(a) 
The planning and zoning commission shall not recommend for approval any plat, development plan, or subdivision which does not make adequate provision for stormwater or floodwater runoff channels or basins and the town council may not approve such plats, development plans, or subdivisions without such provisions. Drainage provisions shall ensure the health and safety of the public and property in times of flood and such drainage facilities shall not cause excessive increases in flood heights or velocities, particularly to adjacent and downstream properties. When calculations indicate that curb capacities are exceeded at a point, no further allowance shall be made for flow beyond that point, and basins shall be used to intercept flow at that point.
(b) 
The applicant may be required by the planning and zoning commission or town council as a condition of preliminary plan and/or final plat approval to carry away by pipe or open ditch any spring or surface water that exists either previous to, or as a result of, the subdivision. Such drainage facilities shall be located in the road right-of-way where feasible, or in perpetual unobstructed easements of appropriate width, and shall be constructed in accordance with the construction standards and specifications of the town.
(Ordinance 17.0 adopted 3/21/00)
The following table sets out design storm frequencies for various areas and facilities:
Design Storm Frequencies
Area or Facility Frequency
Residential street capacity
100 years
Other street capacity and enclosed pipe system (if needed)
100 years
Channels and creeks*
100 years
Culverts and small bridges
100 years
Large bridges**
100 years
Floodways between building lines
100 years
*Channels and creeks shall have one foot of freeboard
**Large bridges are those with a total span greater than 50 feet
(Ordinance 17.0 adopted 3/21/00)
(a) 
The following table sets out the permissible water spread for various types of streets:
Water Spread Limit
Street
Classification
Permissible
Water Spread
Major thoroughfare (divided)
100-year storm - traffic lane must remain open each direction
Major thoroughfare (undivided)
100-year storm - 2 traffic lanes must remain open
Collector street
100-year storm - traffic lane must remain open each direction
Residential street
100-year storm - water flow must not exceed 4" above curb
Pipe system and inlets
100-year storm - hydraulic grade shall be 2' top of curb below
(b) 
The permissible water spreads are based upon and 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.
(Ordinance 17.0 adopted 3/21/00)
Allowable depths of flow across street intersections for 100-year storms are established in the following table:
Street Cross Flow
Street Intersection
Cross-Flow Depth
Expressway
None
Major thoroughfare (divided and undivided)
None
Collector street (one valley crossing)
6 inches of depth or max spread at valley sections
Residential street
6 inches at valley sections
(Ordinance 17.0 adopted 3/21/00)
(a) 
The complete drainage system is composed of: (1) the initial system, consisting of inlets, stormdrains, and the associated appurtenances to convey the initial storm runoff (one-hundred-year); and (2) the major system of the major runoff (one-hundred-year), which consists of swales, creeks, channels, floodways and emergency overflows to prevent water encroachment into residential and commercial facilities.
(b) 
Initial storm systems are required when water spread and street cross flow limits are exceeded (closed pipe generally up to eighteen-inch capacity).
(c) 
Channels.
(1) 
Channels are to be concrete-lined at least to the ten-year frequency channel flow level with additional height to carry the 100-year flow.
(2) 
One-hundred-year flow is to be contained within the right-of-way.
(3) 
Unlined channels will be considered for quantities of floodwater larger than the equivalent flow of a seventy-two inch pipe.
(Ordinance 17.0 adopted 3/21/00)
Utilization of retention ponds, detention ponds and dispersion areas and preservation of major floodplains, etc., shall be strongly encouraged and 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.
(Ordinance 17.0 adopted 3/21/00)
The town's major drainage floodplains that are still functioning in a natural or semi-natural state will require special drainage and other preservation considerations. To implement this policy of the natural one-hundred-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.
(Ordinance 17.0 adopted 3/21/00)
(a) 
Minimum velocity with the pipe flowing full shall be three feet per second.
(b) 
The minimum storm drainpipe diameter shall be eighteen inches.
(c) 
Pipe diameters shall not normally decrease downstream.
(d) 
Pipe crowns at change in sizes should be set at the same elevation.
(e) 
Vertical curves in the conduit will not be permitted, and horizontal curves will be permitted only with the approval of the town engineer.
(f) 
Manholes:
(1) 
Maximum manhole spacing is shown in the following table:
Manhole Spacing
Pipe Size
Maximum Spacing
18-36"
600'
42-60"
1,000'
Larger than 60"
No limit
(2) 
Manholes shall also be placed at pickup points having two or more laterals, at pipe junctions having pipe sizes twenty-four inches or greater, at alignment changes, and at the beginning of the stormdrain system.
(g) 
Inverted crown sections will be permitted only in alleys.
(h) 
Street crowns shall be reduced for approximately one hundred feet on each side of valleys, and only one valley crossing for each street shall be used at an intersection.
(i) 
At streets with culverts or bridges; an emergency overflow shall be provided to contain the one hundred year channel flow within the building lines.
(Ordinance 17.0 adopted 3/21/00)
(a) 
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.
(b) 
The following table gives general guidelines of values for runoff coefficients which may be used in the determination of stormwater runoff.
Runoff Coefficient "C" For All Storm Calculations
Type Area or Land Use
Runoff Coefficient "C"
Park and permanent open space
0.40
Single family residential
0.50
Multifamily
0.65
Industrial and manufacturing
0.70
Business
0.80
Central business district
0.90
(c) 
When the town engineer cannot verify ultimate development, a standard runoff coefficient of .6 can be used.
(Ordinance 17.0 adopted 3/21/00)
(a) 
The following table sets out minimum inlet time of concentration for various types of areas:
Minimum Inlet Time of Concentration
Type Area
Minimum Inlet Time
Parks and permanent open areas
20 minutes
Residential (less than 5 un/ac density)
15 minutes
All other residential
10 minutes
Industrial and business
10 minutes
Roof and paved areas, streets
10 minutes
(b) 
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 town engineer.
(c) 
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.
(Ordinance 17.0 adopted 3/21/00)
(a) 
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.
(b) 
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.
(c) 
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. If necessary easements or letters of permission cannot be obtained, the town engineer will review the downstream damage potential and make recommendations to the planning and zoning commission and town council.
(d) 
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.
(e) 
(1) 
Where it is anticipated that additional runoff incidental to the development of the subdivision may overload an existing downstream drainage facility, whether natural or manmade, or may adversely affect or impact downstream property, the planning and zoning commission may withhold approval of the subdivision until improvements, including storm sewer systems, channel grading, driveway adjustments, culvert improvements, etc., are made.
(2) 
In areas where downstream pipes or channels are adequate to handle proposed increased flows, the town as one alternative may consider accepting cash payment in lieu of actual drainage improvements. The developer must show that the proposed pipe system to handle the flow from 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.
(Ordinance 17.0 adopted 3/21/00)
(a) 
Where topography or other conditions are such as to make impractical the inclusion of drainage facilities within road rights-of-way, perpetual unobstructed easements for such drainage facilities shall be provided across property outside the road lines and with satisfactory access to the road. Easements shall be indicated on the plat. Drainage easements shall be carried from the road to a natural watercourse or to other drainage facilities.
(b) 
When a proposed drainage system will carry water across private land outside the subdivision, appropriate drainage rights must be secured and indicated on the plat or other instrument as approved by the town attorney. In the case of clear public interest, the town may participate in easement acquisition by power of condemnation.
(c) 
The applicant shall dedicate an appropriate drainage easement either in fee or by drainage easement or by conservation easement of land on both sides of existing watercourses to a distance to be determined by the planning and zoning commission.
(Ordinance 17.0 adopted 3/21/00)
Any person, firm or corporation violating any of the provisions or terms of this article shall be subject to the same penalty as provided for in the Comprehensive Zoning Ordinance of the Town of Double Oak, as heretofore amended, and upon conviction shall be punished by a fine in accordance with the general penalty provision set forth in Section 1.109 of this code for each offense; and each and every day such violation shall continue shall be deemed to constitute a separate offense.
(Ordinance 17.0 adopted 3/21/00)