Chapter 19.20
STORMWATER DRAINAGE
Sections:
19.20.020 Purpose and applicability.
19.20.030 General requirements.
19.20.050 Standard drainage system maintenance.
19.20.010 Introduction.
A. The items herein contained are the stormwater drainage standards, conditions, and specifications of the city of Rock Island, and are to be considered in conjunction with other provisions of city plans and/or regulations governing stormwater drainage. These are minimums only and may be increased or altered to fit particular situations. The city by this reference adopts the latest edition of the “Standard Specifications for Road, Bridge and Municipal Construction” (hereinafter referred to as the “Standard Specifications”) as prepared by the Washington State Department of Transportation and the Washington State Chapter of the American Public Works Association to govern all aspects of stormwater drainage improvements including but not limited to materials, labor, equipment for installation, workmanship, testing, etc. Any material, design, standard, or testing not specifically addressed within this document shall be as per the Standard Specifications. The city also by this reference adopts the latest version of the “Standard Plans” (engineering drawings hereinafter referred to as the “Standard Plans”) published by the Washington State Department of Transportation.
B. In addition to the specific standards set forth in this chapter, the city may develop a technical specifications guidebook containing drawings that graphically depict the provisions of this chapter and the Standard Specifications. This guidebook may also specify where the city may choose to add to, remove from, and/or replace (in whole or in part) certain aspects of the Standard Specifications and Standard Plans adopted by reference herein. The guidebook is a technical reference document that may be updated periodically based on changes in engineering standards, whether those be city standards or updated standards in the design and construction documents that are adopted by reference in this chapter. (Ord. 01-062 § 5).
19.20.020 Purpose and applicability.
A. The city of Rock Island has found that future stormwater drainage problems may be reduced or avoided if future developers, both private and public, provide for storm and surface water drainage of their respective properties. Storm drainage standards and guidelines are set forth to protect life and property from loss and damage by flooding and to protect streams, creeks, and lakes from pollution and excessive flows.
B. The storm drainage standards and guidelines contained herein are intended to reduce and prevent adverse storm drainage impacts. They represent the minimum design standards for the construction of storm drainage facilities within Rock Island. Compliance with these standards does not relieve the designer, owner, or developer of the responsibility to apply conservative and sound professional judgment to protect the health, safety and welfare of the general public. Special site conditions and environmental constraints and considerations may require a greater level of protection than would normally be required under these standards.
C. Plans for permanent drainage and stormwater detention facilities shall be provided for property improvements within the city in accordance with the standards for the following types of improvements:
1. All types of land divisions as defined in RIMC Title 16.
2. All developments, including remodeling, reconstruction and new construction adding a total of 5,000 square feet or more of new impervious surfaces, including roof areas.
3. Developments entailing construction which would change the point of discharge of surface waters, discharge surface waters at a higher velocity and/or quantity than that of the predevelopment discharge rate, or tend to add to pollution of surface waters.
4. Any proposed development adjacent to or within frequently flooded areas and/or any stream, river, wetland or other surface water body.
5. At the discretion of the city maintenance supervisor, plans may be required for other types of development not listed herein based on the possibility for future problems related to stormwater drainage.
D. The plan requirement established in the previous section shall not apply when the city determines that the proposal meets all of the following conditions:
1. The proposed development will not seriously and adversely impact the water quality conditions of any affected receiving bodies of water.
2. The proposed development will not substantially alter the drainage pattern or increase the peak discharge.
3. The proposed development will not cause runoff exceeding the available capacity of the existing drainage system where such system was designed to serve the proposed development area.
E. A single-family residence with less than 3,500 square feet of impervious surfaces is exempt from plan requirements. (Ord. 01-062 § 5).
19.20.030 General requirements.
All persons proposing land development and/or approvals as outlined above shall provide a drainage plan for surface water flows entering, flowing within and leaving the subject property. The plan is to conform to the following standards and requirements:
A. When required by this chapter, plans for stormwater management shall be prepared by a registered civil engineer currently licensed by the state of Washington and qualified by experience and education in the field of hydraulics, hydrology, or a closely related field. Storm drainage plans or revisions to any approved plan shall be reviewed and approved by the city prior to any construction.
B. On-site storm drainage improvements must be sufficient to mitigate impacts on runoff, erosion, sedimentation and pollution.
C. All drainage system elements must be designed to allow for adequate maintenance and accessibility at all times.
D. Drainage systems shall be designed to meet applicable state and federal requirements for water quality prior to discharge to any wetland, stream, or lake. The developer shall be responsible for obtaining any local, state, or federal construction or discharge approvals or permits.
E. In no case shall storm drainage facilities be allowed to connect to sanitary sewers.
F. Manhole standards of the sewer design details shall apply to stormwater drainage facilities.
G. No drainage originating inside of a building or structure shall be connected to the storm drainage of surface water systems.
H. All surface water and stormwater runoff from a proposed development that would construct new or modify existing drainage facilities should be discharged at the natural location. Diversions may be allowed if it corrects an existing problem.
I. Proposed developments should identify the upstream tributary drainage area and provide an analysis of the pre-existing drainage volume and quality and an analysis of the impact of the proposal on the drainage system. The post development peak rate runoff shall not exceed the predevelopment peak rates for the site. The methods of peak rate runoff control may include detention, retention and/or infiltration. On-site biofiltration in combination with infiltration systems is the preferred method for management of on-site stormwater and shall be considered before transporting stormwater off-site.
J. For all proposed development requiring a drainage conveyance system, the conveyance system must be analyzed, designed and constructed to handle existing off-site tributary flows and on-site storm drainage flows caused by development of the project.
K. Developments involving clearing and grading and which require new or modification of existing drainage facilities must include an erosion/sedimentation control plan meeting the Washington State Department of Ecology requirements and providing suitable measures to prevent sediment-laden runoff from leaving the site during construction.
L. The long-term maintenance and operation of proposed private drainage facilities is the responsibility of the property owner or properly formed homeowner’s association and shall be performed in compliance with the city’s maintenance standards. The city reserves the right to perform maintenance on a private drainage facility when the facility is out of compliance with maintenance standards, as determined by the maintenance supervisor. Any and all costs associated with this city maintenance will be paid by the property owner or homeowner’s association.
M. Adequate easements shall be provided for operation and maintenance of all drainage facilities located on private property.
N. Construction of storm drainage facilities may be required to comply with the bonding and insurance requirements contained within this title. (Ord. 01-062 § 5).
19.20.040 Design standards.
A. Developments shall be designed and constructed to provide control of the quality and quantity of stormwater runoff both during and after construction. Erosion and sedimentation control plans shall be submitted and approved by the city prior to the beginning of any construction. Please note that a permit from the Washington State Department of Ecology is required if five or more acres are disturbed. Peak discharge control and detention facilities shall be provided in accordance with the development standards.
B. The maximum allowable release rates from stormwater detention systems shall be based upon the predevelopment runoff from the development site as described in this chapter. The allowable release rate shall be determined as specified, and may be modified on a case-by-case basis only due to constraints in the drainage system downstream of the point of discharge. Storm precipitation distribution and runoff modeling will conform to the Natural Resource Conservation Services Technical Report 55 or other acceptable method. The “rational method” used by licensed professional engineers may be used for runoff modeling on parcels under two acres, not subject to upstream runoff. The following are allowable release rates:
1. For sites with tributary basins greater than five acres or sites less than five acres in area which are deemed by the city to have significant impacts due to runoff quantity, the discharge shall be limited to the capacity of the receiving conveyance system. Peak runoff rate shall be computed using the Natural Resource Conservation Services TR-55 method, modified Santa Barbara Urban Hydrograph Method or other appropriate models.
2. Should the proponent desire to discharge at higher flow rates than provided above, then he/she shall provide for sufficient improvements of the downstream conveyance system to handle the increased flow.
C. The on-site drainage system, including conveyance, flow restriction, detention, pollution control, and emergency overflow elements must be properly designed and sized to handle runoff from the site and conveyance through the site. The design should be carefully analyzed for potential problems, flow impediments, construction or maintenance difficulties, and potential erosion or other property damage. Catch basins shall be required at every intersection and shall be spaced a maximum of:
|
Spacing |
Grade |
|---|---|
|
150 feet |
0.5% to 1.5% and 12.0% and greater |
|
200 feet |
1.5% to 3.0% and 8.0% to 12.0% |
|
300 feet |
3.0% to 8.0% |
D. Infrastructure shall be designed to a two inches in 90 minutes storm event.
E. All stormwater discharge shall be treated prior to release using best management practices. The applicant should consider the use of vegetative or other natural filtration means. Effluent discharges from any oil removal treatment device to the storm sewer or surface water system shall be in compliance with State Department of Ecology regulations for discharge to storm drains or surface waters. Whenever determined necessary by the city, paved parking and/or access roadways shall include the installation of appropriate oil/grease separation devices, as approved by the city. They shall be located at a point where they can be easily maintained and where they will intercept floating contaminants flowing off road rights-of-way, parking lots, and other sources of pollutants. Selection and sizing of oil separation device types shall be subject to approval of the city.
F. The property owner assumes full responsibility and liability for proper maintenance and operation of the oil separator, unless the separator is a part of a publicly operated drainage system. This statement must appear on the recorded drawings for the development.
G. Access to the separator shall be maintained.
H. The following minimum standards shall apply to stormwater detention facilities:
1. Detention facilities will be required when the downstream capacity is inadequate to convey the runoff and/or when there are potentially negative water quality impacts to streams, rivers and/or their tributaries that are documented as having threatened or endangered species, pursuant to the federal Endangered Species Act. All stormwater runoff originating from and/or draining from any proposed development shall be controlled and/or conveyed in accordance with all city standards and policies and as described in these standards.
2. Stormwater detention systems should be designed to maximize reliability, ease of maintenance, and water quality of runoff and should minimize hazards to persons or property (both on-site and off-site), nuisance values, and risk of failure.
3. Sufficient detention storage capacity shall be provided to store the excess runoff from the developed site for all storm events up to a “100-year storm.”
4. In calculating the storage volume provided, “dead storage” in wet ponds shall be excluded, i.e., that volume of water which must be assumed to be present in the detention system at the commencement of the design storm. Any volume at a level below that of the outfall invert must be presumed to be dead storage, e.g., catchments.
5. All detention storage facilities should include a provision for control of overflows, and suitable data shall be provided to support the design. Under no circumstances should the overflow discharge over land or over private property unless drainage easements are acquired.
6. The following are site, soil and infiltration data requirements for calculating effective infiltration rates to reduce storage requirements:
a. The proposed site should have favorable topography to preclude high runoff rates. Engineering calculations shall be included with any submittal to show that there will be no adverse impacts due to the reduced storage. Such adverse impacts may include but not be limited to increased frequency of overflows.
b. A log of the soils and infiltration test data should be submitted to reveal site soil conditions and infiltration rates.
c. An adequate number of test holes should be located over the proposed site to substantiate representative conditions for the final layout of the development, and as a minimum condition, test holes shall be located in each area and at the elevation proposed for infiltration.
d. Ground water depth, location, flow and general characteristics shall be considered.
e. The designer shall demonstrate the adequacy of the depth to the impervious layer below the bottom of the proposed infiltration trench or basin.
f. A soil log may be required to describe soil type and depth along with a site map showing the location of each test hole. Classification may be in general terms such as loose sand, sandy silt, clay hardpan, rock, etc., or classification may be in specific terms as described by the U.S. Department of Agriculture (Soil Conservation Service). The soil log should include the depth to ground water table, if less than 12 feet in depth. A falling head permeability test or similar method must be used to demonstrate the infiltration capacity of the least pervious soil layer. (Ord. 01-062 § 5).
19.20.050 Standard drainage system maintenance.
Maintenance of storm drainage facilities on private property shall be the responsibility of the owner(s), unless otherwise provided for in these standards. This responsibility and the provision for maintenance shall be clearly stated on the face of final plats for all types of land divisions, property conveyance documents, and/or drainage improvement plans. In the event the owner(s) does not provide proper maintenance and the city determines the storm drainage facility represents a public safety threat, the city will give 30-day notice to the owner(s) to correct the deficiencies. If the deficiencies are not corrected within 30 days the city may enter upon the property to perform the necessary maintenance at the owner(s) expense. This provision for access will be included as a provision of final approval. (Ord. 01-062 § 5).
19.20.060 Access drainage.
The right to lay a culvert within the street is reserved expressly to the city. A person desiring the installation of a culvert within a street under a driveway or along property frontage shall apply to the city, stating the length, location, and purpose of the culvert. The city shall determine the necessity of a culvert pipe under the driveway approach. In general, a pipe will be required where the approach crosses the roadway ditch, or where a roadside drainage problem exists, or where one will be created by the construction of the road approach. Where a pipe is specified, it shall meet the following requirements:
A. Inside diameter: 12 inches, unless otherwise approved by the maintenance supervisor.
B. Material: concrete, corrugated steel, or corrugated aluminum, culvert pipe specifications, pursuant to the Standard Specifications.
C. Installation: the flow line of the pipe shall be at the same elevation and alignment as the flow line of the roadway ditch or drainage ditch. Concrete pipe joints may be ungrouted. The pipe shall extend a sufficient distance past the toe of the approach fill to prevent sloughing into the pipe ends. (Ord. 01-062 § 5).
19.20.070 Plans and profiles.
The drainage plans submitted shall include at a minimum the following information:
A. The plan shall be prepared and designed by a civil engineer currently licensed to practice in the state of Washington. The plan shall contain the engineer’s stamp and signature certifying the design.
B. A plan and profile of the existing and proposed drainage area and conveyance systems showing all hydraulic and physical data such as:
1. Existing and proposed topography;
2. Invert elevations at all inlets, structures, outfalls and other points of interest;
3. Bottom elevations of all ditches, channels, ponds, swales and streams;
4. Pipe sizes or channel cross sections;
5. Pipe length and material;
6. Grades on all pipes or channel bottoms calculated to the number of decimal places necessary to guarantee 0.01 foot accuracy for pipes or paved channels and 0.1 foot accuracy for earth bottom channels;
7. Design water surface elevations and flow rates for all conveyance pipes or channels;
8. All above-ground pond details, if used, shall include design volume, contours of the finished surface, inlet locations, outlet details including inverts or grate elevations, and secondary overflow paths;
9. Top or grate elevations of all structures, inlets, catch basins or manholes;
10. Retention pipe, trench or drywell details including dimensions, elevations of inverts and maximum water surface elevations;
11. Details of all structures not shown in the standard drawings, including underground retention/infiltration structures, if used;
12. Phasing limits for phased construction projects and any interim drainage control measures required due to the phasing;
13. Control points and benchmarks used for vertical and horizontal control of construction (coordinates and elevations shall be relative to the city’s standard datum);
14. Erosion and sedimentation control plans.
C. A copy of the site survey plan including as a minimum:
1. Property boundaries;
2. Existing topography at one-foot contour intervals on the subject property, the drainage area, and all adjacent properties sufficient to determine all potential topographic impacts of the construction;
3. Existing utility locations including type, material, depth and dimensional locations;
4. Natural or manmade drainage courses or pipes to the extent necessary to determine all hydraulic or hydrologic impacts of the proposed project;
5. Locations of all existing structures or pavement;
6. Locations and description of the proposed drainage system.
D. Engineering calculations shall include, at a minimum:
1. Infiltration/retention volume calculations used;
2. Analysis of all off-site flows upstream of the proposed drainage system insufficient detail to determine they hydraulic and hydrologic impacts;
3. Hydraulic and habitat analysis of the downstream drainage environment in sufficient detail to determine the impacts of the proposed drainage system. A minimum length of one-quarter mile downstream of the proposed discharge point shall be analyzed;
4. A narrative describing the upstream and downstream analysis and detailing the intent and function of the on-site system;
5. Pipe or channel sizing calculations for all conveyance system elements, design flow rates, shall be determined through the use of manual methods or computer programs commonly used by local professionals. Computer output sheets shall be legible and able to be interpreted without additional information by the plan reviewer;
6. All additional pertinent backup information, survey data or calculations.
E. Prior to acceptance of improvements, issuance of a certificate of occupancy or any other final sign-off or approval, an engineered as-built plan shall be submitted showing the following minimum provisions. The as-built information shall be shown in the form of revisions to the approved drainage system plan and submitted in a reproducible form complying with the applicable provisions of this title.
1. Location and type of all catch basins, manholes and/or other structures;
2. Location, lengths and type of pipe installed in the system;
3. Elevation at top, inverts and bottoms of all structures;
4. Locations and volumes of all above-ground ponding/retention areas;
5. All connections to city drainage systems, when applicable;
6. All outfalls to streams or other bodies of water;
7. Signature and stamp of the licensed engineer/surveyor preparing the as-built drawings;
8. Documentation of all easements. (Ord. 01-062 § 5).