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Home My WebLink AboutManaging Storm Drainage Through Improved Land Design ~accamaw a guidance publication ol the Regional Planning and Development Council, Waccamaw Regional Planning and Development Council Georgelown, South Carolina Leslie Mclver, Chairman Kenneth C. "Ken"lhompson, Executive Director William J. "Bill" Schwartzkopf, Director of Planning Larry Schwarlz, Projecl Director Georgetown County Alfred B. "Al" Schooler, Counly Administrator Horry County John Hatchell, County Administrator Williamsburg County Alex Chatman, County Supervisor Roy Mann Associates, Inc. Landscape Architects and Environmental Planners Museum Wharf, Boston, Massachuselts The preparation of this report was made possible lhrough a grant fram lhe U.S. Environmental Protection Agency under Seclion 208 of the Federal Water Pollulion Control Acl of ~1972, as Amended [Public Law 92- 500] and wilh the supporl of George- town. Horry. and Williamsburg Counties April. t982 contents Introduction The region and its problems In brief What the handbook covers Objectives that can be reached by using this handbook From Rain to Run-off to the Sea The landscape and its natural processes Development and its effects on natural drainage Problems and Solutions Principles for improved drainage Agricultural lands Residential areas Shopping centers and olher large developmenls Shore areas Conclusions Glossary For Further Assistance Suggested Reading Acknowledgments and credits 2 5 13 42 44 44 45 45 introduction the region and its problems in brief Drainage problems in Georgetown, Horry, and Williams- burg Counties present many constraints to farming, home but!ding, and commercial development. Results from the Waccamaw Regional Planning and Development Coun- cil's Section 208 Nationwide Urban Runoff Program have · shown overland water drainage from farm fields, residen- 'rial areas, and commercial development carries pollu- lanls into downstream wetlands, esluaries, and ocean beach surf. Problems of drainage, flooding, erosion, and water pollution affecl Ix)Ih the inland areas of lhe lhree- county region and the shoreline of lhe Gr~nd Slrand. Farmers, developers, homeowners, roadbuilders, and courtly officials are all concerned with lhe problems of how land drains. Because lhe topography of the region is rial and Iow, and because soils in many areas do nol readily absorb rainfall, rainwater often remains ponded on the surface or overflows drainageways, causing crop and property damage and health and safety hazards to Ihe region"s people. e On the farm. practices that lead Io erosion of field soil and poor ditch design can also worsen exisling drainage conditions by silting in drainage systems and causing them to back up and overflow. e Within residential areas ditching has long been used to help speed rainfall away from homesites, as is the case with farmland, but most standard ditches cannot cure the problems ot flat homesites on poorly drained soils, especially where high water tables exist. e At shopping centers and other large develop- ments, parking and other hard surfaces force the shedding of water log quickly into municipal drain- age systems or onto streets, overloading them as well. In shore areas, too, lhe nalural capabilities of lhe land lo absorb rainfall are often ignored wilh pave- menls and roofs tunnelling runoff onlo beaches, eroding the beach and beauty of the Grand Slrand's shoreline. T~ dispose of excess water and improve soil drainage so that the land surface can be developed, swales, ditches, and canals have been excavated in order to channel pre- cipilafion more rapidly away tram farm, residential, or olher desirable lands. In some cases, ditches are dug deep enough to artificially lower lhe water fable and thus render the soil above the table better drained and more usable. But more rapid removal of surface runoff at the local level and lhe grealer quantifies of water removed lhrough water lable lowering mean a greater flow intensity and impact on downstream dilches, canals, drainage systems, and elsewhere in the area o~county. The faster it is that water flows from high elevations, the more intense are lhe flow impacts on downstream points within areawide drainage systems. When more water reaches an oullet point at the downstream edge of any drainage area lhan can be accommodated by the outlet within a given lime period, the drainageways upstream of lhe oullet will back up and overflow, causing surface flooding on Iow-lying lands. Because of the flatness of the land, many a residen,~of towns, subdivisions, and other communities of the region has experienced flooded floors, lawns, streets, and farm- lands year in and year out, where drainage syslems are in- capable of overcoming the deficiencies of topography and soils. what the handbook covers The lechniques proposed in lhis handbook will not solve most flooding problems caused by severe slorm runoff. Nor are lhe¥ intended Io salvage lands that, due 1o poor soils, high waler table, or insufficient gradient, are inherenlly un- suitable for development. Bul lhey can help improve drain- age condilions for many siles of average soils, waler fable, and gradient conditions. The techniques proposed can be effeclive in dealing with pollulion problems and the fre- quently occurring rainfalls of Iow and moderate inlensily. Of course, wetlands and high waler table lands on which development cannol occur wilhout environmenlal damage should be left alone. And Ioo, good basic land design cannot cure all drainage problems. Fortunately, there are techniques available to help im- prove drainage for many lands within the region, tech- niques thai may also help reduce flow impacts on down- stream areas as well as improve drainage characterislics on flat lands in variaus condilions. objectives that can be attained by using this handbook The targets that can be reached through use of Ibis hand- book lo benefil homeowners, developers, farmers, com- mercial property owners, and lhe general public are: · Improvement of water quality · Better management of on-site drainage · Reduction of erosion and sedimentation · Reduction of downstream flooding · Enhancement of aesthetlc appearance · Reduction of private and public expendllures on drainage and water quall~ Improvements · Protection of wildlife habitat and the environment In general the next few pages... An understanding of lhe natural processes al work in the region is an imparfanl firsl step towards learning 1o manage storm drainage lhrough improved land clesign, A turn of lhe page is all you need to starl from rain to runoff to the sea landscape and its natural processes An understanding o! the processes by which precipltalion moves across and through the land lo the sea helps reveal many answers lo better storm water managemenl. The drawings on lhe next two pages show how area-wide drainage patterns occur in coastal Carolina. topography [landform] Unlike the Piedmont region below which it ties, the coastal plain is characterized by Iow flatlands interrupted only oc- casionally by shallow rolling landforms never exceeding 75 feet above mean sea level. Along the coaslline Iow dunes are found above the sand flats. hydrology [water patterns] Swamps and marshes edge the lower river reaches and estuarine waters of the region, while the water table found beneath even the better drained lands does not exceed four to five f~)et in most areas. Many areas are shallower than these. Near the coast, the influences of high tide and surging sea levels during storms act to worsen inland flood- ing as river and stream flows are impeded and the drain- ageways that feed into them are backed up. Even when sea levels are Iow, flooding and standing water are often experienced where the landform is too flat, soils are imper- meable or heavy, a high water table prevails, and drain- age systems have insufficient capacities. Infiltration, or percolation, is the process by which precipi- tation moves downward through the upper soil layers. When water reaches the zone of water saturation {the "water table"] or an impermeable layer of hardpan, it moves toward ditches, streams, or other drainageways which lie at lower elevations. When the entire upper soil layers become saturated, as during flood, or where pav- ing or other imperviaus layers cover the land, rainfall will run off the surface as averland flow ["slorm runoff"]. During the initial moments of intense rainfalls, also, water carrying pollutants will shed over land surfaces without infiltrating, particularly where soils are bare of restraining vegetation, and transport pollutants to rivers, downstream wetlands, and to the beach surf. 6 Soils within the Waccomaw region are generally sandy in nature, varying from poorly drained to well-drained, depending on elevalion and amounts of clay. One type of clay commonly found in the region is gumbo. Within the poorly drained Iowlying marsh and riverine areas, eroded silts, clays, and fine organic mafter are found carried by water flow from elsewhere within the region. Percolation is often poor where clay is abundant in soil. Standing water is common where a saturated layer of clay (claypan) exists beneath surface soils. Within the higher elevated and agri- cultural areas, sandy loams exist that contain concentra- tions of clay and fertile organic matter. Along the dune and beach areas coarser, highly permeable and well drained sands exist. Well drained soils yield better building, roadway, and drainageway sites because of better percolation and drainage. However, where both poorly and well drained soils exist in proximily, it may be preferable to locate build- ings, roadways, and drainageways over the poorly drained soils, if foundation conditions allow, and to allow for a larger infiltration area where the soils are better drained. Appropriate measures should be taken when considering use of on-site sewage disposal units near less permeable soils or over a high water table. Because drainage ditches and adjacent areas are sus- ceptible to erosion, proper side slopes, spoil disposal, and drainageway gradients must be designed to minimize soil erosion, maximize infiltration, and decrease runoff and siltation. 8 / / / / ~ / vegetation Vegetation impedes surface flow. Where soils can accept infiltration, vegetation can cut down the quantity and velocity of surface runoff and diminish erosion resulting from rain impact. Debris from vegetation can also clog ditches and reduce effective drainageway capacities, but fine-textured growth, such as mown lawn grasses and other ground-hugging covers, can preserve high drain- ageway capacities while moderately increasing friction in the ditch, swale, or ground surface, resulting in a degree of moderation in the rate of runoff and on impact on down- stream flooding. ' development and its effect on natural drainage Changing land use within Horry, Williamsburg, and Georgetown counties over recent years has had an ad- verse effect on drainage within lhe region. Farm fields, areas developed for housing, shopping centers, commer- cial properties, and industrial areas have been developed in many instances without due regard to off-site drainage impacls, or even to drainage effects on the site itself. Parking areas, paved roads, rooftops, and compaction of soil coupled with raised water tables resulting from septic tank drain field installation, clearing of vegetation from large areas, and elimination of drainageways during de- velopment all have created highly increased runoff. It these effects continue, existing drainage facilities will become increasingly overburdened and unable to handle runoff. In order to improve drainage throughout the three-county region, the U.S. Soil Conservation Service (SCS) has pro- posed a regional drainage network consisting of main canals and lateral drainageways that would be dug .along natural drainage paths to improve the flow of runoff Jn each watershed. ~ Improvement of local drainage techniques, however, guided by belier land design, needs to be adopted by in- dividual property owners and users, step by step with any ~mprovements in area-wide or county drainage systems, if the latter systems are to work effectively. Agricultural land drainage is one system that alters natural runoff for legitimate reasons, but increases the rate of runoff and volumes in public ditches and canals. Where poor farming practices are allowed, however, the runoff problem is compounded with field erosion -- carrying away soil, pesticides, and fertilizers -- which can pollute and silt in drainageways, estuarine marshes, and boating Channels. Residential development laid out in gridiron patterns with flat homesites and straightline ditching causes the rapid flow of runoff from rooftops, homesiles, and impervious paved surfaces to surcharge downstream drainageways. Residential development imposed, on the land without respect 1o nalural drainage patterns causes flooding, erosion, sedimentation, and destruction to private, public, and ecologically important natural areas. Commercial land uses. such as shopping centers and in- dustrial properties, also contribute to a speeding up of the rates of runoff that would exist in the natural landscape. This is so because these areas are surfaced with imper- vious paving designed to carry water quickly and effec- tively away from customer access, parking, loading, and other areas, as well as the buildings, other structures, and valuable open lands on the property. In achieving this legi- timale drainage objeclive, however, large volumes are carried rapidly into public dilches and streams. Shoreline development is the major cause of dune de- struction and beach erosion along the Grand Strand. Sand dunes and the natural areas directly inland that absorb and retain stormwater help protect inland areas from the brunt of major storms. These nalural features, however, have been replaced by seawalls and other hard vedical surfaces lhat cause erosion and narrowing ot the beach face. The rapid flow of runoff from impervious surfaces of pool decks, roadways, parking lots, rooftops, and other im- pervious surfaces, and drainpipes causes extensive erosion to the beach face, embankments, ~nd pavement edges. The following diagrams show how "nat so good" and "good" local land design can affect area-wide drainage in agricullural, residential, and commercial situations. 9 a comparison of agricultural area roadside ditch systems typical layout Bare fields promole erosion of soil causing sedimen- tation of roadside ditches and downstream wet- lands, waterways, and estuaries. Loss of pesticides and ferlllizers is costly to the farmer and damaging to the environment. Poor equipmenl lurns damage dilch edges and cause ditch bl.ockages. Improved design · Cover crops reduce runoff rate, erosion, and pollu- tion of drainageways and downst,ream en- vironment. · Delention ponds slowly release water into main drainageways. · Dilch edge buffer helps maintain ditch walls, alerls equlpmenl operator. a comparison of residential land design layouts typical layout · Ungraded flat land leaves homes and other buildings close to elevations of surface flow. · Stralghl-line swales and ditches carry waler away from homes and forms but ore limited in Infillration capacity. · Head drolnagewoys often short-cut across lopogrophy, resulting in too rapid o runoff rate, turther surcharging downstream dralnageways and causing lhem to overflow. · Cleared vegetation does not allow for rainfall Inter- ception, increases runoff rate, and does not allow for absorption of rainfall. improved design · Land graded into mounds and swales allows buildings to perch higher above the water table. (~urvlng swales take more water and release It more gradually to downstream drainageway.s. · Detention ponds release water more gradually over weirs, set at calculated heights. · Vegetation softens rain impact, decreases overland flow, and allows for absorption. !1 a comparison of commercial land design layouts t2 typical layout · Impervious rooftops and paved surfaces shed runoff off-site with high intensity. · Escaping runoff overloads municipal storm drains or flows across street surfaces to damage other property or beaches. Improved design · Roof pondlng, porous paving, and uncurbed median strips help to reduce the rate and amount of runoff. · Perimeter drainage retains runoff on-site while detention ponds slowly release water to an off-site outlet. problems & solutions principles for improved drainage As we have seen, there are right and wrong ways of con- trolling drainage. The techniques proposed in this hand- book are based on simple principles lhat are by no means new. They have been and are being used in coastal Caro- lina, though not as widely as they oughl to be. To properly control drainage the following principles should be applied to land design. problems and solutions t design and modify landform to create high and Iow points in advance of building construction to create higher elevations for houses and other valuable property areas, and lower elevations for broad swales, ponds and other ¢lrainageways. 2 provide curves In swales and other on-site drainageways so that runoff will travel longer an~l at a lower gradient before reaching outlet points, following landforms wherever possible. 3 maintain absorptive surfaces while planning develop- ment wisely by leaving as much of the absorbent surfaces as lawn, garden, or natural areas, use porous paving instead of impervious paving particu- larly where parking areas are extensive, in conjunction with well-graded planled median strips. 5 design and maintain ditches to last with a minimum of Ciilch wall erosion and sedimentation. If these simple principles are followed by homeowners, site planners, designers and others, it is very likely that an im- provement in local drainage can be achieved. The tech- niques presented on the following pages are the specific aclion sleps that can be taken lo reach this objeclive. 15 agricultural lands Poor farm practices, lack of adequate ditch design, and Improper ditch maintenance have caused erosion, sedi- mentation, and flooding to occur on farm fields, adjacenl drainageways, and downstream poinls. To reduce these adverse effects and the impact of pollution on down* stream wetlands and estuaries, improved managemenl measures need to be adopted. These can be found on the following pages. 18 the problems In agricultural areas, roadside ditches are sometimes silted in as the result of steep wall ditch design, which allows crumbling of erodible soils. Two other typical prob- lems are the mounding of ditch and canal excavation spoil too close to the dralnageways, a negligent practice which allows rainfall to wash the spoil right back into the channel, and farm equipment turns too close to roadside ditch edges, which can result in direct assault on the dilch crest. Another contribution to the sedimentation of ditches has been the practice of some of leaving farm fields bare of cover between cash crops. Unprotected fields are easily eroded by strong rainfalls since there is no foliage to absorb Impact and decrease the flow rate of stormwater runoff. Without the restraining action of crops or other cover vegetation, great amounts of topsoil, fertilizer, and pesticides can be carried away and seriously Impact stream, wetland, and coastal environments. When ditches and drainageways fill in because of these poor practices, the potential for back-up and flooding of crops increases, often resulting in significant dollar losses. Filled in dilches also mean higher county, municipal, and private maintenance costs. the solutions Many improvemenls lo faulty drainage ditch design can be found. Many problems of poor design and mainten- ance can be overcome if the following procedures are followed. ditch design Drainageways should ensure efficient roadside flow by proper vertical placement of ditches, culverts, and pipe inverts. Elevations should be set by a qualified professional to avoid local blockages and overflow. Proper ditch design will make sure that agricultural land is well drained. · Side slopes of sandy unstable soil should be cut at 45%. In areas where clayey soil exists, 60% is ac- ceptable. · Equipment should be carefully selected -- a V- bucket for ditches three feet or less in width and a dragline for larger ditches. · Grasses, such as tight-growing Bahia, Bermuda, and Common Lespedeza should be planted on ditch bottoms, slopes, and adjacent areas to stabilize soil and restrict weed growth. spoil disposal Material excavated from drainageways should be han- dled properly. If it cannot be spread over the land, it should be piled as far from the ditch as possible. · The location of spoil should be set back from ditch edges to prevent it from washing back into the ditch and reduce excess weight on the ditch wall. · Open spacing should be provided between piles for drainage. · Mounds should be seeded with suitable grasses after excavation to provide spoil stabilization. improved field practices Ditch sedimentation can be reduced by eliminating soil erosion resulting from poor field practices. · Crops such as barley, oats, rye, or winter wheat should be planted during periods when fields are otherwise left without vegetative cover. · A planted buffer between the ditch edge and turn- ing border should be left undisturbed to prevent equipment damage to the ditch. drainage ditch design spoil mound placemenl and seeding 19 Adequate setback of tumarounds from ditch edges Is e~entlal. A buffer of thick gra~es can reduce ero~on. · Detention ponds should be excavated at suitable Iow points to detain and retard stormwater runoff and for livestock watering. · Weirs at pond outlets should be sized and located to ensure that outflow to downstream drainageways is managed at the lowest discharge rate possible with- out causing flooding from the pond. maintenance In order to maintain their runoff carrying capacity and reduce flooding, routine maintenance should be per- formed on all ditches. · Plant debris and other matter that can obstruct water flow should be cleaned from ditches and disposed of so that they will not re-enter the drainageway. · The stability of ditch walls and spoil mounds should be checked. · Exposed spoil should be seeded with recom- mended grasses. /am pond. Willan~b~g County residential areas Poor site conditions on flat lands and poorly designed slormwater control melhods in residential ~reas have led Io flooding of on-site housing as well as lands In down- stream areas. Early in the site planning process, problems of excessively flat lopography and stormwater control need to be addressed and appropriate land design measures selected to improve drainage, reduce flood- lng, and enhance land values. Solutions for achieving these goals through Improved land design can be found on the following pages. ii III Ill II II III the problems poor site design Residential areas and subdivisions have often been laid out in gridiron patterns with flat home$1tes and stralghtllne ditching. Even though straightline ditching allows for rapid on-site drainage, such design may allow runoff to accumu- late and surcharge downstream drainageways, with little help to the homes they serve. Instead, flooding to proper- ties, roadways, and natural areas results in times of heavy or moderately heavy rainfall. For disposal of stormwater, landowners and developers have relied on poorly designed drainageways, inherited from the past, that connect and discharge into the larger county system. Often, these inadequately designed local and county draina~neways which lack proper outlets flood because of sedimentation from rapid runoff caused by new residential development. Eroded sediments accumu- late in wetlands and estuaries, upsetting nutrient flows and smothering habitats valuable to fish, shellfish, and wildlife. Then, too, surface shedding from parking lois, driveways, walkways, rooftops, and other impervious surfaces accel- erates the arrival of runoff at outlet points and Increases flooding potential downstream. On flat land, in the ab- sence of a raised homesite, there is little recourse for the .homeowner during a heavy rain except to hope for clear- lng skies. 22 thef'--)lutions .Runoff in residential areas developed on basically flat land can be better managed if the land can be graded into landforms of a more rolling character. On the one hand, the curving, meandering swales that carry runoff will allow somewhat more time, as well as space, for runoff to move across and off lhe site than is allowed in straight-line ditching, providing greater opportunity for water infiltration into the soil (through the swale shoulders) and a less intense outflow into the main ditch system. On the other hand, the higher ground formed with the soil excavated from the swales supports homes at higher elevations and protects them from wet foundations and flooding to a better aegree than existed on the original lay of the land. Because all soil types and depth to groundwater play such an important role in controlling drainage, it is essential that information about the topography and groundwater of the land be obtained before a site plan is drawn. The Soil Con- servation Service or County Agricultural Extension can pro- vide general information on soil characteristics such as ab- sorption and percolation capabilities as well as depth to water table. South Carolina Department of Health and Environmental regulations require avoidance af septic system develop- ment on unsuitable soil types and over shallow water tables. These criteria and minimum drainage requirements of local subdivision regulations, where they exist, should be met in any housing development. For specific investiga- tions of site absorption, percolation, and water table depth, a competent consultant should be retained. Finally, mounding to provide raised elevations for home- sites, if accomplished through sensitive landscape design, can produce a quality image, improved property values, and a more liveable environment. Map drainage patterns, dele[mine soil sultabllltles, and eslabllsh water table depth. 23 Bolh flood 10~otecflon ancI grounclvvatec prolecflofl are Improve~. mounding Where individual lots are large enough, mounding can be accomplished by building on earth fill. Mounds of sandy soil help water drain into swales and away from structures and ensure a safe height above the water table for living areas and septic systems. · Slopes should allow for ease of mowing and permit a uniform, non-erosive rate of runoff flowing into ad- jacent swales. · Land~caplng of mounds will assist in runoff control while improving the aesthetic appearance of home~ sites and residential areas. · On-site sewage disposal units should be placed in porous soil away from less permeable soil, and away from shallow water tables. Ral~ed shuctuxes may appear u~natufal on flat tefraln, while bu#dlng~ on I'al~ed earthform$ can appear compatible with natural terrain. 24 swales Runoff from raised homesites and areas where imperme- able soils predominate should be slowed in ils progress loward the main drainageways by flowing toward and through gently sloped, grassed swales. · The location of swales should be along natural drainage paths in the lower and border areas of silas. · The grading of swales should be designed to assure the flow of runoff into roadside drainageways so that standing water does not occur. · Side Slopes should be designed to allow for a slow, even flow of runoff, enabling the swales to absorb water and reduce downstream flooding. · Grasses. such as tight growing Bahia and Bermuda, among others, should be planted in swales to help slow runoff, stabilize soil, and slow weed growth. roadside drainageways Roadside drainageways collect water from swales, home- sites, and roadways. Steps should be taken to ensure efficient roadside flow by properly locating ditches, culverts, and pipe inverts. Eleva- lions should be specified by a qualified professional to avoid local blockages and overflow. · The design of residential roadside drainageways should follow the guidelines described previously in the agricultural roadside ditch design section. natural vegetation and landscaping Vegetation improves the visual quality of homesites and enhances property values. A local landscape architect can best provide information on plant types, their avail- ability, design, and other pertinent landscaping in- formation. Where homeslte lois are large enough lo be modified from flatland to a varied topography, a valuable flexibility becomes available to the home- owner or developer. For the home can now be set higher and drier, while swales In new Iow points collect and lead water safely away. Proper widths, slopes, and elevations of roadside dralnageways are essential as are adequate culverts. 25 I I III popcorn pavtnglporous o~ha# aggr® .gat® paving surface e Natural vegetation should be retained and trees and shrubs planted on homesites. Low, dense grasses should be maintained on lawns to soften initial rain impact on soil and slow runafl. permeable paving In sites where sandy absorptive soils exisl, lhe use of per- meable type parings should be considered, since they allow infiltration of runoff into the soil and slow down the rapid flow of water ta swales and ditches from hard sur- faces. Water moves through permeable paving into a layer af gravel and then filters naturally into the underlying soil. The Waccamaw Regional Planning and Development Council can provide advice on local sources, costs at per- meable paving, and conditions of use. · Porous asphalt paving, commonly known as pop- corn paving, should be used on driveways, parking areas, and road surfaces because of its ability 1o absorb runoff and reduce surface flow. Its strength and stability make it acceptable for a great range of access and parking conditions. · Lattice block pavers contain hallow'voids and may be filled with sand, crushed stone, grass over porous soil, and other porous material. Their use can be best applied to special parking areas where Iow speeds are suited to their masonry quality. · Precast interlocking and brick pavers, set on sand for increased absorption, can provide a hard support surface for pedestrian use on patios and sidewalks and for vehicular parking. Precast pavers are somewhat more expensive than concrete, al- lhough they are more effective in reducing runoff and are visually pleasing. .e Aggregate paving, such as gravel, crushed stone, sand, or cinders can also be used in areas of pedes- trian and Iow-speed vehicular use. They provide better infiltration than the popcorn paving and pavers set in sand, although they do not possess their stability and require greater maintenance. infiltration structures Infiltration pits, dry wells, Dutch drains, and French drains can receive runoff from roof eaves or downspouts, drive- ways, or large lawn areas, and allow It to be absorbed directly into the lower layers of the soil. Since soil will ac- cept less infiltration with continuing rainfall, pits, dry wells, and other stone-filled drains are most effective at the be- ginning of storms. · Inf;itratlon plt~ dug directly into the ground and filled with gravel or porous rubble that Is separated from the soil by a filter mat can be used to intercept and absorb runoff. The filler mat will prevent soil from clogging the porous gravel and decreasing infiltra- tion. These pits can also be used to lower the water table in desired areas. Linear infiltration pit~,"' or trenches are commonly referred to as Dutch drains. ,/ lattice block paving Infiltration pit/trench 27 ~b~- · Dry wells, pits filled with gravel or stone rubble and surrounded by a mesh soil separator or by a vedi- cally set perforated fiberglass, concrete, or metal cylinder, can be used to collect stormwater from root gutter downspouts. The water will be absorbed into the ground provided the soil is sufficiently perme- able and unsaturaled. · French drains, perforated linear pipes set in a gravel trench, should be used to collect excess stormwater from the base of dry wells in areas where less perme- able soils exist, and lead it into areas of greater soil permeability or to drainage outlets. roof down~pout drainage system 28 shopping centers and other large developments Planners and developers are often faced with the difficult question of how to control the large amounts of runoff generated from shopping centers and other large devel- opments and are often unaware of methods that can be used to increase on-site infiltration wllhout cutting back on drainage efficiency. Improved methods of controlling runoff can be found on the following pages. Porous paving al~orbs rainfall while runoff drains Info planted medians, perimeter swales, and detention ponds. 30 -- the problems Same of the problems of flooding in shopping centers ancl other large developments are the result of the natural per- meability of the original soil giving way to excessive amounts of Impervious paved and building surfaces that are unable to absorb or retard the flow of stormwater. With the large amounts of runoff that occur in commercial parking lots, roadways, walkways, and rooftops, any open land that can be retained and landscaped in their vicinity will help to balance runoff with positive infiltration. the solutions As with residential areas, the major objective of slormwaler control for large developments is to allow far drainage away from the buildings and to hold some partion of runoff safely on-site until it can be absorbed by natural processes or slowly released to other drainage syslems. This can be accomplished by proper site grading, increasing absorp- tive capacities, and detaining stormwater on-site. The use of porous paving, planted areas, detention ponds, and in- filtration pits along the edge of parking lots and at the base of building walls, can increase the total absorptive capacity of the site. These features can reduce flood damages while providing an attractive environmenl for shoppers and other users. Improved grading To limit the amount of sheet runoff causing local and down- stream flooding, parking lots should be graded to direct runoff at a reasonable rate toward areas that can delain and either absorb or discharge rainwater slowly to other outlet points. · The gradient of parking surfaces should not exceed lwo percent where feasible, to slow runoff and in- crease infiltration through the pavement pores. Where parking surfaces are forced into sleeper slopes, flatter bands or landings should be graded Q, ,,0 more than I I/2-2%, at suitable intervals, to in- tercept flow and increase infiltration through the porous paving or other permeable material used. Runoff should be shed toward swales, perimeter ditches, medians, and other detention areas by proper grading design. porous paving In order to allow water to seep into subsurface soils, porous paving should be incorporated inlo existing sites and used on hard surfaced areas. Its ability to absorb and allow stormwater to percolate through will help to slow and re- duce the amounl of runoff. · Popcorn paving may be used on roads and parking surfaces especially where highly porous soils exist. Clogging of pores by silt and sand may occur over lime, but may be reversed by vigorous hosing or by frequent vacuuming and sweeping. Where less per- meable soil exists, borrowed porous fill from on- or off-site can be used in the grading process. · Aggregate paving, such as crushed stone or sand, may also be used on pedestrian and on some vehic- ular surfaces. · Brick, Interlocking, and lattice block pavers, set on sand, as defined in the preceding residential sec- tion, may be used on pedestrian and parking areas among others for better absorption of stormwater. graded landings to Increase Infiltration In porous paving lafllce block paving T popcorn paving/porous asphalt aggregate paving surface "_*i precast concrete wheelstops with runoff cuts median simps Intercept ond absorb runoff perforated wheel stops Wheel stops are used to prevent vehicles from damaging median strips and other planted borders. They are typically precasl in concrete and should have horizonlal voids in their base for runoff flow. perimeter ditches A principal drainage channel should be located around shopping centers and other large developments to ac- cept a portion of parking lot and roadway runoff while pro- viding an opportunity for natural absorption and evapora- tion, and to carry excess runoff into an on-site detention pond where feasible. · The design of perimeter ditches should follow that previously outlined in the agricultural land section. median strips Median strips should be designed to accept safe amounts of runoff from adjacent roadways and parking lots. Depending on their size they can incorporate plantings ot species tolerant of lemporary wet condilions and urban stormwater runoff, along with linear infiltration pits. Not only do median strips absorb stormwater runoff -- passing ex- cessive amounts on 1o other on-site outlet points -- but they also filter out pallutants collected by runoff. Provision should be made through design to prevent vehicular leak- ages from reaching any sensitive plants in the median. · The size of median strips should vary depending on planning requirements and lhe overall dimensions and requirements of each site. · Dutch drains, linear ditches of gravel, should be used along the perimeter of medians to help inter- cept runoff, store it, and allow it to percolate into sub- surface soils. Filter fabric should be used 1o separale gravel from the bordering soil 1o prevent clogging. ®. French drains, perforatea pipe surrounded by gravel should be used to carry excessive stormwater from Dutch drains to on-site outle! points. · Sweeping and vacuuming on roadways and park- ing lots should be performed on a regular basis to reduce clogging of pores caused by fine particles of soil and other debris. controlled rOof drainage Large rooftops can be designed to temporarily detain sformwater, reduce the total amount of runoff on ground suifaces, and allow collected slormwater 1o be absorbed by subsurface soils or passed on to other on-site detention points. · Rooftop drains that utilize a strainer and detention ring to ensure a safe, moderated release of water into a drain pipe may be used. · Infiltration pits dug into the ground directly below arain pipes and filled wilh gravel or porous rubble, should be installed to detain slormwater until it is absorbed by soil. Filter fabric should be used arour~ gravel to keep out soil particles and other debris. Shrubs or other ground covers can be planted around the edges of infiltration pits to soften their appearance. detention ponds Detention ponds are intended to store excess on-site runoff collected from Dutch drains, French drains, swales, and perimeter ditches. During periods of moderate and high rainfall,-detention ponds release excess amounts of storm- wafer into natural water courses or man-made systems. At other periods in time they allow for vegetative absorption, evaporalion, and natural infiltration of detained water. rooftop drainage system 33 · The size and design of detention ponds should not be determined by their stormwater control function alone, but with consideration given to their use as recreation areas, groundwater resuppliers, and aesthetic resources, among other tunctions. · Weirs provide outlet of excess stormwater at pond edges and should be designed and set at a proper elevation by an engineer or other qualified profes- sional. Weirs may also be arranged to hold back a higher head of waler during storms than during fair weather and thus aid in flood control. landscaping controls stormwater and improves aesthetic appearance. 34 landscaping The use of native or ornamental trees, shrubs, and ground cavers should be incorporated into shopping centers and other large developments not only tar lheir visual benefits, as is oftentimes done, but to take advanlage of their ability to control stormwater. A local landscape architect, nur- seryman, or agricultural extension expert can recommend plants best adapted 1o site conditions. · A perimeter planting of Iow grass should be usea on gently sloping drainageway borders 1o stabilize bank soil. Grassing also helps to moderate fluclua- tions in ditch (Iow. · Dense vegetative ground covers and heavy mulches, such as gravel, rocks, and shell hulls, may be used in planted median slrips and areas where the soil is void of cover 1o prevenl erosion. · Root zones of median plantings should be slightly elevated by berming where standing water is likely 1o occur. · Invasive vegetation, including types with intrusive root systems, should nol be used in areas where they may cut down absorptive capabilities of French drains, infillration pits, and olher drainageways. shore areas Development has taken place along the Grand Strand shoreline without respect to the sand dunes and their ad- iacent inland areas. Extensive erosion has occurred be- cause o! the lack of planning or poor design, and only recently have questions been asked on ways to reduce these adverse effects. Modern methods for improving drainage by simulating natural processes can be found on the following pages. _ I 36 the problems the dynamic shoreline and the value of its dune system The beaches, dunes, sand ridges, and swales thal com- pose the Grand Strand's coastal edge are a dynamic and easily damaged component of the South Carolina coastal zone. Formed by a constantly changing regime of wind and water, sand dunes act as barriers, protecting the beach and inland areas from the erosive power of wind and waves. Stabilized by native vegetation such as Sea Oats, dunes are lhe primary shoreline defense against severe storms, high wave action, and strong winds. A stable, undisturbed dune system shelters the area imme- diately behind it. In a natural environment, this buffer pro- tects the delicate ecological balances of the backshore and wetlands in their lee. Dunes and the natural areas that occur behind them absorb and retain stormwater. When these areas are re- placed with hald, impervious surfaces, such as parking lots, pool decks, patios, and rooftops, the stormwater that can na longer flow into the ground must be collected and disposed of. Often, as in Myrtle Beach, such drainage con- sists of stormwater pipes that discharge directly onto the beach. The result has been extensive erosion of the beach face, embankments, and pavement edges, all of which tend to accelerate natural beach erosion where it may exist, or change a stable beach into one susceptible to erosion. Correction of these problems generally requires drastic and expensive remedial action. urban development and dune destruction In developing areas, the dunes help buffer buildings, park- ing lots, lawns, and olher site improvements against ero- sion and potentially destructive storms. If man is nol careful in the planning, design, construction, and maintenance ot areas that affecl the dune zone, serious problems, many of which can be seen in Myrtle Beach, can develop. These pr~oblems, outlined below, can impose high reconstruclion and maintenance costs on private landowners and peren- nial expenses on municipalities and other public bodies. Three destruclive mistakes are common along the sea edge of the Grand Strand: · removal of dunes · construction of seawalls and other built edges as rep;acements to the dunes · direct discharge of runoff via storm drains onto the beach face When dunes are removed, inland properties face a greater risk of danger from storm driven waves and winds unless a seawall or other vertical surface has replaced the dune. Seawalls and other vertical slructures, however, can increase problems of beach erosion by reflecting wave energy that is normally absorbed by dunes, downward onto the beach, causing the breaking waters to scour and erode the beach. This scouring erosive action of waves de- creases the beach width and leaves it more vulnerable to erosion. The removal of the dunes and narrowing of the beach area compromises lhe appearance and natural beauty of the shoreline, reducing the attractiveness of the beach to both residents and tourists. The direct discharge of runoff via storm drains onto the beach causes deep eroded gullies to occur during Periods of moderate and heavy rainfall; afterwards, standing water often occurs beneath these drain pipe Outlels. Runoff from drain pipes and impervious hard sur- faces not only deposits pollutants on the beach, bul il also causes extensive erosion to the beach fac~ and pavement edges and makes possible a greater "bite" by wave ac- tion. Pollutants and runoff from inland and adjacent shore areas cause the beach to become both visually and phys- ically degraded and oftentimes requires costly repair and restoration efforts. 37 The beach [Zone 0 of the coastal edge) Is vulnerable to erosion from both the ~ea and stormwater runoff. The dune line [Zone ~] can hold back and absorb ~urface runoff unabsorbed by the swale. The swale [Zone 2] provides for Infiltration of runoff not absorbed by Zone 3 surfaces. Public and private parking areas, motels, slreet ends, and public walkways form the primary absorption zone [Zone 3]. Existing surfaces In this zone should be redesigned to Incorporate porous materials for parking, drives, and walks, along with extensive landscaping to provide for positive absorption of stormwater. the solutions By proper site selection, site planning, and design aimed at controlling runoff and minimizing erosion, developers can protect and enhance their investments in the dune environment. Owners of existing development can enhance the value of their properties and improve the ap- pearance of their environment by altering the pattern of runoff. Dunes, beaches, and all other elements of a coastline -- whether natural or man-made -- are subject to all the forces of the coastal environment. Before a municipality or a private landowner embarks on a dune reconstruction project, modifies seawalls, or begins a beach nourishment program, the expertise of coastal experts and knowledge- able officials, as well as the appropriate permiis for beach modification, should be obtained. build behind the dune zone Sand dunes play an important role within the coastal com- munity and provide protection to coastal development from wind, waves, and high lictes. · Where construction is yet to begin, new residential or commercial structures should be sited behind the dune and swale zones in the leeward protected area. · Structures should be located on the land so that dune stabilizing vegetation is left Intact. In this way, the dunes can continue 1o act in their natural role as protectors of lands to their lee, while leeward devel- opment can be sited to take advantage of the beauty and image of the dunes, as well as of views of the beach and ocean beyond; landform modification In sand flats leeward of lhe backdunes, where well- drained soils lie sufficiently above the water table, a greater opportunity for landform modification exists than is formed closer to the sea. As new communities developed in recent years have demonslrated, residential layouts with landtorms modified into mounds and swales achieve good drainage and attractive residentail environmenls at the same time. · Pond and swale excavation material should be graded into broad mounds or other natural appear- ing landforms fo elevate houses and their adjacent grounds. · Mounds and swales should be laid out consistent with the overall drainage plan ot the site, allowing water to move off the land al a restrained, non- erosive rate. · Suilable grasses should be planted over exposed sand s_urtaces after landform modifications have beePr'completed. avoid obstruction to estuarine flow The construction ot causeways or the placement ot landfitl in salt marshes or waters can adversely attecf lhe subtle and varied circulation patterns that are crilical to provid- ing nutrients to aquatic and intertidal organisms through lhe food web to coastal fish and wildlife. By slowing the velocily of tidal flow, causeways and other fill typically cause increased sedimentation and yet further loss ot ecological syslems, nol to speak of lost navigation capacity and increased dredging costs. · Pilings or other substructures should be used to ele- vate roadways in order to allow the continuity of natural tidal flow patterns. · Fill should never be placed in wetlands, which are among the most valuable biologically productive re- sources we possess. dune walkover dune reconstruction By limiting development to the leeward side of the dune zone, seawalls and other beachtront protection struclures may not be necessary. Where dunes have been removed or seawalls already exist, the remnant dune may be recon- structed to renaturalize the appearance of the beach and afford some storm protection~ Sand can be bermed up against a seawall, where one has been built, and may be partially stabilized by native vegetation and dune fencing In some areas, sand nourishment may be feasible as a means of restoring beaches. Where this is possible, recon- structed dunes will be afforded aaded prolection tram the sea. If beach conditions allow dune reconstruction, the follow- ing steps can be followed. · Place sand to a height of three to four feet to eslab- lish the dune base. · Plant suitable vegetation such as Sea Oats [Uniola panlculata]. Bitler Panicum [Panicum amarum], and other suitable species, Experts should be consulled before planting to determine site specific suitabilities for particular species, · Dune fencing should be installed just behind the beach toe of the dune to encourage the deposition of more sand by the wind. A line of dune fence should also be installed on the backside of the dune to prevent trampling and erosion by beachgoers. This fence will also help trap windblown sand. IA fence is also typically set along the dune ridge where the dune base is broader than fifty feet.] · Dune walkovers should be conslructed 1o allow people access to the beach and minimize trampling of the reestablished dune. conclusions The use of most land within the Waccamaw region is highly . dependent on adequate drainage. It land resources are to be r,'~ore wisely managed, protected, or developed, im- proved measures 1o control flooding, erosion, and sedi- mentation must be put into practice. For much, if not all, development within the region there are ways to design the land, improve site drainage, enhance property value, reduce public and private expense, and decrease impact on natural areas. Through improved site and land design, together with local and county storm water runoff managemenl, sup- porlecl by both ordinances and voluntary cooperation at both levels, improved drainage and runoff control are achievable. If lhe objectives can be achieved, George- town, Horry and Williamsburg Counties may enjoy new benefits and Iowpred costs throuah the years to come. minimized erosion and sedimentation Proper ditch design, spoil handling, farm equipment turns, ditch slope grassing and maintenance, curved swale and mound design, detention pond development, replace- menl of impervious surfaces with permeable paving, and retaining large areas of landscaping and other high-in- filtration surfaces can all go tar to improve both on-site and off-site drainage. With improvement, houses and other struclures set upon the higher ground ct graded earlh mounds will be betler protected from stormwater. Al the same time, there will be less erosion and sedimentation of drainageways and less flooding from tilled ditches. improved water quality By building homes and septic systems on raised earth to secure satisfactory heights above groundwater, we may be able to decrease chances that septic leachate will reach and pollute water supplies and recreational water bodies. The slowinj down of rapid runoff can also improve water quality in rivers, ponds, and estuaries by reducing erosion and siltation, protected ecosystems By limiting erosion and sediment loads in drainageways, the problems of siltation and filling in of wetlands, water- ways, and esluarine waters can be reduced. Proper drain- age control can thus result in improved environmental quality and help protect fragile ecosystems and the valu- able fisheries and wildlife species they support. minimized public and private costs By employing techniques outlined in this handbook, home and other property owners can limit the expense they incur from the costs associated with clean-up, repair, and re- placement of flood damaged features. Costs to muni- cipal, county, and state government for flood clean-up, ditch maintenance, and pipe and ditch installation can be reduced. The cost of dredging navigable waters to remove sediments can be lowered. Similarly, on agricul- tural lands, a reduction af back-up flooding will decrease crop damage and loss of revenue to the farmer. · · · notes improved appearance By molding landforms and carving meandering swales, mounds, retention ponds, and other landscape elements, we can make residential and commercial areas more~t:~t- tractive. The Waccamaw region is a gaod place to live, and its landscape can be not only protected, but en- hanced by the simple measures discussed in this hand- book. enhanced propedy values The improved appearance and reduced potential for floocling and wet conditions can serve to increase property values and the marketability of new develop- ments. 43