STORMWATER DESIGN MANUAL

Transcription

1 KITSAP COUNTY STORMWATER DESIGN MANUAL Effective Date: December 31, 2016 prepared f

2 This page intentinally left blank.

3 CHAPTER 2 SITE ASSESSMENT AND PLANNING Befre evaluating minimum requirements and starting the prcess fr selecting On-site Strmwater Management, Flw Cntrl, and Water Quality Treatment BMPs, each prject must first assess and evaluate existing and pst-develpment site cnditins. Site planning helps t maximize the ptential develpment site pprtunities while minimizing prject-related strmwater impacts by minimizing impervius areas, lss f vegetatin, and the vlume and rate f runff generated that must be subsequently managed n-site. This sectin utlines requirements fr inventry and analysis f key prject cmpnents, site design cnsideratins, use f Lw Impact Develpment (LID)/Green Strmwater Slutins (GSS) as the preferred apprach t planning and design, site mapping, and submittal requirements. 2.1 INVENTORY AND ANALYSIS OF KEY PROJECT COMPONENTS Perfrming a cmprehensive inventry and analysis is an essential first step t site assessment and planning, and must precede site design. The inventry shall include n- and ff-site natural and built cnditins that wuld affect the prject design. Plicies, land use cntrls, and legally enfrceable restrictins shall als be evaluated and dcumented (Puget Sund Partnership and WSU Extensin, 2012). The fllwing sectins prvide additinal guidance n key prject cmpnents that can significantly influence the prject design. These key cmpnents shall be inventried and analyzed as part f the site assessment and planning step: Prject bundaries and structures Sils Critical areas Dewatering Tpgraphy Hydrlgic patterns and features Vegetatin Land use cntrl Access Utility availability and cnflicts See the Cunty s Gegraphic Infrmatin System (GIS) mapping website fr available infrmatin that may be used as a resurce fr creating this inventry f key cmpnents, as apprpriate Prject Bundaries and Structures Prject bundaries, nearby structures, and ther related issues can directly affect strmwater runff and BMP designs. The fllwing must be addressed befre selecting a strmwater management BMP: Vlume I Prject Minimum Requirements and Site Planning 2-1

4 Prject Bundaries: The prject bundaries typically define the limits f disturbance and can affect the threshlds and applicable minimum requirements. Prject bundaries generally cincide with the right-f-way and/r prperty line. Setbacks: Prperty lines, adjacent right-f-way bundaries, and setbacks required frm each must be identified and cnsidered t evaluate siting f structures. Lcatin f Buildings: All existing and prpsed buildings must be identified, including all existing and prpsed temprary and permanent structures (such as retaining walls) and impervius surfaces (driveways, patis, etc.). Structures n neighbring prperties can als affect strmwater BMP selectin, and shall be identified n plans as apprpriate. Fundatins and Fting Drains: The type f prpsed fundatins and fting drains, including lcatin and extent, must be determined. These features can include: Cnventinal spread ftings Pile shaft Basement Fting drains and their assciated pint f discharge, if applicable Water-tight fundatin withut fting drains Sils The existing sil types present must be evaluated t assess the infiltratin capacity f the site and the applicability f varius strmwater BMPs. General requirements fr infiltratin facilities, including site characterizatin and infiltratin rate determinatin, are presented in Chapter 4 in this vlume and in Vlume II, Chapter Critical Areas Additinal regulatry requirements are placed n prjects that are within r near critical areas, pursuant t KCC Title 19. Such areas include streams, wetlands, frequently flded areas, critical aquifer recharge areas and gelgically hazardus areas. Depending upn the type f critical area, additinal requirements r limitatins regarding strmwater management may apply. See KCC and Vlume II, Chapter 8 f this manual and the Cunty s Critical Areas Ordinance website fr mre infrmatin Dewatering It is imprtant t have early estimatins f the grundwater discharge frm the prject site. The site s prximity t receiving waters r its lcatin in areas where there may be perched, static, tidally influenced r hydraulically cnnected grundwater can have significant impacts n hw the prject is designed and which requirements may apply. Additinal infrmatin n dewatering is described as part f the minimum requirements fr Cnstructin Strmwater Pllutin Preventin (Chapter 4 in this vlume) Tpgraphy Understanding the existing site tpgraphy is imprtant t implementing LID principles, such as minimizing grading and preserving existing flw paths. Tpgraphy will als influence hw and where strmwater facilities are incrprated int the site. Imprtant features t assess and dcument include: 2-2 Vlume I Prject Minimum Requirements and Site Planning

5 Steep slpes Clsed depressins Grade breaks Radway grades and elevatins Hydrlgic Patterns and Features T maintain existing hydrlgic patterns and imprtant features, n-site hydrlgic prcesses, patterns, and physical features must be understd and dcumented. This step requires dcumenting the fllwing feature types, amng thers fund n-site that may influence drainage patterns: Streams Wetlands Native sils and vegetatin (see als Sectin f this manual) Seeps Springs Clsed depressins (see als Sectin f this manual) Drainage swales and ditches Signs f ersin T the extent pssible, this step shuld be undertaken during wet perids. See Figure 2.1 fr an example map f hydrlgic features fr a hypthetical subdivisin develpment. Vlume I Prject Minimum Requirements and Site Planning 2-3

6 Figure 2.1 Example Map Dcumenting Existing Hydrlgic Features Detailed sub-basin delineatin prvides several advantages as fllws: Individual practices receive smaller hydraulic and pllutant lads. Small-scale practices can be arranged in the prject efficiently and save space fr ther amenities. Individual LID BMPs can be accurately sized based n the apprpriate tributary drainage areas and their cumulative perfrmance acrss the site can be evaluated Vegetatin Vegetated areas f a prject site can be very effective in minimizing strmwater runff. Existing site vegetatin shall be characterized as part f the site assessment, and may be accmplished thrugh the use f aerial imagery, bservatins recrded during site walks, r ther apprved methds t delineate frest, pasture, grassed and landscaped areas in the existing cnditins. Effrts shall be made t cnfigure the prject t minimize disturbance f areas cvered by valuable existing vegetatin, such as mature trees Land Use Cntrls Applicable land use cntrls, such as limitatins n impervius surface cverage, minimum landscaping requirements, minimum lt area, setback requirements, parking requirements, and 2-4 Vlume I Prject Minimum Requirements and Site Planning

7 site design standards assciated with building placement and rientatin, shall be analyzed and dcumented using the fllwing steps: Review applicable cmprehensive plan designatin, zning classificatins, and verlay districts that may apply t the site. Overlay districts may include requirements fr special design review r district verlays. Determine whether the Shreline Master Prgram applies t the site and cmply with applicable guidelines and requirements. Cnsult with the Kitsap Cunty Develpment Services and Engineering (DSE) Divisin f Department f Cmmunity Develpment (DCD) t identify ther land use regulatins that may allw clustering r ther practices intended t minimize impervius surfaces. Examples include Perfrmance-based Develpments and Master Plan develpments Access Vehicular and pedestrian access, circulatin, and parking elements f the built envirnment shall be identified as part f the site inventry and analysis. Access can ften represent a cntrlling element fr the design f a site. The designer shall cnsult the Kitsap Cunty Rad Standards and applicable zning in KCC 17 fr site access. These requirements will establish the number f allwed access pints, the width f the access, the spacing f access pints between sites n the same r ppsite side f the adjacent street right-f-way, and pedestrian circulatin requirements alng and thrugh the site t the prpsed use. The fllwing steps shall, at minimum, be used t inventry and assess access: Map the lcatin f rads, driveways, and ther pints f ingress and egress within 100 feet f the site. Refer t the Kitsap Cunty Rad Standards t identify the classificatin f the street that will be prviding access t the site. Knwing the classificatin f the abutting street will allw the designer t understand frntage imprvements, sight distance requirements, allwed driveway widths, and ther gemetric design requirements. Cnsult with the Kitsap Cunty Public Wrks (KCPW) Transprtatin Planning Divisin t understand any mtrized r nn-mtrized plans that may influence the design f the prject Utility Availability and Cnflicts The lcatin f wet (e.g., water, sewer, strmwater, etc.) and dry (e.g., pwer, phne, cable, etc.) utilities shall be identified and the adequacy f these utilities shall be cnfirmed. If new utilities need t be extended t the site, the applicant will need t understand where the utility will cme frm, and ptentially extend t, and the impact that easements and restrictins may have n the site design. The fllwing steps shall be used t assess utilities: Cnsult with the utility purveyr(s) t determine the lcatin f wet (e.g., water, sewer, strmwater, etc.) and dry (pwer, phne, cable, etc.) utilities and discuss the prpsed plans. This cnsultatin shall be initiated during the planning phase f the prject and extended thrugh final design. Utilities Lcate Service number is 811. Vlume I Prject Minimum Requirements and Site Planning 2-5

8 Map existing utilities and utility easements n the site plan. Nte the setbacks frm the easements that may be required. Map existing utilities that may need t be mved and new utilities t be extended t the site. Design apprpriate measures t mve r prtect utilities, as needed. 2.2 SITE DESIGN CONSIDERATIONS T manage strmwater effectively and efficiently, site design fr bth cnstructin and the pstdevelpment cnditin must be dne in unisn with the design and layut f the strmwater infrastructure. Effrts shuld be made, as required and encuraged by Kitsap Cunty develpment cdes, such as Titles 12, 16, and 17, t cnserve natural areas, retain native vegetatin, reduce impervius surfaces, and integrate strmwater cntrls int the existing site drainage patterns t the maximum extent feasible. With careful planning, these effrts will nt nly help achieve the minimum requirements cntained in the Strmwater Cde, but can als reduce impacts frm develpment prjects and reduce the csts f water quality treatment and flw cntrl. Befre designing the site and strmwater infrastructure, cnsider the fllwing: Strmwater: Identify the apprved pint(s) f discharge and cnveyance system flw path(s) based n bth piped cnveyance and natural tpgraphy. Using LID/GSS principles, manage strmwater runff (quantity and quality) as clse t the pint f rigin as pssible. Minimize the use f cnventinal strmwater cllectin (catch basins) and piped cnveyance infrastructure. Use LID/GSS BMPs (e.g., dispersin, infiltratin, and reuse) where feasible. Fit develpment t the terrain t minimize land disturbance and lss f natural vegetatin, especially mature cniferus frest. Landscaping: Maintain and use natural drainage patterns. Preserve natural features and resurces, including trees. Create a multifunctinal landscape using hydrlgy as a framewrk fr landscape design. Cnfine and phase cnstructin activities t minimize disturbed areas, and minimize impacts t envirnmentally critical areas and their assciated buffers. Plant new trees in prximity t grund level impervius surfaces fr n-site strmwater management and/r flw cntrl credit. Minimize r prevent cmpactin f and prtect sils. Amend landscape sils t prmte infiltratin. 2-6 Vlume I Prject Minimum Requirements and Site Planning

9 Impervius and Pervius Surfaces: Fr sites with varied sil types, lcate impervius areas ver less permeable sil (e.g., till). Minimize develpment ver mre prus sils. Use prus sils by lcating biretentin, permeable pavement, r ther apprved infiltratin methds ver them. Cluster buildings tgether. Minimize impervius surfaces (e.g., buildings, sidewalks, etc.). Minimize pllutin-generatin hard surface (PGHS) (e.g., areas subject t vehicular use such as driveways and parking strips). Minimize pllutin-generating pervius surfaces (PGPS) (e.g., fertilized lawns, flwer beds, etc.). Cnsider landscaping with native vegetatin. 2.3 LOW IMPACT DEVELOPMENT Lw impact develpment (LID) is a strmwater and land use management strategy that strives t minimize pre-disturbance t hydrlgic prcesses f infiltratin, filtratin, strage, evapratin, and transpiratin. In Kitsap Cunty and elsewhere, the terms Green Strmwater Slutins (GSS) r Infrastructure (GSI) are als used t describe LID strmwater management practices. LID/GSS gals are accmplished by emphasizing cnservatin and use f n-site natural features, site planning, and distributed strmwater management practices that are integrated int a prject design. LID/GSS strategies can be applied t new develpment, urban retrfits, infrastructure imprvements, and revitalizatin prjects t prtect aquatic resurces. Biretentin in a cul-de-sac f an LID residential neighbrhd in cnstructin in western WA. The biretentin manages strmwater runff frm the radway and cntributing rf and driveway areas. Numerus large existing trees were retained, adding valuable strmwater and cmmunity benefits. LID/GSS principles and applicatins present a significant cnceptual shift frm a purely structural t a primarily surce reductin apprach. Site planning and strmwater management are integrated at the initial design phases f a prject t maintain a mre hydrlgically functinal landscape even in denser settings. Hydrlgy and natural site features that influence water mvement (described abve) are used t guide rad, structure, and ther infrastructure layut. Native sil and vegetatin prtectin areas and landscaping are strategically distributed thrughut the prject t slw, stre, and infiltrate strm flws, and als serve as prject amenities. The gal f LID/GSS is t prevent physical, chemical r bilgical degradatin t streams, lakes, wetlands, and ther natural aquatic systems frm cmmercial, residential r industrial develpment sites. Prperly designed LID/GSS measures can reduce strmwater runff frm a prject site. As a result, the size and cst f cnveyance, flw cntrl, and Water Quality Treatment BMPs can be greatly reduced r, in sme cases, eliminated. A gd descriptin f the Vlume I Prject Minimum Requirements and Site Planning 2-7

10 LID planning, alng with detailed descriptins f LID measures can be fund in the LID Technical Guidance Manual. 2.4 SITE MAPPING Thrugh the assessment prcess discussed in Sectin 2.1 abve, map layers are prduced t delineate imprtant site features. These map layers are cmbined t prvide a cmpsite site map that guides the layut f streets, structures, and ther site features and the verall lcatin f the develpment envelpe(s). This cmpsite site map shuld be used fr all develpment types and will frm the basis fr the site design cnsideratins described in Sectin 2.2 and help t identify LID practices as described in Sectin 2.3. Figure 2.2 illustrates the prcess f develping a cmpsite site map. 2-8 Vlume I Prject Minimum Requirements and Site Planning

11 Figure 2.2 Example Cmpsite Site Map Surce: Mdified frm (AHBL and HDR 2013). Vlume I Prject Minimum Requirements and Site Planning 2-9

12

13 2.5 SUBMITTAL REQUIREMENTS A Site Assessment and Planning Packet (See Vlume II, Chapter 1, Appendix C) must be cmpleted t dcument the assessment f key prject cmpnents and site design cnsideratins discussed abve in this chapter. The packet cnsists f the fllwing elements, each f which must be cmpleted by the applicant and submitted as part f permit review: A. Prject Infrmatin Includes basic prject summary infrmatin. B. Existing Site Inventry and Analysis Checklist Dcuments findings frm the inventry and analysis as described in Sectin 2.1. C. Existing Site Cmpsite Map Cmbines the infrmatin analyzed in Sectin 2.1 int a single site map that is used as the basis fr site design. D. Existing and Prpsed Site Land Cver Areas Summarizes existing and prpsed site land cver areas. This summary infrmatin helps demnstrate cmpliance with the requirement t minimize impervius area, lss f vegetatin, and strmwater runff. E. Prpsed Site LID BMP Matrix - Dcuments LID BMP infeasibility evaluatin and prvides justificatin fr why individual LID BMPs were included r nt included in site plans. Vlume II, Chapter 1 prvides mre detailed discussin f the permit review prcess and Appendix C in the same chapter includes a printable cpy f the Site Assessment and Planning Packet Vlume I Prject Minimum Requirements and Site Planning

14 I-2 Kitsap Cunty Strmwater Design Manual