ITEM 10a: WAKE COUNTY HYBRID STORMWATER DESIGN TOOL SUBMITTAL JORDAN LAKE NUTRIANT STRATEGY COMPLIANCE SUBMITTAL SEPTEMBER 10, 2011

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1 ITEM 10a: WAKE COUNTY HYBRID STORMWATER DESIGN TOOL SUBMITTAL JORDAN LAKE NUTRIANT STRATEGY COMPLIANCE SUBMITTAL SEPTEMBER 10, 2011 Wake Cunty Hybrid Tl Wake Cunty is unique in that it has its wn vlume cntrl SW rdinance, lcal and state water supply watershed regulatins and is subject t three state nutrient management strategies in additin t the Swift Creek Land Management Plan. The cmplexity f applying multiple and verlapping regulatins necessitates a streamlining f develpment submittal and review prcesses thrugh the use f a single hybrid strmwater tl. Wake Cunty administers its wn strmwater rdinance fr unincrprated areas f Wake Cunty in additin t a different urban strmwater rdinance fr three municipalities in eastern Wake Cunty. Fr the unincrprated areas, Wake Cunty uses a vlume-cntrl strmwater rdinance with Target Curve Number (TCN) runff vlume limits fr residential develpment. Bth residential and cmmercial develpments adhere t The Neuse Rules strmwater requirements fr peak flw, nutrient management, and riparian buffer rules. Wake Cunty als adpted the Neuse regulatins cuntywide (including within the Cape Fear Basin). Fr strmwater submittals, engineers are currently required t cmplete the Wake Cunty Strmwater Design Tl, which incrprates the Neuse requirements. The tl calculates pre and pst develpment runff, peak flw, nitrgen lading, and target curve number vlume requirements. With adptin f the Falls Lake and Jrdan Lake Nutrient Strategies, it was necessary t adapt the current Wake Cunty tl t include the Falls/Jrdan nutrient requirements. With administratin f three different regulatry watersheds fr review (Falls, Jrdan, and Neuse/Cape Fear areas) it became necessary t streamline develpment submittals int ne cmprehensive tl. Wake Cunty is submitting fr review/apprval a hybrid Strmwater Design Tl incrprating Wake Cunty Unified Develpment Ordinance requirements and nutrient requirements assciated with the Falls Lake Nutrient Strategy, Jrdan Lake Nutrient Strategy and The Neuse Rules. The submitted tl will incrprate requirements as utlined in the Jrdan/Falls Lake Strmwater Lad Accunting Tl User s Manual and will be as prtective as the Jrdan/Falls tl. The hybrid tl will als cmply with The Neuse Rules. All nutrient lad calculatins fr this tl use the same methds used in the Jrdan/Falls tl.

2 2 Wake Cunty Hybrid Tl Cntents The submitted draft tl is cmprised f the fllwing calculatin sheets: Site Data wrksheet DA wrksheets Site Summary wrksheet DA BMP wrksheets BMP Summary wrksheet Calculatins sheet Nte: Behind the scenes calculatin sheets are als hidden t the public n ther tabs. All hidden sheets may be accessed by DENR by right clicking the sheet tabs and selecting Unhide. Only cells in blue (engineer inputs) will have editing privileges fr public distributin. Site Data Wrksheet The Site Data Wrksheet cntains prject infrmatin including but nt limited t: 1. Engineer cntact infrmatin 2. Site Lcatin (Regulatry Watershed, Gelgic Regin) a. Used t determine target N and P Lads and BMP remval rates 3. Develpment Type (Residential/Nnresidential) and Zning a. Develpment type is used t determine Target Curve Number (TCN) applicability and zning assumptins 4. Precipitatin Amunts 5. Strmwater Narrative a. Strmwater Narrative shuld describe the site cnditins pre and pst develpment including a descriptin f site imprvements and prpsed strmwater BMPs. DA1-6 Wrksheets DA wrksheets are designed t accunt fr prject requirements per Wake Cunty UDO standards as well as the Neuse, Falls Lake and Jrdan Nutrient Strategies. All prject infrmatin is entered by drainage area. The tl allws fr up t six drainage areas fr the prject. DA Wrksheets calculate runff, time f cncentratin, peak flw, and vlume t be managed per drainage area (if applicable). Inputs are als used t calculate the site cmpsite curve numbers fr pre and pst develpment and ttal nitrgen (TN) and phsphrus (TP) lading calculatins by drainage area. Ntes and Assumptins fr DA1-6 wrksheets: 1. Site and ffsite land uses Nt all land uses frm the Jrdan/Falls tl were used in the Wake Cunty tl. All residential land uses use the custm lt ptin instead f the general lt sizing methd. Detailed breakdwn f residential land use is required by Wake Cunty UDO fr Target Curve Number (TCN) requirements. Land uses are assigned curve numbers (CNs) adapted frm TR-55 as well as Event Median Cncentratins (EMCs) as defined by the Jrdan/Falls Lake Strmwater Nutrient Lad Accunting Tl User Manual.

3 3 The fllwing additinal land uses have been added t cmply with Wake Cunty TCN requirements: Transprtatin-High Density (right-f-ways, grassed), Transprtatin-Lw Density (right-f-ways, grassed) and Residential-Radway (right-f-ways, grassed). These land uses have the same EMCs as the assciated radways but have differing CNs. Site land uses are used t determine pre and pst develpment CNs and nutrient lading. Since the lads cntributed by the prpsed new develpment activity shall nt shall nt exceed the unit area mass lading rate fr nitrgen and phsphrus [NCAC 02B.0277(4)(a)], site nutrient lading is nly calculated frm site land uses. Offsite and site land uses are used t determine pre and pst develpment runff and peak flw. All pre and pst nutrient lading back calculatins can be fund n the Pre-Pst NP Calcs_Hidden sheet. T view this sheet, right click the sheet tabs and select Unhide. This sheet clsely resembles the WS Calcs hidden sheet in the Jrdan/Falls tl. 2. The Wake Cunty tl uses the Discrete SCS Curve Number Methd fr runff vlume (the Simple Methd is used fr the lading calculatins) and TR-55 Graphical Peak Discharge Methd fr Type II Distributin fr peak flw. Wake Cunty is currently examining the use f the Ratinal Methd and Simple Methd fr runff and peak flw calculatins. Results f a cmparisn between these methds and the CN methds will be used t determine the best methd fr Wake Cunty runff and peak flw calculatins and cmpliance with applicable nutrient management strategy. Ratinal C values will be cnverted frm inputted CNs fr the Ratinal peak flw apprach. The final versin f the Wake Cunty tl will have this issue reslved if DENR is interested in this aspect f the tl. Site Summary Wrksheet SITE SUMMARY wrksheet summarizes the pre and pst runff and peak flw per drainage area based n inputs frm DA1-6 wrksheets. TCN and cmpsite curve numbers fr pre and pst develpment are als calculated and summarized. If the TCN is exceeded, this wrksheet calculates ttal vlume t be managed fr the entire site based n TCN requirements. Nutrient lading rates fr the site are cmpiled and calculated frm DA1-6 wrksheets. Target rates fr nutrients are als summarized based n regulatry watershed. Ntes and Assumptins fr SITE SUMMARY wrksheet: 1. There is nly ne engineer input n this sheet. Discnnected Impervius - this area will be used t prvide an adjusted pst develpment cmpsite curve number (CN adjusted ) t allw a credit fr the use f discnnected impervius per Wake Cunty strmwater rdinance. DA BMP Wrksheets DA BMP1-6 wrksheets require engineers t input prpsed BMP infrmatin. BMPs are characterized by sub-basins within the drainage area. Engineers input BMP sub-basin land uses, BMP device name, type, and vlume managed by device.

4 4 Ntes and Assumptins fr DA BMP wrksheets: 1. BMPs within a sub-basin are assumed t be in a series with the same drainage area. Engineers may specify if sub-basins cntribute t ther sub-basins within the drainage area. This cncept is very similar t Catchment Ruting in the Jrdan/Falls tl. The exceptin is that the Jrdan/Falls tl allws fr BMPs in a series t have different drainage areas. The Wake Cunty tl accunts fr this with the sub-basin apprach. Bth tls accmmdate the same site scenaris. 2. Engineers input site land uses by sub-basin. Off-site drainage t the sub-basin is als required (if said drainage is ruted thrugh the BMP). Lading calculatins are nly based n site land uses/vlume. Offsite values are used in additin t nsite values t calculate the required water quality vlume (1 runff). Cnsequently, while site lading is based n nsite land uses nly, required BMP strage vlume includes ffsite drainage t btain BMP remval credits. This cncept differs frm the Jrdan/Falls tl assumptin f including ffsite land uses at built ut cnditins. Wake Cunty has several issues with this assumptin: Very ften in Wake Cunty, develpers chse nt t bypass ffsite drainage. Due t cunty vlume cntrl requirements, the device is ften versized. In many cases, it is mre cst efficient t use this additinal capacity rather than design and cnstruct bypass ptins. Nne f the nutrient strategies Wake Cunty administers (Neuse/Falls/Jrdan) state a requirement that new develpment treat ffsite runff at an assumed built-ut cnditin in additin t the lad frm the new develpment. Additinally, requiring built-ut cnditins assumes that ffsite develpment is nt respnsible fr their wn nutrient lading. The built-ut cnditins assumptin verestimates the lad cntributing t the new develpment dwnstream. [15A NCAC 02B.0265] states lads cntributed by the prpsed new develpment activity shall nt shall nt exceed the unit area mass lading rate fr nitrgen and phsphrus [this cannt be interpreted t include assumed maximum built-ut cnditins fr ffsite drainage]. [15A NCAC 02B.0273] distinguishes ffsite nutrient remval as an ffsite ffsetting ptin that the develper may chse t use r nt. 3. Rainwater Harvesting is a BMP type in the WC tl. Hwever, remval f lading and vlume has nt been added t the tl. Rainwater harvesting will be addressed with resubmittal f this tl fllwing DENR cmments and will fllw guidelines utlined in the Jrdan/Falls User Manual. 4. BMP nutrient calculatins are autmatically calculated frm sheet inputs. All BMP nutrient lading calculatins can be fund n the BMP NP Calcs_Hidden sheet. T view this sheet, right click the sheet tabs and select Unhide. This sheet clsely resembles the BMP Calcs hidden sheet in the Jrdan/Falls tl.

5 5 5. BMPs are required in each DA where pst-develpment peak flw is higher than predevelpment peak flw. Only under special circumstances will a BMP nt be required. BMP SUMMARY Wrksheet BMP SUMMARY wrksheet summarizes the pst develpment and pst BMP runff and peak flw per drainage area based n inputs frm DA BMP wrksheets. Nitrgen and Phsphrus lading fr the site is calculated based n the results frm DA1-6 BMP wrksheets. Prject cmpliance with the Wake Cunty UDO, Falls Lake Nutrient Strategy, and/r the Jrdan Lake Nutrient Strategy is presented n this sheet. Ntes and Assumptins fr BMP SUMMARY wrksheet: All BMP nutrient lading calculatins can be fund n the BMP NP Calcs_Hidden sheet. T view this sheet, right click the sheet tabs and select Unhide. There are n engineer inputs n this sheet and all exceedances frm DA BMP wrksheets will be flagged in red with the final versin f the tl. CALCULATIONS Wrksheet All calculatins and table references are included n this wrksheet. Ntes and Assumptins fr CALCULATIONS wrksheet: Sme links t CALCULATIONS wrksheet can be fund thrughut the ther wrksheets. The final versin f the Wake Cunty tl will prvide additinal links fr calculatin references. Additinal cmments regarding the Jrdan/Falls Lake Strmwater Nutrient Lad Accunting Tl: 1. The Wake Cunty Hybrid Tl has been enhanced since the tl versin submitted fr Falls Lake n 8/10/11. The WC Tl prvided with the Jrdan submittal is the current versin. 2. This submittal des nt have all f the user-friendly features planned fr the final versin. Regulatin red flags and warnings messages, cmplete Wake Cunty rdinance requirements, final frmatting, and engineer testing mdificatins will be added at a later date. 3. The Wake Cunty Hybrid Tl (WC Tl) differs in basic structure frm the Jrdan/Falls Tl (JF Tl). The difference primarily deals with the rganizatin f the site by actual Drainage Areas (DA) instead f the JF Tl Catchments (which may include mre than ne drainage area). The WC Tl drainage areas d nt cntribute runff int ther drainage areas as with the catchments in the JF Tl. Alternatively, the WC Tl des allw fr sub-drainage areas (subdas) t cntribute t ther subdas; hwever these subdas are always within the same DA. Example test scenaris are prvided in this dcument t demnstrate this basic difference. We believe

6 6 the use f actual drainage areas is needed in rder t better suit the type f lw-density develpment that ccurs in Wake Cunty. Additinal assumptins regarding the Wake Cunty Hybrid Tl: 1. The mass metric (lbs) used n the BMPCalcs wrksheet f the Jrdan/Falls tl will nt be used in the Wake Cunty tl. Ms. DeBusk cnfirmed that ultimately these values are nt necessary in the Jrdan/Falls tl (Discussed via telephne with Kathy DeBusk n August 9, 2011). 2. BMP versizing The Wake Cunty tl ensures that each BMP has met the water quality vlume (first flush) by calculating vlume f runff frm the first inch f rain (including any ffsite drainage being ruted thrugh the BMP). If the prvided strage vlume n the DA_BMP wrksheets des nt equal at least this amunt, a red flag is raised. In mst cases, BMPs will be versized (greater than the NC BMP Manual water quality vlume) in Wake Cunty due t additinal BMP design requirements (i.e.: TCN vlume). The Wake Cunty tl des nt assume versizing fr nutrient remval if the BMP is designed larger t meet Wake Cunty vlume standards. Based n a meeting with Kathy DeBusk n July 6, 2011, Ms. DeBusk cnfirmed with Bill Hunt that this assumptin wuld have minimal impact t lading calculatins. Ptential Prblems/Issues with the Jrdan/Falls Lake Strmwater Nutrient Lad Accunting Tl: Nte: Ptential prblems identified belw may affect cmparisn scenaris between the Wake Cunty Tl and the Jrdan/Falls Tl as well as prduce inaccurate results fr users f Jrdan/Falls Tl if nt addressed. 1. Jrdan/Falls tl (JF Tl): Watershed lading value n the BMP Calcs wrksheet (Rws 31 and 42) des nt include the Land Taken by BMP except fr BMP#1 in each catchment. All subsequent BMPs exclude this land use frm the N/P ttals. After speaking with Kathy DeBusk, this is a calculatin errr and Land Taken by BMP shuld be added t all f the BMP lads. The Wake Cunty tl has Land Taken by BMP lading entering all BMPs. If nt addressed, this issue has a ptential fr calculatin errrs fr all develpment scenaris using the JF Tl. 2. JF Tl BMP Calcs BMP#3 (in all catchments) _Rws 55 and 66: Previus BMP Overflw TN and TP EMC (mg/l) reference an incrrect value. Cells shuld reference the Cmpsite EMC (Rws 108 and 109) leaving the BMPs (BMP#2) in each catchment. Cells affected are E55, F55, I55, J55, M55, N55, Q55, R55, U55, V55, Y55, Z55, E66, I66, M66, Q66, U66, and Y66. The Wake Cunty tl assumes this crrectin. If nt addressed, this issue has a ptential fr calculatin errrs fr all develpment scenaris with a 3 rd BMP using the JF Tl.

7 7 3. Jrdan/Falls tl: BMP Calcs wrksheet references incrrect Rv values fr Radways in the summary N and P (lb) calculatins prducing underestimated lading fr radway areas. Cells AD516 & AD623: reference t AD353 shuld be AD354 Cell AC516 reference t AC353 shuld be AC354 Cell AE623 reference t AE353 shuld be AE354 If nt addressed, this issue has a ptential fr calculatin errrs fr all develpment scenaris with residential radways. 4. It is assumed fr the Wake Cunty tl that fr BMPs in a series, the ttal drainage area t all three BMPs includes the cntributing drainage area plus the cntributing drainage area frm any additinal subdas that are ruted thrugh the BMP series. Fr scenaris where there are BMPs in a series with cntributing catchments, the JF Tl results in errrs in inflw drainage area calculatins fr sme scenaris and in errrs in inflw vlume calculatins fr thers. Example: Catchment 1 has 3 BMPs (all with the same drainage area f 1 acre). Catchment 1 is als a cntributing watershed t Catchment 2. The JF Tl assigns the same drainage area frm BMP#1 t # s 2-3 in Catchment 1(ie: BMP Calcs, D10, E11 & F11 wuld have the same 1 acre drainage area). A Yes value is required in the Catchment 2 clumn fr BMP#1 (BMP Characteristics, L30) t shw that Catchment 1 Catchment 2. Prblem A: If a N value fr BMP # s 2-3 (BMP Characteristics, N30 & P30) is inputted the reprted values fr Cntributing Catchment (BMP Calcs, I12 & J12) d nt include the 1 acre frm Catchment 1 fr BMP # s 2-3, prducing errrs in lading. Prblem B: If a Yes value fr BMP # s 2-3 (BMP Characteristics, N30 & P30) is inputted, Prblem A is addressed, hwever the ttal vlume entering BMPs #2 and #3 are duble cunted (BMP Calcs, I23 & J23), prducing errrs in lading. As seen with this example, Wake Cunty believes that BMP Calcs, Rws may cntain a errrs. The Wake Cunty tl addresses this scenari thrugh a different platfrm that assumes that BMPs in a series d share the same drainage area. If cmparing lading results t the JF Tl, this difference in thery des prduce differing results. If nt addressed in the JF Tl, this issue has a ptential fr calculatin errrs fr all develpment scenaris using BMPs in a series with cntributing catchments. Wake Cunty requests that DENR advice n ur interpretatin f this issue.

8 8 Example Test Scenaris Perfrmed by Wake Cunty Wake Cunty perfrmed test scenaris with the Wake Cunty Hybrid Tl (WC tl) as well as the Jrdan/Falls Tl (JF Tl). Inputs and results were cmpared and cntrasted fr bth tls. The scenaris prvide demnstratin f calculatin accuracy as well as directin n hw the WC Tl and JF Tl, while different in structure, can be used fr the same scenaris t cmply with the New Develpment Rule. The example test scenaris, cmparisn results, and cmments are listed belw. Additinal testing is still needed and further changes may be necessary. TEST #1 WC Tl Inputs JF Tl Inputs Initial Results/Prblems Identified Cmments 1 Drainage Area (2 Ac) 2 SubDAs 1 BMP in each sub DA SubDA (a) SubDA (b) 2 Catchments 1 BMP in each Catchment Catchment 1 Catchment 2 Test Passed with Mdificatins Pre and Pst Develpment, Untreated, Leaving BMPs, and Pst Develpment with BMPs calculatins validated by bth tls (See Summary tables n BMP Calcs (JF) and BMP NP Calcs_Hidden (WC). Initial Results/Prblems were identified and mdificatins t the JF were made as described in Ptential Prblem #1 & #3 prir t Test#1. Withut these mdificatins, test scenari validatin wuld have failed. TEST #2 WC Tl Inputs JF Tl Inputs Initial Results /Prblems Identified Cmments 1 Drainage Area (2 Ac) 1 SubDAs (1 Ac each) 3 BMPs in each subda N cntributing subdas 1 Catchments (2 Ac) 3 BMP in Catchment N cntributing catchments Test passed with JF Tl Mdificatins N(lb), P(lb), TN EMC, and TP EMC values prduced errrs fr these specific calculatins with a margin f errr f +/ (See BMP Calcs and BMP NP Calcs_Hidden sheets). Initial results included mdificatins t the JF Tl (Ptential Prblems #1 & #3) as described in Test #1 Cmments. With mdificatin t JF Tl as described in Ptential Prblem #2, test scenari was validated by bth tls with n errrs.

9 9 TEST #3 WC Tl Inputs JF Tl Inputs Results Cmments 1 Drainage Area (3 3 Catchments Ac) 3 SubDAs (1 Ac each) 3 BMPs in each subda SubDA(a) subda(b) 3 BMPs in each catchment (all with same DA f 1 Ac) Catchment 1 Catchment 2 Test failed fr Pst Develpment, Untreated, Leaving BMPs, and Pst Develpment with BMPs calculatins (See Summary tables n BMP Calcs (JF) and BMP NP Calcs_Hidden (WC). With mdificatin t JF Tl as described in Ptential Prblem #4, test scenari culd nt be validated by bth tls withut errrs. A cmparisn cannt be made between bth tls fr this type f scenari until Ptential Prblem #4 is addressed. TEST #4 WC Tl Inputs JF Tl Inputs Initial Results/Prblems Identified Cmments 3 Drainage Areas 1 SubDA (1 Ac) 1 BMP in subda 3 Catchments 1 BMP in each Catchment N crsscntributing Catchments Test passed with JF Tl Mdificatins Pre and Pst Develpment, Untreated, Leaving BMPs, and Pst Develpment with BMPs calculatins validated by bth tls (See Summary tables n BMP Calcs (JF) and BMP NP Calcs_Hidden (WC). Initial results included mdificatins t the JF Tl (Ptential Prblems #1 & #3) as described in Test #1 Cmments. TEST #5 WC Tl Inputs JF Tl Inputs Initial Results/Prblems Identified Cmments 3 Drainage Areas 2 SubDA (0.5 Ac each) 1 BMP in subda SubDA(a) subda(b 6 Catchments (0.5 Ac each) 1 BMP in each Catchment Catchment 1 Catchment 2; 3 t 4; and 5 t 6 Test passed with JF Tl Mdificatins Pre and Pst Develpment, Untreated, Leaving BMPs, and Pst Develpment with BMPs calculatins validated by bth tls (See Summary tables n BMP Calcs (JF) and BMP NP Calcs_Hidden (WC). Initial results included mdificatins t the JF Tl (Ptential Prblems #1 & #3) as described in Test #1 Cmments.