Storm Water Design Manual Notes

Last updated October 11, 2024
By Emma Howland

Overview

County must comply with National Pollutant Discharge Elimination System (NPDES) Issued by the Washington State Department of Ecology pursuant to the Clean Water Act.

The manual outlines:

  • Drainage Review and Requirements for water runoff policies
  • Drainage plan Submittal including report and plan formats/scopes
  • Hydrologic analysis and design of methods used to estimate runoff and design flow control, conveyance, and water quality facilities
  • Conveyance system analysis design methods and criteria
  • Flow control design methods and criteria and design of flow control facilities
  • Water Quality Design methods and criteria of analysis and design of water quality facilities

Definitions:

  • Arterial: a high traffic-volume road or street primarily for through traffic and includes roads or streets considered “collectors”. it does not include local access roads which are generally limited to providing access to abutting property.
  • Bioretention: a flow control best management practice (BMP) consisting of a shallow landscaped depression designed to temporarily store and promote infiltration of stormwater runoff. Standards include soil mix, plants, storage volume and feasibility criteria, are specified.
  • Construct or Modify: install a new drainage pipe or ditch or make improvements to an existing drainage pipe or ditch for purposes other than maintenance that either serves to concentrate previously unconcentrated surface water or stormwater runoff or serves to increase, decrease, or redirect the conveyance of surface water or stormwater runoff.
  • Critical aquifer recharge area: the critical area designation, defined and regulated in KCC 21A that is applied to areas where extra protection of groundwater quantity and quality is needed because of known susceptibility to contamination and importance to drinking water supply.
  • Critical Drainage Area: an area where the Department of Natural resources and Parks has determined that additional drainage controls are needed to address a sever flooding, drainage, and/or erosion condition that poses an imminent likelihood of harm to the welfare and safety of the surrounding community.
  • Effective Impervious Surface: those impervious surfaces that are connected via sheet flow or discrete conveyance to a drainage system. Impervious surfaces are considered ineffective if 1) the runoff is fully dispersed as described in Appendix C, 2) residential roof runoff is infiltrated in accordance with the full infiltration BMP described in Appendix C or 3) approved continuous runoff modeling methods indicate that the entire runoff file is infiltrated.
  • Flow Control BMP: Small scale drainage facility or feature that is part of a development site strategy to use processes such as infiltration, dispersion, storage, evaporation, transpiration, forest retention, and reduced impervious surface footprint to mimic pre-developed hydrology and minimize stormwater runoff
  • Groundwater Protection Areas (also, Critical Aquifer Recharge Areas): sole source aquifer areas as designated by the federal Environmental Protection Agency, and wellhead protection areas as mapped by the Washington State Department of Health.
  • Impervious Surface: a hard surface area that either prevents or retards the entry of water into the soil mantle as under natural conditions before development; or that causes water to run off the surface in greater quantities or at an increased rate of flow compared to the flow present under natural conditions prior to development. Common impervious surfaces include, but are not limited to:
    • Roof
    • walkways
    • patios
    • driveways
    • parking lots
    • storage areas
    • areas that are paved, graveled, or made of packed or oiled earthen materials or other surfaces that similarly impede the natural infiltration of surface water or Stormwater.
    • permeable pavement
    • vegetated roofs
    • pervious surfaces with underdrains designed to collect stormwater runoff
  • Low Impact Development (LID): A stormwater and land use management strategy that strives to mimic pre-disturbance hydrologic processes of infiltration, filtration, storage, evaporation and transpiration by emphasizing conservation, use of on-site natural features, site planning and distributed stormwater management practices that are integrated into a project design.
  • LID Best Management Practices (LID BMPs) also Flow Control BMPs: bioretention, permeable pavements, limited infiltration systems, roof downspout controls, dispersion, soil quality and depth, and minimal excavation foundations
  • Rain Garden: a shallow, landscaped depression with compost-amended native soils and adapted plants. The depression is designed to pond and temporarily store stormwater runoff from adjacent areas and allow stormwater to pass through the amended soil profile.
  • Steep Slope Hazard Area: a critical area designation that is applied to areas on a slope of 40% or more within a vertical elevation change of at least 10 feet.
  • Threshold discharge area: an onsite area draining to a single natural discharge location that combine within one-quarter-mile downstream as determined by the shortest flowpath.

There are a total of 9 core requirements as follows:

  1. Discharge at the Natural Location: All storm water runoff and surface water from a project must be discharged at the natural location so as not to be diverted onto or away from downstream properties. Intent: to prevent adverse impacts to downstream properties caused by diversion of flow from one flowpath to another, and to discharge in a manner that does not significantly impact downhill properties or drainage systems.
  2. Offsite Analysis: All proposed projects must submit an offsite analysis report that assesses potential offsite drainage and water quality impacts associated with development of the project site and proposes appropriate mitigation of those impacts
    • initial submittal must include at min. a level 1 downstream analysis-which examines the drainage system within one quarter mile downstream of the project site or farther. It is intended to identify existing or potential/predictable downstream flooding, erosion, and water quality problems that should be mitigated. A secondary component is an evaluation of the upstream drainage system to verify and document that significant flooding and erosion impacts will not occur as a result of the project.
      • Task 1: define and map the study area
      • Task 2: Review all available information on the study area
      • Task 3: Field inspect the study area
      • Task 4: Describe the drainage system and its existing predicted drainage and water quality problems
    • Intent: to identify and evaluate offsite flooding, erosion, and water quality problems that may be created or aggravated by the proposed project and to ensure appropriate measures are provided for preventing creation or aggravation of those problems.
  3. Flow Control Facilities: all proposed projects must provide onsite flow control facilities to mitigate the impacts of storm and surface water runoff generated by new impervious surface, new pervious surface, and replaced impervious surface targeted for flow mitigation. Intent: to ensure the minimum level of control needed to protect downstream properties and resources from increases in peak, duration and volume of runoff generated by new development
  4. Conveyance System: all engineered conveyance system elements for proposed projects must be analyzed, designed, and constructed to provide a minimum level of protection against overtopping, flooding, erosion, and structural failure. Intent: to ensure property design and construction of engineered conveyance system elements (primarily engineered elements such as pipes, culverts, ditches and channels)
  5. Construction Stormwater Pollution Prevention (CSWPP): All projects that will clear, grade, or otherwise disturb the site must provide erosion and sediment controls to prevent, to the maximum extent practicable, the transport of sediment from the project site to downstream drainage facilities, water resources, and adjacent properties. To prevent sediment transport and pollutant discharges as well as other impacts related to land-disturbing and construction activities, Erosion and Sediment Control (ESC) measures and Stormwater Pollution Prevention and Spill Control (SWPPS) measures that are appropriate to the project site must be applied through a comprehensive Construction Pollution Prevention Plan (CSWPP). Intent: prevent transport of sediment and other impacts like increased runoff which can result in flooding from obstructed drainage ways, smothering of salmon spawning beds, algal blooms in lakes, and exceedances of state water quality standards for turbidity. To prevent, reduce, or eliminate the discharge of pollutants to onsite or adjacent stormwater systems or watercourses from construction-related activities such as materials delivery and storage, onsite equipment fueling/maintenance, demolition of existing buildings and disposition of demolition materials and other waste, and concrete handling, washout, and disposal.
  6. Maintenance and Operations: Drainage facilities must be maintained and operated in accordance with King County Standards. Intent: Ensure that the maintenance responsibility for drainage facilities is clearly assigned and that these facilities will be properly maintained and operated in perpetuity.
  7. Financial Guarantees and Liability: all drainage facilities constructed or modified must comply with the financial guarantee required by King County Ordinance 12020. Intent: to ensure financial guarantees are posted to sufficiently cover the cost of correcting, if necessary, incomplete or substandard drainage facility construction work, and to warrant for two years the satisfactory performance and maintenance of those newly-constructed drainage facilities to be assumed by King County for maintenance and operation.
  8. Water Quality Facilities: All proposed projects must provide water quality (WQ) facilities to treat the runoff from those new and replaced pollution-generating impervious surfaces and new pollution-generating pervious surfaces targeted for treatment. Intent: To require an efficient, cost-effective level of water quality treatment tailored to the sensitivities and resource protection needs of the downstream receiving water to which the project site drains, or, in the case of infiltration, protection of the receiving groundwater system.
  9. Flow Control BMPs: All proposed projects must provide onsite flow control BMPs to mitigate the impacts of storm and surface water runoff generated by new impervious surface, new pervious surface, existing impervious surfaces, and replaced impervious surface targeted for mitigation as specified in the following sections. Flow control BMPs are methods and designs for dispersing, infiltrating, or otherwise reducing or preventing development related increases in runoff at or near the sources of those increases. Intent: to provide mitigation of hydrologic impacts that are not possible/practical to mitigate with a flow control facility. Such impacts include increases in runoff volumes and flashiness and decreases in groundwater recharge. Increased runoff volume and flashiness leads to higher and more variable stream velocities at low flows and more frequent water level fluctuations in streams and wetlands that can cause washout of organic matter, loss of vegetation diversity and habitat quality, disruption of cues for spawning, egg hatching, and migration. Decreased groundwater recharge reduces water supply for human use and summer base flows in streams which is critical to water temperature, salmonid use of smaller streams, and the habitat quality of mainstem side channels and wetlands used for spawning, rearing and flood refuge.