Demonstration & Monitoring of Effective Level Spreader Designs
Final Report: Demonstration & Monitoring of Effective Level Spreader Designs A 319(h) project sponsored by NC DENR – Division of Water Quality FY 08 – NC DENR Project # EW08010 Contract Period: 8/15/07 – 4/30/10 Total Project: $169,133 Requested 319 (h) funds: $99,974 Principal Investigators: William F. Hunt, III Associate Professor and Extension Specialist, NCSU Biological & Agricultural Engineering Deanna L. Osmond Professor and Extension Leader, NCSU Soil Science and William G. Lord and Mitchell D. Woodward, Area Environmental Agents, NCSU Cooperative Extension Report Submission Date: June 1, 2010
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Acknowledgements This project was funded under an EPA Section 319(h) grant administered by the North Carolina Department of the Environment and Natural Resources’ Division of Water Quality. Several entities provided tremendous support during this project included the Town of Apex (Jessica Bolin), the Town of Louisburg, and Wake County Schools. The authors are particularly grateful for the efforts of Mr. Ryan J. Winston, former master’s candidate, who oversaw the research project and was responsible for data collection and interpretation. Mr. Shawn G. Kennedy has again served a critical role as project technician. Mr. Jason Wright and Mr. Jon Hathaway also assisted with the design of both sets of level spreaders. Lastly, Ms. Annette Lucas of NC DENR’s 401 Unit is acknowledged for promoting changes to the level spreader and vegetated filter strip chapters in the NC DENR’s Stormwater BMP Design Manual that incorporated the results of this research.
Table of Contents
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Executive Summary 3 Project Deliverables 4 Project Description 6 Site Description 6 Apex 6 Louisburg 8 Methods 10 Data Collection 10 Data Analysis 11 Statistical Analyses 11 Results and Discussion 11 Soil Properties 11 Storm Event Characteristics 12 Reduction in Event Mean Concentrations 13 Load Reductions 18 Comparison of Effluent Concentrations to Ambient Water Quality 19 Comparison of Effluent Concentrations to Other LID BMPs 20 Research Conclusions 21 Overall Outcomes & Conclusions 22 References 22 Budget 25 Appendices (attached under separate cover) 26 Appendix A: Engineering Drawings for Level Spreaders installed in Apex Appendix B: Engineering Drawings for Level Spreaders installed in Louisburg Appendix C: NCSU Design Bulletin - Level Spreader- VFS Performance and Research Appendix C: NCSU Design Bulletin - Level Spreader - VFS Update: Design, Construction & Maintenance Appendix E: Revised NCDENR Level Spreader – Vegetated Filter Strip Chapter 8
2
Executive Summary The purpose of this project was to demonstrate two types of level spreader – vegetated filter strip systems at two locations: Apex High School in Wake County and the Franklin County EMS facility outside Louisburg. Both projects served as both demonstration and research sites and were featured several times on either tours or during workshops. The level spreaders were selected in locations with stable filter strips that did not appear to have an obvious impediment to flow within the filter strip. Each site had one short and one long filter strip. The short filter strip was comprised of 25 feet turfgrass; the long filter strip was 50 feet, one-half turf the other half wooded. Both sets of level spreaders were constructed in winter of 2007/8. The systems were monitored from March 2008 through March 2009 for both hydrology and water quality. Hydrology measurements proved to be accurate only at the Louisburg site; however, both sites yield water quality data. The data were analyzed until summer 2009. At that point, employees of NC DENR’s stormwater and 401 units began a series of meetings with NCSU faculty discussing recommended changes to the level spreader and vegetated filter strip design guidance. The meetings culminated with a draft design guidance released in early March 2010. Following the release of the design guidance, a series of 5 workshops (2 more than expected due to the high level of interest) were held to train designers and regulators/plan reviewers on the new design standards for level spreader – vegetated filter strip systems. The revised manual and the research associated with this project were the underpinnings of these workshops. Principal findings associated with this project include the following: 1. Both lengths of filter strips reduced runoff volume at Louisburg. The cumulative runoff reduction ranged between 40 and 50%. Neither filter strip was appreciably better than the other. 2. Both lengths of filter strips reduced sediment concentrations and loads. This was expected, because this is what filter strips were originally designed to do. 3. Longer filter strips effectively reduced the concentrations of nitrogen species and phosphorus. Whether this improvement was due in part to wooded cover in the 2nd half of the filter strip, or if it were simply due to the filter strip being twice as long is not known. 4. Both filter strips reduced loads of nitrogen and phosphorus. Load reductions exceeded 40% in all cases (of TN, TP, and TSS). Key changes to NC DENR’s level spreader – vegetated filter strip design manual include the following: 1. Filter strips no longer need to be 50 feet in width; 30 feet is sufficient to “earn” pollutant removal credit. 2. Upslope of grass or other herbaceous system, level spreaders are now sized to need 10 linear feet per 1 cfs of flow. Prior standards had required them to be 13 linear feet per cfs. (In wooded applications, the length was set to 50 feet per cfs.) 3. The TSS removal rate was raised to 40% (it had a sliding scale of 20% to 40%). 4. The TN removal rate was increased to 30% (it had been 20%). 5. The TP removal rate remained at 35%. Perhaps the most obvious result of the study was that level spreader – vegetated filter strip systems were an under-utilized practice by most designers throughout North Carolina. In locations with seasonally high water tables (when other site-scale practices like bioretention and permeable pavement and prohibited due to water tables present near the surface), level spreader- vfs systems
3
have particular application. The revised NC DENR credit associated with pollutant removal should drive the use of LS-VFS systems in the future. A few construction pointers were also gleaned by conducting this demonstration and research project: 1. Concrete level spreaders are able to be constructed level. This reaffirmed guidance from 2007 which suggested this material. 2. Minor transverse slopes in the filter strip limit the ability of level spreaders to provide diffuse flow. 3. In tighter soils, turf-lined blind channel behind level spreaders should be underdrained so that water vacates the blind channel within one day. 4. The entry point of water to the blind channel should not be perpendicular to the level spreader. 5. Ideally, construction sequencing should be (1) overflow swale, (2) filter strip, (3) level spreader/blind channel, and (4) forebay. Working uphill prevents damage to the filter strip, particularly. The findings from this study have been summarized in three documents in addition to the revised Level Spreader – Vegetated Filter Strip chapter in the design manual: 1. A North Carolina Cooperative Extension Bulletin entitled: Level Spreader - VFS Update: Design, Construction & Maintenance 2. A North Carolina Cooperative Extension Bulletin entitledLevel Spreader- VFS Performance and Research 3. A Journal of Irrigation and Drainage Engineering article entitled: “Field Evaluation of Four Level Spreader-Vegetative Filter Strips to Improve Urban Stormwater Quality.”
The former two documents are undergoing final review and will be sent to University Graphics for final editing and publication in Summer 2010. The journal article has been accepted and, while no publication date is set, publication is expected in Winter 2010/2011.
Project Deliverables The project was comprised of six deliverables. All deliverables are listed below. Additionally, a brief description of how each was accomplished is given. 1. A total of four level spreaders will be installed in two municipalities. Two sets of level spreaders were installed at the Franklin County EMS operation center in Louisburg, NC, and the Apex High School in Wake County. Two level spreaders were installed at each location. Each level spreader was approximately 13 feet long. The difference between the two level spreaders at each site was the length of the downslope filter strip. Each site had one filter strip that was 25 feet and completely comprised of turf; the other filter strip was onehalf turf and one-half woods/shrubs. The latter filter strip was 50 feet wide. 2. 3 workshops/ field tours will be held to train the development community, government officials, and environmental advocates. By including two “follow up” workshops held in early May, a total of 5 workshops dedicated solely to level spreader – vegetated filter strip systems were conducted in 2010. They were held at the following locations: Apex (March 16), Apex (May 10), Gastonia (May 5), Hickory
4
(March 17), and Washington, NC (March 25). All but the workshop in Gastonia sold out. A total of 173 design professionals and regulators attended. Additionally, a workshop that focused on both level spreaders and water harvesting systems (entitled: Stormwater BMPs for Shallow Water Tables) was held in Wrightsville Beach on April 23, 2010, and attracted 42 more attendees. 3. A factsheet will be produced and made available on the stormwater group website: www.bae.ncsu.edu/stormwater Two factsheets have been produced, but as of May 13, 2010, they are still in final review within the University system. Both factsheets are expected to be available on-line by mid-summer. Drafts of the two factsheets, entitled: Level Spreader - VFS Update: Design, Construction & Maintenance and Level Spreader- VFS Performance and Research are attached as an appendix. Additionally, a journal article, “Field Evaluation of Four Level Spreader - Vegetative Filter Strips to Improve Urban Stormwater Quality” has been accepted for publication in the Journal of Irrigation and Drainage Engineering. Its publication date has not been set, but is expected to be this upcoming winter. The accepted version of the article is also included as an appendix. 4. Two riparian buffer widths will be examined (20’ and 50’). Peak flow mitigation, runoff volume, nutrient concentrations and nutrient loadings will be evaluated both entering and leaving all 4 level spreaders. An assessment of the performance of four level spreader-vegetative filter strip (LS-VFS) systems designed to treat urban stormwater runoff was undertaken in Louisburg and Apex. At each site, one 7.6 m grassed filter strip and one 15.2 m half-grassed, half-forested filter strip were examined. Monitored parameters included rainfall, inflow to, and outflow from each LSVFS system. A total of 21 and 22 flow-proportional water quality samples were collected and analyzed for the Apex and Louisburg sites, respectively. All LS-VFS systems studied significantly reduced mean TSS concentrations (p
1
Acknowledgements This project was funded under an EPA Section 319(h) grant administered by the North Carolina Department of the Environment and Natural Resources’ Division of Water Quality. Several entities provided tremendous support during this project included the Town of Apex (Jessica Bolin), the Town of Louisburg, and Wake County Schools. The authors are particularly grateful for the efforts of Mr. Ryan J. Winston, former master’s candidate, who oversaw the research project and was responsible for data collection and interpretation. Mr. Shawn G. Kennedy has again served a critical role as project technician. Mr. Jason Wright and Mr. Jon Hathaway also assisted with the design of both sets of level spreaders. Lastly, Ms. Annette Lucas of NC DENR’s 401 Unit is acknowledged for promoting changes to the level spreader and vegetated filter strip chapters in the NC DENR’s Stormwater BMP Design Manual that incorporated the results of this research.
Table of Contents
Page #
Executive Summary 3 Project Deliverables 4 Project Description 6 Site Description 6 Apex 6 Louisburg 8 Methods 10 Data Collection 10 Data Analysis 11 Statistical Analyses 11 Results and Discussion 11 Soil Properties 11 Storm Event Characteristics 12 Reduction in Event Mean Concentrations 13 Load Reductions 18 Comparison of Effluent Concentrations to Ambient Water Quality 19 Comparison of Effluent Concentrations to Other LID BMPs 20 Research Conclusions 21 Overall Outcomes & Conclusions 22 References 22 Budget 25 Appendices (attached under separate cover) 26 Appendix A: Engineering Drawings for Level Spreaders installed in Apex Appendix B: Engineering Drawings for Level Spreaders installed in Louisburg Appendix C: NCSU Design Bulletin - Level Spreader- VFS Performance and Research Appendix C: NCSU Design Bulletin - Level Spreader - VFS Update: Design, Construction & Maintenance Appendix E: Revised NCDENR Level Spreader – Vegetated Filter Strip Chapter 8
2
Executive Summary The purpose of this project was to demonstrate two types of level spreader – vegetated filter strip systems at two locations: Apex High School in Wake County and the Franklin County EMS facility outside Louisburg. Both projects served as both demonstration and research sites and were featured several times on either tours or during workshops. The level spreaders were selected in locations with stable filter strips that did not appear to have an obvious impediment to flow within the filter strip. Each site had one short and one long filter strip. The short filter strip was comprised of 25 feet turfgrass; the long filter strip was 50 feet, one-half turf the other half wooded. Both sets of level spreaders were constructed in winter of 2007/8. The systems were monitored from March 2008 through March 2009 for both hydrology and water quality. Hydrology measurements proved to be accurate only at the Louisburg site; however, both sites yield water quality data. The data were analyzed until summer 2009. At that point, employees of NC DENR’s stormwater and 401 units began a series of meetings with NCSU faculty discussing recommended changes to the level spreader and vegetated filter strip design guidance. The meetings culminated with a draft design guidance released in early March 2010. Following the release of the design guidance, a series of 5 workshops (2 more than expected due to the high level of interest) were held to train designers and regulators/plan reviewers on the new design standards for level spreader – vegetated filter strip systems. The revised manual and the research associated with this project were the underpinnings of these workshops. Principal findings associated with this project include the following: 1. Both lengths of filter strips reduced runoff volume at Louisburg. The cumulative runoff reduction ranged between 40 and 50%. Neither filter strip was appreciably better than the other. 2. Both lengths of filter strips reduced sediment concentrations and loads. This was expected, because this is what filter strips were originally designed to do. 3. Longer filter strips effectively reduced the concentrations of nitrogen species and phosphorus. Whether this improvement was due in part to wooded cover in the 2nd half of the filter strip, or if it were simply due to the filter strip being twice as long is not known. 4. Both filter strips reduced loads of nitrogen and phosphorus. Load reductions exceeded 40% in all cases (of TN, TP, and TSS). Key changes to NC DENR’s level spreader – vegetated filter strip design manual include the following: 1. Filter strips no longer need to be 50 feet in width; 30 feet is sufficient to “earn” pollutant removal credit. 2. Upslope of grass or other herbaceous system, level spreaders are now sized to need 10 linear feet per 1 cfs of flow. Prior standards had required them to be 13 linear feet per cfs. (In wooded applications, the length was set to 50 feet per cfs.) 3. The TSS removal rate was raised to 40% (it had a sliding scale of 20% to 40%). 4. The TN removal rate was increased to 30% (it had been 20%). 5. The TP removal rate remained at 35%. Perhaps the most obvious result of the study was that level spreader – vegetated filter strip systems were an under-utilized practice by most designers throughout North Carolina. In locations with seasonally high water tables (when other site-scale practices like bioretention and permeable pavement and prohibited due to water tables present near the surface), level spreader- vfs systems
3
have particular application. The revised NC DENR credit associated with pollutant removal should drive the use of LS-VFS systems in the future. A few construction pointers were also gleaned by conducting this demonstration and research project: 1. Concrete level spreaders are able to be constructed level. This reaffirmed guidance from 2007 which suggested this material. 2. Minor transverse slopes in the filter strip limit the ability of level spreaders to provide diffuse flow. 3. In tighter soils, turf-lined blind channel behind level spreaders should be underdrained so that water vacates the blind channel within one day. 4. The entry point of water to the blind channel should not be perpendicular to the level spreader. 5. Ideally, construction sequencing should be (1) overflow swale, (2) filter strip, (3) level spreader/blind channel, and (4) forebay. Working uphill prevents damage to the filter strip, particularly. The findings from this study have been summarized in three documents in addition to the revised Level Spreader – Vegetated Filter Strip chapter in the design manual: 1. A North Carolina Cooperative Extension Bulletin entitled: Level Spreader - VFS Update: Design, Construction & Maintenance 2. A North Carolina Cooperative Extension Bulletin entitledLevel Spreader- VFS Performance and Research 3. A Journal of Irrigation and Drainage Engineering article entitled: “Field Evaluation of Four Level Spreader-Vegetative Filter Strips to Improve Urban Stormwater Quality.”
The former two documents are undergoing final review and will be sent to University Graphics for final editing and publication in Summer 2010. The journal article has been accepted and, while no publication date is set, publication is expected in Winter 2010/2011.
Project Deliverables The project was comprised of six deliverables. All deliverables are listed below. Additionally, a brief description of how each was accomplished is given. 1. A total of four level spreaders will be installed in two municipalities. Two sets of level spreaders were installed at the Franklin County EMS operation center in Louisburg, NC, and the Apex High School in Wake County. Two level spreaders were installed at each location. Each level spreader was approximately 13 feet long. The difference between the two level spreaders at each site was the length of the downslope filter strip. Each site had one filter strip that was 25 feet and completely comprised of turf; the other filter strip was onehalf turf and one-half woods/shrubs. The latter filter strip was 50 feet wide. 2. 3 workshops/ field tours will be held to train the development community, government officials, and environmental advocates. By including two “follow up” workshops held in early May, a total of 5 workshops dedicated solely to level spreader – vegetated filter strip systems were conducted in 2010. They were held at the following locations: Apex (March 16), Apex (May 10), Gastonia (May 5), Hickory
4
(March 17), and Washington, NC (March 25). All but the workshop in Gastonia sold out. A total of 173 design professionals and regulators attended. Additionally, a workshop that focused on both level spreaders and water harvesting systems (entitled: Stormwater BMPs for Shallow Water Tables) was held in Wrightsville Beach on April 23, 2010, and attracted 42 more attendees. 3. A factsheet will be produced and made available on the stormwater group website: www.bae.ncsu.edu/stormwater Two factsheets have been produced, but as of May 13, 2010, they are still in final review within the University system. Both factsheets are expected to be available on-line by mid-summer. Drafts of the two factsheets, entitled: Level Spreader - VFS Update: Design, Construction & Maintenance and Level Spreader- VFS Performance and Research are attached as an appendix. Additionally, a journal article, “Field Evaluation of Four Level Spreader - Vegetative Filter Strips to Improve Urban Stormwater Quality” has been accepted for publication in the Journal of Irrigation and Drainage Engineering. Its publication date has not been set, but is expected to be this upcoming winter. The accepted version of the article is also included as an appendix. 4. Two riparian buffer widths will be examined (20’ and 50’). Peak flow mitigation, runoff volume, nutrient concentrations and nutrient loadings will be evaluated both entering and leaving all 4 level spreaders. An assessment of the performance of four level spreader-vegetative filter strip (LS-VFS) systems designed to treat urban stormwater runoff was undertaken in Louisburg and Apex. At each site, one 7.6 m grassed filter strip and one 15.2 m half-grassed, half-forested filter strip were examined. Monitored parameters included rainfall, inflow to, and outflow from each LSVFS system. A total of 21 and 22 flow-proportional water quality samples were collected and analyzed for the Apex and Louisburg sites, respectively. All LS-VFS systems studied significantly reduced mean TSS concentrations (p
