Quantification of evapotranspiration (ET) and infiltration from vegetated stormwater control measures (SCMs), such as rain gardens, is necessary to properly assess their volume reduction potential. Weighing lysimeters at Villanova University mimic three rain garden designs and measure water budget parameters to determine how design elements impact ET. The designs compare two soil media with the same unconstricted drainage systems: a sandy loam and sand and two drainage systems with the same media (sand): an unconstricted valve (UV) outflow and internal water storage (IWS) outflow. A custom system distributed water over the lysimeters to mimic the runoff typically delivered to a rain garden during natural storm events. Runoff simulation trials were performed in summer and fall and compared to direct rainfall to the lysimeters during those seasons. Evapotranspiration rates were unaffected in the sandy loam lysimeter and increased in the sand lysimeters by the simulated rainfall in each system. Average ET over the entire study period was 2.9, 2.7, and 4.3 mm/d for the sandy loam, sand, and sand internal water storage, respectively. Evapotranspiration made up 47, 43, and 70% of the water budget for the sandy loam, sand, and sand IWS systems, respectively. The native soil (sandy loam) did not negatively affect the rate of ET. The IWS drainage system enhanced ET over the unconstricted valve drainage system. In an effort to promote inclusion of ET into rain garden design, seasonal and monthly ET rates are provided as baseline values. The ASCE Penman-Monteith ET and Hargreaves ET equations adequately represent the unconstricted valve lysimeter systems and underrepresent the IWS lysimeter system.
Current stormwater control measure (SCM) design often does not include the dynamic process of evapotranspiration (ET) for vegetated systems. This study compared two reference ET equations with a three-year data set from rain garden weighing lysimeters. The outcome was a tool to incorporate ET into SCM design. The weighing lysimeters at Villanova University, located in southeastern Pennsylvania, were used to measure water budget parameters for three scenarios: sandy loam with UO, sand with an unconstricted outflow (UO), and sand with internal water storage (IWS). The two ET models explored were the ASCE Penman-Monteith equation (a robust model) and the Hargreaves equation (a simple model). Estimated ET values from these two equations, both with and without modifications for water availability and crop presence, were compared and calibrated (if modified) with observed data. Comparisons and calibrations were performed on a daily and storm basis to explore the applicability of the two ET models for continuous and storm approaches. The observed ET was 28%–52% of inflow over the continuous three-year period and 16–30 mm on a storm scale, making ET a significant part of the lysimeters' water budget. Due to the experimental nature of the lysimeters, 12 of the 36 study months had additional simulated runoff, such that a smaller ET as a percentage of inflow was expected in the rain garden SCM's water balance. The Hargreaves and ASCE Penman-Monteith equations without modification provided an adequate estimate for rain garden ET for all systems at the storm scale. Modifications to ET estimations produced by both equations through crop coefficients and a soil moisture extraction function provided a good model for storm-scale ET by reducing errors and increasing efficiencies for all weighing lysimeter types. Evapotranspiration estimates from both unmodified equations provided, at best, a marginally better estimate than the average observed rate for continuous daily rain garden ET. The application of crop coefficients and a soil moisture extraction function to both equations reduced errors in ET estimates and increased the equations' predictive power (Nash-Sutcliffe efficiency) for all weighing lysimeter types. Both equations with modifications on a daily scale produced good ET estimates for the IWS system. For both equations, crop coefficients were found in an expected range for UO systems (0.3–1.5) but were high in the IWS system (1.6–2.0). Soil moisture extraction functions were not needed to calibrate the IWS equations on the storm scale. Both the Hargreaves equation and the ASCE Penman-Monteith equation provided an adequate model (especially with modifications) to incorporate ET into a design-storm approach to SCM design. Use of both predictive models on a daily scale has potential use in continuous simulation, as in most cases the ET estimations predicted by the equations provided a better estimate than the average of the observed daily ET rates.
Bioinfiltration systems for stormwater management rely on the ability of the engineered media to adequately infiltrate, filtrate, and store water to reduce runoff volumes and improve water quality. However, the potential clogging of the soil over time due to the migration of fines and deposition of sediment and debris has raised concerns regarding system longevity. To quantify temporal and spatial changes in textural and hydraulic properties of bioinfiltration media infiltrating runoff from an interstate, a comprehensive field and laboratory study was completed for two sites over a two-year period. Despite observed sediment deposition within both basins, there were no statistically significant trends in the saturated hydraulic conductivity of the media over the study period. Soil core sampling and analysis confirmed fines did not migrate through the soil column. Susceptibility to future clogging of the well-graded, loamy sand used at both sites was evaluated based on the collected data and permeability and retention criteria commonly used in geotechnical design of graded filters. Based on the results of this study and the current literature, soils proposed for use in bioinfiltration systems should be evaluated for filter compatibility with the anticipated sediment load and include maximum limits on the plasticity index to help enhance system lifespan and reduce necessary maintenance.
Aggradation at bridges causes the bridge waterway opening to be reduced, possibly resulting in upstream flooding and increased contraction scour. Aggradation results when the sediment load supplied to a reach of river from upstream exceeds its capacity to transport sediment. Solutions to aggradational problems at bridges are often complex and expensive. Solutions include increasing sediment transport through the bridge by modifying the channel, constructing an upstream sediment trap, redesigning the bridge, dredging, and treating the cause of the aggradation. At many bridges, aggradation problems can be severe. As an example, aggradation at a bridge in northern Pennsylvania is described. The benefits, disadvantages, and costs for various possible solutions to the example problem are compared and the most cost-effective solution is presented.
Quantifying evapotranspiration (ET) and infiltration from vegetated stormwater control measures (SCMs), such as rain gardens, is necessary to physically represent their volume reduction potential. Most states and regulatory entities utilize a design storm for rain garden design in which the rain garden’s capacity is considered using a static volume contribution. The static storage volume approach excludes the dynamic functions of infiltration during an event and ET between events. This work seeks to provide a method to incorporate the function of ET during interevent times into a design storm approach. The suggested method to do this is to estimate the void space recovery due to both ET and gravity drainage. An example is used to demonstrate the method for rain gardens in Pennsylvania where the void space recovery was estimated for 6 and 12 days between events. The void space recovery was estimated using a mathematical model based upon the 1D Richards equation coupled with the ASCE Penman-Monteith model. The mathematical model was validated using data from eight rain garden lysimeters in Villanova, Pennsylvania. This location is in the mid-Atlantic region with a Cfa climate in the Koppen-Geiger classification system. This void space recovery ranged from 15% to 40% (for both ET and gravity drainage) depending on soil type, drainage, rooting depth, crop coefficient and days between events. This void space recovery can be used to calculate a semidynamic recovery based on expected performance using the commonly employed design storm approach.
Abstract Rain gardens are increasing in use as the shift from gray to green infrastructure continues. Water that enters a rain garden is removed by three mechanisms: overflow to an outlet, percolation to the underlying soil, and evapotranspiration (ET) to the atmosphere. Despite the importance of ET in aiding a rain garden to recover void space during the time between storm events, it is not often measured or calculated because it is so difficult to do so. This paper explores the use of soil moisture sensors to estimate ET, since they are relatively inexpensive to purchase and install. Three rain garden weighing lysimeters in Villanova, PA, were used for this study. Over a 3‐yr study period daily ET was calculated each dry day and summed during the time in between storm events >25 mm. The cumulative changes of soil moisture readings during the interevent time at three depths were compared with the concurrent cumulative changes in the lysimeter weight readings. Cumulative soil moisture change was found to be strongly correlated to the cumulative ET for the different lysimeter media and drainage types. Using the two soil moisture sensors at the top and bottom of the soil column provided a similar result to using all soil moisture reading depths. The best single soil moisture reading depth was at the bottom of the lysimeter. The ET estimates via soil moisture tracking performed similar to uncalibrated potential ET estimates, but not as well as calibrated ET estimates.
ABSTRACT: The environment surrounding urban streams imposes constraints upon stream enhancement projects. Constraints include bridges, culverts, highways, sewer and water lines, lack of easements, and other floodplain structures. The consequences of failure of these infrastructure constraints can be significant and should be considered in the design process. Fault tree analysis provides a systematic technique for analyzing the interactions of events that could lead to infrastructure failure. A case study of a stream in Pittsburgh, Pennsylvania, shows that fault tree analysis can effectively model the interactions between the stream system and the infrastructure constraints and predict the most likely modes of failure. In addition, the relative success of alternative designs and failure mitigation techniques can be assessed using this analysis tool, lending insight into the urban stream enhancement design process. The method could also provide justification in the design permitting process and input for risk assessment.
The recovery of soil void space through infiltration and evapotranspiration processes within green stormwater infrastructure (GSI) is key to continued hydrologic function. As such, soil void space recovery must be well understood to improve the design and modeling and to provide realistic expectations of GSI performance. A novel conceptual framework of soil moisture behavior was developed to define the soil moisture availability at pre-, during, and post-storm conditions. It uses soil moisture measurements and provides seven critical soil moisture points (A, B, C, D, E, F, F″) that describe the soil–water void space recovery after a storm passes through a GSI. The framework outputs a quantification of a GSI subsurface hydrology, including average soil moisture, the duration of saturation, soil moisture recession, desaturation time, infiltration rates, and evapotranspiration (ET) rates. The outputs the framework provide were compared to the values that were obtained through more traditional measurements of infiltration (through spot field infiltration testing), ET (through a variety of methods to quantify GSI ET), soil moisture measurements (through the soil water characteristics curve), and the duration of saturation/desaturation time (through a simulated runoff test), all which provided a strong justification to the framework. This conceptual framework has several applications, including providing an understanding of a system’s ability to hold water, the post-storm recovery process, GSI unit processes (ET and infiltration), important water contents that define the soil–water relationship (such as field capacity and saturation), and a way to quantify long-term changes in performance all through minimal monitoring with one or more soil moisture sensors. The application of this framework to GSI design promotes a deeper understanding of the subsurface hydrology and site-specific soil conditions, which is a key advancement in the understanding of long-term performance and informing GSI design and maintenance.
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