Stream Flow at Hedge Bridge Road

Hedge Bridge Road: East and West Culverts
Background

Stream flow or discharge is the rate at which water flows through a stream, usually recorded as cubic feet per second (cfs). This rate will change depending on the amount of rainfall, snowmelt and sunshine during the year. As the flow of the stream changes,the elevation of a stream's surface above a given reference point , or stage, changes as well. Stage is measured either by a staff gage which gives the height from the bottom of the stream or by a piezometer. A piezometer is a vertical perforated pipe that allows water in and acts as a stilling tube for more accurate measurements. At the Hedge Bridge Road site, the peizometer is located upstream of the culverts, and an additional stage measurement was taken at a spot in the east culvert marked by an X carved into the concrete.

Discharge is determined by measuring velocity at multiple points along the corss-section of a stream. Velocity can be determined by an electronic current meter, or by time floats over a measured distance; however, both of these methods are time consuming. To account for variation in stream depth and velocity across the chanel, Velocity is measured from 40% up from the streambed and at multiple points.

Hedge Bridge Road was selected as the site for weekly streamflow sampling because it is not affected by backwater as some of the other sites are. Furthermore, the concrete culverts provide for good measurements low flows, which can be difficult to obtain.

Q= A V where Q=discharge, A=Area and V= Velocity

Area Velocity Method: q=discharge, w=width, d=depth, v=velocity

Because it is difficult and time consuming to measure the volume of a river, we can predict the discharge by creating a rating curve in which discharge is a function of stage. Such a rating curve places stream discharge on the y axis and stage on the x axis so that a line of best fit can be created once enough data points have been recorded. Once the rating curve has been established, an equation for the curve can be determined. Having a rating curve enables us to predict the discharge using only the stage.

It is important to be able to monitor stream discharge because it is directly related to many water-quality variables such as turbidity, conductivity, bacteria, metals, and pH. Measuring these fluxes of nutrients and sediment is required in some sections of the Clean Water Act.

Methods

During March and April 2008, our lab group of four people, Mckay, Lindsey, Bryant and Madelyn, went to Hedge bridge road to measure the stage and discharge of Nequasset Stream. We took measurements every few weeks and attempted to get measurements specifically during both low and high-flow periods.

High Flow: East and West Culverts at high flow

Each time we split into two groups and measured stage and discharge in both of the culverts present on the site. We measured velocity using a Swoffer pygmy electronic current meter and stage using a piezometer. In order to create the data points from which the rating curve was established, we added together the measurements from the east and west culvert of a given measurement period.

Results

The piezometer rating curve is described nicely by a third-order polynomial equation and the culvert rating curve is described nicely by a linear equation. This makes their respective discharges easy to predict.

Piezometer Rating Curve: This is the rating curve of the discharge as a function of the stage from the Piezometer. This graph has fewer data points because the piezometer was not installed at the beginning of the project.Culvert Rating Curve: This is the rating curve of the discharge as a function of the stage from the culvert.