§ 22.52.110. Drainage Plan Required.

A.

Requirements. Drainage plans shall be prepared and submitted for review and approval by the Public Works Director, where required by this Title, by Article 9 (Planning Area Standards), or where a project:

1.

Increases or decreases runoff volume or velocity leaving any point of the site beyond those that existed prior to site disturbance activities; or

2.

Involves a land disturbance (grading, or removal of vegetation down to duff or bare soil, by any method) of more than 20,000 square feet; or

3.

Will result in an impervious surface of more than 20,000 square feet; or

4.

Is subject to local ponding due to soil or topographic conditions; or

5.

Is located in an area identified by the Public Works Director or building inspector as having a history of flooding or erosion that may be further aggravated by or have a harmful effect on the project or adjoining properties; or

6.

Is located within a Flood Hazard (FH) combining designation; or

7.

Is located over a known high recharge area identified by the Public Works Director; or

8.

Involves land disturbance or placement of structures within 100 feet of the top bank of any watercourse shown with a blue line on the most current USGS 7½ minute quadrangle map; or

9.

Involves hillside development on slopes steeper than 10 percent; or

10.

May, by altering existing drainage, cause an on-site erosion or inundation hazard, or change the off-site drainage pattern, including, but not limited to any change in the direction, velocity, or volume of flow.

B.

Exemptions. Preparation of a drainage plan is not required where grading is exclusively for an exempt agricultural accessory structure, crop production, or grazing. This shall include any agricultural roads used exclusively for these purposes when they do not require issuance of a County grading permit. Drainage plans may also be waived where authorized by the Public Works Director.

C.

Submittal. Where required by Subsection A, drainage plans are to be submitted with or be made part of the Zoning Clearance, Plot Plan, Minor Use Permit, Site Plan Review, Conditional Use Permit, grading permit, or construction permit application.

D.

Drainage plan content. Drainage plans shall be legible and accurately drawn, at an appropriate scale that will enable ready identification and recognition of submitted information. The Public Works Director may require drainage plans to be prepared by a registered civil engineer.

1.

Basic drainage plan contents. A drainage plan shall include the following information about the site:

a.

Flow lines of surface waters onto and off the site.

b.

Existing and finished contours at two-foot intervals or other topographic information required by the Public Works Director.

c.

Building pad, finished floor and street elevations, existing and proposed.

d.

Location and graphic representation of all existing and proposed natural and man made drainage facilities for storage or conveyance of runoff, including drainage swales, ditches, culverts and berms, sumps, sediment basins, channels, ponds, storm drains and drop inlets. In addition, private water wells and sewage disposal systems must be shown. Include detailed plans of all surface and subsurface drainage devices, walls, cribbing, dams and other protective devices to be constructed with or as a part of the proposed work.

e.

Proposed flood-proofing measures where determined to be necessary by the Public Works Director and in accordance with Federal Emergency Management Agency (FEMA) requirements.

f.

For projects where the Director or Public Works Director determines that increased discharge rates and durations could result in off-site erosion or other impacts to beneficial uses, the project shall incorporate appropriate hydromodification measures as identified in the Low Impact Development (LID) Handbook. Such measures shall be clearly depicted on the drainage plan.

2.

Engineered plan content. In addition to the information required by Subsection D.1, engineered drainage plans are to include:

a.

An evaluation of the effects of projected runoff on adjacent properties and existing drainage facilities and systems.

b.

A map showing the drainage area and hydraulic calculations showing the facilities flow carrying capacities for the design storm event and justifying the estimated runoff of the area served by any drain. Include design discharges and velocities for conveyance devices, and storage volumes of sumps, ponds, and sediment basins based on the design storm.

c.

Estimates of existing and increased runoff resulting from the proposed improvements and methods for reducing velocity of any increased runoff.

d.

Methods for enhancing groundwater recharge that have been incorporated into the project design or an explanation of non-necessity of groundwater recharge for this site

[Amended 1999, Ord. 2863; 2010, Ord. 3188] [22.05.034/036]