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:
with due considera tion for variations in normal pool level during the year. If the reservoir includes flood control storage, the median of the flood control pool levels for the life of the reservoir should be considered normal for events that are likely to occur during the flood season. Records of levels of regulated lakes and reservoirs are usually collected by the operator of the reservoir. In some cases, these data are published by the U. S. Geological Survey, U.S. Army Corps of Engineers, or other Federal, State, or local agencies. 6. Oceans A complete tide cycle averages 12 hours 25 minutes at most points on the shores of the .United States. Because of this relatively rapid rate of change of ocean levels, the duration of the design basis event should be considered. Most such events are of short duration, ranging from essentially instantaneous (e.g., plane crashes, explosions, and most transporta tion accidents) to a few seconds or minutes (e.g., earthquakes, tornadoes, and some transportation ac cidents). However, hurricanes and fires may last suf ficiently long to span the entire tidal range from low to high. For short duration events, the median tide level may be considered as the normal water level. For 5-3 longer duration events, however, it is probable that the tide will be above (or below) the median level for at least part of the time of the occurrence of the event. For these events, therefore, median higher high tide or median lower low tide (see discussion below) should be used as the normal. The choice of median higher high tide or median lower low tide will depend on the event being considered. For example, hur ricane winds blowing seaward from land will lower the level of the ocean (setdown). If this will have an effect on a safety-related water 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