| Weir cleaning: |
Weir cleaning, or more appropriately, cleaning out the
ponding basin, is an annual event whose purpose is both to remove accumulated
material and to determine the amount of material that has accumulated over
the year. There is a continuous "flow" of particulate matter down the stream
along with the flow of the water. "Particles" may range from soil particles
to leaves and twigs, up to large rocks depending on the flow rate and the
ability of the flowing stream to pick up and move material. This movement
in basically negligible most of the time when the stream is flowing at
base flow levels, but with any rain event which may cause the stream to
rise a little, particulate matter is moved down. Once the material falls
over the lip of the flume into the ponding basin it may sink and become
trapped in the basin. This is not only true for sand and rock but also
for organic matter which is saturated and does not float. Very small amounts
of floating material may be lost over the V-notch.
This particulate matter is removed once a year (unless an exceptionally large amount may have accumulated and necessitates a special cleaning). The cleaning takes place on a day of very low stream flow in the summer. At these low flows the amount of water which must continue to pass though the basin is minimal and will not disrupt the cleaning activity. There are times when the stream is dry and the flow is absent. This may happen once every summer for a few days. To start the cleaning, a cap is removed very carefully from a large pipe which is imbedded in the concrete at the bottom of the downstream wall of the ponding basin. It is located at the lowest point in the basin on or very close to the bedrock ledge. Using boards and shovels to block the debris from leaving with the out-rushing water, the water is slowly drawn down such that only the small flow from the stream is left to pass through the edge of the bottom of the basin.
After the basin is empty of water, the material is shoveled out onto a tarp and from there it is put in buckets to be weighed. A handful of material is collected from each bucket for drying back in the lab. In this way the dry weight of the total mass removed maybe estimated. When all material is removed and any repairs made, the cap is replaced, the ponding basin allowed to fill naturally, and the monitoring by the weir continues for another year. The steps in this process are illustrated in the accompanying photos taken in the summer of 2000.
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