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Question for Hydrologists – Soft bottom streams

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    • 2021-05-12 at 9:38 PM #15496
      James_NZ
      Participant

        Hi all,

        Has anyone got any thoughts on how to calculate flow rating curves for streams with highly mobile bottoms? I ran a site check yesterday and it seems like the lower end of the catchment is largely sand or mobile pumice rock. I am looking to use a Mayfly with a Hydros-12 CTD to develop a rating curve for flow, which will eventually enable me to calculate catchment loads. There are a couple of culverts in the catchment which I plan to use to develop solid ratings, but I’m concerned my flow will be out in the sandy area.

        Any help is much appreciated.

        James

      • 2021-05-13 at 10:40 AM #15508
        neilh20
        Participant

          Sometimes there is no easy answers. I recently went to presentation that included this analysis for

          https://creeklands.org/wp-content/uploads/2021/02/CLC-Micro-Flows-2020.pdf

          So partly what range of flow are you looking for and what accuracy.

           

        • 2021-05-16 at 2:12 AM #15527
          James_NZ
          Participant

            Thanks Neil.  I have had a skim through the attached document but the flows we are dealing with are in the order of 2-3 m3/sec during baseflow.  We also have another concern because there is sediment clearance in the lower reaches on an annual basis, which again will impact the rating curve.

            I think I will have to look for fixed structures, i.e. culverts, to provide an accurate estimate of flow, unless there are any other ideas.

            James

             

             

          • 2021-05-16 at 12:18 PM #15530
            neilh20
            Participant

              Hi James, yes there is the challenge.     My take way from the report was that providing its exceeding 1cfs or 0.028m3/sec it can be done fairly accurately with traditionally USGS methods – which sounds like you are well in that range.  For environmentally significant flows in drought California we are often looking at less than 1cfs/0.028m3/sec

              The CTD-12 scale is  0 to 10,000mm with 0.5% accuracy across depth range,   with the specification stating “no sharp changes in temperatures”. Resolution is different than accuracy.  Specifically they are saying no temperature compensation.   So the type of raw sensor (typically piezo electric, resistive) and how it responds to temperature change is critical – I’ve put out a lot of data on instruments I’ve used on how the apparent reported water depth changes with temperature. In some cases with lower cost instruments a small temperature change causes the reading to wildly swing across the range. So for the CTD-12 the range is +50mm to -50mm.     So if you can find a measurement situation where which has pretty stable temperature change  in water temperature (ice melt?) and can calibrate your CTD-12 across that temperature change,  its likely to be provide useful readings.  The places that have stable temperature’s are typically in ground water, if pumping they are also likely to have a large range of water movement – ideal perhaps for the CTD-12.  When I checked with the manufactures over a year ago they where thinking about doing temperature compensation, but no statements yet on that.

              So then I would think you are looking for the water column depth to vary significantly over the range of your instrument.   Or for locations you have, what is the range of depth change for the flows. Can you place a sensor on the upstream side of the culvert

              For environmental flows the hydrologists I’m working find that the Insitu LT500 is their instrument of choice with a 0.1% total accuracy (including temperature compensation) over 0-3m. It is however a top-end instrument when it comes to the sensor, the cable, and the stripped cable to be able to feed it in to a Mayfly modbus.

              I’m personally trying to do more calibration an trialing with the Keller Nanolevel which is 0-3m and is a large capacitive based sensor.  However the first Mayfly I deployed with it, got stolen after 9mnths, fortunately the sensor cable pulled away so still have the sensor.  I’m looking to connect it over WiFi, so still working on that.

            • 2021-05-16 at 5:15 PM #15539
              James_NZ
              Participant

                Hi Neil,

                I’m pretty sure our methods align to that of USGS. However my concern is that the cross-sectional area will change with every large flow event due to the mobile bottom sediments, giving us a poor relationship between depth and flow. Our hydrology team has suggested that an accurate rating curve is very difficult to obtain in such conditions. My question is how other agencies deal with these conditions, or do they avoid them all together (not an option for us).

                As for the accuracy of the CTD, we have an ultrasonic flow site at one of the Mayfly sites. I will use this record to assess the accuracy for our conditions.

                James

              • 2021-05-16 at 10:00 PM #15542
                neilh20
                Participant

                  Hi James, as I understand it you are right – its difficult to get an accurate rating curve under such conditions. That is the geology.  That is why there are hydrologists, and it takes having someone with experience to find  if there is a geological place  where a rating curve can be built.    regards

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