# Presentation: 211958

## Computation of Streamflow using Index-Velocity Methods

**Computation of Streamflow using Index-Velocity Methods**

**Notes:**
The purpose of this lecture is to provide you with a brief overview of the following:
The types of ADCP’s being used and the numbers of those in the USGS,
The various ways ADCP’s are being used for discharge, velocity, sediment concentrations, and other kinds of measurements, and 
 Some examples of software tools for visualization and analysis of ADCP data.

## Why use Index-Velocity Methods?

## All of the Good Sites are Taken

**Index-Velocity Site**

## Index-Velocity Candidate Sites

**Sites with variable backwater conditions**
**Tidally influenced locations**
**Backwater from impoundments**
**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**

**	****	****	****1 = standard cross-section**
**	****	****	****2 = wading measurement section**
**	****	****	****3 = bridge measurement section**
**Q1 = Q2 = Q3**
**Area is **_**always**_** computed at location 1!**

- Redistributes everything
- Makes flow uniform in long term
- Predictable profiles
- Predictable flow distribution
- Large short-term flow variability

- Stratified

- Inhibits mixing
- Long-term flow is less uniform, harder to predict
- Unpredictable profiles
- Unpredictable flow distribution
- Less short-term flow variability

*Slide by Lee Gordon, NortekUSA*

**Turbulent sites desirable!**

**Stratified sites may yield poor results using a horizontal current meter**

**Turbulent vs. Stratified Flow**

## Typical Index-Velocity Rating

**Typical Index-Velocity Rating**

**Notes:**
ADCP’s are also used to develop index velocity ratings such as the one shown here.  At this site, prior to usage of the ADCP, no measurements were possible for velocities ranging from about 1.5 ft/s to 4.00 ft/s, due to the rapidly changing flow conditions and the time required to make a Price AA measurement.  The measurements ranging from 2.4 to 3.2 ft/s (red dots) provided definition of the rating that was unavailable using conventional methods.

## Index-Velocity Instruments

*Updated April 2005*

**Notes:**
282*8500=2,397,000
25*$24,000=$600,000

Total=

## Index Velocity Station

## Acoustic Doppler Velocity Meter

**Rating Development**

**Linear**

- Curvilinear

- Compound

- Rating Forms

**Rating Development**

- Linear Ratings
**Represented by a straight line:**
**	****	****	****V = XV****i**** + C****	****	**
**Where****	****V = computed mean velocity**
**	****	****	****V****i**** = index (measured) velocity**
**	****	****	****X = line slope or “coefficient”**
**	****	****	****C = y-intercept or “constant”**
**Note - this equation is for a velocity-velocity rating – velocity is the only significant parameter.**

**Rating Development – Linear Ratings**

**Multi-parameter linear ratings**
**Where stage is significant:**
**V = Vi(X + YH) + C**
**Where ****	****Y = a stage coefficient**
**	****	****	****H = stage**
**Note – when stage is not significant YH = 0 and we are left with V = Xi V+ C**

**Computing Q**

**For each stage UV we compute area**
**For each velocity UV we estimate mean channel velocity**
**Q=VA**
**The average of UV’s over a day is mean daily Q**
**Now we have a discharge record**

## 2007 USGS Index Velocity Stations