Enforcement on new by-laws involving resident of many years

For legal consequences consult an attorney. Unless your Covenants specifically provide that there shall be no riprap allowed on the banks, then in my non-legal opinion it is well within the Board's purview to add the riprap in order to maintain the bank. IMO if someone failed to pay the special assessment then they would be subject to being liened and eventually foreclosed upon. Have you asked the Board why they are so intent upon adding riprap if there are no problems?

You can get liened for not paying special assessments.

If the appropriate document was properly amended, and the guideline is considered reasonable, not in conflict with a higher precedent document and properly applied to all members, the answer would be yes.

The legal consequences would be based on your governing documents enforcement policy and applicable State laws, VA § 55-513 for example (applicable if your in an HOA vs. a condominium).

Also see: Covenant Enforcement: An Overview a 2012 pdf document/article from an HOA legal firm in the northern VA area.

Now lets also try and clear up terminology:

Covenants are restrictions and agreements that are written in the CC&Rs and are attached to your deed (deed restrictions). These typically specify what you can and can not do with your property.

Architectural Guidelines are standards utilized by an approving authority to standardize what will or won't be approved within the community. They are utilized when a member requests prior approval to make an exterior change to their property. Said approval is required by a covenant. Often, an Association will mistakenly say that a member is in violation of a guideline. In reality, they are in violation of failing to request and receive prior approval for a change to the exterior of their property.

Community Standards are similar to guidelines. These are adopted by the Association and often clarify ambiguous statements within a covenant. For example, a covenant may require that a member maintains their property in good order and repair. A community standard specifying that that the lawn may only have x% of weeds, grass is to be kept no higher than x inches, etc. would be clarifying what the Association considers a properly maintained home. Again, an Association may mistakenly claim that you are in violation of community standards when in actuality, a member would be in violation of the covenant to maintain their property in good order and repair.

Rules and Regulations are for specifying standards of use and personal conduct when using common areas.

Ahh,

You were discussing an issue for the CA home (your State showed up as VA, which is why the advice was given for VA).

In CA, same thing applies - if it's on common area, the Association has the responsibility and requirement to maintain or be fined. Check with City or County water departments, as they likely have the regulations that need to be complied with.