1.5 LID in Eastern Washington

As noted above, climate, landscape, soils and hydrology vary considerably across eastern Washington. Site-specific conditions, as well as the unique goals and objectives of each project must be taken into account when planning, designing, and constructing LID solutions.

The quality and habitat function of receiving waters in arid and semi-arid climates are affected by pollutants carried by stormwater runoff and by the changes in the patterns of runoff from the land following development. Hydrologic and water quality changes caused by urbanization can result in irreversible changes to the biological systems previously supported by the natural hydrologic system.

Some of the unique considerations associated with the arid and semi-arid environments include:
Intense, relatively infrequent storms.
  • High evapotranspiration rates.
  • Sparse vegetation, leaving soil prone to erosion.
  • Development patterns characterized by low density and large amounts of impervious surface area.

With this combination of factors, stormwater runoff can be a significant source of pollution to receiving waters in arid and semi-arid environments, despite the low mean annual rainfall volumes typically defining these areas.

The structural BMPs described in Chapter 4: LID BMPs can generally be described as infiltration-based or non-infiltration based and are generally categorized as follows:

Infiltration-based Practices:
  • Amended Construction Site Soils.
  • Dispersion.
  • Bioretention Swales (without under-drains) and Infiltration Planters.
  • Trees.
  • Permeable Pavement (without under-drain).

Non-infiltration-based Practices:
  • Vegetated Roofs.
  • Minimal Excavation Foundation Systems.
  • Rain water Collection Systems.
  • Bioretention Swales (with under-drain) and Flow-through Planter.
  • Permeable Pavement (with under-drain).

The infiltration-based practices are applicable only where site conditions are conducive to infiltration. This will not always be the case in eastern Washington, where limiting factors such as silty/clayey soils, shallow depth to bedrock, or a high water table may be present. Chapter 3: Designing for LID provides guidelines for evaluating infiltration feasibility and steps for designing infiltration-based practices where they are determined to be feasible.

The non-infiltration-based LID BMPs are potentially applicable on many sites and can be effective at reducing peak flow rates, volumes, and pollutants leaving the site through storage, evapotranspiration, and reuse of collected runoff.

Arid climate and cold weather are important design considerations in eastern Washington. Plant selection will vary among and within the four climate zones illustrated in Figure 1.1. Plant selection should consider tolerance for drought conditions, periods of infrequent inundation, extreme heat, and winter conditions including snow cover and freezing. The varied nature of these factors make plant selection and availability important design considerations.

Research data show that LID BMPs, when designed, maintained, and constructed properly, can perform better than conventional facilities in many instances. For example, studies performed at the University of New Hampshire showed LID systems exhibited less seasonal variability in treatment performance than conventional techniques relying on sedimentation as a primary removal mechanism (Roseen et al. 2009). Houle (2008) found infiltration rates in permeable pavements were retained in winter conditions with frost depths as high as 27 inches. Porous asphalt required 75 percent less salt than conventional pavement due to lower amounts of snow and ice accumulation.

All of the BMPs included in this Manual provide significant advantages above and beyond their ability tomanage stormwater runoff. For example, amended soils improve the health of the soils and promote landscapes that require less water and maintenance. Permeable pavement and vegetated roofs can significantly reduce the heat island effect and vegetated roofs can provide added insulation to buildings and reducing energy demands throughout the year. Minimal excavation foundation systems may allow development on rocky sites where traditional foundations may not be feasible. While LID principles and BMPs are largely applicable in many environments, special considerations for planning and design of LID facilities in eastern Washington are provided, as appropriate, in the chapters that follow.

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Figure 1.1 - Eastern Washington Climate Regions
Approximate Eastern Washington climate regions.
Source: Ecology Stormwater Management Manual for Eastern Washington (2004) and AHBL, Inc.