(A) Drainage Report Required:
1. A drainage plan and report prepared by a registered professional engineer is required for all new subdivisions, and for building permits for commercial construction, or multi-family developments within the planning jurisdiction.
2. This section is intended to define the basic criteria for drainage plans which shall be subject to review and evaluation by the city. Supplemental information may be requested. The design of drainage and treatment facilities to handle development drainage and storm and meltwater cannot be efficiently covered by a set of rigid requirements. Each development and designer may have unique solutions to their specific development project's needs. This section therefore outlines the elements to be considered during planning and design to promote consistency in natural, storm and meltwater drainage and treatment system design and evaluation.
3. Failure to provide required information will result in delays in processing applications and, in appropriate cases, rejection of those applications by the administrator.
(B) Content Of Report: A drainage and treatment plan and report must address each of the following areas furnishing all requested information. Submittals not providing the required information will be returned without review.
1. Basin Characteristics:
(a) Project site.
(b) Upstream contributing area.
(c) Combined drainage areas.
(d) Physical characteristics.
(e) Existing drainage facilities impacted by the proposed development on the site and downstream of the development.
(f) Existing treatment facilities.
2. Hydrological analysis of the project site and total drainage area (including upstream tributary areas). Minimum design frequencies shall be based upon tables 9.6.04A and 9.6.04B in this section. Rainfall intensities shall be based upon figure 9.6.04 in this section. The report shall show all calculations, assumptions, and methods.
(C) Drainage Plan: A drainage and treatment plan shall be developed in accordance with the requirements herein and the McCall drainage management guidelines.
Storm water conveyance systems shall be designed using the 10-year design storm frequency, with the following exceptions:
Land Use Zone | Design Storm Frequency |
Any drainage crossing a roadway | 25 |
Slopes in excess of 12 percent | 25 |
Major structures, such as bridges | 100 |
Where an existing, primary storm conveyance runs through a property (a primary storm conveyance is defined as a channel, ditch, or pipe with a drainage area of 10 acres or larger) | 100 |
Where an existing, secondary storm conveyance runs through a property (a secondary storm conveyance is defined as a channel, ditch or pipe with a drainage area from 3 to 10 acres) | 25 |
Note: The minimum pipe size that will be accepted by the city of McCall for new conveyances will be 15 inches in diameter.
All land improvement activities which contain from 2 to 5 acres of total site area or from 5,000 to 15,000 square feet of proposed impervious surface | Must detain the 2-year 24 hour design storm |
All land improvement activities which contain greater than 5 acres of total site area or greater than 15,000 square feet of proposed impervious surface | Must detain, on site, the increase in runoff that is estimated will be caused by the proposed project for the 10-year 24 hour storm |
Note: Emergency spillways must be designed for the 100-year design storm.
1. Drainage Control And Treatment Facilities:
(a) The plan shall include drainage and treatment plan showing all proposed facilities.
(b) Final design submittal shall show detailed information of all proposed drainage and treatment facilities and improvements.
2. Downstream Impacts: Downstream impacts, if any.
3. Operation And Maintenance: A description of operation and maintenance requirements must be included in the drainage and treatment plan and report. Items to be discussed include access, maintenance procedures, safety procedures, operation and control procedures, ownership, and any unique elements of the system. (Ord. 822, 2-23-2006, eff. 3-16-2006)