Torrey Pines: Flood Control in Torrey Pines Reserve Extension

Visitors hiking in the hidden gem of Torrey Pines Reserve Extension will likely wonder about the giant wire baskets of rocks that don't seem to do anything. Were they part of a building foundation? What was their purpose? How long have they been there? Well, before we get to the answer, let's travel back in time a little bit.

In The Beginning

The earliest aerial photo of the area that I've found is from 1953, and it's perfect. It shows exactly what the area looked like before much of the construction was done around what is now Torrey Pines Reserve Extension.

The "valley" in 1953 was a gently sloped u-shape that emptied out into a large outwash plain that was being used for farming. Sand movement and erosion was minimal due to the large amount of mature vegetation. There is also a noticeable lack of roads, paths, and other human intervention.

Torrey Pines Reserve Extension Area in 1953

Note: to help with orientation, all of the aerial photographs have two points of interest. The orange star is where the Sea Village tennis courts are today, and the yellow star is the junction of two existing trails in the Extension.

Del Mar Heights Run-off

The 1960s were a time of great expansion of Del Mar, with the streets and neighborhood of Del Mar Heights being built out. Del Mar Heights School was built in 1964, and families moved in as houses were built. The biggest issue from this build out was that all the rain that fell on Del Mar Heights was funneled all at once by roads and gutters directly into the Del Mar Terrace canyon. This newly created flash flood farm needed remediation.

Looking at the following aerial view comparison, it is easy to see that something dramatically changed between 1964 and 1966. During that time an access road was cleared through the extension all the way through to Del Mar Heights, and significant sandy areas were primed for the erosion and gullies that soon followed.

Aerial view comparison of 1964 and 1966

The First Attempt at Erosion Control

About 1970 the situation was clear, and local residents took matters into their own hands. They got whatever approvals they needed, and they build a series of earthen dams throughout the canyon. By covering the mounds of dirt with sheets of plastic, they were able to create basins to catch the flood waters and prevent the increasing erosion. What was nice about this system was that every year they could push the newly deposited sand up onto the dam, making it higher, and put the plastic back over the top.

1973 photo of first erosion dams by Jessica La Grange, Torreyana #235, Dec 1995

The Reserve

During the Del Mar Heights build-out, a growing concern started at the grass-roots level to protect some of the green space and natural habitats in the area. In 1970, with money from dances, bake sales, and national media attention, citizens were able to raise enough to get a $900,000 grant in matching funds to purchase the 197 acres that now make up the Torrey Pines Reserve Extension.

1972 Flood

The storms of November 1972 brought a tremendous amount of rain to the freshly graded area that was soon to become Sea Point and Sea Village. The area had never recovered from the bulldozing and trenching a few years earlier, so the rain water cut wide gullies through the canyon sand. Without any vegetation to stop it, water and sand made its way down the hill to completely cover the intersection of Carmel Valley Road and McGonigle/Del Mar Scenic Parkway. This was a wake-up call to the city and developers. Not only was the lagoon filling up with sand, but the forthcoming townhouse owners certainly wouldn't want to be dealing with sand every time in rained.

1973 Sea Point and Sea Village

Between 1972 and 1974, two townhouse communities were built directly on the outwash plain of the canyon. Read my post on Sea Point and Sea Village for more on that.

And the answer is... Gabions

So, the answer to what those large wire and rock structures are is finally here. You've been patient, so here's what they are: hey are flood/erosion control gabions (GEY-bee-uhnz) that were installed in 1977 to help control run-off from the Del Mar Heights neighborhood. And yes, while it seems strange when you're hiking there in the summer, massive amounts of water do run through the reserve about once a year (whether we need it or not).

Torreyana #235, Dec 1995

Who Built The Extension Gabions?

Walking up the trail into the Extension from the south entrance, one sees small dam-like structures that consist of rocks contained by what appears to be fence material. These are called gabions. (American Heritage Dictionary - gabion 1. A cylindrical wicker basket with earth and stones, formerly used in building fortifications. 2. A hollow metal cylinder used especially in constructing dams and foundations. ; from Italian gabbia, cage.) These are actually the second structures to be built to control erosion in this area.
Prior to about 1960 the bottom of the Extension "valley" was covered with shrubs and plants. A fire in 1962, removal of brush a few years later, and then installation of water and sewer lines and a dirt access road up the center of the valley all led to serious erosion, with deep gullies formed by water runoff. About 1970 some of the local residents, with tacit approval of the rangers, started building small dirt dams to reduce erosion. These were over 6 feet high and covered with plastic with rocks on top to hold the plastic. Over the next few years about a dozen residents, with help at times from scout groups, built a number of these dams.
In 1977-78 these were replaced with the present gabions, built by the Youth Conservation Corps (YCC) and the California Department of Forestry. According to Ranger Chris Platis, who worked with the YCC, about 35 gabions were built.
Many of the gabions are now in need to rebuilding, and additional retaining structures appear to be need, which shows the power of rain runoff in even a small area such as the Extension.

1977 The Gabions are Built

In 1977 the make-shift dam system was replaced with concrete forfeited gabions -- wire cages filled with large rocks. These were built by the Youth Conservation Corps (YCC) and the California Department of Forestry (as reported in Torreyana #235 above). While 35 gabions were built, only 18 remain visible today. When installed, these appeared to be a formidable and permanent solution to a growing erosion problem.

The Flood of March 1978

Just after completion of the official final gabion system, they were faced with their first big test. After many years of drought, the first few months of 1978 in Southern California were wet. Very wet. A drought-breaking winter that filled up the reservoirs that had been empty for most of the 1970s. There was a ton of rain in February that completely saturated the ground and had already started eroding away and filling the newly built gabion basins in the Extension.

At 11:00am on Saturday, March 4th 1978, an extremely heavy rain came down all at once in a very short period time over Del Mar Heights. It was calculated to be at least 1 inch per hour.

Report on Floods of Feb & Mar 1978 in So. Calif., U.S. Army Corps of Engineers, pg 5

It was a lot of rain, and many papers reported on it in exhaustive detail nationwide. Here's a local headline example with accompanying photo.

Escondido Times-Advocate the next day

Water came rushing down from the Extension canyon, flowed right over the tennis courts and emptied out onto Del Mar Scenic Parkway on its way down to the lagoon. It carried a great deal of sand with it, jamming up the storm drain system and flooded at least one house downhill (at the corner of Del Mar Scenic Parkway and Caminito Mar Villa).

Photo taken about noon on March 4, 1978, courtesy K. Oesterreicher

The Gabion Flood Control System in Detail

In 2003, Michael Furtaw wrote a very detailed paper for San Diego State University called, Maintaining Erosion Control at the Torrey Pines Reserve Extension. The following summary is an excerpt from his work, along with his numbering system that I use for the updated report that follows.

Located at the mouth of the channel. It is holding back a large mass of sand behind it. Most likely maintained. A drain is located after it that takes the water underneath Del Mar Scenic Parkway.
Filled to the top
Concrete wall with three 3inch diameter drains that are all clogged.
Filled to the top. Secondary drainage forming around East side.
Filled to the top. Drainage is completely routed around East side.
a) Filled to the top. Small step before 6b. b) Filled to the top and tilted forward on East side. Measured at the center of the wall for average height. West side of gabion eroded and exposed, but still erect.
Not filled. Drainage completely routed around East side.
a) Filled to the top. Small step before 8b. Steep cliffs carved out. b) Filled to the top. Drainage completely routed around East Side. Steep
Filled to the top. Drainage completely routed around East Side. Steep
Concrete footing of wall remains. Looks to have been removed by man. Drainage routed around west side of the destroyed gabion.
East side of gabion failed due to channel erosion around the concrete footing . Drainage routed around East side of of gabion. Steep cliffs carved out.
Filled to the top. There is a small rock pile at the base, but not a gabion.
Filled to the top. The channel had previously meandered around the East side of the gabion (widening the entire channel to 50ft), but it was later filled in with sediments as a fix. Current channel width is 20ft.
Filled to the top.
Not filled. Drainage routed around West side. Created a deep narrow cut due to a change in lithology.
Completely buried with sediment, but still erect. Drainage routed around East side. No longer has an effect on the gradient.
Not filled. Tipped over completely Southward. Drainage routed around West side of gabion. It carved a narrow and deep channel, cutting underneath the West footing of the gabion causing it to fail.
Not filled. Erect.

Before and After

Here is a side by side comparison of aerial photographs of the area. The left side was taken in 1953, years before the Del Mar Heights build out and the installation on gabions. The right side was taken in 1980, and you can clearly see the dramatic change in the landscape. The gabions are clearly visible in the 1980 photo when you zoom in, and there is a ton of sand all around them on all sides. By 1980 they had already been rendered useless.

Gabion impact on Torrey Pines Reserve Extension

Here's a really important image of the surrounding area in 1966 as the Del Mar Heights build out was underway. Highway 5 is newly finished, and there appears to be a construction road that connects Mar Scenic at the top of the hill to what will become Del Mar Scenic Parkway to the south. This road would make bringing in the large amount of rocks used in the gabion system much easier.

Notice also that in 1966 many houses in the heights grid are not there at all, and the very first part of the school is complete. Also note that the townhouses in Sea Village and Sea Point are not there yet.

1966 aerial view of Torrey Pines Reserve Extension

Here's a current 2021 Google Maps view of this exact same area. Once the sand was built up, vegetation started growing on it, so a photo of the area today shows less open sandy area.

Aerial view of Torrey Pines Reserve Extension 2021

The Remaining Gabions

Gabion #12

I'm still sorted out photos, and trying to figure out which gabion is which. I do know for sure that this photo is of #12. It is still filled to the top with sand, and water flows straight over it in a waterfall manner.

#12 in 2022

Gabion #11 Erosion

A deepening gash continues to be carved out to the east of gabion #11 as fast-moving water is routed around it during heavy rains. This erosion was noted in the 2003 report, and it continues to this day with exposed roots and steep wall.

Heavy erosion around Gabion #11 in 2022

The stump of Gabion #10

There is something at the location of #10, which Furtaw describes as a concrete footing for a gabion that appears to have been purposely removed. Whatever was there, it's not doing anything. In fact, when I first saw it I thought it was an old foundation for a building of some sort.

The remains of Gabion #10

Gabion #9

Number nine, number nine, number nine... sorry, growing up I listened to the Beatles a lot. Clearly.

In any case, it's almost hard to imagine that Gabion #9 was once actually right smack in the middle of the channel. It's completely filled up, pretty much in front and back, and the channel is more of a flood plain off to the east.

Gabion #8

This one is in pretty good shape, with the water completely ignoring it off to the east.

Gabion #8

Gabion #7

Based on Michael Furtaw's description and map, I think this old photo from 1978 is Gabion #7 in the foreground, with the people climbing on #8. It appears to be Gabion #9 and either the #10 or #11 further upstream.

Climbing on Gabion #8 in 1978, courtesy K. Oesterreicher

Here's an attempted recreation of the old photograph, taken in 2022. It is clearly the same gabion in the foreground based on the cracking pattern in the concrete side foundation, and the tree has grown up a bit in 44 years.

Gabions #7 & #8 in 2022

I couldn't exactly get the right angle, as it appears that the 1978 photo was taken from the top of Gabion #6, which is not exactly in the same place today...

Gabion #6

So far, gabion #6 is by far in the worst shape of the remaining gabions. It appears to be two gabions at this point, "A" on the left side of the photo (west), and "B" completely fallen over under the tree to the east. 6a is tilted down into the channel at a steep angle, and 6b has fallen forward and down. Water flows between them. Taking a closer look, it does appear that there is some connecting wire between them in the middle that is all pushed down the hill pulling the sides forward and down.

Gabions #6, 7, and 8

Gabion #5

This gabion is damaged, with lots of missing rocks and loose wiring where they once were held. In 2003 Michael Furtaw reported that it was filled to the top and water was routed around it to the east. In this picture from 2022, you can see the entire channel has moved east and completely bypassed this once proud dam. The fill behind it has washed away and now nicely slopes into the passage as if it had always been that way.

Gabion #5 in Torrey Pines Reserve Extension, 2022

Gabion #4

Ah good old gabion #4. The first gabion I ever ran into on my first hike up the reserve trail from Del Mar Scenic Parkway in about 2002.

Gabion #4

Since I first encountered #4 in 2002, the trail and area in front of it have greatly changed due to erosion. The basin behind it was completely filled with sand at one point, and now water has carved its way around to the east pretty well. The torrents of water have changed the landscape in this area quite a lot, and now the trail heads to the right in a fairly straight-forward manner where there was once two ways down. The remnants of the original road and a more heavily used path right next to the gabion.

The Gabion #3 Beach

Part of the trail system that goes through the reserve includes a long stretch of sandy beach. This beach is man-made, as a system of concrete walls was placed in the area during the 1960s.

The beach behind Gabion #3 in Feb 2022

Here's the exact same location during the floods of 1978. Of note are the original road coming down the side of Gabion #4, and the now-missing step wall that the water is flowing over in the middle of the photograph.

Gabion #4 on March 4, 1978, courtesy K. Oesterreicher

Gabion #3 is a concrete structure that appears to have been augmented over the years with chunks of found concrete items from other gabion locations. I think that the round concrete plugs were foundation supports for the short concrete block wall that was about halfway between #3 and #4. When taking the "beach" photo in 2022, there is no sign of this short wall but a few broken chunks of cinder block scattered around. Gabion #3 now sort of looks like a dumping ground, and all the drainage pipes are completely clogged with sand.

Gabion #3 in 2022

One issue with this part of the trail is that many people heading south don't realize the actual trail veers left to the east, and end up jumping down this smaller gabion and walking until they hit the much taller gabion #2 and wonder how they ended up in a dead-end flash flood basin. They should turn around and walk back they way they came, but many off-trail it across to the Sea Village tennis courts they can see in front of them. Just a word of warning, there is a $400 fine if the friendly State Park ranger happens to see you.

Gabion #2

There is another large filled basin behind Gabion #2, which is in good repair otherwise. The metal wiring almost looks new, and it's just tall enough to make the jump down a little off-putting and climbing up unlikely. It has not been breached by erosion, and is a good example of a working gabion that no longer helps much due to the huge amount of trapped sand behind it. It could be cleaned out, but perhaps the large sand traps behind Gabion #2 and #3 do soak in a little water until saturation. I don't think they do much when a large amount comes all at once, like in a flash flood situation.

Gabion #2 in good repair but completely filled in 2022

Gabion #1's Flash Flood Basin Today

Rangers digging out basin in April 2020

The last remaining working basin is the one at the very bottom of the hill next to the Sea Village tennis courts. After some complicated negotiations with the City of San Diego over the years, the State Park system has taken over maintenance of this last defense against flash floods. A great deal of water and sand is held behind Gabion #1, and every year the State Park rangers come in and clean it out. They use a big digger and a series of sand tracks to drive the sand under North Torrey Pines bridge and dump it at the northern end of Torrey Pines State Beach. It's a win-win, as Del Mar Terrace is protected from flash floods and Torrey Pines beach gets their annual winter sand replenishment!

This basin does its job, and prevents a great deal of sand from running into the storm drain system that starts directly behind the stone wall.

Gabion #1 at lower flash flood basin, Feb 2022

Partially dug out basin (facing south from Gabion #2 in Feb 2022)

With annual maintenance, this last basin is large enough to hold a great deal of water and prevent a runaway flash flood. It is also big enough that maintenance of any other gabion basin up-stream is unnecessary.

Flood Basin filling with water in Feb 2019

The Last Stand: The Siltation Basins

The final step in the flood control system is the siltation basin at the bottom of the hill, across the street from the lagoon. In 2001, Maryruth Cox wrote a great article about in Torreyana.

Article from Torreyana, May 2001 page 9

Today the homeowners at Sea Point have a beautiful park for walking their dogs and enjoying the sun in private. It's clearly prime real estate that would have been an amazing place for beachside houses. If you were wondering why it's left open, wonder no more. It's a sand basin!

Sea Point Dog Park, aka the Siltation Basin in 2022

The final step in the current flood control system is the outflow into the lagoon catch basin. This was dug-out on state property across the street from Sea Point. It catches any remaining silt, and joins the lagoon out to its mouth on Torrey Pines State Beach.