Wetlands Assimilation
St. Tammany Neighborwoods Project
Mandeville Neighborwoods Overview
Working for the North Shore communities and citizens throughout Louisiana to show that well designed urban natural areas within residential subdivisions can be used to filter and clean storm water prior to its release into freshwater bayous and eventually into lakes. The goal is to show that these neighborwoods can be planned for both recreation and environmental enhancement that focus on storm water control.
The objective is to:
-
Demonstrate that storm water control features for subdivisions can be installed to
improve local water quality.
- Provide a select list of Louisiana water garden plants.
- Show garden design templates which residents and subdivision developers can follow to construct bio-retention basins, vernal ponds, rain gardens, water gardens, buffer edges iris patches, ditch gardens and sunken lawns.
- Provide special educational activities for children.
- Introduce specifications and building standards for natural landscaping for contractors, developers, landscape architects, architects and engineers.
What can you gain, from this?
- Learn how "green space" (i.e. an undeveloped parcel of land) can be used to manage urban storm water runoff and to also serve as a neighborhood nature site.
- The project site also demonstrates several different types of "wetland theme gardens", which emphasize a naturalized approach for mitigating storm water that is cost effective and complimentary to the landscape.
Geography - LIDAR Data
Site elevations are shown on this LIDAR (Light Detection And Ranging) image of the site. For information on LIDAR see: lidar Elevation contours are shown in red and yellow lines. For more detail download LIDAR pdf file LIDAR (PDF).
Water Quality Data
Water quality samples were collected quarterly to characterize stormwater runoff at the
input and output locations. These samples were analyzed for TKN, N02+NO3, NH4, TP, PO4 and
TSS. At the same time that discrete water samples were taken, dissolved oxygen, water temperature,
conductivity and salinity were measured in situ with a handheld discrete probe.
Dissolved oxygen, water temperature, conductivity and salinity were measured in situ
using a Yellow Springs Instrument Co. meter (i.e. YSI-85, www.ysi.com). The measuring probe of the
meter was rinsed with distilled water before and after use. Discrete water quality samples were
collected in acid washed 1 L plastic bottles 5 to 10 cm below the water surface with effort taken not to
stir bottom sediments or include any film that may be present on water surface. The samples were
immediately acidified except for TSS, stored on ice (4<C) and transported to the laboratory within 8 hour
of collection. Analytical & Environmental Testing, Inc., using EPA methods, carried out the nutrient
analysis of the samples.
Nitrate+nitrite (NO3+NO2-N or NOx) concentrations ranged from 1.31 to 0.05 mg/L. Ammonium
(NHx) levels were also below detection limits (<1.0mg/L) at all times, except at the Far site during
May 2007 when levels were 1.4 mg/L (Table 1). Due to lab error, total Kjeldahl nitrogen (TKN) was not
measured in May 2007. TKN ranged from 1.7 to 1.4 mg/L during the other two sampling periods.
Ortho-phosphate (PO4) ranged from below detection limit (<0.02 mg/L) to 0.17 mg/L (found at Near site
during May). Total phosphorus (TP) ranged from 0.497 to 0.227 mg/L. Total suspended sediments
(TSS) ranged from below detection limit (<0.04 mg/L) to 6.8 to 23.6 mg/L. (Table 1)
NOx (mg/L) |
NHx (mg/L) |
TKN (mg/L) |
PO4 (mg/L) |
TP (mg/L) |
TSS (mg/L) |
|
Near 10/25/06 |
1.31 |
<0.1 |
1.4 |
<0.02 |
0.281 |
8 |
3/15/07 |
0.13 |
<0.1 |
n.a. |
0.17 |
0.389 |
6.8 |
7/19/07 |
0.16 |
<0.1 |
1.4 |
0.13 |
0.443 |
<4.0 |
Far |
||||||
10/25/06 |
0.36 |
<0.1 |
1.7 |
<0.02 |
0.227 |
7.6 |
3/15/07 |
0.05 |
1.4 |
n.a. |
<0.02 |
0.497 |
7.6 |
7/19/07 |
0.05 |
<1.0 |
1.4 |
0.05 |
0.335 |
23.6 |
Table 1. Nitrate+nitrite, Ammonium, Kjeldahl nitrogen, Orthophosphate, Total phosphorus, and Total suspended
Sediments levels measured at Near and Far sampling locations.
Mean dissolved oxygen levels were 2.17 ±0.79 and 2.02 ±0.65 mg/L±s.e. at the Near and
Far sites respectively (Table 2). Conductivity had a mean of 182.2±30.11 and 143.3uM±s.e. at the
Near and Far sites, respectively. Neither dissolved oxygen or conductivity were significantly different
between sites. Temperature, Salinity and pH had very similar means at both sites, with overall
respective means of 18.3<C, 0.09 ppt, 0.01 pH, again with no significant differences between sites (Table 2).
Table 2. Dissolved oxygen (D.O.), conductivity, temperature, salinity and pH at the Neighborwoods study sites.
Date |
Site |
D.O. (mg/L) |
Cond. (uM) |
Temp. < C |
Salinity (ppt) |
pH |
12/12/06 |
Near |
dry |
dry |
dry |
dry |
dry |
1/10/07 |
Near |
2.65 |
202.9 |
14.3 |
0.1 |
6.0 |
2/2/07 |
Near |
1.17 |
108.0 |
12.5 |
0.1 |
6.0 |
3/15/07 |
Near |
1.08 |
141.8 |
19.5 |
0.1 |
6.0 |
4/17/07 |
Near |
dry |
dry |
dry |
dry |
dry |
7/4/07 |
Near |
3.82 |
276.2 |
25.5 |
0.1 |
6.0 |
9/19/07 |
Near |
dry |
dry |
dry |
dry |
dry |
12/12/06 |
Far |
dry |
dry |
dry |
dry |
dry |
1/10/07 |
Far |
2.28 |
225.3 |
11.5 |
0.1 |
5.5 |
2/2/07 |
Far |
2.01 |
188.1 |
12.7 |
0.1 |
6.0 |
3/15/07 |
Far |
0.86 |
54.0 |
21.3 |
0.0 |
5.5 |
4/17/07 |
Far |
0.18 |
143.8 |
21.1 |
0.1 |
5.5 |
7/4/07 |
Far |
4.78 |
105.4 |
26.8 |
0.1 |
6.0 |
9/12/07 |
Far |
dry |
dry |
dry |
dry |
dry |
Vegetation Monitoring
A 10 x 100 m quadrate was established at each of the two study sites (Figure 1). The quadrate
was divided into three 10 x 33.3 m subplots. Two 0.25 m2 leaf litter boxes, with screened bottoms and
approximately 10 cm wide sides, were placed randomly in each subplot (six boxes per site). Leaves and
other materials collected in the boxes were gathered periodically starting October 25, 2006. We use the
term 'leaf litter' in reference to all non-woody litter including flowers, fruits, and seeds that typically account for
<10% of the non-woody litterfall total (Megonigal and Day 1988). Large stems and sticks were removed
from the litter, and the cleaned litter was dried to constant mass at 65<C and weighed. The diameter (dbh)
of all trees in the quadrates were measured above and below (»5 cm) an identification tag during the
December 2006. This method allowed measurements to be taken a safe distance from the tag's nail,
which often caused a small localized swell. We assumed that the contribution of wood from stems
<10 cm dbh and herbs was a relatively small fraction of aboveground net primary production. Tree
species composition analysis was carried out using equations 1-4. Basal area is defined as the
trunk cross-sectional area of a given species in cm2/m2.
- 1. Relative density = (individuals of a species) / (all individuals of all species)
- 2. Relative dominance = (total basal area of a species) / (total basal area of all species)
- 3. Relative frequency =(frequency value for each species) / (total freq. value for all species)
- 4. Importance Value = Relative density + Relative dominance + Relative frequency
The Near and Far sites had a total of 8 species each, with shared species being Hackberry
(Celtis occidentalis), Loblolly Pine (Pinus taeda), Pin Oak (Quercus palustris), Red Maple
(Acer rubrum), Sweetgum (Liquidambar styraciflu), and Water Oak (Quercus nigra). Additional
species at the Near site include Privet (Ligustrum sp.) and Willow Oak (Quercus phellos), and at
the Far site include American Elm (Ulmus americana) and Chestnut Oak (Quercus prinus).
The most important species, as indicated by the Importance Value, which is the sum of Relative
Density, Relative Dominance and Relative Frequency, were Loblolly Pine followed by Hackberry,
Water Oak and Pin Oak at the Near site, and Water Oak followed by Pin Oak, Loblolly Pine,
Hackberry at the Far site (Table 3).
Table 3. Tree species found at the Near and Far sites, and calculations of Relative Density,
Relative Dominance and Relative Frequency, and Importance.
Site |
SPECIES |
n |
Relative Density |
Relative Dominance |
Relative Frequency |
Importance Value |
Near |
Hackberry |
17 |
0.31 |
0.19 |
0.19 |
0.70 |
Near |
Loblolly Pine |
12 |
0.22 |
0.42 |
0.42 |
1.07 |
Near |
Pin Oak |
4 |
0.07 |
0.21 |
0.21 |
0.49 |
Near |
Privet |
2 |
0.04 |
0.00 |
0.00 |
0.05 |
Near |
Red Maple |
4 |
0.07 |
0.03 |
0.03 |
0.14 |
Near |
Sweetgum |
1 |
0.02 |
0.00 |
0.00 |
0.03 |
Near |
Water Oak |
13 |
0.24 |
0.13 |
0.13 |
0.50 |
Near |
Willow Oak |
1 |
0.02 |
0.01 |
0.01 |
0.03 |
Far |
American Elm |
2 |
0.04 |
0.01 |
0.01 |
0.06 |
Far |
Chestnut Oak |
1 |
0.02 |
0.02 |
0.02 |
0.05 |
Far |
Hackberry |
11 |
0.24 |
0.10 |
0.10 |
0.45 |
Far |
Loblolly Pine |
5 |
0.11 |
0.22 |
0.22 |
0.55 |
Far |
Pin Oak |
5 |
0.11 |
0.23 |
0.23 |
0.56 |
Far |
Red Maple |
5 |
0.11 |
0.04 |
0.04 |
0.20 |
Far |
Sweetgum |
3 |
0.07 |
0.10 |
0.10 |
0.26 |
Far |
Water Oak |
14 |
0.30 |
0.28 |
0.28 |
0.87 |
Some leaf litter boxes were destroyed by vandals during this study, notes as N/A on Table 4.
Leaf litter had cumulative sum of 352.0 g/m2 at the Near site and 291.7 g/m2 at the Far site
(Table 4), which is an indicator of ephemeral primary productivity.
Table 4. Leaf litter data at the Neighborwoods study sites. The boxes were established 10/25/06.
Site/Box |
12/12/06 |
1/10/07 |
2/2/07 |
3/15/07 |
4/18/07 |
5/18/07 |
9/19/07 |
Near1 |
22 |
14 |
20 |
6 |
6 |
6 |
9 |
Near 2 |
24 |
25 |
13 |
6 |
5 |
12 |
12 |
Near 3 |
19 |
10 |
21 |
8 |
5 |
4 |
14 |
Near 4 |
34 |
27 |
9 |
9 |
1 |
5 |
6 |
Near 5 |
24 |
21 |
17 |
1 |
19 |
3 |
3 |
Near 6 |
N/A |
N/A |
N/A |
N/A |
8 |
6 |
8 |
sum: |
123 |
97 |
80 |
30 |
44 |
36 |
52 |
g/m2: |
98.4 |
77.6 |
64.0 |
24.0 |
29.3 |
24.0 |
34.7 |
Far 1 |
10 |
N/A |
N/A |
N/A |
N/A |
N/A |
N/A |
Far 2 |
25 |
15 |
12 |
6 |
5 |
3 |
8 |
Far 3 |
19 |
24 |
18 |
12 |
3 |
4 |
10 |
Far 4 |
23 |
23 |
16 |
4 |
2 |
2 |
13 |
Far 5 |
21 |
1 |
1 |
2 |
12 |
||
Far 6 |
20 |
7 |
10 |
4 |
4 |
5 |
9 |
sum: |
118 |
69 |
56 |
27 |
15 |
16 |
52 |
g/m2: |
78.7 |
69.0 |
56.0 |
21.6 |
12.0 |
12.8 |
41.6 |
Soils
Soil bulk density measurements were taken from the study sites using a 10 cm long 2.5
cm diameter 120 cm3 syringe with the top cut off. This allowed the application of suction as the core
was taken, greatly reducing compaction. The sample was sliced into 2 cm sections, dried at 100 <C
for 24 hours, and weighed. Bulk density was calculated in g/cm3 units.
Bulk density of soils at the Near site was lower (0.21±0.024 g/cm3±s.e.) than the Far site
(0.30±0.039 g/cm3±s.e.), but not statistically significant (Table 5).
Site/rep |
Sample Volume (cm3) |
Dry Weight (g) |
Bulk Density (g/cm3) |
Near1 |
65 |
16.18 |
0.25 |
Near2 |
70 |
14.14 |
0.20 |
Near3 |
75 |
12.34 |
0.16 |
Far1 |
75 |
19.1 |
0.25 |
Far2 |
70 |
17.14 |
0.24 |
Far3 |
80 |
29.44 |
0.37 |
Water Flux
Prior to site construction, water flow monitoring instrumentation was installed at
the current inlet and outlet of the project site (Figure 1). The location of water flux monitoring
stations is intended to demonstrate and quantify the efficiency of the Mandeville
Neighborwoods Project in reducing peak flow and total water volume of stormwater runoff.
Water volume was measured every 5 minutes using an ISCO 4230 Bubbler Flow
Meters. A stage-discharge relationship was established at each flow measurement location
to determine flow based on Manning's equation, and verified by discrete in-stream velocity
measurements. The resulting data was averaged over hourly intervals.
Water flow meters were deployed over several days from July 5-7, 2007. Water
flow ranged from 0.01 to 0.12 cubic meters per second (CMS). The two sites mirrored each
other, as would be expected give the ditch drainage of the pre-constructed project area. Due
to time constraints, post-construction monitoring has yet to be carried out.
Figure 2. Pre-construction water flow at the Near (blue) and Far (red) sites using
averaged hourly means. Flow is expressed in cubic meters per second (CMS).
Location
The site lies between the West Causeway Approach Rd. and State Hwy 22 and the fresh
water marsh on the northen rim of Lake Pontchartrain. Approximately 25 acres of wooded
bottomland hardwood located between several Mandeville subdivisions. Those subdivisions
include Beau Rivage, Cedarwood, Chateau Village, Woodridge and Beau West. The site may
be accessed from several surrounding streets - Heavens Dr, Garden Ave. and Dorando Drive.
There is an existing oxidation pond on the site that is surrounded by an earthen berm.