SITES SOFTWARE APPLICATION SEMINAR __________________________ SITES INTEGRATED DEVELOPMENT ENVIRONMENT for WATER RESOURCE SITE ANALYSIS 5 Point Distribution

TRAINING OBJECTIVES Know how distribute a 24 hr. PMP rainfall. Given 6hr. PMP – 29.7” 12hr. PMP – 35.5” 24hr. PMP – 39.8”

History Over 50 years of experience with small NRCS dams indicated that the routed top of dam, or height of dam, has provided a design with satisfactory performance. Most of these routings have been done with a 6 hour design storm. Records of NRCS dams with auxiliary spillway (AS) damage indicate that much of this damage is caused by long duration storms with very large volumes of runoff (sometimes spread over days). For auxiliary spillway design a longer duration storm and a different distribution for checking spillway integrity needs to be considered.

Concern Concern has been and still is that setting the top of dam based on a 24 hour PMP storm will significantly increase the height of the dam when compared to the 6 hour PMP storm. This assumes that the 24 hour PMP storm contains a critically nested 6 hour PMP storm.

Distribution Storms have been nested such that the design storm produced a maximum theoretical peak discharge for watersheds regardless of the time of concentration. This distribution delivers the maximum amount of runoff to the site in the shortest period of time thereby giving the highest possible top of dam elevation. This type of maximized PMP storm distribution for durations of 6 to 72 hours will cause the top of dam to be higher with each increase in storm duration. This is because the 6-hour PMP storm (both magnitude and distribution) is included in the longer duration storm. Runoff volume arriving at the site before this intense 6-hour storm will occupy flood storage, thus raising the starting water level.

Distribution - Goal To provide a large volume of runoff, which will cause a long duration of flow through the AS and not raise the top of dam. A distribution based on 6 hour increments with no durations less than 6 hours that are nested, for a 24 hour storm results in 5 points were tested. The HMRs recommend putting the most intense 6-hour period near the center of the storm and alternatively placing the next most intense 6-hour period before or after the most intense period. In this way a “maximized” or conservative storm is developed. The least intense 6-hour periods are placed at the beginning and end of the storm. The 5 point storm distribution was based on an ordering of the 6-hour periods where rainfall would be considered at a uniform rate within each 6-hour period will still provide a large volume of runoff, which will cause a long duration of flow through the AS.

TEST To test different storm distributions on actual dam sites (representing the United States with most of the sites East of the 105 th meridian) a data set for 105 dams was developed. We ran 5 different distributions through the sites program and compared the Hp (flow depth in the spillway) to the Hp for the current 6-hour storm. Each distribution was site specific based on the rainfall obtained at each location.

Distribution A critically stacked distribution is developed using PMP data for a 6, 12, and 24 PMP storms. The 6 hour PMP is applied uniformly in the second increment of the stacked distribution. Each of the remaining incremental rainfalls are applied uniformly in adjoining 6 hour increments.

How to distribute a 24hr PMP storm We will assume that the 24 hr storm will be distributed in 4 6hr blocks. The 6, 12, and 18 hr PMP rainfall will be included in the 24 hr PMP. The 6 hr PMP will be contained in the 24 hr storm and distributed in the second 6 hr block.

This is a table from the new Chapter 21 showing how to distribute the 24 hour storm for a 5 point.

Example How distribute a 24 hr. PMP rainfall. Given 6hr. PMP – 29.7” 12hr. PMP – 35.5” 24hr. PMP – 39.8”

Example 6 to 12 hrs hr. PMP – 29.7” = 5.8” 12 to 18 hrs ” 39.8” -35.5” = 4.3” 4.3”/2 = 2.15” 0 to 6 hrs ” 18 to 24 hrs ”

How to distribute a 24hr PMP storm 1 st ” 2 nd ” 3 rd ” 4 th ” Make this into a unit mass curve.

How to distribute a 24hr PMP storm 2.15”/39.8” = ”/39.8” = ”/39.8” = ”/39.8” = 0.054

How to distribute a 24hr PMP storm Time Rainfall

QUESTIONS- ??????

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