Aguaje de la Tuna

[typing]
The sediment retention dam at Aguaje de la Tuna has resulted in the degradation of the streambed, to a depth of approximately 8 m, reaching the underlying bedrock.
[VMP] Good morning.
We are here
in the Aguaje de la Tuna,
one of the watersheds that drains
into the Tijuana river,
in Tijuana, Baja California, Mexico.
We are accompanied by
Alberto Castro Garcia,
a hydrologic civil engineer,
practicing in Tijuana.

[ACG] Good morning to all.
I wish to thank Dr. Ponce
and his team of
San Diego State University students
for the opportunity
to present to you a few aspects of
Tijuana's hydrology,
particularly about this creek,
which is referred to as
the Aguaje de la Tuna.
In 1991, there was a heavy storm
in the city,
which produced a flood in this basin,
transporting a large quantity of sediment
to the mouth of the basin.
In 1993, an extraordinary storm event
pounded the city, especially
the Aguaje de la Tuna.
About 400,000 cubic meters
of sediment
were dumped into the delta.

As a consequence
of this situation,
the need was felt to construct
flood protection works,
consisting of channelizations
and sediment retention dams,
like the one shown here.

It is practically at the level
of where I am now standing.
This is the base level
of the first retention dam.
The objective of this dam
was basically to retain sediment.

Originally, the dam was designed
to retain a certain amount of solids.
During the useful life
of the structure,
degradation occurred,
which led to the construction
of a first step.
Ten years later,
another step was built
because degradation continued.
The steady erosion by water continued,
and the erosion depth eventually
reached the underlying rocks,
classified as shales,
as observed here.

[VMP] Here we are located at a distance
of approximately 50 m downstream
of the sediment retention dam.
As you can see,
here the amount of degradation
of the creek bottom
has reached the underlying rock,
which appears to be a disturbed shale.
Here we have an example of
a modern-day geologic, anthropogenic process
in which a small canyon
is being formed
in the rocky streambed
of the Aguaje de la Tuna watreshed.

[VMP] Here we can observe
a curiosity of Nature ---
how the water flowing
in this reach of the
Aguaje de la Tuna,
above the rocky shale bottom,
has been able,
over the past 20 years,
to carve out this feature,
basically a natural dam
of irregular shape,
apparently a spillway,
but with intermediate channels.