This article is hereby adopted and shall be known as the “Drainage and Stormwater Control Ordinance” of the city.
(Ordinance 08-1218A adopted 12/18/08)
The purpose set forth herein is to ensure adequate stormwater drainage and flood control within the city, to protect public health and safety, to minimize property damage due to flooding, to limit runoff rates to equitably distribute the cost of necessary drainage improvements, and to minimize the maintenance cost of drainage facilities constructed. Any development or improvement of property affecting storm drainage and flood control in the city is subject to the provision of this article. It also applies to individual building structures, subdivision, excavation and fill operations.
(Ordinance 08-1218A adopted 12/18/08)
Prior to the issuance of any building permit, all construction for storm drainage in the development or improvement of property within the city shall comply with the following standards, regulations and requirements as set forth herein:
(1)
Stormwater.
Storm sewer inlets shall be provided on any and all paved streets at such intervals as are necessary to limit the depth of flow as follows:
(A)
Residential streets.
Based on parkway slopes of one-quarter inch (1/4") per foot behind the curb, the 100-year design frequency flows shall not exceed a depth of one and one-half inches (1-1/2") over the top of curb. A maximum flow of 45 cfs will be allowed in the street.
(B)
Collector streets.
Based on parkway slopes of one-quarter inch (1/4") per foot behind the curb, in industrial and commercial areas, the 100-year design frequency flows shall not exceed a depth of one-half inches (1/2") over the top of curb. A maximum flow of 45 cfs will be allowed in the street.
(C)
Major thoroughfares.
Based on a cross slope of one-quarter inch (1/4") per foot on the pavement, the 100-year design frequency flow shall not exceed the elevation of the lowest top of curb. A maximum of 45 cfs will be allowed in the street.
(D)
Alleys.
The 100-year design frequency flows shall not exceed the capacity of the alley sections. A maximum of 45 cfs will be allowed for an alley with curbs, and 25 cfs for alleys without curbs.
(E)
Positive overflow.
The approved drainage system shall provide for positive overflow at all low points. The term “positive overflow” means that when the inlets do not function properly, or when the design capacity of the conduit is exceeded, the excess flow can be conveyed over land along a paved course. Normally, this would mean along a street or alley, but can require the dedications of special drainage easements on private property.
(2)
Closed stormwater.
(A)
A closed storm sewer system shall be required to accommodate a runoff exceeding the street capacity, as provided in this section, up to and including the design capacity of a seventy-two inch (72") concrete pipe. The following are recommended maximum design velocities:
Culverts | 15.0 fps |
Inlet lateral | 10.0 fps |
Storm sewer | 12.5 fps |
(B)
Discharge velocities cannot exceed the permitted velocity of the channel or conduit at the outfall.
(3)
An open channel may be permitted to accommodate runoff exceeding the design capacity of a seventy-two inch (72") pipe, as provided below:
(A)
Channels draining an area with a “CA” factor (coefficient to runoff and drainage area as used in the hereinafter described “rational formula”) of less than 600 shall be concrete lined to the design depth plus six inch (6) freeboard except that a closed system as provided above may be used. A twenty (20) foot wide access easement shall be provided along at least one (1) side parallel to channel. The top width of the channel at the design depth must not exceed fifty feet (50') unless specifically approved by the city engineer.
(B)
Channels draining an area with a “CA” factor between 600 and 1,000 shall be improved to a capacity of the 100-year design discharge by excavation, straightening and realignment. The construction of the concrete lined channel shall have a width of not less than the bottom width with the concrete lined to a depth of at least three feet (3') on the banks. Earthen side slopes shall be no steeper than 4:1, horizontal to vertical, and shall have approved groundcover to prevent erosion.
Where drainage conditions or velocities of water will exceed that condition which would create erosion, provision shall be made for the placement of riprap, gabion, etc., along the stream, and channel banks by the developer.
(C)
Channels draining an area with a “CA” factor over 1,000 shall be designed to carry the capacity of a 100-year flood frequency storm. The specific design and type of construction improvements for this drainage facility shall have specific [approval] by the city engineer after review of the maintenance, erosion and site conditions.
(D)
All areas of an earth channel section shall be improved by the developer with low maintenance vegetation as approved by the city engineer, prior to planting. The selection of materials shall comply with the current groundcover listing for North Central Texas furnished through the Texas Agricultural Extension Service.
(E)
The setback for the building line shall be as follows:
(i)
A maintenance strip shall be provided with a twenty foot (20') width along each side of the top of the channel unless approved otherwise by the city engineer and shown on the file plat.
(ii)
A drainage flume section which provides for limited flow of stormwater shall be located within a drainage easement of sufficient width which permits future maintenance accessibility.
(4)
A drainage feature which is to remain in its natural state of native growth may be accepted by the city to remain as an unimproved facility so long as the water conveyance capacity of the area is adequate to handle the future drainage requirements.
(5)
In lieu of the improvements of a channel draining an area with a “CA” factor in excess of 600, the city council may elect to accept the dedication of all land within the 100-year floodway of existing drainage channel as a permanent drainage right-of-way.
(7)
Fill and grading operations within the city limits shall be undertaken only after a proper permit has been obtained from the city engineer. Failure to obtain the proper permit shall result in the requirement for the developer to replace the soils, as required by the edition of the International Building Code currently adopted.
(8)
Drainage and floodway easements shall be provided for all open channels. Easements shall encompass all area beneath a ground elevation defined as being the highest elevation of the following:
(9)
Any groundcover or vegetation which is planted and is a part of the improvement project will not be accepted by the city until the growth has been established and maintained by the developer for a one-year time period.
(Ordinance 08-1218A adopted 12/18/08)
(a)
The owner or developer of property to be developed shall be responsible for all storm drainage flowing through or abutting such property. This responsibility includes the drainage directed to that property by ultimate development as well as the drainage naturally flowing through the property by reason of topography. It is the intent of this article that provision be made for storm drainage in accordance with section 3.09.003 above, at such time as any property affected is proposed for development, use or modification.
(b)
Where the improvement or construction of a storm drainage facility is required along a property line common to two (2) or more owners, the owner hereafter proposing developments at the time of development, shall provide the storm drainage facility including the dedication of all necessary right-of-way or easements, to accommodate the improvements.
(c)
Where a property owner proposed development or the use of only a portion of his property, provision for storm drainage in accordance with section 3.09.003 above shall only be required in that portion of the property proposed for immediate development, except as construction or improvements of a drainage facility outside that designated portion of the property is deemed essential to the development of that designated portion.
(d)
When a property owner proposes a development, provisions shall be made in the development plan for control of excess siltation and downstream erosion.
(e)
In the event that a property owner or developer desires to modify an existing pond or lake or desires to impound stormwater by excavation, filling or construction of a dam within a property, for retention or detention, thereby creating a lake, pond, lagoon or basin as a part of the planned development of that property, the standard provisions for storm drainage as established in section 3.09.003 of this article shall be applicable, and shall also provide:
(1)
That an engineering plan for such construction, accompanied by complete drainage design information, prepared by a registered professional engineer, shall have been approved by the city;
(2)
That the owner or developer shall have agreed to retain under private ownership the lake, pond, lagoon or basin constructed, and to assume full responsibility for the protection of the general public from any health or safety hazards related to the lake, pond, lagoon or basin constructed;
(3)
That the owner or developer shall have agreed to assume full responsibility for the maintenance of the lake, pond, lagoon, or basin constructed;
(4)
That the obligations herein shall run with the land and shall be a continuing obligation of the owner or owners of such land;
(5)
That all federal, state, and county laws pertaining to impoundment of surface water are complied with, including the design construction and safety of the impounding structure. Any existing structure which is included in a project development area shall be improved to comply with the applicable federal, state, and county and city safety requirements for structures. The design flows shall be based upon the urbanized drainage flows which can result from a 100-year flood. All improvements shall be made to the dam structure at the expense of the developer, prior to the acceptance of the adjacent street, utilities and drainage improvements as provided for under the subdivision ordinance;
(6)
On any existing structure, the owner will furnish a study by a professional engineer to the city for approval prior to any proposed alteration. Compensatory storage shall be provided in some manner such that equal or comparable flood retention capacity is maintained.
(f)
The maintenance of private drainage facilities shall be provided for by the property owner or assigned agent. The city shall be kept advised of the responsible agent.
(g)
All existing water seepage springs, or flowing water shall be connected into an underground storm sewer system, or they shall be discharged into an appropriate facility which is intended to carry stormwater runoff. Such flow will be permitted to discharge directly into the street gutter line.
(h)
Fences (private and public screening) shall be constructed such that blockage of surface water flow does not occur. This includes the requirement that erosive conditions shall not be created around, under or near a fence structure.
(i)
The developer shall provide detailed off-site drainage plans for the proper transition to natural ground and stream elevations. Criteria for on-site development shall apply to off-site improvements as required by the city engineer.
(Ordinance 08-1218A adopted 12/18/08)
(a)
Drainage design requirements.
(1)
Each storm drainage facility, including street capacities shall be designed to convey the runoff which results from a certain prescribed design storm.
(2)
Drainage design requirements for open and closed systems shall provide protection for property during a storm having a 100-year recurrence interval with this projected flow carried in the streets and closed drainage systems in accordance with the following:
Drainage Facility | Design Recurrence Interval |
|---|---|
Closed storm sewer systems | 10 years with emergency 100-year overflow |
Closed storm sewer system at street low point or sag | 100 years which include emergency overflow |
Culverts and bridges | 100 years (unless otherwise directed) |
Concrete lined channel | 50 years with emergency 100-year overflow |
Earthen channels | 100 years |
(b)
Computation of stormwater runoff.
(1)
Computation of stormwater runoff for drainage areas less than 200 acres shall be by the “rational method,” which is based on the principle that a maximum rate of runoff from a given drainage area for an assumed rainfall intensity occurs when all parts of the area are contributing to the flow at the point of discharge. The formula for the calculation of runoff by the “rational method” is:
Q = CIA, where: | ||
Q | = | The maximum rate of discharge expressed in cubic feet per second (fps). |
C | = | A runoff coefficient which varies with the topography, soil, soil cover, land use and moisture content of the soil at the time the runoff producing rainfall occurs. This runoff coefficient shall be based on the ultimate use of the land as recommended by the land use plan for the city, and shall be selected from table I herein on the basis of the use shown on land use and zoning map on the comprehensive zoning ordinance for the city. If an area has had a change of zoning to give the area land use for which the “C” in table I is higher than use shown on land use and zoning maps, the higher “C” factor shall be used. |
I | = | Rainfall intensity in inches per hour from the applicable curves of figure 1 (on file in the offices of the city). Time of concentration or duration of rainfall for use in figure 1 shall be calculated by velocity data shown in table II. |
A | = | The drainage area, expressed in acres, contributing to the runoff at the point in question. Calculation of the drainage area shall be made from an accurate topographic map, a copy of which shall be submitted with the engineering plans for approval. |
(2)
For drainage areas in excess of 200 acres, where the use of “rational method” does not provide reliable data, the use of unit hydrograph flow determination shall be made. The use of unit hydrograph calculation will be based upon standard and accepted engineering principles normally used in the profession subject to the approval by the city engineer. The Soil Conservation Service Technical Release Number 55 is an accepted method.
(3)
Computation of stormwater runoff shall be based on a fully developed drainage area, or watershed, in accordance with the land use projected in the then-current comprehensive land use plan for the city.
(4)
For any property being developed into a retail, commercial or industrial type land use, as identified in the zoning ordinance, the stormwater runoff rate which will exit the project after development shall be no greater than that volume measured in cfs, or velocity measured in fps as determined through the defined design criteria, outlined in this article prior to development. This requirement may require the design and construction of detention or retention facilities.
(5)
For property being developed into residential use including single-family, detached or attached units, the engineer shall calculate the stormwater runoff according to standard engineering practices and as outlined within this article to determine the anticipated stormwater flows generated by a 100-year storm frequency. Provisions may be made for the drainage system without the requirement of retention or detention facilities.
(6)
All development plans, including site plans and file plats which have a drainage outlet works or outfall line with drainage into the city’s stormwater system, or any other locations which may be proposed for drainage outfall shall be submitted to the city.
(7)
The presentation and design of a comprehensive drainage plan submitted by the developer or their engineer to the city for all plats will be reviewed. The plan, after review and/or proposed modification, shall be approved by the city’s engineer. If the plan is not adequate, the city’s engineer will report the concerns to the city administrator for action to be considered by the city, or change to be made by the developer/owner.
(c)
Predicting volume of runoff.
(1)
The two basic methods suggested for predicting the volume of runoff with time and the peak flow rate are the rational method and the unit hydrograph method. The rational method may be used for drainage areas less than or equal to 200 acres. Drainage basins that exceed 200 acres must use the unit hydrograph method. The Soil Conservation Service Technical Release Number 55 is an acceptable unit hydrograph method.
(2)
When the rational method is used and detention is required, the volume of water supplied by the design storm may be calculated by converting the runoff rate, during a specific duration, to volume. The inflow volume should be determined for a period of at least twice the time on concentration for the site.
(3)
Retention and detention are two generalized types of storm runoff storage used to control the rate of runoff. All detention ponds should be designed to empty within a 24-hours period.
TABLE 1 VALUE OF “C” FOR USE IN “RATIONAL METHOD” FORMULA Q = CIA | ||
|---|---|---|
Slope | Land Use from Land Use Plan | Value of “C” (Runoff Coefficient) |
Flat Terrain 0% to 1% | Park areas - no developable land | 0.20 |
Park and school land tract | 0.30 | |
Single-family residential | 0.45 | |
Duplex | 0.50 | |
Multiple-family | 0.55 | |
Local business | 0.65 | |
Central business | 0.80 | |
Commercial | 0.80 | |
Industrial | 0.75 | |
Moderate Terrain 1% to 3.5% | Park areas - no developable land | 0.30 |
Park school land tract | 0.40 | |
Single-family residential | 0.50 | |
Duplex | 0.55 | |
Multiple-family | 0.60 | |
Local business | 0.70 | |
Central business | 0.85 | |
Commercial | 0.85 | |
Industrial | 0.80 | |
Steep Terrain 3.5% and over | Park areas - no developable land | 0.35 |
Park and school land tract | 0.45 | |
Single-family residential | 0.65 | |
Duplex | 0.70 | |
Multiple-family | 0.75 | |
Local business | 0.80 | |
Central business | 0.85 | |
Commercial | 0.85 | |
Industrial | 0.85 | |
TABLE II AVERAGE VELOCITY FOR USE IN DETERMINING TIME OF CONCENTRATION | ||||
|---|---|---|---|---|
Description of Water Course | 0% to 3% V. in fps | 4% to 7% V. in fps | 8% to 11% V. in fps | Over 12% V. in fps |
Surface drainage | 5 | 9 | 13 | 15 |
Channels | Determine V. by Manning’s Formula | |||
Storm sewers | Determine V. by Manning’s Formula | |||
(4)
The data shown in table II are average velocity of the runoff for calculating time of concentration or duration of rainfall for use in figure 1 (on file in the offices of the city). These average velocities in this table shall be used unless the designer shows calculation of velocities by streets and/or storm sewers, or overland flows.
(5)
Using the average velocities from this table, the designer shall calculate the time of concentration by the following formula unless more data is shown on the plans for calculating time of concentration.
“Inlet Time” = Five (5) minutes for property zoned for multiple-family, local business, central business, commercial or industrial: ten (10) minutes for property zoned for parks, schools, single-family residential and duplex.
T = “Inlet Time” + D/(V x 60), where: | ||
T | = | Time of concentration in minutes for use in figure 1 (on file in the offices of the city). |
D | = | Distance in feet from point of concentration to upper end of drainage area under consideration. |
V | = | Velocity in feet per second from this table or velocity calculated by designer by streets and/or storm sewers. |
(6)
The existing soils are such that erosive conditions are created at certain velocities. The following velocities are considered to be maximum acceptable design conditions:
Earth (with no concrete protection) | 0 to 5 feet per second |
Shale | 5 to 6 feet per second |
Rock | 6 to 10 feet per second |
(7)
The minimum curb inlet size shall be eight feet (8') in length with a capacity of 8 cfs.
(8)
The minimum curb inlet size shall be eight feet (8') in length with a capacity of 8 cfs.
(d)
Paving and drainage specifications.
The construction of all improvements shall be in accordance with the standards set forth in the current Paving and Drainage Specifications Handbook, and design standards of the city. The frequency chart in years (figure 1), the inlet capacity for low point inlets (figure 2), the drainage capacity road sections (figure 3) (all figures on file in the offices of the city), and the city criteria are hereby adopted as a part of this article.
(e)
Engineering plans.
Complete engineering plans for storm drainage facilities shall be prepared by a professional engineer licensed in the state and experienced in civil engineering work. The total cost for such engineering plans and specifications shall be borne by the owner or the developer and shall be furnished to the city engineer for his review and approval.
(f)
Preliminary drainage study.
In any development, alteration, or improvement of property, the owner may be required to provide, at his expense, a preliminary drainage study for the total area to be ultimately developed. This study shall be submitted to the city engineer as a part of the submitted data for consideration of preliminary plat, or site plan approval.
(Ordinance 08-1218A adopted 12/18/08)
(a)
In all areas subject to flooding, the finished floor elevation shall be a minimum of two feet (2') above the high water elevation calculated for the runoff from a rainfall having a recurrence interval of 100 years, in a completely developed watershed. The owner/developer shall furnish, at his expense, to the city engineer, sufficient engineering information to confirm that the minimum floor elevations proposed are as required by this subsection. Residential construction permits will not be issued until lots and/or sites are elevated from the floodplain in accordance with FEMA-approved revision.
(b)
No building or structure, excavation, filling, or construction of embankment or landscaping shall be permitted within a floodplain or channel which will increase water surface elevations or obstruct natural flow of water within that floodplain or channel, unless sufficient engineering design information is furnished to the city engineer in order that he may determine the same will not adversely affect flow characteristics within that floodplain or channel, resulting in damage to that or any other property nearby.
(c)
The provisions of the city’s current flood hazard prevention ordinance (FEMA criteria) shall be observed in preparing land development plans.
(d)
New construction and substantial improvement of any commercial, industrial or other nonresidential structure shall have the lowest floor, including basement, elevated to the level of the base flood elevation, plus two feet (2).
(Ordinance 08-1218A adopted 12/18/08)
No building or structure, including fences, shall hereafter be constructed, reconstructed, or relocated within twenty feet (20') of any open drainage channel. It is the intent of this section to ensure that an unobstructed width of at least twenty feet (20') is maintained between the top of the high bank of any such drainage channel and any building or structure. (See also section 3.09.003).
(Ordinance 08-1218A adopted 12/18/08)
Minimum lot and floor elevations shall be established as follows:
(1)
Lots abutting a natural or excavated channel shall have a minimum elevation for the buildable area of the lot at least equal to the highest elevation of the drainage floodway easement, and a finished floor elevation at least two feet (2') above the 100-year design storm or FEMA floodway elevation, whichever is greater. The minimum finished floor elevation shall be set at an elevation of two feet (2') above the top of curb elevation, except when the terrain feature slopes, such that drainage is not a critical element to the project.
(2)
Where lots do not abut a natural or excavated channel, minimum floor elevations shall be a minimum of one foot (1') above the top of street curb or edge of alley, whichever is lower, unless otherwise approved by the city engineer. Where a lot is adjacent to a drainage flume or channel, the finished floor shall be a minimum of two feet (2') above the high water elevation. Where the structure is below a street or alley, the builder shall grade and construct facilities such that a positive drainage system of swales are capable of discharging the resultant flows which may flow across the yard area into the structure.
(3)
The minimum finished floor elevation shall be shown on the final file plat for record purposes. Prior to final acceptance of utilities and street construction by the city, an elevation certificate statement shall be prepared, by a registered professional land surveyor showing all lot elevations, as developed within the subject project, meet or exceed the required minimum finished floor elevations. This certification shall be filed with the city.
(4)
Existing platted property which is subject to flooding or carries a specified or recorded minimum finished floor elevation shall be surveyed by a registered professional land surveyor prior to obtaining a building permit. The certified survey data shall be furnished to the city for approval. Certificate of compliance with the provisions of this article pertaining to specified finished floor levels shall be required.
(Ordinance 08-1218A adopted 12/18/08)
No building permit shall be issued, nor plat or site plan approval, nor certificate of occupancy approved for any construction, reconstruction, or development upon any land where such construction, reconstruction or development is not in conformity with the requirements and intent of this article. Anyone who violates any of the terms and provisions of this article shall be denied a building permit until the violations are corrected. Residential construction permits will not be issued until lots and/or sites are elevated above the floodplain by FEMA-approved revisions.
(Ordinance 08-1218A adopted 12/18/08)
(a)
The contractor shall guarantee the work which he does against defective workmanship and materials for a period of one (1) year from the date of final acceptance by the city.
(b)
Where defective workmanship and/or materials are discovered requiring repairs to be made under this guarantee, all such repair work shall be done by the contractor, upon failure by the contractor fail to make repair or correct such defective workmanship and/or materials within five (5) days after being notified, the city may make the necessary repairs and charge the contractor with the actual cost of all labor and materials required.
(c)
The contractor shall provide a performance bond for a period of one (1) year after the date of acceptance of the work to cover his guarantee as set forth above.
(Ordinance 08-1218A adopted 12/18/08)