1. PLANNING NEW STORM DRAINAGE FACILITIES
A. The current list of standards to be used in the design of storm drainage is an follows:
1. Construction & Material Specifications by State of Ohio Department of Transportation (latest edition);
2. Construction Standards of Allen County; and,
3. Allen County Water Management and Sediment Control Regulations.
B. Preparation and approval of plans and specifications:
1. Plans and specifications must be prepared by an Ohio Registered Professional Engineer with adherence to the current standards;
2. Plans must be submitted for approval to the City Engineer. If unusual conditions exist, actual requirements should be predetermined by a preliminary review with the City Engineer; and,
3. Plans must be submitted for approval by the administrator of the Allen County Water Management and Sediment Control Regulations.
Layout Planning
When planning a new subdivision, various drainage concepts should be evaluated before decisions are made as to site grading, street location, alignments, and block layout. Plans should be based upon incorporating natural waterways, artificial channels, storm sewers, and other drainage works into the development.
As a part of any Overall Development Plan, a fundamental study of the drainage pattern of areas contiguous to the development must be made. Tributary flows having a direct effect on the storm sewerage of the proposed subdivision shall be determined and included in the design capacities for storm conduits within the development. A map showing the drainage patterns of the surrounding basin shall be submitted with the Overall Development Plan. A U.S.G.S. topographical map may be sufficient for this presentation.
Planning the alignment of a storm sewer system should be done in connection with the street layout and grading plan. The location of catch basins, manholes, storm sewer conduits, and drainage channels shall be approved by the City Engineer. Provisions must be made to accommodate runoff from upstream areas without diversion onto neighboring properties.
System Sizing
All new storm sewer systems must be adequate to convey anticipated runoff of a watershed from a ten (10) year frequency storm at just full flow. Pressure flows for ten (10) year design storms shall be avoided. The existing outlet must be adequate to accept the additional runoff from the proposed subdivision without overloading. If the existing outlet is inadequate for such additional flow, an improved outlet or some time-release method of discharge (detention), satisfactory to the City Engineer, must be provided. Detention of storm water runoff may also be required by the Allen County Water Management and Sediment Regulations.
Pipe sizes will be determined from a hydrologic analysis using either the Rational Method or the method spelled out in the publication U.S.G.S. Techniques for Estimating Flood-Peak Discharges of Rural, Unregulated Streams in Ohio (WRI Rep 89-4126), published jointly by the U.S. Geological Survey and the Ohio Department of Transportation. (Available through ODOT Location and Design Hydraulic Section, 25 South Front Street, Columbus, Ohio, 43215).
Easements
II. RUNOFF ANALYSIS
Methods for Estimating Runoff
The total watershed that produces stormwater runoff across the site being developed, shall be included when estimating flood discharge runoff. Dependent on watershed size, three principle methods shall be used to estimate design discharge.
1. For small watersheds of 25 acres or less, the design runoff shall be determined by the Rational Method. This method is also used for catch basin hydrology.
2. For watersheds larger than 25 acres draining rural undeveloped land with no significant impervious areas and no existing storm sewers, the design runoff shall be estimated by using the method published in the U.S.G.S. Techniques for Estimating Flood-Peak Discharges of Rural, Unregulated Streams in Ohio (WRI Rep. 89-4126) (computer program available), a replacement of the ODNR Bulletin 45 method.
3. For watersheds larger than 25 acres draining developed land with significant impervious areas (parking lots, roofs), or having existing storm sewers, the design runoff shall be estimated by one of the following:
a. U.S.G.S. Estimation of Peak-Frequency Relations, Flood Hydrographs, and Volume-Duration-Frequency Relations of Ungaged Small Urben Streams in Ohio (Rep. 93-135)
b. A method approved by the City Engineer.
The Rational Method
The Rational Method for estimating peak runoff utilizes the Rational Formula, Q= C I A, where:
Q = peak runoff rate in cubic feet per second;
C = runoff coefficient corresponding to surface imperviousness;
I = rainfall intensity in inches per hour corresponding to the storm design frequency and time of concentration,
A = area of the watershed tributary to the point under design in acres.
Coefficient of Imperviousness
The following table lists runoff coefficients to be used in the Rational Formula. The selected factor shall reflect the anticipated land use according to the Planning Commission’s long range plan.
The table also lists coefficients for various surfaces which may be used to develop a composite runoff coefficient based on the percentage of different surfaces within a drainage area.
Developments on highly permeable soils, such as Spinks and certain Casco and Fox soils, with a permeability rate of six (6) inches per hour or greater, may reduce the imperviousness coefficient on agricultural, open space, and yard areas by 50 percent of the tabulated values.
The designer should refer to the Soil Survey of Allen County, Ohio, publication by the Ohio Department of Natural Resources for general locations of various types in this area.
TYPE OF DEVELOPMENT OR SURFACE | LEVEL AREA | HILLSIDE AREA 15% SLOPES |
Open Space (Parks, Golf Courses, Cemeteries, Lawns) | 0.30 | 0.30 |
Wooded Areas, Forest Litter | 0.25 | 0.30 |
Agricultural | 0.30 | 0.45 |
One Family Dwelling: Density: 3 per acre or less Density: 3 per acre or less | 0.35 0.40 | 0.45 0.60 |
Playgrounds, Schools | 0.85 | 0.90 |
Multiple Dwellings and PUD More than 20% landscaped area 20% or less landscaped area | 0.60 0.80 | 0.95 0.95 |
Commercial, Industrial, Parking Lots | 0.90 | 0.95 |
Paved Areas, Drives, Walks, Roofs | 0.95 | 0.95 |
City Engineer may require breakdown of acres by land use for evaluation of runoff rates.
Rainfall Intensity
The basis for computing rainfall intensity shall be the ten-year rainfall intensity duration curve for the Lima-Allen County area. This curve, relating the expected rainfall intensity in inches per hour, to the storm duration in minutes, is expressed in tabular form on Table A, and shall be used to estimate design discharges.
The ten year values can be converted to other recurrence years by applying the following factors:
TO CONVERT TO MULTIPLY TABLE VALUE BY | |
1 year frequency | 0.55 |
2 year frequency | 0.67 |
5 year frequency | 0.86 |
10 year frequency | 1.00 |
25 year frequency | 1.21 |
50 year frequency | 1.43 |
Time of Concentration
One of the basic assumptions underlying the Rational Method is that runoff is a function of the average rainfall rate during the time required for water to flow from the most remote part of the drainage area under consideration to the design point. The time of concentration determines the average rainfall rate on the rainfall intensity-duration-frequency curve.
Four urban storm sewers, the time of concentration consists of an inlet time, or time required for runoff to flow over the surface to the nearest inlet, and time of flow in the sewer to the point under consideration. Inlet time will vary with surface slope, depression storage, surface cover, antecedent rainfall and infiltration capacity of soil, as well as distance of surface flow. Pipe flow time can be calculated by Manning’s Formula.
Overland flow paths should not be taken perpendicular to contours on proposed subdivisions since the land will be graded and swales will often intercept the natural contour, conducting water to the street in less time.
Inlet time or the initial time of concentration for overland flows can be estimated from the following nomograph:
 |
Example: Determine the inlet time for an area where runoff from the most remote point must traverse 160 feet of lawn area (C-0.45), the 270 feet in a paved gutter to the point of concentration. Average ground slope is 2 percent and gutter slope of 1.5 percent.
From nomograph: time across lawn area = 11.8 min. Gutter time = 06.2 min. Total inlet time = 18.0 min. |
U.S. Geological Survey Techniques for Estimating Flood-Peak Discharges of Rural Unregulated Streams in Ohio (WRI Rep 89-4126) Method.
This method is used for computing flood-peak discharges with recurrence intervals between 2 and 100 years and should only be used for unregulated streams with drainage areas between 0.040 and 6,330 square miles. This method should not be used for areas where flood flows are significantly affected by regulations or where there is urban runoff.
Information pertinent to the Lima-Allen County area has been abridged from the Water-Resources Investigations Report 89-4126 (WRI Rep 89-4126) and is included herein. However, since the monitoring of various streams is a continuing effort, the designer should research supplemental sources. He should obtain the most recent data available on stream discharges which may lend application to the area under study.
The regression equations developed for estimating peak flows Qt for recurrence intervals 2, 5, 10, 25, 50, and 100 years are estimated below:
WRI Rep 89-4126 Method:
Qt = (RC) (CONTDA):*(SLOPE) Y(STORAGE+1) Z where:
Qt = Peak discharge in cubic feet per second for the “t” year recurrent interval:
RC = varying regression constants for different year storms for regions A, B, or C in Ohio. Allen County is in region B;
CONTDA = Main channel slope in feet per mile (between 1.3 and 500); computed as the difference between the elevations at 10 and 85 percent of the distance along the main channel from a specified location on the channel to the topographic divide, divided by the channel distance between the two points;
STORAGE = The percentage of the contributing drainage area occupied by lakes, ponds, and swamps (not to exceed 13%) as explicitly shown on U.S. Geological Survey 7.5 - minute topographic guadrangle maps;
x,y,z, = varying regression exponents for 2, 5, 10, 50, & 100 year storm equations.
The multiple-regression equations are applicable to each of three regions in Ohio, in which Allen County falls in region B. The appropriate regression constants and regression exponents must be selected to estimate a flood-peak discharge for specified recurrence interval at an ungaged site within region B.
Regression Constant and Regression Exponent Characteristics for Allen County
Equation Number | Peak Flow Characteristics (Qt) | Regression Constant (RC) | Contrib. Drainage Area Exp. (x) | Main Channel Slope Exp. (y) | Storage Exponent (z) |
1 | Q2 | 40.2 | 0.782 | 0.172 | -0.297 |
2 | Q5 | 58.4 | 0.769 | 0.221 | -0.322 |
3 | Q10 | 69.3 | 0.764 | 0.244 | -0.335 |
4 | Q25 | 82.2 | 0.760 | 0.264 | -0.347 |
5 | Q50 | 91.2 | 0.757 | 0.276 | -0.355 |
6 | Q100 | 99.7 | 0.756 | 0.285 | -0.363 |
EXAMPLES
1. Compute the 100-year peak discharge for a site in Allen County, Ohio
The site characteristics are:
Drainage area = 66.2 square miles Slope = 7.0 feet/mile Storage = 0.0% |
The equation to use is Qt = (RC)(CONTDA)X(SLOPE)Y(STORAGE +1)Z Q100 = 99.7(CONTDA)0.756 (SLOPE)0.285 (STORAGE+1)-0.363 = 99.7(66.200)0.756 (7.0)0.285 (0.0+1)-0.363 = 99.7 x 23.80 x 1.74 x 1.00 = 4129 cubic feet per second |
III. HYDRAULIC DESIGN CRITERIA
Roughness Coefficients | |
The following coefficients shall be used for Manning’s “n”: | |
SURFACE | “n” VALUE |
Polyvinyl Chloride (PVC) Sewer Pipe (SDR35) | 0.009 |
Polyvinyl Chloride (PVC) | 0.010 |
Polyethylene Smooth-Flow Ribbed Pipe | 0.010 |
Reinforced concrete pipe, box, or arch | 0.013 |
Clay Tile | 0.014 |
Corrugated metal or aluminum pipe | 0.024 |
Street pavement, concrete, or asphalt | 0.015 |
Concrete or asphalt line channels | 0.015 |
Gunite channel line | 0.016 |
Riprap | 0.035 |
Earth Channel with revetments or gabions | 0.035 |
Earth Channel, smoothly graded | 0.025 |
Earth Channel, sodded | 0.040 |
Natural stream channel: regular section | |
Grass, weeds, and light brush | 0.045 |
Grass, weeds, and heavy brush | 0.060 |
If trees are present in the channel, increase above values by 0.015. If irregular section with pools and channel meander, increase values by 0.015.
Flow Velocities
Minimum pipe velocities shall be three (3) feet per second.
Where pipes outlet into an erodible channel, measures shall be taken to lessen potentially destructive velocities. Erosion control devices such as stilling basins, riprap, or revetments may be required.
The following table list safe or permissible velocities for erodible channels.
MAXIMUM VELOCITY | |
CHANNEL MATERIAL | (Feet Per Second) |
Sand or sandy loam | 2.5 fps. |
Firm loam or silts | 3.5 fps. |
Clay, find gravel | 5.0 fps. |
Shale, hard pan, coarse gravel | 6.0 fps. |
VEGETAL-LINED CHANNELS (SLOPES TO 5%) | |
Alfalfa, crabgrass, lespedeza | 3.5 fps. |
Grass mixture | 5.0 fps. |
Kentucky bluegrass, brome, buffalograss | 6.0 fps. |
Data
Sufficient data shall be submitted with the construction drawings so the City Engineer can ascertain the storm sewer design adequacy. Information to be submitted shall include:
1. A map of the area to be sewered having two (2) foot contours. (Minimum scale 1 inch = 200 ft.);
2. A map of the contiguous drainage basin;
3. A layout of the area to be sewered showing existing and proposed improvements, including lot grading;
4. Information regarding anticipated land use;
5. Location and elevation of the outfall point of th storm sewer system. If outletting into an open channel or stream, sufficient information must be provided to substantiate calculations for determining the outlet water surface (hydraulic gradient);
6. Information on existing and proposed utilities, sanitary sewers, or other conflicting substructures;
7. Calculations for the determination of inlet times;
8. A storm sewer hydrology computation sheet (See Sheet D-1). A procedural example, Sheet D-2, and various pipe capacity and velocity charts, Charts D-3, D-4, and D-5, are included for use with the computation sheet; and,
9. A gutter spread and inlet capacity computation sheet. (See Sheet G-1). A procedural example, Sheets G-2a and G-2b, and various flow characteristic and inlet capacity charts, Charts G-3 to G-7, are included for use with the computation sheet.
Alignment
Storm sewers shall be on a constant grade between manholes.
All changes in direction, size, or slope of storm sewers shall be made only at manholes unless unusual conditions warrant the use of a concrete collar. In such cases, approval must be obtained from the City Engineer.
Grades
Storm sewer grades shall provide for a minimum velocity of three (3) feet per second for the design flow.
Grades shall be such that a minimum cover of three (3) feet can be maintained over the top of pipe. Minimum cover beneath roadways shall provide twelve (12) inches between the top of pipe and the bottom of subbase, with higher strength pipe being specified.
Substructure Crossings
Crossings with other major underground sewers and utilities should be on an angle greater than 45 degrees. If insufficient vertical clearance is available, a concrete cradle with or without steel reinforcement may be required. The allowable clearance without special support between storm and sanitary seers shall be twelve (12) inches.
Manholes
Manhole spacing shall conform to the following table:
PIPE SIZE | MAXIMUM SPACING |
15" or less | 350 feet |
18" or greater | 500 feet |
The inside diameter of manholes shall conform to the following table:
PIPE SIZE | MANHOLE DIAMETER |
24" or less | 4 feet |
27" to 42" | 5 feet |
For pipe sizes over 42", manhole details shall be approved by the City Engineer.
Materials
Materials for storm seers and appurtenant structures shall be approved by the City Engineer. When alternate types of materials are included for bidding purposes, hydraulic designs must be developed for each alternate material to demonstrate its acceptability.
The designer shall evaluate the trench conditions and pipe loading to determine strength classifications required for various conduits in the design.
Catch Basin Inlets
Catch basin inlets shall be curb opening with gutter grate combinations and shall be approved by the City Engineer. Any catch basin located outside of curb and gutter shall have flat grates with sufficient opening.
To determine pavement inlet spacing, the design discharge shall be based on the Rational Method using a two-year frequency, 15-minute duration design storm. For the Lima-Allen County area, the expected rainfall intensity for such a storm is 3.1 inches per hour.
The spread of water on the pavement shall be limited to eight (8) feet into the outside traveled lane for the design storm. On continuous grade streets the maximum depth of flow shall be five (5) inches.
At all intersections it will be necessary to remove one hundred percent (100%) of pavement flow to eliminate cross street flow. The inlets shall be located at the beginning of the upstream curb return before the crosswalk.
At sag locations, sufficient basins shall be provided to prevent 25-year storm flows from overtopping the street R/W lines. The capacity of grate inlets under sump conditions is presented graphically in G-7 with a sample computation illustrated on Sheet G-2a. The following explanation applies to this same computation.
After the Total Discharge (Column 7) has been determined and a basin type has been selected, the Q/P ratio is calculated. Q is the Total Discharge and P is the greater perimeter as determined from the Standard Drawing or manufacturer’s specifications. From this value, the height of water (H) over the grate can be established from G-7 along line (a). Using this H value then, the spread is determined from the Flow Characteristic Chart G-3.
The maximum spacing between catch basins or from vertical crest point shall be 350 feet on curb and gutter streets and 450 feet on streets with roadside ditches.
Connector Pipes
All catch basin connector pipes shall connect to the main line at manholes unless otherwise approved by the City Engineer.
The connector pipes shall be hydraulically sized to the catch basin capacity. Minimum size shall be 12 inches. Connector pipes under pavement shall be reinforced concrete pipe.
Direction changes shall not be designed between structures except where concrete collars may be necessary to avoid major substructure interference. Such designs will require approval of the City Engineer.
Culvert Design
All culverts shall be designed with a uniform barrel cross section throughout their length. Location alignment, material specifications, and end treatments, (e.g., headwalls, wingwalls, riprap, apron slabs), shall be approved by the City Engineer.
The design discharge shall be computed for a 50-year frequency storm either by the Rational Method or the U.S.G.S. Techniques for Estimating Flood-Peak Discharges of Rural, Unregulated Streams in Ohio (WRI Rep 89-4126) Method, whichever is applicable with respect to drainage area.
All new culvert designs must be of adequate capacity to convey anticipated runoff of a watershed from a fifty (50) year frequency storm at just full flow, using Manning’s equation for capacity calculations. Pressure flows for fifty 50) year design storms shall be avoided. Culverts shall meet manufacturer’s recommendations for minimum and maximum cover, for bedding, and for backfilling. Flow line elevations of proposed culverts may be required by the City Engineer to be up to 1.5 feet below existing ditch flow line, for anticipated ditch clean-outs.
Computation of culvert hydraulics and barrel sizing shall be prepared by a Professional Engineer and shall be reviewed and approved by the City Engineer.
Example: given a “Q” of 1.0 c.f.s., find Normal and Critical Depths for a 12" concrete pipe, “n” -.015, on a slope of .01. For Normal Depth “D”, follow the vertical discharge line marked 1.0 to the intersection of the slope line marked .01. From that point, follow horizontally, intersecting 12" normal depth curve. Thence from that intersection vertically to top of chart, reading a normal depth of 0.36 feet. For Critical Depth “Dc”, follow the vertical discharge line marked 1.0 to the intersection of 12" critical depth curve. From that point, follow horizontally to the left margin, reading a critical depth of 0.42 feet.
Normal depth is the depth at which water will flow in a pipe by virtue of its slope and roughness based on Manning’s Formula A=(1.486/n)(R 2/3) (S 1.2) A.
Critical depth is the depth at which point the control for determining the headwater changes. Always use n=.015 Discharge Scale to determine critical depth.
Example: Given a “Q” of 70 c.f.s., find the velocity in a 48" concrete pipe on a slope of 0.004.
From the Pipe Capacity Curve, the normal depth reads 3.0 feet. Enter Chart c and follow the vertical normal depth line marked 3.0 feet to the 48" curve intercept. From this intersection move horizontally to the left margin reading a velocity of 10 f.p.s. This is the velocity for 100 c.f.s. The velocity for 70 c.f.s. will equal 10 x *70.100) or 7.0 f.p.s.