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106 11.1 Objectives Highway-drainage engineers and environmental profes- sionals require a straightforward and simple-to-apply method for evaluation of potential BMP and LID strategies for management of stormwater runoff. The main purpose of NCHRP Project 25-20(1) has been to provide results and examples of a method that has been developed and docu- mented by the project team. This summary includes a brief description of the critical elements of the project and key conclusions and recommendations for future research efforts. The principal investigators believe these conclusions and recommendations will be valuable for evaluation of BMP and LID facilities and for the successful integration of LID into linear transportation projects from the perspective of water-quality and hydrologic and hydraulic design and analysis. Section 11.2 of this report includes a brief sum- mary and discussion of key hydrologic considerations. Section 11.3 includes the conclusions of the research and recommendations for future research efforts. In most instances, the term âBMPâ is used to describe practices for control of stormwater and other runoff from highways and urban areas. 11.2 Summary of BMP Evaluation Methodology BMP evaluation methodology, outlined in Chapter 9, is holistic (encompassing all aspects of the problem) and uses the following three procedures: ⢠Practicability analysis. This provides an assessment of crit- ical selection factors (e.g., reliability, safety, aesthetics, costs, and maintenance). The practicability analysis is presented in detail in the Guidelines Manual, but some elements of this analysis are also incorporated into Chapters 5, 6, and 7 of this report. ⢠Performance analysis. This is based on monitored BMPs. The procedures and results are presented in Chapter 8 of this report. ⢠Hydrologic analyses. These are the methods used to evalu- ate runoff treated and runoff bypassed and to conduct a frequency analysis of design parameters (e.g., volume and peak flow) on a regional basis. These methods are described in Chapter 10, and hydrologic analysis related to BMP per- formance is described in Chapter 8. The hydrologic evaluation addresses the first two of the three BMP performance questions presented by Strecker et al. (2001): ⢠How much runoff is prevented, i.e., disposed of, on-site? ⢠How much runoff that does occur is captured/bypassed by the BMP? ⢠What is the effluent quality of the treated runoff? The third performance question is addressed through the use of EMC performance evaluations that are discussed in Chapter 8. Simulation modeling (see Chapter 10) can be used to provide both site-specific and regional analyses for runoff prevented (i.e., runoff prevented or eliminated by a BMP) and runoff bypassed through the use of long-term, continu- ous simulation. Several figures of Sections 10.3 and 10.4 provide examples of design (sizing) guidelines for capture of any desired percentage of runoff. Similar curves could be developed for standard sets of imperviousness to reflect various highway pavement per- centages and for multiple locations. Further details, as well as applications to 30 U.S. locations, are presented in Chapter 7 of the Guidelines Manual. The following design criteria were used in the analysis: ⢠Design for a specified peak flow that enters and is processed through or potentially bypasses the BMP. C H A P T E R 1 1 Summary, Conclusions, and Recommendations
107 ⢠Design for a specified loss by infiltration and ET for an overland flow BMP. ⢠Design of an off-line BMP for capture of a specified volume of runoff, for a specified drawdown time. This design incorporates the trade-off between the goal of longer retention of stormwater and the goal of having storage available for succeeding storm events. ⢠Design of an on-line BMP for capture of a specified volume of runoff, for a specified drawdown time. This design depends on sedimentation theory, i.e., to evaluate the trade-off between greater capture with a high release rate and greater TSS removal with a low release rate. Design results can be presented on the basis of either percent removal of TSS (based on treatability, e.g., particle size distribution) or on TSS effluent EMC (event mean concentration). The results of this analysis are used to derive the BMP/LID selection and conceptual design methodology applied in the Guidelines Manual and outlined in Chapter 9. The method- ology includes the following: 1. Problem definition; 2. Site characterization; 3. Identification of fundamental process categories; 4. Selection of BMPs, LID Elements, and other treatment options; 5. Practicability assessment of candidate treatment systems; 6. Sizing the conceptual BMP design; and 7. Development of performance monitoring and evaluation plan. The following conclusions and recommendations are drawn from this research report, the Guidelines Manual, and the LID Design Manual. 11.3 Conclusions and Recommendations Conclusions and recommendations are presented here grouped into four topic areas: BMP/LID Design and Imple- mentation, Monitoring Needs, Modeling Needs, and General. 11.3.1 BMP/LID Design and Implementation 1. Conclusion. Fundamental unit processes of environmen- tal engineering may be applied along with empirical data to significantly improve the evaluation and selection guidance for BMP and LID facilities. Treatment of stormwater, like treatment of water and wastewater, relies on hydrologic/hydraulic and physical, biological, and chemical operations. The difference in application lies mainly in the different characterizations of stormwater and water for water and wastewater treatment. For some operations and processes, empirical data must still be employed, but hydrologic/hydraulic performance of BMPs, as well as settling performance, can be well repre- sented by unit processes. For predicted pollutant concen- trations and loadings, a combination of unit-process and empirical-data approaches can be used to significantly improve selection and design guidance. Recommendation. The drainage engineer should use fun- damental UOPs to guide his/her selection of treatment systems for control of stormwater. A particular advantage is the focus on UOP selection based on specific targeted pollutants, as opposed to a âone size fits allâ approach with typical BMP performance data. It is also recommended that guidance and requirements for stormwater BMPs based on UOP approaches be combined with empirical approaches as they have been in this project. Adhering to simple design rules only, such as using a 24-hr precipita- tion analysis for sizing and considering all BMPs equal, will not result in meeting water-quality goals in most cases. 2. Conclusion. LID concepts are already an unplanned com- ponent of highway designs with open drainage systems that encourage stormwater infiltration at or near the point at which the precipitation occurs. The performance of these systems can be enhanced significantly by incorpo- rating straightforward LID methods such as additional ET, surface roughening, and enhancement of soils and vegeta- tion to promote infiltration. Recommendation. Use LID methods to maximize on-site control of stormwater, typically by infiltration, for new and existing conveyance systems (a retrofit for the latter). Show the result in terms of the proportion of the highway right-of-way that controls stormwater on site as a way of demonstrating to regulators that the highway control sys- tem is achieving a high degree of control. 3. Conclusion. State and local governments often apply site- development BMP regulatory approaches to highways and require linear projects to use site-development BMPs. Although this approach may be convenient from a regula- tory standpoint, in many cases, the BMPs are less effective or not efficient. Moreover, they are sometimes impossible to design according to the site-development criteria, which results in significant design modifications or waivers. Recommendation. The highway and regulatory communi- ties should work together to develop a consistent and real- istic set of BMP design criteria that will meet water-quality and drainage standards for control of wet-weather impacts.
108 4. Conclusion. There are limits to the effectiveness and effi- ciency of using a limited set of BMPs within the right-of- way to address water-quality impairments. Furthermore, different agencies within a state may adopt BMP strategies (e.g., regional, end-of-pipe, and LID) that are exclusive of each other. Effective structural and nonstructural tech- niques outside of the right-of-way may be a better use of resources to address known impairments. Recommendation. Hybrid approaches that combine on- site and off-site strategies, such as stream stabilization, wetland restoration, and stormwater banking, should be explored. Within the context of the BMP/LID stormwa- ter management framework, agencies should develop common metrics, or policies, for BMPs and other strate- gies that use trading approaches (as is done for some NPDES permitting) to mitigate local wet-weather impacts by more intensive controls elsewhere. This can be used to develop more flexible and effective regulatory schemes. 11.3.2 Monitoring Needs 1. Conclusion. Characterization of stormwater for pur- poses of evaluating its treatability depends most strongly on settleability data for pollutants associated with partic- ulates, that is, a frequency distribution of particle size/specific gravity or frequency distribution of settling velocities (see Section 4.5). Currently, such data are rarely available; however, they are essential for design of effi- cient control strategies, especially treatment trains. Recommendation. Treatability data should be collected from the tributary catchment (e.g., highway) prior to detailed drainage design (but see also next conclusion/ recommendation). Collected data should include, at a minimum, EMCs of all pollutants of concern and their speciation and particulate solids characteristics and especially particle size distribution. Intra-event water- quality data can also be used to identify the presence or absence of a first flush and what pollutants are in the first flush. A related need is to identify and evaluate accurate and applicable methods for monitoring the particle size distribution of suspended sediment con- centrations. 2. Conclusion. The characterization and treatability data discussed in the recommendation above will require extra effort and expense to collect (and for a new high- way would be impossible to collect). This is not likely to occur on a voluntary basis because transportation agen- cies have limited resources and only minimal standards for monitoring are currently required by permits. Recommendation. Collect regional treatability data that are representative of combinations of soils, land use, and highway traffic. Make these data available for design as a default to collection of site-specific treatability data in every case. This might be done by state DOTs or regula- tory agencies. 3. Conclusion. The collection of intra-event stormwater monitoring data within BMPs is relatively rare (e.g., based on the International BMP Database). Although 11 candi- date sites were identified during this project (see Section 8.4), none of them included monitoring of all variables necessary to characterize the unit operation mechanisms by which water quantity was reduced and pollutants removed as a storm was routed through the BMP. Recommendation. Mechanistic understanding of BMP performance can only be obtained through funding of research that fully instruments and monitors a set of BMPs of different kinds (e.g., ponds, detention, and swales) so that within-storm analyses can be completed. 4. Conclusion. In spite of the lack of fully monitored BMPs in the sense just described, several studies offer quantity and quality performance data sufficient to support BMP selection guidance. These include studies entered into the International BMP Database as well as the comprehen- sive Caltrans data sets (many of which have been entered into the International BMP Database) and other studies. Recommendation. Data from the International BMP Database, the Caltrans data sets, and other studies should be analyzed and published in a timely way to support highway and other urban drainage professionals as they seek to refine stormwater control options. Examples include the analysis of Caltrans data by Kayhanian et al. (2003), analysis of International BMP Database data by Barrett (2004a) and Strecker et al. (2004a, 2004b), and analysis of NPDES data by Pitt et al. (2004). 5. Conclusion. The water-quality performance of BMPs is typically characterized by a percent removal of given con- stituents, implying that the effluent EMC is some fraction of the influent EMC. However, especially for particulate- bound pollutants, percent removal usually increases for higher influent EMCs, and effluent EMCs often are essentially functionally unrelated to influent EMCs. In this case, the frequency distribution of effluent EMCs is a suitable way of characterizing performance. Recommendation. BMP quality performance should be evaluated by several measures (see Chapter 8) to ensure that the data are being properly interpreted. The effluent probability method is a good way of representing the quality performance when effluent EMCs are not func- tionally related to influent EMCs. 6. Conclusion. DOTs and other agencies construct water- quality control facilities, but they typically do not engage
109 in postconstruction performance monitoring. The coor- dination between the design and construction process has not been adequately studied. It is not clear whether designs are always constructed to plans, whether materi- als meet specifications, and what effect the construction process has on the BMP. The long-term effectiveness of BMPs is poorly understood. Guidelines for the type, fre- quency, and effectiveness of maintenance programs need to be developed. A continuing need exists for post- construction monitoring data. Recommendation. DOTs and other agencies should monitor at least a subset of different types of BMP/LID facilities to obtain regional performance data for a vari- ety of BMP types and watershed conditions. Such data should eventually be entered into the International BMP Database. Monitoring should include maintenance needs and operation and maintenance (and construction) costs. Costs evaluated should represent the net cost dif- ference for the project, including LID measures in new or expansion projects. Many LID techniques can reduce other infrastructure needs. 7. Conclusion. Monitoring that does occur, valuable as it is, is usually for new control facilities, often at new con- struction sites. However, DOTs and other agencies are often called upon to provide water-quality retrofits for existing runoff locations. A retrofit project may not be designed to the optimal size or location that is specified in a design manual. Recommendation. Retrofit facilities should be monitored to evaluate performance under constrained (e.g., space and fixed upstream configuration) settings and resources. 8. Conclusion. BMP performance data for facilities in oper- ation during the winter in cold climates are relatively uncommon. Recommendation. Additional cold-climate BMP per- formance data are needed. In particular, data are needed on the periodic melt-runoff events that do occur, even during the middle of winter. 9. Conclusion. Performance of storage facilities for mul- tipurpose objectives of flood control and water-quality control depends heavily on the design of the outlet structure. Recommendation. Innovative hydraulic designs of such structures should be widely published by agencies responsible for their design. Attainable storage-discharge rating curves should be included that promote extended detention for water-quality control while releasing flood volumes within prescribed drawdown time limits. Design templates should also be provided. 10. Conclusion. Site-specific hydrologic data are needed for efficient (i.e., cost-effective and within performance constraints) design of BMP and LID facilities. Such data include infiltration rates, ET values, soil moisture and bulk density, losses in nonconcentrated overland flow sit- uations, and precipitation records. The goal is to make the best water balance estimates possible. Thus, even if EMC reduction is subject to great variability, some insur- ance of protection of receiving water may still be obtained through hydrologic source controls, whenever it is possible to significantly reduce runoff volumes. Recommendation. Measure infiltration rates (e.g., with a double-ring infiltrometer) on-site, and use the nearest precipitation and ET records available. Because soil char- acteristics are notably heterogeneous, perform the rela- tively inexpensive infiltrometer tests at enough locations to characterize the catchment and/or BMP/LID facility. Postconstruction infiltration data should also be collected to check for the need for soil amendments or tilling. 11. Conclusion. Infiltration estimates for BMP and LID facilities depend not only on soil type and land use but on the nature of disturbances and construction in the vicinity of the project, especially near highways. Although infiltrometer and other hydrologic data are sel- dom collected during highway construction, geotechni- cal data on compaction, grain size distribution (sieve analyses), bulk density, and other soil properties often are collected. Recommendation. Research is needed to relate data commonly collected at construction sites to hydrologic data needed to assess BMP/LID performance. The stud- ies by Pitt et al. (1999, 2001) are examples of addressing such needs. 12. Conclusion.The lack of consistent precipitation records at 15-min intervals and 0.01-in. resolution hampers contin- uous modeling of small, flashy, highly impervious catch- ments such as highways. The available 15-min, 0.1-in. resolution data are insufficient to evaluate systems with times of concentration and water parcel travel times that are often less than 15 min. Microstorm peaks can be missed when rain gauges tip only at every tenth of an inch. Recommendation. Encourage the National Weather Service and other agencies to record precipitation data at 15-min or more frequent intervals at the 0.01-in. resolu- tion because such records would be very useful to drainage engineers. The 0.01-in. data at 5-min intervals of some regional networks (e.g., Portland, Oregon) are much better for assessment and design. 11.3.3 Modeling Needs 1. Conclusion. Regulatory agencies typically rely on single- event modeling techniques (e.g., SCS methods TR-55,
110 TR-20) or less-sophisticated methods (e.g., Rational Method) to determine BMP effectiveness. Continuous simulation models, although they are the most complicated analysis option, offer great advantages (that the project team believes to be essential) related to annual perform- ance estimates of BMP/LID facilities. For instance, percent control of annual runoff volume can be estimated directly from such models, which is not possible with any level of accuracy with a design storm approach. Continuous sim- ulation models are well documented and readily available to the engineering community. Recommendation. Highway-drainage engineers and related professionals should use state-of-the-art tools to refine their design methods. A bridge between the design storm approach and the continuous modeling approach should be developed to ease the transition to the latter. Continuous models can be incorporated into spreadsheet or other decision support systems along with preprocessed local precipitation data and other local data. 2. Conclusion. While very good hydrologic and hydraulic continuous simulation models suitable for highways and urban areas are available in the public domain (e.g., HSPF, HEC-HMS, and SWMM), these models are not as capable at simulating most treatment processes as models designed specifically for simulation of fundamental treat- ment processes in wastewater treatment plants. Moreover, often in hydrologic and hydraulic simulation models (including proprietary stormwater models) water quality is not simulated at all (e.g., HEC-HMS). The urban/ drainage engineer is often required to simulate BMP qual- ity performance and needs reliable tools for this purpose. Recommendation. Simulation models of the type men- tioned above should be enhanced for the urban and high- way-drainage engineer for more accurate simulation of physical, biological, and chemical unit operations within water-quality control facilities for stormwater and urban runoff in general. If unit process approaches are not well- enough documented for the problem at hand, then more refined empirical approaches should be included. 3. Conclusion. The fate and transport of sediment as stormwater passes through a BMP is critical for evaluation of the removal performance of most BMPs for many parameters of concern. This is particularly true for treat- ment trains, in which upstream devices may perform the bulk of the particulate removal. Recommendation. Stormwater models need to be enhanced for better simulation of scour, deposition, and transport of sediment in urban and highway settings. Par- ticulates should be tracked as they progress through a treatment train. 4. Conclusion. The output of continuous simulation models often includes only long-term hydrographs and pollutographs with some additional statistical sum- maries. The output of such models could easily be cou- pled with optimization techniques that can seek least-cost control strategies within specified constraints. The spreadsheet model of Heaney and Lee (2006) demon- strates such model integration within a simplified simu- lation/optimization framework. Recommendation. Enhance continuous simulation mod- els by incorporating optimization techniques directly into the models so that the optimizer can direct the simulation trials toward the best solution. 5. Conclusion. Generalized performance results for BMPs as a function of hydrologic inputs have been produced as part of this study (see Chapter 7 and Appendix C of the Guidelines Manual). However, regionalization was based solely on meteorological parameters such as rainfall depth, duration, and interevent time (Driscoll et al. 1989). Regionalization of runoff quantity and quality results should be based on a coupling of meteorological charac- teristics and hydrological and water-quality characteristics of the catchment. Recommendation. Research should be performed to develop regionalization or other clustering parameters based not only on rainfall but on catchment characteris- tics (e.g., time of concentration), residence time in BMP storage, soil types, traffic density/type, etc., as these catch- ment characteristics are determined to be applicable for this purpose. Selection of MIT for separation of quantity and quality events can also depend on these factors. 11.3.4 General 1. Conclusion. The authors of this report have observed something of a separation between water-resources and environmental professionals in the highway arena and water-resources and environmental professionals in the broader urban setting. Highway professionals communi- cate primarily through the annual Transportation Research Board conference and in the Transportation Research Record, whereas similar professionals within the urban drainage community tend to communicate through the American Society of Civil Engineers (ASCE) and its conferences and journals. (In actuality, both sets of pro- fessionals engage more broadly in professional societies and activities than implied here.) Both sets of profession- als produce an extensive amount of âgray literatureâ (e.g., professional reports) that may or may not see wide dis- semination outside of these professionalsâ immediate community. Highway research is often funded by high- way-related agencies such as DOTs and the NCHRP,
111 whereas the USEPA has funded much of the stormwater and urban flows research applicable to the broader urban setting. Highway engineers may find it difficult to deviate from AASHTO standards, even when such deviation is likely to lead to improved stormwater control. Recommendation. Highway engineers and drainage and water-quality professionals in similar urban settings should work toward better communication through common meetings, common journals, and broader acceptance of techniques developed outside their narrower professional circles. In this way, innovation developed within the broad community of water resources and water-quality profes- sionals can benefit all practitioners. Additional research needs are listed in the final chapter of the Guidelines Manual and in Strecker et al. (2005).
