DelDOT Maintenance Only

SWM and E&S Involvement for Maintenance Projects Decision Flowchart

This decision flowchart is for DelDOT Maintenance and Operations personnel to use to help decide if a particular project or activity will need to be reviewed and approved by the Stormwater Engineer.

The first decision that has to be made is the amount of disturbance that a project will entail (LOD). Disturbance is defined as anywhere bare soils will be exposed during construction, including areas used for staging and stockpiling on pervious surfaces. This would also include all pervious surfaces within an LOC, even if it is not planned on being disturbed.

For any questions, comments, or concerns, please contact the Stormwater Engineer.

Note to be used on all plans:
For full-depth pavement construction, curb ramp construction, sidewalk construction, and/or utility relocations, bare soils shall be covered within three (3) calendar days to prevent erosion. This can be accomplished by placing proposed subbase material such as graded aggregate base course (GABC) or geotextile / plastic sheeting that is secured so as to not blow away or be undermined by runoff.

Standard Plan Definition: A set of predefined standards or specifications for minor land-disturbing activities that may preclude the need for the preparation of a detailed plan under specific conditions.

Linear Standard Plan Criteria (condensed)

1. LOD ≤ 5 Acres
2. No greater than 1 Acre disturbed at any one time during construction.
3. Pre-developed land use is not classified as "wooded" based on 2017 Land Use / Land Cover data.
4. One of the following must be met:
1. Whole project site is currently being treated by an existing SWM facility.
2. Less than one whole number change in CN value per POA.
3. No new impervious added.

Bridge Standard Plan Criteria (condensed)

1. LOD ≤ 1 Acre
2. Any net increase in impervious area will not exceed 5,000 SF.
3. Land cover will be restored to existing condition or better.

Demolition Standard Plan Criteria (condensed)

1. LOD ≤ 5 Acres
2. No increase in impervious area will be created.
3. Disturbance is for demolition and removal of impervious surface that will expose bare soils.

BMP Construction and Retrofit Standard Plan Criteria (condensed)

1. LOD ≤ 5 Acres
2. The construction will provide either RPv, Cv, or Fv reduction via a new SWM facility, retrofitting an existing SWM facility, creating a floodplain, or stabilization of an eroding area.
3. Pre-developed land use is not classified as "wooded" based on 2017 Land Use / Land Cover data.
4. The total impervious area proposed will not excced the lesser of 7,500 SF or 15% of the LOC.

Stream Diversion Guidance

Disclaimer: This erosion and sediment control stream diversion guidance is intended for use by DelDOT Maintenance & Operations forces only.

For those locations/activities that have a plan set developed, coordination with and approval by the Stormwater Engineer is required. Signature approval will be provided within the plan set, usually located on the E&S sheets.

The intent of this guidance is to give DelDOT Maintenance forces practical and easy recommendations to perform effective erosion and sediment control when having to do a stream diversion for pipe replacement, reshaping of ditches (when drier conditions are required), or any location where an incoming flow needs to be diverted for a short amount of time. When a project will take longer than 7 calendar days, a formal E&S plan needs to be designed and approved by the Stormwater Engineer before any work can begin. If a pipe or culvert being replaced is larger than 60” in diameter or has greater than a 20sf opening, then consult with the DelDOT Bridge Design section first.

There are three main components to a stream diversion:

1. Clean water bypass. The existing incoming flow or possible flow needs to be safely conveyed around or through the work area.
2. Containment and dewatering of the work area. The need to keep disturbed areas within the work zone and potentially dewater the work area in a safe manner.
3. Removal of sediment. This would be the filtering process of the water coming out of the work area so as to not pass unwanted sediment downstream.

Clean Water Bypass

Just as the title implies, there is a need to bypass the incoming flow so that it does not impede the work zone as well as not become sediment-laden before it is released further downstream. The main components for this operation are the upstream and downstream cofferdam and a way to transport the water from upstream to downstream. Cofferdams are temporary enclosures to keep out water and allow dewatering and construction of the permanent facility (pipe/structure) in the dry as best as possible. A cofferdam involves the interaction of the dam itself, soil, and water.

The upstream dam will be placed first, followed by the downstream dam. There are multiple options for creating a dam and it is site dependent on how high and wide to make a dam as well as if there is active streamflow or ephemeral/intermittent flow.

Here are some expedient examples (in no particular order):

1. Impermeable Wrapped Soil. This should only be used in ephemeral/dry ditches. On the upstream end of the project, spread an impermeable layer like plastic, geotextile, or a large tarp just upstream of the intended work area. The width should cover the entire ditch bottom and run up the sides a few feet at a minimum. Push/place the existing soil within the work zone on top of the tarp so that it is secured and to a height, as needed. Now wrap the excess tarp over the top of the soil from upstream to downstream (it should look like a soil taco). The end that was wrapped over can be secured with stakes, stone/riprap, small sandbags, soil, etc. Basically, secured enough to keep the tarp from flapping around. After this is completed, do the same thing on the downstream end of the project, but now the tarp will be wrapped from the downstream side over the soil and secured on the upstream side. The work zone is now enclosed. After the work is completed, the soil can be placed back towards the disturbed area and when enough has been removed, the tarp can be pulled towards the work area to remove any remaining soil.
2. Sandbags. This could include small sandbags, large sandbags, or a combination of both. Just like the procedure above, spread an impermeable layer like plastic, geotextile, or a large tarp just upstream of the intended work area. The width should cover the entire ditch bottom and run up the sides a few feet at a minimum. Place the sandbags on top of the tarp so that it is secured and to a height as needed. Now wrap the excess tarp over the top of the sandbags from upstream to downstream (it should look like a sandbag taco). The end that was wrapped over can be secured with stakes, stone/riprap, sandbags, or even on-site soil. Basically, just enough to keep the tarp from flapping around. After this is completed, do the same thing on the downstream end of the project, but now the tarp will be wrapped from the downstream side over the sandbags and secured on the upstream side. The work zone is now enclosed.
3. Steel plate. This could be an excellent expedient option if not encountering rock formations or excessive tree roots. A prefabricated steel plate is pushed into the ground at a height as needed for the dam. Multiple smaller steel plates next to each other would not be advised as they almost always leak and never seem to provide a good enough seal even if overlapped. A good idea to incorporate into the plate is a weir. This is to help direct any storm events that overtop the plate into the center of the stream first. An ideal weir would be an elongated arc with maybe a 6” drop at the middle of the plate. Other weir configurations could work as well, i.e. v-notch, rectangular, etc.
4. Compost Filter Logs. Triple stack or a single compost filter log (CFL), 18” diameter. This would be an option that would be best for ephemeral/dry ditches, but could potentially be used in very shallow water depths, i.e. less than 10” water depth. The triple stack option should be used for any location with active flow and a single log could be used in a dry ditch. Just like the soil or sandbag option, spread an impermeable layer like plastic, geotextile, or a large tarp just upstream of the intended work area. The width should cover the entire ditch bottom and run up the sides a few feet at a minimum. Now wrap the excess tarp over the top of the compost filter log from upstream to downstream (it should look like a compost filter log taco) and place some small sandbags on top of the compost filter log (every 5’ – 10’ depending on the site condition) so that the compost filter log(s) and tarp are secured. The end that was wrapped over can then be secured with stakes, stone/riprap, sandbags, or even on-site soil if needed. Basically, just enough to keep the tarp from flapping around. After this is completed, do the same thing on the downstream end of the project, but now the tarp will be wrapped from the downstream side over the compost filter log(s) and secured on the upstream side. A 12” compost filter log could be substituted for the 18”, but that would only be good for dry ditches, i.e. no active water running or ponding in the ditch.
5. Water Filled Cofferdam. A couple of prefabricated products include AquaDam and Dam-It Dams. These products are basically geotextile tubes filled with water from onsite. They can be installed and taken out rather quickly and work very well in most situations including rocky and uneven ditch/stream bottoms. Due to the need of water to fill the tubes, dry ditches would probably not be the best place to use these.
6. Self Inflating Cofferdam. A couple of prefabricated products include Water Gate (megasecur.com) and Floodams. These are products that would be good for dry and low flow ditches. They can be employed very quickly and are triggered automatically when under active flow. Plus, this would only be employed on the upstream side of the project as it would not work on the downstream end. So, this could be set up upstream and one of the other options mentioned could be set up on the downstream side.

In concert with the cofferdam is the active pumping of the water from in front of the upstream cofferdam to beyond the downstream cofferdam. For the upstream portion where the water is being pumped from, the major obstacle is being able to pump that water cleanly, i.e. not drawing in any of the surrounding soil/stream bottom/debris. This is referred to as a stilling well. Here are some ideas to help accomplish this feat:

1. If the stream is deep enough, the suction end can be floated. This could be as simple as using an old inner tube or potentially closed plastic containers (i.e., milk jugs) secured to the suction end of the hose, i.e. zip ties. The most important part is to make sure the suction end doesn’t get close enough to the bottom to suck up sediment.
2. For situations where the depth is shallow (15” or less) or maybe even no active flow is present and a pump is on standby in case a flash storm event happens, a few options to consider are:
   1. A weighted 5-gallon bucket with slits or very small holes (1/4”) in the bucket. The bucket could be weighted with some stone or even a hardened concrete layer. The main thing is that the bucket should be able to stand up on its own while the suction end of the hose is drawing water. A bucket laying on its side is not effective for multiple reasons. The holes should only be installed on the top half of the bucket, not starting from the bottom. As for the number of holes, there is no set number other than to state, put in a lot so that water can easily pass to the inside of the bucket as well as not cause any integrity issues to the bucket itself. A couple of things to help the bucket stay upright could either be maybe setting it on a small piece of plywood or even constructing a rudimentary stand.
   2. A small cage with a solid bottom and filled with stone. The suction end would be secured to sit in the middle of the stone.

For the downstream portion where the pumped water will discharge, it is very similar to the upstream operation in that any soil disturbance should be kept to an absolute minimum. This stabilized outfall could be of many different varieties. Here are a few suggestions:

1. Place some geotextile, plastic, or a tarp (4’ x 4’ minimum) on the stream bottom (if dry) and place sandbags or R4 around the edge to secure the tarp. Make sure the discharge point is on the tarp and secured.
2. If the stream has active flow or the bottom is covered with water, can use a similar type setup above, but place the discharge hose on the side slope. Make sure the geotextile, plastic, or tarp runs down the slope and onto the bottom. Don’t worry about the tarp being too long. The sides can be secured with sandbags, stone, or stakes and the tarp only needs to be secured to the water’s edge, not on the bottom. Make sure the discharge hose is secured onto the tarp and the flow from the hose does not go outside of the tarp until reaching the bottom.
3. A very easy method would be securing a long piece of geotextile, plastic, or tarp around the end of the hose, i.e., zip ties, wire ties, etc. and just letting the end of the tarp open up downstream. Remember, the end product is not to disturb any soil as best as possible from the discharge of the hose.
4. A small pre-made cage could also work just like mentioned above with the suction end. Use a cage with a solid bottom and filled with stone (R4 preferable). The outflow end of the hose would be secured to sit in the middle of the stone and water would safely disperse.

The sizing of a pump to transport the water from upstream to downstream can be accomplished in a few different ways.

1. Use a pump that may be readily available. If it is not sufficient, then just add another pump.
2. Use a program like StreamStats to determine a predicted flow. With a predicted flow, a pump size could be ascertained fairly easily. For those not familiar with this method, coordinate with the Stormwater section for guidance and training.
3. Rough calculate the actual flow in real-time and determine a pump size. Utilize the ‘Estimated Active Flow Calculation Program’ (spreadsheet) to determine a flow value needed to adequately size a pump. There are four tabs at the bottom of the Excel spreadsheet which cover two different methods for computing an active ditch flow value, a whole section on determining slopes, and the last tab covers pump type and sizing aspects.
   1. With either flow determination method, there are limitations, which are explained on the particular sheets. These methods will produce an estimated flow value, an accurate flow value would involve precise instrumentation, flow meters, manpower, and time all of which would not be available.

Dewatering of the Work Area

Now that the work area has been contained by the cofferdams and the “clean” water is being pumped from upstream to downstream, it is now time to deal with the “dirty” water within the project area. This water needs to be collected and filtered before being released downstream. This water can come from seepage within the work area or possibly from a recent rain event. For the collection aspect, a sump pit is needed. A sump pit is a small hole in the ground within the work zone to collect and pre-filter any water before being sent to the final filtering device, which will eventually release the water downstream. One of the biggest misconceptions about a sump pit is that it is not intended to dewater a work area below the surface. Its main function is to take care of the dirty water on the surface. If needing to dewater a work area in order to keep it completely dry for installing foundations, then well pointing will need to be explored and that is beyond the scope of this document. If needing further information about well pointing, please contact the Bridge section.

A sump pit can take on many forms, but the main components are a small hole no more than 2’ – 4’ in-depth, a perforated container wrapped with some wire and filter fabric (and with a solid bottom) placed in the middle of the hole, and #57 stone surrounding the perforated container and filling the rest of the hole. The hole should be at least twice the diameter of the perforated container being used. The container can be any number of configurations, with the most common being a cut-down metal pipe approximately 3’ – 4’ in height. Again, a weighted 5-gallon bucket could be used, but this time the perforations will be larger. Wire should be placed around the perforated container to help keep the filter fabric from coming through the holes. Chicken wire would probably work best. The filter fabric will be secured on the outside of the wire and can be held in place with zip ties, wire ties, cordage, etc. The filter fabric shall be non-woven, i.e., no silt fence type material. The allowable flow-through rate for silt fence is too slow and would clog up too easily for this operation. The size of the container and sump pit hole would also be dependent on the type of pump being used. For this operation, a submersible or diaphragm pump would work best. The diaphragm pump would just use an intake hose, whereas the submersible pump would possibly need a container and a hole slightly larger. Refer to the below diagram for the basics needed for a sump pit.

When “dirty” water is pumped out from the sump pit, it now must get filtered before returning downstream of the downstream cofferdam. This is easily accomplished by using a dewatering bag (common name: Dirtbag) or a portable sediment tank (PST). Technically, if the flow coming out of the sump pit is not “dirty”, then it can be directed right to the same stabilized outfall as the clean water bypass. But this requires better attention to detail in installing a good sump pit and as soon as any “dirty” water is spotted, then it needs to get redirected through the dewatering bag or PST. There are some other methods that could be used besides the dewatering bag or PST, but they would be more time-consuming to install, take up a lot more area, and would require more intense maintenance; hence, they will not be mentioned here. If using a PST, the outlet hose could outflow in a stabilized manner as previously mentioned under the clean water bypass. If using a dewatering bag, the outflow could still be easily stabilized, but just needs a little more attention. As with any outflow, the main objective is to not disturb any soils. Place a long piece of geotextile, plastic sheeting, tarp, etc. on the ground big enough for the bag to sit on as well as run-down onto the ditch/stream bottom. If using multiple sheets, just layer like shingles on a house where the upper portion overlaps the lower. Make sure the sheeting is secured down to the water’s edge. This is best done by sandbags because they serve two purposes. One, they secure the sheeting, and two, they help keep the flow contained on the sheeting till it reaches the ditch/stream bottom. This could also be accomplished by utilizing compost filter logs where the logs would sit under the edge of the sheeting creating in effect a chute. Just make sure to secure the logs and sheeting, so they stay in place. When placing the bag itself, set it on a relatively flat surface, otherwise, it will just roll downhill when water starts being pumped into it. Just imagine a big water balloon. Another handy option is to place at least a couple of straps under the bag so that when picked up later, it is less likely to bust open while removing. As for what size bag is needed, they come in standard sizes and will be dependent upon how much flow will be going through them.

The above recommendations are for DelDOT Maintenance & Operations only and if the pipe or culvert being replaced is larger than 60” diameter or has greater than a 20sf opening, then consult with the DelDOT Bridge section.

Any questions, comments, or concerns about this document can be sent to the DelDOT Stormwater section.

Estimated Active Flow Calculation Program

Estimated Active Flow Calculation Program