(a) General Requirements. All storm sewer designs shall be in accordance with the following standards. All designs shall protect from flood damage and inundation by storm water to any dwelling. Drainage water entering the watershed shall be collected and routed from the area in approximately the same location and manner that existed prior to the construction of the improvements. Design of the drainage facilities within the drainage area shall be such that they will not divert drainage area from one watershed to another.
(b) Design of Drainage Facilities.
(1) Hydrologic design. All drainage ways shall be designed in accordance with the following criteria:
A. Major waterways: Major waterways are defined as those with a tributary area in excess of four square miles. Major waterways shall be designed for an average recurrence interval of 100 years.
B. Secondary waterways: Secondary waterways are defined as those with a tributary area of between one and four square miles. Secondary waterways shall be designed for an average recurrence interval of fifty years.
C. Minor waterways: Minor waterways are defined as those with one square mile or less of tributary area. Minor waterways shall be designed for an average recurrence interval of twenty-five years for open channels and culverts.
D. Roadway storm sewer networks: All roadway storm sewer shall be designed for an average recurrence interval of ten years.
(2) Hydraulic design.
A. The design hydraulic grade line for any closed or open waterway, bridges or culverts shall be designed to contain the twenty-five-year rainfall frequency within the proposed system. Storm sewers designed as part of a road system with curb shall be designed so that the hydraulic grade line shall be no higher than the inlet grates for a design discharge of the twenty-five- year design discharge (Q).
B. For all roadway design improvements or developments, catch basins shall be so placed along the streets that the width of flow does not impede traffic. For acceptable spread requirements along a roadway, reference Ohio Department of Transportation Location and Design Volume 2, the most current addition.
C. The depth of flow or ponding for a 100-year average recurrence interval storm shall not exceed a level which would cause inundation or damage to any dwelling constructed within the project area.
D. Site grading within the development shall be such that all lots will readily drain. Lots shall have a one percent minimum slope in grass areas. Overland flow on lots shall be limited to a maximum distance of 300 feet unless approved by the City of Wapakoneta.
E. Bridges spanning open waterways shall have minimum freeboard above water surface of two feet.
F. The minimum velocity for any closed or open conduit shall be 2.5 feet per second under gravity flow conditions.
(3) Structural design. Catch basins, manholes, inlet structures, etc. placed within an improvement or development shall conform to standard plans on file with the City. Structural design of all drainage facilities shall be subject to the approval of the City of Wapakoneta.
A. Channels/swales:
1. Minimum bottom width of constructed channels shall be two feet. Each channel constructed shall have side slopes of 3:1 or flatter.
2. All permanent drainage swales shall be shown on the construction plans with easements on the final plat.
3. Maintenance of all drainage swales through a development shall be the responsibility of the developer or property owner.
4. Bank stabilization and stream bed stabilization, along constructed or natural channels, will be required if the channel velocities are sufficient to cause bank or invert erosion. The top of bank shall be so graded that side drainage will enter channels only at points where structures are provided to prevent bank erosion. Side drainage flow shall enter the channel as nearly parallel with stream flow as possible. Earth channels shall be seeded or sodded depending on the velocity of flow within the channel.
B. Closed conduits: The minimum conduit size for all main storm lines shall be twelve inch pipe.
1. Minimum clearance between top of pipe and top of surface shall be two feet. All pipes with clearance less than two feet shall be reinforced concrete pipe. The alignment of closed conduits shall be as nearly straight as practicable without bends and angle points; manholes shall be provided at all angle points and at intervals not to exceed 300 feet along the conduit unless submitted for approval with proper documentation. Inverted siphons shall not be permitted except for temporary structures.
2. All pipe, bedding and backfill shall be designed in accordance with the Ohio Department of Transportation Design Manual. Field tile/single wall storm tile is NOT allowed. Ditch protection shall be required if the ten-year flow velocities in a channel or waterway exceed four feet per second for soil ditches or six feet per second in sodded ditches. If the exit velocity from a storm sewer exceeds the allowable velocities, an energy dissipating device (i.e. stilling basin, dumped rock) may be necessary.
(4) Storm water runoff control criteria for retention/detention basins. Any development which increases the runoff rate and volume shall be required to control the discharge rate of runoff prior to its release to its off-site outlet.
A. Any increase in the volume of site surface drainage water resulting from accelerated runoff caused by site development shall be controlled so that the post development peak discharge rate does not exceed that of the pre-development peak discharge rate, for all twenty-four-hour storms between a one-year frequency and the critical storm frequency as determined below. The method by which an applicant shall determine changes in rates and volumes of runoff is presented in the U.S. Department of Agriculture, Engineering Division of the Soil Conservation Service, Urban Hydrology for Small Watersheds, Technical Release No. 55, June 1986 or most current edition. To find the critical storm frequency for which additional control will be needed, the applicant shall:
1. Determine the percent increase in runoff volume for a one-year frequency, twenty-four-hour storm occurring on the development area.
2. Determine the critical storm frequency for which additional control is needed by using the percent increase in runoff volume, derived in (a), in Table 1.
3. Control the post development storms of a frequency between one year and the critical storm determined in (b), so as to be equal to or less than the pre-development peak runoff rate for a twenty-four-hour one-year frequency storm.
B. Storms of less frequent occurrence (longer return periods) than the critical storm up to the one-hundred-year storm have peak runoff rates no greater than the peak runoff rates from equivalent size storms under pre-development conditions. Consideration of the one, two, five, ten, twenty-five, fifty and 100-year storms shall be considered adequate in designing and developing to meet this standard.
Critical Year Storm Calculation:
(1-year Post – 1-Year Pre) x 100%
(1 Year Pre)
(1 Year Pre)
(5) Post construction runoff control. In order to:
A. Reduce increases in non-point pollution caused by storm water runoff.
B. Minimize the total runoff volume which flows from a site following increases in impervious area by development.
C. Minimize changes in the watershed hydrology inherent with development.
Table 1
Determining Storm Frequency for Which Control is Needed | ||
Percent Increase in Runoff Volume from a One-year Frequency 24-hour Storm
| ||
equal or greater than (percent) | less than (percent) | Storm Frequency (years) |
Determining Storm Frequency for Which Control is Needed | ||
Percent Increase in Runoff Volume from a One-year Frequency 24-hour Storm
| ||
equal or greater than (percent) | less than (percent) | Storm Frequency (years) |
- | 10 | 1 |
10 | 20 | 2 |
20 | 50 | 5 |
50 | 100 | 10 |
100 | 250 | 25 |
250 | 500 | 50 |
500 | - | 100 |
The drainage design shall incorporate post construction storm water management requirements.
1. The post construction requirements of the Ohio Environmental Protection Agency Construction Permit, OHC000002 shall be made part of these regulations. The City of Wapakoneta shall be responsible for reviewing Part III.G.2.e of the current permit. Please note, the current OHC000002 permit is effective for five years. In subsequent OEPA Construction Permits or if the current permit is revised, the applicable post construction parts of the reissued or revised permit shall take precedence. Any revision in the plan need to be reviewed by the City.
2. The site-specific characteristics shall be considered in the design.
a. Minimum orifice sizes should be considered in areas of soils with low permeability rates.
b. Infiltration systems in sites over well field areas shall require the approval of the Health Department.
(c) Drainage Plan. The development drainage map shall include sufficient data for the City to check the feasibility of the drainage system as proposed by the developer. The following data shall be provided:
(1) Hydrologic calculation. At all critical points within the development including catch basins:
A. Tributary drainage areas delineated on the map.
B. Times of concentration.
C. Intensity.
D. Runoff coefficients.
E. Design flow.
(2) Hydraulic calculations.
A. Sufficient documentation to indicate the results of the investigation into the adequacy of the downstream drainage system to handle the runoff from the proposed development. This will determine the maximum allowable release rate for the proposed development and, in turn, the amount of storm water storage that will be required.
B. The plan and profile of all drainage ways shall be provided, imposed upon which shall be the design energy and hydraulic grade lines.
C. Sizes and types of drainage improvements, including special structures, typical sections, right-of-way width and fencing.
D. Supporting calculations for upstream and downstream channel capacities as they affect overflow, erosion or backwater within the development. Such calculations shall be substantiated by such additional information as is required to determine profile and cross section of the upstream and downstream channel reaches under consideration.
E. Sufficient contours and grading details to indicate proposed street grades and elevations throughout the development.
(d) Construction Plans.
(1) The final construction plans for drainage within the development shall conform to the above provisions and to any special conditions as required by the City of Wapakoneta in approving the tentative map. Such construction plans for drainage shall be approved by the City of Wapakoneta prior to construction of any drainage facilities within the development. A grading plan with minimum finish floor elevations shall be submitted along with the construction plans to identify the lot grading and in-tract drainage planned for the development. The final construction drawings shall contain the locations, widths, names of existing streets, railroad rights-of-way, easements, parks, permanent buildings, and corporation and township lines; location of wooded areas and other significant topographic and natural features within and adjacent to the development property for a minimum distance of 200 feet.
Type of Surface | C Factor |
Type of Surface | C Factor |
Roof surface | 0.95 |
Bituminous or concrete pavement | 0.90 |
Gravel pavements | 0.70 |
Impervious soils | 0.65 |
Impervious soils with turf | 0.50 |
Slightly pervious soils | 0.40 |
Slightly pervious soils with turf | 0.30 |
(2) The designer should investigate the capacity of the immediate downstream drainage facilities to determine if they will be adequate to handle the design flow from this particular development. If the downstream facilities are inadequate, it may be necessary to provide on-site detention or retention basins to limit the flow to an amount which the downstream system can accept.
(3) A site grading plan with minimum finish grade shall be required by the design engineer as a part of the construction plans. At a minimum, first floor elevations should be shown. If minimum basement elevations are stated in the plans, developer/owners must follow the plans or assume all risk. The grading plan shall show all intended direction of surface water flow.
(4) All plans, calculations and design work need to be signed and stamped by a professional engineer.
(e) Storm Water Runoff Policy. This design manual is based on the policy that land uses and developments which increase the runoff rate or volume shall be required to control the discharge rate of runoff prior to its release of off-site land. The purposes of this policy are:
(1) Permit development without increasing the flooding of other lands.
(2) Reduce damage to receiving streams and impairment of their capacity which may be caused by increases in the quantity and rate of water discharged.
(3) Establish a basis for design of a storm drainage system on lands below undeveloped areas which will preserve the rights of property owners and assure the long-term adequacy of storm drainage systems. This runoff control policy applies to all land developments not specifically exempted below. Exemptions are appropriate for certain land use activities which clearly do not generate significant increases in storm water runoff.
(4) The following land uses and developments are within this exemption category:
A. Land preparation for agricultural crops, orchards, woodlots, sod farms and nursery operations.
B. Land grading or leveling for erosion control under direction of the local soil conservation district.
(f) Proposed Guidelines for Detention Basins Statement or Purpose. As urban development continues to transform previous watersheds into highly impervious urban areas, the problem of handling the ever-increasing storm water runoff on these watersheds is becoming more pronounced. In recent years, much attention has been focused on detention basins as a means of controlling the storm water runoff by detaining the water in the basin and releasing it at a rate compatible with downstream conditions. Some basins have a multi-purpose function, containing sport facilities, lakes and a park-like atmosphere during dry periods. The recommendations presented in this report are intended to serve this function. By following the recommendations, the detention basin can be designed so as to obtain the maximum recreational aesthetic benefits for the surrounding community while serving as an effective flood control mechanism.
(1) Planning guidelines. Detention basins serve to capture and temporarily store the surface water runoffs which result from urban development. This temporary storage allows for the release of the storm runoff at discharge rates which are acceptable to the receiving waterway. On site provision for detention storage would follow these approaches:
A. The release rate and volume of detention storage would be based on previously mentioned design requirements and standards.
B. Recreational uses should be maximized where possible and feasible.
C. Any development which the drainage exits the corporation limits of the City and enters onto County, Township or State jurisdiction shall be reviewed by the appropriate government official. A statement of comments must be submitted by that official in writing.
D. Stormwater runoff of the pre-developed "sheet flow" characteristics versus post-developed "point source" release of the storm water exiting the site shall be taken into design consideration. Runoff from a proposed development shall provide downstream energy dissipation.
E. Stormwater design shall take into consideration and provide adequate drainage for neighboring upstream property surface or subsurface water entering onto the proposed development or site. Offsite water may flow through the property and is not subject to storm control as long as it does not eventually enter the onsite detention basin.
F. All storm tiles found entering a proposed development or found while excavating needs to be investigated. If the storm tile is found to be in working condition, the storm tile must be replaced with new within the development and rerouted as necessary to maintain proper flow and not conflict with new utilities and/or structures.
G. If a storm tile is going to be abandoned, it must be totally excavated and destroyed within the development.
(2) Detention basins. The bottom of the basin shall have a minimum slope of 1.0 percent.
(3) Retention basins.
A. In order to provide for better management of the water, retention basins shall have a permanent water area of at least one-half acre. The permanent water area shall have an average water depth of four feet and no extensive shallow areas.
B. In excavated lakes, the underwater side slopes shall be stable.
C. A safety ledge of two-foot-deep, five feet wide is required around the perimeter of the basin.
(4) Detention and retention basins.
A. All developments/site improvements requiring detention/retentions shall have a homeowners association established as a part of the plat restrictions. The association shall be responsible for all pond and drainage swale maintenance.
B. There shall be a minimum three-foot-wide berm at a two percent slope between the right-of-way and the top of the basin slope.
C. Slopes on the sides of the basin shall have a maximum slope of 3:1. This includes both sides of any constructed levy.
D. The minimum diameter of any outlet conduit shall be one foot.
E. Safety measures shall be provided for any pipe or opening to prevent children or large animals from crawling into structures.
F. Danger signs should be mounted at appropriate locations to warn of deep water, possible flood conditions that exist during storm periods and other dangers that exist.
G. Retention pond design and maintenance shall include algae control with pond additives. Fountain shall also be considered but are optional.
H. Grass or other suitable vegetative cover shall be maintained throughout the entire basin area. Grass shall be cut regularly, no less than five times a year by the homeowner association.
I. Debris, trash removal and other necessary maintenance shall be performed after each storm or as required to assure continued operation in conformance with the design.
J. There shall be a minimum of one-foot freeboard between the 100-year storm elevations and the flow line of emergency overflow measure.
K. All basins shall have an emergency overflow one foot below the top of the levy or embankment containing the basin.
L. The City shall be notified prior to any adjustments to the approved outlet structure to any detention or retention basin. No adjustment shall be made until approved by the City.
M. The surface area opening for catch basin inlets shall be equal to or greater than the design overland flow contributing water to that basin.
N. The site developer may pass on maintenance responsibilities to the respective owner/homeowner's association via a maintenance agreement. The maintenance agreement must be approved by the City and an approved copy must be submitted to the City for their records.
(5) "As-built" drawings will be required for all basins to assure compliance with all applicable requirements. This requirement shall include calculations to confirm that the basin and outlet were constructed to function as designed.
(g) Plan Approval. These drawings and calculations shall be certified by a registered professional engineer. The construction of storm water drainage improvements shall be subject to the approval of the Director of Public Safety and Service or the County Engineer, depending on jurisdiction.
(Ord. 2018-35. Passed 7-16-18.)