EFH-2 Engineering Field Handbook Chapter 2 Presented By: Rob Williams USDA-NRCS atBasics of Conservation PlanningOctober 21, 2003
What is Surface Runoff? Volume of excess water that runs off a drainage area.
What is Peak Discharge? Peak rate of runoff from a drainage area for a given rainfall.
What does EFH-2 actually do?? Procedure for estimating runoff and peak discharge from a small rural watershed for use in designing soil and water conservation measures.
EFH-2 consists of …. Tables, figures, exhibits, and worksheets that provide a quick and reliable way to estimate runoff and peak discharge for a range of rainfall amounts, soil types, land use, and cover conditions.
EFH-2 data for …. Peak discharge was computed using procedures from NRCS National Engineering Handbook Section 4 (NEH-4).
EFH-2 applies when …. • W/S is accurately represented by CN between 40 and 98. • W/S area is between 1 and 2000 acres. • W/S hydraulic length is between200 and 26000 feet.
EFH-2 applies when …. • Average W/Sslope is between 0.5 and 64 %. • No valley or reservoir routings are required. • Urban land use area<10% of the total W/S area.
EFH-2 limitations …. • Only 1 main stream or, if more than one, both must have nearlyidentical Time of Concentrations (Tc). • If Tc < 0.1 hour - use 0.1 hour. • Cannot use on snowmelt or rainfall on frozen ground situations.
EFH-2 limitations …. • Cannot use if potholes make up > 1/3 of the total DA, or if they intercept the drainage.
Do Not Use EFH-2 …. If you have conditions that exceed the limitations just mentioned, you should use one of the following procedures: TR55(Urban Hydrology)NEH-4(TR20 Project Formulation - Hydrology is the computerized version of NEH-4)Rational Method(Not an NRCS method) USGS Regression Equa. (>64 ac) Drainage Curves(Removal Volumes – Not Peak Runoff)
EFH-2 Factors that affect surface runoff • Rainfall • Hydrologic Soil Groups • Cover Type • Treatment • Hydrological Conditions • Topography
EFH2 Rainfall 5.6”
EFH-2 Hydrologic Conditions • Affects volume of runoff • Good hydrologic conditions indicates lower runoff potential • Grassland - Good if > 75%coverPoor if < 50%cover
EFH-2 Hydrologic Conditions • Grass cover - evaluate at basal area of the plant • Trees and shrubs - evaluate canopy cover
EFH-2 Cover Type • Raindrop impact on soil • Increased evaporation • Increased flow path • Increased surface roughness
EFH-2 Treatment • Reduce erosion • Form small reservoirs • Increase water travel distances
EFH-2 Topography • Slope has major effect on peak discharge • Slope has little effect on amount ofrunoff • Average watershed is fan shaped • Potholes trap small amounts of rainfall, reducing expected runoff
EFM2 - Runoff Curve Numbers (CN) Fill this in for Field 1 Fill this in for Field 2 Area Product Product Area
EFH-2 Peak Discharge Use runoff, Ia/P, Tc and DA to estimate peak discharge from Exhibit 2-II
EFH-2 Ia/P • Determine Ia from Table 2-4 • Compute Ia/P using Ia and P values
EFH-2 Worksheets • Worksheet 1 - Runoff CN • Worksheet 2 - Tc and Peak Discharge (Peak Q)