This article provides a definition of “Integrated Stormwater Management” and discusses its multi-faceted approach. It discusses rate and volume control, ground water and surface water interaction, pollution prevention, and the definition of “BMP.”
Integrated stormwater management is simply thinking about all of the factors that somehow affect precipitation as it moves from the land surface to an eventual receiving water. It is the process of accounting for all of these factors (e.g. rate, volume, quality, ground water impact) in a logical process so that inadvertent mistakes are not made that could eventually harm a resource. The treatment train approach to runoff management mimics the sequence as the stormwater manager looks at the runoff problem and determines how best to address it, starting with the most basic of questions and increasing in complexity only if needed, since simple methods of management are often the most practical. A regulator might view it as a check to see if a simple approach could replace something more complicated and expensive.
The first step in integrated stormwater management is determining the scope of the project and the likely solutions that will be needed. If on-site, simple practices will solve the problem, a non- or minimum-structural approach can be pursued. If problems extend off-site and impact a major regional water body, then a broader scale will need to be pursued and commensurate BMPs chosen.
The decisions will always be influenced by the regulatory requirements associated with the action. That is, a project that creates new impervious surfaces over one acre or is part of a common plan of development will need to comply with the requirements of the State’s Construction General Permit. Additional local or watershed requirements may also be required (Chapter 5). Retrofits or actions not creating new impervious area can introduce creative or innovative solutions, such as supplemental sub-grade infiltration, proprietary filters or wetland polishing. Note that these can also be part of the regulated treatment train.
Minnesota has a long-standing tradition of approaching water management on a watershed system basis. Landmark legislation in the state has mandated watershed-based planning and management for over 50 years. Figure 2.8 from the previous chapter illustrated the large-scale watersheds within the state, but local or watershed agencies should be contacted to obtain fine-scale watershed boundaries, even on a parcel-by-parcel basis.
For every project, the question that should always be asked is “Where does water come from that enters my site and where does it ultimately go when it leaves?” This single question becomes the basis for a future management approach. For example, if the water leaving the site discharges to a trout stream rather than a lake, a different set of BMPs that focuses on temperature control rather than phosphorus removal will be pursued. Proper operation of the watershed as a “system” should always be part of a stormwater manager’s thought process.
The occurrence of natural features, such as wetlands, forest, natural drainage features, original topography, undisturbed soils, and open space on a site should be viewed as a positive thing. These features can be preserved to minimize the impact of development, used as an integral part of the treatment train, or even enhanced to improve site hydrology or the quality of runoff leaving the site.
Many of the basic tenets of “low impact development,” “better site design,” and “sustainable development” are rooted in the preservation, restoration and enhancement of the natural drainage system. Following this approach can lead to cost savings, as well as added environmental protection.
Integrated stormwater management requires a complete look at both the movement and content of runoff water. Focusing exclusively on one or the other might meet a specific regulatory requirement, but will not result in effective overall stormwater management. Discussion of the quality impacts occurred in Chapter 2 and will not be repeated here. However further discussion of quantity impacts is warranted.
In its early stages, stormwater management was primarily concerned only with quantity control. Urban hydrology techniques focused mostly on peak flow rate control and addressed volume in terms of flood control. The standard approaches for rate control have been greatly refined over the years, with more attention on mimicking pre-development or natural conditions (See discussion in Chapter 10). Volume control, on the other hand, has been something more difficult to achieve. The following section addresses the techniques that should be considered when a need exists to address stormwater quantity leaving a site.
Effective stormwater practices are integrated into the urban landscape to improve their function and performance. Twelve principles that help define the successful integration of a stormwater practice in the landscape include:
In the past, rate control was primarily used to prevent downstream flooding. Relying solely on rate reduction for stormwater control led to many system failures as volume and quality factors were left uncontrolled. Although not universally true, advancement in the state of the art for rate control practices generally came about as urbanization increased and greater protection from water leaving these largely impervious places was needed.
Chapters within this Manual take the stormwater manager beyond flood protection to hydrograph frequency matching, downstream channel protection and control techniques designed to maximize water quality improvement from the commonly occurring events that account for most of the runoff. Reference to this chapter and Chapters 4, 8, 9 and 10, as well as Issue Papers B, D, E, F, and G, found in Appendix J, provides some insight to the reasons for rate control and tools available to accomplish it.
The importance of volume reduction become apparent as more and more urban surfaces developed and more stormwater overwhelmed receiving waters. Clearly stormwater management needs to include volume control.
The term volume reduction can be easily confused with infiltration. One does not, however, necessarily equate to the other. There are many additional techniques and BMPs that can be applied to yield volume reductions.
Any technique that soaks water into the ground, makes water available for evaporation and/or transpiration, stores water for re-use, or in any way diverts stormwater away from the drainage system can be considered a volume reduction practice. Infiltration is certainly one of these practices, but it is only one of many. In circumstances where soils are too tight or where infiltration would endanger ground water, alternatives are available (Table 3.1) to reduce volume.
The following categorical methods for volume reduction, while certainly not all-inclusive, can provide some ideas for how a stormwater manager could reduce the volume of runoff leaving a parcel of land. The specific BMPs that use these methods are discussed in Chapter 6 and Chapter 12.
The most commonly used method to reduce site volume is to soak it into the soil. The result of this action is a direct reduction in volume running off of the land surface. The biggest requirement for use of infiltration is the ability of the soil and the shallow ground water system to accept the water.
The distinction between infiltration and recharge is a narrow one that can usually be ignored. Commonly, infiltration is the process of soaking water into the ground, while recharge is the movement of water into the ground water system. Recharge occurs to both shallow and deep ground water systems.
Low impact development (LID), better site design (BSD), sustainable development and other terms (see Chapter 4) are all variations of an approach that mimics natural conditions by soaking water into the ground close to where it falls. Use of these methods along with reduction of impervious areas reduces overall runoff volume and may be a component in many, but certainly not all runoff management plans. Reduction of connected impervious area and retention of natural drainage patterns and surfaces are the heart of these methods. Chapters 12 and 13 address the caution that should be followed whenever infiltration is used as a management technique.
The combined process of evaporating and/or transpiring (vegetative uptake and release of water) is called evapotranspiration or simply “ET.” This process typically results whenever water is held in storage (evaporation, or sublimation of snow in the winter) and allowed to be taken up by roots and released through leaves (transpiration). In areas with tight soils, holding water in wetlands, depressions, swales or any similar land feature that exposes water to the air will result in evaporation of that water. In addition, allowing it to come in contact with roots either in standing water (wetland) or by soaking into the root zone, will yield volume reduction through transpiration. In fact, this and reforestation can be used as stormwater management techniques.
Where soils provide a constraint, under-drains can provide a means through which water can be routed through the root zone for root uptake, but excess can be captured after filtration and drained to a collection system. This option results in some net reduction in volume and adds filtration as a supplemental treatment. Many bioretention treatment techniques take advantage of this method of volume reduction.
The combined infiltration plus ET rates for Minnesota can vary across the state from 11 inches in the northeast to 23 inches in the south. The complex relationship among precipitation, runoff, infiltration, and ET is discussed by Baker et al. (1979). They discuss the details and methods used to divide the water that falls as precipitation into several categories reflective of where it ends up. Obviously, routing water to areas where it can soak into the ground or to areas with vegetation that can take it up through root action are two very good ways to reduce overall stormwater volume if adequate space is available.
Retaining water somewhere along the path from where it falls to where it enters a drainage system is another way to limit volume. Simple contained storage directly connected to buildings or impervious areas are effective volume reducers and provide an opportunity for water re-use, such as irrigation. A rain barrel, cistern, sub-grade storage device, or even a yard ornamental pond can hold enough water to contain much of the volume coming from a home. A green roof can reduce annual runoff by up to 75% because it soaks and stores water that falls on it, then transpires it away.
Even a pond or a wetland can reduce overall volume because they provide a quiescent area where water can collect and evaporate. Pan evaporation in Minnesota can reach as high as 40+ inches (Baker et al., 1979). This is possible even when rainfall is much less because water is routed to these holding areas from a much larger watershed.
Getting rid of water was the common way to deal with stormwater before the results of that action were realized. Rushing water to a drain pipe, then into a receiving water is now considered a last resort. Using pervious approaches such as vegetated drainage swales and native grass filter strips, in combination with check dams give water a chance to soak into the ground or be filtered before it reaches a location where damage takes place. As with the practices above, volume reduction is an outcome of exposing stormwater to a pervious surface even while it is moving. See Chapter 12 for filtration and infiltration BMPs that would fit in this category.
Many of the previous practices could also be included in a general category that stresses the importance of stable landscapes with native vegetation. In many respects, this is LID/BSD with an added emphasis on structuring the land surface to handle moving water from impervious surfaces. Routing water to low-lying (sump) areas where it can soak in, placing planter boxes or grated inlets for watering trees, and contouring slopes to reduce runoff velocity are all variations on the landscaping theme.
Tying low impact drainage features together via corridors or designed natural treatment trains can further enhance overall site volume reduction by creating a string of reduction possibilities. Safety can always be assured by placing an overflow or even an under-drain to capture any excess flow and route it to the next BMP catchment area.
As with all stormwater management techniques, some caution is advised when applying them under certain circumstances. Following are some advisory cautions that would apply:
Table 3.1 lists many, but not all, BMPs that can be used to reduce overall runoff volume. Reference is made in the table to a more complete description of the BMP later in this Manual.
Integrated stormwater management often takes advantage of the interaction that takes place between ground water and surface water. For example, the slow infiltration and movement of surface water into the shallow ground water system results in peak and volume reduction, filtration through cleansing soil and continuation of baseflow to streams. Although stormwater management is often interpreted as a surface water program, many of the BMPs identified in this Manual rely on the ground water system to make them effective. Infiltration BMPs, for example, rely on the soil’s capacity to soak in water and transmit it downward to the ground water system. Soil cleansing via filtration, adsorption and microbial uptake can be a very effective removal process for some of the more difficult to treat runoff pollutants.
For the above reason, there must be caution used when pollution is “removed” through a system that affects ground water. For example, although soil adsorption is an effective scavenger of some soluble pollutants, one could argue that the introduction of chloride-laden water into any system that discharges to the ground is merely trading pollution in one water for another. The same could be said for ground water pump-outs that discharge contaminated ground water into any surface water or onto any land surface.
The Manual will note several instances when the interaction between ground water and surface water could be problematic. Specific cautions are raised in Chapter 13 for active karst areas and other shallow or fractured bedrock, high ground water table, tight soils, source water (wellhead) protection areas, and potential stormwater hotspots (PSHs).
The old adage “An ounce of prevention is worth a pound of cure” is never more appropriate than when used to describe integrated stormwater management. All of the previous elements have described the physical processes involved, but preventing pollution from coming into contact with runoff is a common sense element. A fact sheet presented in Chapter 12 describes some of the ways in which pollution prevention can be formalized, but keeping in mind the simple separation of runoff and those materials that cause pollution, such as oil, fertilizer, salt and sediment, will go a long way toward controlling urban pollution problems at a very low cost.
Pollution prevention methods are far too numerous to cover in their entirety, but include such common-sense practices as keeping yard and animal waste off of impervious surfaces, preventing soil erosion at all construction sites, disposing of household products properly, repairing leaky automotive parts, and careful storage and use of any polluting chemicals.
Following these simple precautions can make a dramatic difference in the type and amount of polluting material available for wash-off or aerial mobilization.
The selection of a proper management approach is a key factor in the success of an integrated stormwater management approach. Knowing which BMP(s) to apply under certain conditions could make the difference between success and failure, or between a low-cost and high-cost project. As pointed out in the principles listed in Chapter 2, the simpler the approach to an effective solution, the better.
The definition of BMP can vary significantly depending upon the individual or entity. While some only use BMP to define a practice that improves water quality, this Manual uses the term for both quantity and quality. That is, Chapter 12 includes many BMPs that reduce runoff rate or volume, but might have little direct effect on water quality. For example, dry ponds reduce runoff volume by allowing infiltration to occur as water flows and temporarily accumulates over a vegetated pervious layer. Some water quality improvement certainly occurs as the volume of water, and hence the load of any pollutant it carries, is decreased. However, dry ponds are not recognized by the MPCA as a water quality BMP because settled material is easily resuspended when the next big flow occurs.
Chapters 4, 6, 7, and 12 all contain discussion of BMPs, techniques for runoff management and selection criteria. These are all tools to assist with choosing structural or non-structural approaches. This Manual does not contain many additional non-structural practices that could be considered as institutional management approaches. Details on such things as zoning, ordinances, plan and permit review, public education, training, and others are not contained in this Manual. However, they have been referenced throughout with links often included if the user would like further information. The Manual is designed to present physical BMPs only to keep the scope manageable.
More detail on integrated stormwater management is part of the discussion in Chapter 4 on better site design. Better site design is used as an all-inclusive term that includes low impact development, sustainable development, design with nature or any other approach to consistent with the treatment train design philosophy.