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Accelerated soil erosion and the sedimentation caused by it pose serious problems for the construction industry and state and local agencies. Runoff from construction sites has high sediment loads which may contain various pollutants, including oil, chemicals, and soil wastes. Natural erosion in ditches and along roadways may result in sedimentation that may result in costly damage to water and land. Drainage channels, culverts, and storm sewers may be filled and plugged by sediment, resulting in frequent and costly maintenance. This report addresses those problems and outlines methods for controlling erosion both during and after construction. Structural and vegetative practices for reducing sedimentation are given, and turf establishment techniques for native grasses and wildflowers are discussed. Information regarding the National Pollution Discharge Elimination System (NPDES) Storm Water Permit Requirements is also given.

Five roadway sections in northeastern Ramsey County, Minnesota were monitored during 1993-95, to evaluate water quality and loading of constituents from roadway runoff. Two snowmelt-runoff and five rainfall-runoff events were monitored per year at each site. Additional samples of rainfall were analyzed to determine if rainfall was a direct source of constituent loading to roadway runoff. Roadway-runoff samples were analyzed for selected physical properties, dissolved solids, nutrients, dissolved ions, selected metals, and semi-volatile compounds. Concentrations of dissolved ions such as sodium, chloride, and metals such as aluminum, chromium, lead, and zinc were detected at much greater levels for snowmelt-runoff samples than rainfall-runoff samples. Analysis of chemical samples from rainfall indicate that rainfall was not a direct source for most constituents. Dissolved nitrate and dissolved ammonia in rainfall, however, can contribute up to one-half the amounts detected in roadway runoff. Concentrations of total phosphorus and fecal Streptococcus bacteria were greater at unguttered sites than at guttered sites. Concentrations of dissolved solids, and some metals were greater at guttered sites than at unguttered sites. This suggests that the vegetated road ditches associated with unguttered sites may filter out heavier particles such as metals and solids, while contributing additional organic matter. Concentrations of aliiminum, copper, lead, and zinc exceeded chronic condition standard limits established by the Minnesota Pollution Control Agency for metropolitan storm water from 96 percent, 52 percent, 9 percent, and 20 percent of the samples collected, respectively. Chemical loadings of specific constituents, such as suspended solids, from an individual rainfall-runoff event accounted for greater than 90 percent of the cumulative loadings of that constituent for all monitored events at site 4, for the entire study period. Length of latent period was statistically compared to constituent concentration levels of total phosphorus, dissolved sulfate, and total zinc and there was a correlation. Constituent loads were not associated with latent period. No correlation was found between traffic volumes which ranged from 1,888 to 7,172 vehicles per day and constituent concentrations or loads for this study.

This report presents the results of a field study on the performance of erosion control products under artificial rainfall conditions, bed shear partitioning using a hydraulic flume, and regression analysis of previously published data. Ninety-six runs of different plot lengths, erosion control treatments, vegetative cover, and initial moisture contents were gathered and analyzed. Above-ground biomass varied substantially within the growing season, runoff was most strongly influenced by initial moisture content, and sediment load was substantially reduced using erosion control products and mulches. In the hydraulic flume experiments, the percentage of the total shear acting on soil particles was less than 13.2% for all tests. Failure to obtain a large data base of product characteristics greatly limited the use of regression analysis to explain the performance of erosion control blankets.

A field-monitoring program began in the spring of 2000 to test the ability of a grassy swale at removing pollutants in stormwater. In 2001, a check dam was designed in conjunction with Minnesota Department of Transportation (Mn/Dot) engineers and installed into the vegetative swale. The check dam system incorporated some unique design features including a peat filter to trap nutrients and metals; and a low rock pool to trap water for biological processing. The check dam was designed for cost effectiveness and simple installation. The entire system was quantified and evaluated hydrologically and qualitatively both before and after the check dam installation. Pollutants monitored included total suspended solids, total phosphorus, and orthophosphorus. The average pollutant removal rates for the three storms following the installation of the check dam were 54 percent total phosphorus, 47 percent orthophosphorus, and 50 percent total suspended solids. The results suggest that properly designed short vegetative strips and swales, which include peat and rock check dams can substantially reduce pollutant levels from the stormwater exiting roadways.

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.

This Technical Summary pertains to Report 2023-21, “Climate Change Adaptation of Urban Stormwater Infrastructure,” published August 2023.

The design for Interstate Highway 94, east of St. Paul, Minnesota included a dead-end storm water retention pond constructed by the Minnesota Department of Transportation (Mn/DOT), for the purpose of collecting highway runoff. Flow into the pond will discharge primarily through the pond bottom into the groundwater system. Concern for potential impacts to both local and regional groundwater levels prompted Mn/DOT to conduct a groundwater study using an analytical element computer model. Presented in this report is a summary of the analytical aquifer modeling method, including basic theory, fundamental equations, and a brief description of how to access the computer program. As part of the study, an analytical element was developed to permit modeling of inhomogeneities in the aquifer hydraulic conductivity, thickness, and/or base elevation. Basic theory and equations for this element are discussed in the report. Use of the model for the storm retention pond is described including hydrogeology, site description, and simplifying assumptions used in the model. Results of the model are presented, including contour plots of groundwater levels prior to construction of the retention pond, as well as predicted levels when the pond reaches a steady state.

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.

Recent interest in detention ponds and constructed wetlands has extended to improving the quality of the runoff by reducing the concentrations of pollutants associated with stormwater runoff. Lead, zinc, copper, cadmium, phosphorus, and chloride are the contaminants of primary concern in the state of Minnesota. This study examined removal mechanisms in detention ponds. Three wetland graminoids, Glyceria grandis (reed manna grass), Scirpus validus (soft stem bulrush), and Spartina pectinata (prairie cordgrass) were studied for their phytoremediation capabilities for the six target contaminants. The uptake rates of the six target contaminants by the sediments of a detention pond were determined. These two removal processes were incorporated in an analytical model that can be used to determine critical parameters for the design of a detention pond that would produce effluent guidelines that meet requirements set by the Minnesota Pollution Control Agency. Removal rates for the three species were determined. Uptake rates were dependent on both the contaminant and species. The removal rates of the phytoremediation and the sorption sediment experiments were combined to develop a numerical model to simulate the removal mechanisms in detention ponds. This model can be used to develop design plots for a detention pond.

Highway infrastructure represents a substantial portion of the total impervious areas that generate runoff water. Because of long winters in congested areas that require frequent applications of de-icing materials, much of the runoff has the potential for affecting downstream water quality. However, storm water management techniques themselves have the potential for compromising the integrity of adjacent highways when they result in significant increases of water content in the soil beneath the roadway. Because of impacts and the costs associated with construction and maintenance, any storm water management system needs to be assessed before any decisions are made regarding new highway development or redevelopment. The authors consider Best Management Practices (BMPs) as they relate to the most commonly used storm water management approaches including dry ponds, wet ponds, infiltration trenches, infiltration basins, constructed wetlands, grassed swales, bioretention cells, sand filters and porous pavements. They provide a framework for considering cost of practices, negative impact on infrastructure, results from a BMP-related survey of highway design and maintenance professionals and cost-estimation formulas for each of the most commonly used storm water management approaches in urban Minnesota.