Anita M. Thompson

Physical and Hydraulic Properties of Engineered Soil Media for Bioretention Basins

The composition of engineered soil media largely determines the stormwater treatment efficiency of urban bioretention basins. Laboratory flow-through experiments were conducted to quantify infiltration, bulk density, and moisture holding capacity as a function of different composite mixtures of sand, soil, and compost, and to assess the effect of compaction on bulk density, moisture holding capacity, and saturated hydraulic conductivity. Eleven mixtures were evaluated that varied in volumetric proportions of sand (30% to 70%), sandy or silt loam soil (0% or 20%), and organic compost (20% to 70%). Steady-state infiltration rates were high for all mixtures, ranging from 87 to 178 cm h-1, and followed the order of mixtures containing sand and compost only > mixtures containing sand, compost, and sandy soil > mixtures containing sand, compost, and silt loam soil. Infiltration rates for mixtures containing sand and compost only and mixtures containing sand, compost, and sandy soil exhibited a significant linear relationship with the ratio of sand to compost. Bulk density of the mixtures was inversely related to the proportion of compost and followed the order of mixtures containing sand, compost, and silt loam soil > mixtures containing sand, compost, and sandy soil > mixtures containing sand and compost only. Conversely, moisture holding capacity increased with the proportion of compost and followed the order of mixtures containing sand and compost only > mixtures containing sand, compost, and sandy soil > mixtures containing sand, compost, and silt loam soil. Compaction as a result of an initial wetting process and the infiltration tests led to increases in bulk density and decreases in moisture holding capacity, with mixtures containing a silt loam component showing the greatest resistance to these effects. Bulk density, moisture holding capacity, and compaction were all linearly related to the ratio of sand/compost in the mixture. Air permeability measurements were used to estimate saturated hydraulic conductivity of four of the mixtures. Reductions of compost and additions of soil decreased saturated hydraulic conductivity. For the same proportions of sand, soil, and compost, the mixture containing silt loam soil was less compactable and incurred greater changes in saturated hydraulic conductivity compared to the mixture containing sandy soil. Although, at least initially, compost controlled the physical density of these mixtures, the textural class of the mineral component appears to help stabilize infiltration and dampen the effect of changing the ratio of sand to compost on the physical and functional characteristics of these mixtures.

SHEAR STRESS PARTITIONING FOR IDEALIZED VEGETATED SURFACES

Vegetation and other surface roughness materials partition the shear force of flowing water into a portion acting on the vegetation (vegetal shear) and the remainder acting on the intervening soil surface (particle shear). The fraction acting on the soil surface is directly involved in subsequent particle detachment. The purpose of this study was to directly measure the components of shear stress and to quantify the shear partition for various densities of idealized elements representative of non-submerged rigid vegetation in overland flow. Insight into the magnitude of particle shear and vegetal shear is necessary for understanding the role of vegetation in reducing particle shear and, consequently, reducing potential erosion. Circular cylinders and idealized elements with differences in the rate of change in upstream frontal area with flow depth were used to model vegetation. Detailed spatial and temporal particle shear measurements were made using a unique hydraulic flume and hot-film anemometry. Drag force was measured on individual elements within test arrays. This combination of measurements allowed for direct determination of the shear partition. The tests were conducted on three uniform element densities at discharges of 0.005 and 0.01 m3/s. Element width-to-spacing ratios ranged from 0.04 to 0.20. Over the range of densities studied, particle shear accounted for 13% to 89% of the total shear, indicating that complete surface coverage is not required to significantly reduce the shear stress acting on soil particles. Existing shear partitioning theory, in which the partition is a function of the ratio of element to surface drag coefficients and the roughness density, was found to represent the observed partition reasonably well (mean squared error = 0.036). The results from this study are important for selecting appropriate plant species and densities for erosion control systems.

Hydrologic Regimes Revealed Bundles and Tradeoffs Among Six Wetland Services

Ecosystem services are often described as occurring together in bundles, or tending not to occur together, representing tradeoffs. We investigated patterns and potential linkages in the provision of six wetland services in three experimental wetlands by measuring: flow attenuation, as peak flow reduction; stormwater retention, as outflow volume reduction; net primary productivity (NPP), as plant biomass; diversity support, as plant species richness; erosion resistance, as stability of surface soils in a flow path; and water quality improvement, as nutrient and sediment removal. Levels of ecosystem services differed in our system because of differences in hydrologic regime brought on by natural variation in clay-rich subsoils. The fastest-draining wetland (with thin clay layer) provided five of six services at their highest level, but had lowest NPP. In contrast, a ponded wetland (with thick clay layer) that was dominated by cattail (Typha spp.) provided the highest level of NPP, but lowest levels of all other services. Hence, in our site, drainage supported several bundled services, whereas ponding supported such high levels of NPP that other services appeared to be limited (suggesting tradeoffs). These outcomes show that high NPP has the potential to be a misleading indicator of overall ecosystem services. Rather than focusing on NPP, we suggest identifying and establishing hydrologic regimes that can support the services targeted for restoration in future projects. Further direct assessments of multiple services are needed to identify bundles and tradeoffs and provide guidance at the scale of local restoration projects.

A New Polyacrylamide (PAM) Formulation for Reducing Erosion and Phosphorus Loss in Rainfed Agriculture

Soil erosion from agricultural lands and the subsequent transport of sediment and sediment-bound nutrients (particularly P) are serious problems contributing to surface water pollution and threatening agricultural sustainability. Raindrop impact on bare soil destabilizes soil aggregates and leads to surface sealing, increasing runoff volumes and soil loss. Surface-applied polyacrylamide (PAM) decreases soil erosion by stabilizing the soil structure and reducing surface sealing. Previous research has shown that surface-applied PAM reduces erosion at relatively high application rates (20 to 80 kg ha-1). The purpose of this study was to evaluate the effectiveness and longevity of a new liquid/emulsion PAM formulation to reduce runoff, sediment, and P loss from rainfed agricultural fields. The formulation was applied using a hand sprayer at a low PAM rate of 5 kg ha-1 (13,333 L of solution ha-1). Plot-scale field rainfall simulations (75 mm h-1) were conducted on individual test plots at three test intervals (rainfall simulated 2 days, 3 weeks, and 10 weeks after PAM application) on two soil types (Ashdale silt loam and Plano silt loam). The new polymer formulation, Soil Net EM-1000-50, reduced runoff volumes an average of 100% at the 2-day test interval, 59% at the 3-week test interval, and 55% at the 10-week test interval. Overall, larger rainfall depths were applied to treatment plots prior to runoff generation compared to controls. The most significant differences were observed at the 2-day test interval, when an average of 141 mm of rainfall was applied to the PAM-treated plots before runoff started, compared to 82 mm for the controls. These runoff reductions are not only important in terms of erosion but may also enhance water supply through increased infiltration. Sediment loss was reduced an average of 100% at the 2-day test interval, 80% at the 3-week test interval, and 74% at the 10-week test interval. Phosphorus loss was reduced an average of 100% at the 2-day test interval, 75% at the 3-week test interval, and 83% at the 10-week test interval. The polymer treatment was more effective on the Ashdale silt loam, which had received manure during the two years preceding the rainfall simulations. Manure application was the primary difference between the two soils (manure was not applied to the Plano silt loam), suggesting that the PAM treatment reacts more favorably to manured soils. The low cost of this new PAM technology (approx.

Urban stormwater thermal gene expression models for protection of sensitive receiving streams

25 ha-1) coupled with its success in reducing runoff, sediment, and P loss over a 10-week period, make EM-1000-50 an attractive and economically feasible management practice for agricultural producers in rainfed regions.

INSTRUMENTATION TO MEASURE DRAG ON IDEALIZED VEGETAL ELEMENTS IN OVERLAND FLOW

Thermal impact of typical high-density residential, industrial, and commercial land uses is a major concern for the health of aquatic life in urban watersheds, especially in smaller, cold and cool-water streams. This is the first study of its kind that provides simple easy-to-use equations, developed using gene expression programming (GEP), that can guide the assessment and design of urban stormwater management systems to protect thermally sensitive receiving streams. We developed three GEP models using data collected during three years 2009-2011 from four urban catchments; the first GEP model predicts event mean temperature at the inlet of the pond; the second model predicts the stormwater temperature at the outlet of the pond; and the third model predicts the temperature of the stormwater after flowing through a cooling trench and before discharging to the receiving stream. The new models have high correlation coefficients of 0.90-0.94 and low prediction uncertainty of less than 4% of the median value of the predicted runoff temperatures. Sensitivity analysis shows that climatic factors have the highest influence on the thermal enrichment followed by the catchment characteristics and the key design variables of the stormwater pond and the cooling trench. The general method presented here is easily transferable to other regions of the world (but not necessarily the exact equations developed here); also through sensitivity and parametric analysis we gained insight on the key factors and their relative importance in modelling thermal enrichment of urban stromwater runoff.

Modeling Water Table Mounding and Contaminant Transport beneath Storm-Water Infiltration Basins

Information on shear stress acting on soil particles is necessary to quantify potential soil erosion. Movement of water across a rough surface generates a resistive force, part of which acts on the large–scale roughness elements while the remainder acts on the intervening soil surface. An important large–scale roughness element is vegetation, which acts to reduce shear stresses on the soil and thereby reduces potential erosion. Insight into the drag force acting on individual vegetative elements is necessary for understanding this dynamic. An instrumentation system was developed to measure the drag force on individual elements representative of vegetation. Vegetative elements were modeled in a hydraulic flume using rigid circular cylinders and idealized shapes to account for differences in the rate of change in upstream frontal area with flow depth. Data were gathered for a horizontal flume and for a 1% flume slope. Flow rates ranged from 0.004 to 0.028 m3s–1 with average flow depths ranging from 1.9 to 9.8 cm and average flow velocities ranging from 0.41 to 1.02 m s–1. Sixteen element shapes were considered, resulting in a total of 80 test scenarios. The test conditions resulted in both partial and complete submergence of the elements. Results are presented as actual drag force and dimensionless drag coefficient. The relationship between drag force divided by velocity squared and upstream projected area is well represented by a straight line for cylinders and all other elements. Drag coefficient is a function of element shape and is represented by an average value over the range of flow depths investigated. The overall error in the estimated drag coefficient is 7.3%.

MODELING THE EFFECT OF A ROCK CRIB ON REDUCING STORMWATER RUNOFF TEMPERATURE

The objectives of this study were to link an unsaturated and saturated flow model for the purpose of evaluating mounding and contaminant transport beneath an infiltration basin, to calibrate and test the combined water table flow model using experimental data collected from an infiltration basin, and to evaluate the potential for contaminant transport with a numerical fate and transport model. Mound formation may reduce the thickness of the soil available to retard pollutant movement, reduce the infiltration rate of the basin if the mound intersects the basin bottom, and facilitate contaminant movement away from the basin. A 0.10-ha infiltration basin serving a 9.4-ha residential subdivision in Oconomowoc, Wisconsin, was instrumented. Two storm events were modeled using the three-dimensional saturated numerical model MODFLOW. Recharge used in MODFLOW was taken from the seepage flux of the unsaturated one-dimensional model HYDRUS. A good fit was achieved between modeled and measured timing and magnitude of...

Effectiveness of Rock Cribs on Reducing Stormwater Runoff Temperature

Impervious surfaces absorb and store energy from the sun. During a rainfall/runoff event, some of that energy is transferred to the runoff as it flows over heated impervious surfaces. High-temperature runoff can be detrimental to cold-water habitat as it enters receiving waters; therefore, structures to cool this heated runoff water are desirable. Rock cribs are underground trenches that stormwater runoff flows through prior to discharging into receiving waters or storm sewers. Since the crib is below the ground surface, its initial temperature is significantly lower than that of the incoming stormwater runoff. Heat is transferred from the higher-temperature influent water to the cooler rocks and water in the crib, reducing the effluent temperature. In this article, we present a thermal mixing model to predict the conductive cooling effect of rock cribs. Two key simplifications were made in order to model this system: (1) laboratory measurements of an empirical mixing function were used to relate predicted mean crib temperatures to crib outlet temperatures, and (2) transient heat conduction from spheres occurs with an effective depth into the sphere of one-tenth of the rock diameter such that transient one-dimensional heat conduction applies. Results from a laboratory study were used to evaluate the simplifications and test the model. Comparisons between model results and experimental measurements showed that the thermal mixing model assumptions were satisfactory. A non-dimensional, graphical look-up approach is proposed for urban engineers to size rock cribs and implement a viable management practice for reducing urban runoff temperature.

New Approach for Estimating Hydraulic Properties of Soils in Cold Regions

Impervious surfaces such as roof tops, parking lots, and roads, absorb and store energy from the sun. During a rainfall/runoff event, some of that energy is transferred to the runoff as it flows over the heated impervious surface. The high temperature runoff can be detrimental to cold water habitat as it enters lakes, rivers, and streams. The purpose of this study was to gain a better understanding of the effectiveness of rock cribs on reducing the temperature of stormwater runoff. Laboratory experiments were performed to test the effect of crib volume, flow rate, influent temperature, and initial crib water content on reducing water temperature. As the flow rate increased, the time for the effluent water temperature to reach the influent water temperature decreased. The time for specific temperatures to be realized decreased as influent water temperature increased. For constant flow rate and influent temperature, the larger crib was more effective at reducing water temperature. Cribs that were initially full of water were more effective than initially empty cribs at reducing runoff temperature. Exponential decay functions were found to adequately represent the relationship between the normalized temperature difference and the dimensionless time variable (tfW/Vp) for both the initially empty cribs (R2=0.93) and initially full cribs (R2=0.96). Rock cribs have the ability to reduce runoff temperature and could be a viable management practice in urban settings.