(a) Introduction.
(1) Every subdivision and land development shall be provided with a stormwater management system which
is adequate to serve the area and meets the requirements of the current edition of the Mason Stormwater Manual and other criteria of the city.
(2) A. Developers are required to design improvements so that in a 100- year storm, the rate of stormwater runoff leaving the project area at strategic points is no more after development than if the project area had remained undeveloped. If necessary, detention/retention facilities shall be constructed to assure that this requirement is met.
B. Where an existing site is being partially or totally redeveloped all requirements of this chapter will be in full force and effect. If conditions warrant on partially redeveloped sites and the developer can show that the application of all requirements would cause a hardship, he or she may request partial relief from Planning Commission.
(3) The Planning Commission may waive requirements for an individual detention/retention basin if a common or regional detention/retention basin of adequate design is available or if the city is reasonably certain one will be constructed and if the major drainage system from the project area to the common or regional detention/retention basin is such that the public health, safety and welfare will not be in jeopardy. If this option is exercised, the developer shall agree in writing to participate in the cost of the common or regional detention/retention basin whether already constructed or planned. The amount of participation and method of collection will be determined by the city.
(4) It is not the intent of this chapter to hinder innovative and creative solutions to drainage problems, however, in the interest of expediting the processing of plans and construction, use of standard procedures, forms, nomographs, charts and computer programs is necessary. Deviation from these standards will cause delay in the approval process.
(5) Although the submission requirements are specific, they are also the minimum requirements. The City Engineer may recommend to the Planning Commission a higher degree of protection than specified if the design results do not appear adequate to protect the health, safety and welfare of the community.
(6) Stormwater management systems shall be designed for the ultimate use of the land.
(7) Subsurface drainage such as underdrains may be required at the discretion of the City Engineer.
(8) Continued maintenance. Once a Stormwater Management Plan has been approved and constructed it shall be the responsibility of the property owner to maintain the facility as designed and constructed and to ensure its proper operation to meet the intent and requirements of this chapter at all times.
(b) Stormwater management system.
(1) The development of a comprehensive stormwater management system requires providing two separate and distinct drainage systems, the minor system and the major system.
A. The minor drainage system is for collecting and transporting runoff from frequently occurring storms. It includes open channels, street curbs and gutters, and underground storm sewers, manholes, catch basins and culverts. This system’s purpose is to lessen or eliminate inconveniences and safety and health hazards associated with frequent storms. Except where indicated otherwise, design criteria and requirements of this chapter are directed to the minor drainage system.
B. The major drainage system is to ensure that stormwater runoff which exceeds the capacity of the minor drainage system has a route to follow to the retention basin. It shall be recognized that the major drainage system exists even when it is not planned and whether or not physical facilities are intelligently located in respect to it.
(2) Submission requirements for subdivisions. Plans, profiles and supporting documentation to verify conformance with this chapter shall be submitted along with the usual plan submissions required in Subdivision Rules and Regulations.
A. Preliminary plans. In addition to the subdivision requirements, a plan showing the total area contributing runoff to the subdivision or project area being considered shall be submitted with the preliminary plans. This plan shall contain, but is not limited to, the following information:
1. A contour plan showing the outline of all areas outside the project area that contributes runoff to it;
2. Estimated runoff (Q) before and after development for terminal points along natural streams, proposed open channels and other strategic points such as existing storm sewers or culverts;
3. Location of proposed detention/retention areas; and
4. Any other information required by the city to clarify intent.
B. Improvement plans. In addition to the subdivision requirements, the improvement plan for the project area shall contain, but is not limited to, the following information:
1. Diameter, length, slope, type pipe and class of all storm sewers, culverts and subsurface drainage;
2. Invert elevations on profiles of all pipes at terminal points such as manholes inlets, catch basins and head walls;
3. Top of grate elevations of manholes and grate flowlines of catch basins and inlets;
4. Type of catch basin, inlet and manhole (ODOT or city designation);
5. Headwall type (ODOT or city designation);
6. Actual existing and proposed cross sections of open channels showing width of bottom, depth of water, erosion control measures and limits, and side slopes at each point of design along with a profile indicating the longitudinal slope and bottom elevations at the terminal points of design;
7. High and low points indicating the direction of runoff flow along the profile of the roadway;
8. Structural details and design data for detention/retention facilities;
9. Details of construction for all structures not included in the city standard construction drawings, or other referenced standards;
10. Easements;
11. Detention/retention facilities; and
12. Any other information required by the City Engineer to clarify intent or design features.
C. Drainage and grading plans. In addition to the improvement plan, a drainage plan shall be submitted. This plan may be the required improvement plan or a similar plan at a scale of one inch equals 100 feet or larger showing at least the following additional information:
1. Contours indicating the existing and final grading at vertical increments of no more than two feet;
2. Discharge (Q), coefficient of runoff (c) and drainage area (A) along with the outline of the drainage area for each inlet, catch basin, culvert and open channel point of design and other locations designated by the City Engineer. Drainage areas that lie partially outside the limits of the drainage and grading plan may be delineated on any contour map acceptable to the City Engineer;
3. Discharge (Q) before and after development at strategic points within and at extremities of the project area;
4. Delineation of the boundaries and contour elevation, along with the track, of the major drainage system through downstream areas to an adequate outlet even though the outlet may be outside the project area;
5. Delineation of the horizontal limits of ponding areas at low points (sags) in the street profile and low points outside the street right-of-way including, but not limited to, culvert headwater, natural stream water surfaces and sump type inlets for storms with frequencies of 25 and 100 years;
6. High and low water horizontal limits and contour elevation of detention/retention/sediment ation facilities along with water surface and control weir elevations, outlet structures and the like;
7. Areas outside of the project area susceptible to sediment deposits or to erosion caused by accelerated runoff;
8. Location of soils that may be limited for the proposed use;
9. All requirements of this chapter; and
10. Any other information required by the City Engineer to clarify intent, specified requirements or design features.
D. Supporting data. All data and design information used for the design of drainage facilities and for determining downstream flood information shall be submitted with the drainage and grading plan. To facilitate review and avoid confusion, legends, descriptions and structure numbering used on design forms or other calculations shall be identical to those used on the improvement plans and the drainage and grading plan. This data shall include but are not limited to:
1. Weighted runoff coefficient calculations for each contributing area;
2. Pavement drainage computations;
3. Storm sewer computations;
4. Culvert design computations;
5. Open channel computations;
6. Detention/retention facilities computations;
7. Inlet capacity computations; and
8. Any other information required by the City Engineer to clarify intent or design features.
E. As-built plans. Amended improvement plans specifying the locations, dimensions, elevations and capacities of all facilities as constructed shall be submitted to the city on construction completion of the project. These shall include all required design features except those waived by the City Engineer. All revisions to the approved plans shall be approved by the city prior to construction.
(c) Stormwater design. See current edition of City of Mason Stormwater Manual.
(Ord. 99-207, passed 1-10-2000)