In Focus: Managing Urban Stormwater

Transcription

1 Currents A publication of Vol. 17, No. 1 - Spring 2016 In This Issue... Green Infrastructure: Reducing the Quantity and Improving the Quality of Stormwater Runoff Monitoring the Benefits of Green Infrastructure Reintegrating Nature into a Dense Urban Environment In Focus: Managing Urban Stormwater In this issue of Currents, we talk about green infrastructure engineered or designed systems to control stormwater that mimic natural hydrologic processes and its increasing role as the alternative of choice for urban stormwater managers over, or in conjunction with, traditional gray construction. Green infrastructure employs green technologies green roofs, bioretention cells, engineered wetlands, ponds, or swales and other manmade features to promote infiltration, to retain and treat stormwater, and to moderate the negative and even damaging effects of excess surface runoff in urban areas. The reasons to consider green infrastructure are numerous. In many cases, green infrastructure projects can be distributed throughout urban areas, thereby reducing stormwater runoff closer to the source. Where green infrastructure increases stormwater infiltration, pollutant loading from nutrients and other pollutants to waterways can be significantly reduced. In cases where sewer separation is planned to address combined sewer overflows (CSOs), green infrastructure may be able to help reduce the size of stormwater gray infrastructure. If municipalities are concerned with sanitary sewer overflows (SSOs), green infrastructure projects can help keep stormwater inflow out of sanitary sewers. Green infrastructure can reduce peak runoff flows from urban areas, which will help reduce problems from stream hydromodification and excessive erosion. Ancillary benefits of green infrastructure are also numerous, potentially including improved neighborhood aesthetics and quality of life, habitat for birds and other wildlife, and educational opportunities. Widespread implementation of green infrastructure in urban areas may someday decrease heat island effects and help reduce greenhouse gases. At LimnoTech we believe green infrastructure is an important tool in the toolbox for solving our clients stormwater management challenges, and we have employed green infrastructure technologies in projects for not only stormwater management, but also CSO reduction, TMDL achievement, and urban river and stream restoration. We ve contributed to development of green infrastructure plans for major urban areas in the United States, including St. Louis, Washington, DC, Cleveland, and Cincinnati. In this issue we also highlight our work in Waller Creek, an urban creek in Austin, Texas, that has historically experienced severe flooding, channel erosion, invasive species, and pollution from stormwater runoff. LimnoTech is part of a team evaluating engineering solutions for this creek. Work has included hydrologic and hydraulic modeling, evaluating the effects of managed flows, and assessing the influence of stormwater inflows. Future phases of the project will include design and construction of instream restoration and preservation features, stream bank stabilization, and innovative stormwater management alternatives. We hope that you will find the topics in this newsletter interesting and informative. Please contact us with any questions or comments that you may have. Scott Bell, PE, BCEE Vice President sbell@limno.com

2 Green Infrastructure Reducing the Quantity and Improving the Quality of Stormwater Runoff Green infrastructure, or GI, is the generic term used to describe engineered or specifically designed systems to control stormwater that mimic natural processes occurring in plants, soils, and physical landscape features such as swales, ponds, or wetlands. These processes provide water quantity and quality benefits such as stormwater volume control, peak flow shaving, channel protection, and pollutant removal, as well as other environmental and social benefits such as decreasing heat island effect, reducing greenhouse gases, and improving aesthetics. The use of green infrastructure for stormwater control evolved as stormwater managers moved away from large regional control structures, such as ponds that served primarily as flood controls or for channel protection, to more distributed management features focused on retention and infiltration. Green infrastructure can fit well into an urban environment because it can take many different forms such as green roofs, bioretention, rain barrels, cisterns, etc. that can work within site constraints. Much of LimnoTech s recent work has focused on GI as a potential tool for regulatory compliance. This work has typically involved one of two areas: GI used for volume control to help reduce sewer overflows, such as in combined sewer overflow (CSO) Long Term Control Plans, and GI used to help reduce pollutant loading, which can contribute to meeting Total Maximum Daily Loads (TMDLs). GI can be a good solution for either regulatory need. Distributed GI practices can help reduce runoff volume by capturing and retaining runoff; GI practices can also reduce pollutants by retaining runoff, allowing pollutants to settle out, and facilitating infiltration into the soil where natural processes remove pollutants. The following examples show how GI has been used to help plan for regulatory compliance for CSO volume and TMDL pollutant load reduction requirements. Green Infrastructure to Control Runoff Quantity DC Water is currently under a Consent Decree to reduce combined sewer overflows (CSOs). The original Consent Decree called for construction of large centralized tunnels to manage CSOs. In January 2016, DC Water s Consent Green infrastructure elements require less area than traditional "gray" treatment structures and may be easier to insert in urban environments. Decree for combined sewer overflow reduction was amended to include requirements for green infrastructure in the CSO control plan. The role of green infrastructure is to capture and infiltrate or evaporate stormwater locally before it enters the collection system, thus diminishing the amount of water in the combined sewer system and preventing overflows caused by surcharged pipes. Specifically, the amended Consent Decree prescribes the construction of GI facilities to manage a 1.2-inch storm event from 498 impervious acres in two combined sewer areas of Washington, DC, with the intention of reducing the need for centralized tunnel storage at the downstream end of the system. The amended Consent Decree holds the District to the same CSO overflow requirements as the original Consent Decree. LimnoTech has provided comprehensive hydraulic and hydrologic modeling of the potential impact of GI on the CSS collection system, and has developed tools, including a SWMM-based GI model, to quantify the benefits expected with the implementation of distributed green infrastructure. The GI model will be used to evaluate sensitivity to several variables, including investigation of maximum beneficial GI volume capture, optimal GI location within sewersheds, and verification of the overall implementation strategy. Pre-construction monitoring data will be collected throughout 2016, post-construction monitoring data will follow, and both will be used to calibrate and validate the effectiveness of GI in the models. Replacing tunnel storage with distributed green infrastructure allows incremental runoff reductions to occur more quickly because GI can be implemented without the long construction periods typically required by large capital projects such as storage tunnels. Using modeling to evaluate the ability of GI to meet runoff reduction goals for CSO control can be an important part of the planning process. The methodology for modeling GI practices described is increasingly being applied in communities throughout the United States. Green Infrastructure to Improve Water Quality Green Infrastructure can be an effective tool to reduce the amount of pollutants discharged to streams and rivers by stormwater runoff. It can remove or trap many of the potential pollutants in stormwater runoff including nutrients, sediments, bacteria, metals, and toxics before they reach

3 streams, rivers, or other waterbodies. The removal mechanism usually occurs either through stormwater infiltration or filtration. Infiltration is the process by which water soaks into the soil. Infiltrating stormwater runoff removes both the water and the associated pollutants from the surface before it can discharge into receiving waters. Filtration is the process by which pollutants are removed from water using engineered media. Filtration allows treated stormwater to pass through media and discharge to receiving waters. Different GI practices use different designs to achieve infiltration or filtration to control stormwater pollutants, and pollutant removal effectiveness varies depending on the GI design. Quantifying pollutant load reductions provided by GI can demonstrate progress towards TMDL requirements. However, this quantification can be challenging. An evaluation of data from the International Stormwater BMP Database ( a data repository for pollutant concentrations in stormwater flows both pre- and posttreatment, demonstrated that the relationship of pollutant concentrations between flow in and out of the GI can be highly variable and dependent on the BMP type and design. For planning purposes, the efficiencies of GI practices are typically reported as an average percent removal rate, regardless of the design parameters that affect green infrastructure efficiency. One method for minimizing the impact of using these average efficiency values is to use published efficiencies that reflect local conditions. In more recent years, however, there has been a shift away from pollutant removal efficiencies for infiltration practices in favor of quantifying efficiencies in terms of runoff reduction capacity based on the depth of runoff capture for which the practice is designed. For example, a bioretention cell designed to retain 1.2 inches of runoff will have a greater annual removal efficiency than a cell designed to retain 0.6 inch of runoff. This is the favored approach for meeting the Chesapeake Bay watershed TMDL requirements. The runoff efficiencies are used to calculate the runoff volume reduction afforded by GI practices, and the associated pollutant load reductions are quantified by multiplying the runoff volume reduction by the concentration of pollutants found in stormwater. The concentration of a pollutant in stormwater is often represented by the event mean concentration (EMC) value, which can be obtained through monitoring or from published literature values. The quantification of green infrastructure efficiency based on the design capacity gives a more precise estimation of the pollutant loads reduced. An additional challenge with quantifying the impact of GI practices on pollutant removal is that green infrastructure efficiencies are less well studied for pollutants such as heavy metals and toxics than they are for traditional pollutants such as nutrients and sediments, making quantification of load reductions for non-traditional pollutants challenging. In the absence of published efficiency data, a common approach for non-traditional pollutants that bind to sediments is to correlate LimnoTech created a GI pollutant load tracking and modeling tool for the District of Columbia DOEE to calculate pollutant load reductions from green infrastructure to demonstrate progress made towards meeting MS4 TMDL requirements. their efficiency to the TSS efficiency using sediment partition coefficients. LimnoTech used the methods discussed herein to create a GI pollutant load tracking and modeling tool for the District of Columbia Department of Energy and Environment (DOEE) to calculate pollutant load reductions from green infrastructure and to demonstrate progress made towards meeting MS4 TMDL requirements. The District of Columbia currently has 26 MS4 TMDLs for 45 waterbody segments, covering 23 different pollutants. To quantify load reductions to meet these TMDLs, existing GI data were centralized in a database and analyzed to determine if enough attribute data existed to quantify load reductions using the runoff reduction method. Many of the older GI practices did not have information on their runoff design capacity. In those cases, the percent efficiency method was used to quantify pollutant load reductions. The modeling tool automatically calculates load reductions based on the available attribute information for each green infrastructure practice that is tracked and accounted for in the District. The District used the modeling tool to develop a Consolidated TMDL IP, which summarized the District s plan to achieve each of its individual MS4 WLAs. GI is an integral part of DC Water and DOEE s plans for meeting their regulatory requirements, and LimnoTech s modeling of the runoff and load reduction that could be achieved through GI helped them integrate GI into their planning. Is Green Infrastructure right for meeting your regulatory requirements? While GI can reduce runoff and pollutant loads, it may not be a magic bullet for every situation, especially in an urbanized environment. Among the challenges for implementing GI in urban environments are: 66Large areas of GI may be needed to meet regulatory requirements because of the large amounts of runoff or load reduction typically needed to meet requirements, large numbers of individual GI practices may be needed. If sufficient public land is not available to install all of the GI practices needed, installation on private land may be necessary. This will require a great deal of coordination with private landowners. 66The effectiveness of GI on the large scales typically needed to meet regulatory requirements has not been well documented. 66Modeling tools will typically be needed to assess the effectiveness of GI implementation, and this may add to the resources needed to implement the GI program. 66In addition to modeling, it may also be beneficial to have monitoring data to validate conclusions of the GI modeling. If these issues can be addressed, the implementation of GI can help to address regulatory requirements while also adding triple bottom line benefits of an improved environment, costs savings, and enhanced societal acceptance.

4 Understanding the Benefits of Green Infrastructure through Monitoring Programs Green infrastructure is increasingly becoming an alternative to traditional gray infrastructure in urban settings because of its cost-effectiveness, low-impact, and multiple other benefits in managing stormwater runoff. A significant component of green infrastructure projects, to help guide initial decisions in choosing green infrastructure elements and then in measuring their effectiveness, is monitoring. Site monitoring is a key component to the success of any green infrastructure project to ensure that the project is operating according to expectations and to measure its success in resolving water quantity and quality issues. GI monitoring may take many forms, depending on the goals and needs of a situation. Sometimes simple observational monitoring (e.g., visiting the site during a storm event to determine its efficiency) and qualitative judgments are sufficient to confirm that a project is operating as intended. If a thorough understanding of site-specific GI hydrologic and/or water quality benefits is needed to meet regulatory or research goals, however, it becomes necessary to plan and implement a monitoring program that will provide more detailed data of appropriate quality and frequency. LimnoTech has helped clients develop comprehensive monitoring plans to support Green Infrastructure development for larger projects. Two such projects involving GI monitoring plans for the City of St. Louis and the District of Columbia demonstrate the various ways in which GI monitoring can be performed to meet project objectives. The Metropolitan St. Louis Sewer District (MSD) is in the process of installing GI components that will reduce stormwater runoff and combined sewer overflows (CSOs) in focus sewersheds that drain to the Mississippi River. This process first required monitoring at pilot GI sites to determine what types of GI to install for full-scale GI implementation, and to estimate the hydrologic impact of a full GI program by extrapolating findings from the pilot program. The monitoring protocol LimnoTech developed quantified stormwater runoff volume reductions achieved by GI elements, including bioretention cells, rain gardens, porous pavement, and soil amendments. Monitoring included site-scale outflow measurements using flumes and weirs, infiltration testing, simulated storm events, and level sensing of the bioretention cell internal water storage zones. Data collected over a two-year monitoring period indicated high stormwater runoff volume reduction (>85%) at all GI sites. The District Department of Energy and Environment (DOEE) RiverSmart programs sought to help Washington, DC, communities improve water quality by implementing green infrastructure to reduce stormwater volumes. To quantify potential stormwater volume reductions from GI, RiverSmart installed a mixture of GI practices at two metered sites in the City to validate model predictions of green infrastructure performance. Monitoring determined that pre-installation modeling indicated that installed green infrastructure practices are capable of treating at least 60% of the impervious area at each site. In 2010, three sites one control site and two GIimplementation sites were chosen to help demonstrate GI effectiveness. A mixture of GI practices were designed for each site, and pre-construction rain gauges and flow metering quantified existing conditions and aided in development of SWMM models. Those same sites had rain gauges and flow meters installed again in 2015 to collect post-construction data on GI performance. These meters and gauges remain in operation, but enough data have been collected to draw preliminary conclusions about GI performance. At one of the GI sites, data analysis indicates that green infrastructure is having a significant impact in reducing stormwater flows. At the other site, construction in the area may have interfered with green infrastructure practices, and monitoring was inconclusive. Metering continues into 2016; at the conclusion of metering, the SWMM model of the sites will be revisited and potentially recalibrated based on analysis of the complete sewer flow and rainfall data sets. Green infrastructure is a relatively new approach to reducing stormwater runoff, so real-world performance data remain sparse. Monitoring programs not only assist local jurisdictions in evaluating their own GI, but help present and future GI communities by increasing understanding of performance, maintenance needs, and other challenges. Site monitoring helps measure the efficiency of green infrastructure elements to ensure that they are meeting expectations.

5 Reintegrating Nature in a Dense Urban Environment: Restoration of Waller Creek Drainage areas in urban environments have been significantly altered by extensive development and impervious surfaces. Constructed drainage networks are designed to quickly capture stormwater runoff from the urban landscape, which can induce flooding in some areas. As the stormwater moves through the urban environment, it picks up many different pollutants along its flow path and carries them to lakes and rivers. Consequently, cities are faced with the challenge of efficiently managing water quantity and protecting water quality of the receiving waterbodies. Stormwater management tools and techniques have advanced considerably in the last decade. While some of the challenges may be the same from city to city, the solutions vary widely, and there is no one-size-fits-all approach to managing stormwater in urban areas. LimnoTech is performing hydrologic and hydraulic modeling and other tasks to help evaluate stormwater management alternatives for Waller Creek, an urban watercourse that has experienced flooding, channel erosion, and pollution from stormwater runoff. In Austin, Texas, storm events can lead to flash floods in urban streams, resulting in extensive property damage and economic loss. Waller Creek, which runs through downtown Austin, is negatively impacted by dense urbanization, a degraded drainage network, slope failures, and an aged, disconnected trail system. The creek corridor is highly urbanized, with 80-90% imperviousness in the downstream reach, and has experienced severe flooding, channel erosion, and degraded water quality. In an effort to control flooding, the City is constructing a large stormwater diversion tunnel to reroute stormwater beneath downtown Austin and deliver excess flows to the receiving waterbody, Lady Bird Lake. Upon completion, the Waller Creek Tunnel will remove approximately 28 acres of downtown land from the 100-year floodplain, creating a new hydrologic condition for the creek and a unique opportunity for urban revitalization and ecological restoration of the Waller Creek Corridor. The ability to control flooding in the Waller Creek Corridor has allowed project partners to realize the large-scale restoration potential for this delicate urban ecosystem. In cooperation with the City of Austin and the Waller Creek Conservancy, the restoration of Waller Creek is being planned by an integrated team of landscape architects, hydrologists, engineers, scientists, and ecologists. An important part of the project has been working with the City, the Conservancy, and regional planners to develop a common understanding of the goals for development of the Creek corridor, including human use opportunities, water quality, aesthetics, ecological restoration, and habitat creation. The overarching project goals include restoration and reconnection of the existing trail system, protection and enrichment of ecology integrity, and transformation of the Waller Creek Corridor into a welcoming public space. LimnoTech s primary role in this project is to analyze the creek's hydrology and hydraulics under existing and post-tunnel conditions. We are using hydrologic and hydraulic models (HEC-HMS and HEC-RAS, respectively) of the creek system to evaluate the effects of varying flow regimes on water surface levels and stresses under the full range of flow conditions to guide channel design and habitat creation. An adapted geomorphology responding to these changed hydrologic conditions requires a fundamental alteration of channel section and plan that varies significantly by reach, requiring combinations of hard and soft channel modifications that coordinate with the planned human use of the restored waterway and restoration goals. The post-tunnel hydrological conditions will be much less flashy and variable in Waller Creek as compared to pre-tunnel conditions. Upon project completion, the Waller Creek Corridor will be revitalized with an integrated trail system and chain of parks that creates an inviting green space and reintegrates nature in a dense urban environment.

6 LimnoTech 501 Avis Drive Ann Arbor, MI FIRST CLASS US Postage Paid Permit #87 Ann Arbor, MI ADDRESS SERVICE REQUESTED Giving Back: LimnoTech Charities We support many charities at LimnoTech as a way to give back to the community, and one of our favorite charities is the Humane Society of Huron Valley. The HSHV s mission is to support all animals in the community through ensuring proper care of animals at its shelter, placing adoptable animals in good homes, caring for the physical wellbeing of animals in the community, providing education and outreach, and stopping animal cruelty. Many LimnoTech employees have pets that were rescued, and others volunteer at the HSHV shelter. We also hold events at our offices throughout the year to raise money for the services provided at the HSHV. And as you can see by the picture below, while LimnoTech staff share a love of our pets, our pets also share our love of water. Photo: Carrie Turner Currents is published for our clients and associates by the employees of LimnoTech. This newsletter and past issues may be viewed on our website at: For more information please contact: Tim Bertsos, Editor tbertsos@limno.com Contributors to this issue: Scott Bell sbell@limno.com Dendy Lofton dlofton@limno.com Anouk Savineau asavineau@limno.com Derek Schlea dschlea@limno.com Tim Schmitt tschmitt@limno.com Steve Skripnik sskripnik@limno.com Brad Udvardy budvardy@limno.com Reproduction of material by permission only. LimnoTech Office Locations: Headquarters Ann Arbor, Michigan Mid-Atlantic Office Washington, D.C Central Region Office Oakdale, MN Southern California Office El Segundo, CA