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:
method requires the establishment of the relationship between river level and discharge (the rating curve) and the application of that relationship to the normal discharge. If the stream has a movable bed, allowance should be made for shifting of the rating curve. 3. The Great Lakes The levels of some of the Great Lakes have shown upward or downward trends over long periods. For example, Lake Michigan-Huron has shown a slight downward trend over a period of more than 100 years. Such trends should be taken into account in determining the normal level. An acceptable procedure is to adjust the last 50 years of data for the indicated trend before determining the median level. This adjusted 50-year median level should be con sidered as representing the normal level. Lunar tides in the Great Lakes may be ignored because their size is negligible. Because of the size of the Great Lakes, their water levels change very slowly. Therefore, it is not neces sary to determine the normal water level from daily data. Mean monthly values are of sufficient accuracy. The Detroit District, U.S. Army Corps of Engineers, collects and publishes monthly data on the levels of the Great Lakes. 4. Unregulated Lakes As used in this guide, the term unregulated lakes means those natural lakes whose outlets are virtually uncontrolled by dams or other structures and those natural lakes that are not appreciably affected by diversions into or out of the lake. Regulatedlakes are discussed as reservoirs in this guide. A median level based on 50 years of data is accep table as the normal water level. This level can be determined from long-term records of the lake level. If only short-term records are available, they may be extended by estimating longer-term data on the basis of records for other lakes, stream discharge and ground water data, and weather records. As with nontidal streams, a shorter record may be acceptable if it can be demonstrated that major wet and dry periods are included. As with the