Hydrology and Flooding - Expert Overview

Residential and commercial development can permanently affect the flow of stormwater, potentially negatively impacting local waterways and neighboring properties. Activities during the construction of these projects can also temporarily affect stormwater flow conditions. In this article, Civil Engineer, Michael Tuttman discusses construction standards relevant to erosion and stormwater control measures.

Hydrology and Flooding - Expert Overview

Erosion can be defined as the gradual destruction or diminution of something. Running water, streaming over exposed ground, causes erosion of the land. This can be an accelerating process – once a gulley or channel is scoured out by a flow, it will grow as time progresses and more soil is carried away by additional water. Unchecked, stormwater can cause flooding, and erosion can undercut and destabilize structures, and/or wash away roadways. The accompanying issues created by erosion are:

  • Deposition: the re-accumulation of eroded material
  • Siltation: the contamination of water with eroded material

Requirements for Construction Activity

Construction activities often strip the ground of vegetative cover that had served to protect the soil and prevent erosion. In addition, poorly planned development can lead to erosion and the subsequent deposit of earth and sediment into lowlands and local streams and lakes. While this is a normal, natural process, if not planned for, new construction can accelerate it and create new problems.

Runoff containing soil, sediment, or other contaminants is a pollutant and it is necessary, and legally required, for construction activity to plan for erosion control and to take measures to prevent contaminated stormwater from entering waterways. Prior to beginning, construction projects must submit a SWPPP, or Stormwater Pollution Prevention Plan, to the local municipality and the state Department of Environmental Conservation. The SWPPP details what erosion control measures will be taken. The federal Clean Water Act also requires that a facility or construction activity that could impact any designated “Waters of the United States” obtain an NPDES (National Pollution Discharge Elimination System) permit.

TEMPORARY EROSION CONTROL MEASURES

Types of temporary erosion control measures used in construction projects include straw bales, silt fence, check dams, and ground coverings of jute or other types of geotextiles. In long-term construction projects, temporary seeding of embankments and slopes with grass is sometimes done. Permanent storm drainage facilities can include swales, check dams, culverts, catch basins, underground piping, and detention ponds.

STORMWATER CONTROL SYSTEMS

Soil will absorb stormwater until it becomes saturated, after which further precipitation will run off. The speed of the water’s flow will depend upon local conditions such as the slope of the land and the degree, and type, of vegetation present. When designing a stormwater control system, local land conditions, along with the intensities of expected storms, must be considered in calculating what capacity is needed. It is not enough to say that a given amount of rain drops so many gallons of water over an area. As a storm progresses and overland flow of water reaches the system, the amount of water carried in a storm drain increases until it peaks, eventually declining after the rain has passed.

Ground that is able to absorb water is referred to as “pervious” while surfaces that will not hold or pass water are called “impervious.” Impervious surfaces include concrete and asphalt pavement. Areas covered by buildings and other structures are also considered impervious. As a location becomes increasingly developed the land becomes less and less able to absorb water, and the need for larger and improved storm drainage systems is increased. Also, water generally is able to flow across an impervious surface faster than a pervious one; therefore, in a built-up area, the volume of stormwater entering the local storm sewer is increased and peak flow conditions are reached more quickly.

Where development turns large areas from pervious to impervious conditions it can be necessary to create one or more detention basins, designed to hold the increased volume of stormwater that the land is unable to absorb. Detention basins are equipped with an outlet sized to slowly release the collected water so that surrounding area is not overwhelmed by a sudden, large flow.

Storm drainage systems are not designed to hold water forever. What goes in will have to eventually come out. In cases where there is no allowance for detention, stormwater collected from a large area will outlet at a single location. While some closed systems outlet directly into a body of water, others discharge into an open swale or even onto open land. This concentrated stream is more likely to cause erosion than the original spread out flow. A closed system can redirect stormwater, thereby increasing the volume running into a specific area possibly leading to flooding problems.

CASE EXAMPLES

Case 1 – Downstream Erosion & Tree Loss

In this case, the Plaintiff claimed that stormwater flow from the outfall of a closed drainage system was causing erosion and killing trees along the forested portion of a hillside below the storm drain’s outfall.

Our expert opined that the newly constructed storm drain did concentrate stormwater that formerly flowed over a significantly larger area; this water was channeled, via a stone swale, to the top of the Plaintiff’s hillside property contributing to the cause of erosion gullies and dead or dying trees.

Case 2 – Downstream Pond Siltation & Infill

In this case, the Plaintiff claimed that pond siltation and infill was caused by long-term runoff from the defendant’s property, as well as short-term infill resulting from soil slide from the defendant’s adjoining property due to a major storm event and improper stormwater management. The Plaintiff proposed a remediation plan that called for the pond to be emptied of water to allow for the removal of sediment and infill soil.

Our expert looked at stormwater flows and soil conditions in the area and opined that the sedimentation was a natural process; therefore, not the fault of the defendant. However, the defendant had some responsibility for the infill soil at one corner of the pond. Our expert proposed an alternate remediation method that would allow for the removal of the infill soil without dewatering the entire pond. This greatly reduced the cost of remediation.

HYDROLOGY / FLOODING INVESTIGATIONS

Our experts in hydrology and hydrogeology are retained when injuries or economic losses occur related to ground or surface water pollution, flooding or water supply depletion, or various other issues related to soil and water quality. Robson Forensic boasts experts in Geology, Hydrogeology, Civil Engineering and Construction who are experienced in assessing and investigating the issues at hand when injury, property damage, or other economic loss has occurred.

To discuss your case with an expert, contact the author of this article or submit an inquiry.