Introduction To The Highway Runoff Manual This introduction focus on: • An overview of the Highway Runoff Manual. • The definition of Minimum Requirements • What is a TDA? • BMP (Best Management Practices) selection process. • More on Runoff Treatment and Flow Control • BMP’s covered in StormSHED 3G
Chapter 1 Background Chapter 2 Planning and design integration into the project development process. Chapter 3 Minimum requirements & Applicability Chapter 4 Hydrologic analysis methods Chapter 5 Permanent BMP selection BMP design criteria Chapter 6 Construction BMP selection ESC, SPCC BMPs BMP design criteria Monitoring guidance Highway Runoff Manual
What is a Minimum Requirement • There are 9 minimum requirements that apply when planning and designing a stormwater management facility. • Which MR applies to a project is dependant several parameters including: the amount of impervious surface added, the presence of wetlands, watershed plans, thresholds and exceptions.
Activies Exempt From ALL Minimum Requirements – Typically Maintenance & Preservation Activities: • Patching • Resurfacing without expanding coverage area • Shoulder grading • Crack sealing • Re-shaping/re-grading drainage systems • Vegetation maintenance
Minimum Requirements 1-4 1 - Stormwater Planning - To demonstrate how water quality will be maintained both during project construction and in the final developed conditions. 2 - Construction Stormwater Pollution Prevention - To ensure that construction projects do not impairwater quality by allowing sediment to discharge from the site or allowing spills of pollutants. 3 - Source Control – Prevent pollutants from contacting and mixing with stormwater. Source control is more cost-effective than treatment. 4 - Preservation of Natural Drainage - Preserve natural drainage systems to the fullest extent and to prevent erosion.
Activities Subject only to Minimum Requirement 2 – Repairing roadway base or subgrade – Some underground utility projects – Some roadways replacements/removals
Minimum Requirements 5-9 5 - Runoff Treatment - To reduce pollutant loads and concentrations in stormwater. 6 - Flow Control - Prevent increases in the stream channel erosion rates; mitigate impacts of prior development and flow modifications. 7 - Wetland Protection - To ensure that wetlands receive the same level of protection as any other water of the state 8 - Basin/Watershed Planning - To promote the development of watershed based resource plans as a means to develop and implement comprehensive water resource protection measures 9 - Operations and Maintenance - To achieve appropriate preventative maintenance and perform checks to ensure that stormwater control facilities are adequately maintained and properly operated
What is a TDA? Threshold Discharge Area - An onsite area draining to a single natural discharge location or multiple natural discharge locations that combine within ¼ mile downstream. Same a basin, but the area is separated by the discharge locations.
TDA Delineation Why do we need TDAs? - consistency in determining flows - determine Minimum Requirements - identify outfalls
Best Management Practice (BMP) Selection • Stormwater BMP’s are the physical, structural, and managerial practices that when used singly or in combination, prevent or reduce the detrimental impacts of stormwater, such as the pollution of water, degradation of channels, damage to structures, and flooding.
Three Distinct Functions of BMP’s • Source control: Prevents or reduces the introduction of pollutants to stormwater (Mostly Construction). • Flow control: Offsets and attenuates the increased rate of discharge caused by impervious surfaces • Runoff treatment: Intercepts and reduces the physical, chemical, and biological pollutant loads generated primarily from highway use
Minimum Requirement 5 –Runoff Treatment Unless an exemption applies (see section 3-3.5 of the HRM), any project that adds 5,000 square feet or more of new Polutant Generating Impervious Surface (PGIS) in a TDA must provide runoff treatment of stormwater runoff. The intent is to reduce pollutant loads and concentrations in stormwater runoff.
Types of Runoff TreatmentTable 3-1 • Basic Treatment • Enhanced Treatment • Oil Control* • Phosphorus Control Based on: through ADT, ADT at an intersection, and/or if area is designated for phosphorus control. Note EWA oil control requirements and have changed.
RunoffTreatment Selection Process Figure 5-3.2
Minimum Requirement 6 – Flow Control Unless an exemption applies (see section 3-3.6 of the HRM), any project that adds 5,000 square feet or more of net-new impervious surfaces in a TDA must provide flow control of stormwater runoff. The intent is to prevent cumulative future impacts from increased stormwater runoff volumes and flow rates on streams and off WSDOT ROW.
Flow Control Selection Process Figure 5.3.1
Types of BMP’s Covered in StormSHED 3G Intermediate Class Biofiltration – basic runoff treatment Vegetative Filter Strip – basic runoff treatment Natural Dispersion – basic and enhanced runoff treatment and flow control Detention Pond – flow control Infiltration Pond – flow control and (if infiltration is within designated parameters) runoff treatment. Bioinfiltration Pond – basic, enhanced, and oil runoff treatment and flow control