Wetlands, Development and Flood Risk on the east shore of the Great Salt Lake

Rebekah Downard

December 7, 2007

CEE 6440

Table of Contents

Introduction 2

Objectives 3

Methodology 4

Results 5

Conclusion 11

Data Sources and Literature Cited 12

Introduction

The Great Salt Lake is a shallow body of water in the Great Basin of the western United States that is fed primarily by snow melt runoff. Because of this, fluctuations in precipitation directly affect lake elevation, and changes in lake elevation alter the lake area a great deal, increasing or decreasing the surface area by up to 46% (Great Salt Lake Basin Hydrologic Observatory).

Figure 1. Great Salt Lake and contributing watersheds

Wetlands around the lake form the interface between water and land, and they regulate the hydrology of the system. The wetlands on the east side of the lake are of particular note because they form a buffer between the lake and major population centers along the Wasatch Front of the Rocky Mountains. Among the benefits of healthy wetlands systems is that they decrease flood pulses by acting like a sponge to absorb high flows and slowly releaseing them. Other benefits include drought mitigation, water quality enhancement and wildlife habitat (National Research Council 2001).

Wetlands are a protected ecosystem under the Clean Water Act (CWA) of 1972, any action involving dredging or filling wetlands requires a permit under Section 404. This program is run by both the Environmental Protection Agency (EPA) and the Army Corps of Engineers. Despite this protection, wetlands are was decreasing nationwide until 2005. However, it is difficult to find statistics on wetland loss by region. Additionally, wetlands that are removed from an area may be replaced elsewhere, statistically making no net loss, but making a regional difference.

The Great Salt Lake flooded and caused $478 million in damages in 1983-84. However, after water levels peaked in 1986, there has been a significant decrease in lake elevation. This decrease has coincided with tremendous growth along the Wasatch Front, about 14.2% per year (U.S. Census Bureau). As is often the case with wetlands, demands for growth often outweigh protection of wetlands, this is likely the case along the Great Salt Lake, where the shoreline is well below its peak of 4,212 m after many years of drought.

Objective

In an effort to analyze the effectiveness of Section 404 of CWA and the national goal of “Not Net Wetlands Loss,” I would like to see whether there has been a decrease in wetland area on the east shore of the Great Salt Lake that corresponds with increased development. I would like to analyze whether growth has decreased the wetlands buffer between homes, and whether there are developments within projected flood plains. Additionally, I would like to find the precipitation conditions that would endanger development in the area.

Methodology

GIS Data:

1. National Hydrography Dataset for HUC Region 16 from NHDPlus
2. 1 arc second National Elevation Dataset from USGS seamless server.
3. County boundaries, major roads, Great Salt Lake shoreline flooding, city shapefiles from Utah AGRC
4. National Land Cover Datasets (NLCD) for 1992 and 2001 from EPA Multi Resolution Land Characteristics Consortium (MRLC). Projected these into UTM Zone 12 NAD 1983 projection.
5. NAIP 1 meter orthophotography from AGRC.

Additional Data:

1. USGS preciptiation and lake elevation data
2. Great Salt Lake Hydrologic Observatory background information on the Great Salt Lake

Initially I looked at the elevation and determined the contours for various lake levels. I found no developments within the recent shoreline elevations. I delineated contours at 4,212 m and 4,216 m to show the historic high shoreline and the 500 year shoreline.

After analyzing NLCD layers I determined to look at developments near the shoreline in three Utah counties: Weber, Davis and Salt Lake. Box Elder and Toole counties also border the lake, but they do not have developments near the shoreline. Further analysis showed that there was little risk to development in Weber County, so I focused on Davis and Salt Lake counties alone.

I first looked at just those areas within the projected 500-year flood shoreline level and identified areas of development based on NLCD that fell within that boundary. I then identified the extent of wetlands around the margin of the lake from the south boundary of Weber County, through Davis County and around to the west boundary of Salt Lake county between 1992 and 2001 by using the measure function in ArcMap to calculate the area of cells identified as wetlands . I then made 7 cross sections in the area I chose to study and analyzed the area along those lines identified as wetlands (including wetlands and barren land under NLCD cover classes), agriculture (pasture/hay and row crops) and development (both high and low intensity residential and industrial/transportaion). I analyzed the cross sections for 1992, 2001 and 2006 data sets.

I also downloaded precipitation and lake elevation data from the USGS website to find how the lake elevation had fluctuated, and the corresponding snowpack.

Results:

Wetland areas have decreased along the eastern shore of the Great Salt Lake from 1992 to 2001. It is more difficult to tell how wetlands have changed from 2001 to 2006 because of questions in how to classify the barren lake bed. Meanwhile, development has encroached on the shoreline.

Figure 2. Study area

In 1992, I identified two areas within the 500 year flood line of the Great Salt Lake. This data layer represents the baseline for wetlands areas in this study.

Figure 3. 1992 National Land Cover Data Set with 500 year flood shoreline projection (1,283 -1,286 m).

By 2001, areas within the study defined as wetlands had decreased from 2,698.58 km² in 1992 to 62.06 km². This represents a 98% loss. The areas of development within the 500 year flood line had also increased from 14.21 km² to 22.87 km².

Figure 4. 2001 National Land Cover Data Set with 500 year flood line.

It is immediately apparent that the wetland buffer between development and the lake has decreased, as have the agricultural areas between them. This poses an increased flood risk that has developed in just the last 15 years. In 2006, it is more difficult to tell how land cover has changed, because it is not classified into cover types. It is apparent that there is less agricultural area than in 2001, but very low shoreline elevation (1,279 m) confounded my calculations.

Figure 5. 2006 NAIP 1 m photography with 500 year flood shoreline elevation and cross-sections

To do a cross-sectional analysis, I identified 6 growing residential areas and 1 industrial/transportation area that had expanded from 1992 to 2001. I numbered these 1-7 with their corresponding city. I found that between 1992 and 2001 the areas that could be identified as wetlands had decreased in general by 12%, as had agricultural areas, by 19%; while developed areas had increased by 180%. Though not noted in the cross-sections, the general trend for development has been westward, toward the Great Salt Lake shoreline.

Table 1. Land cover change in 1992, 2001 and 2006 for 3 cover classes.

	Wetlands			Agriculture			Development
Cross Section	1992	2001	2006	1992	2001	2006	1992	2001	2006
1 (Syracuse)	791	386	2885	4671	4468	3680	1269	2761	2376
2 (Layton)	1127	1132	3430	4693	4374	4516	0	670	779
3 (Kaysville)	1157	546	3439	2777	1983	500	242	1116	2932
4 (Farmington)	816	763	1837	2831	3037	3230	1036	916	1287
5 (Centerville)	5430	3490	5637	1064	151	1211	1291	2164	2233
6 (Woods Cross)	4385	3766	4835	3861	2126	2607	644	1525	1611
7 (Airport)	4510	5954	9345	1950	1610	0	1085	1105	3308
Total	18216	16037	31408	21847	17749	15744	5567	10257	14526
Change		0.88			0.812			1.842

When doing cross-sections, I decided to count barren land as wetlands, because it had to potential to become vegetated later in the growing season. However, this led to an increased wetlands are in 2006, as much of the area that was classified as open water in 2001 was now barren. I do not feel like the wetlands area calculated from 2006 is an accurate description of actual wetland area. I am confident in the calculation of increased development, and feel it supports my conclusion that the developments along the east shore of the Great Salt Lake are in increased danger of future flooding.

Based on analysis of the shoreline elevation and snowpack, it would take a number of years of above average snowpack to produce a major flood event. However, a few years of average snowpack could bring the shoreline elevation closer to homes and allow for better assessment of wetlands extent.

Table 2. Snowpack and corresponding lake elevation (data from USGS).

Year	Shoreline Elevation (feet)	Snowpack (percent of normal)
1982	4200	166
1983	4210	215
1984	4212	188
1988	4206	93
1989	4204	93
1990	4204	82
1991	4203	96
1992	4202	116
1993	4200	65
1994	4198	134
1995	4197	90
1996	4196	184
1997	4198	88
1998	4200	66
1999	4201	54
2001	4202	116
2002	4198	164
2006	4196	57

Conclusions

Areas in the greatest danger for flooding are portions of Layton, Utah and Centerville, Utah, as well as the Salt Lake City International Airport. The increase in development within the 500 year flood line indicates that should another major flood event happen, damages would be higher than in 1983. The goal of “No Net Loss” of wetlands has not been met within the Great Salt Lake Basin. This may be for a number of reasons, including development pressures outweighing environmental concerns, wetlands mitigation occurring outside the watershed, or incomplete knowledge of how best to restore a wetlands system. These problems should be addressed before a major flood event.

Data Sources

EPA Multi-Resolution Land Characteristics Consortium (MRLC). http://www.mrlc.gov/.

National Land Cover Dataset 1992

National Land Cover Dataset 2001

NHDPlus. http://www.horizon-systems.com/nhdplus/.

Region 16, Version 01_01, National Hydrography Dataset

USGS Seamless Server. http://seamless.usgs.gov/.

1 arc second National Elevation Data Set

Utah Automated Geographic Reference Center. http://agrc.its.state.ut.us/.

County Boundaries

GSL Shoreline

GSL ShorelineFlooding

Major Roads

City Locations

2006 NAIP 1 Meter Color Orthophotography

Literature Cited

Great Salt Lake Hydrologic Observatory. http://greatsaltlake.utah.edu/.

National Research Council. 2001. Compensating for Wetland Losses Under the Clean Water Act. National Academy Press, Washington D.C.

United States Geologic Survery – Great Salt Lake. http://ut.water.usgs.gov/greatsaltlake/.

United States Census Bureau. http://census.gov