640.2 Median Drainage

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640.2.2 General

This guidance describes the design of drainage facilities located in grassed medians. Information pertaining to the drainage of paved medians is presented in Pavement Drainage.

640.2.2 Procedure

The general procedure for the design of median drainage facilities is as follows:

  • Establish location of median inlets.
  • Determine the degree of erosion protection necessary in each median channel segment.
  • Size median inlets.
  • Size median pipes.

The determination of the location of the median inlets requires good engineering judgment. Inlet locations are, in many cases, governed by non-hydraulic as well as hydraulic considerations.

Inlets are required at all low points in grade. Additional inlets are then located, as needed, between the high point and low point of a given drainage segment. The most economical locations are usually at points of minimum cut and fill. Therefore, these points should be considered first, as possible inlet locations.

640.2.3 Design Discharge

After the inlets have been located (at least on a trial basis) the peak rate of runoff arriving at each inlet is computed. This determination is made by application of the rational formula. Appropriate design frequencies are 10 years for all inlets located on grade and 50 years for inlets located at low points in grade.

If the median under consideration is a standard 60 ft. (18 m) grassed median, with no additional inflow, then Runoff on Median Grassed Slopes may be used to estimate the 10 year design flow rate. The values obtained from these figures apply to those areas in the state where the rainfall factor is equal to 1.0. For other areas the values obtained from Runoff on Median Grassed Slopes are multiplied by the rainfall factor which is obtained from the Rainfall Factor Map. These adjusted values are then used as the 10 year design flow rate. The 50 year design flow rate may be obtained by multiplying the 10 year flow rate by 1.50.

640.2.4 Inlet Spacing and Channel Design

Inlet spacing may influence the erosion control measures necessary in the median channel. Therefore, in order to achieve a least cost design, these items must be considered collectively rather than individually.

Optimum inlet spacing will be achieved when inlets are placed at those points where the grassed median channels are operating at capacity. Under these conditions no ditch liner will be required. However, in many cases there are only a certain number of discrete points at which median inlets may be built, in an economical manner. There are also points, such as the upstream limit of the grading for narrow median bridge piers or all median openings, at which an inlet is required. Under these conditions it may be necessary to specify ditch liner for some channel segments. The object of correct inlet spacing and channel design is to minimize the number of inlets and the amount of ditch liner, subject to the constraint that the completed median channel will be stable.

Erosion control criteria are set out in Erosion Control and Energy Dissipation. Cross sections and linings may be analyzed by the general methods of Open Channel Flow.

640.2.5 Median Inlet Capacities

Median inlets are of two types: spillways (Type E) and grates (Type S). Only Type S (see Standard Plan 731.10) inlets may be used within the limits of the safety zones. Therefore, Type E inlets (see Standard Plan 731.10) are never used in medians 60 ft. wide or narrower. Outside of safety zone limits both a Type E and Type S inlet may be considered. Median inlets are either located at low points in grade or they are constructed in conjunction with a ditch block. Therefore, they intercept all waters arriving at the inlet.

Design consists of selecting the smallest inlet which will pass the design flow rate at an acceptable depth. This acceptable depth shall be such that a freeboard of at least 6 in. is provided to the top of the ditch block or to the shoulder line of the roadway.

Guidance for specifying and inspecting drop inlets and grates is in Precast Reinforced Concrete Manholes and Drop Inlets.

640.2.5.1 Type S Inlets

Type S inlets are grate type drop inlets and are governed by either weir flow or orifice flow depending upon the depth of water above the grate. Details and available sizes may be found in the standard plans. Capacity of Type S Median Inlets relates the inlet capacity to the depth of water for standard inlet sizes. The "design capacity", as shown on this figure is one half of the theoretical capacity of the inlet. This factor of safety of 2.0 has been applied to the design curves because grate type inlets which must pass all waters arriving at the inlet are prone to clogging. Therefore, only one-half of the waterway area is assumed to function during the design event.

640.2.5.2 Type E Inlets

Type E inlets are spillway type drop inlets. Details and available sizes may be found in the standard plans. The behavior of this inlet is identical to the behavior of a curb opening inlet located at a low point in grade. Methods and design aids which may be used to estimate the capacity of Type E inlets are presented in Pavement Drainage.

640.2.6 Median Pipes

The conduits which drain the median inlets must be sized such that they will pass the design flow rate at a headwater elevation not greater than 150 mm (6 inches) below the bottom of the grate or the flow line of the spillway. That is, the required headwater must be completely contained within the drop inlet structure. Design of these median drains are based on the principles of culvert hydraulics.

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