Forest Service Peak Flow Calculator
Post-fire curve numbers from others
a Hydrologic Soil Group A: 64; B: 78; C: 85; D: 88
Burn severity Post-fire condition Cerrelli Story Stuart Kuyumjian High water repellent soil 94 93-98 - 95 not water repellent 64-88a 90-95 - 90-91 Moderate water repellent soil 94 - 80 90 not water repellent use cover type in Fair hydrologic condition - 80 85 treated - - 75b 60-85c Low water repellent soil 94 - 70-72 pre-fire CN + 5 not water repellent N,E slopes: use cover type in Good hydrologic condition
S,W slopes: use cover type between Fair & Good
- 70-72 pre-fire CN + 5 treated - - 66b 60-85c Unburned N,E slopes: use cover type in Good hydrologic condition
S,W slopes: use cover type between Fair & Good
- 60-64 -
b Combination of seeding, contour-felling, fencing, and road drainage
c Straw mulch with good coverage: 60; seeding with LEBs, 1 year post-fire: 75; LEBs without water repellant soil: 85
Cerrelli, G. A. 2005. FIRE HYDRO, a simplified method for predicting peak discharges to assist in the design of flood protection measures for western wildfires. In: Moglen, Glenn E., eds. Proceedings: 2005 watershed management conference-managing watersheds for human and natural impacts: engineering, ecological, and economic challenges; 2005 July 19-22; Williamsburg, VA. Alexandria, VA: American Society of Civil Engineers: 935-941.
Kuyumjian, Greg. [Personal note]. Greg's Curve Number thoughts.
Story, Mark. 2003. [E-mail circulation]. September. Stormflow methods.
Stuart, Bo. 2000. Maudlow Fire, Burned Area Emergency Rehabilitation (BAER) plan. Townsend, MT: U.S. Department of Agriculture, Forest Service, Northern Region, Helena National Forest.
Hydrologic condition Hydrologic soil condition Poor Fair Good 45 36 30 Soils have a low runoff potential due to high infiltration rates. These soils consist primarily of deep, well-drained sands and gravels 66 60 55 Soils have a moderately low runoff potential due to moderate infiltration rates. These soils consist primarily of moderately deep to deep, moderately well- to well-drained soils with moderately fine to moderately coarse textures. 77 73 70 Soils have a moderately high runoff potential due to slow infiltration rates. These soils consist primarily of soils in which a layer exists near the surface that impedes the downward movement of water or soils with moderately fine to fine texture. 83 79 77 Soils have a high runoff potential due to very slow infiltration rates. These soils consist primarily of clays with high swelling potential, soils with permanently high water tables, soils with a clay pan or clay layer at or near the surface, and shallow soils over nearly impervious parent material. Poor: forest litter, small trees, and brush are destroyed by heavy grazing or regular burning.
Fair: forests are grazed, but not burned, and some forest litter covers the soil.
Good: forests are protected from grazing, and litter and brush adequately cover the soil.
Modified from: ISU 2008, Table 4, p. 9.
To predict peak runoff from total storm runoff:∇ References
Changing the length and steepness of the hillslope is unlikely to change the predicted runoff amount. It will make a bit of difference in erosion.
- Run ERMiT for climate and hillslope typical of the watershed.
- Note the return period runoff volume from first ERMiT table (Rainfall Event Rankings and Characteristics From Selected Storms).
- Estimate the peak runoff rate from the runoff volume using the TR-55 method.
The SCS-TR55 method has been widely used to estimate peak runoff rates from small rural and urban watersheds (SCS, 1986). This method of estimating peak runoff rate is applicable to watersheds that are smaller than 900 ha and with average slopes greater than 0.5 percent with one main channel or two tributaries with nearly the same time of concentration. (Fangmeier et al., 2006. 5.16-TR55 Method For Estimating Peak Runoff Rate.)
Fangmeier, D.D.; Elliot, W.J.; Workman, S.R.; Huffman, R.L.; Schwab, G.O. 2006. Soil and Water Conservation Engineering, Fifth Edition. Clifton Park, NJ: Thompson Delmar Learning. 95-97.
Foltz, R.B.; Robichaud, P.R.; Rhee, H. 2009. Post-Fire Peak Flow and Erosion Estimation; Use of Post-Fire CNs by BAER Specialists. Moscow, ID: USDA Forest Service, Rocky Mountain Research Station. Web page. Last Modified 05/13/2009. forest.moscowfsl.wsu.edu/BAERTOOLS/ROADTRT/Peakflow/CN/supplement.html#CNGuideline
Iowa State University, 2008. Iowa Stormwater Management Manual. Version 2. Chapter 2C.5, NRCS TR-55 Methodology. Iowa State University. www.ctre.iastate.edu/PUBS/stormwater/documents/2C-5NRCSTR-55Methodology.pdf
Soil Conservation Service, 1986. Urban Hydrology for Small Watersheds. USDA Natural Resources Conservation Service, Conservation Engineering Division, Technical Release 55. June 1986. 164 p. www.cpesc.org/reference/tr55.pdf