Stormwater and Sediment 



direct and convey excess runoff in the watershed accelerate the process of hydrologic 

 alteration. Net effects of development on watershed hydrology include: 



• increase of peak discharge two to five times above pre-development levels; 



• increased volume of storm runoff produced by each storm (a moderately developed 

 watershed may produce 50 percent more runoff volume than a similar forested 

 watershed during the same storm); 



• decreased time of concentration (time required for runoff to reach a watercourse) by 

 up to 50 percent, especially if major drainage improvements arc made; 



• heightened frequency and severity of flooding, increasing frequency of bankfull 

 discharges several fold; 



• reduced streamflow during prolonged periods of dry weather due to reduced level 

 of infiltration in the watershed; and 



• greater runoff velocity during storms, due to the combined effect of higher peak 

 discharges, rapid time of concentration, and smoother hydraulic surfaces resulting 

 firom development. 



Stream channel geometry is also affected by urbanization, resulting in the following 

 responses: 



• channel widening, with consequent severe erosion of unconsolidated, highly 

 erodible floodplain soils; 



• increased flood risk due to elevation of stream floodplain to accommodate higher 

 post-development peak discharge rate; 



• gradual undercutting and slumping of streambanks, with loss of vegetation which 

 triggers a second phase of bank erosion; and 



• severe sedimentation of channel areas, such that channel habitat substrate is 

 blanketed with deposits of sediment 



Alterations in Estuarine Hydrology 



Depending upon the flushing rates of estuaries, and other factors related to the nature of 

 the material and the receiving waters, sediment loading can have significant physical 

 impacts. Development of breachways connecting coastal lagoons and open waters can alter 

 circulation patterns so as to induce rapid sedimentation. Impacts documented in Rhode 

 Island salt ponds due to alteration of breachways include: 



• loss of water depth; 



• increased turbidity, reduced light penetration; 



• alterations in temperature regimes; 



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