Tennessee Design Principles for Erosion Prevention and Sediment Control for Construction Sites

Frequently Asked Questions (FAQ) about sizing and dewatering Temporary Sediment Basins [SB]

1. What construction site conditions require a Temporary Sediment Basin?

a. At general outfalls discharging into unimpaired or non-High Quality (non-I/HQ) waters where total (disturbed + undisturbed) drainage area is 10 acres or more
b. At outfalls discharging into Impaired or High Quality waters (I/HQ) where the total drainage area is 5 acres or more
The current 303(d) list of impaired waters of the state can be found in TDEC’s publications list at:

2. What is the basis for sizing a Temporary Sediment Pond?

A temporary sediment basin, working in combination with an overall system of construction site erosion prevention and sediment control practices, must be designed, constructed, and maintained so that discharged water does not cause, have the reasonable potential to cause, or contribute to violations of water quality standards. This includes, but is not limited to, causing an objectionable color contrast with the receiving stream. Discharging into impaired waters should not add loadings of pollutants
identified as causing or contributing to the impairment of a water body on the TDEC current 303(d) list of impaired waters.

Discharging pollutants into unimpaired waters cannot cause or contribute a violation of a water quality standard. Calculations, design plans and specifications, and SWPPP narrative must be sufficient and complete for demonstrating the effectiveness of a proposed sediment pond for helping to meet this performance standard objective.

Size and shape of pond are critical in sediment trapping efficiency. Shape, represented by the flow length to average basin width ratio (L:W), is important in providing sufficient inlet to outlet distance for maximizing travel time of small particles through the basin, while minimizing the possibility of short-circuiting of sediment-laden water from inlet to outlet and reducing dead spaces within the basin during the design storm event. A L:W of 4:1 is recommended, but never less than 2:1. Size, including both surface area and volume, is important to overall trapping efficiency. Sediment trapping efficiency is primarily a function of sediment particle size and the ratio of basin surface area to inflow rate. Basins with a large surface area-to-volume ratio are most effective in trapping sediment. Generally, the smaller the sediment particle size, the lower the settling velocity, and the larger the required basin surface area. Additionally, the larger the storm water flow into the basin, the larger the required basin surface area.

One commonly-used formula for sizing basin area is As = 1.2Q/Vs

where As = Basin surface area in square feet; Q = incoming design storm event flow (cubic feet per second) often assumed to be the average storm water flow; and Vs = settling velocity (feet per second) of the smallest practical particle size to be trapped.

Another formula used to size basin surface area is As = 0.01Qp

where As = Basin surface area in acres and Qp = incoming peak discharge (cubic feet per second) for sediment basin design.

Both formulae produce surface area loading rates that would result in sediment removal rate efficiencies between 70 to 80% for assumed fine silt and larger particles. For higher design efficiencies and high fractional clay sediment content, other treatment measures may need to be taken such as use of flocculents (see below), baffles, and multiple basin cells. In every situation, primary emphasis should be on using practices that prevent erosion of fine sediment particles.

Minimum volume is required in temporary sediment basin design to provide adequate settling zone space in the bottom of the pond between cleanings, to satisfy the permit requirements for incoming discharge loading from the construction site drainage area during the specified design storm event, and to produce minimum permanent pond pool depths of 2 or 3 feet. Sediment Pond volumes are discussed below.

I. Sediment Pond Volume Criteria

Two options for determining total volume needed to accommodate the estimated sediment loading are as follows:

a. General sediment loading estimate
1. Total sediment volume for establishing the crest elevation of the principal spillway riser is 3618 cubic feet (134 cubic yards) x total acres drained
2. Total sediment volume for establishing the permanent pool elevation (PPE) is 1809 cubic feet (67 cubic yards) x total acres drained
3. Total sediment volume for establishing the clean-out elevation (COE) of the pond is 905 cubic feet (34 cubic yards) x total acres drained
4. The emergency spillway crest elevation is determined after routing the 2-year or 5-year, 24-hour flood through the principal spillway and adding a minimum freeboard as discussed in the Spillways Question #3 below.
5. Dam crest elevation is established by routing the 25-year, 24-hour flood through the pond and adding minimum freeboard requirements discussed in the Spillways Question #3 below.

b. Calculated sediment loading estimate
1. Total sediment volume for establishing the crest elevation of the principal spillway riser, is determined by calculating sediment yield from construction site based on Universal Soil Loss Equation factors such as size of total contributing disturbed and undisturbed drainage areas, soil type and erodibility factor, topographic factors of land slope and slope length, applicable cover factors and practice methods, and sediment delivery. Recommended permanent pool and clean out elevations are based on one-half and one-quarter, respectively, of the total sediment volume.
2. The principal spillway, emergency spillway, and dam crest elevations are established by routing the appropriate 24-hour floods through the pond as discussed above and in the Spillways Question #3 below.

II. Equivalent Control Measures

Where the total drainage area is 10 acres or more, but temporary sediment basins are not practical, equivalent control measures may be substituted for a sediment basin. TDEC will review each equivalency case based on the justified merits determined from submitted SWPPP calculations and narrative.

3. What are the bases for the principal and emergency spillway capacities?

a. Principal Spillway
Once the principal spillway crest elevation has been established from the sediment calculations, the combined capacity of the spillway riser and pipe through the dam should be designed for one of the following applicable conditions:
i. For discharges into non-I/HQ waters the 2-year, 24-hour storm is routed through the pond and spillway
ii. For discharges into I/HQ waters, the 5-year, 24-hour storm is routed through the pond and spillway.

b. Emergency Spillway
Starting with an initial pond elevation at the principal spillway riser, the 25-year, 24-hour flow is routed through the pond, giving outflow credit to both the principal and emergency spillway discharges, to determine the maximum flood level (25YR-FL). The emergency spillway crest elevation should be set a minimum of 1 foot above the routed 2- year flood level (2YR-FL) or 5-year flood level, as appropriate.

c. Top of Dam (crest)
The dam crest elevation is established by adding a minimum freeboard of 1 foot to the routed 25-year flood level (25YR-FL). A minimum freeboard of 3 feet should be added if the principal spillway riser also functions as a combined principal and emergency spillway.

4. How is a sediment basin dewatered?

The water contained in the sediment volume space (dry volume between the principal spillway riser crest and the permanent pool level) should be slowly dewatered to allow most of the finer suspended sediment particles (primarily clay and fine silts) contained in the water column to settle out before releasing to a receiving stream. This is best achieved by “skimming” off the upper, cleaner part of the pond water over a relatively long period of time. Proper design, correct installation, and regular inspection and maintenance are vital to effective operation of any dewatering device. Two methods are currently recommended by TDEC for dewatering:

I. Perforated or slotted vertical pipe or tubing attached to the principal spillway riser at the permanent pool elevation and designed to draw down or dewater the volume of water between the riser crest and permanent pool over a recommended period of 72 hours. The number, size, and spacing of the perforations need to be determined and specified. A slide gate type of valve or outlet orifice should be used in conjunction with the vertical pipe to achieve accurate drawdown time control.

II. Floating type of skimmer configured to always draw or skim water through a small orifice opening located just below the pond surface until the volume of water between the riser crest and permanent pool is dewatered over the recommended 72-hour period. The orifice in a floating skimmer, protected with a trash screen, is always under constant head and can be properly sized to ensure a constant discharge and accurate dewatering time. Floating skimmer design guidelines and size selection procedures are available from commercial vendors.

5. When should a flocculent or coagulant aid be used in a sediment basin?

In site situations involving problematic fine-grained soils such as loess or clays, where conventional erosion prevention and sediment control methods either do not or are not expected to satisfactorily achieve the water quality performance criteria of having construction site discharge into receiving streams without an “objectionable color contrast,” design professionals may consider the use of flocculants added to sediment-laden water as it enters the sediment basin. One common and effective flocculant, polyacrlamide (PAM), is sometimes used for increasing the sediment-removal efficiency of a sediment basin. The rate and method of PAM application is critical to its overall performance. Improper introduction of PAM may result in waste, poor performance, or undesirable environmental effects. Design professionals should consult closely with PAM vendors and TDEC officials before specifying its use.

6. Does a sediment basin need to control or limit peak stormwater discharge?

Watershed disturbance, whether during construction or post-construction phases, can increase the peak flow of runoff water for all levels of storms before leaving the project site. Local community, county or city stormwater or development regulations commonly limit the peak flows from project sites to the peak flows that existed prior to site disturbance (i.e., pre-construction) conditions for specified level(s) of storm events. For example, a local government stormwater regulation may specify that the peak flow from a project site shall not exceed the pre-construction peak flow for one or more storm frequency events such as the 2-, 5-, 10-, 25-, 50-, and/or 100-year return period storm event(s). While there is no direct Tennessee statute governing stormwater control requirements, settled case law and requirements established through TDEC’s administration of EPA’s Phase II, MS4 rule for stormwater runoff quality and quantity management, recognize the need for restraint and control. Where there are no local regulations or where the MS4 rule does not apply, the general practice standard is to limit the post-construction peak flow to the pre-construction or pre-disturbance peak flow for a 10-year storm event. There are many methods and Best Management Practices (BMPs) for controlling peak flow releases from project developments. One common and recognized method for limiting peak flow(s) during the construction phase to pre-construction peak(s) is through controlled, slow discharge release from constructed ponds for specified or assumed storm event(s). A temporary sediment basin provides a convenient opportunity to serve a dual purpose of controlling peak discharge(s) while trapping suspended sediment. Modification or adaptation of temporary basins into permanent stormwater detention or retention ponds may extend their use beyond the construction phase into the “post-construction” phase.

Therefore, developers and their design professionals should be familiar with all local government regulations and best management practice (BMPs) requirements concerning stormwater releases from project developments. The TDEC “Guide to the Selection & Design of Stormwater Best Management Practices (BMPs)” is a manual that discusses the requirements for controlling stormwater discharge and is available on-line through TDEC and Tennessee Water Resources Research Center websites:
http://www.state.tn.us/environment/ and http://eerc.ra.utk.edu/WRRC.html

7. Why shouldn’t an opening be placed at or near the bottom of a sediment pond?

An opening placed at the bottom of a sediment pond such as in a principal spillway riser pipe would eventually force the pond to empty, along with most of the fine sediment particles concentrated near the bottom of the pond, during and after the storm. The purpose of having a permanent pool of water is to allow sediment particles to settle out and remain in the pond while skimming off or dewatering the upper layer of relatively clear water near the pond surface. A riser with a bottom opening, surrounded by rock, is not effective in removing fine sediment particles.