Flood Risk and Vulnerability Analysis Project - Atlantic Climate ...

Drainage
Attribute
Meaning
Hydrologic Soil Group
- Not applicable not applicable
VR Very rapidly drained A
R Rapidly drained A
W Well drained A
MW Moderately well drained B
I Imperfectly drained C
P Poorly drained D
VP Very poorly drained D
Table 4-4. Drainage Attribute to Hydrologic Soil Class Relationship
Weighting Group Description
0 A Soils in this group have low runoff potential
1 B Soils in this group have moderately low runoff potential
2 C Soils in this group have moderately high runoff potential
3 D Soils in this group have high runoff potential
Drainage Density
Table 4-5. Dominant Hydrologic Soil Group Weighting
Drainage density is the total length of all the streams and rivers in a drainage basin divided by
the total area of the drainage basin. It is a measure of how well or how poorly a watershed is
drained by stream channels. Drainage density can also affect the shape of a river's hydrograph
during a rain storm. Rivers that have a high drainage density will often have a more 'flashy'
hydrograph with a steep falling limb. High densities can also indicate a greater flood risk.
The watersheds, for this assessment, were defined by the National Frameworks Data,
Hydrology and Drainage Areas. These watersheds were used in conjunction with the NHN
dataset to define stream density within a watershed.
This assessment identified drainage densities ranging from about 0.2 to 0.3. A review of
literature focused on assessment of watershed response and drainage density provided no
indication of what is considered a “high” drainage density vs. a “low” drainage density. As such,
for the purposes of this prioritization effort the average computed drainage density of 0.26 was
used as the dividing line between upper and lower categories. Drainage densities equal to or
less than 0.26 were assigned a priority factor of “0” and those above the average were assigned
“1”.
TA1112733 page 102