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:
r supply, the normal water level should be the median lower low tide. High and low tides do not consistently reach the same levels. The two high tides during a complete 24 hour 50-minute cycle at some locations are of dif ferent heights, as are the two low tides. The higher of the two high tides is designated as higher high tide and the lower of the two low tides as lower low tide. Other cyclic variations have also been identified. Tidal heights vary during each lunar month from spring (high) to neap (low) range. The tidal range also has a cyclic variation of somewhat more than 19 years. In addition, tidal heights are affected by varia tions in barometric pressure and by wind. These meteorological phenomena cause the principal dif ferences between predicted and recorded tidal heights. In order to ensure that these variations are con sidered, normal tide levels should be computed from a continuous record of at least 20 years. A long-term tide record is not available at the exact location of many nuclear power plant sites. Usually, however, a short-term record will have been collected at the site. In this case, the short-term record may be extended by correlation with one or more records for nearby long-term tide stations. Records of tidal heights are collected and main tained by several agencies but principally by the National Oceanic and Atmospheric Administration (NOAA) in the Department of Commerce. This agency regularly estimates mean high tide or mean higher high tide, primarily for coastal locations. Data for other locations may be estimated, using the rela tions given in tide tables, by interpolation. 7.- Estuaries Both freshwater runoff and lunar tides affect water levels in estuaries. Most estuaries are gaged by the U.S. Geological Survey, National Oceanic and At mospheric Administration, U.S. Army Corps of Engineers, or others. A stage-duration curve should be used to estimate the median water level as the nor mal water level. This procedure may be used for