Preparing for Extreme Events
What is Flood Hardening?
First of all, let’s define what is meant when we talk about “Flood Hardening” a surface water monitoring station. A more appropriate description might be “Preparing your Surface Water Stations to Withstand Extreme Events.” That’s a mouthful, so let’s stick with flood hardening. Why worry about flood hardening? The answer is simple. We don’t want to lose valuable data. What good is a monitoring site if it fails to operate during and after a flood event? Consider the case when a flood-warning site is damaged or destroyed. We not only lose information and data that are vital to public safety during the event, but we also lose it for days, weeks, or possibly months following the event.
Why do I need to Flood Harden my stations?
Extreme events, like floods, are relatively rare occurrences, making it even more important that you collect data during those rare events. Data that are collected during and after floods become invaluable to resource planners and managers, modelers, hydrologists, geomorphologists, climatologists, hydraulic engineers, and many others, including the general public! You may not know it at the time, but your data may become invaluable, even if the station isn’t intended for flood monitoring.
All of these points serve to emphasize the great importance of ensuring that monitoring sites continue to operate reliably before, during, and after floods. Regardless of the purpose for the monitoring station, it is still wise to design your installations to survive floods. After all, who wants to lose all that data or have their expensive equipment damaged or lost?
When is the best time to harden my stations?
The best time to prepare for extreme events is prior to installing the monitoring station. Try to determine what extremes have occurred historically at or near your proposed monitoring site, and add an additional margin of safety. Realize that Mother Nature tends to break her own records. There will always be a bigger flood coming.
Look at access roads and trails to a station to determine if you can actually get to it during floods so you can verify the accuracy of its operation. Be sure there is a plan for making measurements or collecting samples to verify or calibrate sensor data. Incorporate telemetry into the instrumentation plan. There is nothing like having the ability to look at your data in near real time. Telemetry also captures the data so if the sensors are lost, you haven’t lost all of the data recorded since your last field visit and data download.
Tips for your Stage Monitoring Stations
Here are some useful tips for preparing your site for extreme events of water stage, water discharge, water velocity, and water quality, which all require specific considerations.
Pick a good location.
Flood-warning sites on reservoirs and rivers need to be flood hardened. The most obvious consideration would be to locate the instrument shelter above historic high water levels. Place the shelter on stilts in a flood plain where velocities and debris impact or accumulation are minimized. Another option would be to house the data logger and telemetry instruments in a smaller weatherproof box and mount the box and telemetry antenna on a telephone pole. This approach can also be used when installing a temporary instrument package for monitoring during long-duration floods. You may even need a boat to access the station.
Pick your sensors wisely.
Next, we need to consider the operational range of the stage sensor. Submersible and non-submersible pressure sensors have their pressure limits, so be sure to select one that can operate over the expected range in stage. Downlooking stage sensors such as acoustic, ultrasonic, or RADAR water level sensors can be used as primary sensors or added as backup sensors. They may be easier to install in streams when flood elevations spill out of the main channel and onto the floodplain, or when floodwaters occupy more than one river channel.
Now protect your sensors.
Providing adequate protection for the in-stream stage sensing instruments is just as important as protecting the instrument shelter. Protective housing for the data cable of a submersible sensor, or orifice lines of a bubble gauge, need to be installed in a way that minimizes the potential for debris impact and bank erosion. Sensor line and cable housings should be constructed with galvanized pipe anchored to concrete or bedrock, or buried in a trench in the stream bank. Protect and secure the housing up to an elevation that is above potential debris impact or high velocity. If at all possible, avoid installing sensor line and cable housings in areas prone to erosion.
Make sure your stage sensors are working during the flood.
Field verification of stage values is vital and may require some creative planning. Recorded stage values are typically field-verified during the event through the use of staff plates or wire-weight gauges. These need to be visible and accessible during extreme events, they need to cover the full range of extreme events, and they also must survive the event. Staff plates for stream gauges are typically installed in several sections up the stream bank covering the range in elevation from low to flood flows.
They are particularly susceptible to damage from floating debris piling up on the vertical staffs, or moving at high velocities. Another option is installing an inclined (sloping) staff. They are mounted flush with a sloping bank, thus minimizing the potential for debris impact and accumulation. They can be made of wood, steel beams, or concrete. These are custom installations that require surveying the rise and run of the sloping staff, determining the slope, and then marking elevations in even increments on the staff after the sloping staff is installed.
Try to locate vertical staffs in protected areas behind large trees or rocks. The debris will hit and entangle on these features instead of the staff plates. Reinforce them to withstand or deflect the debris using wooden or metal beam supports anchored in concrete. Unfortunately, staff plates are often prone to damage or complete loss during flood events when large debris is being transported. Bank erosion during floods can also cause the loss of staff plates.
A common practice is to locate a monitoring station near a bridge. If this is the case for your site, consider mounting staff plates to the bridge piles and/or abutments. One benefit of a wire-weight gauge vs. staff plates is that it can be mounted to a bridge railing or guardrail. Make plans to run a post-flood survey of flood marks using the datum of the stream gauge to validate the recorded data. If the gauge is damaged or lost, you will at least be able to recover the peak stage value.
Make streamflow measurements.
If a water discharge record is being computed from the stage record, direct or indirect streamflow measurements should be made. Be safe! Plan for this! Direct measurements of flood flows are made from manned and unmanned streamgaging cableways, from boats, and from bridges. Each method requires specific safety training. Flow velocity can be made with standard rotating cup velocity meters with sounding reels and weights, or by using Acoustic Doppler Current Profilers (ADCPs). Using an ADCP is by far the safest way to measure flow. The safety factor and measurement quality justify the cost.
Indirect measurements of peak discharge are used when a measuring site is inaccessible during floods, or when it is otherwise not feasible or safe to make a direct measurement. There are several methods that can be used to calculate peak discharge. The Slope/Area survey is commonly used as is the Contracted Opening, Flow over Dam, Road Overflow and Culvert Flow. They all require surveys of high water marks and channel geometry, and have specific site selection requirements. The USGS has several publications that describe the methods used for the various techniques. https://pubs.er.usgs.gov/search?q=TWRI&page=5#
Tips for your Water Quality Monitoring Station
What about my water quality probes?
Flood events are often accompanied by extremes in water quality parameters. To protect your equipment, use the same flood hardening considerations to plan for the physical survival of your water quality monitoring stations during and after flood events. Site access, protection from debris impacts, avoiding locations prone to erosion, adequate and safe site access, field verification of the recorded data through sampling or measurement, are all important factors.
Plan for non-flood extremes.
Keep in mind that extremes in water quality parameters can occur independently of water discharge events. For example, chemical or sewage spills during low water discharge can result in very high contaminant concentrations and very high values of the associated parameters such as pH, conductance, DO, ORP and turbidity. Be sure to select probes or a sonde that won’t be harmed and will continue to operate accurately.
What do I do about high suspended sediment concentration?
Turbidity probes are probably the most prone to having their range exceeded during periods of high concentration of suspended sediment. If you anticipate a large range in turbidity be sure to select a probe with a large range. Fouling due to high concentration of suspended sediment can impact other parameters. Choosing a sonde that incorporates a wiper system will help with reducing or eliminating fouling.
You should talk to your instrument vendor about any accuracy concerns when a probe is calibrated for use over a large range. You may need to select multiple probes. A word of caution about using multiple turbidity probes; not all turbidity probes provide values in equivalent units. Differences in light source, angle of diffraction, and other factors should be evaluated before selecting your probes. The USGS National Field Manual for the Collection of Water-Quality Data, Chapter A-6, Section 6.7 is an excellent resource for anyone involved with turbidity data collection. Be sure to look at the spreadsheet of turbidity parameters and methods. https://water.usgs.gov/owq/FieldManual/Chapter6/6.7_contents.html
If you want to discuss how to flood harden your surface water monitoring station get in touch today. Call +44 (0)121 733 7743 or email firstname.lastname@example.org.
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