totaling $133 million in estimated cost. The projects involve demolition of houses to build large detention basins that receive surface runoff. Lexington may also construct or improve current stormwater channels, inlet and outlet structures, and elaborate systems for conveying water all over the place.

There is an opportunity cost associated with building detention basins. A detention basin is a place where water can be detained. This is used to gather water and cannot be used for other purposes. In urban areas, sites can be used for economic activities rather than for detention of water. The watershed is a complex maze of hydrological interconnected areas. In some locations, a detention basin may be the most economical solution. At the same time, if all surrounding areas use less impervious space and use more porous surfaces (such as porous asphalt), it is conceivable that many houses could be saved from demolition. This is however, almost impossible to investigate without some computer modeling software. There are a few on the market that help community planners decide the best engineering practices for most cost efficiency when it comes to watershed management. (InfoWorks)(TR-20)

One of the worst problems related to stormwater flooding in Lexington is a large impervious area on Southland Rd (picture below). In the 1960’s, this area was paved over a natural stream of water. This area experiences frequent flooding. The existing channel cannot carry all the stormwater. The open channel is also a public health concern because of mosquito-born illnesses such as West Nile virus and Eastern Equine Encephalitis.

Southland Drive Floodplain Delineation

Current engineering does not improve water quality. During the summer, not necessarily in Lexington, but in general, runoff can get very hot, sometimes as much as 120 °F degrees. When you have stormwater at that temperature flowing into a groundwater-fed stream that is at 65 °F, severe impacts can occur in the aquatic ecosystem such as killing of fish and other biological organisms. (Gunderson)

ii. How much does it change runoff?

The techniques described above do not change stormwater runoff at all. Over time, since no incentives exist to change current development, we can reasonably assume that development will continue as it always has. The more the city grows, the more impervious roads, parking structures, school and church parking lots will be built, thus contributing to the stormwater runoff problems. This approach focuses only on dealing with stormwater runoff through open channels and sanitary pipes, not by trying to decrease the stormwater quantity.

iii. Cost

The cost of progressively increasing the capacity of the stormwater system to handle larger and larger quantities of stormwater runoff, resulting from more of the land in the city being impervious to rain water is not trivial. Lexington supported all this cost through different mechanisms of funding. Currently, the “Stormwater Priority Projects Master List” indicates 100 projects related to stormwater issues. The total cost for all is listed as $133,079,000. For the purpose of this paper, we will assume $130 million is the cost of continuing conventional stormwater management techniques without directing at least some of the techniques toward porous asphalt – the status quo scenario.

Total cost (from Stormwater Priority Projects Master List) = $133,079,000

Fayette County has 1,100 lane miles (for snow removal purposes). 1,100 lane miles is 3.40 square miles (average 15 meters wide streets). Calculated rainfall for one year, on average, for Lexington, hovers around 40 inches of rain. (Kentucky Climate Data)

See Appendix A for detail calculations

RUN – Total runoff volume in cubic feet
TRN – Total rain volume
DA – Total drainage area
IA – Impervious area
BCF – biases correction factor

RUN II = (ß0) 62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF) 1.212 = 62,951 x (40) (1.127) x (3.4076) (.809) x (1) (.522) x 1.212 = 13,164,083 cubic feet

$133,079,000/ 13,164,083 = $10.11 per cubic foot of total stormwater runoff

DS – Dissolved Solids in pounds

MJT – mean minimum January temperatures (Appendix E)

DS II = (ß0)2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208 = 2,308 x (3) (1.076) x (3.4076) (1.285) x (1) (1.348) x 26.18 (-1.395) x 1.208 = 7,129.62 pounds of suspended solids

$133,079,000/ 7,129.62 = $18,666.65 per pound of suspended solids in runoff

TP – Total Phosphorus

INT – is maximum 24-hour precipitation intensity that has a 2-year recurrence interval

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT) (1.543) x (BCF) 1.486 = .153 x (3) (.986) x (3.4076) (.649) x (1) (.497) x 4 (1.543) x 1.486 = 162.78 pounds of total phosphorous

$133,079,000/ 162.78 = $817,539 per pound of phosphorous in stormwater runoff

Similarly rather large numbers will be produced for phosphorus and also suspended solids. One should remember that a pound of phosphorous is a large quantity and much less of phosphorous is usually present in water. A much lesser amount can also be dangerous to aquatic life. The numbers calculated also vary greatly based on amount of rain. Precise data would be needed for entire city regions to calculate precise numbers.

• Porous pavement for 1/3 of all suburban roads

i. What is it?

The scenario is rather unrealistic but it is shown here for comparison purposes. To change a substantial part of county roads to porous pavement would greatly improve stormwater runoff problems and would be beneficial for the environment. However, as shown below, the cost of such an undertaking is prohibitively expensive, even under the most optimistic conditions.

ii. How much does it change runoff?

In theory, most days of the year, except for very large storms which happen in Kentucky on average, once a month, the completely porous pavements on urban roads would decrease the runoff volumes from 70 to 100%. See comparison below after the cost is calculated.

iii. Cost

Looking at the map below, almost half of the roads are in urban service areas. Many roads are not causing any stormwater issues. Therefore, for the purposes of comparison, I assume 1/3 of those roads are located in heavily urbanized areas with large impervious areas (parking, malls, schools, etc.) and those would be changed to impervious asphalt to change the runoff. There are 1,100 lane miles in Fayette County (for snow removal purposes). Lane lines width vary greatly from 9 meter to 45 meters. Assume, on average, 15 meters width. The assumption is a smaller number than ‘true mathematical average’ because there are more narrow roads than very wide roads in any city. Wide roads are fewer and designed to carry the majority of traffic over longer distances while there are many smaller neighborhoods roads.

"Urban Areas" for Implementation of LFUCG's Stormwater Quality Management Plan

15 meters = 49.21 feet || 1,100 miles / 3 = 367 miles = 1,937,760feet
1,937,760 feet * 49.21 feet = 95,357,169.6 square feet
95,357,169.6 * $4.54 = $432,921,550
95,357,169.6 square feet = 3.4076 square miles

This cost is for installing new porous asphalt. However, old asphalt has to be first excavated including the rocks underneath. At least 30% of the cost must be added then, according to asphalt constructors from Georgetown, KY. Some asphalt and presumably the rocks underneath could be recycled.

Total cost = ($432,921,550 * .3) + $432,921,550 = $562,798,015

The cost of more than half of a billion dollars is prohibitive for any serious considerations, especially since this is a best-case scenario. True cost would be higher due to specific difficulties the city would have during excavation of existing asphalt (traffic problems, cables and other infrastructure underneath the roads). One also wonders what could be done with excavated material if it could not be recycled.

See Appendix B for detail calculations

RUN – Total runoff volume

RUN II = (ß0) 62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF) 1.212 = 3,949,225 cubic feet

$562,798,015 / 3,949,225 = $142.51 per cubic foot of total stormwater runoff

DS – Dissolved Solids

DS II = (ß0) 2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF) 1.208 = 285.16

$562,798,015 / 285.16 = $1,973,622 per pound of suspended solids in runoff

TP – Total Phosphorus

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT) (1.543) x (BCF) 1.486 = 52.09 pounds

$562,798,015 / 52.09 = $10,804,338 per pound of phosphorous in stormwater runoff

When one compares 100% impervious surface, for example a conventional parking lot, to porous surface, the reductions of total runoff volume, total dissolved solids and total phosphorus are rather significant and they are as follows:

70% decrease in stormwater volume (per year) – from 13,164,083 to 3,949,225 cubic feet
96 % decrease in dissolved solids (per year) – from 7,129.62 to 285.16 pounds
68% decrease in phosphorus (per year) – from 162.78 to 52.09 pounds

• Require porous pavement – new construction or revitalization only

i. What is it?

Corner of Liberty Rd and New Circle Rd

Porous pavement for new construction is a very promising alternative to conventional management techniques. New or revitalized areas designated for development do not carry the additional expense of excavation and removal of existing structures. The picture above is a three image collage of a site in Lexington that could be developed using porous pavement.

ii. How much does it change runoff?

According to the literature, porous pavement decreases runoff volume by 25%. While the calculations above showed a 70% decrease in runoff volume, the calculations may have oversimplified the particular situation and did not take all possible variables into consideration. Literature from the Asphalt Association quotes a 25% runoff volume reduction. The actual percentage varies greatly depending on the particular situation and a variety of factors play a role in the final results. These variables are slope, soil conditions, neighboring practices, as well as the volume of the rock basin underneath the porous asphalt to hold the rain. The same conditions would most likely change the percentage of runoff volume reduction for 1/3 of public roads. There would however, presumably still be a decrease, as the same special variables existed before repaving 1/3 of public urban roads, as after repaving with porous asphalt takes place. The lower volume of runoff and dissolved solids, suspended solids, phosphorus, etc. in stormwater which ends up in water used for consumption, the less treatment of that water will be needed. This will result in savings related to treatment facility filters use, filters cleaning, equipment replacement frequency, etc.

iii. Cost

The installer of the porous surface bears the entire cost of the porous asphalt (unless incentives are present – discussed in next point). Conventional stormwater drainage must still be installed as it is conceivable that, under certain circumstances, incredibly heavy rain will be too much for porous surfaces as well. However, the core of the problem with stormwater runoff is the ‘pick’ flood resulting right away from relatively normal rain, since much rain water is washed off from roads, parking, and other impervious surfaces. Since it significantly reduces runoff volume, the city is saving money by either not having the need to increase capacity of waste water pipes or by having less water to treat for consumption (or both). There is also an environmental benefit by having less pollutants (such as salt) being washed into streams and ultimately rivers.

The cost of porous asphalt is $4.54 per square foot. This price is based on data from Hamilton-Hinkle Paving Company, which constructed porous parking space in downtown Georgetown, KY. Hypothetically, the same parking structure constructed with conventional (nonporous) asphalt would cost $3.08 per square foot. The main difference is the asphalt mix used and height of rocks under the asphalt. Because there are more rocks and higher adhesive asphalt mix used for porous pavement, the cost per square foot of asphalt is higher - $4.54 vs. 3.08.

The cost of porous asphalt is $4.54 per square foot.
0.2 square miles = 5,575,680 square feet
Cost = 5,575,680 * $4.54 = $25,313,587
TR (Total Rain) = 40 inches
Area = 0.2 square mile – NOTE, this is rather LARGE parking lot; ex. Large Shopping Mall
Impervious Area = 10% (porous asphalt)

See Appendix C for detail calculations

RUN – Total runoff volume

RUN II = (ß0) 62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF) 1.212 = 394,903.63
$25,313,587/ 394,903.63 = $64.10 per cubic foot of total stormwater runoff

DS – Dissolved Solids

DS II = (ß0) 2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208 = 7.68

$25,313,587/ 7.68 = $3,296,040 per pound of suspended solids in stormwater runoff

TP – Total Phosphorus

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486 = 8.21
$25,313,587/ 8.21 = $3,083,263 per pound of phosphorous in stormwater runoff

Pound of phosphorus or pound of un-filterable dissolved solids in stormwater cost runs in millions of dollars. Further atomic weight and detail chemical analysis would be needed to know what pound of those takes in volume.

• Require porous pavement – existing parking and other private areas

i. What is it?

Porous pavement for new construction is a very promising alternative to conventional management techniques, as stated before. But the biggest obstacle to existing places is what’s already on the ground – existing pavement. Additional expense of excavation and removal of existing structures is substantial and even 30% in addition to cost of new development may be understated. The picture below is of a place in Lexington that could be developed by using porous pavement and stop this existing technique that allows untreated polluted stormwater run directly to a stream which is located only about 400 yards from a nearby lake.

Woodland Dr. Shopping Mall

ii. How much does it change runoff?

According to the literature, porous pavement decreases runoff volume by 25%. The actual percentage varies greatly depending on the particular situation and variety of variables play a role in final results.

iii. Cost

The installer of the porous surface bears the whole cost of the porous asphalt. Some of the cost can be passed onto consumers but only to limited degree due to competition and free market forces that reward the least expensive goods suppliers. Any developer considering building a new mall or revitalizing closed mall will most likely pass the cost of building and parking construction onto store owners in the form of rent. However, the owner must be careful because if the rent is height in comparison to other malls, the stores will not open in the mall or will have higher prices. Education of public and calling their attention to the importance of stormwater management may be an important factor. Many environmentally conscious customers may be willing to shop in a mall that has porous parking and other green infrastructure methods of dealing with stormwater, even if that means a dollar or two more expensive items in the stores.

The cost of porous asphalt per square foot = $4.54 per square foot
Add 30% for excavation and removal of current asphalt cover

$25,313,587 + ($25,313,587* .3) = $32,907,663

All the assumptions and coefficients are the same. The only variable that changes is cost, from $25,313,587 to $32,907,663. The unit cost therefore is also higher as higher number divided by the same number produces higher final number.

See Appendix D for detail calculations

RUN II= $83.33 per cubic foot per year
DS II = $4,284,852 per pound per year
TP II = $4,008,242 per pound per year

When compared to new constructions, the costs are lower for new versus existing development per unit:

• - $64.10 versus $83.33 = 23%
• - $3,296,040 versus $4,284,852 = 23%
• - $3,083,263 versus $4,008,242 = 23%
  

Not surprisingly, the differences remain around the 30% additional cost of excavation and removal of existing conventional impervious asphalt pavement.

• Explanation of cost per volume, per pound of dissolved solids and pound of phosphorous

The regression calculates the amount of some entity (volume in cubic feet or pounds of material). The coefficient that increases over time and influences the magnitude of that quantity is (TRN) - Total Rain in an area. Drainage area, impervious area, temperatures are more stable and do not change much over time (if at all).

DS II = (ß0)2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208
RUN II = (ß0)62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212
TP II = (ß0).153 x (TRN) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486

Therefore, the paper shows cost per 40 inches of rainfall. Regression differentiates between three regions with different rainfall. Lexington is on the boundary between region II – 20 to 40 inches per year, and region III – more than 40 inches per year. The rain quantity is the only one that could change the amount there over time. An exceptionally dry year would make dollars spent per volume higher, while exceptionally wet year would make the amount spent lower. If we make the rational assumption that roads, parking structures as well as conventional stormwater drainage such as pipes, culverts, detention basins and other open water channels will last about minimum 20 years, then we can calculate money spent for 20 years assuming approximately 40 inches of rain each year (on average). The TRN would then be (20 X 40) = 800 inches of rain. See Appendix G for detail calculations.

The span of time of 20 years makes the cost per unit smaller. If the infrastructure lasts 40 years and we assume upkeep costs are inconsequentially low, the cost per unit is again about half of the values above. The longer that we use something, the less expensive it is.

• Runoff reduction tax credit

i. What is it?

Primarily, because of the large cost of administering a credit for the stormwater tax, the tax credits are usually available only to nonresidential property owners. The standard residential tax is small. It is not practical to provide private homeowners with incentives to demolish their driveways and build a new one with porous concrete or asphalt. One of the problems encountered right away is the question of water entering residential basements. Porous asphalt seems better suited for new development when engineers can ensure proper hydrology and that water will not enter basements. In old developments, a few decades ago no one knew about porous asphalt and therefore, this new technology may not work with the hydrology of some residential areas. Avoiding a small stormwater tax may result in a much larger expense for the homeowner – draining the basement.

From an economic perspective, the extent to which a credit will increase the efficiency of a stormwater program depends partly on the amount of the tax and of course on the credit itself. According to a survey of stormwater utilities, out of 38 utilities that responded, 71% had no fee reduction in form of credit or other incentives. Of the remaining 11 utilities, two major types of fee reduction approaches were reported: 16% had fee reduction for peak runoff controls by 460% (peak runoff is otherwise known as ‘flush’) and 8% had a fee reduction for implementation of water quality best management practices or proper maintenance of onsite stormwater facilities. (Doll) Refer to Storm Water Quality Management Program (SWQMP) for best management practices for Lexington. Green infrastructure is part of it.

Other cities use and magnitude of stormwater tax credit

Source: Credits as Economic Incentives for On-Site Stormwater Management

ii. How much does it change runoff?

If the tax and credit is large enough, it may either force companies to change some impervious surfaces into porous surfaces and decrease stormwater runoffs significantly, say 30%. Or, if the tax is low and there are few incentives to change impervious into porous area, the companies will pay the tax and the stormwater runoff volume decrease will be much less substantial, say 10% or even 0%.

iii. Cost

The cost is split between firms and government depending on the structure of tax and tax credits. Tax “transfers” negative commodity (stormwater) from where it is happening to another location. Too large a tax may make new companies build in other places rather than Lexington, which is not good economically. The stormwater tax in Fayette County, therefore, must not be too large. Currently, 2,585 square foot property is defined as 1 ERU (Equivalent Residential Unit). According to documents related to the stormwater task force as well as the meetings of this task force, currently the fee hovers around $4 per 1 ERU. Other cities’ fees vary. The city size and most likely magnitude of the problem, as well as political support for taxation play a role in accessing the fee amount. The amount is most likely based on the need to cover all the stormwater management costs in particular city.

Typical parking for a commercial lot or doctor’s office may be 50 parking spaces, lot that is approximately 25,000 square feet or ten (10) ERUs. Note - this is substantially smaller parking than the hypothetical above in point c) and d) with 0.2 square mile area. 25,000 square feet is 0.000896 square miles.

• Porous lot of 25,000 square feet (25,000 x $4.54) = $113,500 (cost to build)
• Conventional lot of 25,000 square feet (25,000 x $3.08) = $77,000 (cost to build)

The additional cost for the owner/developer to do porous versus conventional is $36,500.

If a parking lot owner were to save $4.16 per ERU per month, this would translate into $499.20 savings per year. The time it would take to get a return on investment would be $36,500/$499.20 = 73.12 years

We have existing examples of porous pavements lasting 20-30 years. New porous asphalt can also be constructed without the need of putting rocks underneath. The repaving of asphalt is done extensively with conventional asphalt every 12 years or so (on average). The same can be done with porous asphalt, if needed.

The above scenario assumes a 100% credit to completely offset the tax. If the tax is limited to 20%, for example, that's only $499.20 x 0.2 = $99.84 per year savings. It would then take $36,500/$99.84 = 365 years to get a return on investment. Disregard any potential interest rate and opportunity cost of investing $36,500 into for example 6%, 30-year bond. Already, even without the additional incentives of investing the $36,500, there is absolutely no incentive to implement a porous pavement or do anything to decrease stormwater fee based on proposed credit a company can hope to achieve.

Moreover, the cost comparison is true ONLY for new constructions. An existing doctor’s office parking, for example, must add the cost of demolition and excavation of current surface that will make the number look even more disadvantageous for businesses.

Let’s add 30% as before to the cost of putting porous asphalt in existing places. Porous lot of 25,000 square feet (25,000 x $4.54) = $113,500 (cost to build) + $113,500 x .3 (excavation, demolition of existing structure) = 113,500 + 34,000 = $147,500

Since existing structure is a sunk cost, the business is faced with a decision to pay the entire $147,500 to change the impervious surface to pervious surface to save $499.20 a year.

The return on investment with 100% credit against the tax is $147,500 / $499.20 = 295 years. Realistically then, there is no credit possible to provide enough incentive to install porous pavement for existing businesses. The credit would have to be larger than 100% or the fee for 1 ERU would have to be substantially larger. As already noted, too large a tax may cause businesses to move out of Lexington, or, more realistically, new businesses may choose other cities to locate.

Lexington indeed appears to be poised to go “with a larger than a 100% credit”. As it stand right now, according to information obtained from the Stormwater Task Force meeting, City Council will not implement a tax credit but will provide grants that help businesses pay for eliminating impervious surfaces.

Considering the situation described above where it would take 73 years to justify the expense of putting impervious surfaces, this solution does not make economical sense. If a business is to spend an extra $36,500 on porous asphalt construction, a grant in the amount of $10,000 - $20,000 may make many seriously consider the installation of porous rather than conventional asphalt.

Such credit, in the form of a grant, will also most likely provide a positive image for the city from a political and public relations point of view. Instead of simply taxing citizens and businesses without a practical and economically sound way for businesses to avoid this tax, providing grants can be positively received by businesses and limit the resistance to the proposed stormwater tax.

7. Limitations

Limitations of this study come directly from the difficulty in obtaining precise numbers as well as the complexities involved in stormwater management. There exists numerous software that can handle vast number of variables that play a role in establishing best watershed practices. The regression calculations assume a certain volume of rain. If that rain amount is different however, the number calculated per unit can go much higher or much lower. “Three statistically different regions, delineated on the basis of mean annual rainfall, were used to improve linear regression models where adequate data were available. “(Driver)

The regressions developed based on 30 metropolitan areas are subject to a number of limitations, but the most important one is the fact that “the regression models can only define the effects of the explanatory variables that are statistically significant for each regression model. Models do not include physical or land use characteristics that define the effects of major industrial point sources, localized nonpoint sources, or atmospheric sources of pollution. Consequently, the possible effects of these variables on estimates from each model should be considered when applying the model.” (Driver)

What is clear is that porous pavement decreases the quantities and different ways of doing it have different costs per unit. The difference per unit stays about the same – around 10 fold.

Another important limitation is the uncertainty of real cost. How can we assume that $130 million of “Master Project List” is the true current cost of stormwater? Will there be more projects? What about the cost acquired until today’s situation? Would the cost be substantially smaller if we were widely using porous pavement such as asphalt since 20 years ago? On the other hand, as stated earlier, the estimated roughly half billion dollars to replace 1/3 of urban roads is most likely 100% underestimated. The true cost of traffic jams, complications found in particular places during excavation, delays and other unexpected variables of the process would most likely cause the re-pavement to run at least $1 billion.

8. Recommendations

Historically, taxpayers have incurred the full cost of stormwater management (General funds and other funds are partially supported through general income and other taxes). The situation is now about to change. Residents, developers and business owners will directly pay a portion of the cost to manage stormwater runoff from their properties. It is unrealistic to expect change of all or even a substantial portion of existing roads or parking structures to porous pavements due to the prohibitive cost, political resistance as well as common sense. Existing pavements may benefit more from simple demolition if not needed, partial demolition to make parking ‘green’ (green parking) or stormwater harvesting or other “green infrastructure” approaches. If a business wants to change existing pavements to porous pavements, the city should encourage such action and help with the cost as much as it can afford.

New development, however, should very strongly consider using porous pavements as the cost per measured substance is the lowest in this instance. $36,000 additional cost per 0.2 square mile parking spot (as shown in this paper) reduced to $18,000 with dollar for dollar grant, would provide return on investment in 36 years. Through the economy of scale, the cost of porous asphalt may go down in relation to conventional asphalt. Perhaps the number of years could decrease to 20 years, the proven lifetime of porous asphalt.

Strictly speaking in economic cost/benefit terms, the city should not bother with any credits or grants. However, if the city does nothing, the tax will only finance conventional stormwater management techniques. It will do nothing to decrease the stormwater runoff and therefore, nothing to improve water quality by decreasing the stormwater runoff chemicals and other depositions. If the city does not provide grants or credits, porous pavements and other green infrastructure techniques will be just nice buzzwords on paper and will never materialize.

It is important to note that the actual numbers are subject to limitations. Appropriate rain data in a particular place makes a drastic difference in calculations. For example instead of assuming 3 inches in 0.2 square miles, if one assumes 0.03 inches, the cost per cubic feet or dissolved solids can go to a million per pound. In a particular parking lot, there may be little or no dissolved solids due to conditions around the parking spot. The pound of dissolved solids is somewhat theoretical and in particular situation it may never occur. Instead, one should notice that there is a clear difference in cost per unit for each scenario. It is the magnitude of the change in cost that’s more important than the actual numbers, which are rather difficult to calculate. The difference for the city is at minimum 10 fold in cost per unit. Clearly, conventional stormwater management practices should continue in already developed areas.

Overtime, say 20 years, all newly constructed places will represent a substantial portion of the city. With proper public education, as well as proper monetary incentives in the form of help in construction, builders and developers could advertise their environmental pavements to attract new customers. Having this percentage with porous pavements would increase the infiltration of stormwater and decrease the quantity of it. If this happens, in 20 years the city will find itself in a much better place when it comes to water and environmental quality.

9. Conclusion

The Environmental Protection Agency (EPA) has sued the city of Lexington to require improvements in the management of stormwater runoff. In addition to various specific actions the city must take, it is required to establish the stormwater tax/fee. The fee is based on the size of the area covered by impervious surfaces on the property.

Porous pavement is approximately 30% more expensive to install for new development. Per dollars spent for each pound of total stormwater runoff volumes, as well as pounds of dissolved solids in stormwater along with pounds of phosphorous in stormwater, it is the recommendation of the author that porous asphalt be strongly considered, encouraged and subsidized (in the form of grants) for all new construction. The existing development should rely on conventional methods of stormwater management. Existing structures should evaluate other ‘green infrastructures’ (rainwater harvesting, green roofs, green parking) to decrease stormwater runoff or qualify for some or total stormwater fee reduction.

Encouraging, even perhaps requiring porous pavements (where engineering allows) for new development will encourage public education about the importance of stormwater volume reduction. Infiltration of water and revitalization of ground water might lead to better water quality in the future.

Appendix A.

Status Quo – no porous pavements on urban roads

Total cost (from Stormwater Priority Projects Master List) = $133,079,000

There are 1,100 lane miles in city and Fayette County (for snow removal purposes). 1,100 lane miles corresponds to 3.4076 square miles.

Coefficients:

• TRN – Total rain volume. Lexington gets around 40 inches per year.
• DA – Total drainage area
• IA – Impervious area
• BCF – biases correction factor
• MJT – mean minimum January temperatures (Appendix C)
• INT – maximum 24-hour precipitation intensity 2-year recurrence interval (Appendix C)

RUN – Total runoff volume

RUN II = (ß0)62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212
= 62,951 x (40) (1.127) x (3.4076) (.809) x (1) (.522) x 1.212
= 62,951 x 63.90 x 2.70 x1 x 1.212
= 13,164,083 cubic feet of rain runoff

$133,079,000/ 13,164,083 = $10.11 per cubic foot of total stormwater runoff

DS – Dissolved Solids

DS II = (ß0)2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208
= 2,308 x (40) (1.076) x (3.4076) (1.285) x (1) (1.348) x 26.18 (-1.395) x 1.208
= 2,308 x 52.94 x4.83 x1 x 0.01 x 1.208
= 7,129.62 pounds of suspended solids in runoff

$133,079,000/ 7,129.62 = $18,666.65 per pound of suspended solids in runoff

TP – Total Phosphorus

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486
= .153 x (40) (.986) x (3.4076) (.649) x (1) (.497) x 4 (1.543) x 1.486
= .153 x 37.99 x 2.22 x 1 x 8.49 x 1.486
= 162.78 pounds of total phosphorous in stormwater runoff

$133,079,000/ 162.78 = $817,539 per pound of phosphorous in stormwater runoff

Appendix B.

Porous pavement for 1/3 of all suburban roads

The only coefficient that changes in comparison to Status Quo Appendix A is IA – impervious surface. It is now 0.1 or 10% as opposed to 100% in Status Quo.

RUN – Total runoff volume

RUN II = (ß0)62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212
= 62,951 x (40) (1.127) x (3.4076) (.809) x (.1) (.522) x 1.212
= 62,951 x 63.90 x 2.70 x .30 x 1.212
= 3,949,225 cubic feet of rain runoff

$562,798,015 / 3,949,225 = $142.51 per cubic foot of total stormwater runoff

DS – Dissolved Solids

DS II = (ß0)2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208
= 2,308 x (40) (1.076) x (3.4076) (1.285) x (.1) (1.348) x 26.18 (-1.395) x 1.208
= 2,308 x 52.94 x 4.83 x 0.04 x 0.01 x 1.208
= 285.16 pounds of suspended solids in runoff

$562,798,015 / 285.16 = $1,973,622 per pound of suspended solids in runoff

TP – Total Phosphorus

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486
= .153 x (40) (.986) x (3.4076) (.649) x (.1) (.497) x 4 (1.543) x 1.486
= .153 x 37.99 x 2.22 x .32 x 8.49 x 1.486
= 52.09 pounds of total phosphorous in stormwater runoff

$562,798,015 / 52.09 = $10,804,338 per pound of phosphorous in stormwater runoff

Appendix C.

Require porous pavement – new construction or revitalization only

Coefficients:

• TRN – Total rain volume
• DA – Total drainage area
• IA – Impervious area
• BCF – biases correction factor
• MJT – mean minimum January temperatures (Appendix C)
• INT – maximum 24-hour precipitation intensity 2-year recurrence interval (Appendix C)

The cost of porous asphalt is $4.54 per square foot.
0.2 square miles = 5,575,680 square feet
Cost = 5,575,680 * $4.54 = $25,313,587

Assumptions:

• Parking area = 0.2 square miles
• Total rain (TRN) is 40 inches for a year
• Impervious area of porous pavement is = 0.1 (10%)

RUN – Total runoff volume

RUN II = (ß0) 62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212
= 62,951 x (40) (1.127) x (0.2) (.809) x (.1) (.522) x 1.212
= 62,951 x 63.90 x 0.27 x 0.30 x 1.212
= 394,903.63 pounds of rain runoff

$25,313,587/ 394,903.63 = $64.10 per cubic foot of total stormwater runoff

DS – Dissolved Solids

DS II = (ß0) 2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208

= 2,308 x (40) (1.076) x (0.2) (1.285) x (.1) (1.348) x 26.18 (-1.395) x 1.208
= 2,308 x 52.94 x .13 x 0.04 x 0.01 x 1.208
= 7.68 pounds of suspended solids in runoff

$25,313,587/ 7.68 = $3,296,040 per pound of suspended solids in stormwater runoff

TP – Total Phosphorus

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486
= .153 x (40) (.986) x (0.2) (.649) x (.1) (.497) x 4 (1.543) x 1.486
= .153 x 37.99 x .35 x .32 x 8.49 x 1.486
= 8.21 pounds of suspended phosphorous

$25,313,587/ 8.21 = $3,083,263 per pound of phosphorous in stormwater runoff

Appendix D.

Require porous pavement – existing parking and other private areas

The cost of porous asphalt per square foot = $4.54 per square foot
Add 30% for excavation and removal of current asphalt cover

$25,313,587 + ($25,313,587* .3) = $32,907,663

TRN = 40 inches

Area = 0.2 mile (square)
Impervious Area = 10% (porous asphalt)

RUN – Total runoff volume

RUN II = (ß0) 62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212
= 62,951 x (40) (1.127) x (0.2) (.809) x (.1) (.522) x 1.212
= 62,951 x 63.90 x 0.27 x 0.30 x 1.212
= 394,903.63 pounds of rain runoff

32,907,663/ 394,903.63 = $83.33 per cubic foot of total stormwater runoff

DS – Dissolved Solids

DS II = (ß0) 2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208
= 2,308 x (40) (1.076) x (0.2) (1.285) x (.1) (1.348) x 26.18 (-1.395) x 1.208
= 2,308 x 52.94 x .13 x 0.04 x 0.01 x 1.208
= 7.68 pounds of suspended solids in runoff

32,907,663/ 7.68 = $4,284,852 per pound of suspended solids in stormwater runoff

TP – Total Phosphorus

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486
= .153 x (40) (.986) x (0.2) (.649) x (.1) (.497) x 4 (1.543) x 1.486
= .153 x 37.99 x .35 x .32 x 8.49 x 1.486
= 0.64 pounds of suspended phosphorous

32,907,663/ 8.21 = $4,008,242 per pound of phosphorous in stormwater runoff

Appendix E

Temperatures in January in 2007, 2008, and 2009 to calculate mean January temperature as well as maximum 24-hour precipitation intensity that has a 2-year recurrence interval.

From http://wwwagwx.ca.uky.edu/cgi-bin/ky_clim_data_www.pl

Appendix F

2006 September 2006 storms house flooding and sanitary sewer overflows. The map shows only reported incidents and may not be comprehensive.

Source: Lexington Herald-Leader. Sunday, April 12, 2009 | Kentucky.com/opinion

Appendix G

Calculations for Appendix A, B, C and D but for 20 years, 20 times 40 inches per year rainfall.

Status quo – 20 years

RUN II = (ß0)62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212 = 62,951 x (800) (1.127) x (3.4076) (.809) x (1) (.522) x 1.212 = 385,169,094

$133,079,000/ 385,169,094 = $0.35 per cubic foot of total stormwater runoff

DS II = (ß0)2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208 = 2,308 x (800) (1.076) x (3.4076) (1.285) x (1) (1.348) x 26.18 (-1.395) x 1.208 = 79,050.15

$133,079,000/ 79,050.15= $1,683.48 per pound of suspended solids in runoff

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486 = .153 x (800) (.986) x (3.4076) (.649) x (1) (.497) x 4 (1.543) x 1.486 = 3,121.89

$133,079,000/ 3,121.89 = $817,539 per pound of phosphorous in stormwater runoff

1/3 porous asphalt – 20 years

RUN II = (ß0)62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212 = 62,951 x (800) (1.127) x (3.4076) (.809) x (.1) (.522) x 1.212 = 115,551,028

$562,798,015 / 115,551,028 = $4.87 per cubic foot of total stormwater runoff

DS II = (ß0)2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208 = 2,308 x (800) (1.076) x (3.4076) (1.285) x (.1) (1.348) x 26.18 (-1.395) x 1.208 = 7,162

$562,798,015 / 7,162 = $78,581.12 per pound of suspended solids in runoff

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486 = .153 x (800) (.986) x (3.4076) (.649) x (.1) (.497) x 4 (1.543) x 1.486 = 999

$562,798,015 / 999 = $563,361.38 per pound of phosphorous in stormwater runoff

New Construction Porous Asphalt Parking – 20 years

RUN II = (ß0) 62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212 = 62,951 x (800) (1.127) x (0.2) (.809) x (.1) (.522) x 1.212 = 11,555,102.81

$25,313,587/ 11,555,102.81= $2.19 per cubic foot of total stormwater runoff

DS II = (ß0) 2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208 = 2,308 x (800) (1.076) x (0.2) (1.285) x (.1) (1.348) x 26.18 (-1.395) x 1.208 = 192.77

$25,313,587/ 192.77 = $131,315 per pound of suspended solids in stormwater runoff

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486 = .153 x (800) (.986) x (0.2) (.649) x (.1) (.497) x 4 (1.543) x 1.486 = 157.5

$25,313,587/157.5 = $160,721.19 per pound of phosphorous in stormwater runoff

Existing Asphalt change to Porous – 20 years

RUN II = (ß0) 62,951 x (TRN) (1.127) x (DA) (.809) x (IA) (.522) x (BCF)1.212 = 62,951 x (800) (1.127) x (0.2) (.809) x (.1) (.522) x 1.212 = 11,555,102.81

32,907,663/ 11,555,102.81= $2.85 per cubic foot of total stormwater runoff

DS II = (ß0) 2,308 x (TRN) (1.076) x (DA) (.1.285) x (IA) (1.348) x (MJT) (-1.359) x (BCF)1.208 = 2,308 x (800) (1.076) x (0.2) (1.285) x (.1) (1.348) x 26.18 (-1.395) x 1.208 = 192.77

32,907,663/ 192.77= $170,709 per pound of suspended solids in stormwater runoff

TP II = (ß0).153 x (TR) (.986) x (DA) (.649) x (IA) (.497) x (INT)(1.543) x (BCF)1.486 = .153 x (800) (.986) x (0.2) (.649) x (.1) (.497) x 4 (1.543) x 1.486 = 157.5

32,907,663/ 157.5 = $208,937 per pound of phosphorous in stormwater runoff

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