Tuesday, July 16, 2024

Riverbed changes after 2023 flood

The aerial photos shown in January 2023 Flood Overview illustrate how the flood of January 9, 2023 ripped out large areas of vegetation and "re-set" the floodplain.  In response to this 50 year flood, in many places the primary flow of the river shifted into historic channels within the wide floodplain.

This was the first major flood since the Thomas Fire of December, 2017.  As is typical for this watershed, erosion of thousands of acres of steep mountains following the fire introduced large amounts of sediment into the headwaters of the Ventura River.  This is documented in Post fire storm event, Jan 9 2018, Retrospective on the Thomas Fire, Ventura River post-fire sedimentation 2019, and Matilija canyon sedimentation.

Planning for the removal of Matilija dam has been underway since 1999, with the primary concern being predicting the fate of both the 9+ million cubic yards of sediment trapped upstream of the dam as well as the renewed sediment supply from Matilija Canyon once the dam is gone.  Planners and engineers have been relying on a variety of computer models to estimate changes in the riverbed with increased sediment supplies. Until now there has not been any real world data to groundtruth these predictions.    

A July 2024 report Post-Flooding Numerical Model Update and Review - Technical Memorandum generated for the Upper Ventura River Groundwater Sustainability Agency (UVRGSA) provides a hindsight analysis of channel changes following the 2023 flood based on LIDAR and satellite imagery.  The concern of the GSA was whether the changes in the riverbed would affect their groundwater computer model. Although the focus of this work was on calibration of the UVRGSA groundwater-surface water model, the document includes an interesting review of the changes in channel alignment and bed elevation summarized below.

Of interest is the conclusion that, 

"The changes in the channel characteristics generally depended on the location within the basin. The upstream areas were found to more likely increase in streambed elevations in response to sediment deposition and the downstream areas were more likely to observe decreases in streambed elevation in response to scour."


Of note: 

Upstream of Robles;

In the Kennedy hydrogeologic area, there was minimal change in the lateral position of the updated channel network’s position in comparison to the original channel network. However, the streambed elevation increased by approximately four feet on average (Figure 2.2a), indicating significant sediment deposition in this area. The accretion within the channel network increases moving downstream, with a maximum elevation difference of over eight feet occurring at the most-downstream reach. 




Ventura River preserve:

In the Robles North hydrogeologic area, the lateral position of the channel was minimally changed in comparison to the original channel in the northern half of this area. A new braid was formed in the southern half of this area (Figure 2.2b). Similar to the Kennedy zone, the overall channel within the Robles N zone increased in elevation by up to approximately three to five feet in some areas.

 

Within the Robles South hydrogeologic area, the lateral position of the updated channel network is generally in the same location as the original channel network (Figure 2.2c). However, the updated channel network is less tortuous than the original channel network. There is a small vertical change in the streambed elevation. On average, the updated channel network is approximately one foot lower than the original channel network. This zone represents a transition between sediment accretion in the upper portion of the basin and erosion in the lower portion of the basin. 

 


Live Oak Levee;

In the Santa Ana North hydrogeologic area, the updated channel shifted both westward and eastward in different reaches of the stream in addition to the formation of new braids (Figure 2.2d). The stream location begins westward of the original channel in the upstream areas and then crosses over eastward of the original channel until roughly rejoining the original channel. The updated channel network is consistently around two feet lower than the original channel network in this area, indicating the overall erosion of the streambed. 



Also of note, the “initial flush” from the Thomas Fire consisted of very fine sediment (ash and silt) that deposited in the riverbed following erosional flows up to 5,000 cfs (see Ventura River post-fire sedimentation 2019.)  The following winter the fine sediment was flushed and replaced by sand and gravel moving in flows up to 1500 cfs. (March rains.)   Then the big 2023 flood produced flows over 20,000 cfs which flushed the sand up onto the floodplain terraces and out to the beach, with the new channels almost entirely lined with cobble and boulders (including what some call “VW sized boulders”.)    

The 2024 storms had peak flows over 6,000 cfs which was enough to move this slug of sediment further downstream, and the active channel shifted dramatically again in response.  The image below is the 2023 main channel in Robles South that was abandoned in 2024 when sediment plugged its junction upstream.  

Former primary channel, Ventura River above Baldwin Rd, March 2024



Ventura River Flood flows, 2024 



Computer modeling for the Matilija Dam Ecosystem Restoration Project has predicted a sediment wave moving downstream following dam removal.  The results of this analysis demonstrate this effect, with a rise of 5-8 ft elevation in the riverbed seen moving into the upper reaches of the Ventura River following the 2023 flood.  It is likely that a followup analysis using LIDAR data after the 2024 floods would show this sediment moving further downstream, once again shifting the active channels and raising the riverbed in areas that were scoured in 2023.  

The Ventura River is dominated by the regional drought and flood cycles. This steep watershed has the natural capacity to transport huge volumes of sediment generated by the highly erodible sedimentary geology of the the transverse ranges of California.  Much of the Ventura River features a broad floodplain with many channels that shift in response to the ever changing flow of water and sediment.  For over 70 years Matilija Dam has trapped a large portion of the sediment yield from the upper watershed, and this deficit creates a net erosional environment downstream as seen in the downstream reaches of this study.  Although it's hard to quantify the volume, the pulse of sediment following the Thomas Fire is behaving much as predicted for the removal of Matilija Dam.  More on this to come...  
 


Reference:

Post-Flooding Numerical Model Update and Review - Technical Memorandum, Prepared for: Upper Ventura River Groundwater Agency, July 2024


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