§ 16.68.030. Flood control and drainage.

A.

General Provisions.

1.

The minimum design for facilities which control drainage of storm water generated within a subdivision or other residential, commercial, or industrial development, or for floodwater flowing into or crossing a subdivision or other residential, commercial, or industrial development shall be based on a storm having a frequency of once in one hundred (100) years.

Hydrologic and hydraulic calculations for the design of drainage facilities which control drainage water generated within a subdivision or other residential, commercial or industrial development shall be submitted for approval to the city engineer. Hydrologic and hydraulic calculations for the design of flood control facilities to control floodwater flowing into or crossing a subdivision or other residential, commercial or industrial development shall be submitted for approval to the city engineer.

2.

Watercourses shall be shown as easements when required by the staff advisory committee, and storm drains shall be placed in easements when public rights-of-way are not available or adequate. The staff advisory committee shall require watercourses to be placed entirely in underground conduits, adequately fenced, or otherwise improved in accordance with the city's adopted improvement standards and this chapter. Where sumps are approved to handle drainage as an interim solution, feasible easements shall be provided for necessary channels and sump areas. Fences required for watercourses and sumps shall meet the requirements set forth in the city's adopted improvement standards and this chapter.

3.

The use of streets for flood control and drainage purposes may be prohibited by the city engineer if the use thereof is not in the interest of the public health, safety and welfare.

4.

When the city engineer permits the use of streets for flood control and drainage purposes, the ten (10) year frequency design discharge shall be contained between the tops of curbs or asphalt concrete dikes, and the one hundred (100) year frequency design discharge shall be contained within the street right-of-way. If either of these conditions is exceeded, additional flood control facilities shall be provided.

B.

Flood Control.

1.

The city engineer shall review the hydrologic calculations submitted by the subdivider or developer, and determine the adequacy of peak discharges of off-site floodwaters impinging upon the subdivision or other residential, commercial or industrial development from which protection must be provided.

2.

Improvement plans for flood control facilities to control floodwater flowing into or crossing a subdivision or other residential, commercial or industrial development shall be approved by the city engineer.

3.

After receipt of an acceptable tentative map, or other equivalent application for a discretionary permit for development pursuant to this title, the city engineer shall recommend conditions to be imposed. He or she may also require that a flood hazard report be furnished by the subdivider or developer. A flood protection study review fee as established by city council resolution shall be paid upon the submittal of the hydrology report to the planning director.

There shall be no flood protection study required for reverting subdivided lands to acreage.

C.

Drainage.

1.

Improvement plans for drainage facilities to control drainage or storm water generated within a subdivision or other residential, commercial or industrial development shall be approved by the city engineer.

2.

In subdivisions or other residential, commercial or industrial development where grading is not proposed, the following criteria are established.

a.

Where streets on sustained gradients cross natural drainage courses, adequate culverts shall be provided to accommodate, the one hundred (100) year storm with maximum ponding to an elevation two feet below the road centerline profile grade, provided diversion of ponded water into another drainage area will not result therefrom.

b.

Runoff in natural drainage courses exceeding the ten (10) year storm may be permitted to overtop the roadway in dip sections where, in the opinion of the city engineer, topography, soil conditions, adjacent development and available all-weather routes indicate its feasibility. If permitted, the roadway embankment slopes shall be adequately protected.

c.

Culverts of adequate size, but not less than eighteen (18) inches in diameter or equal, to prevent the ten (10) year storm from overtopping the roadway shall be provided in dip section, or as approved by the city engineer.

d.

Streets crossing improved channels shall be provided with culverts of adequate size to permit passage of the channel design flow or such other type of crossing as approved by the appropriate flood control agency and the city engineer.

e.

When a subdivision or other residential, commercial or industrial development substantially changes, concentrates or increases the natural flow of surface water onto adjacent property, one of the following shall be required:

i.

The water shall be directed to an adequate outlet which is either existing or will be constructed as part of the subdivision or development;

ii.

The subdivider or developer shall obtain a recordable easement or written agreement for drainage purposes across the affected property; or

iii.

The subdivider or developer shall, at a minimum, be required to provide onsite retention facilities for the incremental increase in runoff which will be created by the subdivision or development.

D.

Design Requirements for Post Construction Stormwater Compliance.

All new development or redevelopment projects that create and/or replace greater than or equal to two thousand five hundred (2,500) square feet of impervious surface (collectively over the entire project site) must comply with the Design Requirements for Post Construction Stormwater Compliance adopted by the city and incorporated herein by this reference.

(Prior code § 9-15.030)

(Ord. No. 660, § 5, 2-25-2014)