Design velocities for vegetated and gabion-lined channels, victor M. Ponce and Andreas Koch, San Diego State University

Design velocities for vegetated and gabion-lined channels

Victor M. Ponce and Andreas Koch
July 2003

1. INTRODUCTION

The velocity of flow in watercourses has a direct relation to roughness and bank stability. The roughness decreases as the velocity increases when all other variables are held constant. When the velocity increases, the shear stress increases on the bed and bank material. This increases the rate of erosion and sediment transport.

There are several ways to protect the channel against bed and bank erosion. The banks can be protected directly by using different types of sheeting. There are different kinds of concrete and other artificial lining devices available (e.g., crushed stone and gabions). Natural solutions for bank stabilization include the lining of channels with lawns, vegetated mattresses and bundles of wood.

The advantages and disadvantages of bank stabilization using vegetated and gabion-lined channels are presented below.

Table 1. Comparison between vegetated and gabion-lined channels
Vegetated channels		Gabion-lined channels
Advantages	Disadvantages	Advantages	Disadvantages
Higher infiltration and exfiltration Excellent habitat function Sizable reduction in flow velocity Aesthetically very pleasing	Low structural integrity Protection is usually not immediate after installation	High stability Moderate infiltration and exfiltration Immediate protection after installation Moderate reduction in flow velocity	Less aesthetically pleasing Limited habitat function

2. ECOLOGICAL DESIGN

Gabions consist of mesh baskets filled with small riprap. A gabion structure can consist of several baskets (Fig. 1)

Fig. 1 Layout and dimensions of gabion boxes and mattresses.

Since the riprap is enclosed with wire mesh, it has high stability against erosion. Vegetation can be planted and is able to grow between the gabions layers (Fig. 2).

Gabion boxes with woody vegetation.

Fig. 2 Gabion boxes with woody vegetation.

Planting vegetation in gabions provides habitat, decreases the flow velocity during storm events and increases their aesthetic appeal.

More details about gabions are given in manningsn_hydroecological.html.

In bank stabilization using natural vegetation, attention must be paid to the temporal wetting zones to guarantee habitat sustainability.

The temporal wetting zones are:

A. Permanently wetted,
B. Intermittently wetted,
C. Event-wetted, or floodplain.

A. Permanently wetted zone:

Wattle and fascines are used in various types of assemblies.

Wattle (Fig. 3):

Fig. 3 Wattle of branches.

Staked 30-50 cm into the ground.
Stakes are braided with flexible strong brushwood.
Height of assembly 30-80 cm.
Serves as a temporary solution.

Fascines (Fig. 4):

Fascine sausage:

Cylindrical bodies of willow rods with a length of 10-20 m and a diameter of 0.10-0.15 m.
Manufactured from flexible brushwood with a length of 2.5-3.0 m.
Anchored with stakes of 4-5 cm thickness and length of about 1 m.

Fascine roll:

Structure similar to fascine sausage, with a diameter of 0.25-0.40 m.
1/3 to 2/3 of their thickness is placed under the average base flow level.

Installation.

Fig. 4 Preparation and installation of fascines.

Weighted fascines (Fig. 5):

Cylindrical bodies with a diameter of 0.80-1.20 m.
They consist of a 0.15-0.20 m thick coat of brush wood and a core of rough gravel or crushed rock.

Fig. 5 Weighted fascines.

B. Intermittently wetted zone:

Cattails are placed in the intermittently wetted zone.

C. Event-wetted, or floodplain zone:

For successful bank stabilization of the floodplain zone, the area is engineered by ecological means and the roots will take hold. The following natural building materials are used:

Finished lawn:

Lawn pieces:

Square turf sods with a edge length of 25-30 cm and a thickness of 3-8 cm.
Imported from other locations.
The installation can take place as flat lawns or as head lawns.
Lawns are laid flat or in stacks.
Flat lawns use bank slopes of 1.5:1-2:1; 3:1 is rarely used (Fig. 6).

Fig. 6 Flat lawns.

Areas with high shear stress should be anchored with stakes of length 20-30 cm.
Stacked lawns use bank slopes of 0.75:1 for flowing water bodies; at walls use slopes of 0.3:1-1:1.
The lawns are stacked as shown in Fig. 7.

Fig. 7 Stacked lawns.

Lawn rolls (Fig. 8):

The principle is the same as that of flat lawns.
They can be handled over larger surfaces.
They can be grown beforehand on special surfaces.

Lawn roll

Lawn roll

Fig. 8 Lawn roll.

Lawn seeds:

They are disseminated by wet or dry means (Fig. 9).

Fig. 9 Lawn seeding.

Young seedlings can be protected against removal by means of fiber mattresses or by mixing with a biodegradable adhesive during seeding (Fig. 10).

Fig. 10 Fiber mattress for protection of seeds.

Vegetative bank stabilization (Fig. 11):

Consists of individual components of 2 x 6 m and a thickness of 0.2 m.
The materials are a mixture of gravel, plastic mesh, biodegradable geotextiles, and endemic riparian vegetation.
The building materials are combined and spread in layers.
Can withstand large shear stresses.

Fig. 11 Vegetative bank stabilization.

Wood parts:

Collect staking woody debris during the winter.
Assemble these stakes in any of the following forms:

Brush mattress (Fig. 12):

High shear stress embankments require a slope of 1:1 or flatter.
Anchored to the ground with fascines or wire mesh.
Can withstand velocities up to 3.5 m/s.

Design of brush mattress: Top view

Fig. 12 (a) Design of brush mattress: Top view.

Design of brush mattress: Side view

Fig. 12 (b) Design of brush mattress: Side view.

Design of brush mattress: Side view after a few months

Fig. 12 (c) Design of brush mattress: Side view after a few months.

View of completed brush mattress

Fig. 12 (d) View of completed brush mattress.

Live staking:

Use root-able branch ends with length of 1.0-2.5 m and thickness of 4-6 cm.
Insert as shown (Fig. 13).

Schematic drawing Soon after installation

Fig. 13 (a) Schematic drawing.

Fig. 13 (b) Soon after installation.

Sometime after installation 2-5 yr after installation

Fig. 13 (c) Sometime after installation.

Fig. 13 (d) 2-5 yr after installation.

Established live staking

Fig. 13 (e) Established live staking.

Small trees and shrubs (Fig. 14):

Placed above the baseflow.
Able to sustain flooding for several days when mature.
Typically used in combination with other measures such as lawn pieces and/or weighted fascines.
The following functions are provided:

Stabilization of the bank against erosion.
A mature canopy will shade a good portion of the bank.
Shading reduces the temperature of the water.
For tropical and midlatitudinal climates this temperature reduction will improve water quality by discouraging the growth of algae and other nuisance species.
This creates a niche habitat for a particular community of flora and fauna.

Fig. 14 (a) Bank protection with trees and shrubs.

Fig. 14 (b) Longitudinal view.

Fig. 14 (c) Side view.

Fig. 14 (d) Completed bank protection with trees and shrubs.

3. HYDRAULIC DESIGN

Design parameters

The critical velocity v_c and the critical shear stress τ_c are used in the analysis of the stability of the bottom and the banks of rivers against erosion.

Critical velocity

The stability can be determined by comparing the actual velocity to the maximum allowable velocity or critical velocity v_c based on the bed and bank material. The Manning equation can be used:

V = (1/n) R^2/3 S^1/2

in which is v = velocity (m/s), n = Manning's n, R = hydraulic radius (m) and S = slope (m/m).

The critical velocity must be determined by experiments or empirically derived from information in the literature. The critical velocity is that at which erosion of the bed and/or bank material begins. The criterion for stability is:

v < v_c

Typical values of critical velocity are shown in Table 2.

Table 2. Typical values of critical velocitiy and shear stress
Material	Critical velocity	Critical shear stress
Material	(m/s)	(N/m²)
Lawn (short-time loaded)	1.8	20-30
Lawn (long-time loaded)	1.5	15-18
Fascine sausage	2.5-3.0	60-70
Fascine roll	3.0-3.5	100-150
Weighted fascine	2.5-3.0	60-100
Brush mattress	2.5-3.5	150-300
Live staking in rip rap		> 140
Willows/alder		80-140
Gabions	1.8-6.7	80-140

Critical shear stress

The critical shear stress τ_c is a measure of the stability of the bed and banks of a river against erosion. The bottom shear stress can be calculated as follows:

τ_o = γ R S

in which is τ_o = the bottom shear stress (N/m²), γ = specific weight of water (N/m³), R = hydraulic radius (m) and S = slope (m/m). For wide channels, i.e., those with a top width T ≥ 10 R, the flow depth h is substituted for R:

τ_o = γ h S

The critical shear stress τ_c must be determined by experiments or empirically derived from information in the literature. The critical shear stress τ_c is that at which erosion of the bed and/or bank material begins. The criterion for stability is:

τ < τ_c

Typical values of critical shear stress are shown in Table 2.

Manning's n

The variety of vegetation cover available in nature implies that there is also a variety of resistance to flow. Flow resistance is dependent on whether vegetation is completely or partially submerged. The controlling factor is the height of the canopy in relation to the water depth. The three general cases are (Fig. 15):

A. Short vegetation,
B. Average vegetation,
C. Tall vegetation.

A. Short Vegetation is that which is short in relation to the flow depth, its height being on the same scale as the absolute roughness. The velocity distribution in the cross section resembles that due to absolute roughness.

B. Average Vegetation occurs when the plant height is, on the average, the same as the flow depth. It varies between full and partial submersion. The behavior of the resistance to flow demands a special attention to the flow conditions. This is because average vegetation is susceptible to forceful submission (i.e., tilting) when the stream power overcomes the static resistance of the stems. There is a definite relationship between vegetative resistance and the ratio of plant height to flow depth.

C. Tall Vegetation, is that which is tall in relation to the flow depth. This definition excludes plants that bend such that their height is decreased below the water surface.

Fig. 15 (a) Short, (b) Average, and (c) Tall vegetation.

Table 3. Roughness coefficient
Surface structure	k	K_St	n
Surface structure	(m)	(m^1/3/s)	[Fig. 16]
Lawn	0.06	40	0.025
Grass; field without cover	0.2	30	0.033
Grassland; rocky forest soil	0.25	25	0.040
Grass with shrubs	0.3	24	0.042
Herbaceous vegetation	0.4	22	0.045
Field with arable crop	0.6	21	0.048
Irregular flood plains	0.8	15	0.067
Highly irregular flood plains	1	12	0.083

Irregular flood plains (n = 0.055-0.083)

Fig. 16 (a) Irregular flood plains
(n = 0.055-0.083).

Herbaceous flood plains (n = 0.041-0.050)

Fig. 16 (b) Herbaceous flood plains
(n = 0.041-0.050).

Grassland flood plains (n = 0.028-0.040)

Fig. 16 (c) Grassland flood plains
(n = 0.028-0.040).

Grassland flood plains (n = 0.028-0.040)

Fig. 16 (d) Grassland flood plains
(n = 0.028-0.040).

For conceptually based calculations, the Darcy-Weisbach formula can be used:

v = (1/ f ^1/2) (8 g R S) ^1/2

with

f = [(4 A_p,i ) / ( a_x a_y )] c_w,i

for nearly horizontal flood areas, and

f = [(4 A_p,i cos α) / ( a_x a_y )] c_w,i

for hillslope flood areas, with

A_p,i = h_i d_m,i

The geometric characterization and the equivalent diameter are shown in Fig. 17 and Fig. 18.

Geometric characterization of tree population density

Fig. 17 Geometric characterization of tree population density.

Determination of the equivalent diameter.

Fig. 18 Determination of the equivalent diameter.

For an individual specimen, the coefficient of resistance c_w,i is equal to that of a circular cylinder for which c_w = 1.2.

For groups of trees or bushes, the following formulas can be used:

f = [(4 A_p ) / ( a_x a_y )] c_w,r

f = [(4 A_p cos α) / ( a_x a_y )] c_w,r

with c_w,r between 0.6 and 2.4, with mean value c_w,r = 1.5.

4. RIPARIAN HUSBANDRY

Different vegetative species are expected to require different location and flow conditions. All of them are expected to provide vegetative cover to minimize the possibility of channel or bank erosion.

Wood

Remove old and sick wood, particularly when they are an obstacle to the flow.

With existing vegetation, thin elements positioned in untypical locations.

If hydraulically insignificant, leave vegetative debris to encourage habitat (Fig. 19).

Examples of vegetative debris (e.g., deadwood)

Fig. 19 Examples of vegetative debris (e.g., deadwood).

Sedges and shrubs

Cut sedges and shrubs only when hydraulically necessary.

Lawn

Lawn of embankment should be mowed 1-2 times per year.
During mowing, attention should be payed to bird eggs.
Keep some areas unmowed to conserve biodiversity.

5. SUMMARY

Design velocities for vegetated and gabion-lined channels are based on two criteria: (1) critical velocity, and (2) critical shear stress.
Critical velocity is that at which erosion of the bed and/or bank material begins.
Critical shear stress is that at which erosion of the bed and/or bank material begins.
Critical velocities and shear stresses for various types of vegetative bank protection assemblies have been identified.
Roughness coefficients for these bank protection assemblies under various types of submergence conditions have been identified.
Maintenance and husbandry of riparian vegetation is recommended to assure the long-term effectiveness of vegetative bank protection assemblies.

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