12-3-2: DESIGN CRITERIA:
   A.   Streets: Design criteria for gutter capacity and associated lane flooding will depend on roadway type as shown in table below. Street designs must include surface drainage relief points (especially for flat gradient areas, local sumps or depressions and downhill cul-de-sacs) and provide for routing of 100-year recurrence hydrographs to adequate downstream conveyance facilities. For pedestrian safety, 100-year flood flow in streets must be limited by underground storm drain conveyance and street design so that the product of depth (feet) and velocity (feet/second) in gutter does not exceed six (6). 100-year flood flows in streets must be contained within the street right of way and adjacent drainage easements. Where underground conveyance is required to limit street flows, it will be designed for the 10-year flood, or greater.
   STREET GUTTER CAPACITY FOR 100-YEAR EVENT
 
Locals
Flow may spread to crown
Minor collector and residential collector
Flow may spread to crown
Collector and major collector
Flow spread must leave 1 travel lane free of water
Arterial
Flow spread must leave 1 travel lane and center turn lane in each direction free of water
 
   B.   Storm Drains:
      1.   Storm drain design conveyance capacity will be sized for a minimum of the 10-year, 3-hour flood. The 100-year, 3-hour design flood will be used for evaluation and prevention of significant damage due to street overflow. Inlets must have sufficient capacity to prevent local ponding in the 10-year event, with fifty percent (50%) blocking of inlets by debris. Analysis of combined street and storm drain capacity for the 100-year flood must determine maximum ponding depths and water levels and show that these are nondamaging.
      2.   Design consideration will be given for differences in interception capacity of inlets on grade compared to interception capacity of inlets in sag locations. Inlet spacing and locations will be for continuous grade or sag situations, as appropriate.
      3.   All storm drains will be designed by application of the Manning equation. Minimum velocity shall be two (2) feet/second flowing one-half (1/2) full.
      4.   Storm drains will not be allowed to be designed for surcharged pipe conditions unless otherwise approved by LaVerkin City. When storm drains are designed for full pipe flow, or surcharged pipe conditions, the designer shall establish the hydraulic grade line considering head losses caused by flow resistance in the pipe, and changes of momentum and interference at junctions, bends, and structures. The water surface elevation profile and hydraulic grade line will be shown for the 10-year and 100-year design flood as required in the drainage control report and plan.
   C.   Culverts: A culvert entrance blockage factor of up to fifty percent (50%) shall be used for small diameter culverts and culverts placed in drainages with upstream debris as determined by LaVerkin City. The 100-year water surface backwater upstream will be determined (using HEC-2) and shown to be nondamaging. Potential paths of embankment overtopping flows will be determined and redirected, if necessary, so that no significant flood damage occurs.
   D.   Bridges: Bridges must pass the 100-year event with a minimum of one foot (1') of freeboard. No significant increases are allowed in upstream water levels. A HEC-2 analysis of potential upstream water surface increases may be required by LaVerkin City. If the bridge deck is not free span, local and regional scour analyses are required for pier footings and embankments. Appropriate references are "Stream Stability At Highway Structures", "Hydraulic Engineering Circular No. 20", federal highway administrations, and computer programs such as USCOE HEC-6 or FHWA FESWMS.
   E.   Open Channels: The 100-year recurrence flood will be used for design. Side slopes and maximum velocities will depend on criteria shown in the chart below. Drop structures may be required to limit velocities to noneroding values. Supercritical velocities are not permitted for any material. Any use of irrigation ditches, or canals, for flood control must receive written approval from both the irrigation company and LaVerkin City.
   OPEN CHANNEL DESIGN CRITERIA
Material Lining
Maximum Permissible
Mean Velocity
 
 
Natural and improved unlined channels
Material Lining
Maximum Permissible
Mean Velocity
 
 
Natural and improved unlined channels
 
 
 
Alluvial silts, colloidal
3.75
 
Alluvial silts, noncolloidal
2.00
 
Clay
6.00
 
Cobbles and shingles
5.00
 
Course gravel, noncolloidal
4.00
 
Fine gravel
2.50
 
Fine sand, colloidal
1.50
 
Graded loam to cobbles when noncolloidal
3.75
 
Ordinary firm loam
2.50
 
Poor sedimentary rock
10.00
 
Sandy loam, noncolloidal
1.75
 
Sandy silt
2.00
 
Shales and hardpans
6.00
 
Silt loam, noncolloidal
2.00
 
Silty clay
2.50
 
Stiff clay, very colloidal
3.75
 
Volcanic ash
2.50
 
 
Fully lined channels
 
 
 
Concrete
35.00
 
Gabions
15.00
 
Grouted riprap
15.00
 
Loose riprap
10.00
 
Soil-cement
15.00
 
Unreinforced vegetation
5.00
 
   Notes:
      1.   For composite lined channels, use the lowest of the maximum mean velocities for the materials used in the composite lining.
      2.   Deviations from the above values are only allowed with appropriate engineering analysis and/or suitable agreements for maintenance responsibilities.
   Reference:
    Natural: Fortier and Scobey, 1926
    Fully lined: Various sources
Maximum velocities for grass lined channels depend on type of grass mixtures, with higher velocities appropriate for Bermuda grass. Consult "Open-Channel Hydraulics", Ven Te Chow, for details. The minimum bottom width of the channels will be six feet (6') to facilitate access of maintenance equipment.
   F.   Storage Facilities: Storage facilities will generally be used to prevent local increases in 10-year, 72-hour; and 100-year, 72-hour; peak flows due to development or the 100-year, 3-hour storm, whichever case requires more volume. Detention ponds must empty in twenty four (24) hours, or less. Partially wet basins may be allowed for recreational or aesthetic purposes, but storage below permanent spillways or low level outlets cannot be counted into control calculations. Retention ponds intended for infiltration may require extensive subsoil and ground water studies and maintenance requirements and are not allowed unless approved by LaVerkin City.
Energy dissipation and erosion protection is required at all outlet structures where storm drainage is released into a natural drainage course, unless LaVerkin City agrees there is no need shown for such protection. (Ord. 2005-21, 5-4-2005)