Abnormal rainfall at the summit

Abnormal rainfall at the summit

Upper catchment behavior during extreme precipitation events

An abnormal downpour at the summit is one of the most dangerous events for the entire mountain drainage system. While there are no large rivers or wide channels at the upper level, it is here that numerous small streams form within minutes, which further down the slope merge into a destructive flood wave.

The danger is determined not only by the total amount of rainfall. Of critical importance are:

precipitation intensity;

duration of the downpour;

thunderstorm cell area;

wind direction;

soil condition;

slope;

vegetation cover;

presence of snow or ice;

damage after fire;

roads and paths;

condition of the upper troughs;

a sequence of several rains.

The main principle:

At the summit, the goal is not to try to hold back the entire extreme downpour, but to prevent the water from coalescing into one fast flow too early.

The upper level must receive the first volumes of water, disperse them, retain sediments and safely transfer the excess to the next levels of the catchment area.

1. How does an abnormal downpour differ from normal rain?

In a normal rain, a significant portion of the water:

intercepted by vegetation;

retained by bedding;

penetrates into the soil;

gathers in small depressions;

gradually flows into the streams.

During an abnormal downpour, the area's capacity to absorb water is quickly exhausted.

The sequence changes:

leaves and litter become saturated;

soil pores are filled;

microdepressions are overflowing;

a surface film of water appears;

individual streams merge;

the stream enters into paths, ruts and gullies;

the transfer of soil, stones and plant debris begins;

the upper flood wave is formed.

Even a well-restored area can't absorb water indefinitely. Therefore, a safe overflow remains a mandatory part of the system.

2. Main types of extreme rainfall

2.1. Short, intense downpour

It lasts for a relatively short time, but produces very high precipitation intensity.

Typical consequences:

the water does not have time to be absorbed;

surface runoff is formed quickly;

microstructures are overflowing almost simultaneously;

local high-speed flows arise;

The erosion of roads and paths is increasing sharply.

Such an event is particularly dangerous for small headwater catchments.

2.2. Prolonged heavy rain

The main danger lies in the gradual saturation of the soil.

At first the system may work fine, but then:

infiltration decreases;

underground pressure increases;

the slopes are getting wet;

old landslides are activated;

trees lose root stability;

Even moderate additional rain creates strong runoff.

2.3. A series of successive showers

Between events the soil does not have time to dry out.

Each subsequent downpour becomes more dangerous than the previous one.

Possible sequence:

the first rain fills the soil reserve;

the second saturates the lower horizons;

the third one turns almost completely into a drain;

the fourth triggers a landslide or mudflow.

2.4. Heavy rain with strong winds

Wind makes the distribution of precipitation uneven.

He can:

carry rain across a watershed;

increase precipitation on one slope;

reduce them on the other;

direct water under the plant litter;

to destroy the soil with an oblique impact of drops;

fell trees on saturated soil.

The usual calculation, which assumes vertical and uniform rainfall, is not sufficient here.

2.5. Thunderstorm with hail

City:

damages leaves;

knocks down plant litter;

clogs drains;

accumulates in depressions;

temporarily blocks infiltration;

After melting, add additional water.

Damaged vegetation provides less protection for the soil during the remainder of the event.

2.6. Warm rain on snow

Rain combines with melt water.

The flow may be significantly greater than the volume of rain itself.

Particularly dangerous are:

rich snow;

frozen soil;

ice crusts;

snow-blocked canals;

sudden warming;

strong warm wind.

2.7. Downpour after the fire

After the fire:

the plant interception disappears;

roughness is reduced;

ash is formed;

the surface can become water-repellent;

the roots gradually lose strength;

the amount of sediment increases sharply.

Even rain, which was not previously considered critical, can cause flooding and mudflows after a fire.

3. Development of events at the summit

Phase 1. The beginning of precipitation

The first drops moisten:

leaves;

grass;

mosses;

stones;

plant litter;

top layer of soil.

At this stage, most of the water is still held by the surface.

Phase 2. Saturation of vegetation cover

The crowns and grasses can no longer hold additional water.

It begins:

drip drain;

flowing down the trunks;

filling of soil pockets;

movement of water under the litter.

Phase 3. Exceeding the infiltration capacity

The rain comes faster than the soil can absorb it.

The following are formed on the surface:

separate puddles;

thin streams;

temporary runoff lines;

overflows between microdepressions.

Phase 4. Connecting microstreams

Small jets enter:

paths;

tracks;

road ditches;

natural hollows;

old erosion furrows.

It is at this stage that the dangerous consolidation of the flow begins.

Phase 5. Sediment transport

First, water carries:

dust;

small particles;

seeds;

pine needles;

leaves.

Then the following enter the stream:

clods of soil;

a branch;

branches;

small stone;

damaged plants.

Water becomes denser and more destructive.

Phase 6. Structural damage

With further growth of the flow the following is possible:

breakthrough of the stone line;

destruction of micro-terrace;

road embankment erosion;

bush removal;

collapse of the edge of the ravine;

blockage of a small water passage.

Phase 7. Downward transmission of the wave

The water leaves the upper level no longer as a uniform flow, but as a concentrated impulse.

If the next level is not prepared, the flow quickly intensifies.

4. Critical properties of the vertex

4.1. Landform

Convex apex

The water spreads out in a fan shape, but damage to the vegetation can create many parallel gullies.

Flat plateau

A significant temporary accumulation of water is possible, followed by a sudden overflow over the edge.

Sharp ridge

Water is quickly distributed between the slopes. Retention is limited by the thin soil.

Saddle

It is capable of collecting flow from both sides and transferring water between pools.

Upper bowl

Can hold water temporarily, but creates a powerful directional release when overflowing.

4.2. Slope

The higher the slope, the:

shorter contact time between water and soil;

higher speed;

stronger erosion;

the streams merge faster;

greater risk of stone transfer.

However, a small slope can also be dangerous if a large volume accumulates on the plateau and then breaks out through the bottleneck.

4.3. Soil

The behavior of water is influenced by:

depth;

structure;

organic content;

cracking;

compaction;

humidity before rain;

presence of a clay layer;

freezing;

damage after fire.

Thin, rocky soil becomes saturated quickly but can drain water well through cracks.

Clay soil can absorb water slowly and create high surface runoff.

4.4. Vegetation

Sustainable vegetation:

intercepts drops;

increases roughness;

holds the soil;

distributes water;

retains sediment.

But in the event of an abnormal event, the following become dangerous:

poorly secured branches;

fallen trunks;

dense bushes in the emergency channel;

even-aged trees on saturated soil;

heavy floating mulch.

4.5. Stone outcrops

Rocks absorb almost no water.

They quickly transfer the flow to:

cracks;

gutters;

soil pockets;

lower edges of the slabs.

A small area of ​​soil under a large rock can suddenly receive water from an area many times larger than its own.

5. Preliminary soil condition

Abnormal rainfall cannot be assessed without knowing the initial humidity.

Dry soil

Good infiltration is possible, but an over-dried surface sometimes does not accept the first volumes of water well.

After a long drought, the following may also form:

cracks;

hard cork;

water-repellent organic layer.

Moist soil

Part of the reservoir is already filled. Surface runoff begins earlier.

Fully saturated soil

The soil practically stops accepting additional water.

The risks increase:

landslide;

windstorm;

squeezing out groundwater;

destruction of terraces;

slope collapse.

Frozen soil

Even with a porous structure, water cannot penetrate inside normally.

Most of the rain and melt water runs off along the surface.

6. The role of wind

An abnormal wind can significantly change the actual volume of water in each micro-catchment.

He:

deflects the trajectory of drops;

carries sediment over the ridge;

concentrates rain on the windward side;

creates dry leeward areas;

increases evaporation before the onset of a downpour;

breaks branches;

fells trees;

tolerates hail and plant debris.

In saddles and wind corridors, rain can fall almost horizontally.

Therefore, it is necessary to model not only the area of ​​the rain cell, but also the direction of movement of the moist air mass.

7. Calculation of rain volume

For the initial assessment, the relationship between:

amount of precipitation;

micro-catchment area;

losses due to interception and infiltration;

share of surface runoff.

One millimeter of rain on an area of ​​one square meter is equivalent to one liter of water.

Hence:

10 mm per hectare - about 100 m3 of water;

50 mm per hectare - about 500 m3;

100 mm per hectare - about 1 000 m3.

This is the total atmospheric volume before subtracting infiltration, interception, evaporation and local accumulation.

Even a small upper micro-catchment area is capable of generating hundreds of cubic meters of water in a short time.

8. Efficient drainage

Not all rain turns into surface runoff.

The following are taken into account in a simplified manner:

interception by vegetation;

filling of surface depressions;

infiltration;

temporary detention;

evaporation during the event;

wind throw.

For an extreme scenario, the normal average runoff coefficient cannot be used alone.

At least three states must be calculated:

normal soil;

moist soil;

saturated or frozen soil.

After a fire or severe compaction, the runoff coefficient may increase sharply.

9. Concentration time

The concentration time shows how long it takes for water from the most distant point of the micro-catchment to reach its outlet.

At the peaks it can be very short due to:

small size of the pool;

strong slope;

rock surfaces;

roads;

trope;

thin soil.

If the duration of the maximum rainfall intensity coincides with the time of concentration, a particularly high peak discharge occurs.

The main goal of recovery is to increase the time of concentration without creating dangerous accumulations.

This is achieved through:

plant roughness;

micro-drops;

distributed barriers;

rupture of road channels;

soil restoration;

flow distribution.

10. Critical points

At the upper level, places where the flow can quickly increase are identified in advance.

These include:

saddles;

the end plateau;

upper troughs;

road ditches;

tracks;

pipe exit points;

paths along the line of maximum slope;

edges of rock slabs;

destroyed terraces;

areas after fire;

windy places;

old gullies;

soil dumps;

areas of mass grazing.

Each such point receives its own safe overflow scenario.

11. Cascade protection of the summit

The first barrier is vegetation

Tasks:

reduce the impact of drops;

retain fine soil;

increase roughness;

reduce the speed of the first jets.

The second barrier is microrelief

Used:

small soil pockets;

natural depressions;

stone placers;

short micro-terraces.

They hold back small volumes, but should not act like large dams.

The third barrier is contour elements

Low stone lines and plant stripes:

disperse water;

retain sediment;

interrupt the straight path down the slope.

They are made in sections, with controlled overflows.

The fourth barrier is the upper ravines

The hollows receive the already collected flow.

What is needed here:

reinforced bottom;

flexible vegetation;

absence of dense tree blockages;

clear path of water;

sustainable progress to the next level.

The fifth barrier is the emergency spillway

When the entire system is full, the excess must pass through a predetermined route.

An emergency route cannot be designed through:

road;

building;

tourist site;

landslide slope;

source of drinking water;

unstable embankment.

12. Principle of sectionality

One long contour ditch or continuous terrace on the summit is dangerous.

If damaged, it can:

collect water from a large area;

break through at one point;

direct the entire volume downwards;

create a new ravine.

Therefore, the upper structures are divided into short sections.

Each section must have:

limited catchment area;

own working volume;

reinforced overflow;

safe direction of failure;

independence from neighboring sections.

13. Controlled overflow

An overflow is a mandatory part of any retaining element.

He must:

start work before the structure is destroyed;

be below vulnerable edges;

have sufficient width;

be protected from erosion;

direct water onto a stable surface;

do not hit the adjacent structure;

pass plant residues.

The narrow overflow can become clogged and cease to function.

14. Erosion protection

On extreme flow paths the following are used:

dense turf;

rock fill;

flat stone;

stepped differences;

flexible bushes along the edges;

flow expansion;

sediment deposition areas.

The goal is not to cover the entire natural slope, but to strengthen only critical points.

A completely smooth concrete channel may safely hold the banks, but at the same time greatly accelerate the water and transmit the problem below.

15. Sediments and wood debris

An abnormal downpour carries more than just water.

The stream may contain:

soil;

stones;

branches;

leaves;

roots;

remains of fences;

garbage;

road elements.

Therefore, the throughput should be calculated not only for clean water.

In the upper troughs the following is provided:

accessible deposition sites;

extensions;

rough grates with bypass overflow;

wood capture zones;

ability to clean up after an event.

It is not possible to install a fine mesh across an active flow without an emergency bypass.

16. Roads and trails

During abnormal rainfall, the road often becomes the most efficient channel at the summit.

Water is included in:

tracks;

internal ditches;

steps;

notches;

edges of the coating.

Dangerous sequence:

the road intercepts the slope runoff;

water moves along the canvas;

several micro-catchments are combined;

the flow comes out through one pipe;

The lower slope receives an unnaturally high flow rate.

Required:

frequent cross-releases;

breaking long tracks;

outward slope where it is safe;

wide culverts;

protected outputs;

emergency overflow over a stable section.

17. Saddles

The saddle is a special risk area.

During heavy rain, streams may be encountered here:

from two peaks;

from the windward and leeward slopes;

from the road;

from a snow pocket;

from the upper swamp.

The road embankment in the saddle can temporarily retain water and then collapse.

Therefore, it is required:

accurate relief model;

definition of all incoming flows;

wide safe overflow;

protection against water overflow into a vulnerable pool;

absence of buildings in the zone of a possible breakthrough.

18. Upper bowls and plateaus

The wide top bowl can temporarily hold a large volume of water.

But it is necessary to check:

geological basis;

the presence of an old lake bottom;

soil depth;

waterproof layers;

edge stability;

overflow path.

The main danger is a false sense of security.

While the bowl is filling, the water below may be calm. Once the critical level is reached, a large volume is rapidly released.

19. Useful

Prolonged rainfall increases water pressure in the soil and cracks.

Signs of growing danger:

new cracks;

soil heaving;

tilt of trees;

muddy water from the slopes;

emergence of new sources;

closing old cracks;

road subsidence;

movement of stone walls.

In potential landslide zones it is prohibited to:

concentrate water;

create deep infiltration pits;

build heavy storage facilities;

direct the overflow to the upper part of the landslide;

plant large trees without assessing the soil.

20. Mudflow scenario

The mudflow may begin already in the upper ravine.

To do this you need:

intense influx;

loose material;

sufficient slope;

limited channel.

The sources of loose material are:

talus;

fire stations;

road dumps;

destroyed terraces;

windfalls;

collapsed banks;

avalanche accumulations.

A mudflow is significantly denser than water and is capable of moving large stones.

Small pipes and ordinary drains are almost useless for it.

21. Abnormal downpour after drought

After a long drought, the surface may be:

solid;

cracked;

devoid of vegetation;

covered with dust;

partially water-repellent.

The first streams quickly wash away:

dust;

seeds;

dry organic matter;

pollution;

ash.

Water can penetrate deep into large cracks, but hardly penetrate the rest of the surface.

This creates uneven saturation and local instability.

22. Combination with lightning and fire

A thunderstorm is capable of simultaneously:

put out one fire;

cause a new fire with lightning;

damage electrical systems;

turn off sensors;

block communication;

create a flood.

Therefore, the warning system should not depend on one power grid or one station.

Required:

autonomous sensors;

backup power supply;

local data recording;

radio communication;

manual level gauges;

pre-determined inspection routes.

23. Monitoring system

For the upper catchment area the following are used:

automatic rain gauges;

radar data;

soil moisture sensors;

soil temperature sensors;

level gauges in troughs;

cameras;

ground motion sensors;

tree tilt sensors;

flow meters;

acoustic mudflow sensors;

weather stations;

anemometers;

lightning sensors.

It's especially important to measure rainfall intensity at short intervals. Daily totals can mask a critical fifteen-minute peak.

24. Danger levels

Green level

The soil is able to absorb water, runoff is minimal.

Yellow level

The top layer becomes saturated and individual storage tanks fill up.

Orange level

Steady surface runoff begins, overflows and local erosion are possible.

Red level

There is a high probability of a landslide, mudflow, road collapse or cascading failure.

Black level

There was a breakthrough, a major landslide, a mudflow, or communication with the territory was lost.

Threshold values ​​are determined individually for each catchment. A universal number of millimeters of rain cannot equally describe all peaks.

25. Pre-rainfall preparation

Before the extreme precipitation season, it is necessary to:

clear controlled overflows;

check roads and pipes;

remove dangerous unsecured trunks;

restore damaged stone lines;

check the sensors;

clean the sediment traps;

close unauthorized tracks;

check emergency routes;

restrict grazing in vulnerable areas;

update the risk map.

Preparation should not involve complete clearing of the area. Excessive removal of vegetation and trees can increase erosion.

26. Actions during a warning

When forecasting abnormal rainfall:

dangerous paths and roads are closed;

construction work is stopped;

the equipment is removed from the hollows;

increased data transfer is activated;

backup communication channels are checked;

access for tourists is restricted;

saddles and upper bowls are controlled;

Downstream settlements are warned.

People should not be sent to the summit during an active thunderstorm just for a visual inspection.

27. Post-rain survey

After the event, the following are assessed:

new gullies;

damage to vegetation;

condition of overflows;

sediment accumulation;

road erosion;

cracks;

tilt of trees;

new wet outlets;

damage to sensors;

change of riverbeds;

water transfer between basins.

The first visual inspection does not always reveal all problems.

Landslides and tree falls may continue after the rain stops.

28. Recovery after damage

The work is carried out in the following sequence:

ensuring security;

restoration of emergency water route;

removal of dangerous blockages;

stabilization of active gullies;

protection of exposed soil;

repair of roads and crossings;

restoration of vegetation cover;

cleaning storage devices;

digital model update;

project adjustment.

You cannot immediately return the damaged area to its previous layout if it could not withstand a real event.

The downpour is seen as a test of the system and a source of new data.

29. Sustainability indicators

The upper catchment area is considered stable if, after an abnormal downpour:

large new gullies did not form;

the water was not diverted to a vulnerable adjacent basin;

no cascading destruction occurred;

emergency overflows have been activated;

the roads did not become main channels;

sediments were retained in accessible places;

the vegetation cover is mostly preserved;

the lower level received a stretched rather than a shock flow;

the monitoring system continued to operate;

Local damage can be repaired without major intervention.

30. What not to do

At the top you can't:

calculate the system only based on average rainfall;

try to completely block the extreme flow;

create long continuous ditches;

connect all micro-catchments into one channel;

build retaining elements without overflow;

direct water to a landslide slope;

use small pipes in dangerous depressions;

place dense bushes in the emergency channel;

leave road tracks without cross-outlets;

consider dry soil to always be well-absorbent;

ignore hail, snow, wind and fire;

assess the danger only based on the daily precipitation amount;

consider the cessation of rain as the end of the risk.

31. Design sequence

Stage 1. Dividing the summit into micro-catchments

It is determined where each part of the water will come from and where it will go.

Step 2. Calculating normal precipitation

The day-to-day operation of the system is checked.

Step 3. Calculating abnormal rainfall

Different intensities and durations are simulated.

Step 4. Checking the soil condition

Dry, wet, saturated and frozen scenarios are considered.

Step 5: Adding Wind

Oblique precipitation and transfer over the ridge are taken into account.

Step 6. Sediment calculation

Not only the water consumption is determined, but also the transport of soil, stones and wood.

Step 7. Creating a sectional cascade

The flow slows down and is distributed into small independent elements.

Stage 8. Design of an emergency route

The route of the water is determined after filling all structures.

Step 9. Checking for Cascading Failure

The destruction of one or more elements is simulated.

Stage 10. Monitoring real events

Calculations are updated after each significant downpour.

The final principle

An abnormal downpour at the summit cannot be rendered completely safe. But it can be prevented from immediately turning into a single destructive torrent.

Properly organized upper catchment area:

receives the first precipitation;

slows down surface movement;

distributes water between independent sections;

preserves the soil;

retains sediment;

does not turn roads into riverbeds;

safely overflows;

passes the excess to the next level;

maintains functionality in case of local damage.

The safety of a summit is determined not by how much water it can hold, but by how predictably and without destruction it allows the volume to pass through that it can no longer hold.