(A) General.
(1) Plans, profiles, and specifications shall be prepared for storm sewer improvements to be constructed and shall show the locations, sizes, grades, hydraulic gradients, flow arrows, and other details for the proposed pipe, inlets, manholes, culverts, outlet structures, and other appurtenances. Each sheet of the plans and profiles shall bear the seal and signature of the professional engineer who prepared them. Along with plan/profile sheets the engineer of record shall include in the plan set the design calculations for drainage channels, culvert pipes, storm sewer pipes and inlets. All starting hydraulic grade lines shall be tied to a known FEMA 100-year water surface.
(2) Storm sewers, bridges, culverts, inlets, and drainage channels or ditches of a permanent design, adequate to carry off 100-year design storm, shall be installed by the subdivider throughout the entire length of the drainage area where the surface drainage traverses the subdivision. Drainage ditches shall be no deeper than 4 feet, including 1 foot of freeboard with 4:1 sideslopes. All excess storm drains shall be carried by underground storm drainage system to the outfall.
(3) The subdivider shall pay for the cost of all drainage improvements connected with the development of his or her subdivision, including any necessary off-site channels or storm sewers required or attributed including any facilities needed to accommodate increased run-off resulting from the developer’s development of the subdivision, and acquisition of any required easements. The developer shall provide on-site water detention capacity to limit the rate of run-off from the subdivision or make the necessary off-site improvements to handle the off-site run-off rate increase above undeveloped conditions, if the City Engineer determines those facilities are needed.
(a) All flow lines shall be permanently established by natural rock or some other system approved by the city.
(b) In areas containing FEMA streams, the developer shall conform to FEMA requirements and submit related data to the city for review and approval submission to FEMA.
(c) The plans shall indicate the sizes of all driveway culverts and the location and size of all street culverts. All culverts shall be provided with city approved safety (sloped) head walls, and side slopes of drainage ditches shall not be steeper than 4:1, unless approved by the City Engineer.
(d) For control of erosion along drainage ditches and channels in subdivisions, the developer shall provide a complete climate adaptable grass cover or other city accepted erosion material along the ditches and channels prior to acceptance of the work by the city. Grass shall be placed by using solid sod to obtain the required coverage. Alternatively, if adequate precautions are taken for erosion control, hydromulch may be used. During the first growing season, the grassed area shall be fertilized. Water sufficient to obtain complete coverage of the area shall be provided daily for at least 60 days prior to final acceptance by the city. Areas which do not exhibit healthy growth within 90 days shall be
replanted until a healthy growth is obtained. On all open ditch drainage facilities, the developer shall be responsible for keeping silt removed, eroded areas restored, and ground cover in place for 2 years after acceptance of the development or until 90% of the lots have been developed, whichever occurs first. Each lot shall include erosion control devices to minimize silting of open channels. The developer’s assurance of this maintenance shall be in a form acceptable to the city.
(4) Discharge of storm drainage shall be at a point of adequate capacity. If adequate capacity does not exist off-site, the developer shall be required to provide off-site drainage improvements or improve downstream facilities so they are adequate to carry the 100-year storm flow under fully developed conditions.
(B) Design of facilities.
(1) Standards. The design, materials, and construction of storm drainage structures shall be in accordance with the City of Plano Storm Drainage Design Manual as may be amended by this chapter and the City Engineer. Plans and calculations shall be submitted with the plat. Detention systems shall be designed for a series of storm frequencies 2, 25, 50, and 100 year storm events. Each pond shall have a designed outlet structure to release storm water for the various storm frequencies.
(2) Accommodation of upstream drainage areas. A culvert or other drainage facility shall in each case be large enough to accommodate potential run-off from its entire upstream drainage area, whether inside or outside the subdivision or addition. The developer’s engineer shall initially determine the necessary size of the facility based on the provisions of the construction standards and specifications assuming conditions of maximum potential watershed development, subject to approval by the City Engineer.
(3) Effect on downstream drainage areas. The developer’s engineer, shall study the effect of each addition’s storm run-off on the existing land and drainage facilities immediately downstream of the addition. Where it is determined that existing capacity is not available immediately downstream as assessed by the City Engineer, the developer’s engineer shall design a drainage system, detention facility, or parallel system to mitigate the deficiency. The Commission may withhold (if a waiver is on file) or deny approval of the plat until this mitigation has been provided.
(4) Location. Except where otherwise required, drainage shall be provided in a system of earthen channels where depth does not exceed 4 feet, including 1 foot of freeboard or a combination of open channels and underground storm drainage system constructed in street rights-of- way, or in easements.
(5) Construction of underground facilities. In some cases, a developer may choose or be required to install drainage facilities underground. All flows which cannot be conveyed by 4 feet of deep earthen channels will be in underground system or in combination of open channel and underground pipe system. These installations are subject to approval by the City Engineer.
(6) Detention facilities. Lakes, detention ponds, and retention ponds may be constructed in all areas, provided they are approved by the City Engineer. Easements shall be provided to ensure protection of these areas for maintenance purposes. Detention facilities shall include a maintenance plan as part of the design.
(7) Alternate facilities. Other innovative drainage concepts will be considered if approved by the City Engineer.
(C) Creeks and floodplains.
(1) Floodplain restrictions. The City Council shall, when it deems it necessary for the health, safety, or welfare of the present and future population of the area and necessary to the conservation of water, drainage, and sanitary facilities, prohibit development of any portion of the property which lies within the floodplain of any stream or drainage course. These floodplain areas shall be preserved from any and all destruction or damage resulting from clearing, grading, or dumping of earth, waste, or material, or stumps, except at the discretion of the City Council. Floodplains are also subject to the restrictions of the tree preservation requirements in this code, if any.
(2) Creek restrictions. Major creeks shall remain in open natural condition; smaller creeks or drainage ways may be channelized, provided they meet the criteria of the storm drainage requirements of the city, the Corps of Engineers and as accepted by the City Engineer. All creeks and excavated channels shall remain part of the platted lot with a city dedicated drainage easement. The easement may be dedicated to the city or a homeowners association. When a creek or excavated channel is to remain open, or in its natural condition, it shall meet 1 of the following requirements, subject to approval of Council:
(a) Creeks and drainage ways may be retained as a part of a residential lot, and it shall be the property owner’s responsibility to maintain this area, except as otherwise provided. A maintenance drainage easement shall be granted to the city at city request and shall grant the right but not the obligation to maintain and construct drainage facilities if the creek or drainage way is not being properly maintained. A lien may be filed against the property in favor of the city to secure payment of any expenses incurred by the city for maintenance;
(b) Creeks or drainage ways may be owned and maintained by an approved maintenance entity. A maintenance entity may include homeowners association or similar entity. The maintenance entity’s bylaws and covenants filed of record, if any, shall provide for ongoing maintenance. The easement shall authorize a lien against individual abutting lots in favor of the city to secure the payment to the city for any expenses incurred by the city in the event of default; or
(c) All properties may create an entity to maintain creeks or drainage ways, provided the maintenance area is set forth by easement and the entity’s bylaws, filed of record, provide for ongoing maintenance. These easements shall authorize a lien against individual abutting properties in favor of the city to secure payment for any expenses incurred if the maintenance entity is not properly maintaining the creek or drainage way. Adequate floodway easements and drainage easements shall be required that give the city the right but not the obligation to maintain and construct drainage facilities if, in the city’s sole opinion, the maintenance entity is not properly maintaining the creek or drainage way.
(3) Right of approval. In all cases, the city shall have the discretion to approve, or disapprove, the proposed street alignment fronting on city trails, or drainage ways and creeks.
(D) Dedication of drainage easements.
(1) General requirements. On request of the city, when a subdivision or addition is traversed by a watercourse, drainage way, channel, or stream, there shall be provided a storm water or drainage easement conforming substantially to the line of that watercourse, and of a width and construction as will be adequate for the purpose. Wherever possible, it is desirable that the drainage be maintained by an open channel with landscaped banks and adequate width for maximum potential volume of flow and access for maintenance.
(2) Maintenance and access easements. The property owner shall provide sufficient access on each side of and parallel to creeks or drainage ways for maintenance purposes. The access shall be above the base flood elevation and accessible to vehicles and equipment. Access must also be provided at a maximum 1,200-foot spacing along streets. The location and size of the maintenance and access easement shall be determined by the City Engineer. The minimum width shall be 10 feet, and the maximum width of the access easement shall be 15 feet. Permanent monuments, the type and locations of which to be approved by the City Engineer, shall be placed along the boundaries of the maintenance and access easement and private property. This maintenance and access easement shall be included in the dedication requirements of this section, and included in the drainage and floodway easement width.
(3) Drainage easements.
(a) Where topography or other conditions are such as to make impractical the inclusion of drainage facilities within street rights-of-way, perpetual, unobstructed easements at least 25 feet in width, depending on slopes, for drainage facilities shall be provided across property outside the street lines and with satisfactory access to the street. Easements shall be shown on the plat. Drainage easements shall extend from the street to a natural watercourse or to other drainage facilities.
(b) When a proposed drainage system will carry water across private land outside the subdivision or addition, appropriate drainage easements must be secured.
(Ord. 487, passed 7-18-2000; Am. Ord. 737, passed 7-20-2016) Penalty, see § 155.999