2 IntroductionWater is component of all landscape designs that cannot be ignored.Water issues include:Too muchNot enoughWater being at an undesirable pointWater flowing across an undesirable pointFrost heaveToo much water can be handled by drainage.Not enough water can be resolved by using irrigation.Drainage can also be used to move water from unwanted areas.Drainage structures can be used to reroute water.Drainage can also be used to reduce the effects of frost heave.
3 Site AnalysisBefore starting to survey a site for drainage purposes it is important to evaluate the site.If the site adjoins a waterway, do not remove the vegetation adjacent to and along the stream bank.This vegetation is an essential buffer zone that will help maintain the water quality and curb erosion problems.Check your survey or plat for the location of nearby flood plains.If the land is in a flood plane, it is reasonable to expect the area will be inundated with water at some point.It is important that no structures, especially homes, are built within a designated flood plane.
4 Site Analysis--cont. Also check the map for drainage easements. They should be labeled "d.e." on the plat and are usually located along property lines.A drainage easement indicates that water will be probably flow along the easement after rainfall.Erosion can be a problem along drainage easements.Structures, fences, roads, etc. should not be constructed within drainage easements.
5 DrainageDrainage is the natural or artificial removal of surface and sub-surface water from a given area.Drains can be eithersurfaceor subsurface.
6 Need For DrainageA landscape design that does not properly control runoff may cause damage to and devaluation of the property.To prevent damage or devaluation of property, three questions must be answered.What is the elevation of the design property in relation to adjacent properties.Will water run onto the property, if so, were does it enter and were does it exit?How will the landscape plan change the drainage at the site.Drainage is needed to handle rooftop, driveway, and overland run off.Four main issues to consider when caring for soil and grass roots are fertilization, drainage, aeration, and thatch control.
7 Eight Drainage Principles Water flows downhillWhenever it rains you have the potential for runoff.The greater the intensity of the rain--the greater the potential for runoff.Reducing the permeability of the soil increases runoff.Increasing the non-permeabile area will increase runoff.Water or silt on walkways during, or after a rain, is an indication of poor design.A good landscape plan includes drainage in the plan.Drainage plans rely upon slope, pipes, berms or other structures to control the direction the water flows.
8 SlopeAny area that is exposed to rainfall should always have some slope to direct the flow of water.Water will puddle on flat, horizontal surfaces.The amount of slope varies with the surface and the conditions of the site.Turf areas = 2 - 3%Paved areas = 2%Foundations = special requirementsOne recommendation is a six inch drop within the first 10 feet.
9 Surface DrainsSurface drainage is controlling the flow of water using slope and shaped surfaces.Shaped surfacesSwalesDitchesBermsSurface drainage works best with small sites or for sites with a small amount of runoff.
10 Subsurface DrainSubsurface drain is a system of collecting and disposing of rain water.Common means of collection are a drain grate or perforated pipe.
11 Drain Outlet Both surface and sub surface drains must have an outlet. Modification of existing outlets is usually not very problematic, changing the location of an outlet may cause problems.One alternative is to direct the water towards the street.May require a permit.Greater problem if the drain is a redirect and not the natural path.Part of drainage plan that most municipalities require for development.
12 Drain Outlet--cont.If codes do not allowed the redirection of water to the street, what are the options?Unless you already have a landscape drainage system in place (allowing you to route the runoff into that system), you have two (2) options.1. Channel the water to a location on the site (but make sure it’s not a neighbor's!) where it's less troublesome and where it can percolate into the ground.
13 Drain Outlet-cont. 2. Build a pond and direct the water into it. A pond may be constructed of stone or concreteA storm detention cell may be a code requirement.or natural.
14 Estimating RunoffBefore a decision is made on the type and size of drainage structure or storage structure that is needed, the peak runoff rate and total volume of runoff must be determined.The peak rate of runoff is required when sizing drainage channels and pipes.The total amount of runoff is needed to size a pond.
15 Estimating Peak Runoff Rates Several different methods are available.RationalUseful for estimating peak runoff rates from small areas.Does not estimate volume of runoff.USDA-NRS Technical Release 55 (TR-55)Most popular methodTwo methodsTabular methodGraphical discharge methodUS Army Corps of Engineers HEC-1 Model
16 Rational MethodThe rational method is useful for estimating peak runoff rates from small <20 acre areas that are relatively uniform in topography and vegetation.Peak runoff rates are important when sizing drainage structures, especially pipes.Rational method uses a simple equation:The difficulty is getting accurate values for each variable.
17 Runoff Coefficient (C) The runoff coefficient (C) is defined as the ratio of the peak runoff rate to the rainfall intensity.The runoff coefficient mathematically indicates whether the runoff is likely to be high or low for the watershed.The value of C depends on the type and characteristics of the watershed.Values for “C” are usually determined from tables.
19 Rainfall Intensity (I) The rainfall intensity used in the rational method is based on a specific rainfall duration and recurrence interval.The recurrence used depends on the importance of the project.Terraces and waterways are designed for a 10-year recurrence.Spillways for dams may require a design based on a recurrence interval of 100 years or more.The rainfall intensity can be determined from an intensity-duration-recurrence interval chart.
20 Rainfall Intensity, Duration & Recurrence Interval To find the correct value for rainfall intensity from the chart, the time of concentration must be known.
21 Time of Concentration (TOC) The time of concentration for a watershed is defined as the time required for water to flow from the most remote point of the watershed to the outlet.The peak rate will occur when the entire watershed contributes to the runoff.The time of concentration is a function of drainageway length and slope.Tables are available for TOC.
23 AreaThe area used is the number of acres in the watershed above the outlet.Watershed area can be difficult to determine.When a map is available a planimeter can be is used for this purpose.Another method is placing a grid over the map and counting squares.If the map is digital, mapping software can calculate area.
24 Rational Method Example Determine the peak runoff for a1- 1/2 acre lot that has grass planted on heavy soil with an average slope of 3%. The client says a 50 year reoccurrence interval is appropriate. The drainageway is 850 feet long and has a slope of 1.25 %.The first step is determining the C value.C = 0.18 to 0.22Use 0.22
25 Example--cont.The next step is to determine an appropriate value for the rainfall intensity.The time of concentration is used to determine the intensity.A drainageway of 850 feet at 1.25% slope =7 minThis example shows one of the problems of using tabular data.What do you do when the data falls in between columns and/or rows?In this case the lower number was used knowing that this will cause the calculated peak flow to be slightly higher.
26 Example--cont.With a TOC of 7 minutes and a 50 year interval, the IDR graph can be used to estimate rainfall intensity.I = 10 in/hr
28 Mixed WatershedThe previous example assumed that the entire watershed had the same surface and slope.This seldom happens, therefore the equation must be modified to accommodate mixed watersheds.This is accomplished by calculating a “Weighted C”.
29 Mixed Watershed Example Determine the peak runoff for a watershed that consists of .75 acres of impervious surface, 3.4 acres of lawn at 1.8 % slope and sandy soil and 2.2 acres of lawn at 0.75% slope and heavy soil. The drainageway is 400 feet long with a slope of 1.2%.The first step is to determine the weighted C.
30 Mixed Watershed Example--cont. The next step is to determine the time of concentration and rainfall intensity.With a drainageway length of 400 feet and a slope of 1.2% the best number for TOC is 6 minutes.
31 Mixed Watershed Example--cont. With a TOC of 6 minutes and a reoccurrence interval of 100 years, the rainfall intensity can be determined from the chart.Rainfall intensity = 12 in/hour
32 Mixed Watershed Example--cont. The peak runoff rate from the mixed watershed is: