An Analysis of a Flood Event in the Santa Clara River System

In Washington County, Utah

Alan Moller

CEE6440 – GISWR

Introduction

Geographic Information System (GIS) software has been and continues to be a powerful tool in visualizing data of many types and establishing relationships and trends between different variables. This report will focus on the aspects of water resources and meteorological factors involved in creation of a major flood event in the Santa Clara River system of Washington County, Utah. Between December 28, 2004 and January 13, 2005 a combination of rain and other meteorological factors came together to produce significant flooding conditions in the Santa Clara Basin particularly in the towns of Gunlock, Santa Clara, and Saint George, Utah. See figure 1 for reference of the area in question. In analyzing this event an explanation of the available data sources and their associated limitations will be followed by a thorough GIS analysis to produce products that illustrate some of the causes and the significance of this event.

Figure 1 – Santa Clara Basin and Towns of Interest

Data Sources/Limitations

An adequate amount of applicable data is essential to gaining a solid understanding the factors surrounding the said event. As such, work was performed to obtain as much data as possible for the area near and in the Santa Clara Basin. Data useful for this study centered on stream-flow and basin area information as well as meteorological information through means of remote sensing and surface observations. NHDPlus flowline data and the Watershed Boundary Dataset were downloaded and utilized to produce a base map of the Santa Clara Basin. Next, data and site information were found for several surface stations near the basin. These include stations from different networks, namely the SNOTEL network (run by the Natural Resources Conservation Service), COOP network (run by the National Weather Service), and US Geological Survey stream gage stations. The SNOTEL network has 4 active stations that are useful in this study. Among other things these sites provide data for temperature, precipitation, and snow water equivalent (SWE). The COOP network has 7 active stations applicable to this study. These sites provide data for temperature and precipitation. The USGS has 4 stream monitoring sites along the Santa Clara River. These sites are useful to view and understand the stream response to precipitation etc. Figure 2 shows a layout of the stations used in this study and the location of the Santa Clara River Basin and flow lines.

Figure 2 – Santa Clara Basin with Flowlines (cfs) and Data Stations

In addition to the surface data stations, NEXRAD radar data was obtained from the National Climate Data Center. This data was imported into the GIS software and converted to a raster format that could be used to calculate some zonal statistics regarding the precipitation than fell over the Santa Clara Basin.

As with most data, the data used here doesn’t come without some limitations. For this study it was fortunate to find such a nice distribution of stations over the area of study. However, it should be noted that two COOP stations, namely Gunlock Powerhouse and Veyo Powerhouse which are both located near the center of the basin area, had limited data available during the actual storm event. Both stations had data available through December of 2004 but had missing data for January of 2005. Therefore, these two stations could not be included in any GIS interpolations involving the January portion of the precipitation event. Even so, all COOP stations were available to use in studying average values as opposed to just focusing on the event in question.

The SNOTEL stations provide excellent data in addition to that provided by the COOP stations. Still, however, some limitations exist here too. While all the SNOTEL sites were active during the actual storm event, they vary greatly in terms of how long they have been in service. This has an effect on the average values provided by SNOTEL sites. For example, two stations only had 4 years of data while the others had at least 23 years of data. With the said precipitation event included, the averages of the stations with only 4 years of data were more heavily skewed to a higher value than that of the stations with at least 23 years. To get around this bias and to keep all stations available for further analysis, the stations with only 4 years of data were adjusted such that the year of the flood event were not included in the average calculations. This had the effect of correcting the averages to a more reasonable average.

Finally, it should be noted that the NEXRAD radar data, while a very useful addition to this study, is not without limitations either. The most prominent limitations involving radar data are in connection with complex terrain and distance from the radar origin. In complex terrain, mountains can sometimes block a radar beam. This stops the radar beam from reaching any further and effectively hides any precipitation beyond that point. As distance from the radar origin increases, the radar beam width and height above ground also increases. Consider, for example, a storm situated far away from the point of origin. The radar beam could potentially pass by entirely above the storm and miss it completely. On the other hand, the radar beam could be so wide by the time it reaches the storm that it encompasses the entire storm plus some additional area of clear air. In both cases, the intensity value interpreted by the radar would be lower than what is actually occurring at that point on the surface, leading to an underestimation of total precipitation by the radar. To what extent this is or is not occurring over the Santa Clara Basin during this study remains unclear, but it should be noted that such limitations could be in play and should be considered during analysis of the radar data.

Analysis

After producing such a map as found in Figure 2, detailed analysis of the event in question could proceed. Radar data was imported into the ArcGIS software and an animation was created to display the daily precipitation totals over the basin via radar estimates. Zonal statistics were performed on each frame of the animation to determine the mean daily precipitation totals within the Santa Clara River basin. Results of this analysis are found in Figure 3.

Figure 3 – Mean Daily Precipitation over Santa Clara Basin

As seen in this figure precipitation occurred between about December 29 through January 12. The main flooding event occurred between January 10 and January 12, a period when the basin received similar amounts of mean precipitation for each day.

Figure 4 – Radar Image for Jan. 10, 2005

The image in Figure 4 shows the general theme of the precipitation distribution across the basin. This is that the larger amounts of precipitation were found along the west, north and northwestern areas of the basin. Radar images for other days during the main flood event were of a similar nature. However, given the known limitations associated with radar data, some of the areas of lower precipitation within the basin could potentially be higher than what is imaged here. The nearest radar is located just off the northeast corner of Figure 4 as sits at an elevation of nearly 10000 feet, much higher than the elevation near the lower Santa Clara River which is near 3000 feet in elevation. To get a more complete idea of the kind of precipitation that fell over this region, the surface data from COOP and SNOTEL sites must be examined.

Because precipitation was occurring between December 29 and January 12, this is the period that will be considered when examining the COOP and SNOTEL data. Subsequently, the average precipitation totals for the combined months of December and January will be used for comparison of the significance of the amount that fell during this event. The feature class dataset containing the COOP and SNOTEL sites also carried with it a summary of the actual precipitation totals and the average Dec-Jan totals. These values were then used in the Spatial Analyst tool of ArcGIS to produce interpolated raster grids depicting the data over the entire basin. The interpolation method used to produce each raster was the inverse distance weighted method. The results for the total precipitation are shown in Figure 5.

Figure 5 – Total Precipitation (in) Dec 25 – Jan 15

Here, it can be seen that precipitation was significantly greater over the northwestern area of the basin, ranging from near 2.5 inches at the outlet to over 21 inches at the SNOTEL sites in the northwest corner. This seems to correlate well with the pattern found in the radar data.

Figure 6 – Average Dec-Jan Precipitation

Figure 6 shows the average precipitation totals for the months of December and January, helping to gain a better understanding of the significance of this event. The highest value here is nearly 6.5 inches. Keep in mind that this is the average over an entire two-month period and the event in question is only a portion of that period. A better description was then obtained by utilizing the raster calculator to produce an image showing the difference between the event precipitation and the average Dec-Jan precipitation. This result is shown in Figure 7.

Figure 7 – Anomaly of Event Precipitation vs Dec-Jan

As seen in Figure 7, precipitation for this event was as much as 15 inches greater than the two month average for Dec-Jan. Again the obvious bull-eye lies over the northwestern reaches of the Santa Clara Basin. Further inspection with the raster calculator reveals to what percentage the totals for this event were above the two month average. Figure 8 shows this result.

Figure 8 – Event Precipitation % of Dec-Jan Average

Precipitation totals for this event were as much as four times the two month average. Even the lower amounts on the east side of the basin yielded values that were 133 percent of normal. With this kind of precipitation inundating the western reaches of the basin, it is expected that the greatest and most problematic stream flow values will occur near the two USGS gage stations located nearest to the outlet of Santa Clara River, namely the gages at Saint George and Gunlock. This is confirmed in the stream flow data for each of USGS stations found in the basin and shown in Figure 9.

Figure 9 -- USGS Stream Data Dec. 25 - Jan. 15

The stream flow recorded at the Gunlock and Saint George sites was dramatically greater than that at the two locations in the upper portions of the main drainage. This acts to confirm the results of the precipitation analysis indicating that the largest amount of runoff was entering the Santa Clara River from the western reaches of the drainage. It is also notable that the flow values near 3000 cfs are substantially greater than the mean annual flow in the river which is near 45 cfs at the outlet of the basin.

Further investigation reveals that rainfall alone was not responsible for such a large amount of runoff for this event. Rather, it was a combination of rain, snow, and rain on snow which led to the premature melting of additional snowpack. This information was found in the SNOTEL data which measured the daily temperatures, precipitation amounts, and SWE values. An interesting pattern was found over the course of the event in which the precipitation at many SNOTEL sites began mainly as snowfall. Then around January 10 the temperature warmed to above freezing levels and precipitation began falling as rain, melting much of the snow, and thus contributing to the increased runoff and subsequent flooding that was recorded that day.

Figure 10 – SNOTEL Data (Little Grassy site)

Figure 10 illustrates the pattern just described. As seen here the average temperature was below freezing during a time of heavy precipitation. This then transitioned to above freezing while the heavy precipitation continued. The extra snowmelt is indicated by the decrease in SWE.

To further understand the affect this flood event had on the areas near the Santa Clara River, it was necessary to obtain a land use shapefile and input it into the basin geodatabase.

Figure 11 -- Land Use near Santa Clara River

The resulting image is shown in Figure 11. In this image the orange color is classified as residential, green areas are riparian zones, and most other colors indicate various types of crops and farmland. A large portion of the river between Santa Clara and Saint George is populated by residential areas. Thus is can be determined that many homes were affected by the significantly high stream flows that were observed during the event. In fact, there were at least 20 homes completely destroyed by the flood waters. Much of this destruction occurred as the river eroded away hundreds of feet of sandy shoreline, washing many homes into the raging water. Work has been done in these areas since the flood to streamline the river and prevent such dramatic shore erosion in the future.

With the aid of GIS software it was much more effective to analyze an event such as this. It provided a way to visualize where within the basin the greatest amounts of precipitation occurred, where the greatest amount of runoff was expected and recorded, and what types of land use areas were most likely to be affected.