As a municipal engineer, I’ve learned that everything is not as clear-cut as I would like it to be. There are so many variables that must be considered when looking at the feasibility of a project. I have worked as a municipal engineer for approximately 10 years and have had the opportunity to work in a city that is changing on a daily basis. This constant change makes for a wonderful and enjoyable work environment. However, with the constant change, there are always new challenges to face. I can definitely attest to the fact that my worldview in terms of engineering is considerably different today as compared to the day when I left college. Today, projects are considerably more complex and nothing is “absolute” with respect to design criteria and parameters. As I progressed in my career, I migrated toward what I view as the most challenging and dynamic area of concentration: urban stormwater management. As a result, I’ve decided to write this article to share ideas and concepts with other professionals who can feel the same excitement as I do.

“My yard is flooding! What are you going to do about it?” “The streambanks are eroding away! I’ve been living here all my life and the stream never looked like that.” “What is that odor in my backyard?” “It’s all because of development!” “I don’t want a swale in my yard!” “I’m not going to sell you an easement!” “When are you going to put sidewalks along the road?” How many times have you heard these questions and complaints? I suspect that you have heard them in one form or another as often as I have. There has to be a way of mitigating the effects of time associated with rapid growth. It was a great day for me when the concept of low-impact development (LID) was defined as a potential approach to be used in conjunction with standard long-established practices. I was introduced to LID concepts many years ago and believe that the approach of decentralizing stormwater runoff must become the standard practice rather than the exception. The table below outlines my thought process for other design professionals to consider. However, as I mentioned earlier, new ideas create new challenges and are often met with resistance. Fortunately, regardless of the many benefits associated with a new concept, the transition may be easier when it can be associated with the bottom line: How much is it going to cost?

My comparison is based on an analysis of a 44-acre site. The hydraulic grade line is 1.27% and the Soil Conservation Service curve number is calculated to be 77. The site consists of approximately 1,125 linear feet of pipe discharging into a creek at eight separate locations. The project site contains approximately 105 single-family dwellings. This configuration is very typical of a system existing in many of the cities and towns across the United States. The project upgrade was designed using conventional methods, and the project cost is on the order of $288,300. The net impact of this particular project in terms of stored volume of stormwater runoff under the design parameters is an increase in capacity at the site location on the order of 1,800 cubic feet. I chose to make the comparison based on the differential volume to simplify the discussion in this article, as well as to concentrate on the concept to provide the reader with food for thought.

Bear in mind, from my understanding of the concept, there are an endless supply of LID features available to a design profession—the imagination is the limit. However, in the interest of time limits, I decided to use only one feature for comparison purposes. The feature I chose is simply a modular sidewalk premanufactured using porous concrete. For discussion purposes, I used 30% voids in my calculations. The modular cells are 60 inches long by an average width of 57 by 24 inches deep; the first 12 inches in depth are 60 inches wide, and the second 12 inches in depth are 54 inches wide. The difference in width allows for a lifting apparatus to be used in the future for maintenance purposes. In order to be equivalent to the differential volume, 630 cells would have to be placed on this site. They would be placed at locations along the existing right of way intercepting the stormwater runoff before it is collected and directed to the creek—thus the decentralization of the stormwater runoff. As you can see, the concept is relatively simple. These cells can store a differential of the same volume as the conventional design process.

What have I gained from this approach? The partial list of benefits below includes some that I have identified; you may have others that I have not thought of.

• There is a reduction in the elapsed time interval from the investigation phase of the project to the construction phase of the project for the additional real estate acquisitions required for a conventional project, thus utilizing the existing right of way as an asset.
• Based on the soils in the area as well as the concept of differential volume, costs associated with surveying and engineering from outsourcing are eliminated.
• If the project is treated as a maintenance function, the bidding, purchasing, pre-construction, and award phase of the project can be eliminated.
• Construction can be sequenced in such a way that the impact to the residents is minimized. Upon installation of a cell and restoration with sod, the project location is completed and available for use. If it is done as a maintenance item, problem areas can be identified and completed immediately. Additional segments can be added as part of a master plan—that is, one city block at a time.
• The in situ soil at the project location can be removed and transported to another city project location. There are always construction projects in progress where soils are needed, such as regrading new parks for the citizens to use at their leisure or providing suitable soil for pipe system backfilling, which is paid for as part of a project cost at another location.
• Reduced exposure to urban stormwater runoff lessens habitat degradation and creek erosion at the major outfall—with the potential of converting a once highly impervious urban site to a more pervious urban site as features are added over time.
• A sidewalk is installed in a residential neighborhood where it would not be typically placed during a drainage retrofit project.
• Residents’ property values increase as a result of the visible improvements.
• A visible surface feature is used for stormwater control rather than a subsurface feature.
• If modified with a monitoring endcap, this measure provides means to measure water-quality and water-quantity criteria at various locations.
• If the cells become clogged over time, they can be lifted out using the lifting apparatus; a new cell can be placed, and the cell can be brought back to a controlled environment, washed out, and then made ready for use at another location.

Of course, a $117,000 to $121,000 project savings is quite a benefit, especially if annualized and viewed over a 20-year period. Assume the budget allows for five retrofit projects annually. Do the math: Over 20 years, that’s approximately $11,700,000 to be used for other projects.

D.J. Señeres, P.E., is an engineer with the City of Fayetteville, NC.

SW July/August 2005