(A) Roadway (pavement) drainage design.
(1) General. Good pavement drainage design consists of the proper selection of grades, cross slopes, curb types, inlet locations, and the like, to remove the design storm, rainfall from the pavement in a cost-effective manner while preserving the safety, traffic capacity, and integrity of the highway and street system. These factors are generally considered to be satisfied provided that excessive spreads of the water are removed from the vehicular traveled way and that siltation at pavement low points is not allowed to occur. The guidelines included herein will accomplish these objectives, but all design should meet the Florida Department of Transportation Standards as a minimum.
(2) Minimum groundwater and high water clearances. All streets should be designed to provide a minimum clearance of one foot between the bottom of the base and the estimated seasonal high water table as established by a geotechnical engineer, or the artificial water table induced by an underdrain system. Swale drainage will be permitted only when the wet season water table, as established by a geotechnical engineer, is a minimum of two feet below the invert of the swale.
(3) Curbs and gutters, types. All roadway drainage not considered suitable for swale or ditch-type drainage shall be designed as one of the following:
(a) Miami curb and gutter section.
(b) Standard curb and gutter section.
(4) Design storm frequency. The design storm frequency to be utilized for the design of pavement drainage shall be as follows:
(a) Arterial and collector streets: 10-year, hydraulic gradient line one foot below gutter line.
(b) Local streets: 10-year, hydraulic gradient line one-half foot below gutter line.
(5) Runoff determination. The peak rates of runoff for which the pavement drainage system must be designed shall be determined by the Rational Method. The time of concentration, individual drainage areas, and rainfall intensity amounts shall be submitted as part of the drainage plans.
(6) Stormwater spread into traveled lanes. Inlets shall be spaced at all low points, intersections, and along continuous grades so as to prevent the spread of water from exceeding tolerable limits. The acceptable tolerable limits for arterial and collector roadways is defined as approximately one-half the traveled lane width. Acceptable tolerable limits for interior subdivision roadways are defined as a maximum of one inch above the crown of the road.
(7) Maximum inlet interception rates. Types I and III (single) inlets shall be located such that a maximum of five cfs shall be intercepted (Types II and IV, nine cfs maximum). Bypass flow is limited to a maximum of one cfs. Off-site flows from impervious areas more than one-half acre shall be intercepted prior to right-of-way line.
(8) Inlet types. The curb inlet types to be used shall be the latest version of the State Department of Transportation inlet Types I, II, III, IV, and VIII. Ditch bottom inlets shall be Department of Transportation inlet Types C, D, E, and H.
(9) Low point inlets. All inlets at low points (sumps) shall be designed to intercept 100% of the design flow without exceeding the allowable spread of water onto the traveled lanes as defined above. On arterial roadways, in order to prevent siltation and to provide for a safety factor against clogging of single inlets in a sump location, it is required to construct multiple inlets at all sump locations. Preferably, three inlets should be constructed on each side of the roadway, one at the low point and one on each side at a point 0.2 feet higher than the low point.
(B) Storm sewer design.
(1) Design discharges. The determination of design flows for a storm sewer system shall be in accordance with the methods and procedures set forth in the aforementioned Hydraulic Design Criteria, division (A)(5).
(2) Minimum pipe size. The minimum size of pipe to be used in storm sewer systems is 15 inches. Designs shall be based upon 6-inch increments in sizes above 18 inches.
(3) Pipe grade. All storm-sewers shall be designed and constructed to produce a minimum velocity of 2.5 fps when flowing full. No storm sewer system or portion thereof will be designed to produce velocities in excess of ten fps.
(4) Maximum lengths of pipe. The following maximum runs of pipe shall be used when spacing access structures of any type.
Pipe Size | Maximum (Feet) |
15 to 18 inches
|
300
|
24 to 36 inches
|
400
|
42 inches and larger
|
500
|
(5) Design tailwater. All storm sewer systems shall be designed taking into consideration the tailwater of the receiving facility or body of water.
(6) Allowable materials. Allowable material for storm sewers shall be reinforced concrete pipe, Class III, as a minimum. Structures shall be designed according to State Department of Transportation design standards.
(C) Culvert design.
(1) Minimum pipe size.
(a) The minimum size of pipes to be used for culvert installations under roadways shall be 18 inches.
(b) The minimum size of pipes to be used for driveway crossings shall be 15 inches.
(2) Maximum pipe grade. The maximum slope allowable shall be a slope that produces a 10 fps velocity within the culvert barrel. Erosion protection and energy dissipators may be required to properly control entrance and outlet velocities.
(3) Maximum lengths of structure. The maximum length of culvert conveyance structure without access shall be as allowed in the Hydraulic Design Criteria, division (B)(4).
(4) Design of tailwater. All culvert installations shall be designed taking into consideration the tail water of the receiving facility or body of water.
(5) Allowable headwater. The allowable headwater of a culvert installation should be set by the designer for an economical installation. When end walls are used, the headwater should not exceed the top of the end wall at the entrance. If the top of the end wall is inundated, special protection of the roadway embankment and ditch slope may be necessary for erosion protection.
(6) Design procedure. The determination of the required size of a culvert installation can be accomplished by exact mathematical analysis or by the use of design nomographs. The mathematical solution will give precise results, but is time consuming and somewhat non-productive when considering the inaccuracies of estimating design flows and floodwater elevations.
(Ord. 241, passed 11-27-85; Ord. 632, passed 2-4-93) Penalty, see § 154.04