Home Sustainable Land Development Today February 2005

Dirty Deeds Done Dirt Cheap

Written by Daniel Holzman

Wednesday, 02 February 2005

Planning and coordination during construction saves everyone money. Most erosion during the land development process occurs during the initial construction phase. Erosion control during construction (ECDC) typically results when the soil is exposed, the drainage system is not yet operational, and limited protection may be in place. Although most site plans are required by municipal regulations to make provisions for interim erosion control measures, there may be little incentive for contractors to implement what is commonly regarded as an uncompensated expense, especially when enforcement is limited or nonexistent. The result can be catastrophic flooding and erosion damage to abutters. Relatively simple steps can be taken to minimize erosion, and to limit the impact of the erosion that cannot be prevented. When It Goes Wrong ECDC is generally caused by wind, water and gravity, often acting in conjunction. Wind blows around paper, debris, fine sand and silt. Fine-grained soils are highly vulnerable before vegetation is established, and can blow into infiltration structures, thereby damaging the infiltration capacity. Water is a serious threat, especially on newly exposed slopes. A slope that has lost substantial amounts of topsoil after only a few heavy rainstorms demonstrates the damage that can occur despite the installation of two rows of silt fence, and an attempt to hydroseed the slope. Enough silt washed down the slope to block a culvert. Unvegetated slopes are particularly vulnerable to saturation and mass soil movement (slumping). Serious mass movement of a steep, unvegetated slope consisting primarily of dense to medium-dense, silty sand is possible. Sprayed on mulch was added after tons of silt had washed down the slope and damaged a subdivision at the bottom of the hill. Cold weather issues are particularly important in northern states and high altitudes, with limited growing seasons. Cold weather erosion control issues include:     1. Soil stabilization using vegetation is limited to the growing season, which may be as short as May 1st to October 1st. If work proceeds out of season, expensive stabilization using fabric may be necessary.     2. Spring often produces the most difficult conditions, including high groundwater levels, maximum streamflow, and saturated soils. Stormwater control during spring runoff generally places maximum stress on the system.     3. Design of infiltration practices must recognize the substantial variation in soil percolation rates during the year. Soils that are permeable in the summer may exhibit near zero permeability when saturated in the spring, when maximum infiltration is necessary to control runoff.     4. Frost depths up to 5 feet are possible, which can radically alter the properties of soils with respect to runoff potential, slope stability, and percolation rates. Stormwater systems need to function effectively 12 months out of the year, under very different weather and soil conditions. Case Study The following case history involves a forensic examination of the failure of a stormwater management and erosion control system at the Cote d’Or subdivision, a 42-lot, upscale development in Bedford, New Hampshire. Design began in 1999, and construction of the roads got underway in 2001. The subdivision covers approximately 105 acres, with lot size ranging from approximately 2 acres up to a maximum of almost 5 acres. The subdivision is constructed over two drumlins, and includes natural slopes up to approximately 35 degrees. The subdivision is part of a watershed of approximately 550 acres that drains through Whippoorwill Lane, an older subdivision located about 120 feet lower in elevation than the majority of Cote d’Or. All but 5 acres of Cote d’Or drains through the Whippoorwill Lane neighborhood before draining into Bowman Brook, after passing through a dammed pond used as a fire suppression water supply by the town. The soils in the subdivision include silty loams at the tops of the hills, which typically have high groundwater tables and low hydraulic conductivity. The majority of soils within the subdivision are hydrologic group B and C, with a few pockets of group D soils in lower lying areas, and several wetlands between the drumlins. Prior to development of the subdivision, the land was heavily wooded, including a mix of deciduous and coniferous trees. The predevelopment runoff curve number for most of the developed area was estimated to be between 65 and 70. The designer’s hydrologic analysis of the proposed subdivision included several key assumptions that were not realized in practice:     1. The estimated impermeable area per lot (exclusive of the public road system) was assumed to be approximately 4,000 square feet. Due to the large size of the majority of houses, the actual impermeable area per lot was between 6,000 and 8,000 square feet.     2. The design assumed that substantial storage would be realized from ponding upstream of several culvert crossings of the main access road. Due to a variety of factors, some of the culverts were installed lower than anticipated, and very little storage was actually realized. No detention ponds or other artificially created stormwater storage basins were called for in the plan.     3. The design plans specified a maximum of two acres of disturbed earth at any given time within the subdivision. Unfortunately, there were no local procedures in place to enforce this restriction, and up to 15 acres of disturbed soil were present during maximum build-out periods. The summer of 2003 was significantly wetter than usual, including two short but intense storms in August. The storm of August 4, 2003 created flooding conditions at Whippoorwill Lane, including deposition of significant amounts of silt on the main subdivision road. Damage to septic systems, wells, basements, the fire pond, and the road itself resulted. Flooding of this magnitude had apparently not been experienced in the close to 50-year history of Whippoorwill Lane, and occurred after a storm estimated to have been less than two inches of rain in one hour. During the ensuing investigation into the causes of the flooding, several factors became apparent:     1. The lots immediately uphill of Whippoorwill Lane had been severely disturbed, with minimal installation of silt fencing and hay bales to retard flood flow and soil erosion. The result was migration of large amounts of silt from the top of the hill to the bottom, directly onto Whippoorwill Lane.     2. The estimate of 4,000 square feet of impervious surface per lot significantly underestimated the actual development. The effect was to underestimate the average runoff curve number, leading to an underestimate of storm flows.     3. Several of the culverts that were intended to provide storage were apparently installed below design grade, leading to a reduction of flow storage to near zero within the subdivision.     4. The subdivision was popular, and multiple contractors built large houses simultaneously. This resulted in soil disturbance occurring over as many as 15 lots simultaneously, leading to exposure of more acres of soil than anticipated in the erosion control plan. Erosion control measures were ineffective and weakly enforced.     5. Town regulations did not require preparation of individual lot grading plans. The result was that the steepest lots on Burgundy Terrace were stripped with no final grading plan in place, and no stormwater detention structures planned. Preventing Problems The flooding and erosion problems at Cote d’Or were mostly avoidable, and point to several important lessons.     1. Hydrologic assumptions need to be updated and validated during construction. The construction of very large houses, and the consequent development of more impervious area than anticipated, required adjustments in the design that never occurred.     2. Installation of culverts and other structures must be carefully monitored to insure compliance with design intent. At least one culvert was apparently intentionally lowered during construction, without considering the impact of the field modification on the stormwater design.     3. Find ways to maximize storage. The stormwater design was based on the assumption that significant storage would occur upstream of several culvert crossings. Little or no storage was in fact realized, mainly due to field changes to culvert sizes and inverts. Use of flow control structures such as manholes equipped with stoplog bypass weirs would have allowed tuning of the system to maximize storage by adding or removing stoplogs as needed.     4. Determine whether infiltration structures could benefit flow issues. No groundwater infiltration systems were designed for this project. Much of the soil is hydrologic class B; therefore, infiltration using detention ponds, subsurface galleries, or leaky pipes likely could have been implemented. Infiltration structures combined with storage systems could have provided significant flood flow reduction, both during construction and post-construction.     5. Take steps to conduct interim inspections to enhance erosion control. ECDC appeared to be a low priority. Individual lot grading was the source of the majority of sedimentation, however there was no plan for lot inspection during construction. Inspection was limited to the public roadways, and building inspection of the houses during construction (foundation, walls, plumbing etc., but not grading or lot drainage). Effective ECDC starts with a comprehensive plan that addresses during-construction and post-construction conditions. Post-construction erosion control addresses slope stability, stormwater control, and surface treatment of the constructed project, and typically includes elements such as riprap, stone lined swales, closed drainage structures, contouring, retaining walls, vegetation, groundwater control, pavement and other surface treatment. Post-construction conditions are normally shown in the site plan, are budgeted for by the general contractor, and are often carefully inspected by local authorities. In my experience, post-construction erosion control accounts for a small percentage of erosion problems. ECDC is a far more complex problem. Often, the design engineer regards ECDC as a “means and methods” problem for the contractor to handle. Design plans may show simple control measures such as hay bales and silt fence, often in the form of a detail, but may offer no guidelines regarding installation procedure, monitoring and maintenance, and planting requirements. Local regulatory authorities may not be familiar with the wide variety of techniques available to control site erosion, and often provide only sparse inspection during construction. Federal or state oversight is rare, and local ordinances often provide vague or non-existent guidelines for construction practice. As a result, the contractor may be left on their own to select appropriate stormwater and soil erosion control methods. In short, erosion control during construction is often treated as a nuisance, is left to the contractor, and receives minimal oversight. Recommendations Flooding and sedimentation are most likely during construction when soils are exposed. Limiting the total amount of exposed soil at any one time is the simplest, most important erosion control technique. This may require limitations on the number of lots that can be constructed simultaneously, or the amount of soil per lot that can be disturbed. For areas with well-defined wet and dry seasons, stripping and grading should be done during the dry season. Careful attention to construction sequencing can minimize erosion. Installation of the drainage system prior to site stripping is an effective technique to limit uncontrolled runoff. Disturbed areas should be mulched and seeded as quickly as practicable rather than leaving exposed earth piles on-site. Siltation basins designed to capture eroded soil should be installed well before stripping and clearing begins. Photo 5 shows a newly constructed detention pond that effectively limited offsite erosion by capturing stormwater and silt, even though the pond outlet works had not yet been constructed. Stone lined swales can be an effective tool for minimizing erosion and stabilizing steep slopes. The swale suffered no damage, even though it was at approximately a 15 percent grade. Standard tools for erosion control include use of compost mats and blankets, mulch and hydroseed, hay bales, and silt fencing. Silt fencing and hay bales do not stop erosion; rather they are intended to capture eroded soil before it can damage adjoining properties. On steep slopes, silt fencing must be supported using driven steel or wooden posts spaced approximately every 4 feet, or the fence may be destroyed by the weight of silt and water. Similarly, poorly staked hay bales on steep slopes cannot function effectively. Curiously, I have seen many instances where hay bales are used to surround a catch basin or manhole during construction. Presumably the intent is to keep silt from entering the catch basin; however, the hay bales act like a dam; the ponded water can damage surrounding equipment, and the overflow may damage adjoining property. If soil erosion cannot be controlled at the point of discharge, consider installing a temporary suspended solids handling device such as a Vortechs® separator. One of the simplest, yet most effective, techniques for erosion control is to cover exposed soil with jute mat, filter fabric, or crushed stone. Stripped soil should be stored on flat areas (not on the slope), and may be surrounded by concrete blocks if long-term storage is anticipated. Erosion control requires five simple steps:     • Minimize exposed soil during construction     • Store stockpiled soil on flat areas rather than slopes     • Immediately stabilize exposed soil using fabric, mulch, and seed     • Install drainage control structures including culverts, detention ponds, swales, catch basins and manholes prior to site stripping     • Construct during the dry season if possible; if not possible, plan on extra effort and cost to minimize site disruption The general contractor should prepare a written plan, which should be shared among the site contractors. On a site such as a subdivision with multiple site contractors, cost savings can be realized by bulk ordering items such as hay bales, seed, mulch, fabric and crushed stone. Mobilization costs for expensive items like hydroseeding can be reduced by scheduling the seeding for continguous days, rather than ordering service on a day-to-day basis. Erosion control isn’t glamorous, and it isn’t exceptionally difficult, but it does require coordination and commitment to the plan. SLDT