HYDROLOGICAL IMPACTS OF URBANIZATION: WHITE ROCK CREEK, DALLAS TEXAS. Julie Anne Groening Vicars, B.A. Thesis Prepared for the Degree of

Transcription

1 HYDROLOGICAL IMPACTS OF URBANIZATION: WHITE ROCK CREEK, DALLAS TEXAS Julie Anne Groening Vicars, B.A. Thesis Prepared for the Degree of MASTER OF SCIENCE UNIVERSITY OF NORTH TEXAS December 25 APPROVED: Harry F.L. Williams, Major Professor Samuel F. Atkinson, Minor Professor Miguel F. Acevedo, Committee Member Paul Hudak, Chair of the Department of Geography Sandra L. Terrell, Dean of the Robert B. Toulouse School of Graduate Studies

2 Vicars, Julie Anne Groening, Hydrological Impacts of Urbanization: White Rock Creek, Dallas Texas. Master of Science (Applied Geography), December 25, 91 pp., 28 tables, 55 illustrations, bibliography, 22 titles. This research project concerns changes in hydrology resulting from urbanization of the upper sub-basin of the White Rock Creek Watershed in Collin and Dallas Counties, Texas. The objectives of this study are: to calculate the percent watershed urbanized for the period of 1961 through 1968 and the period of 2 through 25; to derive a 196s average unit hydrograph and a 2s average unit hydrograph; and, to use the two averaged hydrographs to develop a range of hypothetical storm scenarios to evaluate how the storm response of the watershed has changed between these two periods. Results of this study show that stormflow occurs under lower intensity precipitation in the post-urbanized period and that stormflow peaks and volumes are substantially larger compared to the pre-urbanized period. It is concluded that changes in watershed surface conditions resulting from urbanization have lowered the precipitation-intensity threshold that must be surpassed before storm run-off is generated.

3 Copyright 25 by Julie Anne Groening Vicars ii

4 ACKNOWLEDGMENTS Sally Rodriguez, Project Coordinator; Parks and Recreation, City of Dallas, Texas John Unruh, Data Manager; United States Geological Society, Fort Worth, Texas Dr Harry Williams, Associate Professor; Department of Geography, University of North Texas, Denton, Texas Dr Miguel Acevedo, Professor; Department of Geography, University of North Texas, Denton, Texas Dr Samuel Atkinson, Professor; Department of Environmental Science, University of North Texas, Denton, Texas iii

5 TABLE OF CONTENTS Page ACKNOWLEDGMENTS...iii LIST OF TABLES... v LIST OF ILLUSTRATIONS...vii Chapters I. INTRODUCTION... 1 II. REVIEW OF LITERATURE... 2 Hydrologic Cycle Impervious Surface Cover Channel Modifications Objectives Study Area III. METHODOLOGY... 9 Data Collection Data Analysis IV. RESULTS V. DISCUSSION VI. CONCLUSION APPENDIX: DATA TABLES AND GRAPHS BIBLIOGRAPHY... 9 iv

6 LIST OF TABLES Page 1. Calculating Unit Hydrograph Ordinates Excess Precipitation and Threshold Value Calculations Events and Threshold Values Used in Analysis Lag Time Values and Calculations Flood Event Values for White Rock Creek Watershed Scenario #1:.5 Inches per Hour for 3 Hours Scenario #2: 1. Inch per Hour for 3 Hours Scenario #3: 1.5 Inches per Hour for 3 Hours Scenario #4: 2. Inches per Hour for 3 Hours Scenario #5: 2.5 Inches per Hour for 3 Hours Scenario #6: 3. Inches per Hour for 3 Hours Scenario #1 Value Comparison Scenario #2 Value Comparison Scenario #3 Value Comparison Scenario #4 Value Comparison Scenario #5 Value Comparison Scenario #6 Value Comparison Lag Time Comparison: Z-Test Storm Event: November 22, Storm Event: April 27, Storm Event: September 7-8, Storm Event: April 28, Storm Event: August 13-14, v

7 24. Storm Event: December 3, Storm Event: April 23-24, Storm Event: June 11-12, Storm Event: December 12, Storm Event: June 9, vi

8 LIST OF ILLUSTRATIONS Page 1. The Hydrologic Cycle Impervious Surface Cover and Its Impacts on Runoff Study Area: White Rock Creek Watershed Land Use Changes for White Rock Creek Watershed, 1962 and Extract of 196s Handwritten Data Example of Bimodal Hydrograph Example of Initial Hydrograph with Precipitation, for April 28, Semi-log Plot Y=aX+b Scatter Plot Initial Hydrograph and Baseflow Unit Hydrograph Procession from Initial Hydrograph to Unit Hydrograph Rainfall Hyetograph showing Excess Precipitation threshold The S-Curve Plot S-Curve Plots and 3 Hour Unit Hydrograph s Unit Hydrographs and Average s Unit Hydrographs and Average Scenario #1 Hydrograph Scenario #2 Hydrograph Scenario #3 Hydrograph Scenario #4 Hydrograph Scenario #5 Hydrograph Scenario #6 Hydrograph vii

9 24. Peak Flow Comparison Storm Flow Volume Comparison November 22, 1961: Procession from Initial Hydrograph to Unit Hydrograph November 22, 1961: Precipitation Hyetograph November 22, 1961: Final 3-Hour Unit Hydrograph April 27, 1962: Procession from Initial Hydrograph to Unit Hydrograph April 27, 1962: Precipitation Hyetograph April 27, 1962: Final 3-Hour Unit Hydrograph September 7-8, 1962: Procession from Initial Hydrograph to Unit Hydrograph September 7-8, 1962: Precipitation Hyetograph September 7-8, 1962: Final 3-Hour Unit Hydrograph April 28, 1966: Procession from Initial Hydrograph to Unit Hydrograph April 28, 1966: Precipitation Hyetograph April 28, 1966: Final 3-Hour Unit Hydrograph August 13-14, 1968: Procession from Initial Hydrograph to Unit Hydrograph August 13-14, 1968: Precipitation Hyetograph August 13-14, 1968: Final 3-Hour Unit Hydrograph December 3, 22: Procession from Initial Hydrograph to Unit Hydrograph December 3, 22: Precipitation Hyetograph December 3, 22: Final 3-Hour Unit Hydrograph April 23-24, 23: Procession from Initial Hydrograph to Unit Hydrograph April 23-24, 23: Precipitation Hyetograph April 23-24, 23: Final 3-Hour Unit Hydrograph June 11-12, 23: Procession from Initial Hydrograph to Unit Hydrograph June 11-12, 23: Precipitation Hyetograph... 8 viii

10 49. June 11-12, 23: Final 3-Hour Unit Hydrograph December 12, 23: Procession from Initial Hydrograph to Unit Hydrograph December 12, 23: Precipitation Hyetograph December 12, 23: Final 3-Hour Unit Hydrograph June 9, 24: Procession from Initial Hydrograph to Unit Hydrograph June 9, 24: Precipitation Hyetograph June 9, 24: Final 3-Hour Unit Hydrograph ix

11 I. INTRODUCTION Urbanization affects the environment in numerous ways, including habitat and wildlife loss, and disruption and alteration of various hydrological processes. Statistics show that more then 75 percent of the United States population lives in urban areas; by the year 23 more then 6 percent of the world's population will live in urban areas (UN Pop Division 21; US Census 21; Paul and Meyer 21). People have flocked towards rivers for centuries. So, following that trend, over 13, kilometers of streams and rivers in the United States are impacted by urbanization (USEPA 2). Urbanization impacts rivers in various ways; channel control, or channelization, grading of land surfaces, building construction, use of storm water drainage systems, removal of vegetation, and increased amounts of litter and waste are all ways in which rivers are affected by urbanization. In Texas, urbanization is a growing trend. Many agricultural communities are becoming suburbanized, as the demand for 'country living grows. With an increase in population, also comes the potential for an increase in flooding. Urbanization changes watershed surfaces in such a way that natural hydrological processes are disrupted. Elements of urbanization that affect hydrological processes include homes, landscaped yards, businesses, concrete and asphalt streets, parking lots, and storm drainage systems. These urban features alter natural infiltration and runoff. Along with river modifications, changes to natural watershed surfaces pose a potential threat to the people who live near streams or rivers due to the potential increase in storm flow. 1

12 II. REVIEW OF LITERATURE To understand the dynamics of a watershed, one must first understand the hydrologic cycle (Figure 1). Figure 1. The Hydrologic Cycle (McCuen 1998). Water in the environment is recycled through several stages. This process, known as the hydrologic cycle, is based on five steps: precipitation, infiltration, evaporation, transpiration, and runoff. Precipitation falling into a watershed occupies specific hydrological storages or pathways. Some of this precipitation will be intercepted by foliage, which will store it until it eventually evaporates. When the foliage storage space reaches capacity, excess water will flow to the ground as stem flow. Precipitation, that reaches the ground may infiltrate and add itself to soil moisture or groundwater. Subsurface water can be transpired by plants, or it can flow down slope into nearby 2

13 channels, supplying stream base flow. If water reaches the ground faster than it can infiltrate, the excess water can occupy surface depression storages. When these storages become filled to capacity, the excess precipitation will flow down slope as surface runoff. Surface characteristics serve as one of the factors that affect rainfall infiltration and runoff. Surface conditions determine whether rainfall infiltrates the ground and flows along relatively slow subsurface hydrological pathways, or if it fails to infiltrate and instead flows along relatively fast surface pathways (Williams Earth Science Lab). For example, soil texture influences the rate at which surface water enters the soil profile, or the infiltration capacity. The infiltration capacity affects the amount of water that enters streams and rivers as direct or surface runoff (McCuen 1998). Surface runoff is generated in two ways: infiltration-excess overland flow, which occurs when the rainfall intensity exceeds the infiltration capacity, the excess rain becomes overland flow; and saturation overland flow, which occurs when the soil is completely saturated, due to a combination of precipitation and subsurface flows, the base of slopes and hill-slope concavities are prone to this type of surface runoff, especially during prolonged precipitation. Surface cover significantly affects the runoff characteristics of a watershed. For example, lag time, storm flow volume, and peak flow may all be affected by a change in surface cover. Peak flows increase with increased urbanization in a watershed. Along with this increase, storm flow volume also increases. Lag time the time between precipitation and resulting stormflow - may also be changed by changes in surface conditions. 3

14 Impervious Surface Cover Land cover greatly affects surface properties and runoff characteristics of a watershed. Urbanization changes the natural surface of the land, often impeding natural infiltration and promoting increased runoff (Figure 2). Rainfall that settles on to areas of impervious surface cover will automatically become runoff, causing streets to become urban channels; rain that falls on to areas of pervious surface will infiltrate into the ground, pool and infiltrate slowly over time, or be intercepted by vegetation, therefore decreasing runoff. Figure 2. Impervious Surface Cover Impacts on Runoff. Storm drainage systems contribute to increased volumes of storm flow and decreased lag times. Many man-made drainage systems were built to the specifications of a 1-year storm, but with increased impervious cover a 1-year storm can produce the same amount of runoff as a 25-year storm, thereby overloading the drainage system (NFIP 1994). These systems can increase flooding, leading to increased property damage. Storm drainage systems divert the majority of surface runoff, which eventually injects large volumes of storm water into one area of the stream. This increases storm 4

15 flow in the channel, promoting flooding and erosion, which will negatively affect the stability of the stream s channel (NFIP 1994). Channel Modification Rivers and streams naturally migrate over their lifespan, changing their course over time. Due to this migration, people have seen the need to keep rivers and streams from moving, thereby channeling them within concrete channels. Examples of channeled rivers can be seen in most of the urban areas surrounding Dallas, one example is White Rock Creek at Greenville Avenue. Though channeling a river does control the issues of boundaries, it also changes the natural movement of the water. In the past many channels built along riverbanks were created using narrower, straighter channels then were previously (and naturally) there, thus, causing water to flow down those channels at a higher velocity (Paul et al 21). For a river to flow through a narrow channel, it must push its capacity with more force, thus increasing the velocity, and possibly overflowing the channel downstream. Along with channelization come culverts and bridges. Culverts, especially, can be hazardous due to debris build-up. If proper maintenance is not used, bridges and culverts can back up flood waters to a point, that once the debris gives way, it can produce flash flooding downstream. Upstream flooding can also occur because of this back up (NFIP 1994). Objectives In this research project I will study the effects of urbanization on the upper subbasin of the White Rock Creek Watershed in Collin and Dallas Counties, Texas. My objectives are: 5

16 1. Calculate the percent watershed urbanized for the period 1961 through 1968 (little urbanization) and the period 2 through 25 (substantial urbanization). 2. Derive a 196s average unit hydrograph and a 2s average unit hydrograph. 3. Compare the two averaged hydrographs to evaluate how the storm response of the watershed has changed between these two periods of contrasting urbanization. Study Area Headwaters for White Rock Creek are located in Frisco and Plano. The rest of the watershed can be found in Addison, Richardson, and Dallas (Figure 3). Presently, hydrologic instruments located on White Rock Creek include two stream gauging stations, located at Greenville Avenue and Keller Springs Road. The closest rain gage to White Rock Creek, operated by NOAA, is found at Dallas Love Field Airport (approximately 4 miles away). 6

17 Figure 3. Study Area: White Rock Creek Watershed Urbanization in White Rock Creek Hydrologic instruments on White Rock Creek, in the 196s, consisted of 14 recording rain gauges, 4 stream gauging stations, 5 crest stage partial recording stations, and 39 flood profile partial record stations. There were 12 weighing rain gauges throughout the watershed area above Greenville Avenue, spaced to sample 5.5 square miles. Upper White Rock Creek, with a total area of 66.4 square miles, was 87 percent rural in 1961 (Ollman 1969). With the decline in farming in 1967, large apartment complexes, shopping centers, and industrial parks began being constructed in the watershed. By 199, the watershed was becoming much more urbanized; a trend which 7

18 has continued into the 2s (Figure 4). Overall, development within the watershed has increased from 13 percent urbanization in 1961, to approximately 95 percent today. Figure 4. Land Use Change for White Rock Creek Watershed, 196s and 2s 8

19 III. METHODOLOGY DATA COLLECTION Data for this study will include: 196s and 2s Stream Discharge data in cubic feet per second. 196s and 2s Hourly Rainfall data in inches per hour. Gauging Stations 196s and 2s Stream flow Data: White Rock Creek at Greenville Avenue, USGS Stream discharge gage This gage was chosen for its completeness of 196s data, as well as 2s data and for its location. Gage 8572 is located on the intersection of White Rock Creek and Greenville Avenue, which is also the border between the upper and the lower sub-basins of White Rock Creek. This gauge records discharge from a 66.4 square mile drainage area. Precipitation data was obtained from the two gauges listed below: 196s Precipitation data: Rain Gauge 9-W, on White Rock Creek, near the Greenville Avenue streamflow gauging station (Figure 3). 2s Precipitation data: NOAA Rain Gauge, located at Dallas Love Field (Figure 3). Precipitation and stream discharge data, for the 196s, was collected from the report, Hydrology Data for Urban Studies in Dallas, Texas, published by the United States Geological Society Water Resources Division ( ). These annual reports record major storm events that occurred each year. Storms recorded in these books are the highest recorded stream discharge peaks of the year. For the White Rock Creek at Greenville Avenue stream flow gauging station, there are at least three major storm 9

20 events per year. For each of these storms, the data was handwritten (Figure 5). Fields and values that were used include: Time and Date (time = 1 hour increments), Stream discharge in cubic feet per second, and Rainfall in inches. Events for the 196s were selected based on the completeness of the data, whether or not precipitation data existed for a particular event, and visual evaluation of the initial hydrograph. For a storm to be used the initial hydrograph must continually recede on the recession limb side; there can be no increase from one point to the next in stream discharge after peak discharge (i.e. bimodal hydrograph; Figure 6). Bimodal hydrographs represent storms that were influenced by another separate period of rainfall. Peak flow, for an event to be used, must be 9 cubic feet per second or greater, so that only those storms that have a significant amount of rainfall are used. Although fifteen storms were recorded for the 196s, these were reduced to 5 storms that met the above criteria. 1

21 Figure 5. Extract of 196s handwritten data. October 18-19,22: Bimodal Hydrograph Stream discharge (cfs) Time (hours) Figure 6. Example of Bimodal Hydrograph. 11

22 2s stream discharge data was downloaded from the USGS Real-Time hydrology website. This data is updated frequently and is in 15-minute increments. Precipitation data was downloaded from the NOAA rain gauge site. This data covers an entire month and is recorded in 1-hour increments. Stream discharge and precipitation data was entered into an Excel spreadsheet, using 1-hour increments. Criteria for selecting the 2s storm events were the same as the criteria for the 196s. Data for twenty storms was obtained for this analysis, but due to the criteria mentioned above, only 5 were used for analysis. Assumptions and Potential Sources of Error Different storm characteristics and watershed conditions can produce variations in unit hydrographs. Hydrographs for this project were averaged for each time period, because many watershed characteristics were not accounted for in the study. One important factor not considered was antecedent moisture conditions. Antecedent moisture, which refers to the moisture content of the soil immediately preceding a rainfall event, can affect lag times and stormflow volumes and peaks. For example, a soil already saturated at the onset of a storm may act like an impervious surface and generate larger amounts of stormflow more quickly than under lower antecedent moisture conditions. Other factors include the season of the year, and in relation to that temperature and vegetation. Season of the year affects the wetness or dryness of the watershed. In the hot summer months the ground will be drier, therefore more accommodating to large amounts of infiltration from precipitation. If there is a rainy season, the ground will be wetter, thus not allowing as much infiltration. Vegetation type and the amount of cover 12

23 also influence the runoff in a watershed. Vegetation not only intercepts precipitation as it falls, it can also impede runoff flowing along surface pathways and promote infiltration, thus slowing and decreasing stormflow. For this project, one rain gauge was used for each time period and precipitation was assumed uniform for the entire watershed. The gauge for the 196s was located near the USGS White Rock Creek at Greenville Avenue stream discharge gauging station, while the 2s rain gauge was located 4 miles from the site at Dallas Love Field Airport (Figure 7). Data records (196s) for this project came from handwritten books; which raises the possibility of some human error during data recording. Data, in the 2s, was recorded using automated monitors, which may be prone to mechanical errors. Sources of possible error within the analysis are the placement of the baseflow separation line and the calculation of duration of excess precipitation. With the placement of the baseflow line, finding the inflection point on the receding limb is somewhat subjective. The calculation of the duration of excess precipitation is also somewhat subjective, due to the fact that some precipitation outliers above the threshold line are ignored as part of the technique employed, when in fact these outliers may have contributed to the storm flow. These potential sources of error are partly addressed by the averaging of the unit hydrographs. It is assumed that averaging provides a closer approximation of the response of the watershed to a storm. 13

24 DATA ANALYSIS Data for this project was analyzed and compared using unit hydrographs. A unit hydrograph results from one inch of excess precipitation (or runoff) spread uniformly in space and time over a watershed for a given duration (Fielder 1999). Excess precipitation means precipitation that generates stormflow and is in excess of precipitation that fills depression storages, infiltrates the ground, etc. Deriving a Unit Hydrograph (UHG) from a Storm Event To derive a unit hydrograph for a particular storm event, the data was entered into an Excel spreadsheet. Initial hydrographs of the raw data were created to begin the analysis. These graphs included precipitation amounts and stream discharge records, plotted on 2 axes. The bar graph, plotted on the primary Y axis, represents precipitation amounts by time, and the line graph, plotted on the secondary Y axis, represents stream discharge by time (Figure 6). 14

25 April 28, Precipitation (inches) Stream discharge (cfs) Time (hours) Precipitation Stream discharge Figure 7. Example Initial Hydrograph with Precipitation, for April 28, 1966 Separation of Baseflow The next step in the process was to separate the baseflow of the stream from the storm flow. Baseflow must be taken out because it is the part of the stream discharge that is not attributable to direct runoff from precipitation (AMS 2). Baseflow is considered to be a straight line connecting that point at which the hydrograph begins to rise rapidly and the inflection point on the recession side of the hydrograph (Fielder 1999). Baseflow was found using two methods: visual inspection of the hydrograph and using a semi-log plot of the stream flow in cubic feet per second. Visual inspection of the hydrograph involved printing out the initial hydrograph and marking the point were 15

26 storm flow began, the point where the hydrograph begins to rise rapidly, and then estimating the location of the inflection point on the recession limb side, which represents the end of stormflow. For the semi-log plot (Figure 8), stream flow values were plotted on the log scale against time, and the hour was noted when the recession side fit an approximate straight line. These two techniques were used in combination to separate the baseflow for each storm. April 28, 1966: Semi-log Plot Log of CFS Hour when recession limb becomes straight line Time (hours) Figure 8. Semi-log Plot Once the beginning and end of baseflow were found, I used a scatter plot and the formula y=ax+b, to find an equation to separate baseflow from stream flow; where x equals time and y equals cubic feet per second. Once the x, y coordinates for both the beginning and the end of baseflow were plotted, a trendline was fit to the points, 16

27 displaying the equation (Figure 9). The equation shown on the graph was used to plot the baseflow on the hydrograph (Figure 1), Baseflow was then subtracted from the initial stream flow records, the product of this calculation provided new ordinates which represent storm flow (Figure 11). Baseflow Separation: Scatter Plot 14 y = x Stream discharge (cfs) Time (hours) Figure 9. Y=aX+b Scatter Plot 17

28 April 28, 1966: Baseflow and Hydrograph 25 2 Stream discharge (cfs) Time (hours) Figure 1. Initial Hydrograph and Baseflow April 28, 1966: Unit Hydrograph Ordinates 12 1 Stream discharge (cfs) Time (hours) Figure 11. Unit Hydrograph 18

29 The next step in the process involved calculating the depth of direct runoff. The value found through these calculations was later used in conjunction with the precipitation values to find duration of excess precipitation. Calculations for direct runoff are as follows: 1. Storm flow ordinates were summed and then multiplied by 36 seconds to create a value that represents storm flow volume in cubic feet. April 28, 1966 Example: Sum of Ordinates = cubic feet second * 36 seconds = cubic feet (storm flow volume). 2. Storm flow volume was then converted into acre-feet by dividing by 43,56 square feet per acre. April 28, 1966 Example: cubic feet / 4356 ft 2 /acre = acre-feet. 3. The depth of direct runoff in feet was found by dividing the total volume of excess precipitation by the watershed area. April 28, 1966 Example: acre-feet / (66.4 square miles * 64 acres) = feet. 4. The depth of direct runoff, for April 28, 1966, in inches, was feet * 12 inches = inches. Obtaining UHG Ordinates To obtain the unit hydrograph ordinates, each flow was divided by the depth of direct runoff/excess precipitation (Table 1; Figure 12). Units for the unit hydrograph in this analysis are cubic feet per second per inch of excess precipitation. 19

30 Stream Discharge Without Baseflow (cfs) Table 1. Calculating Unit Hydrograph Ordinates. Depth of Direct Runoff (inches) Unit Hydrograph (cfs/inch)

31 April 28, 1966: Procession from Initial Hydrograph to Unit Hydrograph Precipitation (inches) Stream discharge (cfs) Time (hours) Precipitation Initial Hydrograph Without Baseflow Unit Hydrograph Figure 12. Procession from Initial Hydrograph to Unit Hydrograph. Determining UHG Duration The duration of the derived unit hydrograph is found by examining the precipitation for the event and determining that precipitation which is in excess (Fielder 1999). Excess precipitation is considered that precipitation which becomes storm flow. To find the excess precipitation for a particularly storm event, I graphed the precipitation for the storm in hyetograph form. I then had to find the threshold value for the storm event. Using the depth of direct runoff value, which was calculated previously, I estimated different threshold values to find which one would create an amount of excess precipitation that equaled the direct runoff (Table 2). Once the correct 21

32 value was found and the sum of the new precipitation values equaled the direct runoff, the threshold value was plotted as a horizontal line across the precipitation hyetograph. Time (hours) Table 2. Excess Precipitation and Threshold Value Calculations New Precipitation Threshold Value Sum of New Precipitation (inches) (inches) Precipitation: (inches) Direct Runoff: Threshold Value: Duration of Excess Precipitation: hour Everything above the threshold value line on the hyetograph is considered excess precipitation (Figure 11). The duration, in hours, of excess precipitation, equals the duration of the unit hydrograph for this storm. 22

33 April 28, 1966: Precipitation Hyetograph Precipitation (inches) Precipitation above this line is considered excess precipitation Time (hours) Precipitation Threshold Value Figure 13. Rainfall Hyetograph showing excess precipitation threshold. Time Period 196s 2s Table 3. Events and Threshold Values Used in Analysis Storm Events Threshold Values (inches) Average November 22, April 27, s September 8-9, Threshold inches April 28, Average: August 13-14, December 3, April 23-25, June 11-12, December 12, June 9-1, s Threshold Average:.436 inches 23

34 Table 3 shows the 1 events that were used for analysis and their threshold values. The threshold values were averaged in preparation for the scenario analysis. Changing the Duration of the Storm Event To average and compare the unit hydrographs, each unit hydrograph must be of equal duration. The average duration of all ten storm events was calculated and rounded up to the nearest whole hour 3 hours. Then the S-Curve method was used to change the duration of all unit hydrographs to the average duration. The S-curve method involves continually lagging the unit hydrographs by their duration and adding the ordinates (Fielder 1999). Theoretically, the S-curve is a representation of what would happen if it rained continually, the top of the curve flattens due to equilibrium between input and output of precipitation (Figure 12). S-Curve Plot Stream discharge (cfs) Time (hours) Figure 14. The S-Curve Plot 24

35 To change the duration of a 1-hour unit hydrograph to a 3-hour duration unit hydrograph, s-curve ordinates were lagged by 3-hours, and the difference between the two curves equals a 3-hour unit hydrograph (Figure 13; Fielder 1999). Because the original unit hydrograph was of 1-hour-duration; the new 3-hour-duration unit hydrograph will represent 3 inches of rain (instead of the required one inch). Therefore the new hydrograph ordinates must be multiplied by 1/3, in order to show a true representation of a 3-hour-duration hydrograph. April 28,1966: S-Curve Plots and 3-Hour Unit Hydrograph Stream disharge (cfs) Time (hours) S-Curve S-Curve 3-Hour Unit Hydrograph Figure15. S-Curve Plots and 3-Hour Unit Hydrograph Average Hydrographs and Threshold Values Once the S-Curve plots were completed for all storms, the ordinates for each event were copied into the same Excel file. These storms ordinates were then 25

36 averaged to find an average unit hydrograph for the 196s and the 2s (Figure 14, Figure 15). A unit hydrograph represents a combination of watershed characteristics and watershed conditions as it reacts to excess precipitation (McCuen 1998). Therefore, any characteristics or conditions that were not uniform through time will be reflected in the shape of the hydrograph. The April 27, 1962 hydrograph is different from the other 4 hydrographs in its shape and time to peak. Its differences cannot be explained, but removing this graph does not significantly change the averaged unit hydrograph ordinates or the lag time. Since the data meets the criteria listed for data usage, completely removing the event from the analysis would disregard those criteria. Lag times for the storm events were calculated using the raw data from each of the 1 storm events using the following steps: 1. Find the center of mass of precipitation. Calculate to fraction of the hour. 2. Find the hour of peak flow. It is assumed that peak flow occurs in the middle of the hour. Therefore, if peak flow occurs in Hour 15, I will record this has Hour The center of mass of precipitation hour is subtracted from the hour of peak flow. 4. Values for each of the time periods are averaged and compared (Table 4). 26

37 Storm Event Table 4. Lag Time Values and Calculations Center of Mass Peak Flow of Precipitation (hours) (hours) Lag Time (hours) Nov 22, Apr 27, Sept 7-9, Apr 28, Aug 13-14, Dec 3, Apr 23-25, Jun 11-12, Dec 12, Jun 9-1, Average (hours) Unit Hydrographs and Average, 196s Stream discharge (cfs) Time (hours) Nov 22, 1961 Apr 27, 1962 Sept 7-9, 1962 Apr 28, 1966 Aug 13-14, 1968 Average Unit Hydrograph Figure s Unit Hydrographs and Average 27

38 Unit Hydrographs and Average, 2s Stream discharge (cfs) Time (hours) Dec 3, 22 Apr 23-25, 23 Jun 11-13, 23 Dec 12, 23 Jun 8-11, 24 Average Unit Hydrograph Figure 17. 2s Unit Hydrographs and Average Scenario Analysis To compare the response of the watershed to storms between the two time periods, a range of scenarios using hypothetical storms of varying precipitation intensity were developed. The limit of these scenarios was set at the 5-year flood event, which, for White Rock Creek Watershed, equals 56,5 cubic feet per second (FIS 24; Table 4). A total of 6 scenarios were used:.5 inch per hour for 3 hours 1. inch per hour for 3 hours 1.5 inches per hour for 3 hours 28

39 2. inches per hour for 3 hours 2.5 inches per hour for 3 hours 3. inches per hour for 3 hours Table 5: Flood Event Values for White Rock Creek Watershed, 24 Cubic Feet per Second Flood Event Percent Chance of Occurrence 25, 1-year 1 37,2 5-year 2 42,8 1-year 1 56,5 5-year.2 Scenario calculations were as follows (Tables 6-11): 1. Subtract the threshold value from the precipitation values and sum the total amount of excess precipitation. 2. To find the storm flow ordinates, multiply the unit hydrograph ordinates by the total amount of excess precipitation. 29

40 Table 6. Scenario #1:.5 inches of rain per hour for 3 hours 196s Time Precipitation Threshold Value Excess Precipitation 196s UHG Total Excess Precipitation: Total Excess Precipitation New Ordinates 5659 Threshold Value Larger then Precipitation: No Storm Flow s Time Precipitation Threshold Value Excess Total Excess 2s UHG Precipitation Precipitation New Ordinates Total Excess Precipitation: Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): 1,519 29,729,8 3

41 Table 7. Scenario #2: 1. inch of rain per hour for 3 hours 196s Time Precipitation Threshold Value Excess Precipitation 196s UHG Total Excess Precipitation: Total Excess Precipitation New Ordinates 5659 Threshold Value Larger then Precipitation: No Storm Flow s Time Precipitation Threshold Value Excess Total Excess 2s UHG Precipitation Precipitation New Ordinates Total Excess Precipitation: Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): 13,34 261,119,8 31

42 Table 8. Scenario #3: 1.5 inch of rain per hour for 3 hours 196s Time Precipitation Threshold Value Excess Precipitation 196s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): 11,552 24,243,935 2s Time Precipitation Threshold Value Excess Precipitation 2s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): 25, ,51,52 32

43 Table 9. Scenario #4: 2. inch of rain per hour for 3 hours 196s Time Precipitation Threshold Value Excess Precipitation 196s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: s Time Precipitation Threshold Value Excess Precipitation 2s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): 24, ,248,367 36, ,91,24 33

44 Table 1. Scenario #5: 2.5 inch of rain per hour for 3 hours 196s Time Precipitation Threshold Value Excess Precipitation 196s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: s Time Precipitation Threshold Value Excess Precipitation 2s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): 37, ,252,798 48,82 955,291,96 34

45 Table 11. Scenario #6: 3. inch of rain per hour for 3 hours 196s Time Precipitation Threshold Value Excess Precipitation 196s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: s Time Precipitation Threshold Value Excess Precipitation 2s UHG Total Excess Precipitation New Ordinates Total Excess Precipitation: Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): Peak Flow (cubic feet per second): Volume of Storm flow (cubic feet): 5,58 894,257,229 6,623 1,186,682,68 35

46 Once this process was completed for each scenario, I plotted the ordinates created and compared the responses of the watershed, in terms of peak flow and volume of storm flow. Calculating Comparison Values Areas of comparison for this analysis included, peak flow, lag time, and volume of storm flow. Peak flow: the highest discharge value, in cubic feet per second (cfs). Lag time: the time between the center of rainfall to the peak rate of flow (to the nearest hour), (Viessman 1977). Volume of storm flow: in cubic feet. 36

47 IV. RESULTS Due to the size of precipitation and the threshold value for the first and second scenarios, there was no storm flow for the 196s. For the 2s, 3 hours of.5 inches of rainfall, produced a peak flow of approximately 1,519 cubic feet per second, and a storm runoff volume of 29,729,8 cubic feet (Table 12; Figure 18). Table 12. Scenario # 1:.5 inches per Hour for 3 Hours 196s Peak Flow (cfs):. Volume of Storm Runoff (cubic feet):. 2s Peak Flow (cfs): 1,519 Volume of Storm Runoff (cubic feet): 29,729,8 Scenario #1:.5 Inches per Hour for 3 Hours Stream discharge (cfs) Time (hours) No Stormflow 196's Ordinates 2's Ordinates 37

48 Figure 18. Scenario #1 Storm Hydrograph:.5 Inches per Hour for 3 Hours In Scenario #2, the 2s produced a peak flow of 13,34 cubic feet per second, and a storm flow volume of 261,119,8 cubic feet (Table 13). Table 13. Scenario #2: 1. inch per Hour for 3 Hours 196s Peak Flow (cfs):. Volume of Storm Runoff (cubic feet):. 2s Peak Flow (cfs): 13,34 Volume of Storm Runoff (cubic feet): 261,119,8 Scenario #2: 1. Inches per Hour for 3 Hours Stream discharge (cfs) Time (hours) No Stormflow 196's Ordinates 2's Ordinates Figure 19. Scenario #2 Storm Hydrograph: 1. Inch per Hour for 3 Hours 38

49 Both averaged unit hydrographs generated storm flow in Scenario #3. The 2s values are much larger then the 196s, with a peak flow of 25,161 cfs. The 196s peak flow value is 11,552 cfs, with a storm flow volume of 24,243,935 cubic feet. Storm flow volume for the 2s is 492,51,52 cubic feet (Table 14; Figure 2). Table 14. Scenario #3: 1.5 Inches per Hour for 3 Hours 196s Peak Flow (cfs): 11,552 Volume of Storm Runoff (cubic feet): 24,243,935 2s Peak Flow (cfs): 25,161 Volume of Storm Runoff (cubic feet): 492,51,52 Scenario #3: 1.5 Inches per Hour for 3 Hours 3 25 Stream discharge (cfs) Time (hours) 196's Ordinates 2's Ordinates Figure 2. Scenario #3 Storm Hydrograph: 1.5 Inches per Hour for 3 Hours 39

50 The fourth scenario, 2. inches per hour for 3 hours, almost produces a1-year flood event for the 196s. Peak flow for the 196s is 24,562 cfs; a 1-year event requires 25, cfs. The 2s peak flow is 36,981 cfs (Table 15; Figure 21). Table 15. Scenario #4: 2. Inches per Hour for 3 Hours 196s Peak Flow (cfs): 24,562 Volume of Storm Runoff (cubic feet): 434,248,367 2s Peak Flow (cfs): 36,981 Volume of Storm Runoff (cubic feet): 955,291,96 Scenario #4: 2. Inches per Hour for 3 Hours 4 35 Stream discharge (cfs) Time (hours) 196's Ordinates 2's Ordinates Figure 21. Scenario #4 Storm Hydrograph: 2. Inches per Hour for 3 Hours 4

51 In Scenario #5, the 2s unit hydrograph produced enough storm flow to produce a 1-year flood event (Table 16; Figure 22). The 196s unit hydrograph generated a peak flow of 37,571 cfs, which is just over the requirements for a 5-year event. Total volume of storm flow for the 2s equaled 955,291,96 cubic feet; the 196s, 664,252,798 cubic feet. Table 16. Scenario #5: 2.5 inches per Hour for 3 Hours 196s Peak Flow (cfs): 37,571 Volume of Storm Runoff (cubic feet): 664,252,798 2s Peak Flow (cfs): 48,82 Volume of Storm Runoff (cubic feet): 955,291,96 Scenario #5: 2.5 Inches per Hour for 3 Hours 6 5 Stream discharge (cfs) Time (hours) 196's Ordinates 2's Ordinates 41

52 Figure 22. Scenario #5 Storm Hydrograph: 2.5 inches per Hour for 3 Hours The last scenario, Scenario #6, each time period created a significant amount of stormflow. The 196s unit hydrograph generated a peak flow of 5,58 cfs, with a storm flow volume of 894,257,229 cubic feet. With a peak flow of 6,623 cfs, the 2s unit hydrograph generated an event that has a.2 percent chance of occurring in any given year, a 5-year storm event (Table 17; Figure 23). Table 17. Scenario #6: 3. Inches per Hour for 3 Hours 196s Peak Flow (cfs): 5,58 Volume of Storm Runoff (cubic feet): 894,257,229 2s Peak Flow (cfs): 6,623 Volume of Storm Runoff (cubic feet): 1,186,682,68 42

53 Scenario #6: 3. Inches per Hour for 3 Hours 7 6 Stream discharge (cfs) Time (hours) 196's Ordinates 2's Ordinates Figure 23. Scenario #6 Storm Hydrograph: 3. Inches per Hour for 3 Hours 43

54 V. DISCUSSION Threshold values were a key component to this analysis and were found during the excess precipitation calculations. These values are of great importance because they make possible the comparison of the two time periods. When looking at the individual threshold values for the 1 storms used in the analysis, there is a large range of variability between them. This could be due to antecedent moisture conditions, for example, a saturated 196s pervious watershed may display the same hydrologic characteristics as the 2s urbanized watershed. The crucial difference between the two time periods is the averaged threshold values. The higher 196s value indicates greater precipitation intensity is necessary to surpass the infiltration-excess threshold and create overland flow, whereas the lower 2s value indicates less precipitation intensity is needed to produce surface runoff (and stormflow). The decreased threshold value for the 2s illustrates a change in the surface characteristics of the watershed (i.e. urbanization). Higher values of storm flow volume and peak flow are seen in correlation to the lower threshold value Lag times for the two time periods, when compared using a z-test, showed no significant difference (Table 18). The results suggest that the difference in lag times is relatively small (on the order of one hour or less) and the coarse resolution of the precipitation and streamflow data (1-hour increments) can not be used to confidently resolve this difference. Farther research into the issue of lag time changes is required using higher resolution data (e.g. 15-minute increments) and a larger dataset. 44

55 Table 18. Lag Time Comparison: Z-Test 196s Lag Time (hours) 2s Lag Time (hours) Mean Known Variance Observations 5 5 z In Figure 2, a comparison of peak flow between the 196s and the 2s shows a substantial difference at lower precipitation intensities. For example, from Scenario #2 to Scenario #3, peak flow values, as well as storm flow volumes, nearly doubled. But, as the precipitation intensity increases (e.g. scenarios 4, 5 and 6), the 196s peak flows gradually increase and become closer to the 2s peak flows. A reasonable explanation for these close values is that so much rain is falling; the 196s soil has become completely saturated and has begun to behave like the 2s impervious surface. Figure 21 shows a comparison between 196s and 2s storm flow volume calculated during the scenario analysis. The 2s values are approximately 3,, cubic feet higher then the196s, starting in Scenario #2. From Scenarios #3 to #6, the 196s and 2s values stay an equal distance apart. 45

56 Peak Flow Comparison Peak flow (cfs) Precipitation (Inches/3 Hours) 196s 2s Figure 24. Peak Flow Comparison 46

57 Stormflow Volume Comparison Stormflow (cf) Precipitation (Inches/3 Hours) 196s 2s Figure s and 2s Storm Flow Volume Comparison 47

58 VI. CONCLUSIONS Results of the study support the hypothesis that increased urbanization in White Rock Creek watershed has altered the creek s hydrological response to storms. The findings suggest that threshold values (precipitation intensity that must be surpassed before the onset of infiltration-excess overland flow) have been lowered by the proliferation of impervious surfaces that accompanies urbanization. The impact of this change is reflected in higher peak flows and storm flow volume values. In this analysis, the 2s exhibit much larger volumes as well as much larger peak flows, resulting in flooding events that would not have occurred, under the same conditions, in the 196s. Peak flows and volumes at lower precipitation intensities, such as between Scenarios #1 and #2, increased as much as 8 times the Scenario #1 value. These large increases, in the bottom three scenarios, are seen in the 2s values only. The 196s shows no generated storm flow until Scenario #3. An increase in peak flow and storm flow volume can negatively impact a river channel. Increased erosion, channel and bank cutting, and eventual deposition can lead to sizeable changes in the hydraulics and hydrology of a watershed. Watersheds that are located in and around areas of large population growth, such as the greater Dallas- Fort Worth area, are susceptible to these negative impacts. Changes in lag times between the two time periods could not be resolved in this study, probably because resolution of the precipitation and streamflow data is too coarse. It is recommended that farther study of lag times be made using higher resolution data (e.g. 15-minute increment) and a larger dataset. 48