Erosion Hazard Areas

The 1990 King County Sensitive Ares Ordinance (SAO) defines significant erosion hazard areas as those soils in King County that may experience severe to very severe erosion hazard. The 1990 KC SAO adopted the soils definition in the US Department of Agriculture Soil conservation Service (SCS) 1973 King County Soil Survey and the then current draft of the Snoqualmie Pass Area Soil Survey (No Date). The full text included in the 1990 Sensitive Areas Ordinance Folio is included below. I: Erosion Hazard Areas. Erosion is a natural process of wearing away of land surfaces by falling and running water, wind, and glacial scour. Of these geologic forces, erosion by running water is by far the most important within the Puget Sound Basin. The susceptibility of any soil type to erosion depends upon the physical and chemical characteristics of the soil, in addition to its protective vegetative cover, topographic position (slope length and gradient), the intensity of rainfall, and the velocity of runoff water. Whereas erosion is the wearing away of material, sedimentation or deposition is the accumulation of eroded matter where water carrying sediment is slowed. The effects of sedimentation are most pronounced where streams enter Puget Sound and in lowland lakes, ponds, wetlands, and stream channels. The natural geologic processes of erosion and sedimentation are frequently accelerated by land use modifications and urban development. The erosion hazard maps show areas of King County where-soils are particularly susceptible to increased erosion as a result of development. II: Definition. The Sensitive Areas Ordinance (SAO) defines erosion hazard areas as those soils in King County that may experience severe to very severe erosion hazard. The SAO adopts the soils definitions in the US Department of Agriculture Soil Conservation Service (SCS) 1973 King County Soil Survey and the current draft of the Snoqualmie Pass Area Soil Survey (ND). III: Methods. The following soil types were identified by the SCS King County Soil Survey as experiencing severe to very severe erosion hazard: Alderwood gravelly sandy loam (AgD), Alderwood-Kitsap (AkF), Beausite gravelly sandy loam (BeD and BeF), Kitsap silt loam (KpD), Ovall gravelly sandy loam (OvD and OvF), Ragnar fine sandy loam (RaD) Ragnar-Indianola Association (RdE), and any occurrence of River Wash (Rh), or Coastal Beaches (Cb). The Snoqualmie Pass Area Soil Survey includes 165 soil types identified as experiencing severe to very severe erosion hazard. These soils and those from the King County Survey were mapped as erosion hazard areas. Where the two surveys overlap, the soil types from the most recent map were used. III: Limitations. The SCS maps were compiled using the following method: 1) classification by aerial photographs; 2) field checking and verification; 3) correction of photographic distortion; and 4) transfer of soil types onto 1:24,000 scale maps. Each stage has an inherent degree of error, so field verification is important, especially along soil type boundaries. Since the final form of the Snoqualmie Pass Area Survey has not been published, the soil types were copied directly from the uncorrected aerial photographs to the 1:24,000 scale folio maps. Care was taken to match soil types logically to landforms and contours on the folio maps; however, the boundaries of erosion hazard areas covered by this survey are not conclusive. IV: Regulations. The regulations in the Sensitive Areas Ordinance for erosion hazards pertain to timing and methods of clearing for development. In general, clearing is restricted to the period between April 1 and November 1. There are two exceptions: 1) clearing of up to 15,000 square feet on individual lots, and 2) timber harvest pursuant to an approved forest practice permit. Clearing for roads and utilities shall occur first with subsequent clearing on individual lots permitted after approval of the associated building permit. A vegetation management plan is required for clearing on individual lots. An erosion control plan is required for all development proposals within erosion hazard areas. Please refer to the Sensitive Areas Ordinance and administrative rules for more details on the regulations. V: Bibliography. King County, Washington; 1990; “Sensitive Areas Ordinance”; Ordinance number 9614, August 29, 1990 - passed. US Department of Agriculture Soil Conservation Service; 1973; Soil Survey of King County, Washington. US Department of Agriculture Soil Conservation Service; No date; Snoqualmie Pass Area, Washington Soil Survey, unpublished final draft.