OTWB Structures

Purpose:  to raise the level of the experimental streams at a number of locations along the stream's length so that water will seep more readily into the stream banks and floodplain soils.  This will enhance "bank storage" of water that can then be "withdrawn" during dry periods to keep the stream flowing.  Click here for some background on structures.

Our protocol requires that structures must:

     -  be inexpensive to install

     -  require little if any maintenance

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and must NOT:

     -  create barriers to fish and invertebrate movement

     -  increase erosive forces on the stream banks

     -  prevent the transport of sediment - after they "settle in"; sediment will be deposited on the upstream side of the structure, which will increase water storage capacity, and reduce flow through and help stabilize the structure. 

The emerging science of natural stream restoration has developed structures that accomplish what we want to do.  One class of such structures is essentially a V-shaped weir, oriented so that the point of the V faces upstream, and constructed bank-to-bank, sloping down from each side toward the center.  When constructed correctly, erosive forces are focused on the center of the stream channel, not the banks.  This often creates a plunge pool below the structure - excellent fish habitat - and does not erode the stream banks (if installed correctly).  We selected a variant of the cross vane, as developed for a stream restoration project at Big Bear, PA; this structure is made out of logs, and is much less expensive to install than rock structures.  The following graphic shows the basic design, which has to be adapted to the unique characteristics of each structure site.  As of November 2004, 15 structures have been installed in the Site 1 meadow - which offers substantial design challenges in creating stable structures.  The stream is very tiny, meanders constantly, has deeply undercut banks just about everywhere (natural for this kind of channel), blowouts, bank calving, much multiflora rose, and often has orphaned "islands" of sedges in the middle of the stream.   The following slide show will show a picture of each structure, provide comments on its construction and challenges, and present a graph of the stream profile.

Structure #5 Under Construction