Autor: Ing. Pavol Suty
Translated by: Peter Bujnak
Technical University of Zvolen, Faculty of Forestry
DESCRIPTION OF THE FOREST LANDSCAPE
Forest landscapes, especially flysch zones are impacted by considerable deforestation due to large scale commercial forestry activity and development. Timber harvesting with heavy machinery has created a large number of approach roads not only in sloping terrain but also in and along streams. In the case of Slovakia, growth in timber harvesting is not accompanied by adequate construction of suitable forestry roads, as such retrieving timber from forests is accompanied by the formation of large erosion gullies and increased destruction of the natural eco-system. These erosion gullies rapidly drain water from the forests, remove the subsoil and sedimentation as well as carry off logging debris. This result is regarded as the degradation of the landscape which causes flooding during intense precipitation.
All the negative aspects within the forested landscape increase the outflow of water from a territory and lead to its gradual dehydration. This process is the cause of flooding and loss of water in Slovakia and is applicable to many other countries and regions around the world. Such a country is akin to being diseased and requires treatment. Treatment consists of the revitalization of commercial forestry landscapes in order to restore the eco-system and reduce the risks of flooding and drought. Effective revitalization is focused on micro-basin restoration in elevated terrains situated above municipalities where the intensity of flood damages can be averted. Flooding intensity and frequency provides us with insights into what direction to look to in order to mitigate these anthropogenic processes. For centuries, man has been interfering in the natural processes of an ecosystem through intense landuse alterations, now we have something to learn from this and gain new experiences.
ELEMENTS CONTRIBUTING TO FLOOD RISKS
- A large quantity of water that causes damage by volume and strength
- Kinetic energy of water during intense precipitation carries soil, gravel, stones, wood and small debris causing clogging of bridges, streams and other water channels and gullies
- Layers of humus and mud carried away from the forest by excess water run-off
EXAMPLES OF DAMAGED FORESTS
Two overused and unmaintained logging roads meet with a large erosion gully running through them.
Erosion gully forming on an unmaintained logging road.
A hiking trail turns into a stream during rain with no diversion throughs in place, water leaves the forest.
EXAMPLES OF ENVIRONMENTAL MAINTENANCE FROM THE PAST
Our ancestors, to the best of their knowledge, took care of the earth responsibly. In addition to the gentler processing of agricultural lands and a more sensitive approach to forestry, they maintained water balance by building various water retention measures such as brick check dams, wooden cascades, riffles, wooden catchment areas for fish, vegetative bank reinforcements of rivers and streams and other measures. In the Kysuce area situated in north central Slovakia, we have found a number of such old structures, especially wooden ones, which serve their purpose of water retention for decades to this day. According to the description by the locals, the oldest wooden dam - approx. 2 m high - is located in the village of Radôstka, in the Hulák district. This relatively large check dam was built in the 1950’s and is still functioning without any required maintenance.
Wooden check dam in the village of Radôstka,
Hulák district was built in the 1950’s and still
Functions to this day.
Wooden cascade in the village of Vysoka
nad Kysucou built in the 1940’s.
Wooden cascades in the village of Vysoká nad Kysucou have been protecting the bridge downstream against flood waves since its inception. Fish catchments built decades ago for trout breeding are scattered throughout the Kysuce area and have survived large floods.
IMPLEMENTED WATER RETENTION MEASURES
STONE CHECK DAMS
Oščadnica – Masonry stone dam with a capacity of 30,000m3 of water retention with an expected life expectancy of 100+ years
Krivany – dispersed stone check dam
Lačnovský potok – Dispersed stone check dam
Stara Bystrica - masonry stone dam with outflow pipe
Kriviansky potok – stone check dam
LAYERED CHECK DAMS
Vyšné Vane- layered check dam with a membrane
Nová Bystrica- Check dam with Willow weaving
Krasňany – layered check dam
Krasňany – layered check dam
Dunajov – Check dam slowing snow melt in the spring
Hlohovec – check dam in eroded gully
Check dam after heavier rainfall
Stará Bystrica – check dam made of Willows
RAINWATER DIVERSION THROUGHS
Nová Bystrica – rainwater diversion trough on logging road
Krivany – rainwater diversion trough
Dunajov – construction of a diversion trough on a logging road
Krasňany – Wooden cascade check dams
Krivany – palisade with membrane
Dunajov – combined check dam made of stone and wood
Palisade check dam with membrane
Simple palisade check dam
During heavy rainfall, run-off is slowed down
Hranovica – small catchment reservoir built above a Roma settlement
Reservoir comprised of a wooden check dam at its edge
Nižné Vane - reservoir for forest fire fighting built along a forest road
Stará Bystrica – small retention reservoir for peak flow control
Hlohovec – check dam made of wood and stones
Diversion throughs, seepage pits and soaking belts are also a key part of the revitalization of micro-territories. Most of these measures use wood as their main structural element.
Retention swale with infiltration pit
STABILITY AND DURABILITY OF CHECK DAMS AND OTHER RETENTION MEASURES
The main element of a stable and durable check dam is its proper construction. The transverse wooden logs should be layered with branches and pulp (fascines) along the profile of the corridor. It is important to reinforce the edges of the logs and fascines with stones and soil to ensure their stability.
Another important static element is the embedding of cross-cuts in between the transverse logs and fascines on the slope spanning a height of 1 to 1.5 m within the space between the dam walls. This space should not exceed the combined diameter of the logs used so that these cross cuts are firmly in place (20-30 cm).
The entire transverse and cross-sectional structure is stabilized by wooden steaks 8-12 cm in diameter which are hammered into the bed of the corridor until they cannot be wedged. These three elements are interconnected by nails of appropriate size. If using coniferous timber, then it is vital to remove the bark from the logs and steaks.
The stability and strength of the check dam is augmented after a first rainfall because sedimentation settles in and around the structure and loose space is filled in.
Assuming the inaccessibility of the terrain, all work is done manually. By the check dam, the cross-sectional logs are not inserted into excavated grooves but are secured on the slope. Fascines are secured using copper wire 2-3 mm in thickness.
CONSIDERATIONS FOR DESIGN AND IMPLEMENTATION OF CHECK DAMS
- Synergy with forestry and agricultural landscape
- Placement of check dam in the narrowest profile of the corridor and directly against the current while simultaneously ensuring that the retention area achieve its maximum size
- Individual check dams should not affect the effectiveness of other individual check dams
- Co-operation with stream managers and landowners is absolutely essential
- It is necessary to take into full consideration the historical value and uses of the water basin
- Incorporate nature conservation values and knowledge during entire design and implementation process
MISTAKES TO BE AVOIDED DURING THE IMPLEMENTATION
- Improper embedding of cross-sectional logs into the slope
- Insufficient width and length of transverse logs and fascines of the check dam
- Improper laying of the base log (log should be placed just below the corridor surface)
- Insertion of thick nails near the edges of the logs (results in split wood)
- Poorly inserted wooden steaks
- Angle of check dam leaning downstream of current
STATE OF CHECK DAMS 7 YEARS AFTER IMPLEMENTATION
Čadca, part Rieka – 2018 during floods within the region
Dunajov - diversion through fulfilling its purpose of diverting water back into the forest even though it is worn down (left) and fish catchment pond (right).
Dunajov - dispersed stone check dam – downstream side above and upstream side below
Catchment pond for fish (left) and channel for fish (right)
Collection of check dams along a gully and a washed out check dam
Sludge pits for capturing sedimentation
Rehabilitated stream bed (left) and dispersed rock check dam that also acts as a bridge (right)
Lodno - layered check dam filled with mud and rocks
Layered check dam filled with sedimentation gradually restores the ravine (left) and cascades consisting of wooden check dams (right)
Poorly built fascines resulted in them being washed out
Check dam with a retention capacity of roughly 150m3
Foundational transverse log was set too high
Check dam with a 130m3 retention capacity