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Chapter Two STORMWATER CONVEYANCE & RUNOFF CONTROL
Storm Water Rules and Regulations (SWRR)
Version: October, 2008
Chapter Two
STORMWATER CONVEYANCE & RUNOFF CONTROL
This section provides criteria and guidance requirements for planning and designing stormwater conveyance and control systems. Properly designed stormwater facilities provide many necessary, beneficial services to developing areas. Services provided are reduction and prevention of flooding, streambank erosion, and water quality impairment in downstream areas. While separate design criteria are provided for stormwater quantity and quality control facilities, proper design can integrate these measures into a single facility.
Section 2.10 Preparing a Stormwater Management Plan
A Stormwater Management Plan is required on all developments. The planning process is one which should provoke thought and consideration of management alternatives relative to stormwater early in the overall site development process.
Section 2.10.1 Data Collection
Inventory the existing site conditions to gather relevant information to develop an effective stormwater management plan. The information obtained should be plotted on a map and included with the calculations portion of the plan.
- Topography
A 200-scale topographic map of the site should be prepared to show the existing elevations at two-foot (2’) intervals unless directed by the ECEO to provide additional information. When drainage areas are large and include areas off-site, USGS quadrangle maps may be used to show these off-site drainage areas. (Note: 2’ contour interval maps are on file at the ECEO).
- Drainage
All existing streams, drainage swales, and tile shall be located and clearly marked on the topographic map. It is also important to illustrate areas that exhibit potentially critical drainage hazards.
- Soils
Major soil type(s) on the site should be determined and shown on the topographic map. Soil information can be obtained from the Soil Survey of Erie County, issued January 2006. Soil information should be plotted directly onto the map or an overlay of the same scale for ease of interpretation.
- Designated Floodplain
Designated 100-year floodplain boundaries established by the Federal Emergency Management Agency (FEMA) shall be plotted directly onto the map or an overlay of the same scale for ease of interpretation where applicable. If no portion of the site is located within a designated 100-year floodplain a notation of such shall be made on the map.
- Existing Vegetation
The existing vegetation on the site should be shown on the topographic map. Such features as tree clusters, grassy areas and unique vegetation should be located on the map. In addition, existing denuded or exposed soil areas should be indicated on the map.
Section 2.10.2 Runoff Calculations Before Development
With the completion of the Data Collection activity the peak rate of runoff and runoff volume for the pre-development situation can be determined for a five (5) year 24-hour storm.
Section 2.10.3 Preliminary Site Design
Properly designed site layouts created with consideration of existing natural features can often reduce the need for additional structures. The primary goal of developing the site layout for stormwater management is to identify areas of critical importance (ie. Riparian zones, critical vegetation, existing drainage, etc.) and develop ways to avoid and preserve them as designated open space. It is the intention of this section to provide guidance for designing the layout of the proposed project through analysis of collected data.
- Topography
The primary topography considerations are the steepness of the slopes and their length. Because of the adverse effect of these two types of slopes, or one that exhibits both characteristics on run-off, special design for erosion control, sedimentation and water quality is needed. BMPs for slope protection will be an important part of the plan as well as their scheduled implementation in the overall site construction sequence. Careful planning should be taken to limit soil disturbance in these areas as much as possible.
- Natural Drainage
Natural drainage patterns including swales and ephemeral drainage without defined channels provide valuable stormwater benefits, as such they shall be identified on the plan so they can be incorporated into the proposed drainage system. Where possible, natural drainage ways shall be used to convey the runoff to avoid the expense and problems of constructing artificial drainage systems.
- Preservation and Protection of Riparian Areas
Riparian areas are natural “green infrastructure” that provide many benefits which include filtering runoff through the buffer zone, flood storage, groundwater recharge, and critical wildlife habitat. Site layout should limit encroachment into the riparian area. Encroachment may include the clearing of trees, filling of the floodplain, or hydromodification of the stream that may require additional permits through Sections 401/404 of the Clean Water Act.
- Soils
The existing soils’ rate of infiltration of rainwater must be considered in determining proper run-off control of a stormwater management plan. Soils of the County have been grouped into hydrologic soil classes, which should be utilized to determine the areas where critical runoff and increased infiltration will occur.
- Existing Vegetation
The type of existing vegetation greatly affects the amount of runoff generated from the landscape. Rates of infiltration change with types of vegetative cover, particularly areas of native vegetation that increase percolation of rainwater into the soil. It is also important to make efforts to preserve large trees or clusters of large trees wherever possible. Maintaining woody vegetation preserves absorption of rainwater while reducing negative impacts on existing wildlife habitat.
Section 2.10.4 Drainage System Development
After analyzing the data, and determining the site limitations, the designer can then develop a proper drainage system. In proper planning, buildings, roads and parking lots should be located, to exploit the strengths of the site and overcome drainage limitations. The following are suggested considerations in design:
- Terrain Considerations
The designer should attempt to tailor the drainage system to the existing site conditions. This will reduce unnecessary land disturbance and therefore reduce the increase in runoff and soil erosion potential.
Confine Construction to Areas Outside Drainage Ways
Land disturbance in drainage ways will necessitate the installation of more costly control measures.
- Cluster Building Sites
Utilizing a Low Impact Development (LID) method such as clustering building sites minimizes the amount of disturbed land, reduces increases in runoff and in most cases, reduces the amount of conveyance system needed within the development. However local zoning regulations should be consulted first to insure cluster development is permitted.
- Minimize Impervious Areas
Impervious surfaces such as building roof, parking lots, roads, and walkways greatly increase the amount of runoff leaving the site. To reduce runoff, buildings shall be clustered together where possible. To increase infiltration of stormwater runoff from the parking lot(s), the site design should include one or a combination of the following: use of impervious pavements in excess parking spaces or installation of infiltration best management practices (BMPs) such as filter strips, infiltration trenches or bioretention cells as described in the Ohio Rainwater and Land Development Manual (RLDM).
- Native Landscaping
Infiltration of stormwater is greater where native vegetation is present compared to non-native vegetation such as domestic turf grass. The benefits of native vegetation also include reduced need for watering, resistance to pests and disease, and are more adaptive to local climate. Native vegetation should be considered to replace large open areas of disturbed soil, within and surrounding detention basins, open drainage swales, and other areas where applicable. In addition to native plantings in open areas, tree plantings should consist of at least 50% species native to Erie County. Erie Soil and Water Conservation District and the Ohio State University Extension Office may assist in providing lists of native species.
Section 2.10.5 Plan for Runoff Control
After preparation of a general site layout of the stormwater control plan must be prepared.
- Sub-Drainage Areas
Determine how much and where the runoff will travel over the site. Consider how runoff can be controlled in each drainage area. Remember, in most cases it is easier to control runoff in smaller areas than to try to handle the entire site at some location downstream as it leaves the site. - Select Runoff Control Practices
Runoff control practices can be divided into three broad categories:- Vegetative Controls
- Structural Controls
- Management Measures
- Local or State handbooks should be used to select and design appropriate vegetative and structural practices. Management measures are common sense types of controls used to help minimize the need for physical practices.
- Vegetative Controls
Keep in mind that the first line of defense is to prevent increase in volume or rate of runoff. This is accomplished by protecting the soil surface as much as possible and not decreasing the over-land flow time. Vegetative controls can include grass swales, bioretention areas, grass filter strips, etc. - Structural Controls
Where large increases in runoff occur, structural practices are generally the only way to control runoff. It is very important that structural practices be selected, designed and constructed according to standards and specifications of the engineer of jurisdiction. Improper installation can create problems which are greater than the structure was designed to solve. - Management Measures
Good site design and construction management is as important as any physical practices used for runoff control. The following are only some management considerations:- Design site to help reduce runoff in open areas and not increase it.
- Use staged construction.
- Use as few pipe systems as possible.
- Allow runoff to meander through the site, increasing flow lengths.
Section 2.10.7 Develop Design Details
Once a development plan and drainage system that meets the runoff criteria has been decided upon, the detail plans for the drainage facilities and control structures should be developed. Detail design plans shall be developed according to generally accepted engineering principles and approved by the County Engineer or the Engineer of Jurisdiction. It is important at this point in time to work closely with the County Engineer or Engineer of Jurisdiction to make sure that all facilities and structures are being designed according to standards and criteria that exist for these types of facilities and structures within that jurisdiction.
Section 2.20 Onsite Stormwater Conveyance
Adequate stormwater management facilities shall be planned for runoff generated by the proposed development. Stormwater generated onsite and offsite shall be conveyed through the development site to adequate stormwater control facilities designed to prevent flooding within the development and negative impacts to upstream and downstream areas.
Section 2.20.1 Offsite Stormwater Conveyance
Runoff from an offsite watershed that discharges to the development site can either be routed around the on-site storage and treatment facilities or accommodated by these facilities. If the runoff is routed around the site it must be discharged into a proper offsite receiving outlet, see Section 2.30. If the offsite runoff is routed through the on-site facility that facility must first be designed such that the maximum discharge is kept to the site’s pre-developed 5-year release. This would mean that additional storage would be required to accommodate the additional volume from offsite discharge so that the meter structure is not enlarged.
Section 2.20.2 Watershed Diversions
Diversion of stormwater runoff from one watershed or receiving stormwater system to another is generally prohibited because such diversions have the potential to cause or increase flooding, erosion, or negative water quality impacts in the receiving conveyance system. Diversion of runoff from one major watershed to another shall require documentation that the diversion is beneficial and will not have detrimental effects on the receiving stream or system. Approval of diversion of runoff shall be at the sole discretion of the County Engineer or his designee.
Section 2.30 Stormwater Outlet
Proposed stormwater systems shall be routed to an adequate offsite outlet. This can be one of the following:
- A natural stream, creek, river or tributary thereof,
- A man-made open channel or ditch system (generally excluding roadside ditches),
- A storm sewer system adequately sized for the design flow.
The applicant shall use a hydrologic method, acceptable to the County Engineer, (see Section 2.40) to demonstrate that the offsite stormwater system is of adequate size to convey existing offsite and proposed onsite flows. The downstream analysis shall be done from the outlet of the onsite system to the following points:
- The next increase in pipe diameter in a downstream storm sewer system,
- The downstream face of the next bridge or culvert crossing an open conveyance system, or
- A point designated by the County Engineer based upon historic drainage issues in the downstream system.
If it is determined that the downstream system is not adequate, the County Engineer may require more stringent release rates and/or require the applicant to provide the necessary downstream improvements to remedy the identified issues. Also see Section 2.20.
Section 2.30.1 Agricultural Drainage Tile
Agricultural tile systems have been installed on agricultural lands for agricultural purposes only and, in general, may not be used as an outlet for development. If field tile are intercepted during construction, they shall be reconnected or connected to the proposed stormwater system. Field tiles that exhibit evidence of conveying septic effluent shall be immediately reported to the Erie County Health Department and should not be used for stormwater conveyance or tied into the onsite stormwater system.
Designers preparing plans on agricultural lands should contact the Erie County Engineer’s Office or the Erie Soil and Water Conservation District for tile system information that may be on file. All visible field tile outlets and locations shall be field located and shown on stormwater management plans with any plan information obtained from county agencies.
An agricultural drain tile main may not be used as an outlet for a major subdivision or commercial development.
If an existing agricultural tile main is proposed as an outlet for a single lot or a minor subdivision all of the following conditions must be met:
- The Owner/Developer shall provide a disc (DVD) with the existing tile filmed so that a determination can be made by the ECEO as to its condition and whether or not it will be permitted to be used. This determination shall be made prior to plans being developed.
- Runoff from the proposed development site shall be restricted to not more than the remaining capacity of full flow of the existing system in the predevelopment condition based on the tributary area within the development area.
- An easement over the downstream owner(s) shall be required for discharges to a private tile main and is limited to a tile that crosses one landowner to an approved outlet per Section 2.30. If the private tile crosses two or more properties to an outlet it is not acceptable.
Section 2.40 Runoff Calculation Requirements
The amount of stormwater runoff depends on many factors. Some of these factors are reasonably fixed and can be determined accurately, such as watershed size, ground slope and natural ponding. Others vary by season, such as frozen soil, soil moisture, evaporation, or transpiration. Other factors vary by land use, such as type of ground cover, impervious areas or method of cultivation. Finally, precipitation is extremely variable by time of year and other natural factors.
Despite the indeterminate nature of these factors, methods for calculating accurate stormwater runoff have been developed
The following methods are acceptable for calculation of runoff within these regulations:
- One of the methods described in Urban Hydrology for Small Watersheds, Technical Release Number 55, US Department of Agriculture, Soil Conservation Service and its Ohio Supplement.
- The Rational Method (see Appendix D) shall be the minimum acceptable design method.
- Procedures outlined in the current edition of the Ohio Rainwater and Land Development Manual (RLDM).
Section 2.40.1 Runoff Control Methods
The criteria established in this section, necessitates the use of stormwater runoff control facilities and/or Best Management Practices (BMPs). In many development situations both manufactured and structural BMPs can be utilized. Facilities designed and constructed under these regulations need to provide the required stormwater services and functionality while providing the desired aesthetics with proper public health and safety considerations. Properly designed and landscaped stormwater facilities can provide an amenity to the urban environment. This positive impact can be achieved by adherence to four basic steps in the implementation of stormwater control facilities. These are:
- Proper selection of runoff control mechanisms and BMPs.
- Proper facility design; site location, landscaping, flow controls.
- Construction of facility in strict adherence to approved design plans.
- Regular maintenance program performed by a designated responsible party.
Section 2.40.2 Stormwater Runoff Control Design Criteria
Stormwater runoff control shall address peak rate, total volume and water quality. The development must have an acceptable outlet as outlined in Section 2.30. If the outlet is acceptable but does not have the capacity to accommodate the allowable release, as per Item #3 below, then a reduced discharge rate not to exceed the existing outlet capacity and additional onsite storage will be required.
- The storm drainage system shall be designed to carry a minimum 5-year frequency 24-hour duration post developed storm. The design and materials shall conform to Appendix D.
- The detention facility shall have a minimum storage capacity for a 10-year frequency 24-hour duration storm based on the post-developed site conditions.
- The peak rate of discharge from the detention area shall be held to a 5-year frequency 24-hour duration storm based on the pre-developed site conditions.
- The detention basin may also provide for the water quality (WQ) requirements. If it is designed for that additional purpose then the required water quality volume (WQV) is in addition to the 10-year storage volume as per Item #2.
- Any off-site land draining to the site must be addressed, see Section 2.20.1
Section 2.50 Stormwater Quality Control Design
Stormwater quality control facilities shall be designed to control runoff from small storm events before being discharged offsite. The design of these facilities is intended to reduce pollutants contained in stormwater runoff and to reduce streambank erosion during frequent, small storm events. The stormwater management plan for the development site shall include appropriate stormwater quality controls, the selection rationale, location and sizing calculations.
Unless exempted, stormwater quality controls should be used for runoff from development sites designed as outlined in Appendix F.
All stormwater quality control facilities shall be sized to fully capture and treat the WQv determined for the contributing drainage area. Stormwater quality facilities may be combined with stormwater quantity facilities.
Section 2.50.1 Illicit Discharge and Illegal Dumping
Any discharge into a storm sewer system that is not composed entirely of stormwater is an illicit discharge. No person shall:
- Construct, maintain, operate or utilize any illicit connection,
- Knowingly cause or allow any prohibited discharge, and
- Act, cause or permit any agent, employee, or independent contractor to construct, maintain, operate or utilize any illicit connection or cause, allow or facilitate any prohibited discharge.
Development within Erie County shall be constructed in a manner that does not result in illicit discharge into the storm system. Discharges allowable under the terms of an NPDES permit are not considered illicit discharges.
Section 2.50.2 Stormwater Quality Control Methods
The BMPs described in the RLDM are acceptable. Four categories of stormwater quality control facilities have been identified for use:
- Stormwater Basins
- Extended Dry Detention
- Extended Wet Detention
- Constructed Stormwater Wetland
- Media Filters
- Bioretention Facility
- Sand Filter
- Vegetated Swales and Filter Strips
- Commercial Proprietary Systems
- Hydrodynamic Systems
- Filtration Systems
- Catch Basin Inserts
- Chemical Treatment Systems
- Package Treatment Plants
- Prefabricated Detention Structures
Criteria to be used in determination of the proper facility include drainage area, hydrologic considerations, sediment management, health and safety, aesthetics, maintenance, accessibility, durability, cold weather issues, mosquito and vector control.
Section 2.60 Maintenance of the Stormwater Drainage and Water Quality Facilities and Control Structures
The maintenance of the storm water and drainage facilities and the flow control structures are maintained by methods in keeping with their individual characteristics. The following criteria will be used to determine how a facility and/or structure is maintained post development:
- Major residential subdivisions constructed under current county subdivision regulations are required to construct their system under Section 6131.63 of the Ohio Revised Code (ORC) and maintained under Section 6137 (ORC) by the Erie County Commissioners.
- Minor residential subdivision of 2 lots or more shall be maintained by the individual residents that rely on the drainage facility as outlined in Section 1.31 Residential Paragraph c. The developer may apply for maintenance under Section 6131.63 (ORC).
- Commercial/Industrial sites that contain 2 divided lots or more may apply for maintenance of their drainage facilities using Section 6131.63 of the ORC. If public roads are a part of the development then using Section 6131.63 is mandatory for the future maintenance of the drainage facilities. If the owner(s) of a site desires and no public road is involved then he can apply for public maintenance as per Paragraph b above. If Section 6131.63 ORC is not invoked by the owner/developer then Paragraph d, below, shall apply.
- Commercial/Industrial sites of a single lot are responsible for the future maintenance of their individual drainage facilities. Upon approval of their plan the project/design engineer shall notify the owner of his responsibility for the maintenance with the necessary documentation of the type of system and how it must be maintained to be functional.
- Permanent wet detention and/or water quality basins cannot be placed on public maintenance and must be maintain by the owner, however the inlet and outlet connected system can be publicly maintained using Section 6131.63 ORC.
Section 2.70 Drainage Plan Preparation for Small Commercial/industrial Sites
Small site developments can result in major increases in runoff, but alternatives for handling these increase flows may be limited. Wherever possible, requirements of Section 2.10 thru 2.10.7 should be utilized. In those instances where very limited options are available, the designer is required to submit plans with sufficient information to demonstrate compliance with runoff criteria and that the drainage facilities and control structures have been designed to required standards.
Section 2.70.1 Data Collection
Analysis of predevelopment conditions for drainage plan preparation requires the same data required for a management plan. Requirements outlined in Section 2.50.1 should be followed.
Section 2.70.2 Pre-Development Runoff Calculations
Upon completion of data collection, the pre-development peak rate of runoff and runoff volume can be determined for the five (5) year storm.
Section 2.70.3 Runoff Calculation and Control Criteria
In most cases for small sites, the developer will know how the surface of the site is to be developed. He can then go directly to calculation of increase of runoff and determination of runoff criteria following the steps in Section 2.10.5.
Section 2.70.4 Plan for Runoff Control
From the results of Section 2.70.3, the designer can determine the type and magnitude of control practices needed, in most cases being some type of structural control. These controls should be shown on the site plan and runoff calculations checked prepared.
Section 2.70.5 Develop Design Details
Once a development plan and drainage system and drainage system that meets the runoff criteria has been decided upon, then detail design plans for the drainage facilities and structures should be developed. It is important at this point in time to work closely with the ECEO to make sure that all facilities and structures are being designed according to these standards.
Section 2.80 Submission of a Storm Water Control Plan
The plan submission shall consist of three parts:
- Site Plan
The site plan is an overall detail of the site improvements and is a series of maps pictorially explaining the information in the narrative. - Engineering Details
The engineering details are detailed drawings, calculations and specifications on the drainage facilities and control structures to be constructed as part of the development. - Erosion Control Details
The site plan shall show placement and types of approved erosion control structures and practices as per examples and/or acceptable manuals as noted in these regulations. A typical drawing of each typed used shall show on the plan for easy reference by the ECEO reviewer and contractor.
CHECKLIST
FOR STORMWATER CONTROL PLANS
_______________________________________________________
SITE PLAN
_________ Vicinity Map –
A small map locating the site in relation to the
surrounding area.
_________ Existing Contours –
The existing contours of the site should be shown on
a map.
_________ Existing Vegetation –
The existing tree lines, grassy areas or unique
vegetation should be shown on a map.
_________ Soils –
The boundaries of the different soil types should be
shown on a map.
_________ Indicate North –
The direction of north in relation to the site should be
shown.
_________ Existing Drainage Patterns –
The dividing lines and the direction of flow for the
different drainage areas should be shown on a map.
_________ Development Plan –
Location and size of all areas to be rendered impervious.
_________ Limits of Clearing and Grading –
Areas, which are to be cleared and graded, should be
outlined on a map.
_________ Location of Drainage Facilities and Control Structures –
The location of the drainage facilities and control
structures to be used on the site should be shown on
a map. Facilities or structures subject to section 2.60
shall be delineated.
ENGINEERING DETAILS
The following shall be submitted for drainage and water quality facilities or control structures to be constructed.
_________ Design calculations ________ Design drawings
_________ Specifications ________ Quantities