Group 9 – K55TTKHMT– Environmental Science – HUS

HANOI UNIVERSITY OF SCIENCE, VNU
FACULTY OF ENVIRONMENT
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WETLANDS - BIOLOGY AND REGULATION

Topic:
APPLICATION HGM THEORY TO CALCULATE THE MITIGATION RATIO OF RESTORED WETLAND
AND REFERENCE WETLAND IN XUAN THUY NATIONAL PARK, NAMDINH CITY, VIETNAM.

Instructor: Prof. Nguyen Thi Loan
Group 9:
1.
2.
3.
4.
5.
6.

Duong Thi Kim Anh
Tran Vu Diem Huyen
Ngo Thi Huyen
Chu Thi Nham
Nguyen Thi Minh Trang
Duong Thi Thu Trang

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APPLICATION HGM THEORY TO CALCULATE THE MITIGATION RATIO OF RESTORED WETLAND
AND REFERENCE WETLAND IN XUAN THUY NATIONAL PARK, NAMDINH CITY, VIETNAM.

I.

II.

INTRODUCTION
We were assigned using HGM theory to calculate the mitigation ratio of restored wetland and reference
wetland in a coastal area in Vietnam (suppose success ratio is 30%). We choose to calculate in Xuan Thuy National
Park, Namdinh city, Vietnam. Hydrogeomorphic HGM approach was applied to calculate the mitigation ratio by
evaluating wetland functions of the mitigation site compared to the reference site.
METHOD
1. Site description
Xuan Thuy National Park is a national park in Red River Biosphere Reserve in the province of
Namdinh, Vietnam.The park was the first wetland area to be announced a Ramsar site in south-east Asia and
is internationallysignificant as a migratory bird habitat, being the 50th site worldwide.
 Landscape and Climate: Xuan Thuy National Park is located in Giao Thuy district (Namdinh province). It is
the largest coastal wetland ecosystem in the north of Vietnam and placed in the south of the Red River mouth.
The Core Zone has a total area of 7.100 hectares. There are two types of soil formed from the alluvium of the
Red River. One is alluvial mud (which becomes loam) and one is sand. Transported by water the alluvial
forms the coastal soil like light soil ( sand and light loam and pure sand), medium soil and heavy soil ( loam
and clay )
 Biological characteristics: biodiversity of fauna and flora

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Group 9 – K55TTKHMT– Environmental Science – HUS


Maintain spatial structure of habitat

The detail information and results are summarized in data table.
 Strategy:
 The reference site was given a score of one for each index of variable.
 The scores assigned for variables of the mitigation site were based on their likeness to the reference site.
 The total function index was the average scores of the four functional categories.
 The ratio of total function index of the reference site to the mitigation site is called the HGM mitigation ratio.
Because forested wetlands have a failure rate of 70%, an adjusted HGM mitigation ratio is necessary (Robb 2002).
The below equation is used to calculate the adjusted HGM mitigation ratio.
Adjusted HGM mitigation ratio =
(the success rate is 30% or 0.3)
III.

COLLECTING DATA AND CALCULATION USING HGM THEORY
1. LONG-TERM SURFACE WATER STORAGE
1.1. Definition:
Long-term surface water storage is the capacity of a wetland to store (retain) surface water for long durations;
associated with standing water not moving over the surface. Sources of water may be overbank flow, direct
precipitation, or upland sources such as overland flow, channel flow, and subsurface flow.
1.2. Description of variables

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Group 9 – K55TTKHMT– Environmental Science – HUS
 VSURWAT, Indications of surface water presence. For the Long-Term Surface Water Storage function to occur, a

wetland must be inundated by ponded or retained water for a continuous period of not less than 1 week. Site
assessments are not always possible when there is the presence of surface water for the requisite continuous


1.0 for 75 to 125 percent of reference standard
0.5 for 25 to 75 percent or >125 percent of reference standard
0.1for 0 to 25 percent of reference standard
0.0 for the absence of variable and indicators
 VTURNOV Annual turnover of detritus. Detritus turnover is “the other half” of nutrient cycling. Detrital stocks are
represented by snags, down and dead woody debris, organic debris on the forest floor (leaf litter, fermentation, and
humus layers), and organic components of minerals soil.
Most detrital components can be observed directly and compared with reference standards. Additional
indicators could include fungi and mycorrhizae, as well as arthropods and other invertebrates, for assessments
conducted in more detail:
Sites within:
 75 to 125 percent of reference standards in detrital stocks score 1.0.
 Where detrital stocks are significantly reduced (25 to 75 percent) or overabundant (>125 percent), the variable
should score 0.5;
 If major disturbance has depleted the site of most soil and detrital organic matter (1 to 25 percent), the function
should receive a 0.1.
 If there are no detrital stocks and the potential for recovery is absent the score should be zero
3. MAINTAIN CHARACTERISTIC PLANT COMMUNITY
3.1. Definition: Species composition and physical characteristics of living plant biomass. The emphasis is on the





dynamics and structure of the plant community as revealed by the dominant species of trees, shrubs, seedlings,
saplings, and ground cover, and by the physical characteristics of vegetation.
3.2. Description of variables
 V
COMP, Species composition for tree, sapling, shrub, and ground cover strata.

the sky is covered by leaves when one looks into the canopy.
 If the percent cover in an assessment site is >75 percent from reference standard sites, a score of 1.0 should be
given.
 If canopy cover is from 25 to 75 percent, Index score is 0.5.
 If an assessment site is from 0 to 25 percent, score should be 0.1.
 A zero is given when there is no tree layer.
 V
DTREE, Tree density. Density (VDTREE) and basal area (VBTREE below) of trees can be used to evaluate the
successional status and stability of plant communities.

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Group 9 – K55TTKHMT– Environmental Science – HUS
 If tree density at an assessment site is between 75 and 125 percent of reference standards, it may be assumed that

the site is stable and a score should be 1.0.
 If the range is either from 25 to 75 percent or from 125 to 200 percent, a score of 0.5 should be assigned.
 If densities beyond the foregoing ranges (i.e., higher or lower), a score should be assigned 0.1.
 The absence of tree species receives a zero.
 V
BTREE, Tree basal area. Basal area of trees (VBTREE ) is proportional to aboveground plant biomass of trees
and is a dependable indication of forest maturity
4. MAINTAIN SPATIAL STRUCTURE OF HABITAT
4.1. Definition: The capacity of a wetland to support animal populations and guilds by providing heterogeneous

habitats.
4.2. Description of variables
 VSNAGS, Density of standing dead trees (snags). Standing dead trees are important in contributing to habitat structure.
Density determinations should focus on the larger size classes of snags (with respect to reference standards), because

 Patchiness between 75 and 25 percent or >125 percent of its reference standard should score 0.5.
 Assessment sites that are between 0 and 25 percent of this reference standard for patchiness should be scored 0.1.
 Sites have no potential for restoring patchiness to the reference standard should receive a zero score.
 VGAPS, Canopy gaps. Death of canopy trees is a normal process that has important implications for the dynamics of
ecosystems.
 Gap area or density that is between 75 and 125 percent of reference standards should receive a variable index
score of 1.0.
 Conditions between 75 and 25 percent or >125 percent of the reference standard should score 0.5.
 Assessment sites that are between 0 and 25 percent of the reference standard should be scored 0.1.
 Sites than have no potential for recovery to the reference standard (development of gaps not possible) should
receive a zero score.
Table: Summary of values and calculation for wetland functions using hydrogeomorphic index at

Xuan Thuy National Park, Namdinh city, Vietnam.
Model measure

Measure

Reference wetland

Index of Restored wetland
Variable

LONG-TERM WATER STORAGE
Indicators
of Visual observation of ̴ 2m flooding observed 1.0
VSURWA surface
water water mark, buttress, from
watermarks,


thickness of leaf litter layer 7cm
productivity
6cm =6cm average
5cm
6cm
Annual
Measuring the thickness of Very thin organic layer if 1.0
VTURNOV
turnover of A horizontal layer
present VTURNOV then index is VTURNOV otherwise use VPROD
MAINTAIN CHARACTERISTIC PLANT COMMUNITY

gymnorrhiza,
bruguiera cylindrical.
SAP: Aegiceras
corniculatum, bruguiera
cylindrical, sanneratia
caseolaris
GC:Acanthus ilicifolius,
kandelia candel,
phragmites australis,

0.25

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Group 9 – K55TTKHMT– Environmental Science – HUS

VREGEN Seedlings/sap Estimated ratio of seedlings
lings and/or to saplings (DBH5cm DBH) in 10m radius
VBTREE Tree
basal Measured DBH in a 10m
area
radius plot for all tree

gymnorrhiza: 18 17 19
30 36 26
17 26 19 28 30 19 19
Aegiceras corniculatum:
Aegiceras
13 25 17 19 36 21 35 27
corniculatum: 18 13 19
19 24
17 19 21 18
Xylocarpus granatum: 25
Bruguiera cylindrical:
30 26 15 18 22 19 17 28
19 13 18 25 21 12 14
25 23 27 32 35
25
Sanneratia caseolaris:34 23
Sanneratia caseolaris:
35 33 25 17 27 19 15 15
19 18 23 27 18 25 27
19 20 27 25 40
17 18 14 20 21
Total: 1224
Total: 754(62%)
Index of function = [(VComposition+VRegeneration+VCanopy Cover) + (VTree Density+VBasal Area)/2]/4
MAINTAIN SPATIAL STRUCTURE HABITAT
Density of Counted standing dead 10
VSNAGS standing
trees in 10m radius plot
dead trees
Abundance


VPATCH

Number and
attributes of
vertical
strata
of
vegetation
Vegetation
patchiness
Gaps
forest

Estimated number of strata Trees: 75%
and their percent coverage
Saplings: 70%
Shrubs: 10%
Ground cover: 30%

1.0

Trees: 30%
Saplings: 50%
Shrubs: 8%
Ground cover: 20%

Viewed aerial photograph Appeared uniform
using Google Earth


2.21:1
7.37:1

TABLE: Index of variables for reference site and mitigation site at Xuan Thuy National Park, Nam Dinh City, Vietnam
Wetland function
Long-term surface water storage
Nutrient cycling
Maintain characteristic plant communities
Maintain spatial structure of habitat
Total function index

Index of variable
Reference site
Mitigation site
1
0.75
1
0.1
1
0.34
1
0.62
1
0.4525
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Group 9 – K55TTKHMT– Environmental Science – HUS

Mitigation ratio

 Prof. Christopher B. Craff, Wetlands: Biology and Regulation, Indiana University, 2011.
 Mark M. Brinson, Richard D. Rheinhardt, F. Richard Hauer, Lyndon C. Lee, Wade L. Nutter, R. Daniel Smith,
Dennis Whigham, A Guidebook for Application of Hydrogeomorphic Assessments to Riverine Wetlands, 1995.
 The Regional Training Course on Sustainable Use and Management of Coastal Wetlands, Mahidol University,
Thailand; A successful case in wetland resource management: Xuan Thuy National Park, Vietnam, 2007.

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