Construction Site Erosion Control

Transcription

1 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

2 Site Description The site has an area of about 15 acres including the adjacent roads. The pre-development terrain is sloped from 2 nd 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 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, 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 ( 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.

3 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 ( 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 2 nd 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)

4 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 in/hr of water depending on the ambient conditions. The K factor is 0.28 at 0-5 inches and 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)

5 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

6 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 preexistent drainage system, therefore is no need for a diversion channel.

7 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 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

8 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

9 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 ( in/hr). Table 1. Soil Survey Characteristics for Site Area Soil Depth USDA Permeability AWC Erosion Organic Name (in) texture (in/hr) (in/hr) Factor (K) Matter (%) Smithdale 0 5 Fine sandy loam Clay loam, sandy clay loam, loam Loam, sandy loam 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;

10 Table 2. Particle-size Distribution for Smithdale Soil Sample number Depth (in) Horizon Clay (<0.002mm) Silt ( mm) Sand ( mm) Caution exchange Capacity (meq/100ml) S77AL S77AL S77AL Ap B21t B22t S77AL B23t S77AL B24t Table 3. Building Site Development Limitations Shallow Local Streets and Dwellings with Lawns and Soil Excavations Roads Basements 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.

11 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 Zone Zone Zone Zone 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 tons to 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.

12 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 = lb/ft 2 Information about the site Q 25 = 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.

13 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 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) n/a n/a 24 P300 (perm) n/a n/a 18 C350 (perm) n/a n/a 18 S250 (perm) n/a n/a 14.4 S75 (temp) S75BN (temp) S150 (temp) S150BN (temp) 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 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

14 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 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 % Paved % Total 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.

15 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) (%) 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) 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 Q 50 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 / ft 2

16 v = 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 yd 3 for sandy loam soil at the site yd 3 = ft 3 = 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 = q o / (3.2*Hw 1.5 ) q o = Q 50 - Q rec. weir = 101 cfs 18.5 cfs = 82.5 cfs Assume Hw = 1 ft and q o = 82.5 cfs Lw = 82.5 cfs / (3.2*1 1.5 ) = 26 ft Assume Hw = 2 ft Lw = 82.5 cfs / (3.2*2 1.5 ) = 9 ft (better design) Therefore, the emergency spillway will be rectangular with a length of 9 ft and a depth of 2 ft.

17 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 Zone :1 Zone : 1 Zone : 1 Zone : 1

18 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

19 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