The following shall govern the design of any improvement with respect to the detention of stormwater runoff.
(A) Acceptable detention methods. The increased stormwater runoff resulting from a proposed development shall be detained on-site by the provision of appropriate wet or dry bottom reservoirs, parking lots, streets, lawns, or other acceptable techniques. Measures which retard the rate of overland flow and the velocity in runoff channels shall also be used to partially control the runoff rate. Detention basins shall be sized to store excess flows from storms with a 100-year return period. Control devices shall limit the discharge to a rate no greater than that prescribed by this chapter (see § 55.40(E) and (F)).
(B) Design storm. Design of stormwater detention facilities shall be based on a return period of once in 100 years. The storage volume and outflow rate shall be sufficient to handle stormwater runoff from a critical duration storm, as defined in § 55.40 (E) and (F). Rainfall depth-duration-frequency relations and intensity-duration- frequency relations shall be those given in Tables 5 and 5A.
(C) Allowable release rate.
(2) In the event the natural downstream channel or storm sewer system, such as Flint Creek, is inadequate to accommodate the release rate provided above, then the allowable release rate shall be reduced to that rate permitted by the capacity of the receiving downstream channel or storm sewer system, and additional detention, as determined by the City Engineer, shall be required to store that portion of the runoff exceeding the capacity of the receiving sewers or waterways.
(3) 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 proportion to the ratio of its drainage area to the drainage area of the entire watershed upstream of the restriction.
TABLE 5 RAINFALL DEPTHS FOR VARIOUS RETURN PERIODS AND STORM DURATIONS FOR LAFAYETTE | ||||||
Depth (Inches) | ||||||
Duration | Return Period (Years) | |||||
2 | 5 | 10 | 25 | 50 | 100 | |
TABLE 5 RAINFALL DEPTHS FOR VARIOUS RETURN PERIODS AND STORM DURATIONS FOR LAFAYETTE | ||||||
Depth (Inches) | ||||||
Duration | Return Period (Years) | |||||
2 | 5 | 10 | 25 | 50 | 100 | |
5 minutes | 0.42 | 0.52 | 0.59 | 0.68 | 0.75 | 0.82 |
10 minutes | 0.64 | 0.79 | 0.91 | 1.04 | 1.15 | 1.25 |
15 minutes | 0.80 | 0.99 | 1.13 | 1.29 | 1.43 | 1.55 |
20 minutes | 0.95 | 1.17 | 1.34 | 1.53 | 1.70 | 1.85 |
30 minutes | 1.11 | 1.37 | 1.57 | 1.79 | 1.98 | 2.16 |
40 minutes | 1.23 | 1.52 | 1.74 | 1.99 | 2.20 | 2.40 |
50 minutes | 1.33 | 1.64 | 1.87 | 2.14 | 2.36 | 2.58 |
60 minutes | 1.40 | 1.73 | 1.97 | 2.26 | 2.49 | 2.72 |
1.5 hours | 1.70 | 2.09 | 2.39 | 2.72 | 3.03 | 3.30 |
2 hours | 1.71 | 2.11 | 2.41 | 2.76 | 3.05 | 3.33 |
3 hours | 1.84 | 2.27 | 2.60 | 2.97 | 3.29 | 3.59 |
4 hours | 2.06 | 2.54 | 2.90 | 3.31 | 3.67 | 4.00 |
5 hours | 2.16 | 2.66 | 3.04 | 3.48 | 3.85 | 4.20 |
6 hours | 2.22 | 2.74 | 3.13 | 3.58 | 3.96 | 4.32 |
7 hours | 2.31 | 2.85 | 3.26 | 3.72 | 4.12 | 4.49 |
8 hours | 2.35 | 2.90 | 3.32 | 3.79 | 4.20 | 4.58 |
9 hours | 2.42 | 2.99 | 3.41 | 3.90 | 4.32 | 4.71 |
10 hours | 2.49 | 3.07 | 3.51 | 4.01 | 4.44 | 4.84 |
12 hours | 2.58 | 3.18 | 3.63 | 4.15 | 4.59 | 5.01 |
14 hours | 2.66 | 3.29 | 3.76 | 4.30 | 4.75 | 5.18 |
16 hours | 2.75 | 3.40 | 3.68 | 4.44 | 4.91 | 5.39 |
18 hours | 2.82 | 3.48 | 3.98 | 4.55 | 5.03 | 5.49 |
20 hours | 2.89 | 3.56 | 4.07 | 4.65 | 5.15 | 5.62 |
24 hours | 3.00 | 3.70 | 4.23 | 4.83 | 5.35 | 5.83 |
TABLE 5A RAINFALL INTENSITIES FOR VARIOUS RETURN PERIODS AND STORM DURATIONS FOR LAFAYETTE | ||||||
Intensity (Inches/Hour) | ||||||
Duration | Return Period (Years) | |||||
2 | 5 | 10 | 25 | 50 | 100 | |
TABLE 5A RAINFALL INTENSITIES FOR VARIOUS RETURN PERIODS AND STORM DURATIONS FOR LAFAYETTE | ||||||
Intensity (Inches/Hour) | ||||||
Duration | Return Period (Years) | |||||
2 | 5 | 10 | 25 | 50 | 100 | |
5 minutes | 5.04 | 6.24 | 7.08 | 8.16 | 9.00 | 9.84 |
10 minutes | 3.84 | 7.74 | 5.46 | 6.24 | 6.90 | 7.50 |
15 minutes | 3.20 | 3.96 | 4.52 | 5.16 | 5.72 | 6.20 |
20 minutes | 2.85 | 3.51 | 4.02 | 4.59 | 5.10 | 5.55 |
30 minutes | 2.22 | 2.74 | 3.12 | 3.58 | 3.96 | 4.32 |
40 minutes | 1.85 | 2.28 | 2.61 | 2.99 | 3.30 | 3.60 |
50 minutes | 1.60 | 1.97 | 2.24 | 2.57 | 2.83 | 3.10 |
60 minutes | 1.40 | 1.73 | 1.97 | 2.25 | 2.49 | 2.72 |
1.5 hours | 1.13 | 1.39 | 1.59 | 1.82 | 2.02 | 2.20 |
2 hours | 0.86 | 1.06 | 1.21 | 1.38 | 1.53 | 1.67 |
3 hours | 0.61 | 0.76 | 0.87 | 0.99 | 1.10 | 1.20 |
4 hours | 0.52 | 0.64 | 0.73 | 0.83 | 0.92 | 1.00 |
5 hours | 0.43 | 0.53 | 0.61 | 0.70 | 0.77 | 0.84 |
6 hours | 0.37 | 0.46 | 0.52 | 0.60 | 0.66 | 0.72 |
7 hours | 0.33 | 0.41 | 0.47 | 0.53 | 0.59 | 0.64 |
8 hours | 0.29 | 0.36 | 0.42 | 0.47 | 0.53 | 0.57 |
9 hours | 0.27 | 0.33 | 0.38 | 0.43 | 0.48 | 0.52 |
10 hours | 0.25 | 0.31 | 0.35 | 0.40 | 0.44 | 0.48 |
12 hours | 0.22 | 0.27 | 0.30 | 0.35 | 0.38 | 0.42 |
14 hours | 0.19 | 0.24 | 0.27 | 0.31 | 0.34 | 0.37 |
16 hours | 0.17 | 0.21 | 0.24 | 0.28 | 0.31 | 0.34 |
18 hours | 0.16 | 0.19 | 0.22 | 0.25 | 0.25 | 0.31 |
20 hours | 0.14 | 0.18 | 0.20 | 0.23 | 0.26 | 0.28 |
24 hours | 0.13 | 0.15 | 0.18 | 0.20 | 0.22 | 0.24 |
(D) Drainage system overflow design. Drainage systems shall have adequate capacity to convey the stormwater runoff from all upstream tributary areas through the development under consideration for a storm of 100-year design return period, calculated on the basis of the upstream land in its present state of development. An allowance, equivalent to the reduction in flow rate provided, shall be made for upstream detention, when such upstream detention and release rate have previously been approved by the city or county, and evidence of its construction can be shown.
(E) Determination of storage volume - Rational Method. For areas of 200 acres or less, the Rational Method may be used to determine the required volume of stormwater storage. The following eleven-step procedure may be used to determine the required volume of storage. Other design methods may also be used, subject to approval of the City Engineer, and as described in § 55.40(F).
Steps Procedure
1. Determine total drainage area in acres “A.”
2. Determine composite runoff coefficient “Cu,” based on existing land use (undeveloped).
3. Determine time of concentration “tc” in minutes, based on existing conditions.
4. Determine rainfall intensity “Iu” in inches per hour, based on time of concentration, and using Figure 1 or data given in Table 5A for the ten-year return period.
5. Compute runoff, based on existing land use (undeveloped) and ten-year return period:
Qu = CuIuA
6. Determine composite runoff coefficient “Cd,” based on developed conditions and a 100-year return period.
7. Determine the 100-year return period rainfall intensity "Id" for various storm durations “td” up through the time of concentration for the developed area using Table 5A.
8. Determine developed inflow rates “Qd” for various storm durations “td,” measured in hours.
Qd = Cd Id A
9. Compute a storage rate “Std” for various storm durations “td” up through the time of concentration of the developed area.
Std = Qd - Qu
10. Compute required storage volume “SR” in acre-feet for each storm duration “td.” This assumes a triangular hydrograph of duration (2*td) hours, with the peak flow of “Std” at “td” hours.
SR = Std (td/12)
11. Select the largest storage volume computed in step 10 for detention basin design.
(F) Determination of storage volume - other methods. Methods other than the Rational Method for determining runoff and routing of stormwater may be used to determine the storage volume required to control stormwater runoff. The procedures or methods used must receive the prior approval of the City Engineer. The ILLUDAS, TR-20 and TR-55 models are approved by the Commission as appropriate only in analysis of the runoff and routing of stormwater. The use of these models or other approved procedures can be defined in a seven-step procedure to determine the required storage volume of the detention basin.
Step Procedure
1. Calibrate the hydrologic/hydraulic model that is to be used for prediction of runoff and routing of stormwater.
2. For each storm duration listed in Table 5, perform steps three through six.
3. Determine the two-year, undeveloped peak flow. Denote this flow by Qu10.
4. Determine the 100-year runoff hydrograph (Hd100) for developed conditions.
5. Determine the hydrograph that must be stored (Hs100) by subtracting a flow up to Qu10 from the hydrograph (Hd100) found in Step 4.
6. Determine the volume of water (Vs) to be stored by calculating the area under the hydrograph Hs100.
7. The detention basin must be designed to store the largest volume (Vs) found for any storm duration analyzed in step 6.
(G) General detention basin design requirements.
(1) Basins shall be constructed to detain temporarily the stormwater runoff which exceeds the maximum peak flow rate authorized by this chapter. The volume of storage provided in these basins, together with such storage as may be authorized in other on-site facilities, shall be sufficient to control excess runoff from the 100-year storm.
(2) The following design principles shall be observed:
(a) The maximum volume of water stored, and subsequently released at the design release rate, shall not result in a storage duration in excess of 48 hours, unless additional storms occur within the period.
(b) The maximum planned depth of stormwater stored (without a permanent pool) shall not exceed four feet.
(c) All stormwater detention facilities shall be separated by not less than 25 feet from any building or structure to be occupied, and ten feet from any right-of-way line, sanitary sewer, or potable water line.
(d) All excavated excess spoil may be spread so as to provide for aesthetic and recreational features, such as sliding hills, sports fields, etc. Slopes no steeper than 2 horizontal to 1 vertical for safety, erosion control, stability and ease of maintenance shall be permitted.
(e) To prevent children or large animals from crawling into such structures, safety screens having a maximum opening of four inches shall be provided for any pipe or opening.
(f) Danger signs shall be mounted at appropriate locations to warn of deep water, possible flooding conditions during storm periods and other dangers that exist. Fencing shall be provided if deemed necessary by the Commission.
(g) Outlet control structures shall be designed to operate as simply as possible, and shall require little or no maintenance and/or attention for proper operation. They shall limit discharges into existing or planned down stream channels or conduits so as not to exceed the predetermined maximum authorized peak flow rate.
(h) Emergency overflow facilities, such as a weir or spillway, shall be provided for the release of exceptional storm runoffs or, in emergency conditions, should the normal discharge devices become totally or partially inoperative. The overflow facility shall be of such design that its operation is automatic and does not require manual attention.
(i) Grass or other suitable vegetative cover shall be provided throughout the entire basin area. Grass should be cut regularly at approximately monthly intervals during the growing season or as required.
(j) Debris and trash removal and other necessary maintenance shall be performed on a regular basis to assure continued operation in conformance to design.
(k) A report shall be submitted to the Commission describing: 1. the proposed development; 2. the current land use conditions; 3. the method of hydraulic and hydrologic analysis used, including any assumptions or special conditions; 4. the results of the analysis; and 5. the recommended drainage control facilities. Hydraulic and hydrologic calculations, including input and output files, shall be included as appendices to the report.
(H) Dry bottom basin design requirements. Detention basins which will not contain a permanent pool of water shall comply with the following requirements:
(1) Provisions shall be incorporated to facilitate complete interior drainage of dry bottom basins, to include the provisions of natural grades to outlet structures, longitudinal and transverse grades to perimeter drainage facilities, paved gutters, or the installation of subsurface drains.
(2) The detention basin shall, whenever possible, be designed to serve a secondary or multipurpose function. Recreational facilities, aesthetic qualities (open spaces) or other types of use shall be considered in planning the detention facility.
(I) Wet bottom basin design requirements. Where part of a detention basin will contain a permanent pool of water, all the items required for detention storage shall apply, except that the system of drains with a positive gravity outlet required to maintain a dry bottom basin will not be required. A controlled positive outlet will be required to maintain the design water level in the wet bottom basin, and provide required detention storage above the design water level. However, the following additional conditions shall apply:
(1) Basins designed with permanent pools or containing permanent ponds shall have a water area of at least one-half acre. If fish are to be maintained in the pond, a minimum depth of approximately ten feet shall be maintained over at least 25 percent of the pond area. The remaining pond area shall have no extensive shallow areas, except as required by subdivision (3) below.
(2) In excavated ponds, the underwater side slopes in the pond shall be stable. In the case of valley storage, natural slopes may be considered to be stable.
(3) A safety ledge four to six feet in width is required, and must be installed in all ponds approximately 30 to 36 inches below the permanent water level. In addition, a similar maintenance ledge 12 to 18 inches above the permanent water line shall be provided. The slope between the two ledges shall be stable and of a material, such as stone or riprap, which will prevent erosion due to wave action.
(4) A safety ramp exit from the pond is required in all cases, and shall have a minimum width of 20 feet and an exit slope of 6 horizontal to 1 vertical. The ramp shall be of material that will prevent its deterioration due to vehicle use and/or wave action.
(5) Periodic maintenance is required in ponds to control weed and larval growth. The pond shall also be designed to provide for the easy removal of sediment, which will accumulate during periods of pond operation. A means of maintaining the designed water level of the pond during prolonged periods of dry weather is also required.
(6) For emergency use, basin cleaning or shoreline maintenance, facilities shall be provided or plans prepared for auxiliary equipment to permit emptying and drainage.
(7) Facilities to enhance and maintain pond water quality shall be provided, if required to meet applicable water quality standards. Design calculations to substantiate the effectiveness of these aeration facilities shall be submitted with final engineering plans. Agreements for the perpetual operation and maintenance of aeration facilities shall be prepared to the satisfaction of the Commission.
(J) Parking lot storage. Paved parking lots may be designed to provide temporary detention storage of stormwaters on all, or a portion of, their surfaces. Outlets will be designed so as to empty the stored waters slowly. Depths of storage must be limited to a maximum depth of six inches, so as to prevent damage to parked vehicles, and so that access to parked vehicles is not impaired. Ponding should, in general, be confined to those portions of the parking lots farthest from the area served.
(K) Facility financial responsibilities. The construction cost of stormwater control systems and facilities as required by this chapter shall be accepted as part of the cost of land development. If general public use of the facility can be demonstrated, negotiations for public participation in the cost of such development may be considered.
(L) Facility maintenance responsibility. Maintenance of detention/retention facilities during construction and thereafter shall be the responsibility of the land developer/owner. Responsibility for maintaining facilities serving more than one lot or holding shall be documented by substantially the following appropriate covenants to property deeds to be executed by the landowner and property indexed and placed of record, at the landowner's expense, unless responsibility is formally accepted by a public body, all before the final drainage plans are approved.
"The undersigned, hereinafter the Grantor, owner of the following described real estate, hereinafter referred to as "Real Estate," located in Huntington County, Indiana, to wit:
(legal description of the tract of land being impressed with maintenance burden)
Plans to construct a storm water detention system on a portion of the above-described real estate as specified in development plans, filed with the Community Development and Redevelopment Department (CDR) and the City of Huntington Engineering Office (CHEO).
The City of Huntington acting by and through CHEO, the grantee, as the governmental entity having power and authority to regulate storm water drainage facilities and systems on real property in the City of Huntington, Indiana, desires that the Grantor, and all of its grantor's successors in interest to the Real Estate, maintain and keep in good repair the storm water detention system located on the Real Estate.
In order to induce CHEO to approve the above-referenced development plans, the Grantor for Grantor and Grantor's successors in ownership of real estate does hereby covenant and agree that the Grantor will construct the storm water detention system in accordance with the above-referenced plans and be responsible for its perpetual maintenance, repair, or replacement if necessary. Maintenance shall include structural as well as cosmetic activities, including but not limited to, mowing and weed-, algae-, and mosquito-control. All maintenance will be done so as to assure that storm runoff will be detained and that the rate of runoff will not be increased after the improvements have been constructed as contemplated.
CHEO, or its successor agency, shall have the right of entry over, across, and through the Real Estate for the purpose of inspecting, evaluating, maintaining, or repairing the storm water detention basin. CHEO, or its successor agency, shall have the right to order Grantor, and its successors in interest to the Real Estate, to perform obligations of normal and emergency maintenance, repairs, and/or replacement of the storm water drainage and detention facilities. In the event the Grantor fails to make appropriate corrections within thirty (30) days of receiving notification from CHEO in writing of the needed repairs or maintenance, then the City, acting by and through the CHEO, shall have the right, but not the duty, to enter upon the Real Estate and perform such obligations of the Grantor and to collect from Grantor the reasonable value thereof.
The Grantor hereby covenants to indemnify and save harmless the City of Huntington, and its appointed and elected officers, employees, and contractors, from and against all loss or expense for the design, construction, maintenance, and operation of the storm water drainage facility, including but not limited to, judgments, settlements, attorney's fees and costs by reason of claims and/or demands upon the City or its contractors for damages because of personal or bodily injury, including death and property damage at any time resulting therefrom, sustained by any persons on account damage to property due to negligence of the Grantor's employees, contractors, or agents and their successors.
The covenants contained herein shall constitute covenants to run with all the land comprising the Real Estate and shall be binding upon the Grantor and all other persons and parties claiming through the Grantor, and shall be a limitation on all future owners of said Real Estate."
Dated this day of , 20
BY
TITLE
"Grantor"
ACKNOWLEDGMENT
STATE OF INDIANA )
) SS
COUNTY OF HUNTINGTON )
Before me, the undersigned, a Notary Public, in and for said County and State personally appeared as for and acknowledged the execution of the foregoing Document as and for Grantor's voluntary act and deed for the uses and purposes therein contained.
WITNESS my hand and notarial seal this day of , 20 .
Notary Public
Resident of County
(Printed Name of Notary)
My Commission Expires:
Prepared By:
(M) Joint development of control systems. Stormwater control systems may be planned and constructed jointly by two or more developers as long as compliance with this chapter is maintained. Assignment of responsibility for maintaining a jointly constructed system shall be submitted to the Commission for review and approval.
(N) Installation of control systems. Erosion control systems shall be installed on the perimeter of the site prior to the commencement of any site excavation. Stormwater management systems shall be installed, and made operational as designed, prior to the placement of any hard surface, such as a building slab or parking area. Detention/retention basins shall be designed with an additional six percent of available capacity, to allow for sediment accumulation resulting from development, and to permit the pond to function for reasonable periods between cleanings. Basins should be designed to collect sediment and debris in specific locations so that removal costs are kept to a minimum.
(O) 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 the location unless compensatory storage is provided.
(P) Off-site drainage provisions. When the allowable runoff is released in an area that is susceptible to flooding, the developer may be required to construct appropriate storm drains through such area, to avert increased flood hazard caused by the concentration of allowable runoff at one point, instead of the natural overland distribution. The requirement of off-site drains shall be at the direction of the Commission.
(Ord. 16-C-98, passed 6-9-98; Am. Ord. 22-C-98, passed 8-11-98; Am. Ord. 22-C-04, passed 11-9-04; Am. Ord. 3-C-05, passed 3-15-05; Am. Ord. 20-C-14, passed 7-29-14)