(A) Manning Equation.
(1) The Manning Equation shall be used to determine the capacity of open channels and enclosed gravity conduits:
Q = AV = 1.486 R
2/3
S
1/2
A n
(a) Q is the flow rate in cubic feet per second.
(b) R is the hydraulic radius in feet.
(c) A is the cross sectional area of the flow in square feet.
(d) S is the slope in feet per foot
(e) V is the flow velocity in feet per second.
(f) n is the Manning coefficient
(2) Values of the Manning coefficient for various pipes and open channels are given in Table 23.31.042(1).
Conduit Material | Manning Coefficient |
CLOSED CONDUITS | |
Cast iron pipe | .013 |
Concrete pipe | .013 |
Corrugated metal pipe | |
Plain | .024 |
Paved invert | .020 |
Fully paved | .015 |
Plastic | .013 |
Vitrified clay | .013 |
OPEN CHANNELS | |
Lined channels | |
Asphalt | .015 |
Concrete | .015 |
Rubble or rip rap | .030 |
Vegetal | .040 |
Excavated or dredged | |
Earth, straight and uniform | .030 |
Earth, winding, fairly uniform | .040 |
Unmaintained | .100 |
NATURAL CHANNELS (MINOR STREAMS) | |
Fairly regular section | .050 |
Irregular section with pools | .100 |
Adapted from Table XIV, ASCE Manual No. 37, 1970 | |
(B) Pipe flow criteria.
(1) Catch basin laterals shall be not less than 12 inches in diameter. All mains or trunk lines shall be not less than 18 inches in diameter.
(2) The minimum velocity in closed conduits shall be one fps when flowing full. Maximum velocity shall be ten fps.
(3) The hydraulic grade line (HGL) of the design storm flow must be computed for all storm drain systems and must be shown on the design profile when it is above the top of the pipe. The HGL shall be a minimum of one-half feet below the elevation of the inlet grates and manhole covers of all structures within the system.
(C) Open channel flow.
(1) Maximum velocities in open channels shall be as follows on Table 23.31.042(2).
Channel Material | Max. Allowable Velocity (fps) |
Fine sand* | 2.0 |
Sandy loam* | 2.5 |
Alluvial silt* | 3.0 |
Firm loam* | 3.5 |
Fine gravel* | 4.0 |
Stiff clay* | 4.5 |
Coarse gravel | 5.0 |
Bottom paved channels (A.C. or P.C.C.) | 8.0 |
Fully lined channels | 10.0 |
*One fps may be added to the above when turf is maintained in channel. | |
(2) Freeboard requirements shall be set by the Engineer or the agency governing the proposed channel facilities.
(3) Channel flow shall be placed in closed conduits where the flow requires a concrete pipe of 48 inches diameter or less.
(D) Backwater effects. When obstructions, transitions, junctions, constrictions or other irregularities in an otherwise uniform channel system create backwater conditions in the system, the consulting engineer shall make computations to determining the effects of the backwater condition. The consulting engineer shall use careful consideration in determining when losses due to channel irregularities are small enough to neglect or when they are large enough to create considerable backwater effects.
(E) Detention basins.
(1) Detention basins other than those shown on the Master Storm Drainage Map may be used for revisions to the Master Plan or the staged development only upon approval of the Engineer.
(2) The volume of water to be stored shall be computed from the following basic formula with credit for outlet flow per division (E)(2)(d) below:
V = | CAR | Formula 3-5 |
12 |
(a) V = the volume in acre-feet.
(b) C = the runoff coefficient (see Tables 23.31.041(1) or 23.31.041(2)).
(c) A = the contributing area in acres.
(d) R = the total rainfall in inches for the 24-hour storm period, using a 100-year storm frequency.
(3) The volume of detention basins shall be determined with no allowance for percolation.
(4) All detention basins shall have outlet facilities providing terminal drainage capable of emptying a full basin within 24 hours. Allowance for outlet facilities to reduce the total volume of the basin can be made only if the consulting engineer can demonstrate by routing computation that the basin will operate properly.
(5) The bottom of the basin shall be above the highest recorded groundwater elevation and shall also meet the requirement of the state’s Water Quality Control Board. It is the responsibility of the developer’s consulting engineer to coordinate with the state’s Water Quality Control Board and submit evidence of their approval to the Engineer.
(6) The design water surface elevation of basin shall be a minimum of one foot below all berm, and of roads, and/or top of curb elevations upstream from the basin.
(F) Retention ponds.
(1) Developments will be required to provide terminal drainage where developments are within one mile of a terminal drainage facility.
(2) When terminal drainage is not available, retention ponds utilizing percolation as a means for emptying the basin may be used only upon approval of the Engineer.
(3) Retention ponds that utilize percolation as a means of emptying the retention ponds shall not be permitted unless bottom elevation of the retention pond is two feet or more above the highest recorded groundwater elevation in the previous ten years.
(4) Retention ponds that utilize percolation as a means of emptying the retention pond shall conform to the following design criteria:
(a) The retention pond design shall be the responsibility of the developer’s engineer and subject to review by the county.
(b) Permeability and differential head available must be considered for the life of the project, not just present values.
(c) Earth material and groundwater condition evaluations shall be made on a site-specific basis by a qualified Geotechnical Engineer and include project-specific subsurface data and tests, all subject to review by the county.
(d) The volume of storage required shall be 100% of the computed volume of storage for the 100-year storm.
(e) Retention ponds shall be designed to empty 100% of the required volume of storage within ten calendar days.
(f) Levees shall be discouraged around retention ponds.
(G) Culvert criteria.
(1) Culverts shall be designed per CALTRANS standards. Maximum headwater depth at inlets shall not exceed one and one-half times the pipe diameter without approval of the Engineer.
(2) The constants Ke = 0.25 and Ko = 0.75 shall be used for concrete pipes and box culverts. For corrugated pipes, Ke = 0.50 and Ko = 1.0 shall be used.
(3) Cross culvert conduits and box structures shall be designed to pass the peak flow from the ten-year storm without damage to the roadway and shall be checked on the basis of the ten-year runoff plus 50% to determine that no serious damage will be incurred upstream as a result of the higher design storm.
(4) Cross culvert profile will be determined by an examination of the overall profile of the channel for a minimum distance of 500 feet each side of the installation.
(5) Outlet velocities of all culverts must be checked. When outlet velocity exceeds the maximum permissible channel velocity listed on Table 23.31.042(2), energy dissipaters must be provided to minimize potential erosion at the outlet.
(H) Inlet criteria.
(1) Inlets shall be spaced so that the flow capacity of the inlet is not exceeded by the design storm flow rate or so that the length of gutter flow does not exceed 700 feet, whichever is less. The maximum flow capacity of a single standard storm: Drain Inlet No. 1 at a sag point is 2.0 cfs. On a continuous grade greater than 0.5%, the maximum capacity is approximately one cfs, and 0.50 cfs on grades less than 0.5%. Larger Basin No. 2 may be used in industrial or commercial areas or when approved by the Engineer. Industrial/commercial catch basins shall conform to county standards, and will have capacities approximately 50% greater than the Standard Drain inlet No. 1.
(2) Drainage in a sump area must be provided with a secondary outlet for flows greater than the ten-year design capacity of the drain inlet. This outlet must be provided as overland, street or
other above ground means of carrying flow, unless it can be shown that a secondary drain inlet to the underground system, along with that underground system can convey the 100-year storm without flooding beyond the limits shown in division (I) below.
(3) Eighteen inch storm drain inlets may be used at sag point only as approved by the Engineer. Capacity of this inlet is dependent on maximum allowable submergence and will be reviewed on a case by case basis.
(I) Surface flow in street and allowable inundation of 100-year storm.
(1) The primary use of roads is for traffic. The allowable depth of flow in streets shall be limited to the following:
Street Classification | 10-Year Storm Encroachment | 100-Year Storm Encroachment |
Local residential | No curb overtopping; flow spread must leave at least one lane in each direction free of water | Depth of flow above street crown must not exceed 12 inches; flow spread shall not exceed building setback lines |
Collectors | No curb overtopping; flow spread must leave at least one lane in each direction of free of water | Depth of flow above street crown must not exceed six inches; flow spread shall not exceed building setback lines |
Arterial, expressway, freeway | No curb overtopping; flow spread must leave at least one lane in each direction of free of water | Depth of flow above shall not exceed three inches; flow spread shall not exceed building setback lines |
(J) On-site drainage.
(1) Minimum pipe size shall be 12 inches or as approved by the Engineer.
(2) Maximum area to be drained to a single inlet is two acres.