High Water Detection Systems Overview and Planning


Flood Statistics
High Water Detection System Planning
Flood Gauging Station Considerations
Communication Systems
Power Systems
Base Station and Monitoring Software
Central Monitoring Service
Protecting The Agency While Protecting Lives And Property
Positive Road Closures Methods
Maintenance Considerations


Flood Statistics

Flash flooding is the leading cause of weather-related deaths in the United States, with approximately 200 deaths per year. Over 50% of flood related drownings are vehicle related. Figure 1 shows the five states with the highest flood fatalities from 1960 through 1995.

Rank State Deaths
1 Texas 612
2 California 255
3 South Dakota 248
4 Virginia 241
5 West Virginia 240

Figure 1 - U.S. Flood Fatalities from 1960-1995 (1)


For the period of 1973 through June 2000 in South Central Texas (known as Flash Flood Alley), there were 274 Weather related deaths. Of those, 203, or 74% were flood or flash flood related. Of those fatalities, 136, or 67% involved vehicles.

Weather-Related Fatalities Number %
Flood/Flash Flood 203 74%
Tornado 39 14.3%
Lightning 12 4.4%
Winter Storm 8 2.9%
Extreme Heat 6 2.2%
Thunderstorm Wind 6 2.2%
Total 274 100%

Figure 2a - Weather Related Fatalities 1973-June 2000 (2)


Flood/Flash Flood Fatalities Number %
Vehicles 136 67%
Permanent Homes 30 14.8%
Walk/Play Near Flood 24 11.8%
Mobile Homes 8 3.9%
Outside 5 2.5%
Total 203 100%

Figure 2b - Flood/Flash Flood Related Fatalities 1973-June 2000 (2)


More recent information provided by the National Oceanic and Atmospheric Administration’s (NOAA) National Weather Service (NWS) shows that in 2014 66% of flood fatalities occurred while driving. Their data also shows that from 2010 through 2014 there were 363 flood related fatalities. (3)

When flash flooding occurs, creeks and rivers rise rapidly & currents are swift. The roadbed can wash away in floodwaters. The washed out area can be concealed by the floodwaters, making the crossing a trap for vehicles.

      Washed out roadway


Figure 3 – From the NOAA website, washed out roadways present a hidden hazard to motorists when flooded.


Even when the roadway remains intact, a swift current can sweep a car off of the road. Cars that are swept away often roll to one side or flip over. Many drivers panic as soon as their vehicle submerges and are found later with their seat belt intact. Ironically, many drivers rescued from flood waters had been in a hurry to get home to safety. Despite what car commercials sometimes depict, driving into flood waters may be the most dangerous thing one might ever try.

Water weighs approximately 62 pounds per cubic foot and floodwaters often flow at 6 to 12 mph. When a vehicle enters the floodwaters, the water's momentum is transferred to the car. For each foot of water, 500 pounds of lateral force is applied to the car. Buoyancy plays the biggest role though. For each foot the water rises up the side of the car, the car displaces 1,500 lbs. of water. In effect, the car weighs 1,500 lbs. less for each foot the water rises. As a result of these forces, two feet of water will carry away most automobiles (4).

To raise public awareness of flooded roadways, NOAA has promoted its “Turn Around Don’t Drown” campaign. The campaign includes promotional materials, signage and other materials.

Road and Public Works departments often manually deploy barriers and warning signs at low water crossings. But, with no way to know a crossing is flooded without driving out to the site, lengthy delays occur which put lives and property at risk. High Water Detection Systems at problematic Low Water Crossing sites provide agencies with the opportunity to deter or prevent motorists from entering flooded areas, potentially saving lives and property.

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High Water Detection System Planning

      High Water Detection System


Figure 4 - Typical Flashing Beacon High Water Detection System with optional video monitoring.


High Water Detection Systems are designed to achieve two primary goals. The first is to warn motorists they are approaching hazardous road flooding conditions, and the second is to give the motorist the opportunity to stay out of harms way. Specifically, a command and response system for automatic remote-controlled activation of warnings on either side of a low water crossing point provides a deterrent to motorists driving into flooded areas. This is normally accomplished by activating warning beacons or Dynamic Message Signs and in some cases by activating automatic barrier gates. Video cameras can be activated and utilized to provide visual monitoring of site conditions. These high water detection systems also notify transportation and emergency management personnel of the flooded roadway condition so action can be taken. A well-designed high water detection system can help provide improved public traffic safety, more thorough roadway condition forecasting and better response to roadway flooding events.

The High Water Detection System's flashing beacon sites provide the primary warning to approaching motorists. These high water detection systems use one or more water level sensors to trigger the flood warning beacons and notifications.

In a High Water Detection System, the Master Gauging site houses a Controller/Transmitter Unit, which communicates with each of the remote sites (flashing beacon or Dynamic Message Signs) in a command and response fashion. The Master High Water Detection Gauging site uses a sensor to determine water level and activates or deactivates the Remote flashing beacon sites when preset upper and lower water level thresholds are met. The Master High Water Detection site should be positioned away from the flood zone to avoid being affected by high water and debris, and well away from vehicle traffic. A single Master High Water Detection Gauging site can control and monitor multiple Remote sites (typically up to eight). The Master High Water Detection site can also serve as a flashing beacon site if appropriate for the low water crossing area; however, equipment must be kept out of the flood zone and away from vehicle traffic.

Communications between the Master High Water Detection Site and Remote sites is normally wireless, but can be via underground cable if best for a particular site.

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Flood Gauging Station Considerations

Experienced project management teams know how important it is to understand the characteristics of each low water crossing before choosing equipment location at the gauging and warning sites. Sensors need to be protected from strong current and debris while still being close enough to the action to reliably gauge flooding. Exposure can be limited by running securely fastened conduit on the downstream side of culvert wing walls or abutments. Sensors mounted on or within poles in the flood-prone area should be avoided in order to limit damage or loss due to floating debris or an errant vehicle.

The most rudimentary systems use a float switch to activate the high water detection system at a preset level. While sometimes adequate, with just two states (off and on), the float switch imparts little insight to developing conditions at the high water detection site. With origins in tank monitoring, slightly more advanced high water detection systems use a series of several spaced contacts in direct contact with water to indicate relative water level. These systems rely on directly detecting moisture at each contact for determining the water level. Resolution is coarse and like the float switch susceptibility to fouling is high, as is the potential for lightning induced damage due to their direct connection with the stream.

      Master High Water Detection System Gauging Station


Figure 5 - Stanislaus County, CA. Master High Water Detection System Gauging Station with Pressure Transducer mounted on culvert wing wall.


State-of-the-art high water detection systems use fully insulated solid state, water level measurement technology to trigger the system and to give accurate, real time data useful in the prediction of an impending flooded roadway. Depending on the local geography and construction, a solid state Pressure Transducer (PT) is often used to provide very accurate (typically 0.01 foot resolution) and continuous level measurement over a wide range of depths. Bubbler sensors have similar accuracy and are used when particularly hazardous conditions might jeopardize the sensor. With a bubbler sensor, only a small easily replaceable run of poly tubing and a bubble chamber are in the flooded stream. Non-intrusive sensors, such as ultrasonic, radar, or laser sensors are used when the conditions dictate. These overhead mounted water level sensors are expensive, but allow water level to be monitored when no other means will get the job done. These instruments are widely deployed in hydrological monitoring systems around the world. A chief benefit of choosing state of the art technology for these applications is that rate of rise of the stream can be determined and used to provide warnings to transportation and emergency management personnel prior to the road actually flooding.

In more urban applications, the sensor can be protected by properly housing it within a storm drain inlet box. Storm drains are only used for this purpose if the drain is located at the or near the lowest point in the road. This provides a means to position the sensor away from potential harm and below road level so water rise can be observed and acted upon early.

Well designed high water detection systems provide for automatic or manual activation of video cameras for observation of current conditions. Further, provision are made for lowering barrier gate arms automatically to provide positive road closure to ingress lanes while leaving egress lanes open for any vehicles within the flood area.

Ultimately, the best high water detection reliability is attained when redundant sensors are used. If a primary sensor is damaged, a backup sensor can activate the system. The best practice involves utilizing a different technology for the redundant sensor. In the case of using a Pressure Transducer as the primary sensor, a float switch might be used as backup. In the storm drain scenario above, the float switch can be housed in the access box behind the outflow box. Of course, the equipment and high water detection system base station software chosen for the system should support activation and reporting with redundant sensors.

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Communication Systems

By nature, High Water Detection Systems for low water crossings are subject to difficult conditions when they are needed most. Roadway flooding generally occurs as a result of heavy rains and high winds. Culverts fill rapidly. Debris is washed downstream. Motorists have a difficult time navigating because visibility is low. Heavy rainfall cuts into the distance a radio signal can transmit, especially at higher frequencies. High winds knock out power lines taking phone lines with them. Internet connectivity is disrupted. Cell phone usage skyrockets overloading cell sites.

The prudent high water detection system architect recognizes that heavy storms or other emergencies often disrupt power and communications infrastructure, while battery powered systems will continue to operate. Building and maintaining high water detection systems that will operate reliably under these adverse conditions requires rigorous planning, technical expertise and thoughtful design.

While tying into existing infrastructure is not uncommon, serious consideration should be given to maximizing high water detection system reliability when conditions deteriorate due to intense storm emergencies. Dense rainfall weakens higher frequency communication signals much more than it does lower frequencies. This means that cellular modems, 900MHz spread spectrum radios, microwave systems, and satellite modems are affected more than terrestrial VHF communications. Radio paths must be engineered shorter or reliability suffers in adverse situations. Landline telephone systems are subject to outages, as are Internet connections. Fiber optic systems are also subject to damage and service interruption.

It is key to get system status information between high water detection sites and back to the Base Station reliably. Although no single communications means is perfect, VHF/UHF radio frequencies offer the greatest dependability in adverse conditions. Because of the risk of infrastructure outages, utilizing the VHF spectrum with battery/solar powered equipment can result in more reliable communications than other the alternatives. VHF hydrological monitoring systems operating on a separate frequency than other systems in the area avoid congested hydrological channels during storm events.

As with data paths in other applications, improved reliability can be attained by providing redundant data paths. Redundancy is best attained by the use of alternate technology. For example, a VHF based system might also use an Ethernet node on a fiber backbone for redundancy.

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Power systems

      Solar Powered High Water Detection System Sites communicating via VHF radio


Figure 6 - Solar Powered High Water Detection System Sites communicating via VHF radio.


While many High Water Detection Systems are in urban settings, others are located in remote areas, away from infrastructure. Regardless of location, most systems are solar powered, even when located in the presence of power infrastructure. Battery powered systems with dedicated VHF communications will continue to operate reliably when storms disrupt power. In addition, the cost of a solar panel is almost always much less than the cost of construction and the ongoing costs of providing utility power to these sites.

Solar panel and battery requirements are light, as the controller/transmitter communicates with short bursts of data only when needed. The use of advanced Maximum Power Point Tracking (MPPT) technology in the charge controller maximizes power extracted from the solar panel. Nighttime dimming of the flashers further extends operational time. Current high water detection systems utilize a single 20 Watt solar panel and a single 100AH deep cycle battery to provide more than 72 hours of site autonomy (operation time without any sunlight). This stand-alone operation provides isolation from infrastructure outages.

Video Cameras and the higher bandwidth transmission equipment needed to transport the images are more power hungry than the controller/transmitter units used to manage the flashing beacons. A solar power system sized to support running these items full time becomes large and expensive. A power conserving approach is to use one of the control channels of the high water detection system controller to power up the camera when viewing the site is desired. This can be accomplished automatically when the flood thresholds are reached, or manually with a command from the Base Station.

If a power outage affects just a single part of a communications path, service to the base station can be interrupted. Be sure to equip a uninterruptible power source (UPS) at the base station.

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High Water Detection System Base Station and Monitoring Software

Comprehensive software systems can track all the high water detection system gauging data, system status, and events, and send text messages, pages, or e-mails directly to Intelligent Transportation Systems (ITS) and emergency management personnel. Data from the High Water Detection System's Base Station software can be automatically fed to other databases in order to populate agency or public websites and Advanced Traffic Management Systems (ATMS) in Traffic Management Centers. Our litigious society pushes us to take precautions to prevent errors in deployment and record keeping. In addition to the data logs kept by the system, manual logs should be kept recording the times barricades were placed and removed, maintenance visits, etc.

Even more confident predictions of an impending flooded roadway can be made by using the data from other hydrological sensors to augment the simple observation that water near the roadway is rising. Equipping a high water detection system site or a nearby site with a rain gauge can provide further insight as to what conditions are likely to occur there. It’s one thing to know that the stream is rising. Also knowing that it’s still raining hard is better. In some cases data from existing local rain gauges operated by the National Weather Service (NWS) can be incorporated using the same wireless system with which the High Water Detection System communicates. Include stream flow data from USGS gauges via NESDIS database (5) and you will also know whether the stream is swelling upstream. The additional minutes of advance warning provided by intelligent high water detection system gauges can mean saved lives and the opportunity to protect property.

Who’s going to keep an eye on all that data and make these determinations? Even the largest communities can’t dedicate staff to continuously monitor numerous high water detection system sites for flooding. Comprehensive software systems can track all the gauging described above and send text messages, pages, or emails to the people who need to know based on a predetermined threshold and/or combinations of events. The high water detection system's software can activate lighted maps, flash beacons, or signal an alarm. The server running the high water detection system's software can serve up web pages for officials and the public to check before venturing out during a storm.

Notifications can be sent to road departments for early barricade mobilization. Flooding at more than one location might call for full deployment of Emergency Management personnel, while automatically alerting Commissioners or the Mayor. Notifications to road crews can keep them informed of other flooded areas so they don’t get trapped between two low water crossings while setting portable barricades or dropping fixed barricade arms. Lesser events, such as the need for maintenance at a site, can be set-up to notify the appropriate departments by e-mail for action the following day.

These software monitoring programs can also feed data to other databases in order to populate ATMS in Traffic Management Centers (TMC). The benefits go even further. Data acquired from a High Water Detection System can be shared with the NWS. When given access to additional real-time rain and stream data, the NWS is able to provide improved forecasting information, as well as to archive data for historical purposes. Implementing high water detection systems using compatible data technology helps provide better forecasts benefiting the agency and the entire region.

High Water Detection Systems designed with the capacity to allow additional sensors at each site provide even more benefits. Valuable real-time insight as to fire danger can be gained by outfitting a few sites with temperature, relative humidity, and wind speed/direction.

The equipment and software systems discussed here isn’t limited to flooded roadways. These systems can be used to automatically activate high wind warnings for trucks and RVs or when equipped with visibility sensors can warn motorists of foggy conditions.

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Central Monitoring Service

High Water Detection Systems can also be monitored remotely by a third party, where the user receives data and information about activated sites, but may not have the ability to override and manually activate or deactivate beacons if needed. While convenient to hand off system monitoring when staffing is limited, be sure you have the controls and notifications in place to confirm high water detection system availability, override automatic beacon activation, and customize notifications

Have a well thought out processes in place. How will the monitoring service communicate with emergency management if communications infrastructure isn’t available due to the emergency conditions? Keep good checks and balances in place to be sure the high water detection system is being monitored properly. Check with your agency’s legal representative to be sure the agency is contractually protected in the event of default or negligence on the part of the monitoring service.

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Protecting The Agency While Protecting Lives And Property

There has been a large deployment of High Water Detection Warning Signs and Beacons in the traffic industry in recent years. There have been vast improvements in the ability to warn motorists and transportation departments of flooded roadway conditions at low water crossings. Protecting your agency means having the right field equipment and processes in place to minimize risk for all concerned.

The location of the High Water Detection Warning Signs deserves careful consideration. The signs must be far enough ahead of the flooded roadway to allow a motorist the opportunity to safely exit and avoid the floodwaters. To provide adequate warning on high-speed roadways, low water crossings may include more than one high water detection warning sign in each direction. Avoid placing signs and beacons in the low water crossing itself because notification is too late there. In poor visibility conditions, the beacon will not provide adequate warning to allow motorists to avoid entering high water. Motorists may instead be lured into the flooded area.

Signs and beacons should be affixed to on Department of Transportation (DOT) approved breakaway bases and pole (per American Association of State Highway and Transportation Officials (AASHTO) Standard Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals). They are designed to shear when hit by a vehicle. Accidents happen and breakaway poles increase survivability.

When roadway flooding does occur and is detected, messages activated and notifications sent, response of crews and emergency management personnel must be prepared to execute predefined plans in order to help protect motorists, and to protect themselves from litigation. Always consult with the legal department for approval of sign wording. Whether a Manual on Uniform Traffic Control Devices (MUTCD) Warning Sign or a Dynamic Message sign, the message must be clear and concise, conveying the hazard and the action required on the part of the motorist. The wrong wording can result in increased liability in the event of litigation.

Provide at least one water level staff gauge for each direction of traffic so motorists can take note of the water depth while still on higher ground. Providing staff gauges at various points along the low water crossing area allow motorists to observe floodwater depth except at night, when most low water crossing deaths occur. These gauges also serve to familiarize motorists with low water crossing areas.

Our litigious society pushes us to take precautions to prevent errors in deployment and record keeping. Logs should be kept recording the times barricades were placed and removed. Maintenance visits should be documented. It may not be sufficient to produce software system logs as documentation of the dates and times of beacon activation; unless you can also demonstrate that the computer clock was accurate at the time. A diligent attorney will challenge when the last time the clock had been checked, the accuracy of the source it was set by, whether daylight savings updates were in effect, etc.

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Positive Road Closure Methods

      Barrier Gate deployed automatically during this January 2008 storm event near Crows Landing, CA


Figure 7 - Barrier Gate deployed automatically during this January 2008 storm event near Crows Landing, CA.


      Railroad style High Water Detection System gate closure in Austin, TX


Figure 8 - Railroad style High Water Detection System gate closure in Austin, TX


Flashing beacons give advance warning of a road closure. Beacons alone provide a warning, but they don’t preclude a driver from exercising poor judgment and continuing into the flooded area. In many cases road department staff place barricades as full road closure to prevent entry into the low water crossing area.

Three types of barricades are generally used. The most common and most labor intensive are portable barricade fence systems loaded into trucks, driven to the site and put in place to block the road. These barricades require the lowest capital investment, highest labor expense to manage and take the longest to deploy.

By contrast, the High Water Detection System can activate automatically operated crossing gates. These gates automatically flash beacons and lower an arm across the road when the high water detection system detects waters have risen to flood level. While capital costs can be high, these barricades deploy immediately and eliminate the need for dispatching maintenance personnel to the site.

One consideration for choosing an appropriate gate operator is the type of drive used. Gearbox driven operators are preferred for road closure applications because the units sit unused for long periods of time (between flooding events). While good for frequent use applications, hydraulic operators have a tendency to develop seal problems over time when used infrequently. These effects are primarily due to the development of flat spots on the seals, and, in humid areas, rust forming on the exposed section of the ram.

In the State of Texas, guidelines set by the Texas Grant Committee require gates that lower automatically and then are locked in place by appropriate personnel until the storm is over and the road has been inspected and is deemed safe for use. If automatic operation is not required or desired, fixed, manually operated barricades are an affordable compromise minimizing the capital expense and operational costs associated with closing frequently flooded roadways.

One example is a barrel type fence. The barrel contains a roll of heavy-duty plastic fencing that personnel must extend across the roadway. This is a difficult exercise during heavy rain and high winds, as the barrel must be securely anchored on either side of the road. Moreover, these fences can pose a hazard if a motorist fails to leave the affected area or is trapped after the fence is deployed. The average motorist may not be able to retract the fence in order to leave the area. These barricades are also subject to fouling because of the mechanism’s proximity to the ground.

Another solution is a manually activated up-swing type gate that is permanently mounted at the side of the road and locked in an upright position until needed. The gate is easily lowered from the roadside and may include its own signage and beacons. The simple mechanism is not easily fouled by debris or vegetation, but consideration to overhead utilities and trees must be given. Units can be sized to block traffic one way or in both directions. The arm can easily be raised to allow a vehicle to leave the flooded area, and will drop back down into position once released.

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Maintenance Considerations

      Regular maintenance programs maximize High Water Detection System availability


Figure 9 - Regular maintenance programs maximize High Water Detection System availability.


In the long run, the reliability of any high water detection system is dependent upon the quality of preventative and remedial maintenance it receives. A high water detection system not maintained regularly may not be available when it is needed. Whether your own maintenance personnel take care of the high water detection system or it is done under contract, site visits should be made two to four times a year depending upon conditions and seasons. Besides general site checks, beacon activation, battery condition, and sensors must be checked. Sensors activate the high water detection system. Be sure they are clean and free of vegetation and debris. Depending on the size and location of the High Water Detection System and available maintenance staffing, the agencies interests may be best served by contracting the maintenance to the manufacturer or their authorized agent.

Also important to high water detection system reliability is how the system is monitored and data disseminated. In most cases, the base station is operated by Emergency Management, the Transportation or Road Department. In this case, periodic checks are warranted, especially before an expected storm. With a well thought out alarm notification scheme, automatic text or email messages are sent when certain parameters are met. Maintenance personnel can know right away if a battery is low or a station is not reporting.

Like any other automated traffic safety or hydrological/meteorological monitoring equipment, a regular maintenance program is key to trouble free operation and long service life of the system.

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(1) www.floodsafety.com, National Climatic Data Center - “Storm Data” publications (1995), July 2008

(2) www.floodsafety.com, Storm-Related Mortality by CDC, July 2008

(3) www.nws.noaa.gov/oh/hic/flood_stats/recent_individual_deaths.shtml, NOAA, NWS Fatality data

(4) National Weather Service, “Flash Floods and Floods...the Awesome Power!”, www.nws.noaa.gov/om/brochures/ffbro.htm, July 2008

(5) USGS data is typically available in fifteen-minute increments hourly, providing insight as to upstream conditions but not entirely timely in the case of locales where rapid flash flooding is common.

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