Erosion Prevention & Repair Elements

Description: A zuni bowl is a geologically armored plunge pool constructed from tightly knit stone that halts the up-watershed migration of a headcut.

Function(s): Zuni bowls create an armored lip and basin over which and and into high-energy water flows. The shape of the zuni bowl creates a small plunge pool, containing standing water, into which high-energy, fast-moving water falls. In this way the erosive force of the high-energy water is mitigated and an opportunity for re-patterning the now lower-energy run-off water is created – either within the gully or often times across the surrounding landscape by linking the zuni bowl with swales or infiltration basins.

Context-Specific Design Criteria for Zuni Bowls

Zuni Bowls are generally a good fit for a specific context when:

• When the grades of the slope are such that excavating a headcut and creating a bowl shape does not require moving much soil (most zuni bowl construction work can be performed by hand).
  • Heavy machinery can be used to shape the headcut and lay in rock in certain circumstances (slopes are such that they can be safely operated, and stone required is too large to be safely or efficiently moved by people alone).
• Irregular, angular stone is available on-site or nearby for creating the plunge pool and associated one-rock dam. If limited or no stone is available, see Log-Drop Structures as an alternative headcut-armoring technique.

Zuni bowls may not be a good fit for a specific context when:

• Terrain is too steep to allow for the creation of the armored bowl – i.e. too much soil would have to be moved in order to create a stable structure. As a general rule, if the vertical drop exiting a zuni bowl would be equivalent or greater than the depth of the incision which the bowl is designed to address, the terrain is too steep and the incision is located within the “transport zone” – i.e. an area where the slope is so steep that de-energization of the water flow is not possible (the water would speed up again immediately upon exiting the zuni bowl).
  • In these situations, the best option is to utilize armored drains to transit the water through the steep section of terrain in a non-erosive fashion, and de-energize it once the slope begins to flatten out at the key point or a pseudo-key point (see Water Infiltration Earthworks > Keyline Subsoil Ripping to learn more about identifying the key points in your landscape).

General Design & Installation Considerations for Zuni Bowls

1. The headcut overhang and sides should be laid back to a 1:1 or 2:1 slope, and all dead or dying vegetation removed from the pour-over lip.
2. Measure the height of the headcut drop. Then measure down-watershed 3-4x this length. This will be the approximate extent of the bowl. Begin by digging a small trench at this point and setting in a rock splash apron. The splash apron should be set at the same height as the native gully bottom.
3. Underseed the area where the zuni bowl is to be constructed with native grass seed or other appropriate erosion control seed mix.
4. Utilizing the largest rocks available, create a tightly knit row of rocks just upstream from and sitting slightly atop the splash apron. These rocks should be approximately 1/2 the total height of the headcut. These rocks will create the exit sill of the zuni bowl.
5. Armor the bottom of the zuni bowl with a single layer of tightly knit stone, working from one end to the other to eliminate any gaps between stones.
6. Starting from the upstream side of the exit sill, begin building up the sides of the bowl to form the walls. Build up towards the native grade knitting the stone as tightly as possible. Cracks and gaps should be infilled with smaller material, such as road base or smaller gravel, to prevent piping of water through the rocks that could lead to failure of the structure. Try to maximize stone-to-earth contact as you go, building up in courses. Use “gravity glue” to tie the rocks together. Test each course by walking up and down on it – the stones should not slip or wiggle underneath your weight.
7. Utilize flat stones for the pour-over lip, set to the same height as the native ground above the headcut lip. Setting the pour over lip higher than the native terrain may lead to the flow path being altered and the water flowing around the zuni bowl or entering it at an unintended location, both of which are to be avoided.
8. Whenever the terrain permits it, always construct a one-rock Dam (ORD) immediately downstream of the zuni bowl. Place the upstream edge of the ORD downstream from the zuni bowl splash apron approximately 6-8x the height of the headcut, or far enough so that the anticipated deposition zone of the ORD will reach up to but not past the splash apron of the zuni bowl (see One-Rock Dams to learn how to construct them and their role in supporting the function of zuni bowls in rehabilitating incised drainages).

Resources for Continued Learning about Zuni Bowls

Description: Stone lined pools installed at the transition point where a high energy / high velocity flow needs to be pacified before transiting through a lower energy / lower velocity area. EDPs are different from zuni bowls in that they are not installed into active headcuts, rather they are built into drainage paths where concentrated run-off needs to be pacified to prevent the formation of a scour pit and initiation of a headcut in the first place.

Function(s): To de-energize / slow down high energy / high velocity water so that they may be patterned into a lower energy system (such as a swale or infiltration basin) to be infiltrated or slowly moved through the landscape. EDPs are typically installed at the bottom of a “no chance” zone (to borrow a Bil Zeedyk term) – an area where there is no chance to drain run-off water, and typically where both flow velocity and volume increase to a point where they would do damage to native soils if abruptly introduced to them without pacification (i.e. where a steep grade transitions to a much shallower grade suddenly).

Context-Specific Design Criteria for Energy Dissipation Pools

Energy Dissipation Pools are generally a good fit for a specific context when:

• There is a concentrated source of run-off exiting a high-energy / high-velocity transport zone and being discharged onto more fragile soils (typically at keypoints or where steep terrain transitions to flatter terrain over a short distance).
• Any place a concentrated flow of run-off has to be sped up in order to transit a drainage structure to keep it from clogging (such as a rolling dip, culvert, armored drain, or other high volume / high energy drain).
• Anywhere that a concentrated flow of water is falling onto native soil on a slope (this often happens with improper culvert installations in steeply sloped terrain).
  • Most often additional flow spreading structures (media lunas, one rock dams, berms etc.) should be installed in concert with an EDP to spread out the water.

Energy Dissipation Pools may not be a good fit for a specific context when:

• When the substrate that the water will be discharged onto is non-erosive (i.e. bedrock, highly porous gravel etc) and EDP is not necessary.
• Terrain is too steep to allow for the creation of the armored bowl – i.e. too much soil would have to be moved in order to create a stable structure. As a general rule, if the grade exiting an EDP is equivalent to the grade of the inbound flow, the terrain is too steep and the incision is located within the “transport zone” – i.e. an area where the slope is so steep that de-energization of the water flow is not possible (the water would speed up again immediately upon exiting the EDP).
  • In these situations, the best option is to utilize armored drains to transit the water through the steep section of terrain in a non-erosive fashion, and de-energize it once the slope begins to flatten out at the key point or a pseudo-key point (see Water Infiltration Earthworks > Keyline Subsoil Ripping to learn more about identifying the key points in your landscape).

General Design & Installation Considerations for Energy Dissipation Pools

1. Begin by digging a small trench at the point where the bowl will end on the downstream side. Set the splash apron here at the same height as the native gully bottom.
2. Underseed the area where the zuni bowl is to be constructed with native grass seed or other appropriate erosion control seed mix.
3. Utilizing the largest rocks available, create a tightly knit row of rocks just upstream from and sitting slightly atop the splash apron. These rocks should be approximately 1/2 the total height of the headcut. These rocks will create the exit sill of the energy dissipation pool.
4. Armor the bottom of the energy dissipation pool with a single layer of tightly knit stone, working from one end to the other to eliminate any gaps between stones.
5. Starting from the upstream side of the exit sill, begin building up the sides of the bowl to form the walls. Build up towards the native grade knitting the stone as tightly as possible. Cracks and gaps should be infilled with smaller material, such as road base or smaller gravel, to prevent piping of water through the rocks that could lead to failure of the structure. Try to maximize stone-to-earth contact as you go, building up in courses. Use “gravity glue” to tie the rocks together. Test each course by walking up and down on it – the stones should not slip or wiggle underneath your weight.
6. Utilize flat stones for the pour-over lip, set to the same height as the native ground above the headcut lip. Setting the pour over lip higher than the native terrain may lead to the flow path being altered and the water flowing around the zuni bowl or entering it at an unintended location, both of which are to be avoided.
7. Whenever the terrain permits it, always construct a one-rock Dam (ORD) immediately downstream of the Energy Dissipation Pool. Place the upstream edge of the ORD downstream from the EDP splash apron approximately 6-8x the depth of the EDP, or far enough so that the anticipated deposition zone of the ORD will reach up to but not past the splash apron of the EDP (see One-Rock Dams to learn how to construct them and their role in supporting the function of zuni bowls in rehabilitating incised drainages).

Resources for Continued Learning about Energy Dissipation Pools

Description: An rock ramp constructed of knitted stone used to stabilize small headcuts.

Function(s): To transition high-velocity flows down a steep or sudden grade change without eroding the native soils underneath.

Context-Specific Design Criteria for Armored Rundowns

Armored Rundowns are generally a good fit for a specific context when:

• The distance to be covered is relatively short (generally up to 3x the height of the drop).

Armored Rundowns may not be a good fit for a specific context when:

• The distance to be covered is relatively long (greater than 3x the height of the drop)

General Design & Installation Considerations for Armored Rundowns

1. Cut or lay back the edge of the headcut or drop to be transited, ideally to a 3:1 or gentler slope. Trim back the soil until live plant roots are exposed. Seed the area with perennial erosion control grasses or other hairnet root plants. These will grow up through the stone and bind them together, enhancing the effectiveness and lifespan of the structure.
2. Make sure the rocks at the pour-over lip are at the same elevation of the headcut so that water flows freely over it.
3. Shape the area to be armored and slightly dish out the center of the water course (to prevent water from moving around the structure.
4. Knit rocks together tightly to prevent piping. Add small, sharp gravel between stones if necessary to better knit them together.

Resources for Continued Learning about Armored Rundowns

Description: Also known as log drop structures, log and fabric drops are a stair stepped series of logs underlain with geotextile fabric, generally utilized on larger in-channel headcuts where the fall is 4′ or greater and/or logs are more easily sourced than stones.

ABOVE IMAGE: National Forest Foundation

Function(s): Stabilizes deep, high-energy headcuts by creating a stable staircase for the water to cascade down without further degrading the leading edge of the headcut. Using logs and cutting them to size allows for large headcuts to be protected easier than using stones that would be so large and heavy as to require a small excavator to move into place.

Context-Specific Design Criteria for Log and Fabric Drops

Log and Fabric Drops are generally a good fit for a specific context when:

• Headcut is deep and high energy.
• Stone is either not availabe on-site, or would be too expensive to haul in or too heavy to work with given the size of the headcut that requires stabilization.
• Logs are available on-site or nearby and can be sourced cost effectively.

Log and Fabric Drops may not be a good fit for a specific context when:

• Stone is available on-site, and is of proper size to handle the anticipated flow volumes.

General Design & Installation Considerations for Log and Fabric Drops

1. Detailed instructions available starting on page 41 of Hand-Built Structures for Restoring Degraded Meadows in Sagebrush Rangelands
2. Materials Required
   • Geotextile Fabric (silt fencing fabric in 3 foot widths works well and is convenient to use).
   • Logs: Logs 6 to 10 inches in diameter and varying lengths from 4 to 8 feet long. (For example, bottom
     tier, 8 feet long, second tier, 6 feet, third tier, 4 feet.) Logs should be straight, trimmed and green, or
     seasoned, but not rotten. Any protruding knots, limbs, or knobs make stacking very difficult and should be
     trimmed.
   • Wire: one roll of smooth fencing wire or barbed wire.
   • Fencing staples: 2 inches long, about 2 lbs.
   • Sod clumps: 6” X 6” X 3”. Dig locally.

Resources for Continued Learning about Log and Fabric Drops

Description: Media lunas are level structures made of knitted stone shaped like a crescent moon.

Function(s): Media lunas are used to manage sheet flow in one of two ways; sheet flow collectors (tips of the moon facing downhill) are used to aggregate small rills (<6″ deep) or runnels (<1′ deep) to prevent headcut formation at the head of rill, runnel or gully by creating a stable transition from sheet flow to channelized flow, and flow spreaders (tips of the moon are facing uphill) to create a soil deposition area on relatively flat (shallow grade) ground where potentially erosive channelized flow can be transitioned to more passive sheet flow.

Context-Specific Design Criteria for Media Lunas

Media lunas are generally a good fit for a specific context when:

• Concentrated, high-energy flows need to be spread out after being de-energized by a change in grade, an energy dissipation pool, zuni bowl, log and fabric drop, culvert, rolling dip, armored drain etc.
• Stone is available on-site or locally.

Media lunas may not be a good fit for a specific context when:

• Grades are too steep – they should either be installed at the heads of rills or runnels before gully formation, or at or below the keypoint in the landscape where the slop changes from convex to concave (becomes shallower).
• No stone is available on-site or locally.

General Design & Installation Considerations for Media Lunas

• Select the two end point locations for the media luna and determine the proper orientation depending on the functional requirements – “tips up” for spreading flows, and “tips down” for concentrating flows. Both of the end points should be level with one another.
• Dig out a small trench in the soil to lay down the splash apron in a shallow arc. These stones should be no more than 2″ higher than the native grade of the soil. The rest of the structure will be keyed into this apron to prevent any fall-induced erosion as water transits the structure.
• Lay the first course of stones, each one keyed into the one immediately downhill of it. Then continue layering on additional courses of stone until the top of the media luna is level (this is critical – if the top of the media luna isn’t level it will not spread or concentrate flows evenly, which can lead to erosion issues)
  • The top of the “tips up” media luna should function similar to an overflowing coffee cup – coffee spills around the entirety of the rim of the cup at the same time.

via GIPHY

Resources for Continued Learning About Media Lunas

Description: A line of knitted rocks that crosses an incised gully bottom perpendicular to flow, generally 6 to 8 courses of stone deep and one high.

There are two parts to a one rock dam – the sill and the dam. The sill is a single line of rocks, knitted together with a relatively broad, flat surface facing up, dug down until their tops are at the existing grade of the drainage and arranged perpendicular to the direction of flow. The dam consists of four to five rows of rocks knitted together tightly spanning the width of the drainage. The lowest row sits atop the keyed-in sill rocks. The sill acts as a non-erosive splash guard during high-flow events to prevent undercutting when water is moving over and through the dam rocks.

Function(s): One rock dams (ORDs) are used to spread concentrated flows, accumulate sediment, and initiate revegetation of incised waterways. ORDs can be employed to raise the thalweg (deepest part of a drainageway) over time by creating sediment depositions upstream of the dam, which increases water and nutrient holding capacity, and thus becomes a more amenable site for vegetation to establish, which further slows down water, provides habitat, and reduces evaporative losses. ORDs utilize the already occurring natural process of erosion to aggrade (raise)  incised drainages and ultimately reconnect them with their floodplains and restore or improve hydrological function. ORDs can be installed sequentially, year by year as conditions warrant, to continue gulley healing and improve overall landscape hydration.

Context-Specific Design Criteria for One Rock Dams

One Rock Dams are generally a good fit for a specific context when:

• Water flows are carrying sediment that can be dropped out of the water column.

One Rock Dams may not be a good fit for a specific context when:

• Water is not carrying sediment.

General Design & Installation Considerations for One Rock Dams

• Select the site for the ORD placement, and dig a footer trench. Into this set stones that are even with the native height of the water course bottom. These will serve as the splash apron for the ORD.
• Seed the remaining footprint of the ORD with perennial grass seeds or other hairnet root plants.
• Starting building the ORD, one course at a time, beginning with the first course half on top of the splash apron.
• Over time, the ORD will create a lense of sediment just upstream of it. Another ORD can then be constructed on top of this sediment lense, in this way gradually aggrading the drainage bottom. The first ORD will become the splash apron for the second.
• ORDs are recommended anywhere a drainage has become incised and disconnected from its floodplain.

Resources for Continued Learning about One Rock Dams

Description:

Function(s):

Context-Specific Design Criteria for ELEMENT

ELEMENTs are generally a good fit for a specific context when:

ELEMENTs may not be a good fit for a specific context when:

General Design & Installation Considerations for ELEMENTs

Resources for Continued Learning about ELEMENTs