Chapter 12 — ROAD STANDARDS, STORM DRAINAGE

Watershed areas of six hundred forty (640) acres and less can be analyzed by the Rational Method. Watershed areas larger than six hundred forty (640) acres shall be analyzed utilizing the Soil Conservation Service (SCS) Unit Hydrograph Method or a method agreed upon by the consulting engineer and the engineer prior to submitting the analysis to the engineer for review.

The submittal of the improvement plans shall be accompanied by the drainage analysis consisting of a watershed area map and drainage design calculations (two (2) copies each).

A.

Design Storms. Drainage design calculations shall show the calculations used to determine the hydraulic load for both the ten (10) year and one hundred (100) year design storms at each drainage facility. The specific design requirements are detailed in Sections 16.12.020 to 16.12.060.

B.

Rational Method of Estimating Peak Runoff.

Time of Concentration. The time of concentration is the time required for the runoff from the most remote point in the watershed to reach the point of concentration at which the flow is to be calculated. It is generally composed of two (2) parts, the overland flow time (sheet flow) and the concentrated flow time.

a.

Overland Flow Time. In undeveloped or lightly developed watersheds, the initial runoff is usually in the form of sheet flow. This overland flow time shall be determined using Standard Drawing D-8.

b.

Concentrated Flow Time. Concentrated flow time is the time required for the water to flow from one (1) point of concentration to the next. This flow may take place in a man-made or natural conveyance. The velocity chosen for use in this calculation should accurately reflect the hydraulic conditions in the drainage system.

Rainfall Intensity. Rainfall intensities for the ten (10)-year and one hundred (100)-year storms shall be determined using Standard Drawings D-11 and D-12.

Runoff Coefficient. The runoff coefficient shall be chosen to reflect the ultimate development of the drainage area. This shall be based on the County's General Plan, the County's Zoning Maps and the Soil Conservation Service Soil Survey of Nevada County. The runoff coefficient shall be determined using Standard Drawing D-15.

C.

Unit Hydrograph Method of Estimating Peak Runoff and Volumes. The Army Corps of Engineers' computer program, HEC-1, shall be used to calculate flow rates for all watersheds over six hundred forty (640) acres. The SCS method shall be the method used with HEC-1.

Copies of the HEC-1 program as well as technical support are available through private vendors. For a list of these vendors, contact the U.S. Army Corps of Engineers, Water Resources Support Center, The Hydrologic Engineering Center in Davis, California.

Lag Time. Lag time can be considered as a weighted time of concentration. It is discussed in detail in the Soil Conservation Service Publication SCS-TP-149.

The equation for calculating lag time is as follows:

L = 0.6tc

Where L = lag in hours

tc = time of concentration

Rainfall Duration-Depth. The rainfall duration-depth relationships to be used in unit hydrograph calculations are shown on Standard Drawings D-13 and D-14.

Loss Rate. Precipitation losses due to land surface interception, depression storage and infiltration shall be determined using SCS Curve Numbers. Hydrologic soil group information may be found in the "Soil Survey of Nevada County Area, California" published by the Soil Conservation Service. Curve numbers shall represent the ultimate development of the drainage area.

D.

Drainage Area Maps. A drainage area map shall be prepared to accompany all drainage calculations and shall reflect the following criteria:

All maps shall be of adequate scale to show a clear representation of the drainage conditions. Contours shall be shown and labeled throughout. Prominent features (roads, lakes, streams, etc.) shall be shown on the map.

All individual watersheds and subwatersheds shall be clearly defined with colored pencil shading and heavy lines delineating the boundaries.

Travel paths of all flow shall be clearly defined. Where concentrated flow exists, the type and dimensions of the conveyance shall be noted on the map. Where sheet flow exists, note this on the map.

Points of concentration for each structure shall be shown throughout. At each point of concentration, note the peak flow rates, generally the ten (10) and one hundred (100)-year flows (Q10 and Q100).

The areas (in acres) of all individual watersheds and subwatersheds shall be noted on the map.

Rational Method "C" values or SCS Method Curve Numbers shall be shown for each watershed and subwatershed.

Unusual features. N.I.D. irrigation ditch spillways for discharging excess storm runoff, upstream drainage facilities discharging onto the project, etc. shall be shown including expected values for Q10 and Q100.

Nevada County uses Manning's Equation to calculate friction losses and the pressure-momentum equations and energy equations to calculate junction or change in cross-section energy losses.

A.

Minimum Freeboard Requirements.

B.

Friction Losses. The Mannings Equation shall be used to calculate friction losses for hydraulic profiles. The following Mannings "n" values shall be used:

Patented Products See Manufacturer's Specs

C.

Junction Losses. At points in the hydraulic profile where there is a change in the flow rate or conveyance geometry, the energy loss being experienced due to these changes and the corresponding change in water surface elevation shall be calculated using either the pressure-momentum method or the energy equation method. The value of the loss coefficient, k, used in the energy equation shall be at the discretion of the consulting engineer and subject to the approval of the engineer.

D.

Minimum And Maximum Velocities.

Velocities exceeding fourteen (14) fps are special circumstances and criteria shall be established on a case- by-case basis to provide for protection from scour and unstable flow conditions.

E.

Detention Facilities. Detention facilities are intended to reduce the peak discharge of a watershed by storing the excess flow in a reservoir and slowly releasing it back into the system after the peak of the storm has passed. Reservoir size and outlet configuration shall be designed using an inflow hydrograph computed using HEC-1. The required storage volume and outflow requirements shall be determined using a flood routing method established in the engineering profession as being acceptable. An example of an acceptable method is the Modified Plus Method.

All detention reservoirs shall be designed such that the water surface returns to its base elevation within twenty-four (24) hours of the end of the storm and shall be placed as required by the engineer.

The following are minimum standards. They shall in no way relieve the consulting engineer from designing culverts to meet actual design conditions. All storm drainpipe intended for use within the roadway shall be checked for maximum allowable cover utilizing manufacturer's design values. The gauge or class of pipe shall be specified on the plans.

A.

Road Cross Culverts. Culverts crossing the roadway shall be adequate to carry the design flows. The culvert shall be reinforced concrete pipe, corrugated steel pipe, corrugated aluminum pipe or high-density polyethylene plastic pipe. The minimum culvert diameter shall be eighteen (18") inches when the culvert length does not exceed eighty (80') feet and twenty-four (24") inches when culvert length exceeds eighty (80') feet. The maximum spacing between cross culverts shall be five hundred (500') feet, and the maximum differential in elevation between successive cross culverts shall be twenty (20') feet unless otherwise approved by the engineer.

All corrugated metal pipe (both steel and aluminum) shall be a minimum of twelve (12) gauge. All reinforced concrete pipe shall be a minimum of Class III. The minimum cover depth for all culverts shall be one (1) foot.

B.

Driveway Culverts. Culverts crossing driveway entrances shall be adequate to carry the design flow and shall be of the materials specified in Section 16.12.030.A, above. Minimum pipe size shall be twelve (12") inches.

C.

Cross Culvert Inlets. In cut areas with other than concrete curb and gutter, cross culvert inlets shall be installed as shown on Standard Drawing D-3, as required. Cross culverts in fill areas shall be provided with a flared end section (FES) as shown on Standard Drawing D-2, as required.

All open drainage facilities shall be adequately protected from erosion by the use of an appropriate lining, and shall, as a minimum, meet the design criteria set forth in "Erosion and Sediment Control Guidelines for Development Areas of the Sierras," prepared by the High Sierra RC&D. Temporary and permanent drainage structures not specifically included in these specifications are "Special Circumstances."

A.

Existing Ditches. Unless justified within the calculations, all abrupt changes in alignment or profile and all underbrush and debris which seriously restricts the flow in existing ditches shall be regraded and improved. Such work shall be shown on the improvement plans. Unless previously approved by the engineer, centerline curve radius of an open ditch shall be a minimum of thirty-five (35') feet.

B.

Diversion. The diversion of natural drainage, subject to engineer's approval, will be allowed only within the limits of the proposed project. All natural drainage must leave the improved area at its original horizontal and vertical alignment unless a special agreement, approved by the engineer, has been executed with the adjoining property owners.

C.

New Construction. When selecting a ditch section or a lining material, consideration shall be given to the esthetics of the area. A trapezoidal section shall take precedence over a deep "V" ditch whenever possible.

A lining material shall be used which will adequately protect the channel from erosion.

Seeded Earth Ditches. This type of ditch may be used, subject to approval by the engineer, only where the velocity does not exceed four (4') feet per second (six (6) fps with Engineer's approval) and the native material, when scarified, will support plant growth. Information regarding approved seed mixtures is available at the Department.

Concrete Poured in Place or Air Blown. Minimum thickness to be four (4") inches with welded wire fabric WW F6x6 - W2/W2, fiber mesh reinforcement or other approved reinforcing method in sides and bottom.

Asphaltic Concrete (Type B). Minimum thickness shall be three (3") inches after compaction (ninety percent (90%) relative).

D.

Roadside Gutters. When roadside gutters are used to convey storm drainage, the flow from the ten (10)- year design storm (Q10) shall not inundate the traveled way, and the flow from the one hundred (100)-year design storm (Q100) shall not inundate more than one-third (⅓) of the traveled way.

The maximum length of a roadside gutter shall be five hundred (500) lineal feet between cross culverts unless otherwise approved by the engineer. The maximum length of an asphalt concrete dike shall be five hundred (500) lineal feet between down drains.

E.

Toe Ditches. Rock-lined toe ditches may be used, subject to the approval of the engineer, only at the base of fills. This type of ditch shall not be used as a roadside ditch. Rock lining shall conform to the provisions of Section 72 of the State Specifications. The minimum size shall satisfy the specifications for No. 2 Backing. The minimum thickness of the rock lining shall be twelve (12") inches.

(Ord. 2531. (10/24/2023))

A.

Capacity. Special provisions shall be made within the drainage system to ensure that the inlet flow line elevation and the capacity of the drainage system is such that it may be extended to serve the entire drainage basin at the time of ultimate development. This is to include the entire upstream portion and the portion of the basin outside the development, regardless of existing conditions.

B.

Pipe. Closed conduit storm drains shall be constructed of reinforced concrete pipe, corrugated metal pipe or high-density polyethylene plastic pipe.

C.

Structures. Unless otherwise approved by the engineer, all manholes, junction structures and catch basins shall conform to the following standards:

Use type 36RX grate or steel cover, redwood cover prohibited.

Use type 24-13 grate.

D.

Outfalls. All drainage outfalls shall be shown in both plan and profile on the improvement plans until a definite daylight condition is established.

When the outfall is from a closed conduit drain into a natural drainage course, an energy dissipater approved by the engineer must be provided.

E.

Cover Requirements. At locations where the general minimum cover requirements cannot feasibly be obtained, the conduit shall be either encased in concrete or provided with a concrete cover, or another method of pipe protection as specified by the consulting engineer and approved by the engineer.

(Ord. 2531. (10/24/2023))

The location and design of all proposed structures shall be considered individually and shall be designed in accordance with the State Standard Drawings and the Nevada County Standard Specifications. The design shall be as approved by the engineer.

(Ord. 2531. (10/24/2023))

The boundary lines of all drainage easements shall be shown on the plans.

A.

For Improved Drainage Facilities. Easements shall be provided for all ditches, culverts and conduit systems whether constructed as newly built improvements or as rebuilt improvements.

B.

For Existing Drainage Facilities. Easements shall be provided for all existing drainage facilities within the boundaries of and/or affected by any land areas to be improved.

C.

Extent. All drainage easements shall extend from the point at which a flow is concentrated to: 1) the point of confluence with a natural drainage course, or 2) the point where the flow is returned to sheet flow.

D.

For Offsite Drainage Facilities. All concentrated drainage leaving the boundaries of the area to be approved in other than natural drainage courses will require either specific easements or drainage release letters from the property owners of the lands from the point at which the drainage leaves the limits of the improvement area to the point at which it is deposited in a natural water course. The requisite easements must include adequate provision for all of the drainage structures to be used in the offsite drainage (i.e., culverts, ditches, dissipaters, etc.).

(Ord. 2531. (10/24/2023))