Appendix 13. Hydromorphology – Hydrological Surveys

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1. Trash line recording

Figure A13.1: Trash lines

After flooding events lines of debris accumulate marking the highest level that the flood waters reached (see Figure A13.1). Knowing the peak flood levels gives the observed some idea of the likelihood of a river going out of bank and into the floodplain and therefore indicates whether the river is connected to the floodplain.

2. Water level measurements

Figure A13.2: Gauge board at gauging station

Gauge Boards

Figure A13.2 shows a gauged board fixed to the downstream wing wall of a gauging station. In this instance the gauge board is there to check the water level downstream of the gauging weir. Gauge boards can be fixed in any location, but need to be surveyed in so when they are read the level can be calculated from a standard datum (such as

ordnance datum). The boards can then be read at regular intervals (daily, weekly or monthly) by anyone from Agency staff to the Wild Trout Trust volunteers to members of action or interest groups.

Stilling Well and Level Logger

A stilling well is a tube or lined well construction in which a float travels up and down according to the vertical movement of the water level of the river. The tube goes directly into the river whilst the well construction is in the bank and is connected to the river via a horizontal pipe. Within the tube or well a transducer can be fixed which record the water level and logs them electronically at a set interval such as every 15 minutes. Water level loggers can also be installed directly in rivers to record water levels.

The levels can be used to determine when a river goes out of banks and spills onto the floodplain. This gives an indication of how well connected a river is to its floodplain.

3. Spot Gauging

Spot gaugings are a series of flow/discharge measurements that are carried out often as part of a set series of flow measurements at predetermined sites. Appendix 5 has an extract from the Environment Agency‟s hydrometric handbook which describes spot gauging and acoustic Doppler channel profiling (ADCP) gauging. Both types of flow/discharge measurement give an indication of the amount of water in the channel at the time of measurement. The method of calculating the flow/discharge is to record a

series of velocities across the channel in a series of panels. The mean for each panel is then multiplied by the cross-sectional area to determine the amount of water flowing in each panel, these are then summed to calculate the total flow/discharge.

These measurements can be used along with macroinvertebrate sample data for example, to assess the relationship, if any, between flow and aquatic invertebrates. A series of spot gauging results taken at the same site can also be used to derive a time series of flows at that site by using regression analysis with a nearby gauging station flow time series.

4. Velocity measurements

The velocity data collated when conducting a spot gauging can be used to assess the velocity pattern across the channel. A velocimeter can be used to take a number of reading both across the channel and at each departure from the bank to build up a more detailed pattern of water movement. Velocity readings can be compared with biotope maps or macroinvertebrate sampling using a surber sampler to determine which plants or invertebrates are associated with particular velocities, and whether a change in velocity changes the aquatic invertebrates or macrophyte assemblages.

5. Rainfall-runoff Modelling (to determine mean daily flows)

Rainfall-runoff models can be used to model flows/discharges at any location within the catchment provided there is sufficient gauged flow data to calibrate the model. Once calibrated, the models can then be run to produce the flows which might be expected with various climate change scenarios.