(a) Drainage Improvements. The subdivider shall construct all necessary facilities including underground pipe, inlets, outlets, catch basins or open drainage ditches, as determined by the County Engineer, to provide for adequate disposal of subsurface and surface water and protection of natural drainage courses.
Such drainage improvements must meet the standards below and also the standards of Section 1107.02(b) (Sediment Control Standards and Criteria) of these regulations.
(b) Storm Sewer and Storm Water Drainage.
(1) Where an adequate public storm sewer is available at the plat boundary, the subdivider shall construct the new storm sewer system and connect with such storm sewer line. If such a storm sewer line is not available, the new storm sewer system shall outlet into a natural drainage course of adequate capacity. The ultimate outlet, whether a public storm sewer or a natural water course, shall be approved by the Commission and County Engineer. Storm drainage, including drain tiles around basements shall not be permitted to discharge into any sanitary sewer facility, but shall connect to an adequate drainage outlet.
2 A. A storm water retention facility shall be installed where required by the County Engineer.
B. The design of any retention facility will also be evaluated in regards to the safety of the area residents, therefore, fencing along with other safety precautions may be requested by the Commission.
(3) Where type "A" streets are to be constructed, storm sewers shall be designed for a ten-year frequency design storm.
A. Catch basins and storm inlets shall not enter a main line sewer directly.
B. Large angular horizontal changes in storm sewer lines are not acceptable.
C. Minimum storm sewer size shall be fifteen inches.
The Commission and County Engineer may require a larger diameter storm sewer if necessary, depending upon grade, anticipated volume and velocity of storm water flow, etc.
(4) Where type "B" streets are to be constructed
A. All newly constructed roadside ditches shall be lined. Storm water velocities of a two year frequency design shall be used to determine the type of lining that shall be constructed. Concrete lining will be required where flow velocity exceeds seven feet per second (fps). Sodding will be required when flow velocity exceeds 4.5 fps. Seeding will be required in all other instances.
B. Depths of flow in roadside ditches shall not exceed one foot during a ten-year frequency design storm.
(5) Within areas of high population density, the Commission and County Engineer may require that all surface drainage be carried within a closed conduit system. Enclosure of less populated areas being subdivided shall be determined at the time of platting. The Commission and County Engineer may require a drainage enclosure system in less populated areas due to the existence of the required utilities and/or due to the encroachment of various development patterns.
(c) Culverts and Bridges. Where natural drainage channels intersect any street right of way, it shall be the responsibility of the subdivider to have satisfactory bridges and/or culverts constructed.
(1) Where culverts are required, minimum requirements shall be as follows:
A. All culverts shall extend across the entire right-of-way width of the proposed street, unless the County Engineer feels that a modification in this requirement based on need and engineering design is warranted. The subdivider's engineer shall prepare all plans and the County Engineer will review and endorse in writing, if acceptable.
B. Minimum culvert size shall be based on a ten-year storm design with an HW/D of one or less under inlet control. Nowhere shall the minimum diameter be less than eighteen inches.
C. Culverts shall be analyzed under both inlet and outlet control to determine headwater depth. Analysis shall include a determination of headwater depth for 10, 25 and 100 year storm design.
D. Headwater depth shall not exceed an elevation of one foot below pavement centerline for 25-year storm design.
E. A concrete headwall shall be constructed by the subdivider where required by the County Engineer.
F. Culverts are designed for controlled inundation, therefore, a structure to be occupied shall not be proposed for any land below the 100 year headwater elevation.
G. If culvert analysis show any adverse tailwater effects on other property or highways, the culvert size shall be increased to eliminate this effect.
H. In areas designed as Zone A on the Flood Boundary and Floodway Map of Mahoning County (National Flood Insurance Program) all above requirements are superseded by the requirements, directives and outlines of the U.S. Army Corps of Engineers and the U.S. Department of Housing and Urban Development.
Note: In addition to up-stream effects, downstream effects shall be considered with corrective measures (detention, channel widening, etc.) and detailed on the plan.
(2) A. Driveway culverts shall have a minimum length of twenty feet and a minimum diameter of fifteen inches. Such culverts shall be designed by the subdivider's engineer and reviewed and endorsed in writing by the County Engineer if acceptable.
B. The material used shall be concrete, vitrified, corrugated or material equal to or greater than these mentioned, if approved by the County Engineer.
C. Headwall construction may be required when deemed necessary by the County Engineer.
(3) Bridge waterways. The determination of adequate waterway openings is essential to the design of safe bridges. Depending upon the type of structure, number of spans and classification of roadway the design package should include applicable parts of the following:
A. Site data.
1. Maps, stream cross-sections, aerial photographs.
2. Data on existing bridges, including dates of construction and performance during past floods.
3. Available high water marks with dates of occurrence.
4. Information on ice, debris and channel stability.
5. Factors affecting water stages, such as high water from other streams, reservoirs and flood control projects.
B. Hydrologic analysis.
1. Compile flood data applicable to estimating floods at site including both historical floods and maximum floods of record.
2. Plot flood-frequency curve for site.
3. Determine distribution of flow and velocities at site for flood discharge to be considered in design of structure.
4. Plot stage-discharge curve for site.
C. Hydraulic analysis.
1. Compute backwater and mean velocities of bridge openings for various trial bridge lengths and selected discharges.
2. Estimate scour depths at piers and abutments of proposed structures.
(4) Structure design and construction.
A. The design of any proposed bridge shall be in complete accordance with the Standard Specifications for Highway Bridges as adopted by the American Association of State Highway and Transportation Officials (AASHTO), Division 1.
B. Construction of any bridges shall be in compliance with Division 2 of the AASHTO Standard Specifications for Highway Bridges.
(d) Pedestrian Footbridges. The Commission may require the construction of a pedestrian footbridge across a stream or natural drainage course or man-made feature in order to help protect the health, safety and general welfare of the residents and to provide access to a facility within or adjacent to the subdivision.
(e) Drainage Design. The subdivider shall file with the office of the County Engineer, drainage calculations supporting size and type of storm sewer systems, culverts and bridges proposed on detailed construction drawings. Generally in most cases, plain concrete pipe, fully coated corrugated metal pipe and vitrified pipe of adequate strength may be used outside the pavement area. Reinforced concrete pipe, fully coated corrugated metal pipe and vitrified pipe of adequate strength shall be used within the pavement area and backfilled with premium material.
(f) Protection of Drainage Courses. The subdivider shall make provisions to assure that all natural water courses and gullies carrying storm surface water runoff shall continue to function in their normal manner. Whenever changes are contemplated for any water courses, plans for such changes shall be submitted to and approved by the County Engineer before any changes on-site are made. The Commission may require an easement of sufficient width to be shown on the plat survey which is to be recorded.
(Ord. 88-1. Passed 6-23-88.)