Conventional stormwater management has proven to be efficient for decades, but only superficially. Also proven are its damaging ecological impacts.

The conventional method is centralized, in that runoff is collected from large areas and discharged, using gutters, drains, and a networks of pipes, to a centralized point.

A primary focus in the planning and selection of best management practices (BMPs) is determining how to disconnect from this centralized system. Through decentralization and treating runoff near the initial contact origin, it is possible to achieve decreased discharge volumes.

Managers and planners must carefully analyze many features in order to select the most effective, appropriate, cost effective, and resilient BMP at a development site best suited to satisfy the local and state stormwater quality criteria.

Flexibility is key. This will allow managers and planners to select multiple BMPs to mitigate the problems specific to the location. The selection of an inappropriate BMP for a site can pose immediate and long-term problems for local resources, create friction between residents and regulators, and could be an overall waste of time, money, and effort. Inappropriate BMP selection can also further misconceptions regarding green stormwater management approaches.

Now, let’s take a look at the nine factors that should be considered when selecting a BMP for a certain site. from interacting with the external environment.

The Minnesota Pollution Control Agency has recognized the rising interest in shifting to better stormwater management. Aiming to provide guidance, the Minnesota stormwater manual was developed “to produce a useful product that helps the everyday user better manage stormwater.”

The manual offers nine factors that should be evaluated in the BMP selection process:

1. Investigate pollution prevention opportunities. Evaluate the site to look for opportunities to prevent pollution sources on the land from becoming mobilized by runoff.

2. Design the site to minimize runoff. Assess whether any better site design techniques can be applied at the site to minimize runoff and therefore reduce the size of structural BMPs.

3. Select temporary construction erosion and sediment control techniques. Check to see what set of temporary sediment control techniques will prevent erosion and minimize site disturbance during construction.

4. Identify receiving water issues. Understand the regulatory status of the receiving water to which the site drains. Depending on the nature of the receiving water, certain BMPs may be promoted, restricted or prohibited or special design or sizing criteria may apply.

5. Identify climate and terrain factors. Climate and terrain conditions vary widely across the state, and designers need to explicitly consider how each regional factor will influence the BMPs proposed for the site.

6. Evaluate stormwater treatment suitability. Not all BMPs work over the wide range of storm events that need to be managed at the site, so designers need to choose the type or combination of BMPs that will provide the desired level of treatment.

7. Assess physical feasibility at the site. Each development site has many physical constraints that influence the feasibility of different kinds of BMPs.

8. Investigate community and environmental factors. Each group of BMPs provides different economic, community, and environmental benefits and drawbacks. Designers need to carefully weigh these factors when choosing BMPs for the site.

9. Determine any site restrictions and setbacks. Check to see if any environmental resources or infrastructure are present that will influence where a BMP can be located at the development site.

These considerations may seem vague, but there are many tools available to assist in determining the details behind the selection and placement of BMPs. Let’s take a closer look at the different options.

Assistance in determining stormwater management strategies is widely available. For example, this table, produced by the University of Arkansas, assists end users in determining the most effective designs. It organizes stormwater controls by increasing level of treatment on the horizontal axis and increasing level of volume reduction on the vertical axis. It depicts flow control devices on the left side of the table to show that this method maintains the least amount of treatment and volume reduction. Conversely, constructed wetlands are on the right side of the table to show that they offer the most treatment and volume reduction.

Another, more detailed assessment of BMPs selection is seen in this table. This depicts 21 LID control features and juxtaposes them against 22 different control measures that are classified as either quality control, pollution control, biological control or other.

For example, flow control devices satisfy only one of the 22 controls: peak flow attenuation, or more simply, controlling high peak flow.

On the other hand, a dry swale satisfies 13 of the 22 controls: peak flow attenuation, runoff volume reduction, infiltration, dispersion, evapotranspiration, runoff collection and usage, sedimentation, filtration, plant metabolism, nitrification/denitrification, organic compound degradation, temperature reduction and disinfection.

Tables such as this can provide more insight on selecting a BMP for a certain location that may have a unique issue, such as unusually high zinc loadings.

See the module in Canvas for a link to the complete version of this table.

In addition to many state-published stormwater management manuals, there are computer-based tools that assist in the selection, simulation and placement of stormwater BMPs. These programs greatly reduce the time commitment and error involved in BMP siting, although it does not completely omit it.

Of the many tools available worldwide, the tool we will focus on is the EPA siting tool (a component of the SUSTAIN tool) developed in the US EPA.

EPA’s Best Management Practices (BMPs) Siting Tool identifies potential suitable locations for installing a variety of BMPs. It supports users by helping to select locations that meet the defined site suitability criteria, such as:

•drainage area,

•slope,

•hydrological soil group,

• buffers – and many more

It is designed to be used by anyone interested in reducing runoff from a property, most popular among site developers, landscape architects and urban planners.

The BMPs available for selection in this program address three main criteria that are critical to managing urban runoff:

• Reduce or delay the volume of stormwater that enters the sewer system

• Reduce the maximum flow rate into the system by decreasing the volume and lengthening the duration of discharge

• Improve water quality through volume reduction, filtering, and biological and chemical processes.

The tool classifies BMPs as scale-based and type-based. Scale-based classifies BMPs according to the size of the application area, such as lot-, community-, or watershed-scales.

The type-base category classifies BMPs into three types according to the geometric properties:

· Point BMPs: capture upstream drainage at a specific location

· Linear BMPs: narrow, linear shapes adjacent to stream channels

· Area BMPs: land-based management practices that affect impervious area, land cover, and pollutant input

The THE EPA SITING TOOL tool allows users to select the type of BMP to be installed. The program is freely available online as a plug-in tool for users that have ArcGIS mapping software along with the spatial analysis extension. A detailed user’s guide describing the installation and use of the tool is available on the EPA’s website.

The tool is rather simple to use. First, the user must upload a map of the area in question. Then, the user must upload information that defines certain parameters for the area including the elevation grid, land use, streams, soil data and land ownership.

Next is the BMP selection. In this particular example, the user has selected to determine the potential sites for a bioretention basin. The user defines the BMP siting criteria based on features such as land slope, soil group and buffers for the bioretention basin. After the parameters of the location in question are selected, the tool can automatically determine the best places for BMP installation, producing a final map that looks like this.

From this data, the user can then plan the best places for the BMP installation based on factors such as funding, acceptance, pollutant loadings, etc.

In short, the EPA siting tool helps to:

· Select BMPs

• Screen BMPs for specific local conditions

· Site BMPs - identifies potential locations for the selected BMPs

The groundbreaking European project, SWITCH, was a major research endeavor, with 33 partners from 15 countries and a budget exceeding €20 million (roughly $22 million US dollars). The project was funded by the European Commission and development of the tool occurred between 2006 and 2011.

The main objective of the SWITCH project was to find new answers to age old concerns: how to increase the efficiency of urban water systems. The SWITCH project team revised old paradigms and produced new solutions for urban water management and offered an improved scientific foundation to ensure that sustainable water management systems were resilient, flexible, and applicable under a wide range of applications.

The SWITCH project included:

· All aspects of the water cycle - water, wastewater, stormwater and natural systems

· A wide range of climatic, socio-economic and institutional situations

· Social, economic and environmental perspectives

· Scales ranging from household to city levels

· Water as part of urban planning and the built environment

· From present time to the 'City of the Future'

SWITCH was recognized for its innovation in the practical realization of sustainable urban water management.

A major product of the SWITCH program was the development of the ‘SWITCH Approach.’ These concepts are visualized as methods to aid in a municipal ‘switch’ to sustainable urban water management practices. The SWITCH approach encompasses aspects of green infrastructure, making use of a ‘gray to green’ approach, and also provides “the environmental foundation that underpins the function, health and character of urban communities” (CABE, 2009).

The key features of the SWITCH approach:

· Establishment of city learning alliance platforms - these multi-stakeholder learning alliances guided and supported SWITCH on the development and implementation of research and demonstration activities, by taking account of local problems and needs

· Implementation of a strategic planning process - this encourages and enables all stakeholders in the city to view the urban water cycle in an integrated way and allows the development of new strategic directions for urban water management

· Establishment of early-action demonstrations representing different aspects of the water cycle that are designed for up-scaling at both the local and global level

· Development of a training toolkit with the city learning alliances to maximize the utility and impact of the SWITCH approach

The SWITCH project engaged stakeholders in 12 different cities around the globe. Situation-relevant demonstrations occurred in Accra, Alexandria, Beijing, Belo Horizonte, Birmingham, Bogota, Chongqing, the Emscher Region of Germany, Lima, Łódż, Tel Aviv and Zaragoza.

The intent was to “…translate the results of the SWITCH research into tangible and socially-relevant "on-the-ground" activities which could be used to test the research assumptions regarding performance, costs, community acceptance and other factors.”

Using tools, such as the EPA Siting Tool, help tremendously with implementing small designs all the way to projects like SWITCH. One of the first things managers must be conscious of is the local watercourse and how to enable suburbia to cater to this cycle. Therefore, management must be decentralized.

The concept of decentralized management, mentioned earlier, is inherent to BMP design and are selected in order to attenuate peak flow volumes, sequester nutrients, retain pollutants and allow for groundwater recharge.

A decentralized system should make use of source controls, such as vegetated filter strips, that will help to disconnect impervious surfaces. Overflow runoff from the source controls will be subject to the stormwater control system, such as a detention pond, where further infiltration and treatment will occur. Finally, the remaining waters will be ready for discharge as they are of a quality high enough to ensure resource protection.

There are two types of BMPs: structural and non-structural. Structural BMPs are designed to work together with non-structural practices. Both structural and non-structural BMPs should be considered in any management plan.

Structural BMPs are the physical constructs that are installed ‘in the field’ (ncforestservice.gov). The vegetated filter strip and the detention pond in the previous example made use of structural BMPs. On the other hand, non-structural BMPs make use of minimization and avoidance practices (ncforestservice.gov).

In short, structural BMPs are things and non-structural BMPs are practices.

Structural BMPs are designed around the natural rhythm of the local environment. These systems make use of vegetation and various soil mechanisms to get the job done. Determining the appropriate structural BMP for any site requires detailed knowledge of the local area, the problem or problems that need attention, and any existing infrastructure or practices that will help or hinder not only the instillation, but the function of the BMP as well.

A few examples of structural BMPs include swales, rain barrels, permeable pavement, bioretention, vegetated filter strips, green roofs, rain gardens, retention ponds, detention ponds, infiltration basins and constructed wetlands.

Non-structural BMPs do not make use of constructed mediums to mediate concerns. Non-structural BMPs are better described as proactive practices that can be performed in order to relieve stresses on the structural BMPs. Non-structural BMPs focus on preserving the natural green spaces that are already established, protecting natural systems, and incorporating existing landscape into designs, such as allowing existing wetlands to assist with nutrient uptake from runoff.

A few examples of non-structural practices include disconnecting impervious surfaces, pre-harvest planning, integrating stormwater management into the initial site design process, street sweeping, marking stream buffers with paint or flagging prior to construction, and locating streams on the site before you begin work.

As we discussed earlier, the EPA’s BMP siting tool, the EPA Siting Tool, assists in the placement of BMPs. With a clear distinction between structural and non-structural BMPs, it is good to note that the siting tool only assists with the placement of structural BMPs. Non-structural BMPs must be incorporated into design processes and people’s everyday habits, particularly those processes that impact the watershed most. Education is the most powerful tool that can be used in order to implement non-structural BMPs.

As you can see, detailed research is required in order to determine the most appropriate BMP for a particular site, whether it is structural or non-structural. It is also important to be able to determine when it is necessary to shift completely to natural BMPs and when it will be necessary to incorporate engineered systems into the management design. Complete reliance on natural BMPs can be quite the achievement for any stormwater manager, yet this is an unlikely circumstance.

Shifting to sustainable stormwater management makes use of many BMPs that are designed to handle the runoff from an event that is typical for that area. Design concepts using natural systems must plan for the best, but must be able to handle the worst. BMPs installed must be able to accept an abnormally large volume of runoff, if it occurs. Stormwater managers and infrastructure designers must be conscious of the statistics of the presiding 2-year 24-hour storm event over the local area.

Let’s take a look at this storm event and see what it can tell us.

If a certain location has an established rain depth of 5.0 inches for its 2-year 24-hour storm event, then it means that there is a probability of 1 (or a 100% chance) that 5.0 inches of rain will fall in any given 24-hour period over the spread of 2 years. It also means that there is a probability of 0.5 (or a 50% chance) that 2.50 inches of rain will fall within any given 24-hour period over the spread of any given year.

Designers must ensure that the structural BMPs that will be installed will be able to tolerate the runoff from the 2-year, 24-hour event that presides over that area. In the usual case of a large storm event, some structures will be overwhelmed. Therefore, these BMPs can be linked with the already existing engineered stormwater system. This greatly reduces the probability of floods. So, if a particular BMP becomes overwhelmed by runoff, the excess will be directed through the gray infrastructure for immediate discharge.

With the 2-year, 24-hour storm event in mind when planning, it makes sense to incorporate the gray infrastructure into the design as a secondary measure to ensure the most optimal performance of the BMP.

At the advent of green infrastructure design, it is imperative to understand the benefits and the deficits of each BMP as well as the green infrastructure concept as a whole. Detailed research and sound locational knowledge is imperative for the proper implementation of any BMP. There are many tools available that can assist with the decision and placement, but those decisions are not absolute. Many other considerations must be included into the decision process, such as the 2-year, 24-hour storm event probability. As this field reaches maturity, there will be more detailed tools and technology to relieve designers of any apprehension toward uncertainty, but until then, it is all well-researched trial and error.