Christine A. Pomeroy

Cost-Estimating Tools for Low-Impact Development Best Management Practices: Challenges, Limitations, and Implications

Tools were developed for estimating costs of vegetative roofs, rainwater catchment systems, and bioretention facilities. These tools provide a detailed framework to facilitate cost estimation for capital costs, operation and maintenance costs, and life-cycle net present value. The tools can provide users with planning-level cost estimates and serve as a format for cost-reporting for past, current, and future projects. Very little cost data was available in the public forum, and prolific inconsistencies of supporting details were found in the available cost data. To address this, design assumptions were established for each facility type and professionally prepared cost estimates based on these design assumptions were used. Electives in design, such as plant selection and media depth, also greatly affected costs. To make the user aware of these effects, the model separates each option into line items that can be elected or excluded as appropriate. To facilitate collecting future cost data, best management practice (BMP) designers and builders should use these tools to record actual costs and report them to a clearinghouse such as the BMP Database.

BMP Performance Algorithms for the BMP Selection/Receiving Water Protection Toolbox

The Water Environment Research Foundation (WERF) has funded a project entitled Linking BMP Systems Performance to Receiving Water Protection to Improve BMP Selection and Design. The project includes the development of a modeling tool called the BMP Selection/Receiving Water Protection Toolbox (Toolbox). After selecting an initial representative list of water quality parameters and stormwater BMPs, algorithms for the BMP Module of the Toolbox were evaluated and compared. Three general types of algorithms were considered for BMP performance modeling: (1) hydraulic algorithms - these determine the volumes captured, stored, and bypassed by the BMP, (2) hydrologic algorithms - these determine the volume losses within the BMP due to infiltration and evapotranspiration and/or use (in the case of cisterns), and (3) treatment algorithms - these determine the concentration reductions provided by the BMP. Based on applicable unit treatment processes, available performance data, and desired level of user input requirements, BMP modeling approaches have been recommended.

Analyzing the Impacts of a Retrofit Detention Basin Flow Control Strategy on Biodiversity in an Urban Stream System

Protecting biological diversity of a stream system while supporting development and growth is critical to sustainability. To this end, the City of Lenexa, Kansas has embarked on the Eastern Lenexa Detention Study. The study focuses on 1017 acres (1.6 sq. mi.) of Little Mill Creek watershed in Lenexa, Kansas, a Kansas City suburb. Little Mill Creek drains to Mill Creek, which is listed as an impaired water body under section 303(d) of the Clean Water Act. Water quality impairment parameters include chloride, fecal coliform bacteria, and biology. This study examined whether modifying 16 existing detention basins to capture and detain smaller storm events would significantly increase biological diversity and reduce erosion potential within Little Mill Creek. The study used biological data and discharge measurements collected at 11 sites within and near Little Mill Creek to develop linkages among hydrologic and biologic metrics.

Hydrological impacts evaluation of pervious pavement based on a storm water management model

Hydrological performances of pervious pavement were examined at a site within Little Mill Creek watershed in Lenexa, Kansas, USA using three hydrological indicators of peak flow rate, total runoff volume and runoff coefficient based on the 2-yr, 10-yr and 100-yr design rainfall events and a continuous ten year hourly rainfall record. Three scenarios of undeveloped, developed with no runoff control and developed with pervious pavement controlled were introduced to represent how much pervious pavement can mitigate development impacts and replicate undeveloped condition. Environmental Protection Agency (EPA) Stormwater Management Model (SWMM) was used to simulate the three scenarios due to its advantage in representing different land use scenarios. Results showed that pervious pavements offer significant stormwater management and peak flow rate, total runoff volume and runoff coefficient are even smaller than those of undeveloped condition. Its deduced that pervious pavement has great potential in replicating the undeveloped runoff through appropriate size and design.

Surface temperature and heat exchange differences between pervious concrete and traditional concrete and asphalt pavements

Permeable pavement has been used successfully for many years to provide stormwater control and has recently become a common feature in low impact development (LID) projects. Recently, the potential benefits of pervious concrete over traditional impervious concrete and asphalt pavements for reducing surface temperatures, sequestering carbon, improving health of adjacent trees, and more have been identified. Isolated studies have quantified these benefits for a select few cases. Additional studies are needed to continue to expand the knowledge base of the benefits of pervious concrete beyond stomrwater management. This paper addresses this need by presenting results quantifying the impact of pervious concrete on surface temperatures in semi-arid urban environments. Surface and internal temperatures were monitored at a new pervious concrete site, an adjacent traditional concrete site, and a traditional asphalt pavement site. The results from the summer of 2009 showed a significant reduction of surface temperature at the pervious concrete site compared to the asphalt site. Interestingly, as the monitoring moved into June the traditional concrete site became shaded, providing a comparison between pervious concrete and shaded traditional concrete. The surface temperatures were very similar, leading to the conclusion that pervious concrete may serve as a UHI (urban heat island) mitigation measure equivalent to shading of traditional concrete. The results of the surface and internal temperature monitoring of the pervious concrete were used to calibrate and validate a numerical heat flux model. The model was used to analyze the relative impact of pervious concrete aggregate selection and thermophysical properties on surface temperatures, heat exchange to the earth, and heat exchange with atmosphere.

Examining the Impact of Detention Basins on Floodplains in Lenexa, Kansas

The impacts of stormwater controls within headwater streams are rarely accounted for when Federal Emergency Management Agency floodplains are delineated even though stormwater controls, and specifically detention ponds, are often designed to control peak discharges from 10-, 25- or 100-year runoff events. Inconsistency in the maintenance of stormwater controls impacts their reliability for control of peak discharges, but it is hypothesized that if these facilities are properly and consistently maintained, their impacts on floodplains should be considered in their delineation. The City of Lenexa, Kansas is currently undergoing a study to develop a strategy for maintaining and improving existing detention facilities with the goal of refining floodplain boundaries, providing long-term stream stability, and improving water quality. As part of this project, an assessment was conducted to inventory existing detention basins within the study area by reviewing aerial/topographic mapping and as-built information combined with a field reconnaissance. It was found that a significantly greater number of basins existed within the study area than was originally anticipated. The impacts of the detention basins were evaluated using HEC-HMS and HEC-RAS models to better understand the function and necessity of the existing facilities, to determine the level of service of the existing basins and to determine their combined impacts on receiving streams from both a flooding standpoint and a geomorphologic standpoint. For the hydrologic analysis, an existing HEC-1 model was converted to a HEC-HMS model, and then modified to provide storage nodes that reflect the existing detention ponds. For the hydraulic analysis, HEC-RAS models were used to simulate the 2, 10, and 100-year design storms using the peak flow rates estimated by the HEC-HMS model. These analyses will be used to create a detention basin strategy that maximizes flood reduction potential for the 10 and 100-year design storms through improved maintenance of existing detention basins, retrofitting of existing basins, and the identification of locations that would be beneficial for new detention basins.

A fresh design outlook: Update of the WEF/ASCE manual of practice "design of urban runoff controls"

The Water Environment Federation (WEF) and the American Society of Civil Engineers (ASCE) are collaborating in the development of the Manual of Practice (MOP) “Design of Urban Runoff Controls” as an update to WEF’s MOP 23 and ASCE’s Manual and Report on Engineering Practice 87. The focus of the manual will be to provide a defensible technical document that addresses the design and maintenance of stormwater controls, for new development as well as retrofit and redevelopment. The MOP provides a contemporary outlook based on recent improvements in stormwater management, for example: • Emphasis will be placed on the combined hydrologic, water quality, geomorphic and ecologic effects of urban drainage systems and integration of these considerations in site planning and design. • Sizing guidance will be based on unit process principles. • A taxonomy of runoff controls will be provided to unify terminology • Numerical design examples will be provided for each of the featured controls. Two audiences are identified: 1) Engineers designing stormwater management infrastructure for developers, and reviewers of those designs; and 2) non-technical government officials who need to know what to ask their engineering staffs regarding the adequacy of a proposed urban runoff control plan, or the development of stormwater management policies. The MOP will reflect the collective knowledge and experience represented in an international task force of over 100 practitioners including designers, manufacturers, planners, regulators, and academics. Vetted by WEF’s and ASCE’s industry-wide review process, the MOP will be an authoritative reference of the state-of-the-practice in stormwater design.