Hint: This Does Not Cause Sea Level to Rise in Portsmouth
This is Part IV of our five-part series on sea level rising in the Portsmouth area.
Is the sea the only source of flooding for Portsmouth? Does the rise in sea level, even if it is tiny, explain why the Portsmouth waterfront is so easily flooded? Our now-wiser Chicken Little says, “No way!”
The story is far more complicated, as we have begun to learn in our earlier articles on sea level. If someone tells you there is only one explanation to anything, be skeptical! You, dear reader, have learned that Earthly issues are more complicated.
We have focused on what’s going on downstream from Portsmouth. Let’s look upstream. There’s a lot going on along the Piscataqua!
What Is Urbanization When It Comes to a River?
We’re not talking about building houses and streets on the river. But we are talking about filling wetlands, building flood walls and building on flood plains. For a good example, you don’t have to look any farther than Prescott Park.
Part I showed us that the Park used to be a stream with its own flood plain. All of that was filled in by dumping garbage on it and eliminating the stream. Now when the Piscataqua River floods Prescott Park, it is simply reclaiming temporarily the flood plain that was and continues to be part of the old stream and the Piscataqua River.
The River has now been urbanized by filling in the old stream, constructing buildings along the River’s stream banks and building flood walls for the docks. These activities eliminate the historical flood plain and reduce the “flood safety valve” features that the River used to absorb floodwaters.
Construction, Construction, Construction
In case you haven’t noticed, New Hampshire is growing. Why? We know Rockingham County is an attractive place to live. Friendly people, less noisy and less crowded than Massachusetts, cost of living is less. Taxes are low. Yet, the State provides many services. New Hampshire is the fastest growing state by population in all of the New England states.
To accommodate this growth, amenities have to be built. New or improved roads, expanded buildings for living and services, and better facilities for water and waste disposal. All of this means encroachment on what makes New Hampshire so physically attractive. Because a greater population means more construction.
As you can see, the Piscataqua River drainage basin covers a significant area of New Hampshire. It is about 1495 square miles. The River itself is about 12 miles long with its main tributaries being the Salmon Falls River and the Cochecho River. Rivers and streams flowing into Great Bay are part of the area.
There are many towns and several cities within the drainage basin. Some of these have seen significant growth over the past 50 years, leading to the urbanization of those towns and cities. Development along the Piscataqua has made several species of fish extinct in these waterways, specifically salmon and the lamprey. Oyster beds in the Great Bay area struggle with increased nitrate loading and the effects of denser population and development in the area.
Hydrology – The Study of Water
Since we are interested in the effects of urbanization on the Piscataqua River, let’s go to an Environmental Protection Agency (EPA) publication on urbanization and hydrology. This study describes what happens to streams with urbanization. The Piscataqua River is tidal and it is not heavily urbanized. But mankind has certainly bent the River to its needs.
The graph below helps explain these generalized features. The graph shows how a typical stream behaves in a rural setting (green) and in an urban setting (yellow). The graph is a plot of stream discharge, or water volume flowing in the stream during a unit of time (for example, cubic feet of water per hour), versus time (for example, over a week’s time).
The Yellow Line is “Spikier” …
Moving from green to yellow, common reported effects of urbanization on stream flow conditions include:
Stormflow
- Increased high flow frequency
- Increased high flow magnitude
- Increased “flashiness”
- Decreased high flow duration
- Decreased lag time
Baseflow
- Decreased low flow magnitude
- Increased low flow duration
… And the Green Line Is More Muted and Spread Out
Since the Piscataqua River is Tidal, These Effects of Urbanization Are Not as Dramatic
While the urbanization effects are likely to be less obvious for the Piscataqua River, still the shift towards those conditions from a rural stream state may be noted.
The term which may be most useful to the layperson is the river’s “flashiness.” Imagine a heavy rain falling in the upper reaches of the Piscataqua River drainage basin. A rural stream, as is shown in the above chart, would respond gradually with more slowly rising stream levels as the river surges downstream due to its storm loading of rainfall and runoff.
As the river becomes urbanized, the river’s response to the rainfall and runoff becomes “flashier.” That is, the river rises more quickly, may flood more easily and then recede more quickly. Conceptually, that is shown by moving from the green line to the yellow line on the graph.
This is because urbanization removes the dampening effects of a rural river due to channel straightening, flood wall construction and destruction of wetlands. Urbanization reduces the opportunities for recharge to occur because of an increase in impervious surfaces being built.
Finally, groundwater withdrawal due to pumping water from domestic and city wells may lower the groundwater table. This reduces flow entering streams from the groundwater. This reduces or eliminates the stream’s base flow, or the minimum flow observed during drier months. This may mean the elimination of stream habitat for some species.
How Does This Affect Residents of Portsmouth?
Generally speaking, think of urbanization as removing the shock absorbers around a stream when it loses its more rural character.
Urbanization Reduces Low-Flow Duration and Magnitude
Regarding the reduction of baseflow, some tributaries of the Piscataqua, especially in the Dover area, show much lower stream flow once a storm has passed and “normal” stream conditions return. Baseflow levels may be reduced and portions of streams in the Piscataqua River Basin may even go dry in the summer months. Such a drop in “low-flow magnitude” can have an effect on biota diversity in the streams. In addition, urbanized streams may also erode and destroy environments for organisms living on the stream beds.
The Piscataqua River Becomes “Flashier”
Perhaps of greater interest for Portsmouth residents is the impact on streamflows during storms. Following the graph above, it would be expected that higher flood levels would be observed along the Piscataqua due to urbanization.
The removal of wetlands and the construction of seawalls and flood walls would all contribute to the river’s inability to absorb rising stream levels. Engineered structures, such as retention ponds and infiltration galleries, are important to mitigate these outcomes, but they may not be widely employed due to expense.
From a management point of view, stream urbanization is clearly a regional issue. The effects of urbanization is cascading and it is important to consider such effects in the upper reaches of the drainage basin as well as in the mid- and lower-reaches. Such coordination requires regional management efforts that cut across town and city boundaries.
Urbanization Does Not Cause Sea Levels to Rise!
It is not a surprise if some residents notice elevated stream levels during severe storms as the Piscataqua River may have become flashier due to the urbanization of the River in its lower reaches. This stream behavior is separate from the rise of sea level.
Chicken Little, don’t confuse the two!
Our final part, Part V, will bring us to tides and storm surge.
