Turning a Floodwater Liability into Biologically Filtered Groundwater Assets

A friend and mentor of mine, Bryan Hummel, has been doing a lot to help educate folks in Texas and beyond on how to apply flood mitigation and groundwater recharging practices.  He applies a lot of beaver biomimicry to his projects and as a result, has had a lot of success turning floodwater liabilities into biologically filtered groundwater assets.

Bryan's YouTube Channel

"Slow it, Spread it, Sink it, Soak it, Store it"

Using contoured beaver biomimicry bioswales, Bryan slows the water runoff, soaking the land, preserving and spreading that water, and reducing the runoff of vital nutrients.  I worked with him on his property in the Texas Hill Country in 2020, and learned so much.  I really appreciate what he is doing and think that the permies community would appreciate it as well.

Since Bryan explains what he's doing best, here are some videos:

Really good points.  Nature generally knows more about how to best regenerate.  That is for sure, but a couple of items did arise as I watched the above videos.  
First, I saw some corrugated metal culverts (CMPs) crossing roads.  I see that Brian was working to keep the outlets clear.  That is a more dated concept that I will address a bit later.  First, CMPs have a couple of quirks.  An important one is that if they flow full, they carry less water than if they are nearly full.  This is due to friction along the inside surface of the culvert.  In addition, as CMPs get close to or run at full capacity they develop a resonant frequency (they begin to shake).  This shaking allows water to work its way along the outside of the pipe and I have seen culverts popped completely out from under a road.  I pioneered a method of mitigation for this process in North Dakota a couple of decades ago.  I suggested that they place some geotextile and then place grouted rip rap on the inlet end.  Grouted riprap alone works, but not quite as well as using fabric to prevent water from getting to the side of the culvert.  Previously, they had problems keeping CMPs from being ripped out every two years or so and now that has been stopped.  Next, to get back to digging out the outlets to prevent plugging.  The United States Corps of Engineers has set out some regulations called the National Water Protection (NWP) regulations.  I do not always agree with the USACE, but these regs do have some utility.  One, culverts must be at least 25 percent wider than the incoming channel and must carry an estimated 50-year flood.  This allows better handling of gully washers.  In addition, any culvert must be set at least one third of its diameter into the bottom of the channel.  This is designed to allow benthic (water organisms) to traverse channels during high water to populate upstream ponds and wetlands.  Without the ease of travel, they are often subject to ecological fragmentation and that can lead to higher stress on populations.  I expect Bryan must know this, but he may not have considered that on that site.
One method I have always pushed for handling flow in channels to reduce siltation and channel sedimentation is to use modified beaver dam structures.  I consider it a reverse check mark system.  Place flow restriction structures of branches and rock that force the channel flow to become sinuous.  First bend it one way and then bend it another.  The change in flow direction will slow the flow and drop sediment out of the flow.  Water energy to carry sediments is primarily the square of the velocity.  Cut the water's velocity by half and reduce energy by at least 4 times.  Depending on soil type, it is important often to keep silt out of a recharge basin.  Silt and clay particles will clog the pores in gravel and slow how fast recharge to groundwater occurs.  I really love Bryan's concept of dropping live branches into a berm or channel and covering with stone or other materials.  Those branches will really slow flow and help to clean the water which will result in faster recharge and less chance of overwhelming berms and sedimenting channels.
Keeping composting organics moist is valuable to keep trees hydrated to allow them to resist burning.  Bryan's practice of trimming tree trunks to force straighter trees not only allows more sunlight to penetrate to seedlings, it reduces the potential for "ladder fuel" which is low hanging branches that a wildfire can use to climb up trees and morph from groundfire to a tree top ravening beast.  Always ensure that if you live in wildfire prone areas to keep ladder fuels at least 130 feet from any wood structure.  National wildfire protocols call for around 90 feet, but that is predicated on defensive space for crews to fight the fire.  Wildfires have been measured in Australia to ignite wood by radiant heat up to about 40 meters or about 131 feet.  Of course this does not consider embers in the wind.

I see that I did not explain the reason for why grouted riprap reduces CMP resonance.  It is due to the fact that the grouted riprap modifies the resonant frequency of the CMP at one end, making resonance much more difficult.  Stop the shaking, really reduce erosion along the outside.