Sediment and Stormwater Current Technical Practices


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Sediment and Stormwater Current Technical Practices
Maryland Department of Transportation State Highway Administration
June 7, 2018
This document details the current technical practices that are used to review Maryland Department of Transportation State Highway Administration (MDOT SHA) projects submitted to the MDOT SHA Plan Review Division (PRD). Some of these practices may be modified or supplemented when the MDOT SHA Sediment and Stormwater Technical Procedures are approved by the Maryland Department of the Environment (MDE).

Sediment and Stormwater Current Technical Practices June 7, 2018
Erosion & Sediment Control
The intent of this section is to provide a supplement to the “2011 Maryland Standards and Specifications for Soil Erosion and Sediment Control” in support of design efforts for MDOT SHA projects. Future revisions and modifications to the Standards and Specifications may result in a conflict between this supplement and the current Standards and Specifications. The designer is reminded that the Standards and Specifications are the overriding design document and sound engineering judgment should be applied to all designs.
The supplemental information listed below is referenced by the section number in the Standards and Specifications that it refers to:
Add the following to Section A-5.I, Content of the Erosion and Sediment Control Plan:
Design Data on Plans Some erosion and sediment control (ESC) measures (e.g. temporary gabion outlet structures, temporary stone outlet structures, traps, basins, etc.) require design data on the plans. For temporary stone outlet structures (TSOS) and temporary gabion outlet structures (TGOS), include: location and/or identification number (ID #), weir elevation, drainage area (DA) size, required storage, and actual storage. The ESC plan should include grading contours for sediment traps and basins. When excavation or additional grading is needed to achieve the required storage volume behind a TGOS or TSOS, show the temporary contours or provide enough information during plan review to demonstrate sufficient storage availability.
Standard Stabilization Note All disturbed areas with slopes flatter than 2:1 must be stabilized with 4 inches of topsoil, seed, and mulch. For slopes 2:1 or steeper, refer to the “MDOT SHA Landscape Design Guide.”
Same Day Stabilization: Same Day Stabilization (SDS) is not a standard ESC measure. It should be limited to small areas where the ESC filtering practices provided in the “2011 Maryland Standards and Specifications for Soil Erosion and Sediment Control” are not feasible or practical. The SDS provision should also be limited to areas where the proposed work can be completed in a single working day, including application of the permanent stabilization. In some instances, temporary plastic sheeting may be used to protect multi-day SDS areas. Seed, straw, and/or SSM are not acceptable methods to temporary stabilize an SDS work area that will be disturbed the next day.
When a plan has an area that calls for same day stabilization, but the rest of the disturbance has ESC measures, the limits of SDS need to be clearly identified on the plans. This can be done with a bubble, shading, or hatching. If shading or hatching is used, the pattern should be identified in the legend.
MDOT SHA projects that include limited amounts of disturbances, such as sidewalk replacement or guard rail installation, often include a provision for same day stabilization in lieu of installation and removal of sediment control devices. A note detailing the requirements
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Sediment and Stormwater Current Technical Practices June 7, 2018
should be provided on the plans and referred to in the Sequence of Construction (SOC). If the note allows for either same day stabilization or the installation of sediment control devices, then the sediment control devices should be shown on the plan.
Removal (pulling) and resetting of W-Beam post and panel that does not require grading or earth disturbance may be excluded from the Limit of Disturbance (LOD). Most end treatment installation includes safety grading and should be included within the LOD. New installation of guard rail and associated safety grading should be included within LOD. The LOD may be shown on a typical section or detail and included in the SOC. If a typical LOD detail is used, any LOD located within environmentally sensitive areas should still be shown individually on the plans.
Add the following after Section A-5.I, Content of the Erosion and Sediment Control Plan, Item H.6 (e):
(f) Details that deviate from the “2011 Maryland Standards and Specifications for Erosion and Sediment Control” must be shown on the plans.
Add the following to Section B-1:
Stabilized Construction Entrance In certain situations, a stabilized construction entrance (SCE) may not be required. This special allowance is made for areas where it is either infeasible or inapplicable to provide an SCE. A typical example would be when the work area is smaller than the disturbance that would be created by an SCE. Where no SCE is provided, the contractor shall designate the construction equipment that shall be allowed within the LOD. This equipment shall be kept within the LOD until the proposed work is complete and shall have treads/tires cleaned prior to leaving the LOD. The method of cleaning shall be specified by the contractor. Washing of treads/tires requires an appropriate sediment filtering practice or capturing devise.
Rumble Pad Pre-constructed rumble pads may be used instead of stabilized construction entrances provided they are installed according to manufacturer’s recommendations and a sufficient number of pads are installed to allow a minimum of four tire revolutions while on the pad. More pads may be needed depending on site conditions. The plan shall specify that accumulated materials be cleaned from the pads daily (or more often if necessary) and an acceptable disposed method be specified on the plan or in the specification.
Add the following to Section B-4-4:
Temporary Stabilization Disturbed areas that are to be paved shall be stabilized with graded aggregate base (GAB). All other areas shall be stabilized according to MDOT SHA specification section 704.
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Sediment and Stormwater Current Technical Practices June 7, 2018
Add the following to Section B-4-8:
Staging, storage and stockpile areas Staging, storage and stockpile areas are typically identified and located by the contractor, but in some cases, may be shown on the plans. The contractor is responsible for obtaining approvals for off-site borrow or waste sites. Off-site borrow or waste sites require local, county, and Soil Conservation District approvals if they are located on private property, PRD approval if on MDOT SHA property, or MDE approval if on federal property or other State property.
Add the following to Section D-4, Conditions Where Practice Applies:
The exit slope must be flat. If the slope exceeds 0%, then use “NRCS Design Guide MD #6 Riprap Design Methods” on riprap channel design and Isbasch equation.
http://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs144p2_025594.pdf
Other Supplemental Information:
Notice of Intent (NOI) For projects with an earth disturbance of 1.0 acre or greater, a National Pollutant Discharge Elimination System (NPDES) Individual Application or a Notice of Intent (NOI) to comply with General Permit to Discharge Stormwater Associated with Construction Activities must be completed online and approved by MDE prior to any earth disturbance.
Notice of Termination (NOT) The online Notice of Termination must be completed by MDOT SHA Compliance personnel or MDOT SHA responsible construction personnel before a project can be closed out. Stormwater facility as-builts should be accepted by the Highway Hydraulics Division and the Plan Review Division prior to the project closeout.
Material Removal by Pressure Washing If a high-pressure water jet is to be used to remove concrete from existing structures, the plans or specifications should direct the contractor to submit a plan for effluent collection, removal, and off-site treatment. This activity may require an Industrial Discharge Permit from MDE.
Scarifying Soils Existing in-situ soils should be scarified below stormwater management best management practices (BMPs). Suggested language for this note to the contractor is as follows: “Scarify the soil surface with a backhoe, skid-steer, or tractor with ripping teeth, cultivator, disk harrow, or other agricultural machinery. Scarify to a depth that will result in soil that is easy to dig for at least the top 12”. Do not scarify within the drip line of existing trees to prevent damage to surface feeder roots.”
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Sediment and Stormwater Current Technical Practices June 7, 2018
Three Day Dry NOAA forecast Three Day NOAA weather forecasts are “Dry” when the probability of precipitation during each of the three consecutive days is less than 20%. National Weather Service precipitation forecasts for a project location can be found on www.weather.gov. Follow the steps below:
1) Enter the city/state or the zip code into the top-left blank. 2) Scroll down the page to see the detailed forecast. 3) Obtain the numerical probability of precipitation (0-100%) by clicking on “Hourly
Weather Graph”. 4) The brown line on the third section of the graph shows the hourly probability of
precipitation. 5) To see this in a tabular form, instead of a graph, click "Tabular Forecast" near the
bottom right of the web page.
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Sediment and Stormwater Current Technical Practices June 7, 2018
Stormwater Management
The intent of this section is to provide clarifications to the current version of the 2000 Maryland Stormwater Design Manual, Volumes I & II (Design Manual) for MDOT SHA projects. Sound engineering judgment should be applied to all designs. The information listed below is referenced by the section number in the Design Manual that it refers to:
Chapter 2, Section 2.1:
Based on the 2007 Stormwater Management Act and the revisions to Chapter 5 of the Design Manual, the criterion for calculating Environmental Site Design Volume (ESDv) is now the same statewide. Therefore, P=1.0 inch should be used statewide for water quality and Tables 5.3 should be used to determine statewide ESDv.
Chapter 2, Section 2.3 page 2.8, Clarification on Cpv requirements for Eastern Shore:
Based on the 2007 SWM Act and the revisions to Chapter 5 of the Design Manual, ESDv, and therefore Channel Protection Volume (Cpv), is required for the Eastern Shore. There are multiple Cpv waivers that Eastern Shore projects may qualify for due to the prevalence of tidal water. However, Chesapeake Bay Critical Area Regulations may require full ESDv on Eastern Shore. Critical Area required management cannot be waived by PRD.
Chapter 2, Section 2.4 and 2.5, The following is a clarification to information provided on page 2.12, and 2.13 of the manual:
Existing agricultural land uses within the project LOD should be modeled as meadow in good condition in the existing condition.
Existing agricultural land uses that are outside the LOD should be modeled as the present land use in both the existing and proposed conditions.
Downstream impacts should be identified and addressed when culverts are enlarged as part of a development project. See Sections 4.1.C and 4.2.C in the “MDOT SHA Sediment and Stormwater Guidelines and Procedures – Part A” (Guidelines).
Chapter 3, Section 3.1.1 page 3.8, Additional guidance on feasibility:
Dry detention facilities may be used to provide quantity management (Qp2, Qp10, or Qf100).
Wet ponds are not permitted in Use III and IV Waters, or within 4 miles of airports.
Watershed and Stream Use information can be obtained from the following web sites:
Interactive 6 and 8 Digit Watershed Map: https://data.maryland.gov/Energy-and-Environment/Maryland-s-8-Digit-SubWatersheds/e9j9-vuxg/data
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Sediment and Stormwater Current Technical Practices June 7, 2018
Interactive Stream Use Map: http://mde.maryland.gov/programs/Water/TMDL/WaterQualityStandards/Pages/Designat edUsesMaps.aspx
Chapter 5, Section 5.1.3.1:
Loss of water quality applies to both new development and redevelopment projects when an existing system that provides water quality is altered. It should be evaluated throughout the design development process. Changing a roadway from open section to closed section is only considered to be a loss of water quality (LOWQ) if the existing roadside swale has an MDOT SHA BMP number.
If the area to be developed was providing stormwater management (SWM) in existing conditions for an adjacent drainage area within MDOT SHA right-of-way (ROW) or through an agreement where MDOT SHA accepted responsibility for providing management, then the management being lost must be replaced. If a new BMP replaces an existing BMP or other SWM feature, such as a grass channel or disconnection, the new BMP must be designed to replace the existing management in addition to satisfying the proposed management requirements.
Chapter 5, Section 5.1.3.1:
A discussion of the erosion & sediment control approach should be included in the Concept SWM narrative.
Chapter 5, Section 5.2, page 5.17:
The manual refers to and encourages the use of “treatment trains” as part of an overall system for meeting the project’s stormwater water requirements. A minimum PE of 1 inch needs to be treated by ESD, but it does not have to be attained in a single facility. It is acceptable to provide practices in parallel or in series. For example, a treatment train of three bioswales is essentially the same as a single bioswale with three separate segments or units for ESDv. The facilities are connected, and one drains to the other, but each individual unit has a sub-drainage area that drains directly to it.
For ESDv design purposes, the drainage area is the portion that drains directly to the individual unit. If each unit in the treatment train is designed to treat the same level of PE from its individual sub drainage area, there should be no overflow into the downhill unit during the ESDv design storm, except for grass swales. However, if the PE treated varies from unit to unit, the downhill facility will have to include overflow from the uphill facility for the ESDv design storm. For quantity analysis (Qp), the drainage area is cumulative and gets progressively larger for each downhill practice, as it does with a storm drain system. See the Design Manual for design constraints for swales.
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Sediment and Stormwater Current Technical Practices June 7, 2018
If the cumulative ESDv provided meets the target Pe and ESDv, then the ESDv treatment provided by the treatment train is satisfactory. Water quality credit is not given for the portion of PE above 1.0 inch, except for TMDL projects.
Add the following to Chapter 5, Section 5.2:
5.2.6: Impervious Area Requiring Treatment The following procedure should be used for identifying, evaluating, and classifying POI/LOI for MDOT SHA projects and calculating treatment requirements:
A. Identify the points of investigation (POIs) and lines of investigation (LOIs) A POI must be identified at every location where concentrated runoff leaves the ROW. A line of investigation (LOI) must be identified at every location where sheet flow leaves the ROW. In some cases, it may be acceptable to place the POI where runoff leaves the LOD, but this is unusual. If new points of concentrated flow are created or if there is a proposed increase in peak flow, the applicant must obtain a letter from the adjacent property owner acknowledging and accepting the impact(s) to his/her property. POIs with shifted drainage area divides may result in increases in runoff and may have additional quantity management requirements.
B. Determine Waiver Applicability Areas that qualify for a 3.3.A waiver are not required to provide stormwater management. If the entire POI qualifies for a 3.3.A waiver, there is no need to proceed with this process for that POI, other than identifying the POI location.
C. Evaluate POIs Provide a drainage area map delineating the drainage area to each POI for both existing and proposed conditions. The drainage area, impervious area, time of concentration (Tc) flow path, and runoff curve number (RCN) are required for each POI unless it qualifies for a 3.3.A waiver.
D. Classify POIs Calculate the percentage of existing impervious area (%I) for each POI. The area used for calculating %I is the Stormwater Study Area (SSA) and will correspond to either the LOD or ROW within the drainage area to the POI. The same method of calculating %I (LOD or ROW) must be used for all POIs in the project. The designer should check the POI classifications using both methods to determine the best choice for the project.
a. If I>40%, the POI is classified “redevelopment” and reconstructed areas require 50% WQ treatment and no Cpv is required. New impervious cover requires 100% ESDv (WQv and Cpv).
b. If I≤40%, the POI is classified “new development”. Both reconstructed areas and new impervious cover requires 100% ESDv (WQ and Cpv).
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Sediment and Stormwater Current Technical Practices June 7, 2018

E. Calculate IART

LOD

= Limit of Disturbance (acres)

Ai

= Impervious area (acres)

Existing Ai = The total impervious area in the existing condition within the LOD(1)

Proposed Ai = The total impervious area in the proposed condition within the LOD(1)

ΔAi

= Net change in Ai within the LOD (Proposed Ai – Existing Ai)(2)

I

= Impervious cover (%)

(1) Exclude impervious area that is maintenance and qualifies for a 3.3.A waiver. Impervious areas associated with isolated small foundations or posts for signs and lighting structures are considered de minimis and should also be excluded from IART computations. Large structures and areas or concrete pads should be included in IART calculations.
(2) This value will be negative when there is a net decrease in impervious area.

For Redevelopment POIs (I>40%): IART = (Proposed Ai – Existing Ai) + 50% (Existing Ai), therefore: IART = ΔAi + 50% (Existing Ai)

For New Development POIs (I≤40%): IART = (Proposed Ai – Existing Ai) + (Existing Ai), therefore: IART = Proposed Ai

The sum of impervious areas treated by all ESD facilities for water quality (PE=1.0”) must equal or exceed IART. ESD facilities must be sized for the area draining to them. If the project has excess WQv from ESD facilities or Chapter 3 bioretention facilities with a total drainage area of less than 3 acres, it will be credited to the MDOT SHA Water Quality Bank. Other Chapter 3 facilities may be used to treat project IART but may not be credited to the WQ Bank.

Generally, when using the IART method, if IART ≤ 0 for a POI then stormwater management treatment is not required for that POI. There are a few exceptions to this such as improvements that generate no additional impervious area, but the disturbed area is expected to be heavily compacted, or when the RCN is expected to increase in post development condition. For those situations, the designer is expected to address SWM requirements using the method of DA to POI (Table 5.3 in the SWM Manual).

Loss of water quality treatment (LOWQ) must be replaced or otherwise addressed.

F. Provide ESD to the MEP

ESDv =(Pe)(Rv)(A)/12

Where: Pe = 2.6 for A or B soils Pe = 2.2 for C soils Pe = 2.0 for D soils Rv = 0.95 A = ΔAi, for redevelopment POIs (I>40%) A = Proposed Ai, for new development POIs (I≤40%)

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Sediment and Stormwater Current Technical Practices June 7, 2018
ESDv is comprised of a water quality component (WQv and Rev) equivalent to the first 1.0 inch of PE and a channel protection component (Cpv) equivalent to the full target PE. When the project includes redevelopment POI’s that have a combination of new impervious and reconstructed impervious, it is necessary to calculate these components separately.
Water Quality Volume (WQv) ESD practices (Chapter 5 of the Design Manual) must be used to treat the WQv (Pe=1”). Quality management does not have to be provided within the drainage area of the POI but must be provided within the same six-digit watershed. WQv provided by a facility is limited to the impervious area within the drainage area to the facility. For example, if the impervious area draining to the facility is 0.3 acres, physically sizing the facility to treat 0.5 acres will not result in additional water quality credit.
If Chapter 5 practices are demonstrated to be impracticable, Chapter 3 facilities may be used. However, a variance from providing ESD treatment for new development impervious surfaces must be justified and approved by PRD for Chapter 3 facilities to be used. If both Chapter 5 and Chapter 3 facilities are impracticable, a debit from the MDOT SHA Water Quality bank may be used if there is sufficient credit for the watershed and the Highway Hydraulics Division (HHD) approves the transaction. A debit from the WQ Bank satisfies both WQv and Rev requirements for the debited acreage. Typically, WQ Bank debits are intended for projects with a limited scope where IART requirements are minimal and there are circumstances that preclude the use of SWM facilities, such as limited ROW, wooded areas, karst, steep slopes, or urbanized corridors. Examples include Americans with Disabilities Act (ADA) sidewalk projects, safety and resurfacing projects, and projects of a similar nature.
Channel Protection Volume (Cpv) This is the portion of ESDv that comes from “new” impervious area. It represents the minimum volume that must be provided in the POI to satisfy the Cpv requirement. The channel protection volume storage requirement must be met at each POI.
a. For POI’s that are classified as redevelopment (I>40%), Cpv is required for the increase in impervious cover (ΔAi).
b. For POI’s that are classified as new development (I≤40%), Cpv is required for all impervious area within the LOD (Proposed Ai).
Cpv requirements are calculated using the ESDv equation and are inclusive of the first 1” for WQv. If the WQ Bank will be used to address the WQv requirements at a POI or the WQ obligations will be satisfied elsewhere on the project, the required ESDv to satisfy Cpv requirements at an individual POI is still the full target Pe and resulting ESDv for all new development pavement (not the target Pe minus 1”). Additional treatment flexibility can be granted if the WQv portion of the ESDv is satisfied elsewhere, since the selected BMP would then not be required to have a WQ component. The total ESDv provided within a POI should equal or exceed the required ESDv for that POI. It is allowable to provide excess ESDv in one facility to compensate for a shortage of ESDv in another within the
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Sediment and Stormwater Current Technical Practices