Document: NRC Regulatory Guide
Document ID: 0b3f08e9-b342-468d-a869-a00f19278ecb
Document Type: regulatory_guide
Title: Normal Water Level and Discharge at Nuclear Power Plants
Source: NRC Regulatory Guide Division 1
Source URL: https://www.nrc.gov/docs/ML0037/ML003740143.pdf
Revision Date: 2023-06
Chapter: 
Section ID: RG-1.135
CFR Part: 
CFR Title: 

Content:
evel (or discharge) that can be expected at the time of occurrence of design basis natural or accidental events.' Subject to the excep tions below, the normal water level (or discharge) will generally be the median water level (or discharge), i.e., that water level (or discharge) that will be equal led or exceeded 50% of the time. 1. Seasonal Occurrence of Design Basis Events If the design basis event under consideration is seasonal, the normal water level (or discharge) should be determined for that part of the year in which the event is likely to occur. 2. Nontidal Streams The normal discharge should be the median dis charge based on a period of mean daily discharge record, preferably at least 50 years in length. The nor mal water level should be either (a) the median mean daily water level based on a record of preferably at least 50 years or (b) the water level corresponding to ' See Footnote 1. 1.135-4 normal discharge, including consideration of shifting rating curves for streams with movable beds. A shorter period of record (not less than 12 years) is ac ceptable if longer records do not exist and it can, be demonstrated that the period used includes major wet and dry periods. 3. The Great Lakes Normal water level should be the median mean monthly level based on the latest 50 years of record, with suitable adjustment for demonstrated long-term trends. 4. Unregulated Lakes (Other Than the Great Lakes) Normal water level should be the median based preferably on 50 years of data. Unless it can be demonstrated that the median based on mean monthly levels is sufficiently accurate, mean daily data should be used. If long-term trends can be iden tified, the 50-year median should be adjusted for these trends. A shorter period of record (not less than 12 years) is acceptable if longer records do not exist and it can be demonstrated that the period used in cludes major wet and dry periods. 5. Regulated Lakes and Reservoirs Normal water level should be the designated nor