River observations at Carding Mill Valley

Today Dave, Jim, Javi and Phil (from the FSC) took me to Carding Mill Valley and showed me the river observation and data collection activities that they do there with groups of students. We walked up the river to get a sense of the location, and Dave and Jim gave me a data collection worksheet that students currently use and walked me through the process of collecting a set of river observations. With secondary school groups they might repeat the observations up to ten sample sites as they walk back down along river, for primary students a similar set of observations would be made at fewer sites.

Although a similar types of measures are made by different types of groups (i.e. primary, secondary, tertiary, etc.) the equipment used and number of data collection sites does vary. The following notes identifies the order and procedures involved in taking these measures, these would be repeated at each data collection site:

Example data collected at one site

  1. Wetted perimeter (m) [x1]: 1.67 meters
  2. Width (m) [x1]: 1.45 meters
  3. Profile data [x11]:
    1. Depth (cm): 14.5, 12, 16, 25, 21, 27, 30, 24.5, 20, 18, 13 centimeters
    2. Bedload size (cm): 5, 3.4, 6, 6.3, 5.4, 9, 14, 8, 5.6, 12, 4.8 centimeters
    3. Bedload roundness (class): 2, 2, 3, 2, 1, 2, 2, 3, 3, 2, 2, 3 Powers Index of Roundness
  4. Velocity [x3]: 1.2, 0.8, 1.4 meters per second
  5. Gradient: 23 degrees
  6. Notes: There is a sharp bend to the right at horizontal angle of about 45 degrees, the left hand side bank shows signs of water erosion and there is some gravel deposited approx 10cm up the bank on that side, large boulder in the right hand side of the channel, approx 2m upstream.

Data collection procedure

Wetted perimeter (m) [x1]: At each site the groups start by measuring the wetted perimeter and width of the river. These are measured directly across the river. The wetted perimeter is a the length along the riverbed of a line that perpendicularly bisects the river. A chain is used to measure this. Starting by placing one end of the chain at one side of the river, the chain is laid out a few inches at a time along the riverbed until you reach the other side of the river. The end point on the chain is then held and the length of chain measured out is noted in meters.

Width (m) [x1]: The width of the river is measured with a measuring tape. Like the wetted perimeter this is measured directly across the river. The width and wetted perimeter together outline the cross-section of the river at that data collection site.

Cross profile data [x11]: The following three measures (i.e. depth, bedload size and bedload roundness) are done repeatedly across the river. Typically, the groups take measures at equal intervals across the river. For secondary school groups 11 data points are collected – 11 is an ‘easy number’ in that to identify the distance between each data point the students divide the river width at that point by the number of measures minus one. So in the case of 11 points the students would divide the river width by ten (e.g. river width = 1.45 meters, so make eleven measures starting 0 centimeters from one one side and work your way across to the other side i.e. at 14.5cm, 29cm, 33.5cm … 145cm from one side).

  • Depth (m): The depth of the river is measured vertically from the top of the water to the riverbed, typically by putting a meter rule or measuring pole into the river. The depth at each riverbank will usually be zero.
  • Bedload size (cm): The bedload size is commonly the longest length of a stone taken from the riverbed at that location. Lifting one of stones out of the river that the meter rule was touching when measuring the depth is a useful method for selecting the bedload sample. The longest side is measured in centimeters using a measuring tape. Before putting the stone back into the river, the students also need to measure the bedload roundness.
  • Bedload roundness (Powers Index of Roundness – Class): The bedload roundness is measured by comparing the shape of the selected bedload stone against a chart showing six classes of roundness. These classes are referred to as ‘Power’s Index of Roundness’. The range is: class 1 = very angular, class 2 = angular, class 3 = sub angular, class 4 = sub rounded, class 5 = rounded, class 6 = very rounded (see example data collection sheet).
River flow rate measuring equipment River flow rate measuring equipment. When the propeller is put into a river the force of the water makes it spin along the threaded bar. The time taken to get from one end to the other is used to calculate river velocity.

Velocity [x3]: After taking all of the profile measures, the students then measure the velocity of the river at a three points (although it could be more or fewer) across the river at the same data collection site as their eleven profile data points. The same equation can be used as before for identifying the separation between data collection points across the river, but this time the students don’t measure the velocity at the river bank, so the students divide the width of the river by four to get the distance between each data collection point (i.e. 1.67 divided by 4 = 0.4175 meters).

The students at Preston Montford use an ‘Owens and Boys hydro-prop’ and a stopwatch to time how long it takes an impellor on a threaded bar to move from one end of the bar to the other (see photo). The time in seconds is recorded at each data collection point across the river and an (spreadsheet) equation is used to calculate the velocity of the river in meters per second (velocity = (3.281 / meantime) + 0.0277). Alternate devices used at other centres include electronic flow rate meters that give a velocity in meters per second, or in some cases in rotations (which can later be converted into meters per second using a spreadsheet equation).

Gradient: The last measure taken at each site is the gradient of the river. Primary students use a clinometer to take this measure, but secondary students and adult groups typically use a tripod mounted surveying sight and measuring staff. The tripod is set up at the data collection site and at a known distance upstream and the measuring staff is placed vertically above the river with the bottom of the staff touching the top of the river, and the survey sight is used to read off the vertical height of the river below the level of the survey sight. The process is then repeated at a known distance downstream and the gradient of the river can then be calculated by subtracting the two vertical measures (downstream height – upstream height) to identify the drop in the river between the two points.

As well as noting the upstream height and downstream height the students need to record the distance between the two measures. From these three figures a (spreadsheet) equation can calculate the gradient angle which is recorded in degrees (i.e. the sine.angle = opposite / hypotenuse).

Notes: Finally, the students are also encouraged to make field observation notes at each data collection site. Photographs are also sometimes taken and would be useful to help illustrate (or remind the students about) about important features of each data collection site.