Continuous Long Term Simulations for Evaluating Storage Treatment Design Options of Stormwater Filters
Stormwater media filters are used to treat a variety of pollutants at different source areas. These can range from being simple rain gardens or biofilters containing soils or special media, to proprietary devices. Historically, sand filters and sand-peat filters were some of the earliest filters used for storm-water control. Austin (1988), Galli (1990), Shaver (1994), Claytor and Schuler (1996), and Urbonas (1999) all include descriptions and performance information for these fundamental stormwater filtration systems. These filters have been used to treat a variety of conventional stormwater pollutants, mostly focusing on suspended solids and nutrients.
Continued research has examined additional media and expanded our understanding of stormwater media filters. Clark and Pitt (1999) include an extensive review of different media, designs, and expected performance. Many proprietary stormwater filters are also now available and usually include cartridges of specialized media that can target specific classes of stormwater contaminants. Descriptions of many of these devices have been described at technical conferences, especially the annual StormCon conference (http://www.stormcon.com/) where vendors have extensive exhibits showcasing these filters. The International BMP Database has much data describing actual field performance for a wide range of stormwater filters (http://www.bmpdatabase.org/).
This chapter focuses on an important issue pertaining mostly to the proprietary filters that are sized to be within the guidelines of regulatory agencies. There is much confusion associated with sizing filter installations in order to meet a specific volume based criterion. As an example, regulatory agencies may require individual stormwater controls to treat at least a half inch of runoff. For sedimentation practices, this has usually been interpreted as the water quality treatment volume (such as the volume in a wet detention pond above the normal dry weather elevation and below the emergency spillway). For a filtering system, and other flow-through controls, storm-water treatment is more clearly associated with flow rates, not volumes.
Some agencies have therefore resorted to transforming the volume objective to a treatment flow rate, using a single design storm and an assumed hydrograph shape. This approach greatly decreases the flexibility in the design and does not adequately consider the interaction between storage and treatment flow rates. This chapter illustrates a simple method using continuous long term simulations that are much better suited in sizing and evaluating these flow-based treatment systems.
The long-term performance of a stormwater treatment filter is dependent on the amount of the annual runoff that is treated by the unit and by the level of treatment that is provided by the filter to the water passing through it. Most performance summaries assume that all of the runoff is treated, and therefore overestimate the level of treatment provided. Over a long period this is not a reasonable assumption, as the largest peak flows are substantially greater than flows that occur most of the time. Most filters usually have maximum treatment flow rates that can be utilized per filter unit (per unit area of filter surface, per filter module, or some other measure) to obtain the stated treatment level of the treated water. However, the use of up-gradient storage can moderate the high flows, decreasing the amount of stormwater that bypasses without treatment. The sizing of this adjacent storage should be done in conjunction with a continuous model that can evaluate many storage-treatment combinations.
This chapter presents a framework, through examples, for sizing storm-water treatment filters using long-term simulations. These simulations can be used to predict performance and to prepare design curves in order to size stormwater filters for specific areas.
The chapter starts with a discussion of the need for continuous long-term simulations for water quality stormwater controls, and then describes some basic aspects of urban hydrology that affect filter performance and design. The use of correctly conceived urban hydrologic processes is critical, especially when calculating flows associated with small and intermediate sized rains. These processes, in conjunction with long-term simulations, allow accurate estimates to be made. Probability distributions of modeling outcomes that relate to many receiving water objectives in urban areas can also be prepared from the results of long term water quality simulations. The use of single design storms and hydrological calculations that focus on larger events do not provide accurate information for the rains which affect receiving water resources and distort information pertaining to the sources of flows and pollutants.
Examples for several different treatment objectives are presented in this chapter for Madison, Wisconsin, using a 5 y rainfall record that was selected as being representative of long term conditions. These examples show how the treatment flow rate is dependent on treatment objectives, and how storage can be used in some cases to reduce the overall expected costs of the treatment systems. The framework presented in this chapter can be used by regulators to assist in the development of regulations pertaining to treatment goals for local conditions; by manufactures of stormwater filters in the preparation of design curves to assist in the sizing of filter units to meet these objectives; and by stormwater designers to help select alternative stormwater treatment systems.
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