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