ML20134K667
| ML20134K667 | |
| Person / Time | |
|---|---|
| Issue date: | 08/06/1985 |
| From: | Codell R, Johnson T NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Knapp M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-39 NUDOCS 8508300344 | |
| Download: ML20134K667 (3) | |
Text
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MEMORANDUM FOR: Malcolm R. Knapp, Chief DISTRIBUTION:
Geotechnical Branch
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MJBell Division of Waste Management NMSS r/f REBrowning WMGT r/f FROM:
Richard Codell and T.L. Johnson RCodell & r/f Hydrology Section TJohnson & r/f Geotechnical Branch MFliegel Division of Waste Management J0 Bunting
SUBJECT:
MEETING WITH NATIONAL WEATHER SERVICE TO DISCUSS PMP On July 31, 1985, we met with E. Marshall Hansen, Chief, Water Management Information Division and Douglas Fenn, Meterlogist, Hydrometeorology Branch, National Weather Service (NWS) in Silver Spring, MD (phone: 301-427-7543),to discuss the Probable Maximum Precipitation (PMP) estimates for very short durations in very small watersheds. The need for these PMP estimates arises in the calculation of peak runoff necessary to design rock armor for erosion protection on uranium mill tailings embankments.
Tables used for the PMP estimates are available only for periods of 15 minutes or longer.
The very small, steep watersheds for the tailings embankments slopes, however, are characterized by time scales on the order of several minutes. Therefore, estimates of rainfall intensities for periods shorter than 15 minutes must be developed.
NWS stated that the network of rainfall gages used in the U.S. is sparse, so that the probability of detecting very-intense precipitation rates is small.
Mathematical and statistical methods are used to extend the period of record and to infer rates of precipitation for small regions and long recurrence intervals.
We asked about the possibility of deterministic mathematical modeling of storms for the purpose of simulating intense precipitation. Mr. Fenn said that although such models have been tried, he was not aware of any useful results which could be applied to our present needs. The PMP estimates by NWS are based on observed storms which have been optimized for highest p'recipitation on the basis of physical limitations. At our request, NWS re-evaluated some of the data used to prepare Hydrometeorological Report (HMR) No. 49, concentrating on small-area, short-duration events. NWS examined data from several first-order stations in the Four Corners area, particularly the incremental rainfall associated with several record storm events.
Based on recurrence-interval depths of precipitation calculated from these data and on proportionalities between PMP and recurrence-interval depths for durations
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beyond 5 minutes, their estimate of the 5-minute depth of PMP accumulation was 45 5% of the 1-hour PMP depth of accumulation.
We inquired about the possibility that the very-intense precipitations are caused by small, short-lived thunderstorms which might be difficult to deal with statistically because they would be small compared to the distance between stations. NWS, however, was able to show from their available records that the maximum 5, 15, and 30-minute precipitation at 4 out of 6 stations occurred on the same day, indicating that the storm or storms causing this precipitation were widespread and of longer duration than would be indicated by a small isolated thunderstorm. NWS stated that a large percentage of the 24-hour rainfall occurred in a 5-minute period in several of the record storms. These were not large-scale weather systems however, since there was little additional rain after 6 to 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
They may in fact have been multiple thunderstorms passing near the stations.
NWS also pointed out that the storms might have been much more intense in an area which did not have a gage to determine the actual rainfall amounts.
We also requested NWS to suggest PMP estimates for durations less than 5 minutes. Their response was that very-intense, short-duration rainfalls tend to be similar at mid-latitudes, because the rates of precipitation are likely to be limited by meteorological considerations such as velocity of updrafts and the moisture content of air. Altitude is a factor, but the approximately 5000 ft. elevation in the Four Corners area would probably not have a great effect.
This leads to a tentative conclusion that the PMP for durations less than 5 minutes can be bounded by historical records of intense rainfalls at mid-latitudes around the earth.
We asked about the common practice in storm hydrology of arranging periods of precipitation within the one-hour event in an arbitrary order so as to maximize the runoff. NWS stated that there is no statistical basis to choose one arrangement over another, and therefore the practice of arranging the segments to maximize the runoff is justified. Guidance given in HMR 49 and other NWS reports states that the most intense segment should go first in a short-duration sequence. NWS stated that this is general guidance only and is not well-founded. Some storms show the most intense portion of the rainfall occurring in later segments.
Our final discussion centered on the possibility of the NWS continuing the study of intense precipitation under NRC contract. They expressed a willingness to pursue such work, although they are presently fully comnitted.
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426.1/DC/85/08/01 In summary, the brief meeting we had with Douglas Fenn and E.M. Hansen of NWS was immensely useful and reassuring. The predictions made by NWS largely support the empirical estimates of rainfall rates used in the NRC modeling studies.
Original Signed By Richard Codell Hydrology Section Geotechnical Branch Division of Waste Management Oligtnal signes ig(
T.L. Johnson Hydrology Section Geotechnical Branch Division of Waste Management
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