Guidance

Annex 2c Indicative designs for leaky woody dams

Published 29 June 2023

Applies to England

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Examples of a small and a large leaky woody dam.

Small leaky woody dam (RP32): 1m to 2.99m wide

Indicative design for 1 to 3 metres wide, leaky woody dam 1. A cross section view of a 3 log high woody dam shows the arrangement of the stacked logs – 2 logs at the base with another 2 logs directly on top of them. The final log is at the top of the log stack, positioned in the dip between the 2 logs below it. Ground level is shown as being between the second row of logs from the bottom and the top log.

  1. The base of the lower logs is positioned at mid-point between the channel bed and bank top, to permit the unhindered passage of low-medium flows and fish. There is scope for adjusting the size of the lower gap to maximise the effectiveness of the dam in relation to critical flood flows.

  2. The lower logs are slotted 50cm into the bank on either side of the channel or braced against a bankside tree or stump. There are 2 rows of logs installed for added strength, one behind the other.

  3. The top log is positioned on top of bank and extends 3 to 4 metres out onto either side of the floodplain to enhance flood water storage

  4. The top log is secured into place using cross stakes driven into the ground and wired together. Bundle of 5 logs also fixed together with wire.

Large leaky woody dam (RP33) – 3m to 5m wide

Indicative design for 3 to 5 metres wide, leaky woody dam

  1. A cross section view of a 4 log high woody dam shows the arrangement of the stacked logs – 2 logs at the base with another 2 logs directly on top of them, then another 2 logs directly on top of those. The final log is at the top of the log stack, positioned in the dip between the 2 logs below it. Ground level is shown as being between the second and third row of logs from the bottom.

  2. The base of the lower logs is positioned at mid-point between the channel bed and bank top, to permit the unhindered passage of low-medium flows and fish. There is scope for adjusting the size of the lower gap to maximise the effectiveness of the dam in relation to critical flood flows.

  3. The lower logs are slotted 50cm into the bank on either side of the channel or braced against a bankside tree or stump. There are 2 rows of logs installed for added strength, one behind the other.

  4. The top 2 logs are positioned on top of bank and extend 5 to 6 metres out onto either side of the floodplain to enhance flood water storage. There are 2 rows of logs used at ground level for added stability.

  5. The top 2 logs are secured into place using cross stakes driven into the ground and wired together. Bundle of 7 logs also fixed together with wire.

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