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 which runs off each piece of property as a result of the development of that property.
Why Drainage Requirements?
Almost any form of development increases the amount of rainfall that tends to run off property after a heavy rainstorm. Removing trees and other vegetation reduces the amount of water absorbed by roots and the subsequent evaporation through the leaves. Regrading the land eliminates natural pockets and depressions in the soil which hold water until it is absorbed into the soil. Covering ground with roofs, pavements, and similar impervious covers prevents the underlying soil from accepting surface water. Covering porous soil with comparatively impervious top soil and lawn decreases the rate the soil can absorb water. Driving over the ground (both during and after construction) compacts the surface, decreasing the small voids in the soil which let the water filter into the ground. Placing plastic films under landscaped areas prevents the underlying soil from absorbing water. These and other factors combine to decrease the ability of the native soil to absorb rainfall as well as it would have prior to development. To minimize the impacts of stormwater runoff associated with development, Clallam County requires appropriate management of that runoff.
When do I need a drainage plan?
In this county, an approved drainage plan is required prior to the issuance of a building permit for either a new structure or for the enlargement of an existing structure. Also, the Department of Community Development will inform land development applicants of any drainage requirements for their project(s).
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), the standard method is to install drywells of a prescribed size.
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.
The Standard Method
The standard method of controlling stormwater runoff from a structure is to route all roof runoff into downspout drywells. If you meet the criteria for this method and want to use it, you must submit a Plot Plan (forms are available from the County). This will usually constitute a pre-approved drainage plan. No further drainage evaluations or plans are usually necessary, and your building permit will not be delayed due to drainage review. (Drywell design information is included later in this brochure.) The standard method can be used if the following criteria are met:
A site on your property must be available for the drywell which does not conflict with any septic drainfield, 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 (see Alternate Methods).
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.
After a proposed drainage system is approved by the Road Department, it must then be installed, adhering to the approved plan. If you use the standard method, you must install the drywell(s) with the required volume of drain rock according to the guidelines in this brochure.
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.
If you plan to utilize downspout drywells to control stormwater runoff, you will need to observe the following:
- Submit a plot plat of your proposed development. Include locations of proposed drywells with their sizes. Draw arrows showing which roof area will be routed into each drywell. (Plot plan forms with instructions are available from the County.)
- Keep the amount of pavement, concrete, driveways, and other relatively impermeable surfaces to less than 10% of the total lot area, thus limiting excess runoff caused by sources other than the roof.
- The location of the drywells must be coordinated with the locations of the septic drainfield, wells, and building foundations so as to not adversely affect them. We suggest drywells be at least 30 feet from the septic drainfield and at least 5 feet from foundations.
- The drywell must be deep enough to penetrate a porous soil strata so the water can percolate out of the drywell.
- The roof runoff must be routed to the drywell, preferably tightlined.
- The drywell(s) must be filled with round, washed drain rock (1 1/2" to 3" in diameter). Any number of drywells may be used, provided the volume of drain rock in each drywell is adequate to receive the rain runoff from that portion of roof area being routed to it. This volume (measured in cubic yards) can be determined using one of the following formulas which are based upon records of rainfall intensity over 30 minute intervals:
Sequim area: (Roof area) x 0.0048
Port Angeles area: (Roof area) x 0.0047
Forks area: (Roof area) x 0.0099
Clallam Bay/Sekiu: (Roof area) x 0.0072
- The top and sides of the drywell must be covered and wrapped with filter fabric so the fine soils do not migrate into the voids of the drain rock. Landscaping cloth (generally available at hardware stores) works well for this purpose. A screen must be provided either at the outlet to the gutter or at the inlet to the drywell pipe to keep debris from entering the drywell.