Weiss, Peter T.

The final analysis of historical (TP-40), current (Atlas 14), and future predicted storm events for three watersheds in Minnesota (Duluth, Minneapolis, Rochester) has shown that current design philosophy is not sufficient to prevent flooding from 10-year and larger design storm events and that flood depth and duration will increase given current climate projections. Several stormwater infrastructure adaptation strategies were assessed for reducing flood depth and duration: Baseline (existing conditions), adding rain gardens (aka, Infiltration Basins), adding new wet ponds, retrofitting existing stormwater ponds to be “Smart Ponds, adding new Smart Ponds while also converting existing ponds into Smart Ponds, or upsizing of stormwater pipes to convey more water. In watersheds that are mixed urban, suburban, and rural like Rochester’s Kings Run or Duluth’s Miller Creek sub-watersheds, the most cost-effective climate change adaptation strategy was to build new stormwater wet ponds (Extra Ponds strategy) to treat the impervious surfaces not currently treated by existing wet ponds and other stormwater BMPs. In the fully developed urban 1NE watershed in Minneapolis, the most cost-effective (excluding land costs) climate change adaptation strategy was building wet ponds (Extra Ponds). Securing property for building new stormwater infrastructure in fully developed urban watersheds like 1NE may be a substantial cost compared to other watersheds. Smart Ponds do not require additional land for implementation and thus represent a relatively low-cost alternative that will be more beneficial in watersheds with numerous existing wet ponds.

Road salt (NaCl) is used predominantly across the state for winter road anti-icing (as brine) and de-icing (as a solid) operations. Road salt is used because it is inexpensive and effective, but the thousands of tons used annually have resulted in increasing chloride concentrations of surface water bodies throughout Minnesota. In many cases, chloride concentrations are above regulatory limits, which results in the loss of aquatic biota and the water body being labeled as impaired. Thus, there is a need for one or more road salt alternatives (RSAs) that are effective, relatively inexpensive, and environmentally friendly. This report investigates the environmental impacts of potassium acetate (Kac), which is effective at lower temperatures than most other potential RSAs and is also less corrosive to steel than conventional road salt. Field measurements indicate that current applications of KAc do not have a substantial influence on biochemical oxygen demand (BOD) and microbiological water quality in Lake Superior. However, KAc concentrations due to application to 25% of the roads in the Miller Creek watershed are predicted to be above the toxic limit for water fleas. We believe that KAc could be used in the most precarious winter driving safety locations, but not over all watershed roads or for all storms. Acetate could be used as a general organic anti-icer, but in combination with another cation, such as sodium or magnesium.

Infiltration stormwater control measures are an important structural practice to mitigate the impacts of urbanization on stormwater quality and quantity. Infiltration stormwater control measures help to mimic the natural processes of infiltration and evapotranspiration. Unfortunately, the failure rate of infiltration stormwater control measures has been observed to be between 10% and 50%. Two common causes of failure are addressed in this work, namely improper siting and improper characterization of saturated hydraulic conductivity. A procedure to calculate a preliminary infiltration rating (PIR) was developed in a geographic information system to identify areas where infiltration stormwater control measures are likely to be successful. The Modified Philip-Dunne infiltrometer, double ring infiltrometer, Turf-Tec IN2-W infiltrometer, and soil texture analysis were used to estimate infiltration capacity in three swales in the Twin Cities Metropolitan area. A correction factor was proposed for the Turf-Tec IN2-W infiltrometer. A protocol for assessing infiltration capacity was also proposed.

Vehicular traffic contributes a large fraction of the pollutant load in stormwater runoff from roadways. While runoff concentrations have historically been characterized for urban roads with high average daily traffic (ADT); the runoff quality from paved rural roads that have relatively low ADT is largely unknown. In this study; runoff from low-volume roads (ADT < 1500) in Minnesota was monitored at 10 locations during 174 rainfall events in 2018 and 2019. The initial concentrations of total suspended solids (TSS); total phosphorus (TP); nitrate+nitrite; and heavy metals in the runoff; and the relationship between measured concentrations and site-specific conditions were analyzed. Concentrations were strongly influenced by the surrounding land use and soil type. Sites with agricultural lands had higher mean TSS; TP; and zinc concentrations; and lower nitrite+nitrate concentrations than wooded sites; which can be related to the type of soil that would get transported onto the roadways. When compared to existing urban runoff quality data; the estimated event mean concentrations (EMCs) in rural road runoff were substantially lower for copper and zinc and marginally lower for TSS; TP and nitrate+nitrite. Based on detailed cost-benefit analysis of various roadside treatment options; roadside drainage ditches/swales are recommended for cost-effective treatment of runoff from low-volume roads over ponds; sand filters and infiltration basins. Example road widening projects were also modeled to determine how stormwater management requirements can be achieved using drainage ditches/swales.

Iron-enhanced ditch checks in roadside swales were developed specifically for capturing dissolved phosphorus and dissolved metals from roadway runoff in both urban and agricultural environments. One iron-enhanced ditch check constructed along CR 15 (formerly TH 5) in Stillwater; Minnesota; was monitored during 40 storm events from 2016 to 2018. The iron-enhanced sand filter insert generally captured phosphate; yielding lower phosphate concentrations and mass load reductions that varied between 22% and 50% during several events. However; the cumulative phosphate retention in the filter insert decreased from 42% in 2015 to 30% in 2016; 25% in 2017; and 23% in 2018. The filter insert was not an effective retention device for dissolved copper and zinc. The overall ditch check's performance; although unexceptional in 2016 and 2017; appeared to improve in 2018. Sampling issues likely contributed to the low performance measured until 2017. The 2018 water sample collection method provided a better estimate of the ditch check's performance and roughly matched that of the filter insert. Synthetic runoff testing supported the level of treatment achieved during storm events. Phosphate load from the degrading topsoil and the overutilization of the bottom filter media most likely affected overall treatment performance. Design improvements and recommended maintenance actions were developed based on the lessons learned from field monitoring. The iron-enhanced ditch check can improve net phosphate retention through roadside swales; as long as the recommended maintenance actions are performed as scheduled.

This document is an extensive review of full-depth permeable pavements including porous asphalt, pervious concrete, and permeable interlocking concrete pavers (PICP). Also included is a brief section on articulated concrete blocks/mats. The main topics, which have been divided into chapters, include structural and mix design, hydrologic design, hydraulic performance (i.e. infiltration capacity), maintenance needs/frequency/actions, the impact of permeable pavement on water quality, results of a highway shoulder feasibility study, knowledge gaps, and several cold climate case studies from the United States and Canada. While progress has recently been made with the relatively new permeable pavement technology, researchers have also identified many unresolved issues that are not well understood. These include a methodology to measure subgrade infiltration rates, filling data gaps related to structural integrity, construction, and related issues associated with permeable pavements, determining what maintenance activities are most effective on various pavement types and how frequently specific maintenance actions should be performed, a better understanding of the processes involved in the observed reduction of contaminant concentrations in stormwater flowing through permeable pavements, and a better understanding of the performance of permeable pavements over a time frame that better corresponds with a life-span of 20 years.

Roadside swales are drainage ditches that also treat runoff to improve water quality, including infiltration of water to reduce pollutant load. In the infiltration study, a quick and simple device, the Modified Philip Dunne (MPD) infiltrometer, was utilized to measure an important infiltration parameter (saturated hydraulic conductivity, Ksat) at multiple locations in a number of swales. The study showed that the spatial variability in the swale infiltration rate was substantial, requiring 20 or more measurements along the highway to get a good estimate of the mean swale infiltration rate. This study also developed a ditch check filtration system that can be installed in swales to provide significant treatment of dissolved heavy metals and dissolved phosphorous in stormwater runoff. The results were utilized to develop design guidelines and recommendations, including sizing and treatment criteria for optimal performance of the full-scale design of these filters. Finally, the best available knowledge on swale maintenance was combined with information obtained from new surveys conducted to develop recommendations for swale maintenance schedules and effort. The recommendations aim toward optimizing the cost-effectiveness of roadside swales and thus provide useful information to managers and practitioners of roadways. The research results and information obtained from this study can thus be used to design swale systems for use along linear roadway projects that will receive pollution prevention credits for infiltration. This will enable the utilization of drainage ditches to their full pollution prevention potential, before building other more expensive stormwater treatment practices throughout Minnesota and the United States.

This research is a field evaluation of the water quality performance of dry water quality ponds with underdrains. The evaluation is performed in terms of pollutant retention by measuring concentrations in the inflow and outflow from the pond. Three dry detention ponds, Mn/DOT pond 4012-03, Mn/DOT pond 4012-04 and a pond operated by Carver County, were investigated for their ability to remove total phosphorus, dissolved phosphorus, total suspended solids and volatile suspended solids. The measured influent concentrations of most parameters in storm water runoff at the Carver County dry detention pond with under-drains were substantially lower than concentrations typically mentioned in other studies throughout the nation and influenced the pollutant retention efficiency of the pond. This study confirmed that dry detention ponds with under-drains are an option for water quality control. The results of this study indicate that various storm water pollutants can be removed by dry detention ponds with under-drains. A comprehensive comparison of pollutant retention efficiencies of various dry detention ponds throughout the nation is also carried out in this study. Comparison of these values with the retention efficiencies of this study indicated that the pollutant retention performance of the Carver County pond was below the average expected performance of dry detention ponds but well within the expected variation, even with the low influent concentrations at the Carver County pond. This is an indication that the under-drains do provide improved pollutant retention when used with a dry detention pond.

Stormwater management practices for treating urban rainwater runoff were evaluated for cost and effectiveness in removing suspended sediments and phosphorus. Construction and annual operating and maintenance cost data was collected and analyzed for dry detention basins, wet basins, sand filters, constructed wetlands, bioretention filters, infiltration trenches, and swales using literature that reported on existing SMP sites across the United States. After statistical analysis on historical values of inflation and bond yields, the annual operating and maintenance costs were converted to a present worth based on a 20-year life and added to the construction cost. The total present cost of each SMP with the 67% confidence interval was reported as a function of the water quality design volume or, in the case of swales as a function of the swale top width, again with a 67% confidence interval. Finally, the mass of total suspended solids and total phosphorus removed over the 20-year life was estimated as a function of the water quality volume. The results can be used by planners and designers to estimate both the total cost of installing a stormwater management practice at a given site and the corresponding total suspended solids and phosphorus removal.