Stormwater step pools are defined by design features that address higher energy flows due to more dramatic slopes than dry or wet swales. Using a series of pools, riffle grade control, native vegetation and a sand seepage filter bed, flow velocities are reduced, treated, and, where applicable, infiltrated. The physical characteristics of the stormwater step pools are similar to Rosgen A or B stream classification types, where “bedform occurs as a step/pool, cascading channel which often stores large amounts of sediment in the pools associated with debris dams” (Rosgen, 1996). These structures feature surface/subsurface runoff storage seams and an energy dissipation design that is aimed at attenuating the flow to a desired level through energy and hydraulic power equivalency principles (Anne Arundel County, 2009). Stormwater step pools are designed with a wide variety of native plant species depending on the hydraulic conditions and expected post-flow soil moisture at any given point within the stormwater step pool.
Stormwater step pools may be located at the end of the stormwater treatment train, the main form of conveyance between or out of BMPs, or designed as off-line configurations where the water quality volume is diverted to the stormwater step pool. In any case, the practice may be applied as part of a stormwater management system to achieve one or more of the following objectives:
Typical applications of stormwater step pools with or without underdrains include the following, where relatively steep longitudinal slopes are present:
Certain site-specific conditions may make use of stormwater step pools without underdrains (infiltration) infeasible. Examples include:
The following site-specific conditions may make use of stormwater step pools swales with underdrains (filtration) infeasible:
One of the goals of this Manual is to facilitate understanding of and compliance with the MPCA Construction General Permit (CGP), which includes design and performance standards for permanent stormwater management systems. These standards must be applied in all projects in which at least 1 acre of new impervious area is being created, and the permit stipulates certain standards for various categories of stormwater management practices.
For regulatory purposes, stormwater step pools fall under the “Infiltration / Filtration" category described in Part III.D.1. of the MPCA CGP. If used in combination with other practices, credit for combined stormwater treatment can be given. Due to the statewide prevalence of the MPCA permit, design guidance in this section is presented with the assumption that the permit does apply. Also, although it is expected that in many cases the stormwater step pool will be used in combination with other practices, standards are described for the case in which it is a stand-alone practice.
The following terms are thus used in the text to distinguish various levels of stormwater step pool design guidance:
Of course, there are situations, particularly retrofit projects, in which a stormwater step pool is constructed without being subject to the conditions of the MPCA permit. While compliance with the permit is not required in these cases, the standards it establishes can provide valuable design guidance to the user. It is also important to note that additional and potentially more stringent design requirements may apply for a particular stormwater step pool, depending on where it is situated both jurisdictionally and within the surrounding landscape.
If adequate space exists, stormwater step pools are suitable for retrofit applications. These systems are used where topographic gradients and hydrology combine to develop greater erosive potential than where dry or wet swales of gentler slopes would suffice. Indicators of suitable locations may include evidence of open gully formation or where recent or proposed changes to site hydrology will increase the current condition’s discharge of flow (e.g., new development, redevelopment, new routing of stormwater, conversion of native landscapes, etc.). However, space considerations often limit their use in ultra-urban and some highway/road settings.
The following table provides guidance regarding the use of stormwater step pools in areas upstream of special receiving waters. This table is an abbreviated version of a larger table in which other BMP groups are similarly evaluated. The corresponding information about other BMPs is presented in the respective sections of this Manual.
Summary of design restrictions for special waters.
Link to this table
|BMP Group||receiving water|
|A Lakes||B Trout Waters||C Drinking Water||D Wetlands||E Impaired Waters|
|Filtration||Some variations NOT RECOMMENDED due to poor phosphorus removal, combined with other treatments||RECOMMENDED||RECOMMENDED||ACCEPTABLE||RECOMMENDED for non-nutrient impairments|
Infiltration BMP design restrictions for special watersheds. This information applies to all infiltration practices.
Link to this table
|BMP Group||Receiving water|
|A Lakes||B Trout Waters||C Drinking Water1||D Wetlands||E Impaired Waters|
|Infiltration||RECOMMENDED||RECOMMENDED||NOT RECOMMENDED if potential stormwater pollution sources evident||RECOMMENDED||RECOMMENDED unless target TMDL pollutant is a soluble nutrient or chloride|
1 Applies to groundwater drinking water source areas only; use the lakes category to define BMP design restrictions for surface water drinking supplies
Stormwater step pools should remain effective water quality improvement systems for many years, even during winter conditions, if designed and constructed properly and it has been shown that hydraulic efficiency and infiltration rates can remain at levels used for design sizing. However, in cold climates, some special considerations are HIGHLY RECOMMENDED for stormwater step pools to ensure sustained functionality and limit the damage freezing temperatures and snow and ice removal may cause.
One concern with stormwater step pools (used for filtration) in cold weather is the ice that forms both over the top of the facility and within the soil interstices. To avoid these problems to the extent possible, it is HIGHLY RECOMMENDED that the facility be actively managed to keep it dry before it freezes in the late fall. This can be done by various methods, including limiting inflow and ensuring the underdrain is functional.
Even if the infiltration properties of a stormwater step pool are marginal for snowmelt runoff during the period of deep frost in the winter, the storage available in the facility will provide water quality benefit if it is dry entering the melt season. However, flow originating in an industrial area, a high traffic area where large amounts of salt are added, or another potential stormwater hotspot PSH should be diverted away from stormwater step pools if pretreatment features have not been properly designed to handle such an increase in loading.
For all BMPs it is HIGHLY RECOMMENDED that snow and ice removal plans including predetermined locations for stockpiling be determined prior to or during the design process. Stormwater step pools cannot be used for significant snow storage areas as debris build-up, plant damage, and lower infiltration rates are likely to occur. Some snow storage is unavoidable when BMPs are adjacent to areas where snow removal is required, but it is critical that the property owner and snow and ice removal contractor have identified other areas for large scale snow storage.
Excessive deicing agents have the potential to create a hot spot in some locations that could lead to reduced infiltration rates or concentrations that exceed surface water or groundwater standards. Locations such as busy intersections on slopes, parking garage ramps or on walkways near the entrances of commercial buildings are likely to be heavily treated with deicing agents to avoid slip and falls or vehicle collisions. This should be taken into consideration when siting any stormwater step pool.
Plant selection is critical to ensure that the damaging effects of snow and ice removal do not severely impact plantings or seedings. Even a small amount of snow storage can break and uproot plants requiring additional maintenance in the spring. Woody trees and shrubs should be selected that can tolerate some salt spray from plowing operations.
Stormwater step pools can help reduce detention requirements for a site by providing elongated flow paths, longer times of concentration, and volumetric losses from infiltration and evapo-transpiration. Generally, however, to meet site water quantity or peak discharge criteria, it is HIGHLY RECOMMENDED that another structural control (e.g., detention) be used in conjunction with a stormwater step pool.
Stormwater step pools can remove a wide variety of stormwater pollutants through chemical and bacterial degradation, sorption, and filtering. Surface water load reductions are also realized by virtue of the reduction in runoff volume.
Properly designed infiltration systems will accommodate a design volume based on the required water quality volume. Excess water must be by-passed and diverted to another BMP so that the design infiltration occurs within 48 hours if under state regulation, or generally within 72 hours under certain local and watershed regulations. In no case should the by-passed volume be included in the pollutant removal calculation.
Design specifications should prevent putting contaminated runoff and excess water beyond that which will infiltrate within the given time frame. Any runoff containing toxic material or excess volume that cannot infiltrate should be diverted away from the infiltration system and reported as inflow to another treatment device.
Water quality performance of stormwater step pools can be diminished when plants die off in the fall and winter months as they are no longer able to uptake water and nutrients.
The following general limitations should be recognized when considering installation of stormwater step pools without underdrains (infiltration).
The following general limitations should be recognized when considering installation of stormwater step pools with underdrains (filtration).
Stormwater step pools are currently not included as a BMP in the MIDS calculator. The swale main channel BMP can be used, but the maximum allowable slope is 4 percent. To determine volume retention for slopes greater than 4 percent, you will need to develop a relationship between the slope and volume retained. To do this, determine volume retention at 0.5 percent slope increments for your site at slopes ranging from 0.5 to 4 percent. Determine the appropriate regression for volume retention and slope and calculate the volume retained at the slope for your site. The relationship is not linear. Links to MIDS calculator information are provided below.