Chapter 19.20 STORMWATER DRAINAGE

Chapter 19.20
STORMWATER DRAINAGE

Sections:

19.20.020 Purpose and applicability.

(1) The city of Entiat has found that future stormwater drainage problems may be reduced or avoided if future developers, both private and public, provide for storm and surface water drainage of their respective properties. Storm drainage standards and guidelines are set forth to protect life and property from loss and damage by flooding and to protect streams, creeks, and lakes from pollution and excessive flows.

(2) The storm drainage standards and guidelines contained herein are intended to reduce and prevent adverse storm drainage impacts. They represent the minimum design standards for the construction of storm drainage facilities within Entiat. Compliance with these standards does not relieve the designer, owner, or developer of the responsibility to apply conservative and sound professional judgment to protect the health, safety and welfare of the general public. Special site conditions and environmental constraints and considerations may require a greater level of protection than would normally be required under these standards.

(3) Plans for permanent drainage and stormwater detention facilities shall be provided for property improvements within the city in accordance with the standards for the following types of improvements:

(a) All types of land divisions as defined in EMC Title 16.

(b) All developments, including remodeling, reconstruction and new construction adding a total of 5,000 square feet or more of new impervious surfaces, including gravel surfaces and roof areas.

(c) Developments entailing construction which would change the point of discharge of surface waters, discharge surface waters at a higher velocity and/or quantity than that of the pre-development discharge rate, or tend to add to pollution of surface waters.

(d) Any proposed development adjacent to or within frequently flooded areas and/or any stream, river, wetland or other surface water body.

(e) At the discretion of the city public works director, plans may be required for other types of development not listed herein based on the possibility for future problems related to stormwater drainage.

(4) The plan requirement established in the previous section shall not apply when the city determines that the proposal meets all of the following conditions:

(a) The proposed development will not seriously and adversely impact the water quality conditions of any affected receiving bodies of water.

(b) The proposed development will not substantially alter the drainage pattern or increase the peak discharge.

(c) The proposed development will not cause runoff exceeding the available capacity of the existing drainage system where such system was designed to serve the proposed development area.

(5) A single-family residence with less than 3,500 square feet of impervious surfaces is exempt from plan requirements. However, all buildings with down spouts or other point sources shall provide a drainage plan. (Ord. 661 § 1, 2006)

19.20.030 General requirements.

All persons proposing land development and/or approvals as outlined in EMC 19.20.020 shall provide a drainage plan for surface water flows entering, flowing within and leaving the subject property. The plan is to conform to the following standards and requirements:

(1) When required by this chapter, plans for stormwater management shall be prepared by a registered civil engineer currently licensed by the state of Washington and qualified by experience and education in the field of hydraulics, hydrology, or a closely related field. Storm drainage plans or revisions to any approved plan shall be reviewed and approved by the city prior to any construction.

(2) On-site storm drainage improvements must be sufficient to mitigate impacts on runoff, erosion, sedimentation and pollution.

(3) All drainage system elements must be designed to allow for adequate maintenance and accessibility at all times.

(4) Drainage systems shall be designed to meet applicable state and federal requirements for water quality prior to discharge to any wetland, stream, or lake. The developer shall be responsible for obtaining any local, state, or federal construction or discharge approvals or permits.

(5) In no case shall storm drainage facilities be allowed to connect to sanitary sewers.

(6) Manhole standards of the sewer design details shall apply to stormwater drainage facilities.

(7) No drainage originating inside of a building or structure shall be connected to the storm drainage of surface water systems.

(8) All surface and stormwater runoff from a proposed development that would construct new or modify existing drainage facilities should be discharged at the natural location. Diversions may be allowed to correct an existing problem.

(9) Proposed developments should identify the upstream tributary drainage area and provide an analysis of the pre-existing drainage volume and quality and an analysis of the impact of the proposal on the drainage system. The post-development peak rate runoff shall not exceed the pre-development peak rates for the site. The methods of peak rate runoff control may include detention, retention and/or infiltration. On-site biofiltration in combination with infiltration systems is the preferred method for management of on-site stormwater and shall be considered before transporting stormwater off-site.

(10) For all proposed development requiring a drainage conveyance system, the conveyance system must be analyzed, designed and constructed to handle existing off-site tributary flows and on-site storm drainage flows caused by development of the project.

(11) Developments involving clearing and grading and which require new or modification of existing drainage facilities must include an erosion/sedimentation control plan meeting the Washington State Department of Ecology requirements and providing suitable measures to prevent sediment-laden runoff from leaving the site during construction.

(12) The long-term maintenance and operation of proposed private drainage facilities is the responsibility of the property owner or properly formed homeowner’s association and shall be performed in compliance with the city’s maintenance standards. The city reserves the right to perform maintenance on a private drainage facility when the facility is out of compliance with maintenance standards, as determined by the public works director. Any and all costs associated with this city maintenance will be paid by the property owner or homeowner’s association.

(13) Adequate easements shall be provided for operation and maintenance of all drainage facilities located on private property.

(14) Construction of storm drainage facilities may be required to comply with the bonding and insurance requirements contained within this title. (Ord. 661 § 1, 2006)