Watershed Flow Frequencies

Transcription

1 Watershed Flow Frequencies By: Debora De La Torre CSU Fresno April 1 st -July 2014 Advisor: Tom Lowe USDA Forrest Service Sierra National Forrest July 31, 2014

2 TABLE OF CONTENTS 1. Acknowledgements pg Executive Summary pg Project Objectives pg Project Approach pg Project Outcomes pg Conclusions pg Appendices pg. 5 a. Project Report pg

3 I. AWCKNOLEDGEMENTS This project was supported by Agriculture and Food Research Initiative Competitive Grant no from the USDA National Institute of Food and Agriculture. Special thanks to the Sierra National Forest and the support of Tom Lowe, Elwood Raley (Cliff), Edward Dietz, and Lisa Bonilla for their assistance and guidance in preparing and completing this project. II. EXECUTIVE SUMMARY The Water Resource Institute allowed me to participate with the Sierra National Forrest and assist them in developing a method for calculating flow frequencies for their watersheds. The project involved finding methods to predict water flows for Sierra National Forrest watersheds/basins. These flows would then facilitate in further design projects, such as culvert design, road maintenance, hydrological analysis projects, etc. In order to find these flows several methods had to be looked at and analyzed. Finding adequate and current data were also key areas of this project. III. PROJECT OBJECTIVES The goal of this project was to develop a process that would determine flow frequencies for watersheds located in the Sierra National Forest. This process would need to consider the hydrologic conditions of the Sierra National Forest and utilize the most current data available. The goals of this project did not change over the course of the internship although more details were added to this project in order to understand its complexity. For example, research in methods to calculate flows were looked at; then current data had to be gathered and the most feasible methods were analyzed. One additional goal that correlated with this project was to collect water meter data from different wells located throughout the forest. This was a goal that was added to the project in order to understand water usage in the Sierra Forrest. 3

4 This project helped me put to use what knowledge I had acquired through school and actually apply it to a real life problem. I was able to experience real work that would facilitate design projects for engineers and hydrologist. This project would assist future engineers/ hydrologist in finding a method to calculate flows. The internship allowed me to become familiar with USDA Forest Service careers and the type of work performed in these careers. It is a career path that is very rewarding and is something that I look forward in obtaining. IV. PROJECT APPROACH At the beginning of this internship some of the steps required to complete the project were not very clear. My first s steps were to research methods that would calculate watershed/basin flows. I was provided with many informative articles and reports that included watershed methods to calculate flows. Once all possible methods were looked at; two methods for calculating these flows were chosen. These methods were chosen since they considered precipitation, snow accumulation, and overall the hydrology of the Sierra Nevada. These two methods were the rational method and the method developed by the USGS using regional regression equations. Once these two methods were chosen, then current data had to be gathered. The data collected was from different agencies such as the Department of Water Resources, National Oceanic and Atmospheric Administration, the US Geological Survey, and others. Data was also obtained from GIS database provided by the Sierra Forest. Once all the data was collected a trial problem was developed to compare both methods. Both of the methods used provided answers that were less than 10% different. Once the trial problem was calculated an Excel workbook was developed to facilitate calculations. This Excel workbook will help future calculations, since only several parameters must be inputted in order to calculate a flow. Originally an Excel workbook was not included in the goals of the project but since it facilitates the overall project it was included as well. The target flow that needed to be calculated was Q 100 but other key flows such as Q 2 and Q 25 were also calculated and included in the Excel workbook. A report was also developed that correlated with the Excel workbook. This report also includes explanations as to why these two methods were chosen and provides explanation of the two methods. 4

5 V. PROJECT OUTCOMES The outcomes of this project were several. One of the most important outcomes was to develop a trial problem and calculate Q 100 using the two methods, and imputing all the data gathered. When the solution to the problem was found using the rational method and the regression equations provided by USGS, the difference between both was only 9%. This result showed that both methods produced very similar answers. For the purpose of finding the right solution we decided that the best answer would be the most conservative from both methods utilized. VI. CONCLUSIONS Overall this project provided me with experience of real problems. The project I was assigned allowed me to implement school material with a real life problem. It also allowed me to become familiar with the type of work performed by engineers with the USDA Forest Service. This internship introduced me to multiple career options that are available thru the USDA. It would be great if in the future I could join the USDA, specifically as a Civil Engineer focusing on water resource/ environmental engineering. VII. APPENDIXES Below is the report developed and prepared for the US Sierra National Forrest. I. INTRODUCTION a. Background The Sierra National Forest is located in California and lies between Yosemite National Forest and Sequoia National Forest. It s located on the western slope of the Sierra Nevada and encompasses more than 1.3 million acres of land. Its elevations range from 900 ft. to about ft. The Sierra Forest is located east of the central valley with nearby towns consisting of Fresno, Clovis, Oakhurst, and Madera CA. This Forest provides recreation along with spectacular views and resources that many valley residents rely on. 5

6 Figure 1: Sierra National Forest b. Problem The Sierra National Forest needs to develop a simplistic method to calculate water flow frequencies for watersheds located in the Forest. These methods must consider flows pertaining to precipitation, snow, evaporation, etc. When looking at all the possible ways to determine these flows two methods were looked at in this report. These methods provide estimates for watershed flows that incorporate all parameters mentioned. In order to calculate the flows for the Sierra Forest the flow of the 100 year flood must be calculated. This means that annual exceedance probability is equal to 1. There are several approaches when calculating Q 100 (flow), but for the purpose of this project two methods can be used. One method involves the use of the Rational Equation, and the second is that of the USGS Regression Equation. Both of these methods provide flow estimates for a watershed. II. PROCEDURE For the purpose of determining flow (Q) for Sierra Forest watersheds there are several approaches than can be utilized. The procedures described below only pertain to ungagged sites. Regression equations are statistical models that must be interpreted and applied within the limits of the data and with the understanding that the results are best-fit estimates with an associated scatter or variance (Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water Year 2006: USGS, pg. 14) 6

7 According to the Highway Design manual (Caltrans), is recommended that if the watershed is smaller than 320 acres (0.5 mi 2 ), then proceed using the rational method. Rational Methods should be used only for estimating runoff from small simple watershed areas, preferably no larger than 320 acres (Highway Design Manual-Chap810-Hydrology-pg.14). For all other watershed areas it is recommended to use the USGS regression equations. For the purpose of this project it is recommended to utilize both methods presented, and use the solution that produces the most conservative answer. Finding Q 100, the flow that will be used for design purposes or road maintenance projects can be found using the following two methods. III. RATIONAL METHOD According to the Highway Design Manual; if watershed is < 320 acres (0.5 mi 2 ) then the rational method can be utilized. Q = CIA (Highway Design Manual, Chap March 7, 2014). For rational method: Q = CIA C = Runoff Coefficient, (dimensionless) Found from the Highway Design Manual. Chap810-16: C can be found using the table provided for Undeveloped Areas. It is a summation of soil parameters in the watershed. (Relief, Soil Infiltration, Vegetal Cover, and Surface Storage) 7

8 Figure 2: Runoff Coefficient C A = Area of the watershed. To find the area of the watershed several procedures can be utilized. One of these procedures is to look at software provided by the Sierra National Forest. With the help of GIS a map can be utilized where the Sierra Forest has been broken down into small watersheds, and areas are given in acres. One other way to calculate the area is to develop Thiessen polygons and utilize the areas developed by this procedure. This procedure is more tedious and time consuming than utilizing GIS watershed information. For the purpose of calculating areas the most simplistic way is to use GIS to obtain this information. I = intensity in in/hr. It is the average rainfall intensity in inches per hour for the selected frequency and for duration equal to the time of concentration. It requires that the storm duration and the time of concentration (T c ) be equal. Intensity can be found using the following equations. (1) Finding T c = Time of concentration= Time of concentration is defined as the time required for storm runoff to travel from the hydraulically most remote point of the drainage basin to the point of interest. 8

9 (2) Once T c is found and an recurrence interval is of 100 is chosen; then utilize IDF curves provided by the National Oceanic and Atmospheric Administration (NOAA) to find I, intensity (NOAA s- To find Time of Concentration (T c ) the summation of three parameters must be performed. These parameters are used or calculated only when the conditions in the watershed represent these. 1. Sheet flow travel time (T t ), 2. Shallow concentrated flow travel time (T t ), 3. Channel flow travel time (T t ). (1) Sheet flow travel time: Sheet flow is flow of uniform depth over plane surfaces and usually occurs for some distance after rain falls on the ground. This can be found using: Where; T t = Travel time in minutes L = Length of flow path in feet flow from the beginning of the channel to the site under consideration (Highway design manual Chap810-4) n = Manning's roughness coefficient for sheet flow P = is the 2-year, 24-hour rainfall depth in inches S = Slope of flow in feet per feet. S can be found using topographic maps and calculating the change of elevation (ft.) over the change of length (ft); Δ Elevation (ft) Δ Length (ft) n can be found utilizing the table provided. (Highway Design Manual: Chap ) 9

10 Figure 3: Roughness Coefficient for Sheet Flow n (2) Shallow concentrated flow travel time. After short distances, sheet flow tends to concentrate in rills and gullies, or the depth exceeds the range where use of the Kinematic wave equation applies. This can be found using the following two equations: Figure 4: Coefficient for Shallow Concentrated Flow k (1). V = (3.28) ks 1/2 Where; V= average velocities (ft/s) S= Slope of flow in percent k = is an intercept coefficient depending on land cover k, can be found using: 10

11 And (2) Where; T t = Travel time in minutes L = length in feet V = velocity in feet per second (3) Channel flow travel time. This parameter is considered for the stream being analyzed as well as any open/closed channels in the watershed. For the most part watersheds in Sierra Forest will not have manmade open channels, but analysis of the watershed needs to be conducted prior to any calculations. When the channel characteristics and geometry are known the preferred method of estimating channel flow time is to divide the channel length by the channel velocity obtained by using the Manning equation, assuming bank full conditions. The Following equation can be used to find Travel time for Channel flow. (1) Manning s equation, V = (1.49/n) (R 2/3 ) (S 1/2 ) Where: n = is manning s roughness coefficient for the channel (dimensionless) (found using table provided) A= Cross sectional area of the channel (ft 2 ) R= Hydraulic Radius, Found by (A/P w ). P w = wetted perimeter of the cross-sectional area of flow (ft) S= Slope of the channel V= velocity in ft/s And T t = L/60V Figure 5: Manning s Roughness Coefficient (Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains United States Geological Survey Water supply Paper

12 If all three parameters are found in the watershed then the summation of these (Sheet flow, Shallow Concentrated flow, and Channel flow) will produce Time of Concentration (T c ). Once this time of concentration has been calculated then IDF curves provided by NOAA can be utilized and intensity can be found. Watersheds travel time (T t) will be dependent on the watershed being analyzed. a.) Example problem (Rational Method): A culvert needs to be replaced in Sierra Watershed. This culvert is located on road 9S07 near Pine Ridge. The coordinates for the location can be obtained from a map and they are 37 o N and 119 o W. In order to size the culvert correctly, the flow will need to be calculated. (Q 100 ). In order to do so the two procedures will be utilized. The first method will be that of the rational method. Solution: Rational Method: Q= CIA (C)= Runoff Coefficient can be obtained from Figure 2 provided in this paper. Relief= High- Hilly with average slopes of 10%-30% = 0.20 Soil Infiltration= Normal = 0.06 Vegetal Cover= Low = 0.04 Surface Storage= Normal= 0.04 C= 0.34 (I)= Intensity can be found using the following equations: I.)Travel time for sheet flow: 12

13 For example: P = 4.19 in. (Obtained from ( using the coordinates provided) n= 0.40, obtained from Figure 3. S = 0.20 (assumed 20% for this example problem, but can be obtained from topographic map) L= 9,400 ft (Obtained from Google maps, GIS, etc.)(length of stream from furthest upstream location to the site under consideration) T t = 0.42 *(9400 4/5 )* (0.40 4/5 )/ [(4.19 1/2 ) *(0.202 /5 )] T t = minutes or 4.07 hours. II.) Travel time for shallow concentrated flow (Using Upland Method) For Example: K= (obtained from figure 5) S= 0.20 V= 0.11 ft/s And travel time is Where; L= 9,400 ft (length of flow path) V= 0.11 ft/s Tt= 9400 ft/ [60*(0.11ft/s *60 sec/min)] T t = minutes or.39 hours. III) Channel flow travel time. (Manning s Equation) 13

14 For Example: V = (1.49/n) A (R 2/3 ) (S 1/2 ) And T t = L/60V Since area and wetted perimeter of the stream is only found after performing a survey of the stream we will not include this parameter in the example calculations. For real problems a survey on the stream must be conducted to get a profile which will provide the information for the wetted perimeter and the hydraulic radius. All other parameters such as slope and Manning s roughness coefficient can be obtained using the tables provided. Area = A = by + zy 2 Wetted perimeter = P w = b + 2y (1 + z 2 ) 1/2 Hydraulic radius= R= (by + zy 2 )/ [b + 2y (1 + z 2 ) 1/2 ] Figure 6: Profile of the channel/stream (finding Hydraulic Radius) Therefore Time of concentration (T c ) = min min giving a total time of concentration of 306 minutes or 5 hours. T c = 5 hours. From here we utilize NOAAS webpage and input latitude and longitude for the location of the example problem and obtain IDF curves with different duration and frequencies. Since there is no duration of 5 hours we look at the closest duration of 6 hours and the recurrence interval of 100 years. The intensity given from the graphs and table is in/hr. (I)= in/hr 14

15 Figure 7: IDF Curve for example problem. (A) Area can be found using GIS provided by the Sierra National Forest. Under the watershed area tab HUC8 can be found. This map contains small watersheds and their areas are given in acres. The example problem would lie in a watershed with 2159 acres. Area (A) = 2159 acres or 2159 acres Figure 8: Area of watershed for example problem. Therefore the solution for this example problem would be Q=CIA. Q = (0.34) * (0.794) (2159) = , or 583 cfs 15

16 Rational method solution Q= 583 cfs IV. REGRESSION EQUATION If watershed has an area > 320 acres utilize USGS Regression Equations. (Methods for Determining Magnitude and Frequency of Floods in California, Based on Data through Water year 2006, USGS SIR ). For Sierra Region this equation is Q 100 = 20.6 (A) (E) (P) 1.24 Where; (A) = Area of watershed in mi 2. This can be found by using GIS, and loading HUC8 watershed area information. If the area that is to be analyzed is only a portion of the whole HUC8 area, then multiply by that factor. Flood-frequency relations at sites near gaged sites on the same stream (or in a similar watershed) can be estimated using a ratio of drainage area for the ungagged and gaged sites. (Methods for Determining Magnitude and Frequency of Floods in California Based on Data through Water Year 2006: USGS, pg. 18). For example if the watershed that is being analyzed is only 1/3 of the whole HUC8 area, then multiply the area by that factor. Other forms of calculating the area have been mentioned above and include Thiessen Polygons. (E) = mean average basin elevation in ft. This elevation can be found using any topographic map that might be available. It is recommended that ten different points along the basin be taken and averaged out to obtain the mean elevation. (P) = basin mean annual precipitation in inches. This can be found using current information available thru NOOAS National Weather Service ( or thru the department of water resources. One other key available source can be found using GIS provided by the Forrest Service. This can be done using precipitation information from GIS. 16

17 Figure 9: Areas: This map shows watershed areas in acres. This section is the northern part of the Sierra Forest. a.) Example Problem (Regression Equation) A culvert needs to be replaced in Sierra Watershed. This culvert is located on road 9S07 near Pine Ridge. The coordinates for the location can be obtained from a map and they are 37 o N and 119 o W. In order to size the culvert correctly, the flow will need to be calculated. (Q 100 ). In order to do so the two procedures will be utilized. The first method will be that of the rational method. Solution: Regression Equation Method: (A)Area: Q 100 = 20.6 (A) (E) (P) 1.24 To find area of the watershed, GIS can be utilized. HUC8 can be used and the location of the culvert can be found by utilizing its coordinates. Using figure 7 the area of the watershed is given = 2159 acres. This can then be converted to mi 2. (E) Elevation: A= 2159 acres / (640 acres/mi 2 ) = 3.4 mi 2 To find elevation use a topographic map. The elevation for the example was found using National Oceanic and Atmospheric Administration-National Weather Service. ( The elevation was found to be 7092 ft. 17

18 E= 7,092 ft (P) Precipitation: To find mean annual precipitation several options are available. A GIS map can be utilized showing mean annual precipitation. This was found using Sierra National Forrest GIS watershed information. The map below illustrates annual mean precipitation. Figure 10: GIS map showing precipitation data. From figure 8 the culvert lies between inches of precipitation, but it is much closer to 30 inches. If utilizing data from the Department of water Resources- California Data Exchange Center the mean average precipitation is inches. ( Therefore utilize the data that represents a larger number; use inches. (P) = in. Once all these three parameters are found then Q 100 can be calculated; this calculation can be performed using Excel or a calculator. Q 100 = 20.6 (2159 acres *1mi 2 /640 acres) (7092) (34.94) 1.24 = r 533 ft 3 /s. Regression Equation Solution = 533 ft 3 /s. 18

19 CONCLUSION: When comparing both solutions we can obtain a 9% difference between both. Therefore utilize the estimate that is more conservative. For the example problem we can utilize the answer provided by the rational equation which is of 583 cfs. For future problems associated with calculating flow there is one other source which can provide estimates. This source is found online through the USGS website and the application is Stream Stats. This source is an interactive map where a location is inputted and the flow is given along with the area of the basin/watershed. This interactive map can be found in It is recommended to utilize all sources since these are estimates for the design flow. It is up to the designer to utilize the flow that he considers is the most adequate one. Choosing an estimate should be done once a full analysis of the watershed has taken place. 19

20 REFERENCES 1. Cafferata Pete, December 13, Stream Crossing Alternatives, California Department of Forestry and Fire Protection. 2. Cafferata Peter, Spittler Thomas, Wopat Michael, Bundros Greg, and Flanagan Sam. February 10, Designing Watercourse Crossings for Passage of 100-year Flood Flows, Wood, and Sediment. 3. Clarkin Kim, Connor Anne, Furniss Michael J., Gubernick Bob, Love Michael, Moynan Kathleen, Wilson Musser Sandra. November NATIONAL INVENTORYAND ASSESSMENT PROCEDURE For Identifying Barriers to Aquatic Organism Passage at Road-Stream Crossings 4. DeVault Brooke, TEAMS Enterprise Unit, USDA Forest Service Aquatic Organism Passage (AOP) at Road Stream Crossings Assessment. Sierra National Forest. 5. Highway Design Manual. Chapter 810 Hydrology. March 7, Highway Design Manual. Chapter 850. Physical Standards. March7, Keller Gordon, PE, Sherar James, PE. July Low-Volume Roads Engineering Best Management Practices Field Guide. USDA, Forest Service, International Programs. US Agency for International Development (USAID). 8. Gotvald, A.J., Barth, N.A., Veilleux, A.G., and Parrett, Charles, 2012, Methods for determining magnitude and frequency of floods in California, based on data through water year 2006: U.S. Geological Survey Scientific Investigations Report , 38 p., 1 pl., available online only at 9. G.J. Arcement, Jr. and V.R. Schneider. Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains United States Geological Survey Watersupply Paper ( 10. McCuen Richard H., Johnson Peggy A., Ragan Robert M., Hydraulic Design Series No. 2, Second Edition, Highway Hydrology. U.S. Department of Transportation. 20