Large Woody Debris Index
Why are Large Woody Debris (LWD) important?
Large woody Debris are a critical part of a healthy stream habitat. Woody debris are important for the formation of pools which create great habitats for fish. The debris also assist in stabilizing banks, creating dams, and helping with the sinuosity of the stream channel. In stream restoration there is a strong push to add large woody debris because they will help rebuild the critical parts of the river ecosystem.
The Index
Created by two groups; Stream Mechanics and Ecosystem Planning & Restoration. The index allows for us to get a point total for the amount of fallen trees or trees. There are two types that index looks at individual debris and debris dams. The equation to find the total; [Lwdi=PS+ (DD* 5)]
Individual debris; are called pieces these pieces are judge on six criteria. Length, diameter, location, type, structure, Stability and orientation. Length means the length of the pieces. Diameter is the width of the piece. Location is where the pieces falls within the river plane, zone 4 is above the banks, zone 3 is within the banks but above water, zone 2 is along bank line and can be partially in the water, zone 1 is below water line. The type is a what the piece is classified as. Bridges are typically in zone 3 and they stretch across bank to bank, Ramps extend from the banks down towards the water. Submersed, Pieces in zone 1 that sit within the water. Buried, is in zone 1 and is underneath the sediment. Structure, is if there are branches coming off the pieces. Stability is how anchored the pieces are in their place. Orientation is the degree a piece is to the river direction. if the piece is parallel to the stream it is 0º-20º but if there is a piece that is perpendicular across that is 80º-90º.
The image on the left would meet the criteria of:
Length: >1.0 meters
Diameter: > 50 Centimeters
Location: Zone 3
Type: Bridge
Structure: Plain
Stability: Secured
Orientation: 80º-90º
Debris Dams are a collection of pieces the dams are scored on similar to the pieces index except there are only five criteria. The length and the height are based off the percent of the stream corridor that is covered. The structure is based off the type of debris that are within the dam. The location is where the dam is in terms of speed of water. Stability is how anchored the dam is.
The image on the left would meet the criteria of:
Length %: 80-100
Height %: 80-100
Structure: Intermediate
Location: Low flow
Stability: Secured
This link leads to the document of how we learned to score the debris and taught us the importance of the Woody debris.