§ 42-194. Storm drainage facilities.

Latest version.

(a)

General. The developer shall have prepared by a registered professional engineer the plans for the installation and construction of all necessary drainage facilities. These plans shall be approved by the city engineer and shall include details for the construction of the proposed storm drain system.

(1)

Determination of storm drain facilities necessary and construction requirements shall be the responsibility of the city engineer. Where there is a question as to the justification of size of the facility required, the question will be resolved in favor of additional drainage capacity.

(2)

Coordination with the comprehensive master drainage plan (when available) is required. The developer shall provide those drainage improvements which traverse or abut the proposed subdivision, where specified in the comprehensive plan. All cost for such improvements shall be paid by the developer, except where the city manager shall determine that the improvements benefit other citizens more than that of the proposed subdivision and shall determine the equitable city participation in such improvements. Such city participation, or any appeal of such requirements, shall be approved by the city council.

(3)

Drainage facilities shall be provided and constructed by the developer in accordance with current design criteria herein adopted by the city and the standard specifications for public works construction and the following basic requirements:

When conditions upstream from a proposed channel or storm drain outside the limits of the ownership of the developer do not permit maximum design flow, the drainage facilities shall be designed based on potential and fully developed conditions.

When conditions downstream from a proposed channel or storm drain outside the limits of the ownership of the developer do not permit maximum design flow, water surface elevations for a 100-year design frequency shall be indicated considering the downstream condition in order to define the potential flood hazards. Solutions to protect the property shall be developed.

All drainage improvements shall be designed to an acceptable outfall.

Where there is a question as to the justification or size of drainage facilities, doubt will be resolved in favor of additional drainage capacity.

The combined capacity of storm drain pipe, street and surface drainage shall contain the 100-year design flow at the building lines. The finish floor elevation shall be at least one foot above the 100-year flood elevation.

All streets and storm drains shall be designed so that the stormwater runoff from a 100-year frequency design storm is contained within the available right-of-way or drainage easements. Underground storm drains shall be designed and provided in accordance with the following standards:

Local streets shall have the capacity to carry a five-year stormflow without topping the curb and the 100-year stormflow shall be contained within the right-of-way.

Collector streets shall have the capacity to carry a five-year stormflow without topping the curb and at least one lane of traffic shall be kept open at all times. In addition, the 100-year stormflow shall be contained within the right-of-way.

Thoroughfares shall have the capacity to carry a five-year stormflow without topping the curb and maintain at least one open lane of traffic in each direction. In addition, the 100-year stormflow shall be contained within the right-of-way.

The size of the underground storm drain shall be sufficient to contain the portion of the 100-year stormflow not carried by surface flow within the street right-of-way or drainage easement. The development shall install an underground storm drain beginning at the point where the calculated stormwater runoff is of such quantity that it will overflow the street at curb height during a five-year frequent event. The storm drain shall be constructed from this point to an approved outlet where stormwater can be discharged safely without damage or flooding to the adjacent property. All drainage facilities shall be constructed on public right-of-way or easements dedicated for this purpose and of sufficient size to permit access for maintenance of the drainage facility. Drainage facilities shall be handled by one of three methods:

A concrete channel liner designed for the 100-year water flow as based upon total development of the drainage area involved;

Underground reinforced concrete pipes; or

A natural drainage channel may be retained only in those areas specifically designated in the comprehensive plan. Where natural drainage is retained, the entire limits of the 100-year floodplain shall be dedicated as a drainage easement and encroachments of fill into the floodway fringe shall be limited to less than 50 percent of the total floodway fringe designated on the FEMA maps. Normal maintenance of these areas shall remain the responsibility of the adjacent landowner or homeowner's association, but adequate access shall be provided for the city to perform emergency maintenance, if required.

The developer may install a concrete lined channel in lieu of installing pipe larger than 60 inches. This open channel shall be at the rear of residential lots. In the event it is necessary to locate the drainage facility adjacent to and parallel to a street it shall be a closed conduit even though pipe sizes larger than 60 inches are required.

A permanent chainlink fence or other fence meeting the requirements of the city shall be constructed along the top of any channel exceeding three feet in depth to enclose the area where it is adjacent to residential lots and also in other cases, where it is deemed necessary to restrict access to the channel.

All drainage facilities shall be constructed on public right-of-way or easements dedicated for the purpose. Drainage easements shall be of a sufficient size to permit access for maintenance of the drainage facility. The easement shall be designed to facilitate maintenance access to the drainage channel by city crews and equipment. Additional easements shall be required at any access points and the access points shall be designed to restrict access by unauthorized personnel. An access point will typically be required at every intersection of the drainage easement with street right-of-way.

When a drainage ditch or storm drain pipe, culvert or bridge is proposed, calculations shall be submitted showing basis for design.

When a drainage channel, storm drain pipe, culvert or bridge is proposed, completed plans, profiles and specifications shall be submitted, showing complete construction details and detailed cost estimate.

The city has adopted the six-inch high curb as the standard for residential streets. The gutter shall be 24 inches wide.

Detention/retention ponds: Retention (maintains a permanent pool elevation) and detention (no permanent pool storage) may be constructed only with the approval of the city. The design of such facilities shall be in accordance with a commonly-accepted design method. The method used shall be well documented and will be subject to the approval of the city engineer.

Retention/detention ponds shall be encompassed by an easement. The facility will remain the maintenance responsibility of the owner/developer, unless otherwise accepted by the city. Acceptance by the city will be contingent upon the facility being a part of a dedicated park or other such property which meets with the city's approval.

In some instances the city may require detention to prevent the exacerbation of existing drainage problems, or to prevent drainage problems where existing drainage systems are at capacity.

The owner or developer of property to be developed shall be responsible for all storm drainage flowing on his property. This responsibility includes the drainage directed to that property by ultimate development as well as drainage naturally flowing through the property by reason of topography.

Adequate consideration shall be given by the owner in the development of property to determine how the discharge leaving the proposed development will affect adjacent property.

On lots or tracts of three acres or more where stormwater runoff has been collected or concentrated, it shall not be permitted to drain onto adjacent property except in existing creeks, channels or storm sewers unless proper drainage easements or notarized letters of permission from the affected property owners are provided.

The subdivider shall pay for the cost of all drainage improvements required for the development of the subdivision, including any necessary off-site channels or storm sewers and acquisition of the required easements.

Where it is anticipated that additional runoff incidental to the development of the subdivision will overload an existing downstream drainage facility, whether natural or manmade, the planning and zoning commission may withhold approval of the subdivision until appropriate provision has been made to accommodate the problem, and plans shall be provided which include all necessary off-site improvements including storm sewer systems, channel grading, driveway adjustments, culvert improvements, etc.

In areas where downstream pipes or channels are inadequate to handle proposed increased flows, the city, as one alternative, may consider accepting payment in lieu of actual drainage improvements. The developer must show that the proposed pipe system to handle the flow form his development would not function properly without substantial downstream improvements. Prior to permitting any development that will significantly increase flood heights downstream or upstream, a hearing before the planning and zoning commission is required with special notice to the adjacent property owners.

(b)

Design criteria.

(1)

For watershed areas less than 200 acres, the rational method may be used. The rational method of design is based on the direct relationship between rainfall and runoff, and is expressed by the following equation:

Q = CIA

Where

Q = Discharge or surface runoff in cubic feet per second.

C = Coefficient of runoff.

I = Intensity of runoff in inches per hour as derived for United States Weather Bureau, technical paper 40.

A = Drainage areas in acres.

(2)

For watershed areas greater than or equal to 200 acres, the engineer shall use one of the following methods:

SCS Tabular method;

SCS Graphical method;

SCS TR-20;

HEC-1; or

Other hydrograph method approved by the city engineer.

(c)

Design storm frequency. All drainage facilities must be designed using the following criteria:

Type of Facility	Description of Area To Be Drained	Maximum Time of Concentration (Minutes)	Recommended Design Frequency (Years)
Storm sewers	Residential, commercial and manufacturing	30	5
Culverts, bridges, channels and creeks	Any type of area less than 100 acres	30	100
Culverts, bridges, channels and creeks	Any type of area greater than 100 acres but less than 1,000 acres	45	100
Culverts, bridges, channels and creeks	Any type of area greater than 1,000 acres	60	100

A minimum time of concentration of ten minutes shall be used, with detailed computation made to determine the exact time of concentration to each inlet facility.

(d)

Water spread limit. Permissible water spread is as follows:

Street Classification	Permissible Water Spread
Major thoroughfare (divided)	Ten-year storm—two traffic lanes must remain open
Collector street	Ten-year storm—one traffic lane must remain open
Residential street	Ten-year stormwater flow must not exceed two inches above curb
Pipe systems and inlets	Ten-year storm hydraulic grade shall be two feet below top of curb

The permissible water spreads are based upon the initial storm frequency (ten-year), but consideration must be given to street conveyance of the major storm (100-year) and possible flooding. All streets shall be capable of conveying a major storm without water encroaching into adjacent buildings. Therefore, the maximum spread limit in streets for a major storm shall be the building lines. This requirement of utilizing the streets to convey the major storm runoff may require increasing the capacity of the enclosed drainage system.

(e)

Street cross flow . Allowable depths of flow across street intersections for initial year) are established as follows:

Street Intersection	Cross Flow Depth
Major thoroughfare (divided and undivided)	None
Collector street (one valley crossing)	20 cfs
Residential street (one valley crossing)	20 cfs

(f)

Storm drainage design. Storm drainage facilities shall be designed in accordance with Storm Drainage Criteria and Design Manual, City of Fort Worth, 1967 with the following modifications:

(1)

Surcharged flow. All storm drains shall be designed for surcharged flow.

(2)

Outfalls. Outfalls in natural watercourses shall begin the hydraulic gradient elevation for surcharge design at the top of the conduit plus V /2 _g .

(3)

Open channels. Open channels shall be designed to carry the stormwater from a 100-year frequency storm with a minimum of one foot additional freeboard. Culverts and crucial low point drainage facilities shall be designed using the 100-year frequency stormflow with a minimum of one-foot freeboard. Bridges shall be designed with a two-foot freeboard between the 100-year water surface and the low point of the bridge.

(4)

Upstream entry. Flows entering a project from upstream shall be calculated for fully-developed conditions in the contributing watershed.

(5)

Drainage system requirements. The complete drainage system is composed of the initial system, consisting of inlets, storm drains, and the associated appurtenances to convey the initial storm runoff (ten-year); and the major system or the major runoff (100-year), which consists of swales, creeks, channels, floodways and emergency overflows to prevent water encroachment into residential and commercial facilities:

Initial storm systems are required when water spread and street cross flow limits are exceeded (closed pipe generally up to 48-inch capacity).

Channels:

100-year flow is to be contained within the building lines.

Unlined channels will be considered for quantities of floodwater larger than the equivalent flow of a 72-inch pipe.

(6)

Additional storm drain criteria.

Utilization of retention ponds and dispersion areas and preservation of major floodplains, etc., may be required if a proposed drainage improvement is found to create actual or potential upstream, adjacent or downstream property damage due to the creation of excessive flood velocities or heights.

The city's major drainage floodplains are still functioning in a natural or seminatural state will require special drainage and other preservation considerations. To implement this policy of the natural 100-year floodplain for flooding areas draining one square mile or more, it may be recommended that these areas be zoned for planned development when zoning requests are made so that channel improvements and preservation efforts will be coordinated and defined on the site plan before detailed plans are submitted.

Minimum velocity with the pipe flowing full shall be three feet per second.

The minimum storm drain pipe diameter shall be 15 inches.

Pipe diameters shall not normally decrease downstream.

Pipe crowns at change in sizes should be set at the same elevation.

Vertical curves in the conduit will not be permitted, and horizontal curves will be permitted only with the approval of the city engineer.

Maximum manhole spacing is shown below:

Pipe size (inches)	Maximum spacing (feet)
15	400
15—36	600
42—60	1,000
Larger than 60	No limit

Manholes shall also be placed at pickup points having two or more laterals, at pipe junctions having pipe sizes 24 inches, or greater, at alignment changes and at the beginning of the storm drain system.

Inverted crown sections will be permitted only in alleys.

Street crowns shall be reduced for approximately 100 feet on each side of valleys, and only one valley crossing for each street shall be used at an intersection.

At streets with culverts or bridges an emergency over flow shall be provided to contain the 100-year channel flow within the building lines.

(7)

Runoff coefficients. Storm drainage shall be designed for ultimate development of the watershed and, therefore, runoff coefficients used shall consider these fully developed conditions. Master plans, zoning maps and land use plans shall be used to determine the ultimate development. The table below gives general guidelines of values for runoff coefficients which may be used in the determination or stormwater runoff:

Runoff Coefficient C for All Storm Calculations

Type Area or Land Use	Runoff Coefficient C
Parks and permanent open space	0.20
Single-family residential	0.50
Multifamily residential	0.65
Industrial and manufacturing	0.70
Business	0.80
Central business district	0.90

When ultimate development cannot be verified, a standard runoff coefficient of 0.80 can be used.

(8)

Time of concentration. The table below describes the minimum time of concentration:

Minimum Inlet Time of Concentration

Type Area	Minimum Inlet Time
Parks and permanent open areas	20 minutes
Residential (less than five units per acre density)	15 minutes
All other residential	10 minutes
Industrial and business	10 minutes
Roof and paving areas, streets	10 minutes

When inlet times of concentration which are in excess of these minimums are used, the techniques and assumptions used in computing these times must be submitted with the plans and approved by the city engineer. In cases where it is evident that the actual time of concentration is less than that indicated above, a shorter time of concentration should be used.

(9)

Design flows. All streets and storm drains shall be designed so that the stormwater runoff from a 100-year frequency design storm is contained within the available right-of-way or drainage easements. Underground storm drains shall be designed and provided in accordance with the following standards:

Local streets shall have the capacity to carry a five-year stormflow without topping the curb and the 100-year stormflow shall be contained within the right-of-way.

The size of the underground storm drain shall be sufficient to contain the portion of the 100-year stormflow not carried by surface flow within the street right-of-way or drainage easement.

Open channels shall be designed to carry the stormwater from a 100-year frequency storm with a minimum of one foot additional freeboard. Culverts and crucial low point drainage facilities shall be designed using the 100-year frequency stormflow with a minimum of one foot freeboard. Bridges shall be designed with a two-foot freeboard between the 100-year water surface and the low point of the bridge.

Flows entering a project from upstream shall be calculated for fully-developed conditions in the contributing watershed.

(10)

Drainage modification permit.

No land shall be modified in any 100-year flood drainageway until a drainage modification permit is issued by the city secretary upon direction by the city engineer. The city engineer, as a condition of the permit, shall certify that such modification will, as a minimum, fulfill the requirements of this article with special attention that peak flow heights or velocities will not be increased on adjacent properties.

Drainage modification permits for drainageways whose basin exceeds one square mile may be approved by the planning and zoning commission after notification of adjacent property owners. Drainage modification permits for drainageways less than one square mile may be approved by the city secretary upon direction by the city engineer. Smaller drainageways which do not carry runoff from upstream properties and do not change the downstream concentration point will not require permits.

Any decision by the city engineer can be referred to the planning and zoning commission by either the city engineer or the applicant for final determination. Upon such referral all adjacent property owners will be notified of the hearing at least ten days prior to the scheduled planning and zoning commission meeting.

(Code 1994, ch. 9, art. 52; Ord. No. 248, exh. A(art. 52), 3-24-1997)

                    
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