GREEN STORMWATER INFRASTRUCTURE (GSI) Planning Guidelines

Transcription

1 GREEN STORMWATER INFRASTRUCTURE (GSI) Planning Guidelines Last Updated: June 2015 The purpose of this document is to outline the department s standard workflow for GSI planning and analysis. This packet provides detailed guidance for drawing drainage areas, analyzing potential GSI locations, and tracking planning level decisions. To ensure the outputs of planning level work can transfer seamlessly into design phase, the information provided here aligns with the current GSI design guidelines. HOW TO USE THESE GUIDELINES: Planning analysis conducted in house or contracted out to a consultant team follows the basic process outlined below. This document provides more detail on each of these steps and lays out a suggested framework for decision making based on current design knowledge. A consistent approach to analysis and data collection is a key component of the overall planning strategy for successful implementation of short and long term projects. Steps 1 and 2 are listed as preparation that is needed before analysis can occur. Steps 3.1 and 3.2 should happen simultaneously with each other since drainage areas should be classified with a proposed management location. Step 3.3 should be conducted to confirm the results of the previous desktop analysis. The results of this analysis process are presented in a refined data tracking spreadsheet, GIS layers, maps, and other appropriate graphics (Step 4). STEP 1 PREPARATION SETTING UP THE GIS BASEMAP STEP 2 INVENTORY PLANNING INITIATIVES EVALUATION AND DRAINAGE AREA MAPPING STEP 3.1 ANALYSIS CATEGORIZING DRAINAGE AREAS STEP 3.2 ANALYSIS EVALUATING POTENTIAL GSI LOCATIONS STEP 3.3 ANALYSIS SITE VISIT TO CONFIRM RECOMMENDED LOCATIONS STEP 4 RESULTS RECORDING RESULTS IN THE DATA TRACKING SPREADSHEET, GIS LAYERS, MAPS, AND OTHER GRAPHICS Page 1 of 14

2 STEP 1 PREPARATION SETTING UP THE GIS BASEMAP PWD will provide a GIS basemap to ensure the same data layers and basic symbology will be utilized. This basemap should be used as a starting point; additional layers may be necessary based on the project specifics. The table of contents of the basemap will contain a list of layers; some will be provided by PWD, while some are available to the public and will need to be downloaded from the web. Both the layers provided by PWD and the ones downloaded from the web will need to be re linked to the layers listed in the table of contents. The layers on the basemap and their preferred symbologies will then be maintained. The layers under Conceptual Work are working layers (bolded below), while the remainder of the layers should be referenced to evaluate the feasibility of managing stormwater at a given location. GIS BASEMAP LAYERS Conceptual Work o GSI SMP Footprint Concept o GSI Drainage Area Concept Planning Group Layers o Project Opportunity Parcels in CSO o Planning Study Areas Existing Projects o CAPIT Projects o SW Permit Tracking o GSI Project Areas Green Stormwater Infrastructure Green Stormwater Infrastructure Drainage Area PWD Infrastructure o Sewer Combined System (CSO) Inlet Manhole Sewer Inlet Pipe Sewer Waste Water Vent Pipes Separate & Green System (SS) Stormwater Manholes Stormwater Inlets Stormwater Stormwater Inlet Pipes Stormwater Vent Pipes o Water Fire Hydrant Water Valves Water Water Service Line Pipes Water Hydrant Line Pipes Environmental o Flow Network Ridges Nodes Valleys Streets o Impervious and Sewersheds o Philadelphia Major Watersheds o Sewersheds o Modelsheds Reference o Neighborhoods o Council Districts 2016 o Land Use o Philadelphia Planning Districts o Philadelphia Parks & Recreation o Philadelphia City Limits Base Layers o Streets o PWD Parcels o Topographic 2 Contours o Curbs o Hydrology Centerline o Hydrology Polygon o Aerial Imagery 2012 See the metadata for each layer for a layers description and any relevant information pertaining to the data. Page 2 of 14

3 STEP 2 INVENTORY PLANNING INITIATIVES EVALUATION AND DRAINAGE AREA MAPPING PLANNING INITIATIVES EVALUATION: All existing planning initiatives that affect the study area or site under review should be referenced to determine their impact on stormwater management potential before physical analysis begins. Consultants tasked with analysis of large study areas may be asked to provide a planning iniatives memo that summarizes relavent planning efforts and provides information about priority locations and a map identifying potential projects to be explored in later phases. DRAINAGE AREA MAPPING: Before analyzing a street or parcel for its stormwater management potential, it is recommended that the drainage areas for the area in question be drawn. It is recommended that all existing drainage areas be drawn prior to project location selection so opportunities can be analyzed comprehensively. To delineate the drainage areas for a given area, the GIS Drainage Area layer should be used. This layer includes a standardized list of minimum attributes that should be populated. After planning studies are completed, PWD merges these drainage areas with PWDs master GIS Drainage Area layer. For that reason, it is important to populate the attribute tables in a consistent fashion. DRAINAGE AREA NUMBERING: As each polygon is drawn, a unique drainage area ID must be created that PWD will use to link with its planning database. Drainage Area ID s should be assigned based on a combination of the contract work number and a sequential numbering system. PWD uses a unique numbering methodology to link analyzed drainage areas drawn in PWD s drainage area GIS feature class and corresponding potential project locations tracked in PWD s PlanIT database. The numbering begins in the GIS feature class and is carried over into PlanIT. The following drainage area ID numbering methodology must be utilized both within the drainage area GIS feature class and the Data Tracking Spreadsheet. The numbering must match between the two data sets. [SIF Planning Study Work Number] [Sequential number] For Example: If a project s work number was 9330, then the Drainage Area ID s would be assigned sequentially as , , , and so on. A five digit sequential number should be used in order to maintain a consistent amount of characters within this numbering schema. DRAINAGE AREA DELINIATION: When drawing a new drainage area, the associated polygon is labeled with the square footage. This information can then be used in sync with the other data layers to reference and analyze a suitable location for the management of this drainage area. When drawing the drainage area for a street segment, it is important to note that most streets are crowned, and therefore two drainage areas (one for each side of the crown) exist. (See the image below.) Figure 1: Sectional representation of the crown of a road. This shows how two drainage areas are formed for a single street segment. Page 3 of 14

4 When drawing each street drainage area in GIS, the following layers should be turned on so that the polygon can snap to the appropriate layers: pwd parcels, streets centerline, flow network, (stormwater inlets for reference). These layers should be used to create the polygon for a drainage area so that it snaps to (1) the Street Centerline vertex in the center of an intersection, (2) the parcel corner along one end of a street, (3) the parcel corner along the other end of the street, and (4) the Street Centerline vertex in the center of an intersection at the other end of the street. Because the polygon snaps to the parcel lines along a street, the drainage area therefore includes runoff from the footway (sidewalk) in addition to the runoff from the cartway. Drainage areas that are managed by an existing GSI project do not need to be drawn. These drainage areas can be found under Existing Projects, GSI Project Areas, on the layer Green Stormwater Infrastructure Drainage Area. This layer shows final design as built drainage areas collected by PWD and private projects. Projects that are still in design do not have drainage areas drawn yet, but can be identified by the CAPIT projects layer Figure 2: This image shows the steps of snapping the edges of a drainage area polygon to the correct layers in GIS. In some situations, (1) a street segment may split to flow in two opposite directions along the length of a street or (2) there may be a midblock inlet (see examples on next page). If the drainage area splits, this is denoted by the "Flow Ridges" layer and a dot will be shown on the street centerline. In these scenarios, draw the drainage area to end perpendicular to the flow ridge or midblock inlet along the parcel line and the street centerline. At this point, the location of high points should be verified using City Plan. However, both the Flow Ridges and City Plan may not always be accurate, so site visits (see Step 3.3) may be helpful to determine the surface flow. Ultimately, all flow directions will have to be verified by survey during the detailed Design Phase. Page 4 of 14

5 1 Figure 3: This is an example of a city block with a high point, thus splitting the typical drainage area. 2 Figure 4: This image shows an example of a street with a mid block inlet that separates the drainage area. Page 5 of 14

6 To draw non street drainage area polygons, such as a parcel or roof, turn on the Impervious Cover GIS layer which delineates rooftop impervious cover and ground level impervious cover. The drainage area polygons can snap to these boundaries, but the provider is responsible for drawing drainage areas that may not be accurately accounted for by the Impervious Cover layer (e.g. a new building, a building that has been demolished, new impervious areas, etc). See example below. Figure 5: This image shows a new building and parking lot, where drainage areas were drawn to represent the new impervious surfaces that were added to the site. Since new development may change the existing conditions, the drainage areas that are drawn should be verified during a site visit as the aerial imagery may be out dated when the analysis is being completed (see Step 3.3). This should be noted if a system sheet is created (see Step 3.3) Page 6 of 14

7 STEP 3.1 ANALYSIS CATEGORIZING DRAINAGE AREAS Following the delineation of the drainage areas, planning level analysis should be conducted to categorize the management potential of each drainage area. This should be done in sync with the analysis of the potential SMP locations (Step 3.2). DRAINAGE AREA CATEGORIZATION: Each unique polygon (i.e. drainage area) should be categorized based on its ability to be managed in a potential location. In the example shown below, the school parcel has been identified as a potential location for GSI placement. In addition to evaluating the management options for onsite drainage areas, the surrounding drainage areas were examined for feasibility to be conveyed and managed on the site. The drainage areas were categorized based on the amount of current constraints. Figure 6: This image shows a recommended site where drainage areas are classified based on their feasibility of being conveyed and managed on site. Page 7 of 14

8 The management potential of each drainage area should be classified as either (1) high (2) medium, or (3) low based on current design guidelines. Classify the drainage areas based on the following definitions of the three terms: High Potential: Can be managed on a recommended street segment or parcel within the framework of current design practices*. Medium Potential: Drainage area flows towards a potential GSI location, but there is uncertainty of whether it can be managed at that location due to a physical constraint such as a street crossing, higher loading ratio, or other design constraint. Low Potential: Major physical constraints exist making the drainage area difficult to manage following current design guidelines. It is critical to consider all alternatives for management before selecting this designation. *It is important to note that these classifications are not finite, but are ever changing based on constraints that may be overcome as the program and suggested guidelines develop. The classifications should be determined based on the desktop analysis, information gathered from site visits, and from suggested guidelines listed in the following sections. The following fields should be populated in the attribute table for the layer. Figure 7: This image shows a snap shot of the fields that are to be populated for each drainage area that is drawn in the Drainage Area layer in GIS. The notes field can be used to (1) give more specific information that is needed to convey a more specific constraint, (2) more details in relation to where the drainage area is proposed to be managed or (3) detailed information relevant to the drainage area that cannot be listed anywhere else. The following section reviews how to select sites for potential GSI placement. As mentioned previously, it is suggested that this occur simultaneously in the analysis process. Page 8 of 14

9 STEP 3.2 ANALYSIS EVALUATING POTENTIAL GSI LOCATIONS In order to evaluate each potential GSI location in the study area, a Desktop Analysis can be conducted using the GIS basemap, City Plan, Highway Supervisor Plans, and PGW gas plats (provided by PWD). These resources will provide information on existing physical constraints that may affect the placement of proposed GSI. A PA One Call may be conducted for project locations that do not have these data resources available, or for project locations where more detailed information is necessary to evaluate potential GSI. Existing utilities, trees, and other infrastructure can add cost and complication to design; this should be factored into the decision to proceed with a potential project location. These constraints may limit GSI and should therefore be recorded in the drainage area layer in the specific field(s). Constraints may or may not preclude a project, but should be considered when determining project recommendations. SMP level information should be recorded in the GSI SMP Footprint attribute table in GIS. When performing the Desktop Analysis using the GIS basemap, the requirements and guidelines below should be followed to determine the management potential as defined in step 3.1. Requirements are indicated by the words must and will, while guidelines use words such as can and should. However, PWD s design criteria is constantly evolving as the GSI Implementation Program evolves. New design ideas or proposed approaches that deviate from the guidelines presented below can still be considered. It is important to be mindful of situations where it may be appropriate to deviate from the guidelines to accommodate a GSI project. Reasoning for pursuing a project in this scenario should be provided (recorded in Data Tracking spreadsheet step 4). The following section lists the applicable design guidelines to be considered during the planning phase. For additional information on GSI Design requirements, please refer to the following resource: pdf Page 9 of 14

10 GENERAL GUIDELINES APPLICABLE FOR PLANNING LEVEL ANALYSIS: 1. Systems must be properly sized to manage the runoff from the specified drainage area(s). For reference, systems should provide static storage for a target of two inches of runoff from the contributing impervious drainage area. Systems must, at minimum, provide static storage for one inch of runoff. 2. Generally, for GSI project to be cost effective, the drainage area being managed by one SMP should be at least 5,000 square feet. This guideline includes multiple drainage areas that flow towards one SMP and total at least 5,000 square feet. However, smaller drainage areas in close proximity to one another can sometimes be cost effective. 3. Within the right of way, grass strips or other highly compacted areas that are normally considered pervious areas should be considered impervious. 4. Infiltration loading ratios (contributing impervious drainage area to infiltration area), should be minimized as feasible. A maximum impervious loading ratio of 10:1 should be targeted for subsurface SMPs and a maximum impervious loading ratio of 25:1 should be targeted for surface SMPs. Higher loading ratios may be evaluated on a case by case basis as directed by the PWD project manager. 5. Ideally, systems should be located directly upstream of an existing inlet whenever possible in order to maximize drainage area and allow for a convenient underdrain connection. a. Exception: Some locations may be acceptable for GSI systems where convenient underdrain connections are not available, such as areas with adequate infiltration rates or steep streets that require a tiered system approach. 6. In order to protect right of way (Green Street) systems from impacts of future construction on adjacent parcels, a 3 foot buffer must be maintained from the right of way line. Closer offsets are permitted from the right of way line if the adjacent parcel is a City owned property 7. In order to protect off street systems from future construction on adjacent parcels, a minimum of a 5 foot buffer must be maintained from the adjacent property lines, though 10 feet is preferred 8. Typically, at least 10 feet of separation between infiltrating GSI systems and buildings or structures should be maintained. Systems located less than 10 feet from a building may be designed as slow release systems with an impermeable liner. 9. Trench widths in the sidewalk should typically be at least 4 feet wide, even though wider is recommended. Subsurface storage can extend into the cartway if space is available. a. Current policy does not allow for subsurface storage in the cartway on State Routes. However, limited infiltration under the cartway has been permitted in the past. It should be noted on the Site and Drainage Area Feasibility Map and Data Tracking Spreadsheet if there are recommended GSI projects on State Routes. 10. Projects should evaluate the potential costs and greened acre benefit when determining whether to impact existing utilities, poles, and lights. If a project is designed with the intention to not impact utilities, poles, and lights, the following guidance is suggested: a. A 3 foot horizontal distance from existing utility lines should be maintained. b. A minimum of inches vertical distance from existing utility lines should be maintained, depending on the utility type, age, and condition. c. A 5 foot distance from telephone poles or other comparable existing infrastructure should be maintained, where possible. d. A 3 foot distance from traffic lights must be maintained. Page 10 of 14

11 e. Setbacks from existing retaining walls should be considered. f. Locations where many utility service laterals would conflict with SMPs are generally categorized as low potential for GSI 11. The width of a street should be considered when proposing bumpouts. The width of the driving lanes must not be less than 20 feet (total) with bumpouts. General Principles for SMP Selection: 1. Surface features are preferred over sub surface features. 2. Vegetated features are preferred over non vegetated features. 3. Tree planting should be provided on all GSI. SMP Preference: Designers should maximize the Triple Bottom Line benefits by careful selection of SMP types. The following list shows the preferred SMP functions in descending order. The hierarchy presented is a generalized list and doesn t take into consideration site specific constraints that might otherwise preclude certain SMP types. 1. Surface Inflow into Vegetated Systems with Woody Vegetation: Maximizes surface features & vegetation 2. Surface Inflow into Vegetated Systems with Herbaceous Vegetation: Smaller surface features and vegetation footprint 3. Subsurface Inflow with Pretreatment and Vegetation: No surface feature or vegetation footprint, but includes tree planting 4. Systems with No Vegetation: No surface feature or vegetation footprint, additional tree planting recommended. EVALUATING COMMON CONSTRAINTS FOR STREET AND PARCEL PROJECTS 1. Does the drainage area meet the minimum size requirements for GSI? (Based on sizing guidelines #1 and #2) 2. Is there sufficient space for an SMP footprint? (Based on loading ratio guideline #4) 3. Does the location of water or sewer infrastructure pose a challenge? (i.e. multiple residential water service laterals, utilities near the curb line running parallel to the side walk, lateral connecting to a restroom on a park site)? 4. Have Highway Supervisor Plans and PGW gas plats been referenced to confirm there are no additional utilities present in these locations? 5. Are there mature trees or other constriant (utility poles, lights, etc.) present where the SMP footprint would be located? 6. Is steep topography present? (the SMP footprint should generally fit between two 2 foot topographic contour lines) 7. Make note of any potential programming conflicts that should be verified in the field. (ball fields, playgrounds, gardens, plazas, entrances, etc.) Page 11 of 14

12 STEP 3.3 ANALYSIS SITE VISIT TO CONFIRM RECOMMENDED LOCATIONS The purpose of visiting potential locations for GSI after Step 3.2 is to verify information found in the Desktop Analysis phase. During a site visit, the following things may be valuable to consider when determining potential project locations. These considerations for both recommended Street & Parcel Projects should be recorded on the system sheets, as well as in the Evaluation Notes section of the Data Tracking Spreadsheet (see step 4). Constriant and SMP level information will be recorded in the GIS concept layer. In a similar way as the drainage area layer, the concept layer will utilize a unique numbering system as shown below. [SIF Planning Study Work Number] S [Sequential number] For Example: If a project s work number was 9330, then the SMP ID s would be assigned sequentially as 9330 S0001, 9330 S0002, 9330 S0003, and so on. A four digit sequential number preceeded by an S should be used in order to maintain a consistent amount of characters within this numbering schema. The following fields should be populated in the atribute table of this layer. Figure 8: This image shows a snap shot of the fields that are to be populated for each SMP that is drawn in the concept layer layer in GIS. SYSTEM SHEETS: After site visits are complete, a final list of GSI systems will be reviewed by PWD. For each recommended system, a system sheet should be created that includes the following (see the template that has been provided). System overview General notes Utility information for proposed street crossings (type, size, depth, and data source) System summary table including GSI system number, site name, address, owner, drainage area IDs managed, estimated DA, max potential DA, SMP footprint size, SMP location, and recommended SMP functionality Map showing drainage areas managed, SMP footprints, and utility information Photographs Relevant site notes Page 12 of 14

13 SITE VISIT CONSIDERATIONS: 1. Confirm parcel name (if applicable) 2. Inlet locations identify type and current condition 3. Tree locations identify location (distances from corner should be measured and recorded); note condition. 4. Access points identify locations of gates, doors, curb cuts and driveways) distances from corner should be measured and recorded 5. Utility mark outs identify type (electric, gas, sewer, telephone, water, unknown) and distance from front of curb 6. Valves and manholes identify type (gas, sewer, water, unknown) 7. Handicap ramp locations and condition 8. Bus stops and other public transit 9. Sidewalk widths measure width of first sidewalk block and width to building and label condition (particularly at the low end of the block) 10. Sidewalk condition 11. Curb reveal identify material and reveal (particularly at the low end of the block) 12. Parking identify no parking zones and the type and location of parking spaces 13. Bike lanes note location 14. Overhead wires identify location and clearance 15. Runoff flow directions (i.e. surface flow directions) 16. Other miscellaneous features such as fire hydrants, bollards, traffic lights, and poles Page 13 of 14

14 STEP 4 RESULTS RECORDING RESULTS IN DATA TRACKING SPREADSHEET, GIS LAYERS, MAPS, AND OTHER GRAPHICS The Data Tracking spreadsheet will record whether a street segment or parcel is suitable for GSI, and which drainage areas are being managed by that street or parcel project. It is suggested that this spreadsheet should be (1) filled out as drainage areas and potential management locations are being analyzed, (2) altered when a site visit is conducted and the information has changed, and (3) edited once more after the results are reviewed and vetted by PWD. Since PWD imports this data directly into its planning projects tracking database, the provided template should be utilized to ensure that data is filled out in a consistent manner. For this reason, street segments and parcels should use the following naming conventions: Streets: X St from Y St to Z St Vacant Land: Address Schools, Parks, Facilities: Site Name The following categories should be used when selecting the Site Evaluation Result field for all public parcels, street segments, and selected private parcels in the Data Tracking Spreadsheet: Recommended for GSI: No physical constraints and can manage one or more drainage areas Under Evaluation: Some uncertainty about GSI suitability due to data limitations or physical constraints (This is a temporary status that will be eventually changed to recommended or on hold during the PWD vetting process). On Hold: Major physical constrains exist making it difficult to place GSI footprints following current design guidelines. In many cases, a street segment or parcel will have multiple drainage areas associated with that location (e.g. drainage areas on each side of a street segment being managed by a single SMP). If some of these drainage areas can t be managed at a recommended location, the Site Evaluation notes should note this. Once all information from the spreadsheet has been vetted by the PWD Project Manager, a final Site and Drainage Area Analysis Map will be created that shows GSI project locations and associated drainage areas. Update history June 2015 Added Planning Initiatives Evaluation to Step 2 Updated names of Steps 2 and 4 Updated drainage ID numbering system from a 4 to 5 digit number Update language about recording SMP level data in GIS attribute table instead of the data tracking spreadsheet Page 14 of 14