Construction Site Erosion Control Site...
Transcript of Construction Site Erosion Control Site...
Construction Site Erosion Control
Site: New Residence Halls for the Board of Trustees of the University of Alabama,
Tuscaloosa
Location North of UA Campus: North of 2nd Street (north of AIME and Bevill Buildings), east of
McCorvey Drive, west of Hackberry Lane and south of Rose Towers
Map 1: Topographic map showing the site location and surrounding receiving waters
Site Description The site has an area of about 15 acres including the adjacent roads.
The pre-development terrain is sloped from 2nd Street (~220 ft) toward the former
Rose Towers Pond (175 ft) which used to be located at the north of the site. The level
difference between AIME and Bevill buildings and the Rose Towers parking lot is about 35 ft
There are two unnamed creeks near the site. One was connected the Rose Tower Pond
and the other one is drained in Black Warrior River.
The pond was drained in order to facilitate the construction. The empty space was
raised with 10-25 feet and aligned with the Rose Towers parking lot. 60 inches decreasing to
18 inches pipes are used to collect the creek water and stormwater and to transport it to a new
detention pond that was excavated close to McCorvey Rd. at the entrance into Rose Towers
parking lot.
Clearing and grubbing began on April 16, 2004. The present stage of the construction
is of terrain leveling and demolition (already completed), installation of the sewer lines,
stormwater pipes, electrical lines, and building and roadways paths (completed). Foundation
of building 3 is completed and the work at building 1 will start soon.
Plan Development
Step 1: Data Collection
A) Topography: The general topographic map showing the site location and surrounding
receiving waters was downloaded from TerraServer (http://terraservice.net/)
B) Small scale (1 ft) contour maps showing the predevelopment and final grading plans
were supplied in the blueprints secured from Almon Associates & WSV Arhitects
(Maps 3 & 4).
C) A description of the planned project activities for the site was obtained. The original
schedule for phases is not possible to be respected because of the bad water and heavy
rains that affected Tuscaloosa in the months of May and June.
D) Drainage Patterns: Flow patterns were drawn on the above maps (Maps 5 & 6). In
order to calculate the peak flow hydrograph, a representative watershed was delineated
(the same area for pre and post development) and used for this calculation (see Excel
table)
E) Soils: Soil Data was obtained from the Tuscaloosa County Soil Survey. The soil type
was determined to be Smithdale with K values of 0.28 for depths 0-5 inches and 0.24
for depths 5-42 inches. The soil map has been included in the appendix.
F) Ground Cover: Ground Cover prior to construction was established from aerial
photography (http://terraservice.net/). The ground cover consisted of vegetation (grass
covered slopes and patches of pine) and previously built constructions such as
Riverside Pool, Riverside Parking Lot, Riverside Theater and Bevill Parking Lot (see
aero photograph)
G) Adjacent Areas: The construction site is located at UA Campus among 2nd Street
(south), Rose Towers Dorms (north), Hackberry Lane (East) and McCorvey Drive
(West). To the south are AIME and Bevill buildings with their parking lots and to the
North are Rose Towers Dorms with the adjacent parking lots.
Step 2: Data Analysis
A) Topography: The primary topographic considerations are slope steepness and length.
Four areas of similar steepness were determined:
0 – 2% - Low erosion hazard potential
2 – 5% - Moderate erosion hazard potential
5 – 10% and > 10% - High erosion hazard potential
Analysis of the contour maps, both pre and post development, show that the slopes fit
into the medium and high erosion hazard category. Parts of the site (East and North
Parking lots) with low erosion potential due to the slopes fit into the high erosion
hazard category because of their slope lengths (> 300 ft)
The construction site was divided into five zones based on location and similar slope
characteristics for easy use of RUSLE calculation (Map 7). These areas are detailed in
the RUSLE calculation and slope protection section of this paper.
B) Drainage Patterns: This site is located on a previously constructed area which
contained some inlets and drainage pathways. This means that the grading will alter
the existing drainage pathways.
C) Soil: The soil at the site was identified as Smithdale soil (#33) and hydrologic class
“B”. This soil has large amounts of sand and silt in the underlying layers. The soil is
capable of infiltrating 0.6-6.0 in/hr of water depending on the ambient conditions. The
K factor is 0.28 at 0-5 inches and 42 – 72 inches and 0.24 at 5- 42 inches.
D) Ground Cover: Due to the size of the new buildings, none of the trees or the grass on
the property could be left, and a majority of all ground cover was excavated during the
grading process.
E) Adjacent Areas: The potential impact to and from the adjacent areas of the site are
minimal. The site is surrounded by road with an existing drainage system.
F) The basic erosion calculation for the site area (post development) has been completed
using RUSLE formula. A comparison among the site with no cover (C=1), the site
with straw cover (Table 3-12, pag 24, Module 3) and the site with vegetative cover
(C=0.01) has been performed. The calculation shows that the total erosion per unit
area significantly decreases from 279 tons to 44 tons respectively 2.25 tons when the
soil is covered with vegetation.
G) The TR-55 software was used to calculate peak flow rates for the 1, 5, 10 and 25 year
storm for that site area (see attached Excel Calculation and the hydrographs)
Step 3: Facility Plan Development
A) Fit Development to Terrain: The site development does not employ the previously
occupied foundations. Building 3 is on the top of the former pond and Pool parking
lot, whereas buildings 2, 3 and 4 occupy parts of the former Swimming Pool and
Amphitheater.
B) Confine construction activities to the least critical areas: As mentioned above, the
topography and soil types of the site indicated that all slopes were “Medium and High
Erosion Hazard”
C) Cluster buildings together: Due to the nature of the project all buildings are joined
together. The office section, the storage and parking lot are close to the access road.
D) Minimize impervious areas: The plans call for one parking/storage lot and two
adjacent roads already in place. No access road is necessary. The paved area is about 3
acre and the construction area is about 12 acres.
E) Utilize the natural drainage system: The natural drainage system at the site was not
preserved and it was replaced with storm sewers. The use of urban development
previously developed is implemented.
Step 4: Planning for Erosion and Sediment Control
A) Divide the site into drainage areas: The site was divided into five regions based on
their relative slopes and location (see map in appendix). Zone 1 is located at the north
of the AIME building. It is a flat area (0.5%) that will accommodate buildings number
2, 4 and 1. Zone 2 is located to the north of the site. It is also a flat area (0.57%) and
accommodates building 3 and the North Parking Lot which is now used as a
parking/storage area. Zone 3 located to the east of the site, contains the East Parking
Lot. It is also a flat area (1.27%). Zone 4 which is the new Hackberry Lane and part of
McCorvey Drive has an average slope of 6%. In the middle of the site, between the
buildings, is located zone 5. It is a zone with steep slopes (average slope of 23%) and
high erosion potential.
B) Determine the limits of clearing and grading: All site area (15 acres) needed clearing
and grading in order to accommodate the construction.
C) Selection of erosion and sediment control measures: Erosion Control Measures to be
implemented [Map 2: Erosion Control Plan]
1. Silt / Filter Fabric Fences (type “A”)
2. Rip Rap Barriers
3. Straw Bale Barriers – around drainage inlets that drains an area with slopes < 5 %
4. Gravel and wire mesh filter around the inlets where heavy concentration of flow is
expected
5. Sand bags at inlets
6. Slope protection (roughening)
7. Slope Protection Grasses (Seed / Sod) and mulching (final erosion control phase)
A small sediment pond was designed: Dead Storage = 12 ft
Necessary Depth to prevent Scour = 3 ft
Live Storage = 0.5 ft
Emergency Spillway
Outfall device: rectangular weir (depth = 0.5 ft, length = 16 ft)
Emergency spillway: rectangular weir (depth = 2 ft, length = 9 ft)
No need for diversion channel
D) Management Controls:
1. Minimize Upslope Contributions: The site is surrounded by road with a pre-
existent drainage system, therefore is no need for a diversion channel.
2. Down slope Controls: Filter fencing will be applied around the site perimeter
to ensure that sediment is unable to leave the site. Inlet protections will be
assembled, where appropriate.
3. Protect Disturbed Areas: The site was graded in stages (zone 2, zone 1 and 5,
zone 3, and finally zone 4) so that the appropriate stabilization measures
(Seeding, sodding, mulching, and gravel) may be applied. Unfortunately, the
grading period was between May and August and the seeding was not possible.
4. Vegetative controls: Temporary seeding and mulching will be use for slope
protection. The permanent stabilization measure by sodding and seeding will
be applied.
The following sections outline the steps / calculations performed in designing an
effective erosion control plan for the” New Residence Halls for the Board of Trustees
of the University of Alabama” site
A. Planned Project Activities
The work at the Residence Halls began on April 16, 2004 and is stipulated to be done
until June 2005. There are two contractors working on the site. One is doing the civil work
planned in four phases and the second one is doing the construction work (schedule in four
phases, too).
Sequences of Construction:
Site work:
Phase 1: Preparation of Building Pads including secure of all the necessary permits for
performing work as part of the site contract, place all erosion control devices necessary for
current and upcoming construction necessities, reconstruct detention basin for stormwater
runoff from the site, began demolition, clearing and grubbing activities, start to construct
sanitary and storm sewer lines, etc
Phase 2: East Parking Lot and Hackberry Lane Realignment including place of temporary
construction fences, complete clearing and grubbing, construct sanitary, storm sewer lines and
water distribution lines for this part of the site.
Phase 3: Hackberry Lane and McCorvey Road including demolition of the Bevill parking lot,
complete grading activities, continue with storm sewer construction, modify McCorvey Road.
Phase 4: Pres Pavillion Road, North Parking Lot and Miscellaneous including placing
temporary construction fences around North parking Lot area, place curb and gutters for
buildings 1 thru 4, complete construction of access roads for buildings 1 thru 4, etc.
Due to the unpredictable weather and heavy rains, the work phases were not able to be
followed.
Building work:
Phase 1: Building 3 foundation and pluming. At this point start groundwork for building 1.
Phase 2: Building 1
Phase 3: Building 2
Phase 4: Buildings 4 and 5
The landscaping phase included the installation of several erosion control devices (erosion
control mats, protected slopes, etc.).
B. Description of the site soils
According with the “Soil Survey of Tuscaloosa County, Alabama”, the site soil is type
33 (Smithdale) and hydrologic class “B”
Hydrologic soil groups are used to estimate runoff from precipitation, when soil is not
protected by vegetation. Group “B” soils have moderate infiltration rates when systematically
wetted and consist of moderately deep to deep, moderately well to well drained soils with
moderately fine to moderately coarser textures. They have moderate rate of water
transmission (0.15 -0.30 in/hr).
Table 1. Soil Survey Characteristics for Site Area
Soil
Name
Depth
(in)
USDA
texture
Permeability
(in/hr)
AWC
(in/hr)
Erosion
Factor (K)
Organic
Matter (%)
33
Smithdale
0.5 – 2.0
0 – 5 Fine sandy loam 2.0-6.0
0.14-
0.16 0.28
5 – 42
Clay loam,
sandy clay loam,
loam
0.6-2.0 0.15-
0.17 0.24
42 – 72
Loam, sandy
loam 2.0-6.0
0.14-
0.16 0.28
Smithdale soil has slopes ranging from 6 to 15 %. Permeability and available water
capacity are moderate. The soil is low in natural fertility and has a low content of organic
matter. The bedrock bed is greater then 60 in deep.
Water features for Smithdale soil: Flooding – none and Water table depth > 6.0 ft
According with the TTL, Inc (Technology and Tradition) whom perform the soil tests
at the site, there are several on-site types of soil:
• Between Pool and Amphitheater the soil is Yellowish-Red Silty Clayey Sand;
• The Amphitheater - the soil is Reddish-Brown Silty Clayey;
• On the parking lot between the Pool and the Rose Towers Pond the soil is Tan Silty
Sand with gravel;
• North of Bevill parking lot - the soil is Tan Sandy Clay;
• The former swimming pool (manhole L5) - the soil is Tan and Red Siltty Sand with
Trace;
Table 2. Particle-size Distribution for Smithdale Soil
Sample
number
Depth
(in) Horizon
Clay
(<0.002mm)
Silt
(0.05-
0.002mm)
Sand
(2.0-
0.05mm)
Caution exchange
Capacity
(meq/100mL)
S77AL-125-
11-1 0-5 Ap 2.8 29.2 68.0 3.65
S77AL-125-
11-2 5-20 B21t 22.2 34.9 42.9 9.02
S77AL-125-
11-3 20-42 B22t 20.2 29.1 50.7 5.36
S77AL-125-
11-4 42-52
B23t
12.3 26.5 61.2 4.06
S77AL-125-
11-5 52-72 B24t 21.2 12.8 66.0 3.52
Table 3. Building Site Development Limitations
Soil Shallow
Excavations
Local Streets and
Roads
Dwellings with
Basements
Lawns and
Landscaping
33(Smithdale) Moderate (slope) Moderate (slope) Moderate (slope) Moderate (slope)
The on site soil coming from the former swimming pool was used to fill the part of
Rose Towers Pond that was drained out and its adjacent creek. On top of the former creek and
pond will lay out the building no. 3.
Under the buildings no. 1 and no. 2 there is some off- site soil (Red Clayey Sand)
about 60% and some on site soil (Brown Clayey Sand w/ Gravel) about 40 % coming from a
cut beneath Hackberry Lane.
C. RUSLE Calculation
The site area was divided in five zones (based on slope) for easy calculation (see appendix).
The areas have the following general characteristics:
Zone ID Average Slope (%) Average Slope Length (ft) Average LS Factor
Zone 1 0.5 164 0.42
Zone 2 0.57 192 0.22
Zone 3 1.27 283 0.24
Zone 4 6.13 106 0.62
Zone 5 23.4 24 1.35
A comparison among the site with no protective cover (C=1), the site with straw cover (Table
3-12, pag 24, Module 3) and the site with vegetative cover (C=0.01) has been performed. The
calculation shows that the total erosion per unit area significantly decreases from 279.35 tons
to 44.43 tons respectively 2.25 tons when the soil is covered with vegetation.
D. TR- 55 Calculations
In order to calculate the peak flow rate using TR- 55, one representative watershed
was selected. This restriction was imposed due to the size of the construction site, the
complexity of the watershed and multiple sub watersheds.
TR-55 was run for the watershed characteristics in the pre-development stage and then in the
post development stage. The peak flow rates for 1, 5, 10 and 25 year storms were calculated
and their hydrographs were built (see attachment).
The pre-development watershed was divided in two sub watersheds: parking lot and grass
area, whereas the post development watershed has a building and a grass area. The parking lot
was pretty much replaced by the building. The upper part of the pond was filled with soil
from the former swimming pool and the terrain will be seed with grass.
Because of these changes, the post development watershed will have about 1 acre more grass
area.
E. Slope Stability
Introduction
Slope stability design will be performed for each zone area. There are five zones at the
site and two of them need temporary slope protection. Two methods were use in slope
stability design: the first one is step by step manual calculation and the second one is
performed with the help of the North American Green – Erosion Control Design Software
version 4.3.
Method (1) – manual step by step design
All the necessary calculations are attached (Excel sheet)
General information about the site soil
Soil type = sandy loam
K = 0.28
n (for the soil) = 0.02
Allowable shear stress for the soil = 0.075 lb/ft2
Information about the site
Q25 = 25.13 cfs (calculated using WinTR-55 for a representative site area)
n (for the grass prairie cover) = 0.15
The site is divided in 5 zones with different slopes (%)
For zone 1, 2 and 3 (flat terrain, slope 0 – 2%) the maximum calculated C is greater then 1.
Therefore, the entire erosion control mats available from the North American Green products
are suitable.
Zones 1 and 2 are located between the buildings and need a permanent vegetative cover. A
good vegetation density (75-95 % cover) is appropriated. S250, C350, P300 and P550 erosion
control mats are all suitable vegetated blankets.
Zone 3 (flat terrain, a future road and parking lot) and zone 4 (steep terrain, future road) need
a temporary protection mat for about 12 months, beginning with April 2004. The most
suitable temporary mats are S75BN or S150BN.
Zone 5 (steep terrain, located between the buildings) needs a temporary mat during the
construction and then a permanent vegetated cover. An excellent vegetation cover density is
most appropriated. The best vegetated cover available among North American Green products
is P550.
Method (2) – North American Green Software for Slope Stability
Product / SF Zone 1 Zone 2 Zone 3 Zone 4 Zone 5
P550 (perm) 99.9 99.9 n/a n/a 24
P300 (perm) 99.9 99.9 n/a n/a 18
C350 (perm) 99.9 99.9 n/a n/a 18
S250 (perm) 99.9 99.9 n/a n/a 14.4
S75 (temp) 30 26 10 2.4 5
S75BN (temp) 30 26 10 2.4 5
S150 (temp) 48 42 16.3 3.8 13.2
S150BN (temp) 99.9 99.9 97.8 22.8 89.9
Slopes belonging to Zone 1 and zone 2 (flat terrain, located between buildings) need
temporary protection during the construction phases and permanent vegetated cover
protection for esthetic proposes. All available North American Green products (S250, C350,
P300 and P550) are suitable for permanent protection. They have a safety factor of 99.9.
Among the completely unvegetated blankets with a functional longevity of 12 months (S75,
S75BN, S150, S150BN) the most appropriate is the cover type S150BN (SF = 99.9).
Slopes from Zone 3 (flat terrain, a future road and parking lot) and zone 4 (steep terrain,
future road) need only temporary protection. All the completely unvegetated blankets with a
functional longevity of 12 months are suitable. The cover type S150BN is the most
appropriate for temporary protection for both zones.
Zone 5 slopes are very steep and are located between the buildings. They need a temporary
protective mat during the construction phases and a permanent vegetated mat for protection
and for esthetic proposes.
For temporary protection the S150BN product is the most suitable with a SF of 89.9.
An excellent vegetation cover density is most appropriated. Among the available products
(S250, C350, P300 and P550) the most suitable permanent cover is P550 with a SF of 24.
F. Temporary Detention Pond Design
New Residence Halls and a Community Building at UA Campus have about 15 acres. The
detention pond has to be design to accommodate the runoff from the buildings area, two
parking lots and the adjacent streets.
The drainage area is mostly active construction site, but some paved area (roads and
parking lots) drain to the pond. The actual paved area is about 3 acres. A pond size factor of
3 % for paved area and 1.5 % for construction area are taken.
Area (acre)
Pond Size
Factor
Pond Surface (acre)
Water
Quality factor
Pond WQ Volume (ac-in)
Pond WQ Volume (ac-ft)
Construction 12 1.5 % 0.18 0.6 7.2 0.60
Paved 3 3.0 % 0.09 1.1 3.3 0.28
Total 15 0.27 10.5 0.88
The pond surface area during dry weather should be a minimum of 0.27 acre, or about 1.8
% of the drainage area. The total water quality volume of the pond is 10.5 acre-in or 0.88
acre-ft.
Detention Pond Slope Calculation
Required pond side slope should be between 4% and 10%.
The pond Volume (ac- ft) = 0.88 and Bottom area (ac) = 0.27
Using the trapezoidal method, the multiplier used to calculate top surface area is:
X multiplier = [(2 * Volume) / (bottom area * depth)] – 1
Assumed depth X Multiplier Top Area Top Radius Bottom Radius Slope (ft) (acre) (ft) (ft) (%) 0.46 13.17 3.56 1.06 0.92 3.22 0.48 12.58 3.40 1.04 0.92 4.04 0.5 12.04 3.25 1.02 0.92 5.20
0.52 11.54 3.11 1.00 0.92 6.96 0.54 11.07 2.99 0.98 0.92 9.92 0.55 10.85 2.93 0.97 0.92 12.29
Outfall control device:
The outlet device for the control of 5 mm particles was calculated using Table 6-3 and
the method of interpolation. The most appropriate outfall device for this pond will be a
rectangular weir with a head of 0.5 ft and a length of 16 ft.
Head (ft) Flow (cfs) Storage (ac-ft) Req. Area (ac)
0.5 18.5 0.88 3.25
Peak flow calculation
Using TR-55 the peak flow for a 50 year storm for a representative area of 7.12 acre
was calculated (Q = 29 cfs). The Q50 for the entire area was calculated using the extrapolation
method and is about 101 cfs. The output TR-55 form and the hydrograph are attached.
Particle settling velocity:
v = Q / A = 101 cfs / 0.88 acre = 101 cfs / 38333 ft2
v = 0.0026 ft/s = 2.6E-3 ft/s for 5 mm particle
Sacrificial Storage Calculation
In order to minimize scour, 3 ft of standing water is needed above the maximum
sediment depth. In addition, sacrificial storage must be provided in the pond. To estimate this
storage, the sediment load from the watershed must be calculated. RUSLE has to be used to
find the erosion rate. The erosion rate has been found to be 279 tons /acre-yr (from
assignment nr.1)
The construction area is about 12 acre and the construction period is about 1.5 year.
The total sediment loss is estimated to be 5022 tons = 5122 yd3 for sandy loam soil at the site.
5122 yd3 = 138306 ft3 = 3.18 ac-ft
The pond has a bottom width of 0.27 acres. So, the sacrificial storage will have a
height of 3.18 / 0.27 = 11.8 ft
Emergency Spillway
For the emergency spillway I chose a rectangular weir. The length (ft) of the
emergency spillway for a given stage (Hw) is Lw = qo / (3.2*Hw1.5)
qo = Q50 - Q rec. weir = 101 cfs – 18.5 cfs = 82.5 cfs
Assume Hw = 1 ft and qo = 82.5 cfs
Lw = 82.5 cfs / (3.2*11.5) = 26 ft
Assume Hw = 2 ft
Lw = 82.5 cfs / (3.2*21.5) = 9 ft (better design)
Therefore, the emergency spillway will be rectangular with a length of 9 ft and a depth of 2
ft.
G. Filter Fences
Silt fences are temporary erosion control items, constructed of wood or steel fence
posts and a suitable permeable geotextile. They retain suspended solids by acting as a filter
and also slow runoff velocity giving sediments time to settle. Silt fences are most effective
when areas draining to the barrier are 2.5 acres or less.
Silt fences are preferable to hay bales because they can trap a much higher percentage
of suspended solids; they have a longer life and are cost effective.
Design Criteria
• Silt fences shall be in place prior to any construction operation
• Silt fences are limited to sheet or overland flow
• The drainage area behind the silt fence should not exceed 0.25 acre per 100 linear feet
of silt fence for non-reinforced fence and 0.5 acre per 100 feet of wire reinforced fence
• They should be placed opposite erodable areas (newly graded slopes, adjacent to
channels and streams); across a flat are they shall be constructed in a shape of
horseshoe to facilitate sedimentation
• Silt fences have to be places along edge of clearing limits to allow room for a back up
fence if first became full
• Minimum of 24 to 36 inches of support are required above the ground in order to have
effective sediment control
Filter fences Recommendation for “New Residence Halls” Construction Site
The site is divided in five different areas based on slopes and location on the site.
ID Length (ft) Slope (%) Slope fraction
Zone 1 164 0.5 200 : 1
Zone 2 192 0.6 167 :1
Zone 3 283 1.3 77 : 1
Zone 4 106 6.1 16 : 1
Zone 5 24 23 4 : 1
Based on Table SB-2 (Slope Limitations for Silt Fence) from “2003 Alabama
Handbook for Erosion Control” and the site slopes, I have decided to provide a type “A”
fence (36” wide with wire reinforcements) for zone3, 4 and 5 and around the detention pond.
A type “B” silt fence and straw bales are required for inlets protection where the inlets drain a
relatively flat area (less than 5%).
Zone 3 is a relatively flat area, bordered by a small section of high slopes. Therefore,
the fences must be installed along this edge.
Zone 4 has an average slope of 6% and an average slope length of 106 ft. However,
there is a long portion of high slopes adjacent to the roadway that required silt fence presence.
For that reason, the silt fences have to be installed at the border between hilly area and road.
Zone 5 is the most critical zone at the site. It has high slopes (23 % average) with
relatively short lengths. Inside the high slopes area, there is a flat area (zone 2) containing
several inlets. The silt fences have to be installed around the flat area to prevent sediment
coming from zone 5 and entering zone 2. The maximum slope length above the fence must
not exceed 15 ft, but zone 5 has a slope length average of 24 ft. As a result, the fence must be
located at the middle of the slope.
Detention pond must be surrounded by silt fences. Rip-rap barriers at the pond will
definitely help trapped the sediment.
Maintenance Issues
• Sediment deposits should be replaced after each heavy storm event. They must
be removed when deposits reach one-half the height of the barrier
• Fences should be inspected immediately after rainfall. Periodical inspection are
also required
Appendix
1. Site Location at UA Campus
2. Site Aero photograph
3. Soil Map
4. Topo Map – Predevelopment Topography
5. Grading Plan – Post Development Topography
6. Predevelopment Topography – Drainage Path
7. Post Development Topography – Drainage Path
8. Zone Map
9. Watershed Area
10. Erosion Control –Silt Fence Location