Presentation on theme: "Villanova Urban Stormwater Partnership www.villanova.edu/vusp1 Chapter 3 Stormwater Management Principles and Recommended Control Guidelines Robert G."— Presentation transcript:
Villanova Urban Stormwater Partnership Chapter 3 Stormwater Management Principles and Recommended Control Guidelines Robert G. Traver, Ph.D., PE, D. WRE Department of Civil and Environmental Engineering Director, Villanova Urban Stormwater Partnership
Villanova Urban Stormwater Partnership Mission Statement The mission of the Villanova Urban Stormwater Partnership is to advance the evolving comprehensive stormwater management field and to foster the development of public and private Partnerships through research on innovative SWM Best Management Practices, Directed Studies, Technology Transfer and Education. Research and directed studies will emphasize comprehensive watershed stormwater management planning, implementation, and evaluation. Technology transfer will provide tools, guidance and education for the professional. Partnership Goal is to promote cooperation amongst the private, public and academic sectors. www. villanova.edu/VUSP
Villanova Urban Stormwater Partnership Disclaimer What follows is my interpretation of Chapter 3 as a member of the BMP Manual Oversight committee –It is not PaDEPs official position –It is not the VUSPs Corporate Partners position – like other groups, the VUSP made many recommendations during the process. Some were, and some were not included.
Villanova Urban Stormwater Partnership BMP Manual Vs Model Ordinance BMP Manual –Guidance Option and Tools –Living document –Chapter 3 – relates to Ordinance Model Ordinance –Act 167 start point –Assistance to municipalities –Relates to chapter 3
Villanova Urban Stormwater Partnership Chapter 3 - Introduction This Chapter provides guidance for municipalities striving to improve their stormwater management programs. It presents stormwater management principles and recommends site control guidelines to address volume, water quality and flow rate. These guidelines can serve as the basis for municipal stormwater regulation……. All municipalities, regardless of their specific setting, are encouraged to enact the most comprehensive stormwater management ordinances possible. They should also work with their watershed neighbors to integrate their individual municipal actions within the watershed as a whole.
Villanova Urban Stormwater Partnership Model Ordinance POLICY: The Stormwater Management Program, and other DEP staff, will recommend to counties that they use this model ordinance as a template for developing municipal stormwater management ordinances when preparing Act 167 Stormwater Management Plans, to municipalities without an otherwise suitable stormwater management ordinance that they adapt and enact this model ordinance to meet NPDES MS4 permitting requirements, and that other municipalities may adapt and enact this model ordinance.
Villanova Urban Stormwater Partnership 10 – Stormwater Guidelines Managing stormwater as a resource; Preserving and utilizing existing natural features and systems; Managing stormwater as close to the source as possible; Sustaining the hydrologic balance of surface and ground water; Disconnecting, decentralizing and distributing sources and discharges; Slowing runoff down, and not speeding it up; Preventing potential water quality and quantity problems; Minimizing problems that cannot be avoided; Integrating stormwater management into the initial site design process; and Inspecting and maintaining all BMPs.
Villanova Urban Stormwater Partnership Recommended Site Control Guidelines Effective The morphologic impacts on streams from increased volumes of runoff during smaller storms are prevented. The guidelines will be effective on a site-by-site basis, as well as on a broader watershed-wide scale; Proportional The stormwater controls will produce approximately the same post-development stormwater discharge for all types of development in almost any location; Equitable The requirements are based on project characteristics rather than project location so that physically similar projects will have similar storm water controls; Flexible The diversity among Pennsylvanias 2,566 municipalities is accommodated by the guidelines. This diversity in physical conditions presents a major challenge that requires flexibility to achieve a uniform stormwater management program across the state.
Villanova Urban Stormwater Partnership Recommended Site Control Guidelines Water Volume –Focus Impervious and Compacted soils and smaller storms –Baseflow - Water Quality - Stream Bank Protection Peak Flows Flooding (assisted by Volume Reduction) Water Quality Additional Guidelines
Villanova Urban Stormwater Partnership Volume Controls Goals –Protect stream channel morphology; –Maintain groundwater recharge; –Prevent downstream increases in flooding; and greatest extent possible. –Replicate the natural hydrology on site before development to the greatest extent possible. Alternatives –Infiltration; –Capture and Reuse; and –Vegetation systems that provide ET, returning rainfall to the atmosphere
Villanova Urban Stormwater Partnership VOLUME CONTROLS B C
Villanova Urban Stormwater Partnership Volume Control Design Storm Method - may require more complex and detailed analyses - greater opportunity to select stormwater controls that require fewer resources to construct and operate Simplified Method - fewer computations and less design effort - slightly larger stormwater controls - for sites with less then an acre of disturbance - NOT allowed when routing required
Villanova Urban Stormwater Partnership Volume Control – Design Storm Do not increase the post-development total runoff volume for all storms equal to or less than the 2-year/24-hour event. –NOTE – the Model Ordinance uses REAL preexisting Existing (pre-development) non-forested pervious areas must be considered meadow (good condition) or its equivalent. Twenty (20) percent of existing impervious area, when present, shall be considered meadow (good condition) in the model for existing conditions for redevelopment
Villanova Urban Stormwater Partnership Design Storm Method – Scientific Basis –The 2-year event provides stream channel protection and water quality protection for the relatively frequent runoff events across the state; –Volume reduction BMPs based on this standard will provide a storage capacity to help reduce the increase in peak flow rates for larger runoff events; –In a natural stream system in Mid-Atlantic States, the bank full stream flow occurs with a period of approximately 1.5 years. If the runoff volume from storms less than the 2-year event are not increased, the fluvial impacts on streams will be reduced; –The 2-year storm is well defined and data are readily accessible for use in stormwater management calculations.
Villanova Urban Stormwater Partnership Design Storm Method – Practical Consideration –The 2-year 24-hour events for precipitation and runoff are well defined. –Data and software are readily accessible for application of 2-year 24- hour events in stormwater management calculations.
Villanova Urban Stormwater Partnership Volumes of Infiltration - 2 Year Storm Impervious Grass
Villanova Urban Stormwater Partnership Design Storm Method – Dr Ts Opinion & FAQs Is this a lot of water? –Depends on site Predevelopment Land Use Soil conditions Ability to reduce problem Is it harder then Detention? –Yes – But detention doesnt work. Is it replicating nature? –Depends how much you ET! –Surface Water – Probably –Ground Water – Depends on how much ET you replicate – Rock beds definitely no Is it desirable everywhere? –Lots of places Yes –Some places No Where is this easy? –Single Family homes (low % Imp) –Infiltrating soils Where is this difficult? –High % Impervious –Clay soils (brownfields, etc) Is maintenance a concern? –YES Maintenance is a critical design component.
Villanova Urban Stormwater Partnership Simplified Method Stormwater facilities shall be sized to capture at least the first two inches (2) of runoff from all contributing impervious surfaces. At least the first one inch (1.0) of runoff from new impervious surfaces shall be permanently removed from the runoff flow i.e. it shall not be released into the surface Waters of this Commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration. Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first one-half inch (0.5) of the permanently removed runoff should be infiltrated –Note – detention / retention BMPS must empty in between 24 and 72 hours Note < 1 acre disturbed area
Villanova Urban Stormwater Partnership Simplified Method – RTs Opinion and FAQs Use where it makes sense $$$ Use - retrofits, small disturbed areas Where you aren't planning to develop a hydrologic model Use where you have space –Wetlands / Wet ponds More versatile then Design Storm Method, but lots of times Design Storm is easier –Believe environmentally equivalent to Design Storm Method Is it harder then Detention? –Yes – But detention doesnt work. Is it applicable anywhere? –NO! < 1 Acre disturbed restriction (PaDEP Restraint) Is maintenance a concern? –YES Needs to be designed in. However if focused on green designs, (bioretention, bioinfiltration, green roofs), should have less maintenance and operational costs
Villanova Urban Stormwater Partnership BioInfiltration Traffic Island - Watershed 1.3 Ac 46% Impervious
Villanova Urban Stormwater Partnership Bioinfiltration Traffic Island (PA Growing Greener Grant -2001) Watershed – 1.3 acres Bowl Size -.3 Watershed Inches -.6 off impervious. NEVER any runoff until over 1 Land Use - The watershed includes a student parking lot, roadway and lawn areas. It is approximately 50% impervious. Design….. The island is designed to control smaller storms ( inches); infiltrating runoff, reducing downstream stormwater volumes, stream bank erosion, and nonpoint source pollution to the headwaters of the Darby Creek..
Villanova Urban Stormwater Partnership William Heasom
Villanova Urban Stormwater Partnership Example 2005 TI 77 Events 48 Rainfall (not all snow included) 7 Events Overflowed Yearly Summary Overflow 2.5 Pre (Meadow B)
Villanova Urban Stormwater Partnership Comparison to BMP Manual Design Storm Method –Would not meet criteria if preconstruction land use was all woods or meadow. Probably would as there was paved areas on the original site. Simplified Method –Would meet the criteria as we get no runoff with less then 2 or rainfall. However the site is 1.3 acres so not applies. Design Standards –Site foot print is MUCH smaller then 1:5 ratio –Ponding depth is well over a foot.
Villanova Urban Stormwater Partnership Retention and Detention Considerations Infiltration areas should be spread out and located in the sections of the site that are most suitable for infiltration. In all cases, retention and detention facilities should be designed to completely drain water quality volumes including both the permanently removed volume and the extended detention volume over a period of time not less than 24 hours and not more than 72 hours from the end of the design storm.
Villanova Urban Stormwater Partnership Peak Flow Do not increase the peak rate of discharge for the 1-year through 100-year events (at minimum); as necessary, provide additional peak rate control as required by applicable and approved Act 167 plans.
Villanova Urban Stormwater Partnership Water Quality Achieve an 85 percent reduction in post-development particulate associated pollutant load (as represented by Total Suspended Solids), an 85 percent reduction in post-development total phosphorus loads, and a 50 percent reduction in post-development solute loads (as represented by NO3-N), all based on post-development land use. Generally assumed met with either of the control measures – may require use of swales, or other Green practices
Villanova Urban Stormwater Partnership What if I cant do Design Storm Method and my project too big for the Simplified Method? RTs opinion GET YOUR COUNTY TO DEVELOP A TAILORED ACT 167 PLAN TO YOUR REGIONAL CHARACTERISTICS! Or… Meet with PaDep to discuss the problems, and develop an alternate design to meet the site conditions. ALTERNATIVE DESIGN –Minimize problem –Start with Simplified Method –Use Bio-Retention / Infiltration and Green Roofs –Use Green Extended Detentio –Take credit for infiltration during the storm –Calculate Pollutant Loadings –PE License –Design for Maintenance –DONT IGNORE THE ISSUES!
Villanova Urban Stormwater Partnership Infiltration Risk Idea Rock Bed First 1 Runoff goes to Raingarden Pretreatment Next 1+/- Runoff goes to Rock Bed As most rainfall is less then 1 This will extend the life of the rock bed as its use would be less frequent, and the runoff would be cleaner