A final drainage plan shall be prepared consistent with the following criteria:
(a) Minor System Design.
(1) General. The minor system design shall be based on a post development 10-year storm. This system includes all closed conduits, open channels, inlets, manholes and all features designed to collect storm water from streets and lots, including the necessary piping for the collection of sump pump drains. The Rational Method will normally be used when calculating peak rate of runoff in determining conduit size and grade.
(2) Hydrologic analysis. Storm runoff can be calculated using the Rational Method or the SCS Technical Release 55 Urban Hydrology for Small Watersheds (TR55). In cases of large channels, regression equations may be used. The Rational Method shall not be used for drainage areas greater than 20 acres.
(3) Conduit design.
A. General. All conduits are to be designed on a 10-year flowing full design using the Manning Formula. The minimum diameter shall be 12 inches.
B. Material. Pipe material for culverts and storm sewer shall comply with current Ohio Department of Transportation (ODOT) specifications.
C. Roughness coefficient (n). Manufacturer's "n" value plus .004 to account for bends and junction losses is to be used.
D. Velocity. The allowable velocity in the storm sewer shall not be less than 2 feet per second or greater than 14 feet per second for corrugated metal pipe and 20 feet per second for concrete and plastic pipe based on a 10-year frequency storm.
E. Rock channel protection and/or other means must be used when the outlet velocity is higher than the permissible erodible velocity for the channel it enters. The outlet velocity is to be checked on a 10-year frequency storm.
F. Cover. All pipes shall meet the manufacturers minimum requirements of depth of cover so as to not crush or deform the pipe in any way.
G. All changes in storm sewer, grade, alignment and the intersection of 2 or more sewers must take place in a manhole or catch basin.
(4) Inlets. The curb inlet shall be spaced a maximum distance of 300 feet from another catch basin or high point. Closer spacing may be necessary if the capacity of the inlet is less than the flow tributary to the inlet. Curb inlets shall be placed in a manner such that no storm water shall be permitted to flow through any intersection.
(5) Sump pump lines. No sump pump lines may be tied to the sanitary sewer or to the curb. A separate pipe of 4 or 6-inch diameter is to be run behind the back of the curb and tied into the nearest possible catch basin or storm manhole.
A. Standard "Y's" or "T's" should be installed 10 feet from the property line on the downhill side of the lot and marked in the field. Cleanouts should be spaced approximately every 400 feet.
B. No downspouts may be tied to the sump line. All downspouts must be run separately to catch basins or released on splash blocks.
C. All sump pumps must be connected to the sump lines provided at the street.
(b) Major System Design.
(1) General. The intent of planning a flood routing system is to ensure that storm water runoff which exceeds the capacity of the storm sewer system shall have a route to follow in order to prevent a major loss of property or life.
A. Minimum opening elevations are to be set by the City's consulting engineer along all 100-year flood routes. These elevations are to be a minimum of 1 foot above the 100-year floodwater elevation. These elevations are to be clearly labeled on the plan and record plat.
B. Since streets may be used as a routing path, the major system must be taken into account in the initial design of the development. It shall be designed in a manner that will direct the storm water into a detention or retention area.
(2) Capacity. The combination of the major and minor system shall have the capacity to carry runoff from a 100-year frequency storm.
A. Where the street is designed as the major drainageway, the depth of flow shall not exceed 8 inches at the face of the curb.
B. When the major drainageway is located outside a street right-of-way, such as in a retention pond or detention basin, easements shall be provided and a grading plan is to be submitted with detailed elevations showing the flood being contained in this area.
(c) Culvert Design.
(1) Technical Release 55 and the Hydraulic Circular No. 5, which can be obtained from the Superintendent of Documents, U.S. Government Printing Office, are the recommended procedures for design.
(2) A single-span culvert should always be used in lieu of a multiple-span opening. The only time a multiple-opening culvert shall be considered is when no other single- span structure will function.
(3) All culverts that are a part of the major system shall be designed for the 50-year storm with headwater and a flood route provided to accept a 100-year storm.
(4) Maximum allowable headwater:
A. 18 inches below the top of the curb;
B. 12 inches below the edge of the pavement (non-curb and gutter section).
(5) The design of the drainage system shall not cause backwater into any adjacent property. If additional backwater is caused, an easement from the affected property owner must be obtained. The backwater condition should be checked on the 50- year frequency storm.
(6) A structure having a clear opening of 10 feet or more shall be classified as a bridge and subject to special conditions under the direction of the Service Director or other consultant that the City has hired for engineering advice.
(7) Entrance loss coefficient: Appropriate standard entrance coefficients are to be used in culvert design.
(d) Open Channel Design.
(1) General. Any newly designed open channel shall only accommodate a small drainage area (i.e., lot swale). All other design drainage features shall be enclosed with storm sewer, with the exception of large, major channels.
(2) Design.
A. Open channels are to be designed using a full flow 10-year frequency storm unless the channel is part of the major storm system, then it must be designed using a 100-year storm frequency.
B. Special provisions such as a check or drop structure will be addressed on an individual basis.
C. The longitudinal slope of a channel should be 1% or greater. Any channel with a slope below 1% or with a small continuous flow must have a paved bottom, or be under drained with a low-flow pipe. All side slopes shall be 4:1 or flatter in residential areas and 3:1 or flatter in commercial and industrial areas unless otherwise approved by the City's engineering consultant.
(3) Permissible velocities. The maximum permissible design velocity shall not be greater than the erodible velocity of the channel. The velocity is to be calculated using a bank full condition or a minimum 10-year frequency storm, whichever is greater. Channel linings will be required as per the ODOT Design Manual. In designing an open channel, the low flow and grade shall be addressed as to prevent stagnation. All seeding, mulching and sodding shall be installed immediately after construction as per ODOT 659 and 660 and kept in ideal growth conditions until established.
(e) Detention and Retention.
(1) Introduction. The objective of a detention/retention facility is to regulate the rate of runoff and control the peak discharges to reduce the impact on the downstream drainage system.
(2) Design criteria. The allowable peak rate of runoff from the developed area shall not exceed a pre-developed 10-year frequency storm. The basin shall have capacity to store a post-developed 50-year frequency storm from the same area.
(3) Design considerations. The following considerations for maintenance and appearance of a detention/retention facility shall be taken into account in the design:
A. The bottom of the detention area should be constructed with sufficient slopes to drain properly (0.5% minimum).
B. Paved gutter or a low flow under drain shall be constructed from all inlet pipes to the outlet structure. Where two gutters intersect, large radii shall be used so that the change in direction of flow is gradual.
C. Side slopes for detention facilities shall be no steeper than 4:1 unless existing natural conditions do not make this possible.
D. Anti-seep collars shall be used on all pipe outlets of retention basins or ponds.
E. There shall be a minimum of 6 inches of freeboard between the top of the dike and the water surface in the reservoir with the emergency spillway flowing at the 100-year design flow.
F. The emergency spillway shall safely pass the peak flow for a 100-year frequency storm with a safe velocity (8.0-fps max). The spillway is to be cut on existing ground or other protection on the fill slope to be provided.
G. A 20-foot access easement shall be provided for easy ingress and egress to and from the detention or retention basin. This access shall have a maximum slope of 25%.
H. Trash guards should be installed when clogging of the outlet structure is probable.
I. Anti-vortex devices should be used at the top of all riser pipes in retention ponds.
(Ord. 12-2002. Passed 7-2-02.)