SITUATION

Primary Problem – Immediate Scouring

Learning through trial and error: Attempt #1 – Unlined HESCO with Sandbags Inside

We had HESCO flood barriers available but we knew the power of the river against the product would immediately want to push it downstream before we had a chance to fill it. To solve this problem, we decided to remove the fabric from within the individual cells so water could pass through the barrier while we filled it with sandbags. If we were building the HESCO on some sort of river rock base, we may have been more successful. Unfortunately, the problem remained that it would be sitting on highly erosive sand and under scouring was immediate and constant.

RESULT

It was initially promising but as we worked our way into the floodwater, the sand around the base of product was quickly lost to scouring. This was causing the product to lean as the sand it was sitting on was washed away.

LEARNING POINTS 

• We could have created some buttress cells within the line of HESCO (seen below) to reinforce it and/or keep it from falling over as sand scoured away from below the barrier

• A plastic liner over the barrier and laying on the river bottom in front of it would help mitigate against scouring but in this case, the plastic liner skirt would need to be done simultaneously with filling the HESCO with enough sandbags to stop/redirect the water which would have been extremely challenging

• NOTE: When building a traditional soil filled HESCO flood barrier in an area where scouring could be a big problem, we like to take the additional step of placing about 2 layers of sandbags within the bottom of the HESCO itself to mitigate against under scouring from sucking out all fill material when confronted with under scouring.

Attempt 2 – NOAQ Boxwall in long sections

RESULT

The NOAQ Boxwall comes in pieces that are each 3’ long. In this case, we wanted to link together 45’ long sections of this product and cover it with plastic in an attempt to fight the immediate scouring that would take place. However, this product is not designed to be carried in 45’ sections that were pre-joined and some of the joins where coming disconnected while we carried it into place.

LEARNING POINTS

• If we want to deploy more than about 3 or 4 units of the NOAQ Boxwall flood barrier all at once, additional steps should be taken to ensure they stay connected.

• In the next attempt, we simply screwed 12’ long boards to them that were overlapped so we could carry long sections.
  (This was done outside of manufacturers suggested operating procedures and naturally negated any product warranties.

• Flood Defense Group loves the capability of cutting, screwing and modifying this product in a variety of ways which would naturally forego any manufacturer warranties but... the ability to cut/modify this product proves extremely useful, simple, and super effective.)

Attempt 3 – NOAQ Bnoxwall in Long Sections Screwed to Wood – Winning!!

It was extremely important that we could quickly install long sections of the NOAQ Boxwall. This was because every time a section was installed, all the water was being redirected toward the leading edge of our flood barrier. Therefore, after each section, the water would become deeper, faster, more erosive, and even more challenging to work with.

To solve the problem of our units becoming disconnected while we carried them into place, we simply screwed the product to long pieces of wood below them such that we had multiple sections of about 30’ each. We could carry these into place in a single piece, place them into the water, and immediately cover them with a plastic liner to prevent water from scouring the sand from below them. This liner at the base of the barrier is commonly referred to as a ‘skirt’ or an ‘apron’ which is further illustrated below.



By deploying the NOAQ Boxwall to redirect the incoming floodwater, we were able to create a somewhat dry working space behind the barrier where we could build a longer-term defensive line using HESCO barriers.

NOTE: For most working surfaces we encounter, we do not experience the same level of erosion and under scouring that we experienced in this situation. In this case, we were building on sand while fighting the rushing floodwater within a working environment that was hard to access. It was a challenge!

Attempt 3.5 – HESCO Flood Barrier Behind the NOAQ Boxwall

In an attempt to create a longer-term capability of redirecting the water, we were able to stretch out a line of HESCO, with buttress cells attached, in the dry space behind the NOAQ Boxwall. We covered the HESCO with plastic sheeting that was pulled out to also create a ‘skirt’ at the base of the HESCO which we then weighed down with sandbags.

RESULT

Water was able to push many of the sandbags off the skirt of the HESCO and under scouring was able to take hold. 

LEARNING POINTS

• Our initial line of HESCO did not have sandbags attached to the plastic liner skirt which were washed away and allowed scouring to take place.

• We knew scouring would be a problem and did not fill the HESCO past the water point in order to prevent them from getting top heavy if/when a lean caused by scouring began to occur.

• The buttress cells were crucial in holding the barrier in place as the ground scoured out below it.



• Even though this is not the result we were looking for, this initial HESCO line of defense continued to hold the water back with overtopping, did not fall over from the scouring, or experience material failure. This is a good example of testing a product to its limit and we were happy with how it held strong!

FINAL STEPS – CREATE ADDITIONAL LINES OF DEFENSE

Once we diverted the water and created a foothold within the riverbed that allowed us to bring in machinery, we were able to fortify our flood defenses and create additional layers of protection.

Additional Details

• It took about 2-3 hours for local Navajo volunteers to redirect the river with the NOAQ Boxwall by hand

• 2 front-end loaders, 2 backhoes, a bulldozer, and 3 skid steers were unable to redirect this river after 3 days of operation



• Approximately 1,000 linear feet of barrier redirected floodwaters past nearly 1.5 miles of vulnerable levee