KEYWORDS Detention basin retrofit, Hydromodification, Stormwater Management, Erosive flows, Streams

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1 Change the Game with Green Infrastructure - Retrofits of Existing Detention Basin may be Orders of Magnitude More Cost-Effective than New BMP Construction: A Preliminary Report James A. Goodrich 1 *, Robert J. Hawley 2, Matthew S. Wooten 3, Craig Frye 3, Mark Jacobs 4, Jake J. Beaulieu 1, Nora L. Korth 2 1 USEPA Office of Research & Development 2 Sustainable Streams, LLC 3 Sanitation District No. 1 of Northern Kentucky 4 Boone County Conservation District * Goodrich.James@epa.gov ABSTRACT This report discusses alternatives to traditional stormwater management, with a focus and case study highlighting detention basin retrofits. The case study, a novel retrofit in Northern Kentucky, modifies only the outlet control structure for a cost-effective solution. Hydrogeomorphic modeling suggests that the pilot installation can fully restore the natural disturbance regime in the receiving channel to pre-developed conditions, and hydrologic modeling shows an approximate doubling of detention time in the typical year, suggesting substantial improvements in water quality treatment. Preliminary monitoring results show a decrease in peak discharge compared to pre-installation events. Therefore, this novel method of stormwater management has the opportunity to reduce downstream erosive flows thereby improving water quality at a substantial cost savings as compared to typical stormwater management approaches. KEYWORDS Detention basin retrofit, Hydromodification, Stormwater Management, Erosive flows, Streams INTRODUCTION Urbanization has been found to be a leading cause of hydromodification (Hawley and Bledsoe, 2011), which includes the altering of a stream system from erosive flows, channelization and dam construction. As urban areas continue to sprawl, the problem continues to exacerbate, as even minor changes to imperviousness can greatly impact a stream s stability (Hawley and Bledsoe, 2013). In many watersheds, streambank erosion has been found to be a dominant source of sediment (Simon and Klimetz, 2008). As communities face the challenge of mitigating stormwater runoff from developed watersheds, the scale of such tasks underscores the importance of identifying BMP strategies that are substantially more cost effective than many of the approaches tried to date. Traditionally, stormwater management has focused on flood control. Recently, an additional focus on water quality treatment has become more common. Flood control is designed for large events (e.g., 2-, 10-, 50- and 100-year events), whereas the water quality volume control is focused on small events/the first flush (e.g., the first 0.8 inches of every event). Focusing only on

2 these two tiers of stormwater management leaves a large portion of events uncontrolled (Figure 1). Figure 1. Three tiers of stormwater management for precipitation and recurrence intervals of Northern Kentucky Flood control typically follows the method of peak matching, meaning that the post-development peak flow cannot exceed the pre-development peak flow for a given recurrence interval (e.g., 2-, 10-, 50- and 100-year events). As demonstrated in Figure 2, this conventional methodology extends the hydrograph, causing flows to be higher than Qcritical for a longer length of time than even the post-development without detention alternative. Qcritical is the critical flow of a stream, Pre-Developed Post-Development No Detention Post-Development Conventional Detention (Peak Matching) Figure 2. Example hydrograph analysis of various detention controls for the 2-year, 2-hour event in Fort Collins, CO (Adapted from Bledsoe (2002))

3 above which causes excessive erosion (Hawley, 2012). It is clear that standard detention alone cannot control the necessary flows to achieve stream stability. To achieve water quality benefits, green infrastructure technologies, such as green roofs and stormwater planters, can be utilized but can come with extremely high cost. Even constructing more cost-effective volume-based controls such as bioinfiltration basins can come with significant cost when considering the scale of the problem. For example, published efforts to restore a more natural flow regime and improve water quality at the pilot watershed scale have ranged up to $11.6M to $15.0M per square kilometer of drainage area ($45K to $65K per acre, King County, 2013). In contrast, retrofitting existing detention basins for greater channel protection and water quality performance while maintaining the design intent of flood control may be orders of magnitude more cost-effective than constructing new BMPs. Moreover, conventional detention basins are ubiquitous, valuable assets that communities already have in the ground. For example, one developed, 96 square kilometer (37 square mile) study watershed in Northern Kentucky has approximately 535 existing detention basins that could be worth an estimated $60M. On average, that's over $0.6M of existing stormwater BMP assets per square kilometer (nearly $2M per square mile). Most of the existing detention basins were designed to meet conventional peakmatching regulations, resulting in basins that have little or no attenuation for ~97-99% of the storms in a typical year (Emerson et al., 2003; Hawley, 2012). Furthermore, the extended durations of high magnitude flows can result in exacerbated channel erosion in receiving streams. For example, matching the 2-year pre-developed peak flow magnitude may extend its duration from 30 minutes to 90 minutes, which could result in approximately triple the cumulative erosion in the receiving channel. There are numerous methods in which existing detention ponds can be retrofitted to improve performance. Costs of pilot installations that involve re-grading and/or amending the soils tend to be an order of magnitude higher than retrofits that focus exclusively on the outlet control structure. This paper will present the results of a pilot installation of an outlet Figure 3. Toyota Figure North 4. American Installed Parts Detain Center H2O retrofit Kentucky device detention pond study site control structure retrofit on a large industrial site in Northern Kentucky (Figure 3). The DetainH2O retrofit device (patent pending, Figure 4) restricts the capacity of the basin s lowflow pipe (conventionally designed for the 2-year storm) in order to minimize the erosive forces

4 of the flows caused by smaller storm events (i.e., less than the 2-year storm) while maintaining the current level of service during flood events (e.g., 50- and 100-year storms) using a bypass. This device is novel in that it maintains the current level of service by also provides channel protection by mitigating the in-stream flows for small events. Additionally, smart real-time flow control could be further incorporated in order to make even greater use of the basin s capacity during storms that do not approach flood level, such that flows could be detained and released over even longer intervals to provide summer base flow and greater water quality treatment. This report will discuss the preliminary results obtained from the field deployment of the DetainH2O retrofit device. METHODOLOGY The development and testing of DetainH2O was completed in many stages. The genesis of this exercise sprang from the intent to design and test a device that could inexpensively retrofit existing detention pond outlet structures to optimize their stormwater mitigation potential while minimizing the impacts of Qcritical. The objectives of DetainH2O s development included: Control of small, frequent storm events that are normally overlooked in traditional detention basin design (e.g., 6-month, 1-year, and 2-year flows); Continued performance of the detention basin for flood events (i.e., 50-year and 100- year), and; Continued or improved water quality benefits. Design Modeling and Installation Modeling was completed as the basis for the retrofit design. Determining the configuration of the retrofit device utilized the notion that conventional detention exceeds Qcritical more than predeveloped conditions (Bledsoe, 2002). While the discharge rate is important, minimizing the time that a stream is experiencing erosive flows can be more important to its overall stability. Modeling was conducted to determine the number of times and length of time that Qcritical was exceeded with DetainH2O installed. This modeling utilized 40 years of hourly precipitation data and compared multiple detention types. In completing this step, a comparison of residence times could be achieved. By extending the residence time in the basin, additional sediment can settle out of the water, and it is possible to have more contact time with the sun s UV rays for disinfection and additional time for nutrient processing. The extent of each is a function of many factors including the original design, the design of the retrofit, and the quantity and quality of the stormwater. The retrofit was installed on December 21, The specific parameters of the retrofit include: 75% restriction of low-flow outlet (24-inch circular opening) to reduce stream erosion and enhance water quality treatment; 18-inch bypass at 3 feet above inlet of 24-inch inlet to maintain flood control performance in large events, and; Monitoring Both pre- and post-installation monitoring was conducted, with post-installation data still being collected at the time of submittal. Figure 5 shows the multitude of locations where data were

5 collected. Specifically, inflow data were collected at both pipes entering the basin, and outflow data was collected at the overflow. Rain gage data came from two sources: the rain gage from the Cincinnati/Northern Kentucky International Airport, located approximately 1 mile from the site as well as a rain gage installed at the site. Stream gage data (net water level) were collected in the spur and the tributary channel which was monitored both upstream and downstream of the spur. Downstream Inflow2 Inflow1 Site Rain Gage Outflow Spur Pre-installation monitoring began August 2013, with outflow data and inflow data from only Pipe 1. Pipe 1 (Inflow 1) data collection stopped in September 2013 and began again in January, This was also when Pipe 2 (Inflow 2) data collection began. Outflow data was not collected for a few days in January Upstream NWS Rain Gage < 1 mile (Airport) Figure 5. Monitoring locations around the basin Rain gage data from the airport are available online as an hourly instantaneous rainfall amount and are available for the entire length of this study. The rain gage at the site began collecting data in September, This instantaneous rainfall data were collected in 15-minute intervals for the remainder of the study. The data loggers in the stream began collecting data in April, 2013 and have continued since. In addition, video equipment installed at the site collects real-time photographic data to document ponding. RESULTS The results can be categorized into modeled and field results. Modeled Results Modeling results provided the anticipated outcomes, displayed in Table 1, that the retrofit scenario would have fewer storm events exceeding Qcritical and nearly equal cumulative hours exceeding Qcritical as the pre-developed scenario. These results reaffirmed the notion above that today s standard stormwater management strategy, traditional detention, does not manage Qcritical flows. Table 1. Hydrogeomorphic modeling results of retrofit Detention Type Cumulative Number of Storm Cumulative Hours of Flows Events Exceeding Qcritical Exceeding Qcritical Pre-developed No Detention With Detention With Retrofit 8 300

6 Additional modeling presented the results on the residence time of the basin, and are provided in the figure below (Figure 6). The results show that the existing configuration would result in up to ~ 30,000 cubic feet of storage, whereas the post-retrofit configuration would provide 50,000 cubic feet, during the typical year. This means that the pre-retrofit configuration would provide only ~300 hours of detention in a typical year, whereas the post-retrofit configuration would provide ~670 hours. Figure 6. Comparison of pre-retrofit and post-retrofit retention periods from modeling analysis of the typical year. (Durations for storage volumes less than 75 cubic feet are not depicted due to the potential uncertainty surrounding small water depths of less than 2.5 inches.) Field Results Hydrographs of both pre- and post-installation have been reviewed. Although a period of inflow data are missing, as noted above, the extremely cold winter resulted in ice buildup that prevents an accurate evaluation of the device in any event. During the winter, temperatures trended below average around much of the United States during the winter of ; Northern Kentucky was not excluded. January and February had a combined 43 days below freezing. In addition to freezing the water in the outflow pipes and the flow monitor installed in the pipe (Figure 7), much of Figure 7. Gage on 1/9/14 the precipitation during this time is justifiably assumed to have fallen as snow, rendering the rain gage data inaccurate as well. As soils in Northern

7 Kentucky have very minimal infiltration rates, it is not anticipated that the frozen ground would have impacted the outflow at all. By springtime, the inflow data was back in line. The spring-season data supports the modeling effort that the retrofit had flow control benefits (Figure 8). Date of installation: December 21, Figure 8. Summary hydrograph of collected data Two events have been compared that occurred on the pre-installation date of October 31, 2013 and the post-installation date of April 3, 2014 (Figure 9). By the numbers, rainfall on October 31 totaled 0.9 inches with a peak intensity of 0.94 inches per hour. In April, the event was recorded with 2 inches of rain and a peak intensity of 1.18 inches per hour. The pre-installation peak discharge was 6 cubic feet per second, and the post-installation peak outflow was 5.3 cubic feet per second. While inflow data was not available for the October 31 event, the comparisons of total rainfall, peak rainfall intensities, and peak outflow all support that there is a reduction in outflow for comparable, or even worse, post-installation storm events. The authors believe that current additional monitoring will confirm these results.

8 Figure 9. Comparison of October 31, 2013 (left) and April 3, 2014 (right) hydrographs Real-time data was collected during the April 3 event to document ponding. Although the timing is slightly off between the flow monitoring data and the presented photos (Figure 10), the two, displayed together, provide compelling proof of the feasibility and function of DetainH2O.

9 (a) (b) (c) (d)(e)(f) (g) (h) (i) (a) 4/2/14 17:00 (b) 4/3/14 7:00 (c) 4/3/14 8:00 (d) 4/3/14 12:00 (e) 4/3/14 13:00 (f) 4/3/14 14:00 (g) 4/3/14 18:30 (h) 4/4/14 9:00 (i) 4/4/14 12:00 Figure 10. April 3, 2014 post-installation event hydrograph and photographic comparison

10 The stream flow data collected at the spur and in-stream locations correlates well with rainfall data; a spike in rainfall is followed by a commensurate spike in in-stream flow. However, to date, useful trends between pre- and post-installation have not been found. The authors believe that as monitoring continues at this site, the stream data will provide valuable data for passing judgment on the efficacy of this technology. DISCUSSION The cost to install the DetainH2O retrofit was approximately $10,000. This included the efforts by engineers to optimize the design, along with material and installation costs. This is equivalent to ~$1.50 per cubic foot of new storage volume added to the basin. Not included are any time/cost associated with identifying appropriate basins and their capacity to be retrofitted, engaging property owners, and determining basin access. The benefits that are gained from installing such a cost-effective retrofit are numerous. In addition to reducing stream erosion, there are quantifiable benefits from managing flows with the Qcritical methodology. Erosive flows in streams will continue to impact local infrastructure, including roadways, sewers, and other utilities. A recent study conducted by Hawley et al. (2013b) extrapolated Northern Kentucky infrastructure damages to national yearly projections. The research found that state-funded highways could incur $1.1B of damage yearly from flooding and channel instability, or roughly $25,900/mi 2 /y. The study then looked at sewers and gas mains, which could average $3,200/mi 2 /y in combined damages. Control of the erosive flows can reduce the cost of rehabilitating these infrastructure pieces. Also, The National Water Quality Inventory: Report to Congress (USEPA, 2004) has identified hydromodification as the second leading impairment to streams. As streams are interwoven, complex networks, the impacts of hydromodification can touch not only the physical components of the stream, but the biologic as well. To date, a large number of Total Maximum Daily Loads (TMDLs) have been established throughout the country. The key to delisting stream segments from the 303(d) list is to mitigate hydromodification. A more proactive approach would be to stop a TMDL from being developed in the first place. By improving the conditions in a stream before a state is moved to create a TMDL, the costs associated with creating and then delisting the stream segment could be avoided altogether. CONCLUSIONS New stormwater regulations (already occurring in some States) are beginning to require that detention basins provide a water quality benefit. This paper shows that retrofitting detention facilities with a novel low-cost technology can cost-effectively control and treat runoff and optimize flows below the Qcritical threshold for stream bed erosion. By virtue of a treatment train approach within a watershed utilizing green, grey, and remote telemetry expertise, stormwater managers can begin to address both water quality and hydromodification issues in developed watersheds using their existing assets. REFERENCES

11 Bledsoe, B.P., Stream erosion potential associated with stormwater management strategies. Journal of Water Resources Planning and Management, 128: Emerson, C.H., Welty, C. and Traver, R., Application of HEC-HMS to model the additive effects of multiple detention basins over a range of measured storm volumes, World Water and Environmental Resources Congress. ASCE. King County Department of Natural Resources and Parks (2013) Development of a Stormwater Retrofit Plan for Water Resources Inventory Area 9: SUSTAIN Model Pilot Study. Hawley, R.J A Regionally-calibrated Approach to Channel Protection Controls How Meeting New Stormwater Regulations Can Improve Stream Stability and Protect Urban Infrastructure. In Proceedings of the Water Environment Federation Stormwater Symposium, Baltimore, MD, July Hawley, R.J. and Bledsoe, B.P., How do flow peaks and durations change in suburbanizing semi-arid watersheds? A southern California case study. Journal of Hydrology 405 (1 2), Hawley, R.J. and Bledsoe, B.P., Channel enlargement in semiarid suburbanizing watersheds: A southern California case study. Journal of Hydrology 496, Hawley, R.J., MacMannis, K.R. and Wooten, M.S., 2013b. How Poor Stormwater Practices Are Shortening the Life of Our Nation's Infrastructure--Recalibrating Stormwater Management for Stream Channel Stability and Infrastructure Sustainability, World Environmental and Water Resources Congress. American Society of Civil Engineers, Environmental and Water Resources Institute, Cincinnati, OH, May Simon, A. and Klimetz, L., Relative magnitudes and sources of sediment in benchmark watersheds of the Conservation Effects Assessment Project. Journal of Soil and Water Conservation, 63(6): USEPA National Water Quality Inventory: Report to Congress, 2004 Reporting Cycle. EPA 841-R United States Environmental Protection Agency, Washington, DC.