- Public Works
- Roads Division
- Drainage Plans
A drainage plan is a proposed method for containing rainwater on your property. Clallam County requires drainage plans to control any increase in the amount of rainwater that runs off each piece of property as a result of the development of that property.
Drainage Plan Requirements
The requirements depend on your situation, generally falling into one of three categories:
Single-family residential development not governed by a pre-approved drainage plan: If you build in an area with no approved drainage plan (most of Clallam County).
Residential development governed by a previously approved drainage plan: If you build in one of these areas you must conform to the approved drainage plan. The County maintains a list of areas subject to pre-approved drainage plans. Any deviations from the plan must be prepared by a licensed civil engineer.
Non-residential developments, multi-family residential developments, or land divisions: These developments require a drainage plan prepared by a licensed civil engineer.
All drainage plans must be approved by the Road Department before a building permit will be issued.
As of January 1, 2022 Clallam County adopted with modifications the 2019 Storm Water Management Manual for Western Washington.
Residence have the option to utilize the Small Project Drainage Manual.
View the Stormwater Management Checklist and Application (PDF)
A site on your property must be available for the stormwater runoff which does not conflict with any septic drain field, structure foundations, or public wells.
Most of the rest of the lot must remain with vegetative cover such as grass, field, forest, or landscaping, and must not be covered with asphalt, concrete, driveways, or other relatively impervious surfaces. If the amount of such impervious surfaces exceeds 10% of the lot area, the County may require a drainage evaluation to be submitted.
Your soils must be sufficiently permeable. Soils which allow the use of either a conventional sewage disposal system or a pressurized sewage disposal system (due to excessively porous soils) are considered to be sufficiently permeable.
The above guidelines are intended to simplify the screening of drainage plans by the County and are not necessarily all-inclusive. The County reserves the option to require submittal of a drainage plan prepared by a licensed engineer for any development.
Drywell: This design works best in A, B and some C soils. CANNOT be placed on parcels under ½ acre with C soils.
Rain Garden: These work best in any soil, and on fairly flat parcels where the slope does not exceed 10%.
Rain Catchment: This works best when your well is not producing enough to water your landscaping.
Infiltration Trench ALTERNATIVE: This design works best with A, B (coarse sand and cobble) soils, max trench length 100’.
Dispersion Splashblock: This design works best for parcels over an acre in size with a slope of at least 5% and with 50’ or more of established lawn, pasture or wooded area.
Infiltration Trench: Works best in A, B soils, maximum trench length will not exceed 100’.
Engineered Drainage Plan: An engineered drainage plan will be required if a geotechnical report asks for it, the plat your parcel is in requires it, or if on slopes greater than 25% and/or critical areas present.
After a proposed drainage system is approved by the Road Department, it must then be installed, adhering to the approved plan.
The drainage system must be inspected by the Building Division prior to certifying the structure for occupancy. Do not backfill over any underground installation until it has been inspected. Drainage system inspections will be conducted only during regularly scheduled building inspections prior to the final inspection, and installation must be complete prior to the final inspection. The inspection requirements vary depending upon the type of system approved for your development. You can find out more about the inspection procedure when you obtain your permit.
Liability and Responsibility
You are responsible for damage caused by stormwater runoff due to your development. Clallam County's drainage requirements represent a good faith effort to address the potential problems associated with stormwater runoff due to development. However, the County has no control over the accuracy of information submitted and does not assume responsibility for damage which may occur due to stormwater runoff.