§ 155.084 DRAINAGE IMPROVEMENTS.
   (A)   The subdivider shall construct all necessary facilities including underground pipe, inlets, catch basins, or open drainage ditches, to provide for the adequate disposal of subsurface and surface water and maintenance of natural drainage courses. The velocity of flow in an open ditch shall not exceed two feet per second in soil ditches or five feet per second in turf gutters. Paved gutters will be required if velocities of flow are greater than those specified or if it is otherwise likely that destructive erosion will result. Drainage ditches shall not be permitted to discharge into any sanitary sewer facility.
   (B)   All stormwater drainage facilities within a development shall be designed to have capacity for the total tributary area, at the design storm frequency, and based on the rate of single family, residential runoff.
   (C)   All proposed development with a runoff rate greater than that which the downstream system is, or will be designed for, will be required to control the rate of stormwater discharge.
   (D)   All developments having existing controls located downstream from the site, will be required to control the discharge flow rate of stormwater to that rate which existed prior to development.
   (E)   If the decision is made by the city to handle stormwater runoff by storage in a detention or retention facility, the facility will be transferred to the city by deed with a reverter clause to developer if no longer used as a water storage facility. However, the developer will be required to maintain the facility for two years after approval or acceptance by the city.
   (F)   All land developers wishing to develop in the city are required to prepare and submit a Master Drainage Plan for the total development area. The Master Drainage Plan shall include:
      (1)   A topographic contour map, with the drainage area delineated, with a plan for draining the total upstream tributary watershed through the proposed development.
      (2)   A topographic map with at least two-foot contours, with general layout of the proposed inlets and storm sewers for the total development showing all existing drainage structure with size and invert elevations.
      (3)   The capacity of the downstream open channel, culvert or storm sewer that may be used for an outlet.
      (4)   The points downstream that may be used as a control to determine the maximum allowable rate of stormwater runoff for the design storm. Existing culverts are not necessarily controls.
      (5)   The routing path to be provided for runoff in the event the drainage facilities' capacity is exceeded. This path will become part of a grading plan which will be submitted with detail plans.
      (6)   A routing of excess stormwater through the site discharge to off-site lands. The routing path shall be continuous from one development to the next.
      (7)   If a decision is made to handle stormwater runoff by storage and releasing it at a regulated rate to prevent flooding downstream, the storage basin will be located in and become part of the routing path.
      (8)   Excess stormwater will be kept out of proposed habitable structures.
      (9)   The Master Drainage Plan shall be approved by the city prior to the developer initiating detail engineering designs.
   (G)   The rational method can be used to determine the peak rate of runoff for all design storms for drainage areas of 200 acres or less.
   (H)   For drainage areas over 200 acres and for determining major storms, the method explained in “Urban Hydrology for Small Watersheds”, Technical Release No. 55, may be used to provide peak rate of runoff. T.R. 55 can be obtained from the Soil Conservation Service Engineering Division, U.S. Department of Agriculture.
   (I)   Storm sewer criteria.
      (1)   All storm sewer systems are to be designed using Manning's Equation.
      (2)   The sewer must be deep enough to receive the flow from all its sources within the watershed.
      (3)   The size of the sewer must be adequate for just flowing full, based on the design storm.
      (4)   The gradient of the sewer must be sufficient to avoid deposition of solids.
      (5)   The storm sewer material will be concrete or vitrified clay. Other approved material may be used for special design.
         (a)   Design storm frequency.
            1.   72 inches and under - flowing full for two-year storm.
            2.   Over 72 inches diameter - flowing full for ten-year storm.
         (b)   Hydraulic gradient.
            1.   Based on five-year storm, shall not exceed window or grate elevation for an inlet or catch basin.
            2.   Grade line based on tailwater or 0.8 D at outlet or other critical points within the system.
          (c)   Design flow. All design flow will be based on Rational Method Q = CiA. Minimum time of concentrations:
            1.   Curb inlet - ten minutes;
            2.   Ditch C.B. - ten minutes.
         (d)   Runoff coefficient. Based on Appendix C of this chapter.
         (e)   Manning's “n” value. All storm sewers shall be based on an “n” of 0.013.
         (f)   Velocity in sewer for design flow.
            1.   3 F.P.S. Minimum.
            2.   15 F.P.S. Maximum.
         (g)   Maximum length between access structure.
            1.   Pipes under 60 inches - 300 feet.
            2.   Pipes 60 inches and over - 500 feet.
   (J)   Curb inlet spacing criteria.
      (1)   Spacing will be 350 feet maximum for mountable curb and 500 feet maximum for full height curb.
         (a)   Design storm frequency using Rational Method - two-year storm.
         (b)   Minimum time of concentration will be ten minutes.
         (c)   Roughness coefficient will be 0.015.
         (d)   Maximum spread of flow:
            1.   Twenty-six-foot streets or less, the flow may spread to width of eight feet.
            2.   Twenty-six-foot to 36-foot streets, the flow may spread to width of nine feet.
            3.   Thirty-six-foot to 52-foot streets, the flow may spread to a width of ten feet.
      (2)   Type of inlets.
         (a)   Continuous grades.
            1.   Forty-two-inch, 60-inch or curb and gutter.
            2.   For grades over 2%, curb and gutter inlets will be used.
         (b)   Sag or low area. Forty-two-inch, 60-inch or curb and gutter.
         (c)   Street intersections. Forty-two-inch, 60-inch or curb and gutter.
   (K)   Roadway culvert criteria.
      (1)   Design storm frequency.
         (a)   Ten-year storm minimum.
         (b)   Twenty-five-year storm for arterial streets.
      (2)   Runoff coefficient. Based on Appendix C of this chapter.
      (3)   Maximum allowable headwater.
         (a)   Eighteen inches below top of curb.
         (b)   Twelve inches below edge of pavement.
         (c)   One and one-half times diameter.
         (d)   Property damage.
         (e)   Diameter or rise plus four feet or 2D, whichever is lower, in deep ravines.
      (4)   Manning's “n” value.
         (a)   Box culvert            0.011
         (b)   Slab top culvert         0.03 to 0.05
         (c)   Concrete pipe            0.013
         (d)   Corrugated metal         0.019 to 0.032
      (5)   Entrance loss coefficient.
         (a)   Box culvert and slab top culvert   0.2 to 0.5
         (b)   Concrete pipe                     0.2
         (c)   Corrugated metal                  0.9
      (6)   Minimum cover. Desirable, 30 inches to pavement subgrade.
      (7)   Maximum cover. The structural design criteria for culverts will be the same as that required by the Ohio Department of Transportation.
      (8)   Maximum allowable outlet velocity.
         (a)   Bare earth channel        6 F.P.S.
         (b)   Rock protection       18 F.P.S.
         (c)   Stilling basin          Over 18 F.P.S.
         (d)   The ability of the downstream channel to handle the flow satisfactorily.
      (9)   End protection.
         (a)   Full height headwall with flared wings.
         (b)   Other special type headwalls must be approved before use.
   (L)   Open watercourse criteria. Access by the city to storm drainage ditches and channels shall be guaranteed by means of maintenance easements. Such maintenance easements shall be not less than 20 feet greater in width than the width of the ditch or channel. Maintenance easements are to be kept free of obstructions.
      (1)   Design storm frequency. Flowing full for ten-year storm.
      (2)   Design flow.
         (a)   Areas under 200 acres use Rational Method Q = CiA.
         (b)   Areas over 200 acres use TR SS.
      (3)   Runoff coefficient. Based on Appendix C of this chapter.
      (4)   Allowable velocities new ditches.
         (a)   3.0 F.P.S. with seeding.
         (b)   5.0 F.P.S. with sod or jute mat lining.
         (c)   Over 5 F.P.S. special lining.
      (5)   Allowable velocities existing channels. Ability of the channel to handle the flow
satisfactorily.
      (6)   Manning's “n” value.
         (a)   Sod or jute mat lining       0.05
         (b)   Paved lining          0.015
         (c)   Rock protection          0.08
         (d)   Existing lining          0.025 to 0.20
      (7)   Minimum slope.
         (a)   Desirable for new channels       0.40%
         (b)   Absolute            0.10% - with a minimum velocity of 2 F.P.S. based on a two-year storm
      (8)   Side slopes.
            Desirable            4:1
   (M)   Routing path. The routing path or the major drainage system is that part of the storm drainage system which carries the runoff which exceeds the capacity of the designed facilities. The major drainage system shall have the capacity to carry runoff from a storm with a return period of not less than 50 years without causing significant threat to property or public safety.
   (N)   Detention or retention.
      (1)   Parking lot storage. Parking lot storage is surface storage where shallow ponding is designed to flood specifically graded areas of parking lot. Controlled release features are incorporated into the surface drainage system of the parking lot. Parking lot storage is a convenient multi-use structural control method where impervious parking lots are planned. Design features include small ponding areas with controlled release by pipe-size and slope, and increased curb heights.
      (2)   Tank storage. Tank storage is an underground tank or chamber, either prefabricated or constructed in place, which has a special controlled release feature.
      (3)   Surface basins or ponds.
         (a)   Wet ponds or retention basins. Wet ponds are permanent ponds where additional storage capacity is provided above the normal water level and special features for controlled release are included. Historically, wet ponds have proven extremely effective in abating increased runoff and channel erosion from urbanized areas. They are a major soil conversation land treatment practice.
         (b)   Dry basins or detention basin. Dry basins are surface storage areas created by constructing a typical excavated or embankment basin. There is no normal pool level and a specific controlled release feature is included to control the rate of discharge. The detention flow control structure is usually a multi-stage device, and the retention flow control structure is usually a single-stage device.
      (4)   Design criteria.
         (a)   Design frequency.
            1.   Ten-year storm for site development with shallow ponding areas for drainage areas of two acres or less.
            2.   One hundred-year storm for drainage areas exceeding two acres.
         (b)   Design flow.
            1.   Area under 200 acres use rational method (Q=CiA).
            2.   Areas over 200 acres use Technical Release No. 55.
         (c)   Runoff coefficient. Based on Appendix C of this chapter.
         (d)   Release rates. The release rate for all developments shall not exceed the storm water runoff rate, from the discharge. The detention flow control structure is usually a multistage device, and the retention flow control structure is usually a single-stage device.
(Ord. 20-89, passed 6-28-89)