Lesson 9:
Forest and Floods



Learning Outcomes

  1. Understand floods and explain processes involved:
    • What is a flood?
      • Flood stages
      • Flood damage
      • Flood stage or water level
      • Duration of inundation
      • Sediment delivery and deposition
      • Kinetic energy of flood flow
      • Increased earth mass
      • Failure to anticipate floods
      • Failure to zone flood plains
    • Types of flood
      • Long-rain floods
      • Snowmelt floods
      • Flash floods
      • Frozen-soil floods
      • Tidal floods

  2. Discuss the flood control controversy and understand associated factors:
    • Structural measures for flood control
      • Dikes and levees
      • Flood reservoirs
      • Floodways
      • Channel improvement
      • Flood-proofing
    • Flood control by land management
      • Contour cultivation
      • Strip cropping
      • Contour terracing
      • Grass waterways
      • Basin listing
      • Vegetal control
      • Supplementary structures
    • Other methods of flood protection
      • Flood warning
      • Flood plain zoning

  3. Understand the role of forest versus other types of land use, and explain the associated processes:
    • Forest cover holds soil in place
    • Forest cover provides additional soil water storage potential
    • Forest cover maintains infiltration
    • Forest cover favors granular soil frost




Reading Asignment

Along with the online lecture, read chapter 9 in your textbook.



Erosion and Sedimentation Control Devices

Erosion and Sedimentation

In a previous lesson, we showed how erosion can result in the formation of a gully.  Erosion is problematic in other ways as well.  By removing the topsoil from an area, erosion reduces the soil productivity.  Erosion also causes a problem called sedimentation.

Sedimentation occurs when the sediments removed from the ground during erosion are deposited elsewhere.  Although sedimentation helped reclaim the gully in the previous lesson, sedimentation in natural waterways reduces the quality of these waters.  

When the ground is disturbed during construction of roads, buildings, etc., erosion is greatly accelerated.  This lesson will be concerned with methods used to minimize the erosion and sedimentation at construction sites.  


Erosion and Sedimentation Control Devices

Many erosion and sedimentation control devices exist.  In this lesson, we will consider the six devices which are regularly used by the Department of Transportation when widening and improving secondary roads.  When used correctly, these six devices provide good protection of nearby waterways during and after the construction process.



Types of Control Devices

Temporary Silt Fence

The temporary silt fence is the erosion control device most frequently used.  Its purpose is to prevent sedimentation in off-site waters caused by erosion on the construction site.  

Temporary Silt Fence


The temporary silt fence is simply a band of fabric running along the ground which holds back the sediments and allows the water to pass through.  

Silt fences are often installed incorrectly.  The fabric should be buried at least 8 inches deep, but most people do not take the time to bury the fabric.  If the bottom is not buried in the ground, the silt fence is rendered less effective.

Location of the Sedimentation Fence

A silt fence is place at the toe of a fill site, a location where loose dirt and rocks have been placed during a grading operation.  The fill site is very prone to erosion since the dirt has no vegetation covering it.


The temporary silt fence filters sediment out of the runoff coming out of the fill site, preventing it from contaminating off-site waters.  The silt fence should remain in place until the disturbed area has been stabilized, meaning that grass has established on the loose soil.  


Special Sediment Control Fence

Special Sediment Control Fence


A special sediment control fence works in essentially the same way as the fabric silt fence described above.  But it is more effective at filtering out sediment and is often used in areas of increased environmental sensitivity, such as near a spring.  

The special sediment control fence is constructed of hardware cloth on metal posts with number 5 or number 57 crushed stone a minimum of 1 foot high at the base of the fence.  The stone slows the velocity of the runoff and retains sediments.  

The hardware cloth should be ¼ wire mesh and should be at least 2 feet high.  The posts should be 5 feet high and should be spaced 3 feet apart.  


Type C Rock Inlet Sediment Trap


Special sediment control fences are also often used around stormwater drop inlets on the shoulder or median of a highway, as shown above.  A drop inlet is a structure which allows water to descend in elevation through a series of pipes without eroding the soil as it would if flowing over such a steep incline.  The special sediment control fences prevent sediment from entering the drop inlet.  In this case, the fences are known as Type C rock inlet sediment traps.



Silt Basin Type B

Silt Basin Type B


A silt basin is a simple excavation in the earth located so that it intercepts drainage.  Runoff is collected in the basin, where silt and fines settle out.  (Fines are very small particles, including silt and clay.)  Once the silt basin becomes full of sediments, it should be cleaned out.  

The width of a silt basin depends on the width of the channel draining into it.  Its length should be twice its width, and it should be at least 2 feet deep.  The total storage capacity of the silt basin should be 1,800 cubic feet per acre of disturbed ground.  



Temporary Rock Silt Check Type B

Temporary Rock Silt Check Type B

A temporary rock silt check type B, also known as a check dam, is simply a pile of 8-inch rip-rap placed in a channel to act as a dam.  Rip-rap is broken rock, cobbles or boulders placed on earth surfaces such as the face of a dam or the bank of a stream for the protection against erosive forces such as flow velocity and waves.  

The check dam slows the velocity of the stormwater in the channel so that it is below the suspension velocity of silt, allowing the silt to settle out.  The middle of a check dam is slightly lower than the sides, creating a weir effect.

A channel will typically have a series of check dams along its length, with the number of dams depending on the slope of the channel.  Steeper channels will require more check dams to slow the velocity of the stormwater sufficiently to remove the silt.  

To determine how far apart to space check dams, use the following formula:


The following chart summarizes this formula:

Slope of Channel
Spacing of Rock Silt Checks



Pipe Inlet Sediment Trap Type A


Pipe Inlet Sediment Trap Type A

The pipe inlet sediment trap type A is designed to filter all water entering a drain pipe.  The sediment trap is simply a horseshoe-shaped, 8-inch structure around the inlet of a pipe.  This structure should be at least 3 feet high.  The outside of the trap is lined with #5 or #57 stone to capture sediment.  

An excavated storage basin should surround the outside of the device.  As with a silt basin, this excavation should have a volume of 1,800 cubic feet per disturbed acre.

Rock Silt Screen

Rock Silt Screen


The final type of erosion and sedimentation control device which we will consider in this lesson is the rock silt screen.  A rock silt screen is a 8-inch rip-rap dam placed in a live natural stream below a construction area, such as a pipe installation or stream bank restoration.  The silt screen causes the water to ripple, which helps suspended fines settle to the bottom of the stream.  The silt screen will also filter sediments out of the water.  

Cross-section Through a Rock Silt Screen


The top cross-section of the dam should be about 1.5 feet wide, and the side slopes should be 2:1.  The upstream side should be lined with #5 or #57 stone to a thickness of 1 foot.   The top of the silt screen should be near the water surface, but no more than 1 foot above it.  

Before the silt screen is removed from the water, it should be carefully cleaned.




Most of the devices considered in this lesson have specific symbols which are used on stormwater control plans.  These symbols are shown in the chart below:


Temporary Silt Fence

Special Sediment Control Fence

Silt Basin Type B

Temporary Rock Silt Check Type B


North Carolina Department of Transportation, Division of Highways, Roadside Environmental Unit. "Erosion and Sediment Control Measures."




Six devices are commonly used by the Department of Transportation to prevent erosion and sedimentation at construction sites.  These devices are: temporary silt fence, special sediment control fence, silt basin type B, temporary rock silt check type B, pipe inlet sediment trap type A, and rock silt screen.  The devices remove sediments from runoff by slowing the water's velocity so that the sediments settle out.  





In this lesson we will answer the following questions:

  • What three aspects of stormwater management must be considered when choosing a BMP?
  • What two methods can be used to determine the required volume in a detention basin?
  • How is the Graphical Storage Method used to size a detention basin?


Choosing a BMP

Types of BMPs

When choosing a BMP, you must consider three aspects of stormwater management:

Each BMP which you have read about in your text was designed to deal with only one of these aspects, though it may be suitable to control other aspects as well.  The purpose of each type of BMP is shown in Table 14.

Depending on your locality, regulations may require you to deal with some or all of these three aspects when you develop a stormwater management plan.  For this course, you will consider all three aspects of the stormwater management plan.  A summary of the three aspects of stormwater management are given below.  The requirements listed may vary in different localities.

Order of consideration
Type of treatment
The post-developed peak runoff rate for a selected design storm must not exceed the pre-developed peak runoff rate.
Stream channel erosion
Additional volume, velocity, and peak flow rate of post-development runoff must not cause erosion in the downstream channel.
Water quality
The first 0.5 inches of runoff must be treated to enhance water quality.

Since the requirements for preventing flooding tend to demand the largest treatment volume of water, and since the requirements for flooding are the most straightforward, we will choose a flooding BMP first.  Once we have chosen a flooding BMP, we can consider whether additional BMP(s) are necessary to deal with stream channel erosion and water quality.


Detention Basins

As Table 14 shows, the only BMP which has been specifically designed to deal with flooding is the detention basin.  Detention basins are the most commonly used type of stormwater BMP and are usually the least expensive and the most reliable method of controlling runoff after development.  As a result, this lesson will be concerned  


A stormwater detention basin acts as a constriction in a stream, only allowing a certain amount of water through at a time.  When the capacity of the outlet structure is exceeded, a portion of the stream's flow backs up and is temporarily stored in a pond-like structure.  The stored water can be released over an extended period of time, thus preventing flooding downstream by delaying discharge of runoff.



Introduction to Flow Routing

What is Flow Routing?

Flow routing, or flood routing, is the procedure used to determine the volume of water that will be stored in a detention basin during a rainfall event.  Two flow routing procedures commonly used are the Storage-Indication Method and the Graphical Storage Method, both of which will be briefly introduced in the sections below. 


Storage-Indication Method

One of the most widely used methods of determining the required storage volume in detention basins is the Storage-Indication Method.  This mathematical flow routing procedure consists of a trial and error process based upon the Continuity Equation.  The basic premise is that the volume of water entering the basin minus the volume of water leaving the basin (over a given time interval) equals the required storage volume.  The design procedure for implementing the Storage-Indication Method can be quite lengthy and time consuming when done manually.

Rather than present an in-depth explanation or an oversimplified explanation of the Storage-Indication Method in this lesson, the reader is referred to the Soil Conservation Service National Engineering Handbook.  The reference provides a good explanation of flood routing along with design procedures for the Storage-Indication Method and other acceptable techniques of calculating detention storage volumes.  For this course, you will not be required to use the Storage-Indication Method.  


Graphical Storage Method

A simpler, but less accurate, method of estimating detention storage volume is the Graphical Storage Method.  This method was developed by the Soil Conservation Service and is explained fully in the SCS Technical Release No. 55.  It involves the use of one graph which was developed based upon average storage and routing effects of many structures using the Storage-Indication Method of flood routing.

The primary advantages of this method are its simplicity and its compatibility with SCS runoff calculation procedures (such as the Graphical Peak Discharge Method and the Tabular Hydrograph Method.)  It is particularly suited for small detention basin design and for estimating the required size of basins during the project planning phase. 

The rest of this lesson will be concerned with using the Graphical Storage Method to estimate detention basin volume.  The design procedure will only determine the required storage volume of the basin.  The design of an appropriate discharge structure, which will maintain the allowable release rate at the design storage elevation, should be done by a qualified engineer.  A sample detention basin design is shown below.



BMPs for Virginia Agriculture

What Do I Do Now?

Need help with your conservation plan? The place to begin is your local soil and water conservation district. It will always help a landowner willing to support Virginia's efforts to protect water quality.



Table 1



Do you want financial assistance with your conservation efforts? If so contact your nearby conservation district for a cost-share application form and/or tax credit assistance for the practices needed to implement your conservation plan. The district board must then approve the request.

Other agencies that offer assistance include the USDA Farm Service Agency and Natural Resources Conservation Service, Virginia Cooperative Extension, Virginia Department of Game and Inland Fisheries and the Virginia Department of Forestry. More information is also available from the Department of Conservation and Recreation.



The Need

Virginia's soil and water conservation districts (SWCDs) are interested in nonpoint source (NPS) pollution. This pollution is so named because it doesn't come from a single point, such as a sewage outfall or industrial discharge pipe. NPS pollution isn't readily traced to a specific source. But it still contributes excess nutrients, pesticides, sediment, heavy metals and toxic substances to our waters.

Sources of NPS pollution are many, but one of the most significant is agriculture. Because agriculture requires many acres, its potential impact on water quality is great. For example, one EPA study estimates that 27 percent of the phosphorus and 60 percent of the nitrogen entering the Chesapeake Bay originate from cropland. These pollutants need to be controlled in order to protect the environment.



The Solution

The farm community has a tradition of being stewards of the land. By managing it wisely, farmers have made food both inexpensive and abundant in this country. SWCDs wish to help strengthen this ethic.

The business of farming requires as much planning and organization as any other. Strategies to protect surface and ground water should be in those plans.

Locally, districts concentrate on helping farmers in designated priority watersheds. Assistance is available year-round to individuals willing to carry out an approved conservation plan. Many plans qualify but all must be approved by the local district board to participate in some programs. Districts seek and recruit individuals whose efforts can make the greatest positive impact upon water quality.



The Tools

One tool SWCDs use to help farmers is the Virginia Agricultural Best Managment Practices (BMPs) Cost-Share Program. It provides funds to help install conservation practices that protect water and make farms more productive. Funding availability varies by district. The state provides districts funds to target areas with known water quality needs. Areas with the greatest need receive the greatest funding.

The cost-share program supports using various practices in conservation planning to treat animal waste, cropland, pastureland and forested land. Some are paid for at a straight per-acre rate. Others are cost-shared on a percentage basis up to 75 percent. In some cases, USDA also pays a percentage. In fact, the cost-share program's practices can often be funded by a combination of state and federal funds, reducing the landowner's expense to less than 30 percent of the total cost.

Because demand for cost-share assistance is great, districts support the implementation of only those plans which meet local water quality guidelines. Since all requests can't be satisfied, priority ranking of practices must be used to make sure money is distributed and spent wisely.

The most an individual may receive is $50,000. In any case, the state cost-share payment, combined with federal payments, will not exceed 75 percent of the total eligible costs.

Cost-share funds are also availalbe for approved innovative BMP demonstration projects intended to improve water quality. Districts and individuals design the project and install and demonstrate the innovative technology or management system.

All practices in the program have been included because of their ability to improve or protect water quality. Many will also increase farm productivity by conserving soil and making wise use of other farm resources.

Another tool is the Virginia Agricultural BMP Tax Credit Program, which begins with the 1998 tax year. The program supports voluntary installation of BMPs that will address Virginia's nonpoint source polllution water quality objectives.

Agricultural producers with an approved conservation plan can take a credit against state income tax of 25 percent of the first $70,000 spent on agricultural BMPs. The amount of the tax credit can't exceed $17,500 or the total state income tax obligation.

Your BMPs, if approved, will be inspected by the district after they're installed. Soon after this certification, you'll receive cost-share payments or a tax credit approval from your local SWCD.

A third tool, also a tax credit, encourages farmers to use conservation equipment. Sprayers for pesticides and liquid fertilizers, pneumatic fertilizer applicators, manure applicators, tramline adapters and starter fertilizer banding attachments for planters qualify for a tax credit equaling the lesser of 25 percent of the equipment purchase or $3,750. Additionally, conservation tillage equipment is eligible for a 25 percent tax credit not to exceed $2,500. The equipment must meet state-established criteria, and the farmer must have a nutrient management plan approved by the local SWCD.