(A) General. The design and construction of stormwater facilities in the city is under the jurisdiction of the city engineer. Within the public right-of-way all work must meet the approval of the Public Works Department.
(B) Water management and sediment control. Storm drainage proposals for any development shall reflect through investigation of the measures intended to control sediment and manage stormwater as required in Chapter 151. Water management and sediment control plans are required of all development and redevelopment unless specifically exempted in Chapter 151.
(C) Storm retention/detention. In order to minimize storm runoff damage to downstream properties and overloading of existing drainage courses, the criteria found Chapter 151: Stormwater Management shall be followed on all development of parcels one acre or larger in size, and on all redevelopment of existing buildings or site usage as determined by Planning Commission.
(D) Storm Sewers.
(1) Minor Systems. All on-site storm sewers shall be designed and sized to convey the peak flow rate of stormwater runoff for a 10-year storm event, such that the hydraulic grade line remains below the crown of the pipe during peak flow conditions. The peak flow rate of stormwater runoff for a 25-year storm event shall be evaluated as a check to verify the hydraulic grade line remains at least 12 inches below the rim elevation of the storm sewer system to prevent surcharging onto the ground surface. In addition. the peak flow rate of stormwater runoff for a 100-year storm event shall be safely conveyed through an overland flood route to avoid flooding damage. The retention outlet shall be designed based on the runoff calculated in (C) (3) above.
(2) Major System.
(a) A designated routing shall be designed to convey stormwater runoff which exceeds the capacity of the minor drainage system, i.e. storm sewer without causing loss of property or any loss of life.
(b) Surface runoff for the major drainage system shall be determined using a storm frequency of 100 years. The runoff which the major storm routing path shall convey shall be equal to the peak flow minus the flow in the minor drainage system.
(E) Drainage calculations. A professional engineer licensed to practice in Ohio shall submit detailed sewer calculations and drainage maps of sufficient scale and contour interval to verify the proposed hydraulic design.
(F) Surface run-off.
(1) To determine the quantity of surface runoff for areas up to 200 acres, use the “Rational Method”. (Q=CIA)
(2) For areas over 200 acres, appropriate SCS methods as approved by the City Engineer shall be used.
(G) (Reserved).
(H) RAINFALL INTENSITY DURATION-FREQUENCY CURVES. The rainfall intensity for a particular storm occurrence frequency should be determined using the calculated time of concentration and the appropriate intensity-duration-frequency curve or table. The table below shall be used and is based on the latest National Oceanic and Atmospheric Administration (NOAA) Atlas 14 point precipitation frequency estimates in Springdale.
Time of Concentration (min) | 1- Year | 2- Year | 5- Year | 10- Year | 25- Year | 50- Year | 100- Year |
10 | 3.52 | 4.18 | 4.89 | 5.41 | 6.03 | 6.47 | 6.86 |
15 | 2.88 | 3.41 | 4.00 | 4.44 | 4.97 | 5.34 | 5.68 |
30 | 1.90 | 2.28 | 2.74 | 3.08 | 3.51 | 3.81 | 4.11 |
60 | 1.16 | 1.40 | 1.72 | 1.96 | 2.27 | 2.51 | 2.74 |
120 | 0.68 | 0.82 | 1.01 | 1.15 | 1.35 | 1.49 | 1.64 |
(I) Duration of storm in minutes (tc). TR-55 criteria for time of concentration determination shall be utilized.
(J) Inlet time. At the head of the system the inlet time may vary from 10 to 15 minutes, depending upon the size of the area and factors affecting rapid runoff.
(K) Runoff coefficient. Compute a weighted value of the drainage areas, using 0.9 for roof areas and hard-surfaced paved area, 0.9 for sidewalk and macadam driveways, and 0.4 for unpaved areas, yards, and lawns.
(L) Size of sewer. The size of the sewer shall be determined on the basis of the hydraulic gradient to provide adequate capacity for the computed runoff, using n = .015 for concrete pipe, bituminous paved corrugated metal, and n = .013 for corrugated polyethylene smooth lined pipe. In no case shall the size be less than 12 inches in diameter.
(M) Depth. The minimum depth for storm sewers shall be planned to provide clearance for all utilities and to permit inlet leads to be laid on not less than 2% slope, with the invert of the inlet pipe at the manhole, no lower than the top of the bench wall.
(N) Minimum and maximum velocities. Velocities in storm sewer pipe, when flowing full at average peak flows, shall be no less than 2.5 feet per second nor more than 12 feet per second. For velocities greater than 12 feet per second, special provisions shall be made to protect the sewer pipe against erosion and against displacement by shock, or for checking the flow velocity.
(O) Gradients of pipe. The sewer pipe shall be laid on such gradients so that the full flow velocities shall be kept within the foregoing stated minimum and maximum. The pipe sizes should be so selected as to avoid large differences in velocities between consecutive reaches. Hydraulic grade analysis to be submitted to ensure the water level is not within 12" of the proposed top of grate elevation, based on the 25-year storm event.
(P) Manholes. Manholes shall be placed at intersections and termini of sewers; at all changes in size, alignment, and slope of sewer; and at intermediate intervals as required for maintenance. Manholes shall be constructed in accordance with the latest edition of ODOT standard construction drawings, or as approved by the City Engineer.
(Q) Inlets.
(1) Capacity. The capacity of the inlet should not be less than the quantity of flow tributary to the inlet. Inlets at low points or grade pockets should have extra capacity as a safeguard for flooding from flows in excess of design flows. Calculations shall be submitted verifying the capacity of each inlet. Special inlets may be required for streets with steep gradients to provide the extra capacity such situations require.
(2) Type. Single or double gutter style inlets shall be used and installed as shown on the latest edition of ODOT standard construction drawings for “CB-3" and “CB-3A,” or an approved detail. Any inlets having a depth in excess of 5 feet shall be provided with manhole-type steps. Wingwall and ditch- type inlets shall be used where required to drain storm water from watercourses and drainage channels.
(3) Gutter spread and inlet capacity calculations shall be submitted.
(R) Outfalls.
(1) When a storm sewer system outfalls into a flood plain of any major watercourse, the outfall must not be subject to frequent floods or backwaters. Standard wingwalls with erosion control shall be constructed for all outfalls. Suitable baffles or other energy dissipaters shall be provided if determined to be necessary by the city engineer.
(2) The invert of the first storm sewer appurtenance upstream of the outfall structure shall be above the elevations of the flood plain.
(S) Specifications for construction and materials. In all other respects, the materials and construction shall be as specified in Sections 603, 604, 706, and 707, State of Ohio Department of Transportation "Construction and Material Specifications".
(T) For additional requirements of the Water Management and Sediment Control Regulations, see Chapter 151.
(Ord. 39-1987, passed 4-15-87; Am. Ord. 40-2010, passed 12-15-10; Am. Ord. 20-2023, passed 4-19-23)