Randel L. Dymond

Bioretention Hydrologic Performance in an Urban Stormwater Network

Although many studies have evaluated the hydrologic effects of bioretention at the site-level, few have investigated the role bioretention plays when distributed throughout a watershed. This study aims to assess bioretention’s effects on an urbanized watershed in Blacksburg, Virginia by using two modeled scenarios: one where runoff from many land uses was routed through the practice, and another in which only runoff from large impervious areas was routed. Peak flows, volumes, and lag times from these models were compared to the watershed’s current and predeveloped conditions. Both scenarios provided reductions in peak flows with respect to existing conditions for modeled storm events, sometimes to levels below the predeveloped condition. Neither case was able to reduce volumes to predevelopment levels; the option to treat impervious areas had a negligible effect on runoff volume. Both cases were able to extend lag times from the existing development condition. On the basis of these results, bioretention a...

Development of an Improved Approach for Selecting Storm-Water Best Management Practices

This paper discusses the development of a software-aided approach to best management practice (BMP) selection. Supported with input from a geographic information system (GIS), this software provides an algorithmic, mathematically based alternative to the inherently subjective approaches currently used to choose BMPs for management of storm-water runoff. The BMP selection software was applied to a study site located in the town of Blacksburg, Virginia. The software was utilized to identify BMP alternatives for a distributed storm-water management approach on the site. When compared with a base model with no runoff management, the traditional, detention-based management approach yielded 13.0% greater runoff rate reduction than did the distributed model. By contrast, the distributed storm-water management approach greatly outperformed the detention-based approach in removing pollutants from surface runoff. Although both runoff management strategies removed a significant portion of suspended sediment from the...

From Landscapes to Waterscapes: A PSE for Landuse Change Analysis

We describe the design and implementation of L2W – a Problem Solving Environment (PSE) for landuse change analysis. L2W organizes and unifies the diverse collection of software typically associated with ecosystem models (hydrological, economic and biological). It provides a web-based interface for potential watershed managers and other users to explore meaningful alternative land development and management scenarios and view their hydrological, ecological, and economic impacts. A prototype implementation for the Upper Roanoke River Watershed in Southwest Virginia, USA is described.

Evaluation of Variability in Response to the NPDES Phase II Stormwater Program in Virginia

AbstractAuthorities of municipal separate storm sewer systems (MS4s) in small urbanized areas (population less than 100,000) are required to implement stormwater control measures (SCMs) to mitigate and reduce the impacts of urbanization on stormwater runoff under Phase II of the National Pollutant Discharge Elimination System’s (NPDES) stormwater program. This 16–year-old policy has been challenged in its effectiveness in maintaining or improving water quality, but reviews are scarce because of the policy’s subjective requirements, and because it governs MS4s across a wide variety of characteristics, objectives, and institutional capacity. This research models SCM selection as a function of these differences, thereby systematically evaluating the policy’s outcome in its constituents. The results show that certain characteristics of an MS4 community significantly affect the selection of SCMs, suggesting that regulations may need to be refined to address distinct groups of MS4s. The results also reveal inef...

An Examination of the Demographic and Environmental Variables Correlated with Lyme Disease Emergence in Virginia

Lyme disease is the United States’ most significant vector-borne illness. Virginia, on the southern edge of the disease’s currently expanding range, has experienced an increase in Lyme disease both spatially and temporally, with steadily increasing rates over the past decade and disease spread from the northern to the southwestern part of the state. This study used a Geographic Information System and a spatial Poisson regression model to examine correlations between demographic and land cover variables, and human Lyme disease from 2006 to 2010 in Virginia. Analysis indicated that herbaceous land cover is positively correlated with Lyme disease incidence rates. Areas with greater interspersion between herbaceous and forested land were also positively correlated with incidence rates. In addition, income and age were positively correlated with incidence rates. Levels of development, interspersion of herbaceous and developed land, and population density were negatively correlated with incidence rates. Abundance of forest fragments less than 2 hectares in area was not significantly correlated. Our results support some findings of previous studies on ecological variables and Lyme disease in endemic areas, but other results have not been found in previous studies, highlighting the potential contribution of new variables as Lyme disease continues to emerge southward.

An Analysis of Patterns and Trends in United States Stormwater Utility Systems

Several factors, such as municipality location and population, are thought to influence trends among stormwater utilities (SWUs); however, no analysis of the relationship between these factors and SWU characteristics has been performed. This article corroborates hypothesized relationships and identifies trends and patterns in the establishment, funding mechanism, and magnitude of SWUs by analyzing location, population density, home value, and year of establishment for a comprehensive national SWU database with data for 1,490 SWUs. The equivalent residential unit (ERU), a SWU that charges based on impervious area, was the most prevalent funding mechanism in all National Oceanic and Atmospheric Administration Climate Regions of the United States except the West and West-North-Central. The ERU was also found in larger cities with high population densities, whereas the Flat Fee, a SWU that charges a single rate for all properties, was found in smaller towns. Higher home values were correlated with higher monthly fees for 28% of the municipalities analyzed. The residential equivalence factor, a SWU that charges based on runoff produced, was popular in municipalities with higher home values, whereas the Flat Fee was popular in municipalities with lower home values. The number of SWUs established increased with Phase I municipal separate stormwater and sewer system (MS4) permit and Phase II small MS4 permit deadlines. Summary tables provide guidance to aid municipalities considering a SWU.

A Review and Evaluation of Uncertainty Classification and the Error-Band Geometry Model

Advances in computer technologies have improved the quality of maps, making map comparison and analysis easier, but uncertainty and error still exist in GIS when overlaying geographic data with multiple or unknown confidence levels. The goals of this research are to review current geospatial uncertainty literature, present the Error-Band Geometry Model (EBGM) for classifying the size and shape of spatial confidence intervals for vector GIS data, and to analyze the interpretability of the model by looking at how people use metadata to classify the uncertainty of geographic objects. The results from this research are positive and provide important insight into how people interpret maps and geographic data. They suggest that uncertainty is more easily interpreted for well defined point data and GPS data. When data is poorly defined, people are unable to determine an approach to model uncertainty and generate error-bands. There is potential for using the EBGM to aid in the development of a GIS tool that can help individuals parameterize and model spatial confidence intervals, but more research is needed to refine the process by which people use the decision tree. A series of guiding questions or an “uncertainty wizard” tool that helps one select an uncertainty modeling approach might improve the way people apply this model to real-world applications.

Comparison of Stormwater Management Strategies with an Urban Watershed Model

AbstractA detailed hydrologic/hydraulic model for an urbanized 381-acre watershed in Blacksburg, Virginia, is used to evaluate downstream results from simulation of seven stormwater management (SWM) strategies currently practiced within the United States. Each strategy has unique design criteria for specified design storms and is intended to achieve predevelopment peak runoff rates at the site outfall or a baseline peak runoff rate at a watershed point of interest (POI). Model simulations are run that represent each strategy implemented throughout the watershed within areas currently subjected to SWM. Runoff hydrographs at each site outfall and the watershed POI are evaluated. Evaluation of model results at the site outfalls and a downstream watershed POI lead to a number of observations about the performance of the various strategies and their respective design criteria. Results demonstrate that strategies met the predevelopment peak runoff rate targets for design storms specified in their design criteri...

FloodwayGIS: An ArcGIS Visualization Environment to Remodel a Floodway

Floodway modeling has been performed extensively using HECRAS in floodplain studies. The model output is typically exported in GIS format and the floodway boundaries are overlaid on other spatial data to further edit or remodel the floodway to meet FEMA and local development requirements. In this article, a tightly coupled system comprised of a commercial GIS (ArcGIS) and HECRAS is presented. FloodwayGIS provides a comprehensive visual environment to edit, remodel, spatially analyze, and map floodway boundaries. The environment uses the HECRAS executable engine for every remodeling iteration. Four different encroachment editing options are provided within FloodwayGIS, which eliminates the need for a modeler to switch between HECRAS and GIS in the floodway modeling process, and results in savings of modeling time. FloodwayGIS also provides a mapping algorithm based on TIN intersection to produce smooth floodway boundaries that can be mapped in Digital Flood Insurance Rate Maps (DFIRMs) with minor editing.

Evaluation of Continuous Monitoring as a Tool for Municipal Stormwater Management Programs

AbstractThe purpose of this study is to evaluate the uncertainty attributable to inadequate temporal sampling of stormwater discharge and water quality, and understand its implications for meeting ...