NCDEQ s New Stormwater Nutrient Calculator The SNAP Tool Version 4.0 DEPARTMENT OF ENVIRONMENTAL QUALITY Trish D Arconte NC-APWA Stormwater Management Division Conference September 19, 2017
What is the Stormwater Nitrogen and Phosphorus (SNAP) Tool? Spreadsheet calculates nutrient runoff export from a development site Models nutrient reductions by Stormwater Control Measures Based on Jordan-Falls Tool v2 which succeeded Tar-Pamlico Tool which replaced Neuse calculation method Proposed for Jordan & Falls rules compliance - New and Existing Development Stormwater Designed for usability across the State Department of Environmental Quality 2
Why Use the Tool? Document treatment of new development runoff Determine site-specific treatment targets, buy-down targets, and nutrient credits/debits Track new development nutrient export even if reductions are not required Document existing development retrofit treatment for rule compliance, grant reporting, watershed restoration Early in concept design evaluate alternative SCMs and alternative land use scenarios Stormwater retrofitting test undersizing and oversizing scenarios Easy data collection platform for project info Department of Environmental Quality 3
How does the Tool work? Site nutrient export calculated with Simple Method Annual runoff volume * landcover-specific nutrient concentrations Aggregated into annual load N and P (lb/ac/yr) SCM nutrient treatment Inflow partitioned through SCM to hydrologic fates Effluent - fixed concentrations, SCM-specific Optional input data and decision tree determines watershed-specific treatment targets, buy-down targets, and load balance (credits/debits) Department of Environmental Quality 4
What s New? All SCMs that have Minimum Design Criteria Use SCM design variants and HyPerTool outputs Custom SCMs and land covers Reports under/over-treatment vs. loading rate targets Determines, reports watershed-specific compliance Automated Nutrient Offset reporting form Online GIS map to find your watershed, delivery zone Export data to other spreadsheet or database Simpler set of land cover types with clear definitions All background calculations visible to user Department of Environmental Quality 5
What s Improved? Refined science on land cover export values Refined SCM inflow partitioning, effluent concentrations Correct calculation of SCM under- and oversizing Explicit retrofit credit calculation Applicable across entire state Easier to use, simpler layouts, better print formats Better error checking and user feedback Extensive testing and bug fixes Greatly expanded, revised User Manual No more Master-Client confusion Clearer SCM and catchment routing Department of Environmental Quality 6
Nutrient SCMs Now Available for New Development and Stormwater Retrofit Previously available: Bioretention w/ or w/o IWS Permeable Pavement Wet pond Stormwater wetland Sand Filter Rainwater Harvesting Green Roof Level Spreader Filter Strip Grassed Swale Dry Pond New in SNAP 4: 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 StormFilter Silva Cell w/ or w/o IWS Custom SCMs Department of Environmental Quality 7
8 Uses: Document New Development Runoff Treatment
Uses: Determine Site-Specific Treatment Targets, Nutrient Credits/Debits 9 9
10 Uses: Alternative Land Use Scenarios
11 Uses: Alternative SCM Scenarios, Under/Oversizing
12 Uses: Track New Development Nutrient Export When Treatment Not Required
13 Uses: Easy Data Collection of Project Info
14 SCM & Catchment Summaries
15 Detailed SCM Summaries
Tool Release and Future Development In review by Department available Fall 2017 Check website for a draft version & user manual: https://deq.nc.gov/about/divisions/waterresources/planning/nonpoint-sourcemanagement/nutrient-offset-information User Listserv Tool training course NCSU BAE, Raleigh Nov 2017 Future updates: testing modeling runoff with modified Curve Number Method Contact Trish D Arconte for questions and updates: trish.darconte@ncdenr.gov 919-807-6439 Department of Environmental Quality 11
Live Demo & Questions? Department of Environmental Quality 17