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.
(1) Hydraulic condition is based on combination factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or close-seeded legumes, (d) percent of residue on the land surface (good>=20%), and (e) degree of surface roughness.