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Basement flooded? Here’s why…

Many Palmyra residents and business owners have been experiencing basement flooding issues over the last several months – even in homes and neighborhoods where flooding has never before occurred. If you are one of these residents, we sympathize and want you to know you are not alone. While of little comfort, you are actually part of a larger group of residents, not just in Palmyra, but within the entire region, that are experiencing this problem. We know that municipalities up river as far north as Burlington and as far east as Mount Holly and Medford are experiencing these same issues, as are Riverton and Cinnaminson residents. For proof of this, just take a ride through any of these residential neighborhoods and note the number of hoses extending from the various homes actively pumping water into the street.

So why is this happening now and what is the Borough doing about it? While it may seem difficult to believe, it appears that it is being caused by something as simple as too much rainfall. The ground is simply saturated, and especially hard hit are those homes located in the floodplain, which by definition, are constructed at a much lower elevation and therefore subject to ground water issues at a greater rate than those constructed further inland and at higher elevation. This year especially has seen a significant increase in rainfall over previous years, as listening to any weather report will tell you. We have illustrated this in the following graphs.

Precipitation Bar Graph

Precipitation Line Graph

What even a cursory review of these graphs will tell you is that by any measure we’ve seen a substantial increase in rainfall over any point in recent years. For instance, during the previous peak year in 2015 we received 36.9 inches of rainfall. So far this year — and it’s not yet over — 55.21 inches have fallen on our community! That represents a 50% increase over 2015 — and a 65% increase from the previous four year average! Given it’s rainfall that is the basic cause, there really is little the Borough can do about it — it’s all Mother Nature at this point. Having said that, we successfully manage any storm water that we collect in our drainage system located in our streets (which are all in operating order) but we can’t do anything about the rain, the rising water table, or the water that collects in your basement or yard.

Perhaps a more technical explanation is necessary, in which case you need to understand the “hydrologic cycle”; a complex topic that has had hundreds of books written about it. In overly-simplistic terms, water evaporates from the oceans, condenses in clouds and falls back to the earth as rain. Once the rain hits the ground, it has two prominent pathways that it takes to get back to the ocean. The first route is through direct surface water runoff. This would be the surface water that you see running down the street to a storm drain. As noted above this pathway is the one area where the Borough is able to effectively assist — and we do. The various storm drains that are located in the streets, called inlets, and their connecting pipes, provide a means to collect this surface water and generally route it (in a vast majority of cases) to either the Delaware River or the Pennsauken Creek. These water bodies flow out through the Delaware Bay and into the Atlantic Ocean, completing the cycle.

This time of year however, the storm drainage system may not function at peak performance due to the leaves that accumulate in the streets, which can block storm water flow into the inlets. During this time of year the Borough’s Public Works Department therefore, continually runs daily crews operating leaf collection machines to minimize this impact. Residents can assist the Borough with this effort as well by not piling leaves in the street gutters (keeping them up on your curb is preferred) – and especially over inlet grates — and by clearing leaves and debris from inlet grates if they notice an accumulation. This would assist with keeping the Borough’s storm water system functioning at peak performance. Please know that the Borough constantly inspects its storm water system and with the exception of the leaves issue discussed here, has found them to all be functioning properly.

While the sight of standing water around an inlet may seem like an issue, it’s actually the second prominent pathway that the water takes to get back to the ocean that is the problem. Whatever rainfall does not fall onto an impervious surface (roadway, roof, etc) or runs off a lawn into the street, is soaked into the ground and becomes groundwater. The ground, especially in southern New Jersey is comprised of a combination of sand, silt and clay — our soil composition. It should be noted that sands are more likely to allow water to flow through than silts, which are more likely to do so then clay. The combination of these soil types makes up a porous material that essentially acts like a sponge. The rain water that soaks into this “sponge” passes through it both vertically and laterally, the latter flowing towards creeks and rivers like the Delaware River and Pennsauken Creek, which again flow out through the Delaware Bay into the Atlantic Ocean, completing the cycle.

When, as in this year, we have significant amounts of rainfall, the “sponge” essentially “fills up” faster than the groundwater can be transferred into the creeks and rivers. This is known as saturation. If the ground water level rises enough it can flow into basements, though the “tightness” of individual basement construction can impact the severity. At this point the only recourse a homeowner has is to pump the water out. However, until the water level in the ground drops below the level of the basement, water will continue to infiltrate into the basement. And, unfortunately, there is no way to lower the water table on the scale that this issue is occurring throughout the community (remember it’s occurring all over south Jersey) other than to let nature run its course, which will take time.

Source: Winter, Thomas C., et al (1998) Groundwater and Surface Water A Single Source. USGS Circular 1139.

Areas where basement flooding occurs most are in low lying areas, as homes built there have their basements extending deeper into the ground relative to the elevation of the water table. This is sometimes a difficult concept to grasp, but one must remember that the ground is not flat, it has high areas and low areas. The ground water elevation is usually relatively flat in comparison, though sloped slightly towards the rivers and creeks. So a home built where the ground level is low (bottom of a hill) is closer to the water table then a home built at the top of a hill. These low lying areas are also the more flood prone areas from surface water runoff too, the first prominent pathway that water takes to get back to the ocean. However, basement flooding is caused by a rise in the groundwater table and not surface water runoff.

A review of the Borough Flood Plain Map therefore, can indicate areas where basements would be more likely to flood as it notes areas that are at a lower elevation, and therefore closer to the water table. The 100 year flood elevation for the Borough is 11.1 ft, the blue dotted area on the map therefore shows areas where the ground elevation is below 11.1 ft. By way of example, assuming a home built in this area has a first floor elevation of 11.1 ft. and a typical seven foot high basement, the basement floor is at a relative elevation of around 3 ft. (assuming one foot thick first floor framing). As the normal high tide in the Delaware River is around elevation 7, which based on the previous discussion is where the water table is trying to drain to, it should be easy to see how high rainfall can lead to additional ground water, which leads to a higher water table that is clearly above many potential basement elevations. Additionally, while homes outside of the 100 year flood areas would be at a higher elevation, their basements can still extend into the ground below current elevated water table levels. Select the map below for more information.

FEMA/Floodplain Map for the Borough of Palmyra

It should be noted that the Borough is considering installing a number of piezometers around the Borough to allow for monitoring of actual groundwater levels. While this will allow the Borough to gain a better understanding of the relative water table levels it will not provide a solution to the current issue. The Borough has no control over this issue anymore then it has control over the weather. Again, until the water level in the ground sufficiently drops, basement flooding will continue to be a problem. And again, unfortunately, there is no way to lower the water table on the scale that the issue is occurring until Mother Nature is ready to do so.

In the interim, the one item we do recommend to those who have to pump out their basements is to direct the flow to the Borough’s storm water system in a responsible manner (and please avoid discharging to any area where water can soak back into the ground!). Pumping water to the street will help direct the water on its journey back to the ocean without continuing to impact the water table. And please note where your discharge is going; avoid flooding areas or roadways and be considerate of your neighbors as the cold weather can lead to icing issues. Also, and this is critical, please do not direct pumps, including your sump pump, to discharge the water in your basement into the sanitary sewer system! This is very, very important! Aside from being illegal to do so this water will end up in our sewer plant where it will threaten the biology and operation of our wastewater treatment facilities! Violators can be fined so please discharge to the street or an area of your property that does not affect your neighbors.