Section 250-RICR-150-10-8.11 - Minimum Standard 5: Overbank Flood Protection (Qp)A. Downstream overbank flood protection must be provided by attenuating the post-development peak discharge rate to the pre-development levels for the 10-year and 100-year, 24-hour Type III design storm events. In addition, designers must demonstrate that runoff from the site for storms up to the 100-year, 24-hour Type III design storm events actually reach proposed structural practices designed to meet this criterion.B. Peak flow attenuation is required for the 10-year and 100-year, 24-hour Type III design storm events.C. For facility sizing criteria, the basis for hydrologic and hydraulic evaluation of development sites are as follows: 1. The models TR-55, incorporated above at § 8.4(A) of this Part, and TR-20, incorporated above at § 8.4(B) of this Part, (or approved equivalent) will be used for determining the required storage and outlet structures for attenuating the peak flows from the 10-year and 100-year, 24-hour Type III design storms.2. The standard for characterizing pre-development land use for on-site areas shall be woods, meadow, or rangeland. For agricultural land, use a CN representing rangeland.3. For purposes of computing runoff, all pervious lands prior to development shall be assumed to be in good condition regardless of conditions existing at the time of computation.4. If an off-site area drains to a facility, an applicant must also demonstrate safe passage of the 100-year event, based on actual conditions upstream.5. The length of sheet flow used in tc calculations is limited to no more than 150 feet for pre-development conditions and 100 feet for post-development conditions.6. An applicant must demonstrate that flows from the 100-year event will be safely conveyed to a practice designed to manage the 100-year event.D. The Overbank Flood Protection criterion can be waived for sites that: 1. Direct discharge to a large river (4th-order stream or larger), bodies of water > 50.0 acres in surface area, or tidal waters.2. A downstream analysis indicates that peak discharge control would not be beneficial or would exacerbate peak flows in a downstream tributary of a particular site (i.e., through coincident peaks).E. A downstream analysis is required for projects meeting the project size and impervious cover characteristics in the table in § 8.11(E)(1) of this Part, or when deemed appropriate by the approving agency when existing conditions are already causing a problem, to determine whether peak flow impacts are fully attenuated by controlling the 10- and 100-year events. The criterion used for the limit of the downstream analysis is referred to as the "10% rule." Under the 10% rule, a hydrologic and hydraulic analysis is extended downstream to the point where the site represents 10% of the total drainage area. 1. Table 3-5. Projects for Which a Downstream Analysis Is Required Area of Disturbance Within the Subwatershed (acres) | Impervious Cover (%) (I / disturbed area contributing to discharge locations) |
>5 to10 | >75 |
>10 to 25 | >50 |
>25 to 50 | >25 |
>50 | all projects |
2. If flow rates and velocities (for Qp) with the proposed detention facility increase by less than 5% from the pre-developed condition, and no existing structures are impacted, then no additional analysis is necessary. If the flow rates and velocities increase by more than 5%, then the designer must redesign the detention structure, evaluate the effects of no detention structure, or propose corrective actions to the impacted downstream areas. Additional investigations may be required by the approving agency on a case-by-case basis depending on the magnitude of the project, the sensitivity of the receiving water resource, or other issues such as past drainage or flooding complaints.3. Stormwater designers must be able to demonstrate that runoff will not cause downstream flooding within the stream reach to the location of the 10% rule. The absence of on-site detention shall not be perceived to waive or eliminate groundwater recharge (Rev), water quality control (WQv), or stream channel protection requirements (CPv).4. A typical downstream analysis will require a hydrologic investigation of the disturbed area draining to a proposed detention facility and of the contributory watershed to the location of the 10% rule for the 10- and 100-year, 24-hour Type III storms. The approving agency may also request analysis of the 1-year, 24-hour Type III storm on a case-by-case basis. A hydraulic analysis of the stream channel below the facility to the location of the 10% rule will also be necessary. Depending on the magnitude of the impact and the specific conditions of the analysis, additional information and data may be necessary such as collecting field run topography, establishing building elevations and culvert sizes or investigating specific drainage concerns or complaints.250 R.I. Code R. 250-RICR-150-10-8.11
Amended effective 11/13/2018