(a) Drainage. A drainage system shall be designed and constructed by the subdivider to provide for the proper drainage of the surface water of the subdivision and the drainage area of which it is a part. To this end the subdivision shall be graded as follows:
(1) Drainage plan.
A. Prior to the start of any construction (houses or streets) the subdivider shall furnish a plot plan showing by use of arrows how he proposes to surface drain each lot. This information shall also be shown on the street plan and profile sheets.
B. The subdivider shall submit topographic maps showing the area to be drained with calculations prepared by a registered civil engineer in determining the proposed storm water collection system. The maps shall be checked and approved by the Engineer.
(2) Drainage requirements (Grading). The subdivider shall grade each subdivision in order to establish street block and lot grades in proper relation to each other and to existing topography as follows:
A. Street grading plan. A grading plan shall be prepared for streets along with street improvement details. The grading of the roadway shall extend the full width of the right-of-way. Planting strips shall be graded at a gradient of not less than two percent (2%) or more than three percent (3%) upward from the curb to the sidewalk of property line.
B. Block grading. Block grading shall be as follows:
1. From a high point of the lot draining into the streets or from drainageway.
2. Parts of all lots draining to a natural waterway, or drainage system along rear lot lines and portion of all lots draining to the street.
3. Draining across rear or side lot lines provided that drainage onto adjoining properties shall be engineered by a professional engineer and inspected by the City.
C. Lot grading. Lot grading shall be as follows: Any open storm water drainage course or basin which is adjacent to the rear, side or front of a dwelling shall be a minimum of 50 feet from the dwelling at the top bank of drainage course or basin and the slope shall be a minimum of 4 to 1. This includes all drainage courses or basin, active and non-active and if the Engineer requires slope protection, it shall be included.
1. Lots shall be graded so that water drains away from each building at a minimum grade of one percent (1%) to a storm water swale.
2. Surface storm water drainage swales shall have a minimum grade of one-half percent (0.5%) and shall be designed so that surface water will drain into a driveway, street gutter, storm sewer, drain inlet or natural drainage course.
3. The minimum grades of driveways shall be four-tenths percent (0.4%) and a maximum of ten percent (10%).
(b) Topsoil and Ground Cover. If grading results in the stripping of topsoil, the developer/builder shall uniformly spread to a depth of two inches over the lot or lots as grading is finished. Where areas are filled or vegetation is removed, the areas shall be fertilized and seeded with suitable ground cover to prevent erosion. If any area remains stripped of vegetation in excess of 45 days, this entire area shall be seeded. If construction is in progress and the driveway and sidewalks are not installed within 180 days the City may use the construction bond money to install any private sidewalk or driveway and plant a lawn in the front yard and side yards.
(c) Trees. As many trees as can be reasonably utilized in the final development plan shall be retained and the grading adjusted to the existing grade at the trees. Existing trees shall be shown on the Grading Plan.
(d) Road Drainage System. The road drainage sewer system shall serve as the prime drainage system. It shall be designed to carry roadway, adjacent land and house storm water drainage.
(1) Road storm sewers. The design discharge used to determine pavement inlet spacing shall be based on the following engineering specifications.
Design Criteria for Storm Sewers.
Quantity of runoff shall be determined by the Rational Method.
Design Storm Frequency | |
Residential | 10 Year Frequency |
Multi-family Schools Industrial/Commercial | 10 Year Frequency |
Downtown or Major Urban Business Area | 25 Year Frequency |
Additional Minimum Criteria
Flow between 0 - 150 cfs | 5 Year Frequency |
150 cfs - 500 cfs | 10 Year Frequency |
500 cfs - 1500 cfs | 25 Year Frequency |
1500 cfs - and over | 50 Year Frequency |
Rainfall Intensity - Duration
5 year storm | i = 1.50 inches/hr. |
10 year storm | i = 1.80 inches/hr. |
25 year storm | i = 2.00 inches/hr. |
50 year storm | i = 2.25 inches/hr. |
100 year storm | i = 2.50 inches/hr. |
Runoff Coefficient
Zoning | Lot Area (ft. 2) | C= |
Residential | 0 - 5000 | 0.7 |
5,000 - 10,000 | 0.6 | |
10,000 - 25,000 | 0.5 | |
25,000 - and over | 0.4 | |
Multi Family | 0.75 | |
Schools | ||
Industrial/Commercial | 0.90 | |
Shopping Centers | 0.90 | |
Major Urban | ||
Business Areas | 0.09 | |
The above runoff coefficients assume typical ground cover and average slope.
Residential Areas
The concentration times to the critical inlet varies between 12 and 20 minutes with 15 minutes to be used as the general case based upon full development of the land.
Industrial - Multi-family - school Areas
The concentration time to the critical inlet varies between 10 and 15 minutes with 12.5 minutes to be used as the general case based upon full development of the land.
Major Urban Business Areas and Shopping Centers
The concentration time to the critical inlet varies between 5 and 12 minutes with 10 minutes used as the general case based upon full development of the land.
Standard Rainfall Intensity-Duration Tables
The Standard Rainfall Intensity-Duration Tables shall be used to determine the rainfall intensity occurring at the time of concentration to the inlet under consideration.
The Standard Rainfall Intensity - Duration Tables
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(e) Off-Road Drainage System. The design of the off-load drainage system shall include the watershed affecting the allotment and shall be extended to a watercourse adequate to receive the storm drainage. Based on the following requirements in subsections (f), (g) and (h) hereof.
(f) Drainage Easements. Easements for drainage purposes shall be a minimum of twenty feet (20') in width. Where the watercourse is large, easement widths shall be increased as determined by the Engineer. Where watercourse crosses plotted lots diagonally, the subdivider shall straighten such courses where practicable and shall substantially follow sublot lines. Easements shall be shown on the record plat and deeds, and shall cover all existing or reconstructed watercourses. Additional easement requirements shall be as follows:
(1) Easements for drainage purposes shall be staked out or be laid out on the ground before construction is started.
(2) In instances where the existing watercourse is well defined as a result of past storm drainage, the Planning Commission may require the subdivider to plat the lots to take advantage of the natural watercourse. Where it is not feasible to so plat such lots, the subdivider is required to reconstruct the watercourse along sublot lines; such reconstruction shall be piped in conduit and shall be completed to the satisfaction of the Engineer.
(g) Protection of Drainage Systems.
(1) All adjoining land where the vegetation has been injured or destroyed or is in need of protection to prevent erosion, deposits in the drainage facilities, and/or unsightly conditions shall be restored and protected as directed by the Engineer.
(2) In all cases, any drainage facility within the subdivision shall be in a stable condition, free from either erosion or sedimentation, and/or other debris.
(h) Pipe Policy. All pipes shall be installed as specified by the Subdivision General Notes and Standard Details of the Municipality of Twinsburg.
(Ord. 173-2000. Passed 12-17-00.)