The past, present, and future hydrology of Devereux Slough

Thu, 05/04/2017 - 17:03 -- Ryan Clark

 

The 2016-17 rainfall season turned out to be wetter than expected, and this was reflected by repeated flooding of the former Ocean Meadows golf course and NCOS project site. A series of water level monitoring devices in the creeks that flow into Devereux Slough, and in the slough itself, tell an interesting story of how the precipitation and the hydrology of the site interacted prior to the restoration project.

Devereux slough once extended throughout and beyond the boundaries of the former golf course (Figure 1). In the mid 1960s, the upper portion of the slough was filled in to create the golf course and housing sites, as can be seen in this 1967 aerial photo (Figure 2). The 3.2 square mile watershed reaches the slough through 4 tributaries that funnel their way through the former upper arms of the slough. From the west is Devereux Creek, from the north is Phelps Creek, from the northeast is the Whittier channel, and from the east is a small, unnamed tributary from the Storke Ranch housing area that once connected Devereux Slough to Goleta Slough (Figure 3). These channels merge together in the middle of the NCOS restoration project site, and the flow into Devereux Slough is controlled by a sheet-pile grade control structure that separates the creek system from the slough.

 

Figure 1. Portion of a U.S. Coastal Survey Map from 1871-73.                         Figure 2. Aerial photograph from 1966.

 

                                        Figure 3. Map of the tributaries and flow of water through Devereux Creek and Slough.

 

CCBER maintains water level sensors in these tributaries and in the slough. The two graphs below show the changes in water levels in the creeks from October 2016 through April 2017, with a close-up of the January-February period when the heaviest rains fell. These figures show that water levels near the junction of Phelps creek and Devereux creek (purple line) had peak flows of 17.5 feet above sea level during a heavy rainfall event (more than 2 inches in a few hours) on January 20th, and 16 feet above sea level during the large storm (more than 4 inches of rain) on February 17th. The ground level elevation of the former golf course ranges from 10 to 15 feet and, therefore, stormwater easily rises over the edges of the channelized creeks and floods the site during relatively minor to moderate rain events of 2 inches or more.

 

 

 

The NCOS restoration project will remove 1 to 6 feet of soil from the filled flood plain and former estuary. This will enable stormwater flowing down the creeks to expand over ground that will be at elevations ranging from 5 to 10 feet (rather than 10 to 15 feet). Therefore, water levels will be further below the adjacent homes once the project is completed. This aspect of the restoration project design was developed in part with models documented by project hydrologists showing a drop in flood elevations of 1.5 to 2 feet in peak 100-year storm events.  

In addition, rather than having the flows channelized into the narrow creeks and then flooding over the banks during significant rain events, the restored system will allow the flows to spread out more naturally and persist for a longer period of time, providing important habitat for foraging shorebirds, and for water fowl, fish and other wildlife.

Devereux Slough is an intermittently tidal system, which means that there is a sand bar at the mouth where the slough meets the ocean. This sand bar changes height depending on the waves and is breached when water levels in the slough reach almost 10 feet, the average height of the berm. When the water in the slough breaches the berm, the system becomes tidal. This pattern is clearly seen in the data recorded by the level logger in the slough and displayed in the two charts below. The figures show that the height of water in the slough ranged from 4 to 5 feet in elevation during the fall, then began to rise as winter rains started to fall and fill the slough. On January 9, the water level peaked at almost 10 feet following consecutive storms, the sand berm was breached and the water level quickly dropped by more than 4 feet as the water in the slough emptied into the ocean. This breach event began a period of several weeks when water levels in the slough periodically fluctuated daily with the tides, until the mouth gradually filled with sand, replacing the berm and holding the water level near 6.5 feet in elevation through early spring.

 

 

 

During summer, the sun’s energy will evaporate water from the slough surface at a rate of approximately 4 to 6 inches per month. Once the NCOS restoration project is complete, the whole system will be connected to the mouth and should follow patterns very similar to what is shown in the figures above. Rain events will fill up the system, and when water levels reach 8 to 10 feet, depending on waves and tides, the sand bar will be breached and the system will become tidal for 1 to 3 weeks until the mouth is slowly closed up again.

Another aspect of the restoration project design takes into account the potential effects of future sea level rise in a couple of ways. The first is that the amount of soil excavated will be less than what could have been moved in order to provide what is called a ‘transgression zone’. This zone will be high enough in elevation to continue to support salt marsh vegetation that might otherwise get flooded out with just 1 to 3 feet of sea level rise that is projected for the coming 50 to 100 years. The second way the system is designed to consider sea level rise is that, with the increased capacity of the system, especially at higher sea levels, the mouth will be open for longer periods of time. When it is tidal, water levels will be lower, on average, than they would be in a smaller system, where the sand bar would build up higher and higher with sea level rise. With these considerations factored into the restoration design, the public access and recreation values of the site, and the diversity of habitats created, will be preserved.

As part of the long term management of the Devereux Slough system, we will be monitoring many of the aspects its hydrology in order to see how it performs, and to document sea level rise and carbon sequestration processes in the restored system.

 

Date: 
Thursday, May 4, 2017 - 16:45