prepared by nathaniel todea, hydraulic engineer, … by . nathaniel todea, hydraulic engineer,...

Prepared by Nathaniel Todea, Hydraulic Engineer, USDA-NRCS, Salt Lake City, Utah June 2010

Z:\Mill_Site\HH\writeup\Probably Maximum Precipitation and Freeboard Hydrography Study.doc

25TChapter 1 - Probably Maximum Precipitation-USUL/HMR 49, Modified Runoff curve number (treated-slope) and Time of concentration study25T .......................................................... 4 25TAbstract25T ...................................................................................................................................... 4 25TIntroduction25T ................................................................................................................................ 4

25THistory25T .................................................................................................................................... 4 25THydrologic Characteristics25T .......................................................................................................... 4

25TDelineation of Subbasins25T ......................................................................................................... 5 25TTime of Concentration (Tc)25T .................................................................................................... 6 25THydrologic Soil Groups25T .......................................................................................................... 7 25TRunoff Curve Number25T ............................................................................................................ 8 25TRunoff Curve Number (ARC II 24-72 hour)25T ......................................................................... 11 25TReaches25T ................................................................................................................................. 11

25TSITES25T ....................................................................................................................................... 11 25TProbable Maximum Precipitation (PMP)25T ................................................................................... 12 25TUSGS Gage Analysis25T ................................................................................................................. 14 25TResults25T ...................................................................................................................................... 15 25TConclusion25T ................................................................................................................................ 16

25TChapter 2: Sedimentation Study25T .................................................................................................. 17 25TChapter 3: Delay Effects of Raising Dam 2 ft. Concerning Excess Run-off from Auxiliary25T ..... 19 25TChapter 4: 2-500 Year Economics25T ............................................................................................... 22 25TChapter 5: 10-Day Analysis25T ........................................................................................................ 28

25TUSGS Stream Gage versus NOAA Atlas 1425T .............................................................................. 28 25TResults25T ...................................................................................................................................... 30

25TAppendix – HEC-SSP output25T ...................................................................................................... 35 25TAppendix – USUL and HMR 49 General Storm25T ......................................................................... 45 25TAppendix – Time of concentration study Kirpich Tc and Synder’s Lag Equation25T ..................... 50

List of Figures

25TUFigure 1. Delineated Subbasins with basin IDU25T ............................................................................................. 5 25TUFigure 2. Comparison between average velocities from multiple Time of ConcentrationU25T ................... 7 25TUFigure 3. Example of location of breaks within reach.U25T ............................................................................... 7 25TUFigure 4. Hydrologic soils groups with subbasinsU 25T ....................................................................................... 8 25TUFigure 5. Watershed conditions – poor (slope), fair (no treated or slope), and good (treated).U25T ........... 9 25TUFigure 6. Schematic from SITESU25T ................................................................................................................. 12 25TUFigure 7. Comparison between NWS special study and 500-yr 24-hour spatial distributionU25T ............. 13 25TUFigure 9. HEC-SSP 70 peak streamflow plotU25T ............................................................................................. 15 25TUFigure 11. Bathymetric survey results slowed as depth sediment accumulations.U25T ................................ 17 25TUFigure 12. Stage Storage CurveU25T .................................................................................................................... 18 25TUFigure 13. Stage Relative to Storage and Years of LifeU25T ............................................................................. 18 25TUFigure 14. Accumulated volume over 10 daysU25T ........................................................................................... 19 25TUFigure 15. Flood FrequenciesU25T ....................................................................................................................... 22 25TUFigure 16. Area modeled with Stations within Ferron, Utah.U25T .................................................................. 24 25TUFigure 17. 2-500 year event inundation mapsU25T ........................................................................................... 26 25TUFigure 18. Comparison from back calculated USGS stream gage and NOAA Atlas 14 10 day 100

year rainfallU25T ...................................................................................................................................... 28

25TUFigure 19. USGS Stream 10-day VolumesU25T ................................................................................................. 29 25TUFigure 20. 10-day frequency plot from HEC-SSPU25T ..................................................................................... 29 25TUFigure 21. Breach inundation areaU25T ............................................................................................................... 31 25TUFigure 22. Hazard depths and velocities at structures inundation areaU25T .................................................. 31 25TUFigure 23. TR-60 Breach Criteria and Breach Q resultsU25T.......................................................................... 33

List of Tables

25TUTable 1. Watershed CharacteristicsU25T ................................................................................................................ 6 25TUTable 2. Associated land use code and curve number for specific hydrologic soil groupU25T..................... 9 25TUTable 3. Conversion from ARC II to ARC III with normalized valueU25T .................................................. 11 25TUTable 4. HMR 49 General Storm accumulated PMP valuesU25T ................................................................... 13 25TUTable 5. Probable maximum precipitation by subbasin.U25T .......................................................................... 14 25TUTable 6. HEC-SSP results at 133 square milesU25T .......................................................................................... 15 25TUTable 7. HMR general and USUL general scenariosU25T................................................................................. 15 25TUTable 8. Modelled flow compared to USGS stream gageU25T ........................................................................ 23 25TUTable 9. Frequency discharges at bridges in Ferron, UtahU25T ....................................................................... 24 25TUTable 10. House inundated by depth relative to storm eventU25T ................................................................. 25

Chapter 1 - Probably Maximum Precipitation-USUL/HMR 49, Modified Runoff curve number (treated-slope) and Time of concentration study Abstract A study was completed to analyze the results of probably maximum floods to generate the freeboard hydrograph for the Rehabilitation Project Millsite located near Ferron, in Emery County, Utah. USDA-NRCS Technical Release 60 (TR-60) and Utah Division of Water Rights Dam Safety Section rules provided the framework to complete the study. The study consisted of multiple general storms using Hydrometeorological Report No. 49 (1977), an Update for Probable Maximum Precipitation, Utah 72-hour Estimates (Jenson, 2002). Substantial time was used to derive the Runoff curve number (RCN). Treated areas by the National Forest Service, high sloped areas, and land use were taken into consideration during the derivation of the RCN. The Probable Maximum Precipitations (PMP) was generated for the general storms which were later routed to understand the inflow and outflow at the Millsite Reservoir. Introduction

History Previous PMP studies were completed in 1964 by the NRCS formally known as the SCS, the State of Utah Division of Water Resources in 1978, and a special PMP study was completed by the National Weather Service (NWS) in 1993. Furthermore NRCS has continued to work on the project. Work completed by the Utah engineering staff was incorporated in this study. Final results were not undertaken until this study. During the 1964 study it is assumed that the auxiliary spillway was designed using an 89.9 runoff curve number, with 10.2 inches of rainfall in a 6-hour period in a 20 square mile area. This resulted in a 26,464 cfs inflow and inflow design hydrograph of 12,220 cfs, which was the result of a criteria “b” freeboard hydrograph analysis. The PMP study completed by the Division of Water Resources (Division Water Resources, 1978) illustrated that the probable maximum storm hydrograph peak discharge is 88,480 cfs using a 6.4 inch rainfall and 87 curve number over a 155 square mile area. A later study referred by the RB&G Engineering (2006) stated that in 1998 the Division of Water Resources, Dam Safety Section performed a study that resulted in a inflow design flow of 23,000 cfs. The PMP study completed by the NWS modified the areal distribution for the general storm over the entire watershed and also adjusted the barrier to 40 percent rather than 50% to adjust for height. Although the areal distribution was hypothesis it was not used in this study but the barrier adjustment was used. Hydrologic Characteristics Hydrologic characteristics of the Ferron watershed were derived from ESRI GIS applications. Reach characteristics was gathered during a 1996 study from NRCS (formally SCS) Hydraulic Engineer, Bryan Hill. This data was used to generate rating table information that would be entered in SITES to route hydrographs using the Muskingum-Cunge flood routing method.

Time of concentrations was generated using the Kirpich method, segmenting the slopes of each longest flowpath for each subbasin. A separate study was completed to validate the accumulated modified Kirpich Tc method used in this study and was compared to Synder’s lag equation that is used by the Utah Division of Water Resources. This study is located in the appendix.

NRCS GeoHydro and the HEC-GeoHMS extensions were used in ArcView to generate hydrologic characteristics. Specifically NRCS GeoHydro is used to generate the Runoff Curve Numbers (RCN) and HEC GeoHMS was used to derive the all other hydrological characteristics.

Delineation of Subbasins 18 subbasins were delineated. The elevation of the watershed ranged between 11,157 feet to 6074 feet above the mean sea level with the mean elevation being 8763 feet. Located in Figure 1 is the delineated 153 square mile watershed with the subbasin identifications used during this study. Located in Table 1 are the results of the delineated subbasins (i.e. area, longest flow path, etc.)

Figure 1. Delineated Subbasins with basin ID

Table 1. Watershed Characteristics

Subbasin ID

Centroid Elevation

(ft) SLP_1085

Longest Flowpath

(ft) Area (sq

mi)

Tc Kirpich

(hrs)

Average Velocity

(fps) 7 9693 0.044 47038.7 12.4 2.71 4.82 10 8184 0.102 17965.8 4.0 0.98 5.09 9 9907 0.067 32524.7 5.1 1.55 5.83 8 9335 0.059 51339.6 16.6 2.72 5.24 11 7962 0.115 24548.4 5.5 1.21 5.64 14 7993 0.107 11432.1 1.0 0.74 4.29 12 9292 0.073 40226.7 13.8 2.26 4.94 19 7382 0.070 38203.5 11.5 1.88 5.64 20 8877 0.115 31913.2 6.6 1.36 6.52 15 8962 0.093 36631.6 7.7 1.44 7.07 27 8382 0.067 29506.2 5.9 1.44 5.69 13 9127 0.068 42002.3 14.1 1.61 7.25 25 6926 0.110 17413.1 2.1 1.19 4.06 28 8546 0.096 37094.1 11.2 1.73 5.96 23 8290 0.110 38057.3 9.1 1.73 6.11 26 8613 0.118 31502.6 7.1 1.27 6.89 30 6250 0.043 36804.6 8.6 2.21 4.63 31 7899 0.077 45104.1 10.6 2.26 5.54 Total 153.0

Time of Concentration (Tc) Significant time and effort was spent understand and generating the Tc. In the 1996 study

two methodologies were used to generate the Tc. This included the Cowan and Jarrett methodology. The average velocities that were generated were too slow. In any case these methodologies could not be recreated since the watershed was delineated differently. To achieve slower velocities manipulations were made to the reach and upstream/downstream elevations using the Kirpich Tc equation. This resulted in high average velocities and was not deemed appropriate. Finally a segmented approach was taken using the Kirpich equation. Breaks in the reach were identified and these breaks were used to develop the Tc and compiled to obtain the entire Tc for each reach (Figure 3). A separate study was completed to validate the accumulated modified Kirpich Tc method used in this study and was compared to Synder’s lag equation that is used by the Utah Division of Water Resources. This study is located in the appendix. Below, in Figure 2 are comparisons between different methodologies and an example of the segmented accumulated modified Kirpich Tc (Figure 3).

Velocity Compare All

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

6.00 11.00 16.00 21.00 26.00 31.00Subbasin ID

Velo

city

(fps

)

10 85 no reduction 10 85 100 SEGment Cowan Jarrett

Figure 2. Comparison between average velocities from multiple Time of Concentration

WS 14

7200

7400

7600

7800

8000

8200

8400

8600

0 2000 4000 6000 8000 10000 12000Distance (ft)

Elev

atio

n (ft

)

Figure 3. Example of location of breaks within reach.

Hydrologic Soil Groups The majority of the watershed is located on the Manti-La Sal Forest managed by the

National Forest Service. Soils data was provided by the Manti-La Sal National Forest, Price, Utah. The SSURGO2 has not been published but is the latest version to date. Below in Figure 4 are the hydrologic groups with the respective subbasins.

Figure 4. Hydrologic soils groups with subbasins

Runoff Curve Number Three conditions were identified, poor, fair, and good. These areas consisted of treated area

- good, no treated and sloped (< 50 %) – fair, and sloped (> 50% ) – poor. Below in Figure 5 are the three types of conditions.

Figure 5. Watershed conditions – poor (slope), fair (no treated or slope), and good (treated).

NRCS GeoHydro was used to estimate the antecedent runoff condition II runoff curve number. A zonal statistics command was executed to estimate the curve number for each subbasin. This was done three times for each condition. These values were later combined for each subbasin. Below in Table 2 are the associated land use code and hydrologic soil group to derive the runoff curve number for each condition. The National Land Cover dataset was downloaded from the USGS-Seamless web site.

Table 2. Associated land use code and curve number for specific hydrologic soil group Lucode Classifica Hyd_a Hyd_b Hyd_c Hyd_d Slope 11 Water 98 98 98 98 21 Low Int Residential 76 85 89 91 22 High Int Residential 76 85 89 91 23 Comm,Indust,Trans 76 85 89 91 31 Rock,Sand,Clay 87 93 98 41 Deciduous Forest 66 74 79

42 Evergreen Forest 75 85 89 43 Mixed Forest 71 80 84 51 Shrubland 63 77 85 88 52 Shrubland 63 77 85 88 71 Grassland 71 81 89 81 Pasture,Hay 80 87 93 85 Urban/Rec Grass 68 79 86 89 90 Woody wetland 98 98 98 98 92 wetland 98 98 98 98 No Slope/treated 11 Water 98 98 98 98 21 Low Int Residential 76 85 89 91 22 High Int Residential 76 85 89 91 23 Comm,Indust,Trans 76 85 89 91 31 Rock,Sand,Clay 80 87 93 41 Deciduous Forest 48 57 63 42 Evergreen Forest 58 73 80 43 Mixed Forest 53 65 72 51 Shrubland 55 72 81 86 52 Shrubland 55 72 81 86 71 Grassland 62 74 85 81 Pasture,Hay 71 81 89 85 Urban/Rec Grass 49 69 79 84 90 Woody wetland 98 98 98 98 92 wetland 98 98 98 98 Treated 11 Water 98 98 98 98 21 Low Int Residential 76 85 89 91 22 High Int Residential 76 85 89 91 23 Comm,Indust,Trans 76 85 89 91 31 Rock,Sand,Clay 62 74 85 41 Deciduous Forest 30 41 48 42 Evergreen Forest 41 61 71 43 Mixed Forest 36 51 60 51 Shrubland 49 68 79 84 52 Shrubland 49 68 79 84 71 Grassland 53 66 81 81 Pasture,Hay 62 74 85 85 Urban/Rec Grass 39 61 87 80 90 Woody wetland 98 98 98 98 92 wetland 98 98 98 98

Runoff Curve Number (ARC II 24-72 hour) An Antecedent Runoff Condition (ARC) II was used for the PMP study. A 72-hours ARC

II RCN was interpolated from the 24 hr and 10 day RCN supplied in TR-60 Table 2-3. The normalized 24-hour RCN value over the entire watershed for the ARC II is 72 and 72-hour ARC II RCN is 68 (Table 3).

Table 3. Conversion from ARC II to ARC III with normalized value Watershed

ID Area

(Mile P

2P)

RCN AMC II-24hr

RCN AMC II-10 day

RCN AMC II-72-hr

7 12 72 54 68 8 17 71 53 67 9 5 71 53 67 10 4 66 47 62 11 6 67 49 63 12 14 71 53 67 13 14 72 54 68 14 1 67 49 63 15 8 73 56 69 19 12 67 49 63 20 7 74 57 70 23 9 70 52 66 25 2 68 50 64 26 8 72 54 68 27 5 67 49 63 28 11 72 54 68 30 8 79 64 76 31 11 77 61 73

TOTAL 152 72* 54 68* * Normalized values

Note that the areas is 1 square mile smaller than the derived subbasins, this is due to some of the slope areas no being included in the RCN computations. The raster grid size of the Hydrologic Soil Groups and National Land Cover Dataset are 98 feet in size. These areas will not significantly alter the results.

Reaches River channels were surveyed during the 1996 NRCS study. Also Manning’s n values were

determined in this study. This data was to describe the reaches in the watershed. Muskingum-Cunge was used to route the hydrographs. HEC RAS was used to create rating table for the reaches. The 1996 study also determined the base flow for the watershed.

SITES SITES was used to model the PMP local and generals storms. Muskingum-Cunge was used to route the flood hydrographs. Below in Figure 6 is the schematic from SITES. A comparison was made using WinTR20 and the final results taken from WinTR20.

Figure 6. Schematic from SITES

Probable Maximum Precipitation (PMP) An areal distributed and non aerial distributed for the HMR 49 general storms were derived. The PMP study completed by the NWS modified the areal distribution for the general storm over the entire watershed and also adjusted the barrier to 40 percent rather than 50% to adjust for height. This value was used to derive the general storm.

An Update for Probable Maximum Precipitation, Utah 72-hour Estimates (Jenson, 2002) (USUL General) studies was completed as required by the Utah Dam Safety. The 40 percent barrier adjustment from the NWS was not factor in.

Due to the size of the watershed a 6-hour storm was not deemed appropriate by the Utah Division of Water Resources and results from this study was not incorporated.

Rainfall distributions were generated for the HMR 49 general and USUL general storms. These rainfall distributions were entered into SITES with the associated rainfall depth. The rainfall values were entered as areal correct rainfall since the studies already incorporated the areal correction.

Areal rainfall totals were calculated using a percent of total normalized values from a 500 year 24 hour storm from NOAA atlas 14. These values were compared to the interim procedures made by the National Weather Service (NWS) special study of the area. Below is Figure 7 is the comparison between the NWS and 500 yr 24 hour normalized precipitation distribution.

Figure 7. Comparison between NWS special study and 500-yr 24-hour spatial distribution

Located in the appendix are the HMR 49, USUL 72 hour, USUL 24 hour-worksheet to determine rainfall depth and distribution for the general storm.

Table 4. HMR 49 General Storm accumulated PMP values PMP Study1993 6 12 18 24 48 72 Convergence 2.46 3.17 3.53 3.79 4.47 4.75 Orthographic 1.56 2.96 4.15 5.19 8.15 9.6 Total 4.02 6.13 7.68 8.98 12.62 14.35

The USUL 72-hour general storm is 8.54 inches. And the USUL 24-hour general storm is 6.75 inches. Below in Table 5 are the rainfall depths for each subbasin.

Table 5. Probable maximum precipitation by subbasin.

Watershed ID

Area (Square

Mile)

USU general zonal PMP 72-hour

(inches)

USU general zonal PMP 24-hour

(inches) 7 12 8.88 6.96 8 17 9.14 7.16 9 5 9.00 7.05 10 4 8.41 6.59 11 6 7.97 6.24 12 14 9.79 7.69 13 14 10.42 8.16 14 1 8.59 6.73 15 8 9.54 7.47 19 12 7.57 5.93 20 7 8.81 6.90 23 9 8.01 6.27 25 2 7.12 5.58 26 8 8.16 6.39 27 5 7.30 7.30 28 11 7.89 6.18 30 8 6.77 5.30 31 11 7.08 5.55

Normalized over Area 8.54 6.75 USGS Gage Analysis

USGS stream gage 09326500 Ferron Creek near Ferron, Utah was analyzed to understand the applicability of the using PMP hydrology calculations. USGS 09326500 drainage area is 138 square miles. The stream has been recording information between 1912 to present. The peak stream flow count is 70. HEC-SSP was used to conduct the statistical analysis. Weibul was used and data was collected from the internet through HEC-SSP protocol. The 70 peak stream flow records were used and no skew was applied. Projections were made from the 100 year to 1,000,000 year flood occurrences and are presented in Table 6. Below is a graph of the 70 peak stream flows as well as the results from HEC-SSP.

Figure 9. HEC-SSP 70 peak streamflow plot

Table 6. HEC-SSP results at 133 square miles Confidence Limits Flow in cfs %

Probability Flood Year Occurance

Computed Curve Flow in cfs

Expected Prob Flow is cfs

0.05

0.95

0.00001 10000000 32,842 52,679 60,831 20,565 0.00002 5000000 30,874 48,512 56,624 19,475 0.0001 1000000 24,729 36,282 43,780 16,017 0.0002 500000 22,115 31,444 38,465 14,515 0.001 100000 16,736 22,185 27,854 11,353 0.002 50000 14,735 18,987 24,036 10,146 0.01 10000 10,759 13,043 16,706 7,686 0.1 1000 6,494 7,282 9,326 4,915 0.2 500 5,509 6,051 7,716 4,247 0.5 200 4,380 4,696 5,927 3,463 0 100 3,640 3,841 4,794 2,936

Results Below in Table 7 are the results from the HMR 49 and 2 USUL general storm scenarios. Due to the riser being gated flooding routing from the freeboard hydrograph is started at the principal/auxiliary spillway crest. Since the dam has a concrete principal/auxiliary spillway no stability design hydrograph was created.

Table 7. HMR general and USUL general scenarios FBH72hr-

1generalAMCII-krsg-rcn-dis

USUS72hr-1generalAMCII-krsg-rcn-dis72hr

USUS72hr-1generalAMCII-

krsg-rcn-dis72hr-areal

Site Identification UT UT UT Watershed Runoff Curve Number—composite 68 68 72 Total Watershed Drainage Area (Sq.Miles) 152.95 152.95 152.95

FBH or Storm Rainfall Total (Inches) – weighted

14.35 8.54 6.75

FBH or Storm Rainfall Duration (Hours) 72 72 24 FBH or Storm Inflow Peak (CFS) 59899.4 28058.3 31521.3 Initial Reservoir Elevation (Feet) 1055 1059 1059 Maximum WS FBH or Storm (Feet) 1079.23 1064.41 1064.87 Storage at Max. WS FBH or Storm (Acre-Ft) 29248.4 22575.3 22782 Top Dam (Feet) 1079.23 1064.41 1064.87 Storage, Top Dam (Acre-Ft) 29248 22577 22782 PS Crest (Feet) 950 950 950 AS Crest (Feet) N/A 1059 1059 Storage, AS Crest (Acre-Ft) N/A 20126 20126 Hp FBH or Storm (Feet) N/A 5.41 5.87 AS Peak Discharge FBH/Storm (CFS) N/A

U27489 U30961 Conclusion

As citied from Jenson (2002), “Unofficially, PMP “probability is considered as about 1 in 10,000-year occurrence, but it is not the 1 in 1,000 or 1 in 100,000-year occurrence”. It is reasonable to disregard the HMR 49 scenarios and only consider the USUL scenarios. Since the 24 hour USUL general storm is larger than the 72-hour USUL general storm the 24-hour USUL general storm control. There the IDF or FBH is 30,961 cfs.

Chapter 2: Sedimentation Study

Figure 11. Bathymetric survey results slowed as depth sediment accumulations.

The sponsors surmised that the reservoir was filling at a faster rate than design. The sediment pool was design for a 100-year life at an annual accumulation of 58 acre-feet per year. The life of the reservoir was expected to last until 2071. A sonar survey was completed at the reservoir by the Department of Interior Bureau of Reclamations. A bathymetric survey was completed in summer 2006 on what had accumulated in the reservoir floor. The original survey was compared with the bathymetric survey and maximum depths were just less than 50 feet. The maximum depths were at the inlet to the reservoir. In 2006 2611 acre feet of sediment was estimated to have accumulated in the reservoir. The average annual sediment yield per year is estimated to be 74.6 acre-feet. The design sediment pool is expected to fill 22 years faster than intended.

Stage Storage Curve

940

960

980

1000

1020

1040

1060

1080

0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 22000 24000

Storage (Acre-feet)

Ele

vatio

n (f

eet)

Figure 12. Stage Storage Curve

To achieve a 100 year life from 2009, the spillway crest would be raised to a stage elevation of 1065.5 or 10.5 feet than currently. The top of dam is currently at 1062.5.

Stage Relative to Storage and Years of Life

1058.9 --> 2009 + 62 year = 100 year total

1750017700179001810018300185001870018900191001930019500197001990020100203002050020700209002110021300215002170021900

1054

.010

54.5

1055

.010

55.5

1056

.010

56.5

1057

.010

57.5

1058

.010

58.5

1059

.010

59.5

1060

.010

60.5

1061

.010

61.5

1062

.010

62.5

Stage (feet)

Stor

age (

acre

-feet)

20382041204420472050205320562059206220652068207120742077208020832086208920922095

Yea

r

1056.9 --> 2009 + 50 years = 88 year total

1055 --> 2009 + 0 years = 77 year total

Figure 13. Stage Relative to Storage and Years of Life

Chapter 3: Delay Effects of Raising Dam 4 ft. Concerning Excess Run-off from Auxiliary USGS 09326500 Ferron Creek (Upper Station) near Ferron, Utah stream gage was used to determine the delay affects of adding 1 acre-feet of storage to the Millsite Dam. Stream gage 09326500 is upstream drainage area is 138 square miles. The Millsite dam has a contributing area of 153 square miles. The period of record on stream gage is between 1912-06-03 and 2005-05-25.

In a previous study conducted by Nathaniel Todea, USDA-NRCS Hydraulic Engineer, a 100 year 10 day analysis was completed, chapte 5. The USGS gage just upstream of the dam was used to make the 100 year 10 day analysis. Data was sorted by water year by Thom Garday, National Hydraulic Engineer of the NWMC in Little Rock, Arkansas. Calculations were made for each ten days throughout the year and the maximum values were identified. These maximum 10-day volumes started in May 2P

ndP and end around June 30P

th P(seeP

Pbelow). This data was used to isolate potential

overflow of the auxiliary spillway at the Millsite dam.

USGS Stream 10 Day Volumes (ending day)

100030005000

70009000

110001300015000

170001900021000

8-May 18-May 28-May 7-Jun 17-Jun 27-Jun 7-Jul

Time (Date)

Volu

me

(Acr

e-fe

et)

Figure 14. Accumulated volume over 10 days

Three periods were analyzed to identify what the effects of adding 2000-acre feet of storage to the Millsite dam. The highest number of peak 100 year 10 days floods occur between May 24 and June 2. For this study the dates of accumulating the 2000 acre feet start on May 15/16, 20/21, and 25/26. Below are the average days of delay for 70 years for the three periods. Calculating these volumes assume that the gate at the principal spillway is closed.

Average Days of Delay to Accumulate 2000 acre-feet Days of

Delay May 15/16 4.48 May 20/21 3.80 May 25/26 3.44

Average Days of Delay to Accumulate 1700 acre-feet Days of

Delay May 15/16 3.81 May 20/21 3.23 May 25/26 2.92

Only 12 days of accumulated flow was analyzed. In each of the three cases water year 1977 was the only year that did not accumulate more than 2000 acre feet in twelve days. This data is highly variable. Many factors can affect the outcome, snow pack, pre snow pack conditions, and management of the dam. On the good years a delay may only be one day or on the bad years if could take as long as 10 days and on average less than 5 days.

That is . . .

Given: • 300 cfs between May 25P

thP – June 14P

thP

• Upper bound is 363 cfs – 86400 seconds in a day – @ 300 cfs this equals 25,920,000 cubic feet / day or 595 acre feet /day

Therefor 1700 acre feet / 595 acre feet / days = 2.86 days the first year while being reduced every year since sediment is accumulating.

No. of Years of sediment accumulation

Accumulated Sediment at 74.6 acre-feet / year

No. of Days of Depletion*

1 0 2.86 2 74.6 2.73 3 149.2 2.61 4 223.8 2.48 5 298.4 2.36 6 373 2.23 7 447.6 2.10 8 522.2 1.98 9 596.8 1.85

10 671.4 1.73 11 746 1.60 12 820.6 1.48 13 895.2 1.35 14 969.8 1.23 15 1044.4 1.10 16 1119 0.98 17 1193.6 0.85 18 1268.2 0.73 19 1342.8 0.60 20 1417.4 0.47 21 1492 0.35 22 1566.6 0.22 23 1641.2 0.10

* based on a 300 cfs daily average

Chapter 4: 2-500 Year Economics Two data sets were used to conduct the analysis for the 2-500 year flood frequencies and economic impacts without project. The two data sets consisted of high resolution data within the town of Ferron and 10-meter data with supplemented survey data. Stream gage data was used to calibrate hydrographs and design storms were used. In all, 3 bridges do not have capacity to handle all the storms analyzed and up to 5 structures are inundated. No other public utilities were inventoried. A detailed survey was provided by a local engineering firm. This data was used to analyze the 2-500 year frequency flows within the town of Ferron. 10-meter data supplemented with survey data was also used to conduct the same frequency floods where the detailed survey was not provided. USGS 09326500 Ferron Creek (Upper Station) near Ferron, Utah stream gage was used to determine the 2-500 year flood frequency flow. Stream gage 09326500 is upstream of the dam with a drainage area of 138 square miles. The Millsite dam has a contributing area of 153 square miles. The period of record on stream gage is between 1912-06-03 and 2005-05-25. Below is a graph depicting flood frequencies.

Figure 15. Flood Frequencies

A 6-hour Type II storm was used to generate the 2-500 year flow occurrences. The hydrology and hydrologic characteristics are provide in the entitled Probably Maximum Precipitation-USUL/HMR 49, Modified Runoff curve number (treated-slope) and Time of concentration study, Millsite Watershed Rehabilitation Planning near Ferron, in Emery County Utah by Nathaniel Todea, USDA-NRCS Utah hydraulic engineer. Initially NOAA Atlas 14 24-hour precipitations values were used along with a Type II distribution to model the flood frequencies. These values were 3 times the values of the correlated stream gage values. Efforts were made to develop unit hydrographs but large storm hydrographs were not available. These storms event occurred in the 1950s. Rain gage data was also limited. Due to the high variation in elevation and scale of the watershed estimating precipitation and rainfall

distribution is variable, therefore unreliable. It was determined to use a 6-hour precipitation values (NOAA Atlas 14) and a 6-hour Type II distribution. These values straddled the 2-100 year stream gage frequencies. The 2-year flows being below the stream gage derived values and the 100-year flow being above the stream gage derived values. Modifications were made through an iterative process to determine the calibrated precipitation value. Below in Table 8 are those results.

Table 8. Modelled flow compared to USGS stream gage USGS

Stream Gage WRI 83-4129

NOAA Atlas 14 6-hour

Calibrated Precip 6-hour

NOAA Atlas 14 24 hour

Flow (cfs) Bulletin 17B

Precip-in/Flow-cfs*

Precip-in/Flow-cfs*

Precip-in / Flow-cfs**

2-year 919-853 783 0.88 / 48 1.21 /807 5-year 1540-1440 1324 1.16 / 622 1.31 /1245 10-year 2010-1880 1778 1.36 / 1500 1.38 /1610 25-year 2700-2530 2296 1.63 / 3355 1.48 /2228 50-year 3260-3060 3108 1.85 / 5414 1.56 /2798 100-year 3880-3650 3841 2.11 / 8235 1.63 /3355 3.27 /15649 *Flow at 133 square miles; same as stream gage **Flow at 153 square miles HEC-RAS unsteady flow was used to route the hydrographs. The low resolution data, 10-meter grids and supplemental data, was used to route the storm to the town of Ferron and to analyze the impacts upstream and downstream of Ferron, Utah. A total of three bridges were analyzed. Below is a map and position of the bridges relative to the stream corridor of interest. The bridge located at River Station 2290 or on Ferron Canyon Road has the capacity to allow flow below the 5-year flood, without dam. The bridge located at River Station 1240 or on 450 West Street has the capacity to allow flow below the 25-year flood, without dam. And the bridge located at River Station 240 or on State Route 10 has the capacity to allow flow below the 50 year flood, without dam. Below are the results of those flows in table 9.

Figure 16. Area modeled with Stations within Ferron, Utah.

Table 9. Frequency discharges at bridges in Ferron, Utah River Sta

Plan Q Culv Group

Q Weir

Delta WS

Culv Vel US

Culv Vel DS

(cfs) (cfs) (ft) (ft/s) (ft/s) 2290 Culvert #2 500yr 216.71 3591.54 6.48 10.95 12.3 2290 Culvert #3 500yr 131.99 3591.54 6.48 10.49 11.49 2290 Culvert #1 500yr 1402.2 3591.54 6.48 16.67 16.67 2290 Culvert #2 100yr 204.65 1939.15 9.7 10.34 11.89 2290 Culvert #3 100yr 123.95 1939.15 9.7 9.85 11.04 2290 Culvert #1 100yr 1330.99 1939.15 9.7 15.82 15.82 2290 Culvert #2 50yr 200.61 1515.96 7.8 10.14 11.75 2290 Culvert #3 50yr 121.24 1515.96 7.8 9.63 10.89 2290 Culvert #1 50yr 1304.94 1515.96 7.8 15.51 15.51 2290 Culvert #2 25yr 184.9 1025.78 7.53 9.34 11.24 2290 Culvert #3 25yr 108.2 1025.78 7.53 8.6 10.2 2290 Culvert #1 25yr 1182.07 1025.78 7.53 14.05 14.05 2290 Culvert #2 10yr 186.85 513.21 4.57 9.44 11.3 2290 Culvert #3 10yr 111.64 513.21 4.57 8.87 10.38 2290 Culvert #1 10yr 1150.34 513.21 4.57 13.68 13.68 2290 Culvert #2 5yr 184.9 266.48 5.94 9.34 11.24 2290 Culvert #3 5yr 107.83 266.48 5.94 8.57 10.18 2290 Culvert #1 5yr 956.37 266.48 5.94 11.37 19.03 2290 Culvert #2 2yr 136.28 6.88 7.45 9.75

Bridge at River Station 2290

Bridge at River Station 1240 Bridge at River

Station 240

2290 Culvert #3 2yr 76.87 6.88 7.05 8.68 2290 Culvert #1 2yr 795.56 6.88 12.23 16.27 1240 Culvert #1 500yr 2607.31 2726.04 7.18 13.08 19.63 1240 Culvert #1 100yr 2436.16 1158.82 7.48 12.22 18.89 1240 Culvert #1 50yr 2357.94 781.06 7.49 12.96 14.48 1240 Culvert #1 25yr 2240.34 368.51 7.4 12.72 14.24 1240 Culvert #1 10yr 1818.67 6.2 11.77 13.28 1240 Culvert #1 5yr 1508.9 5.37 10.98 12.48 1240 Culvert #1 2yr 1007.15 3.99 9.42 10.91 240 Culvert #1 500yr 2881.8 2403.25 11.79 14.57 26.55 240 Culvert #1 100yr 2721.26 846.3 11.81 14.3 26.28 240 Culvert #1 50yr 2645.92 396.65 11.83 14.16 26.15 240 Culvert #1 25yr 2367.91 11.35 13.65 25.63 240 Culvert #1 10yr 1752.78 9.84 12.35 24.3 240 Culvert #1 5yr 1420.15 8.94 11.51 23.41 240 Culvert #1 2yr 976.84 7.51 10.16 21.86

Finally five houses were inundated. Below is Table 10 with the houses and respective flood frequencies, without dam. Below is also a map of these inundated areas.

Table 10. House inundated by depth relative to storm event ID Type VALUE Floor

dscrptn 2 year (m)

5 year (m)

10 year (m)

25 year (m)

50 year (m)

100 year (m)

500 year (m)

95 hg m-h 1 0.0 0.0 0.0 0.0 0.1 0.2 0.4 96 hg m-h 1 0.0 0.0 0.0 0.3 0.5 0.6 0.8 97 hg l-m 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 98 ob l 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 99 ob l 0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

100 hbc l-m 2 0.0 0.0 0.0 0.3 0.6 0.7 0.8 101 ob l 0 0.0 0.0 0.0 0.5 0.8 0.8 1.0 102 hbg l-m 2 0.4 0.8 1.2 1.7 2.0 2.0 2.2

Figure 17. 2-500 year event inundation maps

Z:\Mill_Site\HH\HECRAS\Economics\compareDEM-hghrsltn.mxd

Chapter 5: 10-Day Analysis USGS Stream Gage versus NOAA Atlas 14 A 10-day analysis was completed using the USGS stream gage just upstream of the Millsite Dam.

Values for the 10-day 100-year precipitation values from NOAA Atlas 14 averaged 6.30 inches (see below for subbasin 10-day 100 year rainfall). These values were not used since values from the USGS Stream gage yield higher results. Below is a comparison between the NOAA Atlas 14 and back calculated USGS Stream values of precipitation.

Comparison Back Calculate USGS Stream Gage and NOAA Atlas 10 day 100 yr Rainfall

0

2

4

6

8

10

12

ID7

ID8

ID9

ID10

ID11

ID12

ID13

ID14

ID15

ID19

ID20

ID23

ID25

ID26

ID27

ID28

ID30

ID31

Subbasin ID

Rai

nfal

l (in

ches

)

NOAA Atlas 14USGS StreamGage

Figure 18. Comparison from back calculated USGS stream gage and NOAA Atlas 14 10 day 100

year rainfall

The USGS gage just upstream of the dam was used to make the analysis. Data was sorted by water year by Thom Garday, National Hydraulic Engineer of the NWMC in Little Rock, Arkansas. Calculations were made for each ten days through out the year and the maximum values were identified. These maximum 10-day volumes started in May 2P

ndP and end around June 30P

th P(seeP

Pbelow).

USGS Stream 10 Day Volumes (ending day)

100030005000

70009000

110001300015000

170001900021000

8-May 18-May 28-May 7-Jun 17-Jun 27-Jun 7-Jul

Time (Date)

Volu

me

(Acr

e-fe

et)

Figure 19. USGS Stream 10-day Volumes

Using the HEC-SSP software that uses Bulletin 17B the 100-year 10-day volume is 21903 acre-feet at the gage (133 sq.mi). Below is the 10-day frequency plot, provided by HEC-SSP. Attach is the results from this analysis.

Figure 20. 10-day frequency plot from HEC-SSP

The volume was used to derive a precipitation depth. The precipitation-storage-RCN was used to calculate the depth of precipitation. The correlated 21903 acre-feet is 3.05 inches of runoff that is equivalent to 8.57 inches rainfall for an average 54 Runoff Curve Number (composite 10-day RCN).

Results WinTR20 was used to derive the 10-day 100 year hydrograph and to evaluate the storage. The study consisted of operating the Millsite dam as if no sediment and no permanent pool were present. The study tried to answer ‘at what elevation or volume would be sufficient to not allow the auxiliary spillway to operate’. Both WinTR20 and SITES were used. Initially WinTR20 was used and SITES results were compared. Rainfall distributions were used from SITES in all scenarios. Without any permanent pool or sediment pool the dam could safely pass the 10-day 100-year storm without the auxiliary spillway operating. If the option 3 scenario was accepted this design would allow the dam to over flow 1650 cfs or 1 foot. If this option was viable the dam could have a permanent pool at 7950 acre-feet or 1025 feet.

Chapter 6: Hazard Classification A breach analysis with a flood inundation map was completed with this study to document the hazard classification of Millsite dam. The dam was originally design and built as a high hazard dam. The dam classification has not changed and remains a high “C” hazard dam. A TR-60 breach criterion was used in this study. HEC GeoRAS and HEC RAS unsteady flow was used to create geometry data and model the breach discharge and map the final breach inundation area (see figure 21). 10-meter DEM data from USGS Seamless was used for terrain data. TR 66 (simplified dam breach) was used to route the breach Q approximately 1000 feet downstream of the dam to derive a breach hydrograph. As defined by TR 66 the flow just downstream of the dam is supercritical. A curvilinear hydrograph was used to route downstream breach hydrograph. A total of 95 building (houses and other buildings were mapped as being inundated by the breach study. A total of 90 houses were identified as being flooded. Below in figure 18 the figure illustrates the depth and velocities at these structures. From the hydrology and hydraulic investigation used in the economic study 3 bridges were modeled to be overtopped. Working upstream to downstream the first bridge on Ferron Canyon Road could only pass the 2 year storm without dam, the second bridge on 450 West could pass the 5 year storm without dam, and third bridge on State Route 10 could

Figure 21. Breach inundation area

Figure 22. Hazard depths and velocities at structures

pass the 10 year storm without dam. This being case the bridge was not modeled and were all assumed to be overtopped. Located in figure 23 is the TR-60 breach parameters used for the peak Q discharge.

Figure 23. TR-60 Breach Criteria and Breach Q results

Appendix – HEC-SSP output

Bulletin 17B Frequency Analysis 26 Feb 2007 03:40 PM ------------------------------- --- Input Data --- Analysis Name: 10day 100year Description: Millsite Data Set Name: Millsite DSS File Name: Z:\Mill_Site\HH\EXCEL\GageStation\10dayexcel\10dayexcel.dss DSS Pathname: ///FLOW-PEAK/01jan1900/IR-CENTURY// Report File Name: Z:\Mill_Site\HH\EXCEL\GageStation\10dayexcel\Bulletin17bResults\10day_100year\10day_100year.rpt XML File Name: Z:\Mill_Site\HH\EXCEL\GageStation\10dayexcel\Bulletin17bResults\10day_100year\10day_100year.xml Skew Option: Use Station Skew Regional Skew: 0.0 Regional Skew MSE: 0.302 Round adopted skew to nearest tenth Plotting Position Type: Weibull Upper Confidence Level: 0.05 Lower Confidence Level: 0.95 Round ordinate values to 3 significant digits Display ordinate values using 0 digits in fraction part of value --- End of Input Data ---

--- Preliminary Results --- << Skew Weighting >> --------------------------------------------------------------- Based on 69 events, mean-square error of station skew = 0.127 Default or input mean-square error of regional skew = 0.302 --------------------------------------------------------------- << Frequency Curve >> Millsite --------------------------------------------------------------- | Computed Expected | Percent | Confidence Limits | | Curve Probability | Chance | 0.05 0.95 | | FLOW-PEAK, cfs | Exceedance | FLOW-PEAK, cfs | |------------------------|------------|-----------------------| | 20,800 21,200 | 0.2 | 25,300 17,700 | | 19,600 19,900 | 0.5 | 23,700 16,800 | | 18,600 18,900 | 1.0 | 22,300 16,000 | | 17,400 17,600 | 2.0 | 20,700 15,100 | | 15,500 15,700 | 5.0 | 18,300 13,600 | | 13,900 14,000 | 10.0 | 16,100 12,300 | | 11,800 11,900 | 20.0 | 13,500 10,600 | | 8,120 8,120 | 50.0 | 9,020 7,330 | | 5,070 5,030 | 80.0 | 5,660 4,470 | | 3,800 3,750 | 90.0 | 4,330 3,250 | | 2,930 2,860 | 95.0 | 3,420 2,430 | | 1,700 1,600 | 99.0 | 2,100 1,300 | |------------------------|------------|-----------------------|

<< Systematic Statistics >> Millsite --------------------------------------------------------------- | Log Transform: | | | FLOW-PEAK, cfs | Number of Events | |------------------------------|------------------------------| | Mean 3.8801 | Historic Events 0 | | Standard Dev 0.2243 | High Outliers 0 | | Station Skew -0.7502 | Low Outliers 0 | | Regional Skew 0.0000 | Zero Events 0 | | Weighted Skew -0.5277 | Missing Events 0 | | Adopted Skew -0.8000 | Systematic Events 69 | |------------------------------|------------------------------| --- End of Preliminary Results --- --- Final Results --- << Plotting Positions >> Millsite --------------------------------------------------------------- | Events Analyzed | Ordered Events | | FLOW | Water FLOW Weibull | | Day Mon Year cfs | Rank Year cfs Plot Pos | |--------------------------|----------------------------------| | 10 Jun 1912 13,689.92 | 1 1984 20,171.9 1.43 | | 02 Jun 1913 4,058.18 | 2 2002 18,825.119 2.86 | | 05 Jun 1914 10,599.67 | 3 1952 17,301.82 4.29 | | 10 Jun 1915 7,898.18 | 4 1998 15,070.41 5.71 | | 15 Jun 1916 10,099.83 | 5 1986 14,499.17 7.14 | | 23 Jun 1917 7,317.02 | 6 1958 14,419.83 8.57 | | 13 Jun 1918 7,352.73 | 7 1957 14,384.13 10.00 | | 27 May 1919 5,807.6 | 8 1912 13,689.92 11.43 |

| 01 Jun 1920 8,528.93 | 9 1983 13,523.31 12.86 | | 15 Jun 1921 3,703.14 | 10 1997 13,416.2 14.29 | | 08 Jun 1922 9,590.08 | 11 1980 12,979.83 15.71 | | 29 May 1923 5,164.96 | 12 1995 12,737.85 17.14 | | 26 May 1948 6,733.88 | 13 1975 12,577.19 18.57 | | 18 Jun 1949 11,016.2 | 14 2003 12,426.45 20.00 | | 01 Jun 1950 5,879.01 | 15 2001 11,861.16 21.43 | | 01 Jun 1951 7,543.14 | 16 1965 11,625.12 22.86 | | 11 Jun 1952 17,301.82 | 17 1993 11,067.77 24.29 | | 16 Jun 1953 7,422.15 | 18 1949 11,016.2 25.71 | | 21 May 1954 4,318.02 | 19 1914 10,599.67 27.14 | | 15 Jun 1955 3,893.55 | 20 1979 10,419.17 28.57 | | 28 May 1956 6,305.45 | 21 1969 10,218.84 30.00 | | 14 Jun 1957 14,384.13 | 22 1916 10,099.83 31.43 | | 01 Jun 1958 14,419.83 | 23 1922 9,590.08 32.86 | | 08 Jun 1959 2,703.47 | 24 1999 9,201.32 34.29 | | 08 Jun 1960 5,787.77 | 25 1973 9,191.4 35.71 | | 31 May 1961 3,782.48 | 26 1970 8,923.64 37.14 | | 14 Jun 1962 8,643.97 | 27 2004 8,901.82 38.57 | | 05 Jun 1963 6,388.76 | 28 1968 8,864.13 40.00 | | 29 May 1964 6,737.85 | 29 1962 8,643.97 41.43 | | 15 Jun 1965 11,625.12 | 30 2000 8,552.73 42.86 | | 12 May 1966 3,939.17 | 31 1920 8,528.93 44.29 | | 31 May 1967 5,799.67 | 32 1982 8,467.44 45.71 | | 07 Jun 1968 8,864.13 | 33 1996 8,463.47 47.14 | | 31 May 1969 10,218.84 | 34 1978 8,300.83 48.57 | | 04 Jun 1970 8,923.64 | 35 1915 7,898.18 50.00 | | 23 Jun 1971 6,398.68 | 36 1974 7,689.92 51.43 | | 02 Jun 1972 4,702.81 | 37 1951 7,543.14 52.86 | | 25 May 1973 9,191.4 | 38 1953 7,422.15 54.29 | | 04 Jun 1974 7,689.92 | 39 1918 7,352.73 55.71 | | 15 Jun 1975 12,577.19 | 40 1917 7,317.02 57.14 |

| 01 Jun 1976 4,133.55 | 41 1991 7,287.27 58.57 | | 05 Jun 1977 1,118.68 | 42 1994 6,997.69 60.00 | | 13 Jun 1978 8,300.83 | 43 1985 6,900.5 61.43 | | 06 Jun 1979 10,419.17 | 44 1964 6,737.85 62.86 | | 19 Jun 1980 12,979.83 | 45 1948 6,733.88 64.29 | | 05 Jun 1981 4,403.31 | 46 1971 6,398.68 65.71 | | 02 Jun 1982 8,467.44 | 47 1963 6,388.76 67.14 | | 26 Jun 1983 13,523.31 | 48 1956 6,305.45 68.57 | | 07 Jun 1984 20,171.9 | 49 1987 5,938.51 70.00 | | 02 Jun 1985 6,900.5 | 50 1950 5,879.01 71.43 | | 07 Jun 1986 14,499.17 | 51 1919 5,807.6 72.86 | | 21 May 1987 5,938.51 | 52 1967 5,799.67 74.29 | | 30 May 1988 5,327.6 | 53 1960 5,787.77 75.71 | | 14 May 1989 3,163.64 | 54 1988 5,327.6 77.14 | | 13 Jun 1990 3,800.33 | 55 1923 5,164.96 78.57 | | 16 Jun 1991 7,287.27 | 56 1972 4,702.81 80.00 | | 15 May 1992 4,131.57 | 57 1981 4,403.31 81.43 | | 31 May 1993 11,067.77 | 58 1954 4,318.02 82.86 | | 21 May 1994 6,997.69 | 59 1976 4,133.55 84.29 | | 30 Jun 1995 12,737.85 | 60 1992 4,131.57 85.71 | | 21 May 1996 8,463.47 | 61 1913 4,058.18 87.14 | | 07 Jun 1997 13,416.2 | 62 1966 3,939.17 88.57 | | 05 Jun 1998 15,070.41 | 63 1955 3,893.55 90.00 | | 01 Jun 1999 9,201.32 | 64 1990 3,800.33 91.43 | | 29 May 2000 8,552.73 | 65 1961 3,782.48 92.86 | | 25 May 2001 11,861.16 | 66 1921 3,703.14 94.29 | | 22 May 2002 18,825.119 | 67 1989 3,163.64 95.71 | | 02 Jun 2003 12,426.45 | 68 1959 2,703.47 97.14 | | 28 May 2004 8,901.82 | 69 1977 1,118.68* 98.57 | |--------------------------|----------------------------------| * Outlier

<< Outlier Tests >> --------------------------------------------------------------- << Low Outlier Test >> ----------------------- Based on 69 events, 10 percent outlier test value K(N) = 2.888 1 low outlier(s) identified below test value of 1,707 Statistics and frequency curve adjusted for 1 low outliers. ----------------------- << High Outlier Test >> ----------------------- Based on 68 events, 10 percent outlier test value K(N) = 2.883 0 high outlier(s) identified above test value of 29,735 --------------------------------------------------------------- << Skew Weighting >> --------------------------------------------------------------- Based on 69 events, mean-square error of station skew = 0.083 Default or input mean-square error of regional skew = 0.302 ---------------------------------------------------------------

<< Frequency Curve >> Millsite --------------------------------------------------------------- | Computed Expected | Percent | Confidence Limits | | Curve Probability | Chance | 0.05 0.95 | | FLOW-PEAK, cfs | Exceedance | FLOW-PEAK, cfs | |------------------------|------------|-----------------------| | 27,600 29,100 | 0.2 | 34,800 23,100 | | 24,300 25,300 | 0.5 | 30,100 20,600 | | 21,900 22,600 | 1.0 | 26,700 18,800 | | 19,500 20,000 | 2.0 | 23,300 16,900 | | 16,300 16,600 | 5.0 | 19,100 14,400 | | 13,900 14,100 | 10.0 | 16,000 12,400 | | 11,500 11,500 | 20.0 | 12,900 10,400 | | 7,820 7,820 | 50.0 | 8,570 7,130 | | 5,280 5,250 | 80.0 | 5,830 4,700 | | 4,280 4,230 | 90.0 | 4,790 3,730 | | 3,590 3,530 | 95.0 | 4,080 3,060 | | 2,570 2,480 | 99.0 | 3,020 2,090 | |------------------------|------------|-----------------------| << Synthetic Statistics >> Millsite --------------------------------------------------------------- | Log Transform: | | | FLOW-PEAK, cfs | Number of Events | |------------------------------|------------------------------| | Mean 3.8897 | Historic Events 0 | | Standard Dev 0.2001 | High Outliers 0 | | Station Skew -0.1268 | Low Outliers 1 | | Regional Skew 0.0000 | Zero Events 0 | | Weighted Skew -0.0995 | Missing Events 0 |

| Adopted Skew -0.1000 | Systematic Events 69 | |------------------------------|------------------------------|

References Jensen, Donald T. 2002 Update for Probable Maximum Precipitation, Utah 72-hour Estimates Area to 5,000 SQ Miles Utah Climate Center. HYDROMETEOROLOGICAL REPORT NO. 49 Probable Maximum Precipitation Estimates - Colorado River and Great Basin Drainages Prepared by E. Marshall Hansen, Francis K. Schwas, and John T. Riedel Hydrometeorological Branch Office of Hydrology National Weather Service Silver Spring, Md. September 1977 Technical Release No. 60 (TR-60 2P

ndP Edition), Earth Dams and Reservoirs, USDA, Natural

Resources Conservation Service, 2005 Part 630 Hydrology Chapter 10 from the NRCS National Engineering Handbook Table 10-1 Division of Water Resources (State of Utah). 1978. Flood Hydrology Analysis of Millsite Reservoir, Emery County, Utah. RB&G Engineering Inc. 2006. Millsite Dam-Phase II Dam Safety Study. Provo, Utah

Appendix – USUL and HMR 49 General Storm

USU Modified HMR49 PMP GENERAL STORM Millsite Dam UTAH DIVISION OF WATER RESOURCES 06/02/2010 15:47 General Storm PMP Computation USU Don Jensen ------------ Millsite Dam ------------ Area (mi2) 153.00 Lattitude: 39.1610d 0m 0s Longitude: 111.3412d 0m 0s UTM Coordinates East,North 470521.17 4334490.29 One Square Mile Convergence PMP = 10.08 One Square Mile 72-Hour PMP = 10.08 Areal and Duration Reduced 72-Hour PMP(inches) = 8.54 ====== USU Modified HMR49 PMP GENERAL STORM Millsite Dam UTAH DIVISION OF WATER RESOURCES 06/02/2010 15:49 General Storm PMP Computation USU Don Jensen ------------ Millsite Dam ------------ Area (mi2) 153.00 Lattitude: 39.1610d 0m 0s Longitude: 111.3412d 0m 0s UTM Coordinates East,North 470521.17 4334490.29 One Square Mile Convergence PMP = 10.08 One Square Mile 72-Hour PMP = 10.08 Areal and Duration Reduced 24-Hour PMP(inches) = 6.69 ====== *** Warning: Only the 72-hour Duration is fully documented ***

HMR49 PMP GENERAL STORM 06/02/2010 15:51:07 Millsite Dam August ------ August General Storm PMP Computation HMR 49 Millsite Dam Area (mi2) 153.00 UTM Coordinates East,North 470521.17 4334490.29 Convergence PMP Drainage Average Value 11.38 Reduction for Barrier Elevation 0.450 Barrier-Elevation Reduced PMP 5.120 Durational Variation 0.687 0.857 0.940 1.000 1.150 1.213 Convergence PMP for Indicated Durations 3.518 4.388 4.813 5.120 5.888 6.210 Incremental 10 sq. mi. PMP 3.52 0.87 0.42 0.31 0.77 0.32 Areal Reduction 0.78 0.92 0.96 0.99 1.00 1.00 Areally Reduced PMP 2.76 0.80 0.41 0.30 0.77 0.32 Drainage Average PMP 2.76 3.56 3.96 4.27 5.04 5.36 Orographic PMP Drainage Average Orthographic index 5.98586321 Areal Reduction 0.917691588 Adjustment for Month 1.00000000 Areally and Seasonly Adjusted PMP 5.49317646 Durational Variation 0.30 0.57 0.80 1.00 1.57 1.85 Orographic PMP for Given Durations 1.65 3.13 4.39 5.49 8.64 10.18 Total PMP for Given Durations 4.41 6.69 8.36 9.76 13.68 15.54 September --------- September General Storm PMP Computation HMR 49 Millsite Dam Area (mi2) 153.00 UTM Coordinates East,North 470521.17 4334490.29 Convergence PMP Drainage Average Value 11.39 Reduction for Barrier Elevation 0.450 Barrier-Elevation Reduced PMP 5.123 Durational Variation 0.687 0.857 0.940 1.000 1.150 1.213 Convergence PMP for Indicated Durations 3.522 4.393 4.816 5.123 5.892 6.212 Incremental 10 sq. mi. PMP 3.52 0.87 0.42 0.31 0.77 0.32 Areal Reduction 0.78 0.92 0.96 0.99 1.00 1.00 Areally Reduced PMP

2.76 0.80 0.40 0.30 0.77 0.32 Drainage Average PMP 2.76 3.56 3.97 4.27 5.04 5.36 Orographic PMP Drainage Average Orthographic index 5.98586321 Areal Reduction 0.917691588 Adjustment for Month 1.00000000 Areally and Seasonly Adjusted PMP 5.49317646 Durational Variation 0.30 0.57 0.80 1.00 1.57 1.85 Orographic PMP for Given Durations 1.65 3.13 4.39 5.49 8.64 10.18 Total PMP for Given Durations 4.41 6.69 8.36 9.76 13.68 15.54 October ------- October General Storm PMP Computation HMR 49 Millsite Dam Area (mi2) 153.00 UTM Coordinates East,North 470521.17 4334490.29 Convergence PMP Drainage Average Value 10.81 Reduction for Barrier Elevation 0.450 Barrier-Elevation Reduced PMP 4.862 Durational Variation 0.658 0.840 0.930 1.000 1.162 1.242 Convergence PMP for Indicated Durations 3.201 4.084 4.521 4.862 5.648 6.036 Incremental 10 sq. mi. PMP 3.20 0.88 0.44 0.34 0.79 0.39 Areal Reduction 0.80 0.93 0.96 1.00 1.00 1.00 Areally Reduced PMP 2.57 0.82 0.42 0.34 0.79 0.39 Drainage Average PMP 2.57 3.39 3.81 4.15 4.94 5.32 Orographic PMP Drainage Average Orthographic index 5.98586321 Areal Reduction 0.917691588 Adjustment for Month 0.987592638 Areally and Seasonly Adjusted PMP 5.42502069 Durational Variation 0.30 0.57 0.80 1.00 1.57 1.85 Orographic PMP for Given Durations 1.63 3.09 4.34 5.43 8.53 10.05 Total PMP for Given Durations 4.20 6.48 8.15 9.57 13.47 15.38 Total PMP (inches) General Storm HMR 49 ------------------ Millsite Dam ------------ 6 hr 12 hr 18 hr 24 hr 48 hr 72 hr ----- ----- ----- ----- ----- -----

August 4.41 6.69 8.36 9.76 13.68 15.54 September 4.41 6.69 8.36 9.76 13.68 15.54 October 4.20 6.48 8.15 9.57 13.47 15.38

Appendix – Time of concentration study Kirpich Tc and Synder’s Lag Equation

INTRODUCTION

A modified Kirpich method, called the Kirpich Accumulated method, to determine Time of

Concentration (Tc) for subbasins less than 20 square miles is proposed. Results from the Accumulated Kirpich Tc and Synder Lag Time are compared. Times of concentration methods from various editions of the Department of Interior Bureau of Reclamation (BOR) Design of Small Dams are noted. Time of concentration values in the Design of Small Dams 3P

rdP Edition are

compared to the proposed Accumulated Kirpich results.

BACKGROUND INFORMATION

The original BOR Design of Small Dam 1P

stP Edition (1960) and 2P

ndP edition (1973) cite the

Kirpich’s Time of Concentration (Tc) method to calculate Tc for the Unit Hydrograph with reference to the Soil Conservation Service (now the USDA Natural Resources Conservation Service or USDA-NRCS). BOR Design of Small Dam 3P

rdP Edition (1987) changed to Synder’s Lag

Equation. The first two editions used equation 1.

Equation 1. TRLR = SLLca /

The Design of Small Dams 3P

rdP edition changed to equation 2.

Equation 2. SLLCT caL /*=

which was expanded to

Equation 3. SLLKT canL /**26= .

Kirpich’s Equation is equation 4.

Equation 4. 385.03 )/*9.11( HLTc =

KRnR = C/26 – associated to Manning’s roughness of entire drainage L = Length (miles) of longest watercourse LRcaR = Length (miles) along the longest watercourse from the point of concentration S = Overall slope (feet/miles) of longest watercourse H = Overall slope (feet/feet) of longest watercourse TRL R=R RLag Time (hours) Tc = Time of Concentration (hours) The New Mexico Engineering Field Handbook Chapter 2 (NM-EFH2) adopted Kirpich’s

methods discussed in the Design of Small Dams 1P

stP and 2P

ndP Editions. The NM-EFH2 methods

were analyzed to determine appropriate velocities.

In Millsite Hydrology & Hydraulic Study Millsite Watershed Rehabilitation Planning near Ferron, in Emery County, Utah, (NRCS, 2010)) Kirpich’s Tc with breaks for change in slope were used to derive an accumulated Tc, or Accumulated Kirpich Tc. Section Tc states:

Finally a segmented approach was taken using the Kirpich’s equation. Breaks in the reach were identified and these breaks were used to develop the Tc and compiled to obtain the entire Tc for each reach (Figure 3).

WS 14

7200

7400

7600

7800

8000

8200

8400

8600

0 2000 4000 6000 8000 10000 12000Distance (ft)

Elev

atio

n (ft

)

Figure 3. Example of location of breaks within reach.

The profile of the longest flowpath in the subbasin is indicated in blue. The magenta points

represent breaks where Kirpich Accumulated Tc calculations were made along the subbasin longest flowpath.

ANALYSIS Four study areas were taken from BOR Design of Small Dams 3P

rdP Edition (1987) (see Table

1) to recalculate Synder’s Lag times and Kirpich Accumulated Tc. These values were significantly different from the original values.

Table 1. Pertinent data from Design of Small Dams (BOR, 3P

rdP edition 1987)

Index

No. Station and Location

Drainage Area (miP

2P)

Basin Factor SLLca /

Lag Time, h Kn

18 Coal Creek nr Cedar City, UT 92. 6.6 2.4 .050 19 Sevier R. nr Hatch, UT 260. 41.0 5.1 .058 21 Centerville Cr. Nr. Centerville, UT 3.9 0.4 2.4 .124 22 Parrish Cr. Nr. Centerville UT 2. 0.3 2.2 .126

Note that Coal Creek near Cedar City stream gage is 80.9 square miles, the study area is 80 square miles, and BOR Design of Small Dams (BOR, 1987) Study area is 92 square miles.

Note that Sevier River Near Hatch Stream Gage is 340 square miles, the study area 198 square miles, and the BOR Design of Small Dams (BOR, 1987) study area is 260 square miles.

To the left. Right Label is Centerville Creek above Diversion near Centerville Stream Gage is 3.15 square miles, the study area is 3.025 square miles, and the BOR Design of Small Dams (BOR, 1987) is 3.9 square miles. Left Label - Parrish Creek above diversions near Centerville stream gage is 2.08 square miles, the study area is 2.09 square miles, and the BOR Design of Small Dams (BOR, 1987) is 2 square miles.

Stream gage locations were assumed to be the logical place for calculations watershed area in Design of Small Dams 3P

rdP Edition (1987). Watersheds were delineated using HEC GeoHMS to

hopefully find the same longest flowpaths and areas used in BOR Design of Small Dams 3P

rdP Edition

(BOR, 1987). However, this was not possible. Stream gage location and contributing areas in BOR Design of Small Dams 3P

rdP Edition (BOR, 1987) does not correlate.

The Coal Creek watershed is 12 square miles smaller than what was used in this study. The 92 square mile (BOR, 1987) Coal Creek watershed would have significant out of bank flows in the valley flat and does not seem realistic for a study location. As a result, the watershed size is questionable.

The Sevier River watershed near Hatch was listed as 260 square miles (BOR, 1987). Breaks in subbasin from 190 to 315 square miles do not allow hydrologic connection between the subbasins to equal 260 square miles. Again, the watershed size questionable. Therefore, the HEC GeoHMS watershed area of 198 square miles was used in this study.

The difference between Parrish Creek near Centerville was insignificant but a smaller value was used nonetheless. These stream gages are located on the Wasatch Front, with high steep slopes (>0.15 ft/ft).

The HEC GeoHMS watershed for Centerville Creek near Centerville was also smaller than Design of Small Dams 3P

rdP edition (1987). A 3.9 square mile watershed would extend beyond the

canyon into a flat valley. The size of the Centerville Creek watershed documented in the BOR 3P

rdP

edition (1987) is also questionable. New calculations were created using HEC GeoHMS for ArcGIS 9.2. Variables gathered

from HEC GeoRAS concerning Longest Flowpath, Centroid Longest Flowpath, and Slope is presented in Table 2. Kirpich Accumulated Tc is calculated and presented in Table 2. An attempt to calculate the same base data from the topography was done; in other words the same profile was used as well as values derived from HEC GeoHMS to calculate Synder’s Lag equation and Kirpich Accumulated Tc. The lag equation was converted to Tc using the equation 5.

Equation 5. Tc = TRLR*1.67.

Table 2. Synder’s Lag (derived Tc) and Kirpich Accumulated (Ack) Tc derived from HEC

GeoHMS Index

Drainage Area (mi2)

Basin Factor SLLca /

Lag Time, h

Kn Tc, h Velocity fps

Tc, h

Velocity, fps AcK

18 CC 80.4 6.98 2.47 .05 4.12 5.67 4.26 5.49 19 H 198 60.75 5.85 .058 9.77 5.61 10.64 5.14 21 C 3.025 0.3967 2.376 .124 3.97 1.76 0.98 7.12 22 P 2.09 0.314 2..236 .126 3.73 1.73 0.91 7.08

The Tc and velocities for the large basins Coal Creek and Sevier River near Hatch are not significantly different. The current guidance states velocities should range between 4-5 fps. The Tc and velocities for Centerville and Parrish between Snyder’s Lag (converted to Tc) and Kirpich Accumulated Tc are significantly different. Canyon slopes are steeper than 15%, which would

suggest that velocities should be higher than Synder’s derived velocities. The velocities derived from Kirpich Accumulated velocities are higher but are reasonable for this terrain and slopes.

The Millsite Hydrology & Hydraulic Study Millsite Watershed Rehabilitation Planning near Ferron, in Emery County, Utah, (NRCS, 2010) used the Accumulated Kirpich method, not the standard direct Kirpich method. Table 3 shows the results had the direct Kirpich method with no accumulations was used to compute Tc. The direct Kirpich Tc is short and corresponding velocities are very high. As a result the direct Kirpich method does not seem applicable to these watersheds. Table 3. Straight Kirpich’s Tc not using breaks, values surmise from DWR Index No. Kirpich’s Tc direct, h Velocity fps

direct 18 - Coal Creek near Cedar City, UT 2.46 9.54 19 - Sevier River near Hatch, UT 7.29 7.51 21 – Centerville Creek near Centerville, UT 0.65 10.65 22 – Parrish Creek near Centerville, UT 0.58 11.09 Millsite near Ferron, UT 4.83 7.35

Utah DWR guidance states that for local storms a coefficient of 1.6 (C) or Kn = 0.062 and for general storms a coefficient of 3 (C) or Kn = 0.11 could be used. As stated in Design of Small Dams 3P

rdP Edition (1987) states:

Additional analysis of these data have led investigators to conclude that C should be 26 times the average Manning’s n value representing the hydraulic characteristics of the drainage network. This average Manning’s n value is identified as Kn, in subsequent consideration of lag time in this manual. Thus, C = 26Kn. It should be emphasized that Kn is primarily a function of the magnitude of discharge and normally decreases with increasing discharge, a Kn of 0.11 is significantly rough and may not be representative of watershed roughness. The Kn of 0.062 may be reasonable. Table 4 presents is a comparison between the

Snyder’s Lag (derived Tc) and Kirpich Accumulated Tc. Table 4. Synder’s Lag compared to Kirpich Accumulated (AcK) Tc (Local (C=1.6) and General

(C=3) Storms and calibrated) Basin Factor

LLca

Lag Time, h C Kn Tc, h Velocity Tc,

Velocity fps AcK

21.65 4.41 1.6 0.06 7.37 4.817 8.96 3.96 21.65 8.27 3.0 0.115 13.82 2.56 8.96 3.96 21.65 5.38 1.95 0.075 8.98 3.95 8.96 3.96

To determine a matching Synder’s derived Tc

a Kn of 0.075 was used. This value is between the local and general storms that DWR guidance uses. Millsite Comparisons

Earth Dams and Reservoirs Technical Release 60 (USDA-NRCS, 2005) recommends subdividing a 153 square mile watershed into smaller basins.

When the area above a proposed dam approaches 50 square miles, it is desirable to divide the area into hydrologically homogeneous sub-basins for developing the design hydrographs. Generally, the drainage area for a sub-basin should not exceed 20 square miles (USDA-NRCS, 2005). Eighteen subbasins were created for the

Millsite study. A local and general storm Synder’s coefficient and Kirpich Accumulated derived velocities associated with their Tc were compared and presented in Figure 3.

Figure 3. Synder’s Derived and Kirpich Accumulated (AcK) Tc for Millsite Subbasins

Overall, the local (C=1.6) and general (C=3.0) storm Synder’s derived velocities are significantly slower. Kirpich Accumulated derived velocities are somewhat fast and average slopes are 0.087 ft/ft, which would dictate a faster velocity. In order to make Synder’s derived velocities

Figure 2. Screenshot of Millsite Watershed

Synder's Derived and Kirpich's Accumulated velocities

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 2 4 6 8 10 12 14 16 18 20

Subbasins

Velo

cities

(fps

)

fps (C=3)

fps (C=1.6)

fps (C=0.96)

fps (AcK)

comparable to Kirpich Accumulated Tc a coefficient of 0.96 (C) or Kn of 0.037 must be used. This relatively small roughness may be adequate for the steep terrain.

CONCLUSION This reports documents the proposed Accumulated Kirpich Tc method used in Millsite Hydrology & Hydraulic Study Millsite Watershed Rehabilitation Planning near Ferron, in Emery County, Utah, (NRCS, 2010). In contrast, Synder’s Lag equation subjectively generalizes a large subbasin. For large basins with moderate slopes Snyder’s Lag seems appropriate. However, for steep watersheds Synder’s Lag values may not be appropriate. DWR local storm coefficient of 1.6 seems to be a reasonable value for the entire 153 square mile Millsite watershed. But when using smaller subbasins the DWR-suggested Local Storm Synder’s coefficient of 1.6 (C) is not appropriate. Comparisons between Kirpich Accumulated Tc and Synder’s Lag equation showed that Synder’s Lag equations to be subjective. The Kirpich Accumulated Tc method is less subjective and may be appropriate to calibrate to Synder’s’ Lag equation. Observations between results show that Kirpich Accumulated Tc is more variable (scatter of values) and Synder’s Lag equation are less variable. Kirpich Accumulated Tc uses multiple slopes and sections rather than a single general calculation throughout the basin, like Synder’s Lag Equation or direct Kirpich’s Tc. Chapter 15 National Engineering Handbook Section 4 Hydrology (USDA-SCS, 1972) states:

Many natural streams have considerable sinuosity, meander, etc. as well as overfalls and eddies. Tendencies are therefore, to underestimate the length of channels and overestimate average velocities through reaches

and Compute the average velocity. In watersheds with narrow flood plains where the depth of overbank flow may be 10 to 20 feet during a major flood event, it may be desirable to use correspondingly higher velocities for frequencies of 10 to 100 years or greater.

Design of Small Dams (BOR, 1987) also states: It should be emphasized that Kn is primarily functioned of the magnitude of discharge and normally decreases with increase of discharge,

These statements suggest that time of concentration should be calculated for each frequency storm event. Time of concentration is still unknown for large discharges. Time of concentration would be shorter for Probable Maximum Floods or Inflow Design Floods and correspond to higher velocities. Generally, time of concentration values are the least conservative and highest risk because time of concentration is calibrated for frequent storms and applied to more infrequent events.

References United States Department of Interior Bureau of Reclamation (US-DOI-BOR). 1960 1P

stP Edition.

Design of Small Dams. US-DOI-BOR. 1973 2P

ndP edition (Revised Reprint, 1977). Design of Small Dams. A Water

Resources Technical Publication. US-DOI-BOR. 1987 3P

rdP Edition (Reprint, 2004). Design of Small Dams. A Water Resources

Technical Publication.

USDA-NRCS. 2005 Earth Dams and Reservoirs Technical Release 60 (TR60). Conservation Engineering Division.

USDA-NRCS, 2010. Draft-Millsite Hydrology & Hydraulic Study Millsite Watershed Rehabilitation Planning

near Ferron, in Emery County, Utah. Salt Lake City, Utah USDA-SCS (now NRCS). 1972. Chapter 15. Travel Time, Time of Concentration and Lag. National

Engineering Handbook, Section 4. Hydrology. 25TUhttp://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=18389.wbaU25T

http://directives.sc.egov.usda.gov/OpenNonWebContent.aspx?content=18389.wba

prepared by nathaniel todea, hydraulic engineer, … by . nathaniel todea, hydraulic engineer,...

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