La Manzanilla

1
La Manzanillas Basin Surface Morphology

• An exploration of the various processes
  responsible for the geometries and morphologies
  present

2
Overview of Morphology

• Plan view of the area shows several intermittent
  streams terminating on the relatively small
  bajada on the eastern shore of Tenacatita Bay.
• The bajada is the result of coalescing alluvial
  fans and rockfalls along the interface of bedrock
  exposure and the bajada.
• There is a long, narrow beach with a back barrier
  mangrove lagoon with a filled tidal inlet at La
  Manzanilla.

3
Overview of Morphology
4
The Streams

• Locals say that the streams only flow for a few
  weeks out of the year.
• Data needs to be collected to confirm those
  assertions.
• Regardless of the exact amount of time the
  streams flow, it is true that flow is violent,
  evidenced by boulder sized material in the stream
  channels.

5
Streams Continued

• Although the streams have strong competency, they
  do not flow all the time.
• This translates into short lived influxes of
  sediment into the bajada.

6
Beach Processes

• Because of the geometry of Tenacatita Bay, waves
  enter from the west and refract along the edges
  of the bay, and they eventually end up breaking
  on the bajada.
• Wave energy has created a beach on the bajada,
  leaving a low energy back barrier mangrove bay
  behind that.

7
Wave Refraction and Longshore Drift
8
Beach Continued

• Due to the morphology of the beach and its
  spatial relationship to nearby intermittent
  streams, I suggest that longshore drift processes
  have transported sediment along shore from the
  northern terminus of the beach where a stream is
  present, to the south, creating a long narrow
  spit.

9
Longshore Drift and Spit
10
The Back Bay

• The back bay is characterized by fringing
  mangrove stands along the channel.
• I suggest that the geometry of the channel is
  induced in part by beach processes along the
  northern tier beach side of the channel and
  alluvial fan deposition to the south landward
  side of the channel around La Manzanilla.
• The edges of the bajada are supplied with
  sediment from the adjacent hillsides during rock
  falls, not the streams.

11
The Two Regions of Morphologic Control
12
Additional Considerations

• The morphology and geometry of the sand bodies in
  the La Manzanilla basin are the responses to
  sediment supply, wave energy and sea level.
• Sediment supply is intermittent.
• Wave energy is medium to low and constant.
• Sea level is debateable.

13
Sediment Supply

• The intermittent streams supply sediment
  periodically to the system.
• The sediment supplied by the streams ranges from
  cobble to clay sized terrigenous clasts.

14
Wave Energy

• Wave energy during the dry season is low, waves
  approached one meter during our January 2008
  meeting.
• Wave energy is inferred to be greater during the
  rainy season because of offshore and periodic
  nearby landfall of tropical cyclones.
• Close proximity to an ocean-continent convergent
  zone exposes the area to occasional tsunamis.

15
Sea Level

• During the past 20,000 years, world wide sea
  level has risen dramatically.
• Tenacatita Bay lies in a graben, an extensional
  terrain feature that is a downthrown block of
  crust that results in a negative topographic
  feature.
• The bay is bounded by two normal faults, so it is
  intuitive that fault slip would result in a local
  transgression.
• However, regional volcanism is related to
  additional heat in the lithosphere and there may
  be uplift in the country rock, resulting in a
  local regression.
• Basically, the tectonics are complicated and it
  is not known how sea level has fluctuated
  recently around Tenacatita Bay.