Interpreting L-THIA's Results: TR-55 - Urban Hydrology for Small Watersheds
Technical
Release 55 (TR-55) presents simplified procedures
for estimating runoff and peak discharges in small
watersheds. While this TR gives special emphasis
to urban and urbanizing watersheds, the procedures
apply to any small watershed in which certain
limitations are met. The methods presented in
TR-55 are identical to those used by L-THIA. L-THIA
repeats the method for each storm over a 30 year
period and then reports the average annual runoff
value.
With
advanced analyses, the correct land use may not
be available in the drop-down menus in L-THIA
(for a list of land use/ soil type combinations
and the related curve numbers that are available
in the drop down menus click here.)
In this case, the custom land use option in the
detailed input may be used. The appropriate curve
number is needed in this case. Below is a table
showing curve number for many different land uses.
There are many more available in TR-55 along with
instructions on finding curve numbers for urban
areas based on percent impervious area.
A full
copy of TR-55 in pdf form is available here.
Description
and Curve Numbers from TR-55
|
|
Curve
Number for Hydrologic Soil Group |
Cover
Type and Hydrologic Condition
|
|
|
|
|
Small
grain - straight rows + crop residue cover -
good condition (1) |
|
|
|
|
Close-seeded
or broadcast legumes or rotation medow - straight
rows - good condition (1) |
|
|
|
|
Streets
and roads - paved; curbs and storm sewers
(excluding right-of-way) |
|
|
|
|
Streets
and roads - paved; open ditches (including
right-of-way) |
|
|
|
|
Streets
and roads - gravel (including right-of-way) |
|
|
|
|
Farmsteads
- building, lanes, driveways and surronding
lots |
|
|
|
|
|