Flood Frequency and Hazard Analysis of Santa Barbara s Mission Creek

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1 Flood Frequency and Hazard Analysis of Santa Barbara s Mission Creek Dylan Berry Andrew Donnelly Bruce Stevenson Dr. Professor Ed Keller ES 144: Rivers UC Santa Barbara Spring 2013 Abstract: The objective of this study was to compare the Weibull and Log Pearson Type III methods in order to analyze the correlation between flood recurrence intervals and discharge in Santa Barbara s Mission Creek Watershed. This relationship was then used to analyze the flood hazard in the downtown Santa Barbara area. Santa Barbara has a long history of large flood events. While flood hazard prevention measures have been taken, the uncertainty associated with the size, location, and type of flood event increase

2 the potential hazard. The study concluded that the Log Pearson Type III model is more accurate in describing long term flood patterns, and that the Santa Barbara downtown is not entirely protected from a potential large flood event. 1. Introduction 1.1 Objectives The objective of this study was to compare the Weibull and Log Pearson Type III methods in analyzing the relationship between flood recurrence interval and discharge in Mission Creek as an assessment of flood hazard in the downtown Santa Barbara area. 1.2 Study Area Santa Barbara is located between the south side of the Santa Ynez Mountains and the Pacific Ocean in Southern California. (Figure 1) Mission Creek runs down the mountains and through the downtown area before ending in a coastal lagoon, separated from the ocean by a sand barrier. (Figure 2) The entire city is located on an alluvial fan, and has had a history of flooding into remnant paleochannels. (Figure 3) Santa Barbara has a Mediterranean climate with high concentrations of riparian and chaparral vegetation.

3 Figure 1 Santa Barbara, CA is located south of the Santa Ynez Mountain Range.

4 Figure 2 Mission Creek Watershed, Mission Creek, Rattlesnake Creek, Skofield Park Reach, Santa Barbara Botanic Garden Reach, Rocky Nook Park reach, Oak Park Reach, Channelized Reach,

5 Lagoon Figure 3 Map of Downtown Santa Barbara showing Mission Creek paleochannels and El Estero.

6 2. Methods 2.1 Weibull Method The Weibull Method of calculating flood recurrence intervals utilizes the following equation: RI = (n+1)/m where n is the number of years on the flood record, and m is the relative rank of the given peak flood measurement in the same period. Plotting the recurrence interval (RI) against the given peak discharges (m), and creating a trendline will allow for interpolation. Specifically, peak discharges associated with floods of 2, 5, 10, 25, 50 and 100 year recurrence intervals can be estimated. This is a simple and easy to use method in comparison to other models, such as the Log Pearson Type III. 2.2 Log Pearson Type III Method The Log Pearson Type III is a more complex procedure, including numerous steps in order to calculate peak discharge from given recurrence intervals. Given annual peak flood data arranged in a time series, first the mean annual flood (MAF) is calculated. Next, the logarithm is taken for each entry of the original peak flood data, and the the mean of the log-transformed data is taken. Next, the standard deviation (S) is taken from the log-transformed data. Next, a corrected skew coefficient is obtained from the equation: G(corrected) = G*[1+(6/n)] Then, the Frequency Factor (K) is obtained from a frequency factor table, for each of the recurrence intervals of interest. In this study, these are 2, 5, 10, 25, 50 and 100 years. Using these previous calculations, the discharges can be calculated by the equation: Q = 10^[LMAF+(S*K)] where LMAF is the mean of the log transformed data, S is the standard deviation, and K is the frequency factor. By taking both sides of the equation an yields the final peak discharge estimation (Q). Graphing these against the recurrence interval yields a trendline that can be used to estimate peak discharge from recurrence interval.

7 A second analysis included the estimation of flood recurrence interval from historical flood discharge data in the Santa Barbara Mission Creek. The trendline equation derived for discharge (Q) is rearranged to solve for recurrence interval (RI), and applied to historical peak discharge values. 2.3 Laguna Drain The Rational Method is used to estimate the discharge for minor (Q5) and major (Q25) storm events in the Laguna Drain. The Rational Method is expressed in the equation: Q = CIA where C is the peak discharge (cfs), I is the average rainfall intensity (inches per hour), and A is the drainage area (acres). The Laguna Drain was designed to carry a peak discharge of 950 cfs, so the calculations for storm event discharges will determine if the drain can contain high flow events. 3. Data 3.1 Flood Frequency Analysis Weibull Method The data tables for estimated discharge in cfs can be seen below: Figure 4: Estimated Discharge using the Weibull Method. 3.2 Flood Frequency Analysis Log Pearson Method The data tables for estimated discharge in cfs can be seen below:

8 Figure 5: Estimated Discharge using the Log-Pearson Method. The tables for estimated recurrence interval (RI): Figure 6: Historical recurrence intervals using the Log-Pearson Peak Discharge as conducted by USGS. 3.3 Laguna Drain The data tables for estimated discharge in cfs can be seen below: Figure 4: Estimation of discharges for a 5-year and 25-year flood for the Laguna Drain.

9 4. Discussion 4.1 Weibull and Log Pearson Type III Distribution Models The Weibull and Log Pearson Type III distributions are two approaches used to analyze flood frequency. The Log Pearson Type III is more complex than Weibull method, and includes a logarithmic transformation. The Interagency Advisory Committee on Water Data recommends the Log Pearson Type III as the standard model for flood frequency studies in all U.S. Government agencies. The Weibull distribution, also called plotting position, utilizes probabilities of peak flood events to construct a flood frequency curve. Furthermore, the Weibull distribution employs two parameters in describing the distribution (compared to Log Pearson s three). This limits the flexibility of use. Application of these two models to the Mission Creek watershed yielded different flood pattern analyses. For higher frequency floods (the 2, 5, and 10-year floods), the Log Pearson Type III method estimated smaller discharges than the Weibull method. Conversely, it estimated higher discharges for lower frequency floods (20, 25, 50, and 100- year). This is a direct result of the log-based distribution that the Log Pearson Type III applies. The following figures show the distinctions between the models Figure 7: Trendline created using the Weibull Method.

10 Figure 8: Trendline created using the Log Pearson Type III Method. The estimations for the recurrence interval of four Mission Creek flood events in the past 30 years yielded a maximum recurrence interval of 22 years, suggesting that the Log Pearson method generally underestimates RI. It is important to recognize the strength of the model in order to better assess the area s flood hazard. In addition, the estimations for the 20-year and 150-year flood discharges were 4,492 cfs and 46,936 cfs, respectively. While we can have confidence in the 20 year estimation because it falls between two measured values, the 150 year discharge is extrapolated and may not be accurate. Extrapolation is dangerous in modeling because the observed relationship may not be consistent over longer time scales than are measured. This study used mean annual (peak) flood data for Mission Creek which can create potential inaccuracies in the study s results. By only using one peak value for each year, the study ignores year-to-year rainfall trends. It also ignores the consequences of a storm event lasting one day against one that lasts a week. These conditions can have huge influence on flood frequency analysis. Mean annual (peak) flood data is sufficient for creating flood frequency curves like the ones designed in this study, but a broader historical data record must be used if flood hazard protection is under analysis.

11 Figure 9: Paleochannels and El Estero in the Santa Barbara downtown area 4.2 Landforms and Flooding: Role of Alluvial Fan and Paleochannels Alluvial fans are of particular importance in considering flood hazard, because they are prone to form and later re-occupy many older paleochannels. Paleochannels are historical, inactive stream channels that have been buried by younger sediment. Though often not directly adjacent to the current channel, these paleochannels are still at high risk for flooding. In addition to occupying paleochannels, it is possible that the flow will break free and form a new channel entirely. This is harder to predict and plan for, but should be noted as a factor in considering alluvial fan flood hazard. Alluvial fans are zones of deposition, where sediment transport capacity decreases due to increases in channel width or a reduction in channel gradient, among other factors (National Research Council 1996). This often occurs at mountain fronts, where a steep confined channel meets a wider, flatter area. The channel forms meanders, probably as a means for decreasing energy loss, and this lateral movement of channel disperses the depositing sediment into a fan-shaped landform. (National Research Council 1996). Alluvial fans are most active where sediment loads are high, such as in arid and

12 structurally weak mountain regions (National Research Council 1996). These conditions are consistent with the Santa Barbara Mission Creek study area. In Mission Creek low flow conditions, flow is confined to a single channel. In high flow conditions, such as those caused by large precipitation events, discharge may exceed the main channel s capacity and be diverted into remnant paleochannels. (Figure 9) Many of the paleochannels in the downtown Santa Barbara area are due to the Mission Ridge Fault, which diverts Mission Creek as it propagates westward. Flow paths are cut as the present channel down-cuts the anticline. This creates wind gaps when the paleochannels are empty, and water gaps where flow is experienced. Presently, Mission Creek runs along the south west boundary of the downtown Santa Barbara area and ends in a small coastal lagoon by Stearn s Wharf. (Figure 9) However historically, Mission Creek flowed into El Estero, a large coastal lagoon that once resided on the northeastern side of downtown. The paleochannels that once emptied into this lagoon run directly through the downtown area, and are therefore a cause for concern. Similarly, the area previously occupied by El Estero is subject to flooding and is cause for concern as well. Flood drainage channels were installed in some areas to remove floodwaters from these paleochannels in case of flooding events. One of particular importance is the Laguna Drain, which empties water from a paleochannel on Laguna Street into the present coastal lagoon. (Figure 10)

13 Figure 10: Location of Laguna Drain. Created using Google Earth. 4.3 Main reasons for flood hazard in Santa Barbara Santa Barbara has a long history of flood events. Between 1862 and 2010, Santa Barbara experienced 15 significant floods, eight of which received Presidential Disaster Declarations (Abrams 2012). While flood hazard prevention measures have been taken, the uncertainty associated with the size, location, and type of flood event increase the potential hazard. The processes of sediment transport and deposition are especially important, as they can potentially alter the flow path and create debris flow conditions. During high discharge events, the abrupt deposition of sediment or debris during a flood may substantially alter hydraulic conditions and initiate new, distinct flow paths of uncertain direction (Larson, 2000). This uncertainty is dangerous because it is difficult to predict and therefore difficult to prepare for. While high risk areas are focuses of flood proofing and flood preparation efforts, the creation of new flow paths would affect unprepared areas and therefore pose a larger hazard of human and property damage. In addition, sediment deposition increases the risk of landslides and debris flows. In Santa Barbara, the risk of debris flow is minimized by the high stability of sediment in the Mission Creek tributaries. However, if a flood event were to occur after a fire event, which creates large volumes of sediment, the potential for a debris flow would be significantly increase. Due to the high recurrence of fires in the area, it is important to consider this possibility. Flow path uncertainty and debris flow potential are major factors in considering the Santa Barbara flood hazard. 4.4 How the Santa Barbara flood hazard might be minimized Planning flood proofing infrastructure urban runoff prevention Emergency response spatial modeling

14 Cities built on alluvial fans such as Santa Barbara must develop flood hazard protection projects to protect the city from flooding. Projects that have been completed in recent years reveal an acknowledgment of the comprehensive reform that must occur to mitigate the significant local hazard of flooding. In mitigating the flood hazard, Santa Barbara County s Office of Emergency Management has summarized the city s aim for flood hazard protection in the 2011 Multi-Jurisdiction Hazard Mitigation Plan. Its three major goals include promoting disaster-resistant future development, building and supporting capacity and commitment to existing infrastructure to become less vulnerable to hazards, and enhance hazard mitigation coordination and communication. The report also details significant flood control work in progress, including improvements of storm drain and channelization infrastructure. The Laguna Drain, a concrete, trapezoidal channel that collects urban runoff, was designed to carry a peak flow of 950 cfs (Keller 2013). This would be sufficient to carry the estimated 5 year discharge of 823 cfs. However, in considering the estimated 25 year discharge of 2100 cfs, the Laguna drain would be insufficient to carry the flow. This suggests that the downtown area is vulnerable to flooding if Mission Creek re-occupies one of its paleochannels. Because the creek has been known to reoccupy its paleochannels during large flood events, this could pose a significant danger. In order to offset the inability of the channel to accommodate the 25-year flood discharge, flood hazard mapping, monitoring the progress or storms, channel modifications (channelization and engineering approaches) and regulatory approaches to reduce vulnerability. (Nelson, 2012) Flood hazard mapping can be used to assess susceptible areas to flooding. By using historical data on rivers and discharges, as well as topographical data, flood-hazard maps can be created that will spatially visualize regions of the area that would be expected to be affected by the various discharges and recurrence intervals. (Nelson, 2012) Geographical information systems/science would be able to highlight the regions of this report s study area, such as in the image below:

15 Figure 11: Map visualizing the impact of various recurrence intervals and discharge stages. By monitoring the progress of storms with geographical information systems/science (GIS) to determine significant factors such the degree of ground saturation, amount of vegetation, amount of rainfall or the degree of permeable soil, a relationship from the integration of the various factors can result in short-term prediction. (Nelson, 2012) Comprehensive forecasting can result in flash flood warning being issued to the affected areas. Channels are dynamic systems that modify themselves in response to changes in physical watershed features regardless of human involvement. Channel modifications include deepening, widening, straightening, and construction of new channels. (Figure 12) Figure 12: Cross-section of channelization methods. Other engineering approaches include dams, retention ponds, levees and floodways. Dams are able to store a volume of water to encourage retention and can be regulated to be released at a desired rate (Nelson, 2012). Retention ponds also serve a

16 purpose that is akin to those that dams accommodate for, by trapping water and releasing it at a controlled discharge to prevent flooding downstream (Nelson, 2012). Floodways provide a means of channeling discharge toward a stream and store it in an area where the land is used for agricultural or recreational intentions. These areas of agricultural or recreational land are located where the threat of a flood is minimized, yet will provide an opportunity for adequate quantities of flood waters to be diverted and stored during the period of high discharge (Nelson, 2012). Figure 13: Flood Management Floodway 5. Conclusions The Weibull and Log Pearson Type III flood analysis models yielded distinct estimations of recurrence interval given the same historical peak flow data. This study reveals that the Weibull method is more accurate in describing smaller discharge events, whereas the Log Pearson method is better at analyzing long-term trends of flood frequency. In analyzing the Santa Barbara flood hazard, it was determined that the downtown region could utilize additional tools, such as infrastructure improvements, spatial modeling (GIS), flood proofing, and flood planning in order to reduce the flood hazard. Large discharge events (>10 year flood) would cause flooding if Mission Creek occupied one of its paleochannels. The uncertainty associated with this flow path is a cause for concern. Flood frequency analysis is a simple and effective way to analyze the flood risk over time, and is important to mitigate risk and avoid property damage.

17 Works Cited Abrams, Richard Santa Barbara County Multi-jurisdictional Hazard Mitigation Plan. County of Santa Barbara ftp://pwftp.countyofsb.org/water/ftp/state request/flood management/multi_hazard Report/Cover Page 2011.pdf Analysis techniques: Flood frequency analysis. Civil, Construction and Environmental Engineering Department. Oregon State University. Corvallis, OR Bengtson, Harlan. Hydrology (Part2)- Frequency Analysis of Flood Data. Continuing Education and Development, Inc Keller, E. Lab 3: Flood Hazard Analysis. Environmental Studies 144: Rivers. University of California, Santa Barbara Larson, Matthew et. al. The Natural Hazards on Alluvial Fans: the Venezuela Debris Flow and Flash Flood Disaster. USGS National Research Council. Alluvial Fan Flooding. The National Academies Press. Washington DC Nelson, S. (2012). Flooding hazards, prediction & human intervention. Unpublished manuscript, Earth and Environmental Sciences, Tulane University, Retrieved from Equations RI = (n+1)/m G(corrected) = G*[1+(6/n)] Q = 10^[LMAF+ (S*K)]

UGRC 144 Science and Technology in Our Lives/Geohazards

UGRC 144 Science and Technology in Our Lives/Geohazards Flood and Flood Hazards Dr. Patrick Asamoah Sakyi Department of Earth Science, UG, Legon College of Education School of Continuing and Distance Education