§ 16.25

These instructions have been prepared for the use of the designer of public improvement within the city of Corning, Tehama County, California. The purpose is to encourage uniformity of design criteria and to aid in the preparation of plans and specifications.

(Ord. 550 (part), 1994).

The purpose of this section is to diminish threats to public health and safety caused by the runoff of excessive stormwaters, reduce economic losses to individuals and the community at large, enhance broader social and economic development of land and water resources. The provisions of this section further regulate, guide and control:

A.

The subdivision, layout and improvement of lands within the city of Corning;

B.

The excavating, filling and grading of lots and other parcels or areas;

C.

The construction of buildings and the drainage of the sites on which those structures are located, to include parking and other paved areas;

D.

The design, construction and maintenance of stormwater drainage facilities and systems.

(Ord. 550 (part), 1994).

16.25.030 - Definitions.

As used in this chapter:

"Based flood elevation" means the elevation at all locations delineating the maximum level of high water should the adopted design flood occur.

"Capacity of storm drainage facility" means the maximum ability of a storm drainage facility to convey stormwater flows without causing substantial damage to public or private property, and in the case of a pipe, without surcharging.

"Channel" means a natural or artificial open watercourse with definite bed and banks which periodically or continuously contains moving water, or which forms a connecting link between two bodies of water.

"Compensatory storage" means an artificially excavated volume of storage within a flood storage capacity with artificial fill or structures.

"Conduit" means any channel, pipe, sewer or culvert used for the conveyance or movement of water, whether open or closed.

"Detention basin" means a facility constructed or modified to restrict the flow of stormwater to a prescribed maximum rate, and to concurrently detain the excess waters that accumulate behind the outlet.

"Detention storage" means the temporary detaining or storage of stormwater in storage basins, on rooftops, in streets, parking lots, school yards, parks, open space, or other areas under predetermined and controlled conditions, with the rate of drainage therefrom regulated by appropriately installed devices.

"Discharge" means the rate of outflow of water from detention storage.

"Drainage area" means the area from which water flows from detention storage.

"Dry bottom detention basin" means a basin designed to be completely dewatered after having provided its planned detention of runoff during a storm event.

"Excess stormwater runoff" means the volume and rate of flow of stormwater discharged from a drainage area which is or will be in excess of that volume and rate which pertained before urbanization.

"Flood fringe" means the higher portion of the floodplain, immediately adjacent to and on either side of the floodway occupied by quiescent or slow-moving waters during floods.

"Floodplain" means the special flood hazard lands adjoining a watercourse, the surface elevation of which is lower than the base flood elevation, which are subject to periodic inundation during floods.

"Floodway" means the channel of a water course and those portions of the adjoining floodplain which are reasonably required to carry and discharge the design floods.

"High water elevation" means the elevation of floodwater of a flood of specified frequency or occurrence at any given point.

"Hydrograph" means a graph showing, for a given point on a stream or conduit, the runoff flow rate with respect to time.

"Lag" means the elapsed time between the center of mass of a rain event and center of mass of the resultant hydrograph at a specific site.

"Leach trench" means a trench designed to hold storm-water and percolate the water into the ground, usually consisting of a horizontal perforated pipe and backfilled with leach rock.

"Off-site detention basin" means a feature or structure for temporarily storing excess stormwater originating at two or more urbanized locations, having devices for controlling the rate of release of the stored waters, and located downstream of all controlled areas.

"One-hundred-year storm runoff" means the stormwater runoff from a rain event of specific intensity and duration having a one-percent probability of occurring in any one year.

"One-hundred-year storms" means rainstorms of varying durations and intensities, having a one-percent probability of recurring in any one year.

"On-site detention basin" means a feature or structure for temporarily storing excess stormwaters, having devices for controlling the rate of release of the stored waters, and located within the urbanized site where the runoff originates. (See "off-site detention basin.")

"Peak flow" means the maximum rate of flow of water at a given point in a channel or conduit resulting from a predetermined storm or flood.

"Retention basin" means a structure or feature designed to retain stormwater over a period of time, with its release being positively controlled over a longer period of time than in detention basin.

"Storm sewer" means a closed conduit for conveying collected stormwater.

"Stormwater drainage facility" means any element in a stormwater drainage system which is made or improved by man.

"Stormwater drainage system" means all means, nature or man-made, used for conducting stormwater to, through or from a drainage area to the point of final outlet, including but not limited to any of the following: conduits and appurtenant features, canals, channels, ditches, streams, culverts, streets and pumping stations.

"Stormwater runoff" means the water derived from melting snow or rain falling within a tributary drainage basin, flowing over the surface of the ground or collected in channels or conduits.

"Time of concentration" means the elapsed time for stormwater to flow from the most distant point in a drainage basin to the outlet or point in question.

"Urbanization" means the development, change or improvement of any parcel of land consisting of one or more lots for residential, commercial, industrial, institutional, recreational or public utility purposes.

"Watercourse" means any stream, natural or artificial depression, slough, gulch, reservoir, lake, pond or natural or man-made drainage way in or into which stormwater runoff and floodwaters flow either regularly or intermittently.

"Wet bottom detention basin" means a basin designed to retain a permanent pool of water after having provided its planned detention of runoff during a storm event.

(Ord. 550 (part), 1994).

16.25.040 - General criteria and standards.

A.

Applicability. This title shall apply to any new residential, commercial, industrial, institutional or utility development.

B.

Maximum Allowable Release Rate. This maximum allowable release rate of stormwater originating from the proposed development shall not exceed 0.15 cubic feet per second per acre and shall not exceed the predetermined safe carrying capacity of any limiting downstream restriction. If more than one detention basin is involved in the development of the area upstream of the limiting restriction, the allowable release rate from any one detention basin shall be in direct ratio that its drainage area bears to the entire drainage area of the watershed.

C.

On-Site Detention of Excess Stormwater Runoff. The increased stormwater runoff resulting from the proposed development will be detained on site by the provision of appropriate wet or dry bottom reservoirs, by leach trenches, by storage on flat roofs, parking lots or street, or by other acceptable techniques. Storage will be sufficient to store flows from twenty-five-year storms of four-hour durations in excess of the runoff from the site before development. Control devices shall limit the discharge from storage to a rate no greater than that prescribed by this title.

D.

Other Retarding Measures. Measures which retard the rate of overland flow and the velocity in runoff channels such as swales or ponding are encouraged to partially control runoff rates.

E.

Joint Development of Control Systems. Stormwater control systems may be planned in coordination by two or more property owners as long as flood or stormwater hazards are not increased at intervening locations.

F.

Detention Facilities in Floodplains. If detention storage is provided within a floodplain, only the net increase in storage volume above that which naturally existed on the floodplain shall be credited to the development. No credit will be granted for volumes below the elevation of the regulatory flood at that location unless compensatory storage is also provided.

1.

Facilities in Floodway Fringes. An exception to the above policy is authorized when the floodways and floodway fringes have been established for floodplains by state or federal agencies. Under those conditions, detention facilities may be authorized in floodway fringe areas and no compensatory storage will be deemed necessary.

G.

Flows from Upland Areas. The total tributary area must be used in calculating the allowable release rate. The required storage volume will be based on the project area only, with extraneous flows from upland areas being by-passed or discharged via overflow spillways or other devices.

H.

Certification of Documents. All computations, plans and specifications related to the implementation to this title must be prepared and sealed by a professional engineer registered in California.

(Ord. 550 (part), 1994).

A.

Required Information-Preliminary Plans. Accompanying the preliminary plans of each proposed subdivision, there shall be furnished the following information and data:

1.

The location of streams and other floodwater runoff channels, their normal channels, and the extent of the floodplains at the established high water elevations, and the limits of the floodway (if available), all properly identified;

Storm drains and sewers;

3.

Septic tank systems and outlets, if any;

4.

Wells and springs.

B.

Preliminary Drainage Plan. A preliminary drainage plan is intended to describe the present conditions and the general method and features proposed for handling storm-water runoffs after development. The plan will be accompanied by preliminary maps or other descriptive materials prepared by and bearing the seal of a registered civil engineer showing the following:

1.

Drainage sub-areas;

2.

General alignment of storm sewers and other drains;

3.

Areas where special provisions may be necessary to reduce the impacts of high stormwater flows;

4.

Existing streams and floodplains and proposed realignments or modifications;

5.

Locations of existing and proposed culverts, detention ponds and basins, and other features which now affect, or will affect, stormwater runoffs and areas inundated by high stream flows.

C.

Site Plan. A plan showing the dimensions of the site with existing and currently proposed structures properly located, together with elevations and/or contours of the terrain before and after the proposed grading and filling, if any, has been completed.

D.

Lot Coverage. For the purpose of increasing rainfall infiltration and reducing storm runoff, a minimum percentage of the development lot area should be left in open space. Development plans shall be submitted showing the proportion of each development lot which will be left unimproved or with previous surfaces. The portion of this type of area is inversely related to development density. (Cluster or density transfer sites in planned development projects will be exempted from these provisions with approval of the

planning commission.) The percent of each lot which must remain in pervious open space for each zoning density district follows:

(Ord. 550 (part), 1994).

Accompanying the final plans of each proposed subdivision, there shall be furnished the following information and data prepared by and bearing the seal of a registered civil engineer.

A.

Contour Map. As described above.

B.

Comprehensive Drainage Plan. A comprehensive drainage plan is a plan designed to handle safety of the stormwater runoff following the rainstorms which exceed the pre-development capacity of storm sewer systems by detention of the increased stormwater runoff. The plan shall provide or be accompanied by maps or other descriptive material showing the following:

1.

The extent and area of each watershed tributary to the drainage channels in the subdivision;

2.

The storm drains to be built, the basis of their design, the outfall and outlet locations and elevations, receiving stream or channel and its high water elevation, and the functioning of the drains during high water conditions;

3.

The part of the proposed street system where pavements are planned to be depressed sufficiently to convey or temporarily store overflow from storm drains and runoff over the curb resulting from the heavier rainstorms and the outlets for such overflow;

Existing streams and floodplains to be maintained, enlarged, altered and eliminated; and new channels to be constructed, their locations, cross-sections and profiles;

5.

Existing culverts and bridges, drainage areas, elevation and adequacy of waterway openings; and new culverts and bridges to be built, their materials, elevations, waterway openings and the basis of their design;

6.

Existing detention ponds and basins to be maintained, enlarged, and altered and new ponds or basins to be built with dams, if any;

7.

The estimated location and extent of impervious surfaces existing and expected to be constructed when the subdivision is completely developed;

8.

The slope, type and size of all storm drains, culverts and other waterways;

9.

For all detention basins, a plot or tabulation of storage volume with corresponding water surface elevations and of the basin outflow rates for those water surface elevations;

10.

For all detention basins, design hydrography of inflow and outflow for the twenty-five-year design runoff events for the site under developed conditions and the calculated twenty-five-year peak flows from the site under natural and developed condition, unless the hydrograph requirement is waved by the city engineer.

C.

Site Plan. As described in Section 16.25.050(C).

(Ord. 550 (part), 1994).

The following rules shall govern the design of improvements with respect to controlling the runoff of stormwaters:

A.

Design. Streets, lots, parks and other public grounds shall be located and laid out in such a manner as to reduce the velocity of overland flow and allow the maximum opportunity for infiltration of stormwaters into the ground, and to preserve and utilize natural streams, channels and detention basins.

B.

Channel Straightening. Meandering streams or channels may be partly straightened and minor changes made in other channels where it can be demonstrated that downstream flooding will not be increased in frequency or depth.

C.

Streets Designed for Detention. Cross sections of streets selected by the developer for the detention of stormwater runoff shall be constructed above flood elevations and shall be designed to temporarily store and convey flows in excess of storm drain capacities, together with over-the-curb runoff. Those streets also shall be provided with adequate outlets for the safe disposal of flows. Where grading of lots is necessary to avoid damage to adjoining buildings, the lot grade adjacent to each building should be at least two feet higher than the crown of the pavement, and the minimum sill elevation of openings in the outer walls of such habitable buildings shall be at least two and one-half feet above the crown elevation.

D.

Manholes. All sanitary sewer and storm-drain manholes constructed in a floodplain, in a street designed for detention, or in an area designed for the storage or passage of flood or storm water, shall be provided with either a watertight bolted manhole cover, or be constructed with a rim elevation of a minimum of one foot above the high water elevation of the design flood or the high water elevation of the design storm, whichever is applicable to the specific area.

(Ord. 550 (part), 1994).

Basins may be constructed to temporarily detain the stormwater runoff which exceeds the maximum peak flow rate authorized by this title.

A.

Storage Volumes. The volume of storage provided in these basins, together with such storage as may be authorized in other on-site facilities, will be sufficient to control the runoff from the twenty-five-year storm of four-hour duration.

B.

Maximum Depth. The maximum planned depth of stormwaters stored shall not normally exceed six feet.

C.

Side Slopes. The side slopes of the basin will conform as closely as possible to natural land contours and preferably shall be under ten percent. If the side slopes exceed twenty percent, erosion control and safety measures shall be provided.

D.

Outlet Control Structures. Outlet control structures shall be designed to operate simply and automatically. They will limit discharges into existing or planned downstream channels or conduits so as not to exceed predetermined maximum authorized peak flow rates.

E.

Emergency Spillway. Emergency overflow facilities must be provided in all instances so that stored waters will not exceed the safe capacity of the basin.

(Ord. 550 (part), 1994).

16.25.090 - Wet bottom basins.

For basins designed with permanent pools:

A.

Wet bottom basins shall not be permitted unless specifically approved by the city council.

(Ord. 550 (part), 1994).

16.25.100 - Dry bottom basins.

For detention basins designed to be completely dewatered:

A.

Interior Drainage. Provisions must be incorporated to facilitate complete interior drainage of dry bottom basins, to include the provision of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facilities, pumps, or the installation of subsurface drains.

B.

Multipurpose Features. These shall be designed to serve secondary purposes for recreation, open space or other types of use which will not be adversely affected by occasional or intermittent flooding.

C.

Aesthetics. Designs should result in aesthetically pleasing configurations which will enhance public acceptability.

(Ord. 550 (part), 1994).

A.

Rooftop Storage. Detention storage requirements may be met in total or in part by detention on flat roofs. Details of such designs to be included in the building permit application shall include the depth and volume of storage, scuppers, design loadings for the roof structure and emergency overflow provisions.

B.

Parking Lot Storage. Paved parking lots may be designed to provide temporary detention storage of stormwaters on all or portion of their surfaces. Outlets will be designed so as to slowly empty the stored water, and depths of storage must be limited to a maximum depth of seven inches so as to prevent damage to parked vehicles and access to parked vehicles is not impaired. Ponding should be relegated to those positions of the parking lots farthest from the area served.

C.

Other Detention Storage. All or a portion of the detention storage may also be provided in underground or surface detention facilities, to include basins, tanks, swales or leach trenches.

D.

Maintenance. Designs of detention facilities will incorporate features which facilitate their inspection and maintenance. The growth of obnoxious weeds, the creation of insects, and the decrease in available storage by accumulated sediments will all be controlled. The cleanup of accumulated debris and other materials after runoff events have subsided will be assured.

Assignment of responsibility for maintaining facilities serving more than one lot or holding will be documented by appropriate covenants to property deeds or by other methods acceptable to the city unless responsibility is formally assigned to a public body.

1.

Inspections. All privately owned detention storage facilities may, as resources permit, be inspected by representatives of the city not less often than once every three years. A certified report will be submitted covering the physical conditions, required storage capacity and operational conditions of key elements of the facility.

2.

Corrective Measures. If deficiencies are found by the inspector, the owner(s) of the detention facility may be required to take the necessary measures to eliminate nuisances and correct structural deficiencies. If the owner(s) fails to do so, the city may undertake the work and collect from the owner(s) using lien rights if necessary.

(Ord. 550 (part), 1994).

Designs of detention facilities will incorporate safety features, particularly at outlets, on steep slopes, and at any attractive nuisances to include, as necessary, fencing, hand rails, lighting and steps in order to restrict access during critical periods and to afford some measure of safety to both authorized persons.

(Ord. 550 (part), 1994).

A.

Responsibility. The administration of this chapter shall be the responsibility of the director of the department of public works.

1.

Variances. No variance shall be issued without the prior concurrence of the city council.

2.

Special Use Permit. No special use permit shall be issued without the prior concurrence of the city council.

B.

Interpretation. In the interpretation and application of this title, the provisions expressed herein shall be held to be the minimum requirements and shall be liberally construed in favor of the city of Corning and shall not be deemed a limitation or repeal of any other powers granted by state statutes or exercised by Uniform Building Code requirements.

(Ord. 550 (part), 1994).

Chapter 16.26 - DEVELOPMENT PATTERNS

Sections: