This chapter defines the reasons stormwater management is important in the state and introduces the general stormwater management principles that are used throughout the Manual. The unique framework and stormwater management approach needed in Minnesota to address the variation in physical conditions that might affect surface water management are discussed.
The material contained in this Manual, and especially the background material comprising this chapter, can be used to educate public officials and citizens on the necessity to plan adequately for stormwater. Although the average Minnesotan is very water-savvy, there is a continual need to keep our youth and those desiring to learn better served.
Minnesota is very fortunate to have a great many educational programs available to its citizens. Such efforts as the University of Minnesota Extension, Project NEMO, Watershed Partners, and all of the Phase II education programs developed by MS4 communities are but a few of the many available. Because this list is far too long to include in this document, the reader is referred to Minnesota Sustainable Communities Network “Next Step” to obtain a comprehensive list of education programs and contacts.
Stormwater is an all-inclusive term that refers to any of the water running off of the land’s surface after a rainfall or snowmelt event. Prior to development, stormwater is a small component of the annual water balance. However, as development increases, the paving of pervious surfaces (that is, surfaces able to soak water into the ground) with new roads, shopping centers, driveways and rooftops all adds up to mean less water soaks into the ground and more water runs off. Figure 2.1 is a variation on a classic diagram that has appeared in many documents describing the effects of urbanization. This adaptation from the University of Washington shows how the relative percentages of water soaking into the ground change once development begins in a forested area. Note that the numbers assigned to the arrows depicting the movement of water will vary depending upon location within Minnesota. General information on regional precipitation, infiltration, evapotranspiration, etc. in Minnesota is contained in Chapter 8, but local details should be obtained from an appropriate source knowledgeable about local water data.
The Center for Watershed Protection has helped document the adverse impact that increased imperviousness (that is, water not able to soak into the ground) has on the health of receiving streams. Further information is contained on its Web site. Similar impacts occur when the watersheds surrounding lakes experience an increase in impervious cover, although in both stream and lake cases this simplistic explanation is only part of the problem. Other factors such as morphology, landscape setting, inherent soils and geology, and land use history could be equally as important.
It is important to note that the Minnesota Stormwater Manual has an urban or developing/developed area focus. This is not meant to ignore or minimize the impact that agricultural or silvicultural activities can have on our receiving waters. Rather, the Manual focuses on the transition from rural and open space to urban uses, and on the management of stormwater from the increased impervious surfaces that result. Readers are referred to the Minnesota Pollution Control Agency Web site, the Minnesota Department of Natural Resources Web site, or the Minnesota Department of Agriculture Web site for further information on agricultural and silvicultural activities.
The passage of the Federal Clean Water Act (CWA) in the 1970s initiated a change in the view of pollution in the U.S. No longer was it acceptable to pollute our country’s water resources. The initial focus of implementing the provisions of the CWA was logically on point sources of pollution, or those discharges coming from the end of an industrial or municipal wastewater pipe. Progress in addressing these discharges was made rapidly, although vigilance is still required to assure continued protection.
In the 1990s the United States Environmental Protection Agency (USEPA) began to apply requirements of the CWA to stormwater runoff. Owners and operators of certain storm drainage systems are now required to comply with design, construction, and maintenance requirements set by the MPCA for the State of Minnesota (www.pca.state.mn.us/water/stormwater/stormwater-manual.html) Manual users are also encouraged to check the Center for Watershed Protection for much more information on the behavior of stormwater and links to many additional sources of information.
The changes in the landscape that occur during the transition from rural and open space to urbanized land use have a profound effect on the movement of water off of the land. The problems associated with urbanization originate in the changes in landscape, the increased volume of runoff, and the quickened manner in which it moves. Urban development within a watershed has a number of direct impacts on downstream waters and waterways, including changes to stream flow behavior and stream geometry, degradation of aquatic habitat, and extreme water level fluctuation. The cumulative impact of these changes should be recognized as a stormwater management approach is assembled.
Urban development alters the hydrology (rate and volume) of watersheds and streams by disrupting the natural water cycle (Georgia Stormwater Manual, 2001). The changes in streams draining altered watersheds are very apparent (Figure 2.2) as they respond to the altered hydrology during this transition. Although similar changes can occur from intensive agricultural or silvicultural activities, the Manual focuses on the impacts of changes associated with development. Notable responses include:
The changes in the rate and volume of runoff from developed watersheds directly affect the morphology, or physical shape and character, of urban streams, rivers, and often ravines and ephemeral (intermittent) drainageways. Some of the impacts due to urban development include (adapted from the Georgia Stormwater Manual, 2001):
Perhaps the most significant impact that results from the physical change to urban streams occurs in the habitat value of streams. Impacts on habitat include (adapted from the Georgia Stormwater Manual, 2001):
As impervious surfaces increase, more water flows off of urban surfaces and is delivered faster to receiving waters. The increased activity on these surfaces means that more polluting material is available, as well. Minimizing the mobilization of this material and its impact is the goal of good runoff management and the purpose of this Manual.
Diffuse sources of pollution, such as that resulting from construction (Figure 2.3), roadways, parking lots and farm fields, have been a focus for Minnesota water management because they surpass point sources in severity for many pollutants of concern. The conversion of rural and open space land to urban uses is the particular focus of this Manual.
The problems associated with the conversion of land emerge as the land surface changes from one that soaks water into the ground to one that inhibits this infiltration. What used to be a small portion of runoff from a rainfall or snowmelt event becomes a major source of runoff volume. Water that used to soak in collects and flows from these new surfaces with enough energy to erode soil that was formerly held in place with protective vegetative cover and strong roots. Streams generally depend on ground water supplies during dry periods of the year. When infiltration is reduced or eliminated, this ground water is no longer available to supply baseflow and support the life of the channel. For the same reason, deeper ground water aquifer units receive less recharge.
Quantity is not the only problem resulting from changing runoff patterns. The water that washes over these new urban surfaces picks up materials laying upon those surfaces. The sediment from construction erosion, the oil, grease and metals from many automobiles, the fertilizer and pesticides from lawns, and many more new pollutants can adversely impact the receiving waters. Table 2.1 lists some of the many nonpoint pollutants of concern and the sources of these pollutants.
The impacts of the various pollutants listed in Table 2.1 are felt to varying levels. It is important to recognize that the hydrologic balance of most receiving water depends on this runoff water. Simply diverting all of the flow around a water body might help reduce a pollution load, but it could also cause the water body to dry up.
The receiving water quality impacts from urban runoff vary depending upon the quality and quantity of the stormwater and the assimilative capacity, or its natural ability to absorb or accommodate certain pollutants without adverse effects, of the receiving waterbody (Conservation Toronto and Region, 2001). Depending on the chemical, biological and physical character of the waterbody, its assimilative capacity can be quite different and tolerance to pollutants may vary greatly. Some waterbodies are inherently more sensitive to types or classes of pollutants than others; for example, lakes are more sensitive to phosphorus than streams and trout streams are more sensitive to increased temperature than non-trout streams.
Potential water quality concerns resulting from stormwater include (among others):
Awareness of the potential for pollution of Minnesota’s water is an important beginning, but action must follow. A performance based approach to action means that a management plan is put together focused on achieving or maintaining a certain goal. The methods used to achieve the goal are not entirely prescriptive. This allows the stormwater manager the flexibility to be innovative.
There are several principles consistent with integrated stormwater management and the treatment train approach that this Manual uses to promote proper runoff management. They are:
Minnesota is a large and varied state. Physical elements such as climate, occurrence of water, ecology, geology, soils and topography, and cultural features such as land use vary dramatically from one end of the state to the other. Stormwater managers in Minnesota know that conditions in the state can complicate solutions that might be simple elsewhere in the country. The extreme weather conditions (cold and hot) and physiographic variability under which we operate makes it impossible to generalize a single accepted approach for the entire state under all conditions. Flexibility in approaching problems site by site is stressed in this Manual. The following section describes some of the statewide variability that can be addressed with variable techniques in the Manual. Appendix A contains a compilation of several additional graphics illustrating the differences in factors that can influence stormwater.
The climate of Minnesota is characteristic of a transition zone from the moist and temperate eastern U.S. to the dry and droughty western U.S. Minnesota’s large size also means that much variation can occur within the state in any given year. Add to that extremes in temperature, and the difficulty in trying to describe Minnesota’s climate can be appreciated. Although the temperature discussion is interesting, this Manual has been developed to address water, so other than the fact that we experience very cold winters, temperature will not be discussed.
The major factors to focus on for good statewide stormwater management are rainfall and snowfall (snowmelt). A complete picture of Minnesota stormwater runoff cannot be painted without a discussion of both. Issue Paper B in the Manual Sub-Committee series (see Appendix J) examined some questions associated with the frequency of precipitation in Minnesota. The discussion was oriented around which design events to use as a basis for unified sizing of stormwater facilities across the state.
Figure 2.4 depicts the normal average annual Minnesota precipitation (rain plus snow) pattern. The statewide variation from less than 20 inches in the northwest to about 35 inches in the southeast is evident on the map. Figure 2.5 is a similar representation showing the areal variability in snowfall, varying less regularly from about 40 to 64 inches.
The real impact of the precipitation that falls in the state is felt when it runs off either as rainfall or snowmelt. Issue Paper B (Appendix J) contains a substantial amount of discussion on the proper statistical representation of “design events” based on the relationship of precipitation to runoff. The discussion was intended to set the stage for selection of the unified sizing criteria contained in Chapter 10. Much of the discussion with the Manual Sub-Committee concerned the use of the US Weather Bureau’s 1961 Technical Publication 40, commonly known as “TP 40”. Even though this publication is generally considered out of date because it does not reflect recent climate changes, there is no acceptable substitute at this time (see Issue Paper B discussion). Until such time as an acceptable replacement exists, the graphics presented in Appendix B define design events that should be used in Minnesota. Appendix B (Supplemental Graphics B.1 – B.7) contains the TP 40 graphs showing the 1-year through 100-year, 24-hour rainfall events. Table 2.2 summarizes the TP 40 data for the state and the seven-county metro area. Further breakdown of aerial precipitation frequencies across the state are presented in Chapter 10 (see also Issue Paper B in Appendix J).
The determination of snowmelt volumes is more complicated that rainfall because it depends on two factors – snowfall depth and the amount of moisture (or the equivalent water moisture) in the snow. Figure 2.5 shows the average annual snowfall amount for the state, but this graphic is somewhat misleading because it does not show the amount of moisture running off as snowmelt when the snowpack melts.
Initial determination of the average amount of snowmelt runoff can be determined using the information presented in Figure 2.6a. This shows the average snowfall depth on March 10th, an approximation of the initiation of melt in much of Minnesota, plus isolines that show the last occurrence of three-inches of standing snowpack. Figure 2.6b shows the average snow water equivalent for March. The total runoff is the product of the snowpack depth times the water equivalent. For example, St. Paul would be 7 inches (0.58 feet) times approximately 0.11 inches of equivalent water equals about 0.77 inches of water that will be available for runoff during a total melt of the snowpack. For Tower, the snow on the ground at melt is closer to 20 inches, but the snow water equivalent is only 0.09 inches, so the amount available for runoff is about 1.8 inches. Details on the data presented in Figures 2.6a and b and much more regarding Minnesota climate in general can be obtained at the Minnesota Climatology Working Group Web site.
Of course not all of the meltwater runs off. Figure 2.7 is based on research in Canada and illustrates that some of the meltwater enters the ground as infiltration. This graphic needs local adjustment based on knowledge of ground conditions, but it does give an approximation of the amount of melt that will soak into the ground and hence be removed from the total runoff volume. (See the equation describing this below).
Many of the physical features that influence the behavior of stormwater are not mapped at a level of sufficient enough detail for the state. This section will generally describe the features of importance and refer the user to sources of better information.
Due to the richness and variety of Minnesota’s water resources, several classes of waters have been identified for special protections through legislation or programs designed to protect these unique resources (Table 2.3).
Watershed-based water management began in earnest in the state in the mid-1950s and has had several additional mandates put in place since then. Watershed districts, watershed management and watershed-based planning are all common terms within the state. Figure 2.9 shows the eight major watershed basins across the state. Details on local watersheds are available from local sources, the Minnesota Department of Natural resources at www.dnr.state.mn.us/watersheds/map.html or the Minnesota Association of Watershed District’s “Where is my Watershed?” Web site.
The reality associated with so many watershed units occurring in the state is that a complex planning and regulatory framework exists for water management. Many of the sub-watersheds contained within the major watershed units have watershed management organizations that typically have some level of authority through a Watershed District or Watershed Management Organization. Information on the location and operations of these organizations can be obtained from the State Board of Water and Soil Resources at its Web site www.bwsr.state.mn.us
Another primary watershed mapping unit for Minnesota waters is based on MPCA’s ecoregion concept. These are geographic areas reflective of similar ecological character assembled to define causative factors for water behavior. Figure 2.10 illustrates the ecoregions as they are mapped for the state. Although not universally true, waters within each ecoregion should generally be similar in character, when all other factors (like rainfall, land use, and land cover) are similar. MPCA uses these as basic planning units for setting water quality standards and evaluating water quality variation. Keeping in mind the watershed and ecoregion within which water is being managed is an important part in structuring an effective management approach for stormwater.
The variable ecology across the state can be presented in many different ways. Figure 2.11 is one of those depictions from the DNR, but again it should be verified with local data when used as a consideration in stormwater design.
The geologic variability across Minnesota is reflective of billions of years of igneous and sedimentary history, plus geologically “recent” glaciation which is responsible for much of Minnesota’s vast natural beauty and abundance of water related resources. In most cases, the debris left behind by the glaciers provides a thick cover between the land surface and the buried surface of the underlying bedrock. In other cases, this glacial material either by-passed a location or has been eroded away, exposing bedrock to material (and possibly pollution) that comes from the land surface.
Figures 2.12 and 2.13 are depictions of the bedrock and surficial (Quaternary) geology, respectively, within Minnesota. Manual users are referred to the Minnesota Geological Survey (MGS) Web site (http://www.geo.umn.edu/mgs ) for details on the geology of the state.
In many portions of the state, bedrock occurs at or near the surface. The “red rocks” of the southwest, igneous intrusions along the St. Croix River and North Shore, and scattered sedimentary outcrops all around the state present some challenges in stormwater management because of their proximity to the surface. Among those difficulties are a lack of soil depth for use of infiltration techniques, structural impairment to best management practice (BMP) installation and steep slopes. The stormwater management implications of shallow bedrock affect infiltration, ponding depths, and the use of underground practices.
Figure 2.14 illustrates just one example of shallow bedrock along the North Shore. Again, details can be obtained from the MGS or a reliable local source, such as the county or a local well driller.
Carbonate and possibly other forms of bedrock can erode or dissolve in a manner that opens up pathways for movement of water into and through the rock. Such karst features, if sufficiently close to the land surface or to a ground water flow pathway, can present an opportunity for surface contaminants to enter the ground water system with very little or no treatment. This has important implications with respect to geotechnical testing, infiltration, pre-treatment and ponding of runoff.
Karst regions are predominantly found in the southeastern portion of the state, as shown in Figure 2.15 (Alexander and Gao, 2002). A statewide map of karst regions is shown in Appendix A. These areas have important implications with respect to geotechnical testing, infiltration, pretreatment and ponding of runoff. Figure 2.16 shows that caution must be used in interpreting the geographic depiction of “Karst lands”. The figure shows the difference between a generalized map (2.15) of active karst versus a county-scale map (2.16) of actual karstic features.
In karst settings where active karstic conditions (within 50 feet of the surface) are known to exist, additional constraints and considerations need to be evaluated prior to implementing most structural BMPs. Of particular concern in karst settings is the formation of sinkholes as a result of hydraulic head build up and/or dissolution of rock present underneath or adjacent to BMPs. Concerns also exist for ground water flow interruption, interflow and recharge particularly as it relates to stormwater facility, location, and size and the relationship of ground water to surface water. Where karst conditions exist, there are no prescriptive rules of thumb or universally accepted management approaches because of the variability intrinsic to karst terrain. An adaptation of a familiar old saying is very appropriate: the only thing predictable about the behavior of water in a karst system is its unpredictability.
In general when underlying karst is known or even suspected to be present at the site, stormwater runoff should not be concentrated and discharged into known sinkholes, but should rather be dispersed, or soaked into the ground after adequate pre-treatment, or conveyed to a collection and transmission system away from the area. In other cases, it may be impossible to remove water from an area with sinkholes or away from karst geology, so common sense clean-up of the water and discharge into the karstic area is a reasonable management approach, especially if some filtering soil is available between the land surface and the karst formation.
One of the first steps in the selection of BMPs is an assessment of the type of soils present on a site and the inherent ability of those soils to soak-up water. Soils are extremely variable throughout the state, but fortunately good information on local soil conditions is usually available. Details on surficial soils (generally to a depth of about six feet) are contained in county soil surveys, which are available from the U.S. Department of Agriculture’s Natural Resource Conservation Service (NRCS). Soil surveys for much of the state have been digitized to make electronic use practical. Figure 2.17 shows the early 2005 status of the digitizing effort. Note, however, that these surveys are not accurate enough to determine site specific characteristics suitable for many BMP applications, so a detailed site analysis is recommended. The primary reason for this is that soils can vary substantially with depth, and the county soil surveys depict only surficial mapped units.
Soils with low infiltration capacity are found throughout the state. On a local scale the absence of good soils that can absorb runoff (i.e., infiltrate) can be a major detriment to good stormwater management. Stormwater management limitations in areas with “tight” soils generally preclude large-scale infiltration and ground water recharge (infiltration that passes into the ground water system). These soils will typically be categorized under Hydrologic Soil Group (HSG) D and have other characteristics as shown in Table 2.4. The infiltration rates noted in this table are conservative estimates of long-term, sustainable infiltration rates that have been documented in Minnesota. They are based on in-situ measurement within existing infiltration practices in Minnesota, rather than national numbers or rates based on laboratory columns.
Use of HSG C or D soils for BMPs that rely on infiltration is generally not recommended unless a pre-development condition is trying to be simulated. That is, these soils can certainly be used in a system that relies only on a small amount of infiltration similar to the small amount that inherently exists on site. If a manager wants to match pre-development volume for all soils, it is apparent that D soils will continue to yield low infiltration. More details on these systems are found in the Chapter 6 and Chapter 12 discussions of BMPs. Pre-development condition is defined in Table 10.1.
On the opposite side of the infiltration spectrum are those soils that rapidly infiltrate water from the surface. Soils with large percentages of sand and separate from the water table transmit water very quickly and might work extremely well for infiltration practices provided precautions are taken to protect the ground water from the introduction of polluting materials. The level of treatment in sandy soils is quite variable. Although the sands can act similar to a sand filter for particulate material, soluble pollutants generally move through the soil quite rapidly and unattenuated. Figure 2.18 shows an example of a large-scale sandy soil condition in the Anoka Sand Plain. Similar large expanses of sandy soils exist elsewhere in Minnesota and should be recognized when planning a BMP strategy.
The elevation and topographical changes evident in Minnesota also present variable challenges to local stormwater managers. For example, the steep slopes along the North Shore and along many major river banks requires a far different approach than those practices where a deep soil cover exists on a flat plain or slowly rolling hills. Figure 2.19 illustrates the state variation in elevation, but again, local attention is required when information on slope, topography and physiographic character is part of the stormwater management deliberation.
Most of the cultural variation in the state relates to the land uses that have developed. Figure 2.20 illustrates the statewide differences in land use that have resulted as the state developed over the past 100+ years. Although the major focus of this Manual is on urbanized land uses, many urbanizing type activities, such as road building, transcend a single land use and apply throughout the state. Also in many cases urbanization occurs on land that was previously altered by agricultural, silvicultural, or pre-development activity.
The citizens of Minnesota long ago realized the potential for worsening water quality as the state grew. The solution they discovered was not to stop growth, but rather to plan for how it happens and to institute protective actions to prevent many of the negative impacts. It is virtually impossible to prevent all negative impacts, but there is a realistic expectation that efforts to minimize the impact should occur. This is the basis for the stormwater regulatory program in place in the state.
There are also many new and ever-evolving ways to manage the runoff and eliminate some of the pollution associated with it. These best management practices are proven effective measures that are readily available in both structural and non-structural ways. There are no “best” solutions that apply universally to all situations across the state. There are best solutions that can be chosen for specific applications to solve specific problems, hence the name best management practice.
The Minnesota Stormwater Manual provides insight for Minnesota stormwater managers on the nature of the stormwater problem in the state, as well as guidance on how to manage it using many available tools. We do not have to accept the situation portrayed in Figure 2.3, for example; instead, we can protect our valuable receiving waters through a reasonable set of practices applied equitably across the state. This is a major objective of this Manual.