Why use Index-Velocity Methods? Traditional Sites – Stage vs Discharge is predictable

Because All of the Good Sites are Taken Variable Backwater – Multiple Discharge for same Stage

Better relation between Index Velocity and Mean Channel Velocity

Index-Velocity Site View upstream View Downstream

Index-Velocity Candidate Sites Sites with variable backwater conditions 1. Tidally influenced locations 2. Backwater from impoundments 3. Anywhere traditional stage-discharge methods aren’t providing acceptable results (not a guarantee!)

Index Velocity Method Q = VA Q = Discharge (ft 3 /s) V = Mean channel velocity (ft/s) A = Channel cross-section Area (ft 2 ) Channel area is computed at a specific, selected location using a stage-area rating Mean-channel velocity is computed from the velocity measured by an instrument using an index-velocity rating

Index-velocity Methods Channel Area Channel area changes with stage. To compute channel areas for a range of stages, a stage-area rating must be developed for a selected “standard” cross-section Channel must be “surveyed” – in some streams can use ADCPs for this A stage-area rating is developed from the survey data

Index-velocity Methods 1 = standard cross-section 2 = wading measurement section 3 = bridge measurement section Q1 = Q2 = Q3 Area is always computed at location 1! 1 23 Gage ADVM

Typical Index-Velocity Rating

Computing Q For each stage value we compute area For each velocity we estimate mean channel velocity from index-velocity rating Q=VA The average of UV’s over a day is mean daily Q