A. General Requirements. The subdivider shall provide storm drainage facilities that will convey stormwater runoff, whether originating within the subdivision or in adjacent areas, to an existing drainage channel or drainage system. Adequate access for maintenance of the system shall be provided. The capacity of an existing drainage system must be large enough to accommodate the additional runoff generated by the subdivision. Drainage patterns existing prior to construction of the subdivision shall be maintained, and full consideration must be given to the rights of adjacent property owners with regard to surface water drainage.
The city will determine the capacity of an existing storm drain system.
The subdivider’s engineer shall prepare an analysis and design of the proposed storm drainage system. When stage construction is proposed, the analysis shall provide for the design of the entire storm drainage system.
The analysis shall consider all existing and future contributory drainage area, regardless of whether or not said area is in the subdivision.
The preliminary analysis shall accompany the tentative map.
B. Hydrology.
1. Storm Runoff. Runoff shall be computed by the rational method.
(Q = CIA) where:
Q = rate of runoff in cfs
C = coefficient of runoff
I = intensity of rainfall in inches/hr during the time of concentration tc (min.) – the elapsed time between the beginning of the storm and peak flow at the drainage structure
A = drainage area, acres
Computations should be clear and complete with all assumptions clearly stated. In making such computations, the following information shall be used:
a. Coefficient of Runoff. Typical values for runoff coefficients are set forth in Table 3.
b. Intensity of Rainfall. A rainfall intensity versus duration design chart for the Chico area is shown on Table 4. A minimum time of concentration of 10 minutes should be used whenever computations indicate a shorter time. For urban area drainage, the maximum initial time of concentration to the first drainage facility shall be 20 minutes. For unimproved areas, drainage time of concentration shall be determined by the method shown for small basins in the Highway Design Manual. The method of computation of time of concentration should be clearly indicated.
c. Design Storm Frequency. The design storm frequency shall be as follows:
(1) Bridges, 200 years;
(2) Open channels, 10 years;
(3) Culverts, 10 years;
(4) Major outfall lines, 10 years;
(5) Collector lines, 5 years;
(6) Local lines, 2 years.
A minimum freeboard of three feet shall be provided for bridges and box culverts, two feet for open channels, and one foot for storm drainage pipe inlets and outlets.
C. Roadway Drainage.
1. Grade. The minimum grade for side ditches and gutters will be 0.25% if paved, 0.50% if earth.
2. Limits of Flooding. Street drainage facilities shall be designed to keep flooding within six (6) feet of the face of curb for a design storm frequency of two (2) years for local streets and ten (10) years for all other streets. The depth of flow at gutter flow line shall not exceed 0.25 feet.
Concentrated flow across the traveled way is prohibited.
D. Conduit Design.
1. Type. For storm drain systems, circular pipes of reinforced concrete or cast-in- place concrete may be used. Class II pipe shall be the minimum for nonroadway areas. The minimum required strength for all pipe in the roadway area shall be Class III as designated by ASTM Specification C-76.
Culverts may be of any of the above materials in any standard manufactured shape. Reinforced concrete box culverts, if used shall be constructed in accordance with state standard plans.
2. Size. Pipes shall have a minimum diameter of 10 inches. For flows exceeding the capacity of 54-inch diameter pipe, open channels meeting the requirements of subsection H below may be acceptable.
3. Slope. Slope will be controlled by physical conditions and velocity criteria. Abrupt changes in slope are undesirable and are to be avoided wherever possible.
4. Velocity. Minimum velocity at full flow shall be two (2) feet per second (fps). The maximum velocity for storm drains shall be critical velocity at full flow. Culverts may have velocities greater than critical provided full consideration is given to the effects of abrasion.
5. Head and Head Losses. To facilitate the passage of debris and detritus, storm drains shall, unless otherwise approved, be designed to pass the design flow with a free water surface. Culverts shall be designed to provide a minimum freeboard of one foot from top of culvert to top of ditch bank at the entrance and exit points.
6. Roughness Coefficient. Suggested values for Manning’s roughness coefficient (n) are:
Reinforced concrete pipe . . . . . .0.012
Cast-in-place concrete pipe . . . . 0.013
7. Alignment. Alignment should be as straight as possible without undue bends and angle points. Where dictated by physical conditions, curved alignment is permissible as long as there is no reduction in the quality and soundness of joints. The minimum radius of curvature shall be 500 feet.
8. Cover. Except for culverts, outside the hinge point, the minimum cover shall be two (2) feet, measured from the top of the pipe to the roadway or ground surface. Cast-in-place concrete pipes shall have a minimum cover of two and one-half (2.5) feet except under roadways where three (3) feet is required. Where less than minimum cover is necessary the concrete cradle shown in the improvement standards shall be used.
9. Pipe Strength. The class of conduit recommended should be adequate for most conditions. Unusual situations may dictate selection of a higher strength conduit.
10. Location. The location of storm drains relative to roadway centerline shall be in accordance with the improvement standards. Care should be taken that storm drains and other underground facilities do not conflict with each other. Location and elevation of existing and proposed sanitary sewer laterals shall be a primary consideration in the design of the storm drainage facility.
E. Drop Inlets.
1. Types. The standard S-7 drop inlet as set forth in the improvement standards shall be used with pipes up to 30 inches in diameter. A modified S-7 drop inlet or a manhole will be used for pipe larger than 30 inches. Special situation drop inlets are shown in Standards S-7A and S-26.
2. Laterals. Laterals shall have a minimum slope 1%.
3. Location. Drop inlets shall be installed at all gutter low points and at locations such that the flooding limitations of subsection C above are met. They should not be spaced further than 500 feet apart.
F. Manholes.
1. Type. The type of manhole to be utilized shall be as set forth in the improvement standards.
2. Location. Manholes shall be placed:
a. Where two or more storm drain pipes join;
b. Where the conduit changes in size;
c. At angle points;
d. At points where a change of slope in the conduit occurs;
e. At changes in type of pipe.
3. Spacing. The maximum manhole spacing shall be 1,200 feet for pipe diameters of 48 inches or more. Spacing may vary from 350 to 700 feet for diameters less than 48 inches to 33 inches. Maximum spacing shall be 350 feet for conduit 30 inches or smaller.
4. Access Shaft. The access shaft shall be centered over the axis of the drain for conduits less than 42 inches in diameter. The shaft shall be offset and made tangent to one side of the pipe when the drain diameter exceeds 42 inches.
5. Special Structures. Special structures may be required for larger diameter pipes and shall be designed on an individual basis.
6. Grade. The crowns of all conduits intersecting at a manhole shall generally match. A minimum fall of 0.10 foot across the manhole shall be provided except in cases where the conduit is continuous through the manhole.
G. End Structures.
1. General. Headwalls and other end structures shall be installed to increase hydraulic efficiency, prevent erosion adjacent to the conduit and provide a counterweight to prevent flotation.
2. Entrances. When a drop inlet is not installed, flared end sections should be used. Headwalls may be used where dictated by physical conditions. Both installations shall conform to the state standard plans.
3. Exits. Where exists are installed, headwalls or flared end sections should be used for culverts. Where drainage systems discharge into a channel, standard headwalls shall be installed in accordance with the improvement standards.
An approved energy dissipater shall be installed at outlets where velocities are erosive.
H. Open Channels. The director may approve the use of open channels on an individual basis.
The finished channel shall have maintenance free bottom and sides. Minimum bottom width shall be three feet. Side slopes shall be no steeper than1-1/2:1.
All open channels shall be located in dedicated easements. An access road 12 feet wide shall be provided adjacent to the channel.
I. Bank Protection. Bank protection such as slope paving, sacked riprap, and facing rock may be required to protect drainage facilities, property or structures. The need and nature of bank protection will be determined by the director on an individual basis.
J. Temporary Leach Field Type Storm Drainage System. In accordance with the provisions of the “Nitrate Action Plan - Greater Chico Urban Area - Butte County,” adopted by city council Resolution No. 141 84-85 on March 19, 1985 as subsequently amended, temporary leach field type storm drainage systems may be installed for temporary use in cases where the public works director determines that storm water cannot be conveyed to the city’s storm drainage system or drainage channel because facilities are not available. The following criteria shall apply to design of such systems:
1. Percolation tests shall be conducted in accordance with environmental health department procedures. Tests shall be taken at the proposed depth of the drainage trench(es) at such locations as required by the public works director to verify the drainage capacity of the soil. Percolation rate shall be converted from minutes/inch to cubic feet per second/square foot.
2. The trench(es) shall be designed to contain a one-in-ten year frequency storm.
3. The bottom of the trench(es) shall be at least ten feet above the high water table and there shall be at least ten feet of soil capable of percolation below the bottom of the trench(es).
4. The rational formula, Q=CIA, shall be used to determine inflow into trench(es).
5. One-third of the trench(es) volume as void area shall be used in computing amount of storm water storage available in trench(es). Rock size in trench(es) shall be from one-half inch to four inches in size.
6. Fifty percent of the trench(es) bottom area and one-half of the depth of the trench(es) side walls and end walls shall be used in determining the area available for percolation out of the trench(es).
7. Where more than one trench is utilized, there shall be a minimum separation of four (4) feet between trench walls.
8. Limitation on Use of Infiltration Best Management Practices (BMPs). Three factors significantly influence the potential for storm water to contaminate ground water. They are: (i) pollutant mobility, (ii) pollutant abundance in storm water, and (iii) soluble fraction of pollutant. In addition, the distance of the groundwater table from the infiltration BMP may also be a factor determining the risk of contamination. A water table distance separation of ten feet in depth in California presumptively poses negligible risk for storm water not associated with industrial activity or high vehicular traffic. Site specific conditions must be evaluated when determining the most appropriate BMP. Additionally, monitoring and maintenance must be provided to ensure groundwater is protected and that the infiltration BMP is not rendered ineffective by overload. This is especially important for infiltration BMPs in areas of industrial activity or areas subject to high vehicular traffic (25,000 or greater average daily traffic (ADT) on a main roadway or 15,000 or more ADT on any intersecting roadway). In some cases pretreatment may be necessary.
K. Post-Construction Structural or Treatment Control Best Management Practices. Post- construction treatment control Best Management Practices (BMPs) shall incorporate, at a minimum, either a volumetric or flow based treatment control design standard, or both, as identified below to mitigate (infiltrate, filter or treat) storm water runoff:
1. Volumetric Treatment Control BMPs
a. The maximized capture storm water volume for the tributary area, on the basis of historical rainfall records, determined using the formula and volume capture coefficients in Urban Runoff Quality Management, WEF Manual of Practice No. 23/ASCE Manual of Practice No. 87 (1998) pages 175-178 (approximately the 85th percentile 24-hour storm runoff event); or
b. The volume of annual runoff to achieve 80 percent or more capture, determined in accordance with the methodology in Section 5 of the CASQA Storm Water Best Management Practice Handbook, New Development and Redevelopment (2003), using local rainfall data; or
2. Flow Based Treatment Control BMPs:
a. The flow of runoff produced from a rain event equal to at least two times the 85th percentile hourly rainfall intensity as determined from local rainfall records; or
b. The flow of runoff produced from a rain event equal to at least 0.2 inches per hour intensity.
(Res. No. 9 77-78 (part), Res. No. 57 82-83 §5, Res. No. 201 84-85 §1, Res. No. 59 90-91 §§8-10, Res. No. 11 95-96 §1, Res. No. 113-07, Res. No. 65-08, Ord. 2468 §6)