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IMPERVIOUS SURFACE METHODOLOGY A Methdlgy fr Defining and Assessing Impervius Surfaces in the Raritan River Basin New Jersey Water Supply Authrity, fr the Raritan Basin Watershed Management Prject May
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IMPERVIOUS SURFACE METHODOLOGY A Methdlgy fr Defining and Assessing Impervius Surfaces in the Raritan River Basin New Jersey Water Supply Authrity, fr the Raritan Basin Watershed Management Prject May 2000 DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT METHODOLOGY FOR DEFINING AND ASSESSING IMPERVIOUS SURFACE AREAS TABLE OF CONTENTS EXECUTIVE SUMMARY... iii ACKNOWLEDGEMENTS... iv 1.0 DEFINITION OF IMPERVIOUS SURFACES Reasns fr Cncern Abut Impervius Cver Discussin f Optins BASIN-WIDE ANALYSIS NJDEP 1995/97 Land Use/Land Cver Data Imprving Estimates Using Radway Data Analysis f 1 st and 2 nd Order Streams SUBWATERSHED ANALYSIS Methds fr Analysis Cmpnents Used in the Subwatershed Analysis Strm Sewers Remtely Sensed Satellite Imagery Building Permits and Subdivisin Apprvals OPTIONS FOR RAPID WATERSHED IMPROVEMENT PROJECTS CONCLUSION ACRONYMS LIST OF REFERENCES...9 APPENDIX ii DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT EXECUTIVE SUMMARY In an effrt t estimate and evaluate impervius cver thrughut the Raritan Basin, this methdlgy was develped t explre the technical and plicy decisins invlved in the estimatin prcess. This methdlgy defines impervius surface areas, states reasns t be cncerned abut impervius cver, and presents several methds f deriving impervius surface area measurements at the Basin-wide scale, at the subwatershed level, and at an even smaller scale fr rapid watershed imprvement prjects. Impervius surface areas will be analyzed and mapped fr the Raritan Basin prject using the fllwing appraches: The Basin-wide analysis will be cnducted at a HUC-14 (subwatershed) level fr the entire Basin and will be aggregated t HUC-11 (watershed) and Watershed Management Area levels. The analysis will include the fllwing: NJDEP 1995/97 Land Use/Land Cver Data NJDOT Rad Netwrk Data (t determine the amunt f impervius surface attributable t rads). Other supplemental NJDOT data include the fllwing, Straight Line Diagrams, Cngestin Management Systems, Pavement Management Systems Analysis f 1 st and 2 nd rder streams using Digital Elevatin Mdel t define their watersheds (if feasible) The subwatershed analysis will be cnducted fr a select few subwatersheds if there is a need based n the Basin-wide analysis, and if time/csts permit. The subwatershed analysis wuld include the fllwing: Optins fr Analysis: Refine 1995/97 Data determine what impervius surfaces dn t cntribute directly t stream flw Update 1995/97 Data update t current cnditins using new imagery and building permit data Create new layer digitize exact impervius surfaces using 1995/97 aerial phtgraphy Create new layer use new (1999) satellite imagery t digitize exact impervius surfaces Additinal cmpnents used in the subwatershed analysis Strm sewers t understand which surfaces cause flw t streams Building permit data t estimate impervius surfaces fr new develpment In additin t the subwatershed analysis, a mre detailed analysis can be cnducted as part f rapid watershed imprvement prjects. This analysis wuld include the fllwing: Review f subdivisin plans and sil ersin certificatins Incrpratin f data frm USGS Natinal Water Quality Assessment study iii DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT DRAFT ACKNOWLEDGEMENTS Primary Authrs: Tm J. Stanuikynas, Gegraphic Infrmatin Specialist, New Jersey Water Supply Authrity Daniel J. Van Abs, Ph.D., Manager Watershed Prtectin, New Jersey Water Supply Authrity Cntributing Authr: Debrah J. Newcmb, Watershed Prtectin Specialist, New Jersey Water Supply Authrity An extended thank yu t the Raritan Basin Watershed Management Prject Characterizatin Cmmittee and Prject Team fr all f their cmments, hard wrk and suggestins. Prject Team members include: New Jersey Department f Envirnmental Prtectin New Jersey Water Supply Authrity Nrth Jersey Resurce Cnservatin and Develpment Cuncil Rutgers Center fr Envirnmental Cmmunicatin Suth Branch Watershed Assciatin Stny Brk-Millstne Watershed Assciatin United States Department f Agriculture Natural Resurces Cnservatin Service United States Gelgical Survey Upper Raritan Watershed Assciatin iv 1.0 DEFINITION OF IMPERVIOUS SURFACES Impervius surfaces are defined in watershed management as surfaces that prhibit the mvement f water frm the land surface int the underlying sil r dirt. Buildings and paved surfaces (e.g., asphalt, cncrete) are cnsidered impervius cvers. A natural cnditin (e.g., bedrck clse t the surface, very dense sil layers such as hardpan that restrict water mvement) is nt cnsidered impervius cver. As such, the watershed field applies the term impervius cver t cnditins that are created by human actin. Natural cnditins are addressed in ther ways, such as thrugh the assessment f grund water recharge. One majr benefit t the fcus n human structures is that buildings and pavement are fairly easy t identify using aerial phtgraphy while subsurface, natural cnditins are nt as bvius. Mre difficult t fit int the discussin are semi-hardened surfaces that greatly impede water flw and are als caused by human actin. Examples include cmpacted sils in develped areas, which can be nearly cncrete-like in their imperviusness. Gravel rads, driveways and parking lts can als have highly cmpacted surfaces. In this methdlgy paper, the definitin f percent ttal impervius area (%TIA) fllws current thinking that the majr issue revlves arund the density f impervius cver within a watershed, rather than the actual area (e.g., square feet) f impervius cver. A related term is percent effective impervius area (%EIA) which recgnizes that sme impervius areas are cmpletely surrunded by pervius areas and therefre have less f an impact n aquatic ecsystems. Effective impervius area is basically the impervius cver that prvides strmwater flws fairly directly and quickly t streams. Fr example, a paved basketball curt surrunded by grass lawn at a schl might have less impact than the same curt surrunded by paved parking areas. A lk at %EIA culd exclude the basketball curt surrunded by a grass lawn frm the estimate f impervius cver. 1.1 Reasns fr Cncern Abut Impervius Cver Impervius cver has been implicated in a number f significant watershed impacts. Mst f these impacts are related t hydrlgic changes the flw f water int and within the stream system. Increased impervius cver generally results in mre strmwater runff and less grund water recharge, unless deliberate steps are taken t mitigate thse impacts. Mre runff, in turn, increases stream flws during strm perids. Stream banks erde, mre sediment is carried int the streams frm surrunding lands, and aquatic habitats are disrupted and degraded. Less recharge means less grund water discharges t streams during dry perids. The reduced stream flw and mre extreme stream temperatures will stress aquatic ecsystems. In additin, pllutants tend t be mre cncentrated because dilutin is reduced. When strmwater mves mre quickly int streams, it als has a greater capacity t carry nnpint surce pllutants int the streams. High levels f impervius cver are assciated with dense develpment, which will have greater pllutant lads available fr 1 transprt t thse streams. Bth higher strm flws and lwer dry weather flws can have water supply impacts. Eclgical studies n the impacts f impervius cver n aquatic ecsystems have been cmpleted in numerus parts f this cuntry. Tm Schueler and his clleagues f the Center fr Watershed Prtectin have been cmpiling and interpreting the results. Althugh every ecsystem will react smewhat differently, and althugh mst impacts shuld be described statistically rather than thrugh the use f fixed numbers, Schueler has fund that increased impervius cver is strngly related t increased degradatin f aquatic ecsystems. Further, he has suggested certain levels f impervius cver abve which stream degradatin shuld be expected (apprximately 10 percent) and abve which extensive and perhaps permanent stream damage shuld be expected (apprximately 25 percent), fr watersheds f apprximately 10 t 20 square miles. 1 Impervius cver is used as an indicatr; the assumptin is that the hydrlgic changes and nnpint surce pllutant ladings frm such land cver cause mst f the ecsystem degradatin, rather than the impervius cver itself. Sme f the research indicates that the situatin is cnsiderably mre cmplex, and that multiple factrs influence aquatic ecsystem health. Pllutant ladings frm pint surces are certainly ne such factr. Others include stream channelizatin, remval r presence f large wdy debris, rad and utility crssings, etc. The expectatin is that these additinal factrs may explain sme f the variability in ecsystem impacts demnstrated by the impervius cver research. (Sme f these factrs are addressed in ther technical reprts fr the Raritan Basin Watershed Management Prject.) Hwever, impervius cver des seem t explain a great deal f stream damage in these studies as well. Table 1 in the Appendix is a schematic f hw varius factrs may play int the health f aquatic ecsystems. Based n the research available, it is imprtant t nte that impervius cver is an indicatr f the ptential fr degradatin it is nt by itself deterministic. Unfrtunately, very little research n this tpic has been perfrmed in New Jersey. The United States Gelgical Survey s Trentn Office has been pursuing this tpic thrugh its Natinal Water Quality Assessment (NAWQA) prject in New Jersey and New Yrk. The first step assessed crrelatins between water quality and aquatic ecsystem degradatin, while the secnd step lked int crrelatins between impervius cver and degradatin. In additin, the New Jersey Gelgical Survey (NJGS) has develped a methd fr identifying and mapping areas that are imprtant fr grund water recharge. This methd (described in the GSR-32 reprt) will be used t identify grund water recharge areas fr the Basin. 1.2 Discussin f Optins There are a variety f ptins available fr assessing impervius cver in the Raritan River Basin. Hwever, the mre cmplex the apprach is, the higher the cst. Several f the ptins culd be implemented in specific subwatersheds as case examples, rather than implementing them ver the entire basin at great expense. The appraches 2 that fllw include thse that will be perfrmed fr the entire Basin as well as thse that may be perfrmed at the subwatershed level based n cmplexity and cst. 2.0 BASIN-WIDE ANALYSIS The analysis fr the entire Raritan Basin will be perfrmed at the HUC-14 (subwatershed) level and will aggregate up t the HUC-11 (watershed) and Watershed Management Area levels. Tw appraches used t derive estimates f impervius cver fr the Basin are discussed in this sectin. The first apprach relies n readily available land use/land cver infrmatin. The secnd apprach calculates the amunt f impervius cver assciated with the transprtatin netwrk. The tw methds are described belw and will be used in cmbinatin fr this prject. 2.1 NJDEP 1995/97 Land Use/Land Cver Data Discussin The New Jersey Department f Envirnmental Prtectin (NJDEP) has generated a statewide Gegraphic Infrmatin System (GIS) data layer cntaining land use/land cver (LU/LC) classificatins. The LU/LC data were digitized using 1995 and 1997 digital rth-pht quarter-quads (DOQQ) and LU/LC values were assigned t individual plygns at a ne-acre minimum mapping unit. In additin t LU/LC cdes, an Impervius Surface (IS) value was estimated fr each plygn. The estimates were made visually based n the 1995/97 DOQQ s and are represented in 5% increments, ranging frm All water categries are cded 0. Any plygn that cntains pavement (e.g., a radway) within its bundaries r has a radway fr a bundary will be assigned an IS value f n less than 5. The Quality Assurance/Quality Cntrl prcess cnsists f frequency analysis f the IS values per varius LU/LC categries t determine if the values are in line with the categry ranges. 2 Befre the data are fficially released fr public usage, they will be field checked by the cntractr (Aerial Infrmatin Systems, Inc.) that develped the cverage. Methdlgy Currently, the NJDEP LU/LC data is the mst useful surce f impervius surface estimates available and will be used t cnduct the analyses fr the entire Basin. The data are in Envirnmental Systems Research Institute s (ESRI) ArcInf and ArcView GIS frmat and can be dwnladed at n charge via the Internet. All plygns are individually measured fr bth %TIA and ttal IS acreage. The frmat is cnsistent with ther NJDEP GIS data that are widely used thrughut the State f New Jersey. The ne-acre accuracy is nt a cncern fr mapping purpses, since mst data used fr this prject have been at a reslutin between 2-5 acres. Hwever, the age f the data is cause fr sme cncern. T rectify this deficiency and accunt fr grwth in the Basin, ther frms f data may be incrprated int the mapping prcess at the subwatershed level. These ther methds will be discussed in later sectins f this methdlgy. 3 2.2 Imprving Estimates Using Radway Data Discussin The secnd cmpnent fr assessing %TIA fr the entire Basin is t cnsider t what extent paved radways affect the amunt f impervius surface cver in the Basin. Calculating impervius surface estimates exclusively fr the rad netwrk will supplement the NJDEP data results twfld. Nt nly will it be utilized t spt check areas f particular interest, it will als allw fr mre in-depth analyses. As described in the previus sectin, the NJDEP LU/LC cverage assigns a single Impervius Surface (IS) value fr each land use plygn. Many f the multi-lane, divided highways are digitized as separate land use plygns and therefre have unique IS values. Hwever the remaining rads (which encmpass a large percentage f the ttal) are nt dealt with as separate areas but are absrbed int the land use plygn in which they are lcated. Therefre, the amunt f impervius surface these rads cntribute t the plygn is cmbined int the ttal IS value. As a result, a separate impervius surface cver layer specific t the rad netwrk will be created. T accmplish this task, a segmented, linear GIS cverage that cntains the entire rad netwrk, including ff-ramps, is required. The New Jersey Department f Transprtatin (NJDOT) pssesses such data. Mre accurate than the cmmnly used TIGER files, the NJDOT line wrk was digitized frm 1991 aerial phtgraphy. Utilizing a GIS, the NJDOT rad layer wuld prvide the basis fr calculating the ttal square mileage f radway within the Basin, which in turn culd be used t estimate the amunt f impervius surface attributed t rads. The first step wuld be t estimate the ttal miles f radway (length) within the Basin using a simple GIS query. Using attributes, which exist within the NJDOT rad layer, and a basic knwledge f the surrunding gegraphy, ne can further analyze the data by rad type (Interstate, turnpike, freeway, highway, and lcal). The secnd variable needed fr this equatin is rad width. Remembering that the NJDOT rad layer is linear, which by definitin is ne dimensinal, rad width data will have t be extracted frm anther surce. Since rad widths vary greatly due t numerus cnditins (number f travel lanes, shulders, parking, intersectins, turning lanes, acceleratin and deceleratin lanes, etc.), an average rad width fr each rad type is used. The techniques that will be used fr apprximating average rad widths by rad type are discussed belw NJDOT Straight Line Diagrams (SLD) The NJDOT Straight Line Diagrams cntain much f the data needed t cnduct this analysis. Functinal class, speed limit, number f lanes, lane width, shulder width, median type, median width, and traffic vlume are prvided fr all Federal Aid System rads. The Federal Aid System includes all Interstates, U.S. Highways, State Highways and a few cunty and lcal rads. Due t the vast amunt f incremental changes in each field fr all rad segments, the attribute tables encmpass hundreds f pages. As f nw, this infrmatin is in a frmat that is nt cmpatible with GIS. Hwever, the NJDOT des 4 prvide sme f the SLD data, including rad widths, in a spreadsheet frmat that is suitable fr querying. This will be a valuable resurce fr spt-checking specific rads. NJDOT Cngestin Management Systems (CMS) Utilized by transprtatin planners t map traffic cngestin, this surce f data includes many f the same data fields as the SLD. Data fields are averaged ver large segments fr a selected quantity f primary arterials. Fewer line segments are easier t mange and therefre will prvide average rad width estimates quickly and effectively. NJDOT Pavement Management Systems (PMS) This dataset includes ttal estimates fr paved surface area using centerline mile, lane miles, and an estimatin f rad width fr selected rad types, including ff-ramps. This data is a useful tl fr spt-checking final results. Once the rad widths are estimated, the simple task f multiplying the ttal length by the average width fr each rad type wuld prduce the square mileage f impervius surfaces attributed t the highway netwrk. Methdlgy Examining the NJDEP LU/LC data as a standalne layer will nt reveal the amunt f impervius surface that is directly prduced by rads. In rder t find the impervius surface area f the rad netwrk, a separate analysis will be cnducted using the NJDOT rad layer t prvide the ttal miles f radway fr each rad type. A new field will be created in the NJDOT rad cverage fr average rad width. An analysis f the NJDOT CMS was used t prduce the average rad widths by rad type. The estimates that will be used fr the Basin analyses are as fllws: NJ Turnpike Interstate Highways Other Freeways Majr Arterial Highways All Other Rads 140 feet wide 110 feet wide 90 feet wide 60 feet wide 36 feet wide Depending n the rad type, the average rad width value will be assigned t each segment. The length and width f each segment will then be multiplied t prduce a ttal square mileage fr each segment. The NJDOT SLD will be referenced fr mre precise calculatins. Data frm the NJDOT SLD can be used fr rads that d nt fit the average ranges r t spt-check certain segments. The final utput f the analysis will be cmpared t the data frm the Pavement Management System fr quality assurance/quality cntrl purpses. 5 2.3 Analysis f 1 st and 2 nd Order Streams If available data permit, analyses will als be dne fr the watersheds f all 1 st and 2 nd rder streams within the Basin. T cnduct the analyses at this level, use f the USGS Digital Elevatin Mdel (DEM) wuld be required. Hwever, due t technical inadequacies within the DEM, accurate results may nt be able t be prduced. First and secnd rder stream analyses will nt be cnducted until a crrected DEM is available. 3.0 SUBWATERSHED ANALYSIS A small number f subwatershed pilt studies may be undertaken t ffer a mre detailed investigatin f impervius surface cnditins thrughut the Basin. Examining a much smaller land area will allw mre fcused analyses that wuld be t cstprhibitive at the Basin scale. Depending n the utcme f the Basin analyses, the value f subwatershed pilt studies will be determined. If pilt studies are desirable, time and cst cnstraints will influence the extent f analysis. 3.1 Methds fr Analysis There are several ways t perfrm a mre precise, subwatershed analysis. Methds will be described frm the least cstly and time intensive, t the mst cstly and time cnsuming. Hwever, the mre labr intensive the apprach, the better the end prduct. Methds fr analysis include the fllwing: Refining r updating the 1995/97 LU/LC data Crrectins and additins t the 1995/97 LU/LC data will imprve impervius surface results and can be accmplished in tw ways. Using the same 1995/97 DOQQ s the NJDEP used t digitize their cverage, existing plygns can be manipulated and re-wrked t better prtray cnditins fund n the DOQQs. Fr example, %EIA can be extracted frm %TIA. Think f a plygn that was digitized as an Educatinal Institutin. This plygn may nw be split int several parts including buildings, grass fields, parking lts, and hard surface recreatin areas, which culd nt be dne previusly. (See belw) By btaining new satellite imagery and building permit data, the same analysis can be dne fr current cnditins. Digitizing new cverages
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