Overview of the Stormwater Nitrogen and Phosphorus Accounting Tool - PowerPoint Presentation

Overview of the Stormwater Nitrogen and Phosphorus Accounting Tool SNAP v4 Department of Environmental Quality Trish D’Arconte Water Quality Committee Meeting | October 9, 2018

What is the SNAP Tool?2 Department of Environmental Quality Latest compliance accounting method for nutrient stormwater rules Estimates project-scale runoff N, P loading before and after stormwater treatment Required for Falls New Development beginning April 2019 Required for Neuse, Tar, Jordan? following rules readoption Available for DMS alternative buffer mitigation credit statewide

Evolution of NC Stormwater Nutrient Tools Neuse - 1999Runoff N export (lb N/ac/ yr) from development via (literature) land cover exports weighted by acres% N reduction via 7 stormwater BMPsTar-Pamlico - 2003 NCSU - Simple Method spreadsheet tool, N and PSame BMP % reduction approachJordan-Falls - 2010 Revised Simple Method, 10 SCMs NCSU precedent – SCM fixed effluent concentrations New and Existing Development Stormwater rules SNAP – 2018 DWR w/ NCSU, DEMLR, stakeholder input Scientific advances, added functions, 21 SCMs, bug fixes 3 Department of Environmental Quality

How Do These Tools Work?Developed land covers, site scale Annual stormwater runoff volume x [nutrient] = export rates (lb N or P/ac/yr )Weighted by land cover differencesEarly methods – BMPs simple % N, P reductionsLater methods - SCM volume reductions, fixed effluent concentrationsSNAP - (optional) compares results to rule-specific load targets, buy-down targets, gives load balance (credits/debits) 4 Department of Environmental Quality

Hydrologic Fates Concept 5

SNAP Improvements Over Jordan-Falls ToolRefined science on land cover [N], [P] values SCMsExpanded to 21Variable design flexibility added to 4 Under/oversizing flexibility addedCapture advances in design, performance scienceAll-new rules compliance check New Development Stormwater Determines, reports watershed-specific complianceReports under/over-treatment vs. loading rate targets Existing Development Stormwater – retrofit credit calc (continued) 6 Department of Environmental Quality

SNAP Improvements (continued)Applicable statewide (precipitation, soils) User-friendliness, quality controlEasier to use, simpler layouts Easy to share results, export to a databaseAutomated reporting form - nutrient offsetImproved error checking, user feedbackMany bugs found and fixed Greatly expanded & revised User Manual7 Department of Environmental Quality

Example Under- and Oversizing Equation in SNAP Department of Environmental Quality

Example Project Info Data Entry Sheet 9 Department of Environmental Quality Dropdown menus help minimize data entry errors Online map helps you get watershed info from an address Buttons on every page to clear or print forms Cell colors help the user tell what’s required from optional

Online Clickable Map 10 Click anywhere to find your watershed, subwatershed, and delivery zones Latitude and longitude of map center visible to user (offscreen in this example) Search for your site with an address Named points are precipitation stations Average annual precipitation shown with colors

Example Land Cover Data Entry Sheet 11 Department of Environmental Quality Custom land covers allow for special site conditions Consistent look and feel across sheets Red text indicates potential data entry errors “Flyover” orange triangles show popups with definitions and other info

Example Results Sheet 12 Green text shows when you’ve met buy-down targets Blue text draws attention to most important results Project summary shows how much over- or undertreatment with selected SCMs Nutrient Offset form automatically filled in from treatment balance

Nutrient SCMs Now Available forNew Development and Stormwater Retrofit 13 Department of Environmental Quality Previously available:Bioretention w/ or w/o IWS Permeable Pavement Wet pond Stormwater wetland Sand Filter Rainwater Harvesting Green Roof Level Spreader – Filter StripGrassed SwaleDry Pond New in SNAP: Bioretention variants Permeable Pavement variants Pollutant Removal Swale variants Sand Filter variants Infiltration Systems Disconnected Impervious Surface Floating Wetlands for Wet Ponds LS-FS with Virophos ® Silva Cell ® w/ or w/o IW S Custom SCMs

Expanded Set of Uses New Development – project designerEarly in design – evaluate alternative site design scenarios, alternative SCMsDetermine site nutrient credits/debits vs. rule treatment & buy-down thresholdsNew Development – local government Document site load reductionsTrack site nutrient export even where reductions not requiredExisting DevelopmentRetrofitting – test undersizing, oversizing scenariosDocument retrofit treatment – for rule compliance, grant reporting, watershed restorationEasy data collection platform for project info 14 Department of Environmental Quality

SNAP Development Process Worked closely with DEMLR SPU, NCSU Bio & Ag Engineering, 2015 - 2018Coordinated with DEMLR Minimum Design CriteriaDeveloped systematic SCM crediting approachDeveloped under- and oversizing design, credit standards NCSU literature search for data, used for calibrationUpdated land cover nutrient concentrations, SCM hydrologic partitioning, effluent concentrationsRequests from local gov’ts on tool functions Goal: a user-friendly tool using best stormwater science, greatly expanded options & flexibility, helps you navigate complex regulations, usable statewide15 Department of Environmental Quality

SNAP Vetting ProcessNSAB reviews Fall 2016, Winter 2017 30-day informal comment periods Fall 2016, Spring 2017Most comments were technical / bug reports Strong interest from local governments, consultants, DMSReview by DWR Director16 Department of Environmental Quality

Questions? 17 Department of Environmental Quality For more information: https://deq.nc.gov/about/divisions/water-resources/planning/nonpoint-source-management/nutrient-offset-information#stormwater